Study of Large Deformations in Automobile Crash Box with ...

i

Study of Large Deformations in Automobile Crash

Box with Novel Geometric Shapes

THESIS

Submitted in partial fulfilment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

by

NASIR HUSSAIN N ID. No. 2013PHXF0113H

Under the Supervision of

Prof. Srinivasa Prakash Regalla

&

Under the Co-supervision of Prof. Yendluri Venkata Daseswara Rao

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI 2020

ii

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI

CERTIFICATE

This is to certify that the thesis entitled “Study of Large Deformations in Automobile

Crash Box with Novel Geometric Shapes” and submitted by NASIR HUSSAIN N, ID No.

2013PHXF0113H, for the award of Ph.D. of the Institute embodies original work done by

him under our supervision.

(Signature of the Supervisor) Date:

Dr. Srinivasa Prakash Regalla

Professor, Department of Mechanical Engineering, BITS-Pilani, Hyderabad Campus,

Jawahar Nagar, Kapra Mandal, Hyderabad – 500 078, Telangana, India.

(Signature of the Co-supervisor) Date:

Dr. Yendluri Venkata Daseswara Rao

Associate Professor, Department of Mechanical Engineering, BITS-Pilani, Hyderabad

Campus, Jawahar Nagar, Kapra Mandal, Hyderabad – 500 078, Telangana, India.

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BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI

DECLARATION

I hereby declare that the thesis entitled “Study of Large Deformations in Automobile

Crash Box with Novel Geometric Shapes” is conducted under the supervision of Prof.

Srinivasa Prakash Regalla and Prof. Yendluri Venkata Daseswara Rao, Department of

Mechanical Engineering, BITS-Pilani, Hyderabad Campus for the award of Ph.D.

I also declare that this thesis represents original work done by me after the registration for

degree of Ph.D. at BITS-Pilani, Hyderabad Campus and has not been included in any other

thesis or dissertation submitted to this or any other institution for a degree, diploma or other

qualifications.

(Signature of the Candidate) Date:

Name: Nasir Hussain N

ID. NO.: 2013PHXF0113H

Research Scholar,

Department of Mechanical Engineering, BITS-Pilani,

Hyderabad Campus, Jawahar Nagar, Kapra Mandal, Hyderabad – 500 078, Telangana, India.

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Dedicated to my beloved Parents

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Acknowledgements

First and above all, sincere and deep hearted thanks to God, the Almighty, for

providing me everything including this opportunity and granting me the support and

capability to proceed successfully. With His blessings only, I have accomplished this huge

task.

I am grateful to my honourable supervisors Prof. Srinivasa Prakash Regalla and Prof.

Yendluri Venkata Daseswara Rao for their constant support during the entire course of the

Ph.D. program. Their timely guidance and continuous encouragement helped me in

completing the thesis in time.

I express my sincere thanks to all those who directly or indirectly helped me in

completing my Ph.D. in the Mechanical Engineering Department at Birla Institute of

Technology & Science, Pilani (BITS Pilani), Hyderabad campus. I take this opportunity to

thank my doctoral advisory committee members, namely, Prof. N. Suresh Kumar Reddy and

Dr. Arshad Javed, for their valuable suggestions during the entire course of Ph.D.

I express my sincere thanks to Prof. Jeevan Jaidi and Dr. Sabareesh Geetha

Rajasekharan for their support and guidance during the process of submission of the Ph.D

thesis and also for their valuable suggestions during the course.

I am thankful to Prof. B.N. Jain (former Vice-Chancellor, BITS Pilani), Prof. V.S.

Rao (former Director, BITS Pilani Hyderabad campus and former acting Vice-Chancellor,

BITS Pilani), Prof. Souvik Bhattacharyya, Vice-Chancellor, BITS Pilani and Prof. G. Sundar,

Director, Hyderabad campus for giving me this opportunity and providing the facilities for

research in the institute. I am thankful to Prof. S.K. Verma, former Dean, Academic Research

Division, BITS Pilani, Prof. Vidya Rajesh, former Associate Dean, Academic Research

Division BITS Pilani, Hyderabad campus and Dr. V.V. Vamsi Krishna, Associate Dean,

AGSRD of BITS Pilani, Hyderabad campus for their encouragement and co-operation in

carrying out this doctoral work. I am thankful to the convener and members of the DRC for

their continuous support in fulfilling the academic requirements.

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I would extend my sincere thanks to all the teaching and non-teaching staff members

of the Department of Mechanical Engineering, BITS-Pilani, Hyderabad campus for

supporting and helping me whenever I needed.

I would like to express my sincere thanks to Prof. Tatacipta Dirgantara, Prof.

Leonardo Gunawan, Dr. Annisa Jusuf, Rizkyansyah Alif Hidayatullah and Eka Curie of

Bandung Institute of Technology and Research University for allowing and also supporting

me in conducting the drop weight impact testing of crash box specimens in the Impact

Testing Laboratory, Mechanical and Aerospace Engineering Department, Bandung Institute

of Technology and Research University located in Bandung, Indonesia.

Lastly and importantly I would express special thanks to my supportive family. My

sincere thanks to my parents, for their unconditional support and making all my dreams come

true. Their love, support, motivation and all physical help have been immeasurable. Special

thanks to my wife for all the sacrifices she has gone through for the successful completion of

my thesis.

NASIR HUSSAIN N

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ABSTRACT

Passenger car vehicle safety requirements are growing steadily due to the increased

user awareness. To ensure conformance of vehicle design to these safety requirements,

different countries have put in place regulations with respect to crash safety. These

regulations require a particular vehicle to satisfy certain criteria for obtaining good ratings in

the crash test, thus forcing the manufacturer to make safer cars. Therefore, it has become

more important for automobiles manufactures, in order to pass these regulations, to improve

vehicles through innovative design of the critical parts of the vehicle having bearing on crash

safety. The crash box is one such very important component. The challenge for the

manufacturer is always to decrease the weight of the automobile by reducing the mass of the

vehicle, at the same time provide greater safety through stronger body parts and more energy

absorbing crash box. Making the automobile parts with thinner sheet metals to reduce weight

of the vehicle makes it more difficult for the automobile structural components to qualify by

absorption of sufficient crash energy in case of an impact to the vehicle. The solution to this

contradicting requirement lies in using composite materials that offer higher strength to

weight ratio as compared to the conventional metals and alloys. Composite materials can be

good alternatives for metals and metallic alloys in vehicle structural safety applications such

as crash boxes. Crash box is used in a vehicle for the purpose of absorption of the collision

energy in a frontal collision or frontal crash impact. It is mounted at the front portion of the

front rails in a BIW (body-in-white) or structure of a vehicle. In the event of an impact due to

collision of vehicle, crash box absorbs the impact energy by collapsing with large

deformation so that there is minimum damage to the vehicle parts mounted behind it.

The behaviour of crash box made of composite materials under impact loads has to be

studied for a better understanding of the influence of geometry and type of material on the

crashworthiness. One interesting method of improving the geometric shape of the crash box

is the use of triggers. Triggers are geometric features applied on the crash box to initiate,

modify and improve the deformation pattern so that the crash box may deform in the desired

pattern that helps in achieving the target force and energy level during the deformation of the

crash box. Triggers of certain shapes have been used in the past in metal crash boxes but the

usage of triggers for composite material crash boxes is still under development in automobile

industry. External triggers (external devices with intended trigger configuration and attached

to the crash box) were explored more in composite shell crashworthiness studies. On the

viii

other hand, the work involving triggers intrinsic to geometry (triggers integrally incorporated

into the geometry of the crash box itself, also known as geometric intrinsic triggers) was

done more for metals but less for composite materials. In the present work, for the

improvement of crashworthiness of glass fiber reinforced plastic (GFRP) composite crash

box, triggers have been designed to be integral with the geometric shape/design of crash box.

Triggers can be very helpful for obtaining the target peak force value, energy absorption and

desired deformation pattern.

In this research work, crashworthiness of composite crash box made of GFRP

material designed with different types of geometric cross sections, along with application of

various geometrically intrinsic novel triggers is studied extensively. Initially, crash boxes

made of glass fiber reinforced plastic with four different types of cross sections are

considered. The cross sectional shapes selected for the present work are square, cylindrical,

hexagonal and decagonal geometries. Later, various types of novel triggers, such as; Notch

triggers (different types of notch triggers), Thickness variation triggers (different types of

thickness variation/front end triggers) and Slot triggers (different types of slot triggers) are

used with combination of different geometric cross-sectional shapes.

Numerical simulation is done to understand the effectiveness of each type of cross

section on the crashworthiness behaviour of the GFRP crash boxes when subjected to impact

at low velocity, as per the standard of vehicle testing procedure known as Research Council

for Automobile Repairs (RCAR) test. This was followed by a comparative numerical analysis

for understanding the effectiveness of each type of trigger on the crashworthiness

characteristics of GFRP crash boxes. Force versus displacement plots also known as Force-

Displacement (F-D) diagrams have been constructed and studied in detail to understand the

relationship between the force and deformation of the GFRP crash boxes under impact

loading. Specific Energy Absorption (S.E.A) values are compared for all the combinations of

the cross sections as well as the trigger types used for the crash box for better understanding

of the crashworthiness characteristics. Later carefully chosen variations of the crash box are

manufactured by hand lay-up process to conduct the experiments. The crashworthiness of the

crash boxes is studied by impact testing experiments using drop weight impact testing

machine. The experimental results are correlated with numerical simulation and a good

agreement between them is achieved.

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Keywords: Automobile crash box, composite materials, crashworthiness, drop weight impact

testing, glass fiber reinforced plastic (GFRP), low-speed impact testing, numerical

simulation, RCAR, specific energy absorption (S.E.A), triggers.

x

Index

Certificate..…………….……………………………………………………………….. ii

Declaration..…………….……………………………………………………………….. iii

Acknowledgements……...…………………………………………………………….. v

Abstract……….………………………………..………..………………………..……. vii

Table of Contents………………..……………………………………....………….….. xi

List of Tables………………..………………………………………..………….……... xv

List of Figures………………………………………………………………………….. xviii

List of Abbreviations………………………………...………………….………..……. xxvii

List of Symbols……………………..………………………………..………..….…..... xxviii

xi

Table of Contents

Title Page No.

Chapter-1 Introduction…………………………………………………………… 01

1.1 Automobile safety and crashworthiness………………………………………... 01

1.2 Composites in Automobiles…………………………………………………….. 05

1.3 Potential for Composites in Electric Vehicles………………………………….. 07

1.4 Research Background…………………………………………………………... 09

1.5 Background of Deformation and Fracture in Composite Material Crash Boxes.. 11

1.5.1 Types of Composite Materials…………………………………………….. 11

1.5.2 Failure Modes in Compressively Loaded Composite Material Structures... 12

1.5.2.1. Breakage of fiber…………………………………………………….. 12

1.5.2.2. Matrix-Deformation or Matrix-Cracking……………………………. 13

1.5.2.3 Separation of Fibers from the Matrix………………………………… 13

1.5.3 Axial Crushing of Composites…………………………………………….. 15

Chapter-2: Literature Review……………………………………………………. 19

2.1 Review of the Past Work……………………………………………………….. 19

2.2 Gaps in existing Research………………………………………………………. 26

2.3. Research Objectives of the Present Work……………………………………… 27

2.4. Scope of Study…………………………………………………………………. 28

2.5. Research Methodology…………………………………….…..………………. 29

2.6. Organization of the Thesis Report……………………….…..………………… 31

Chapter – 3: Numerical Simulation of GFRP Crash Boxes..…………………... 34

3.1. Introduction…………………………………………………………………….. 34

3.2 Merits of Pre-Test Numerical Simulation in Crashworthiness ………….……... 34

3.3 Numerical simulation of composites in LS-DYNA…………………………….. 35

3.4 Analysis Procedure of Composite Crash Boxes……...………………………… 40

3.5 Numerical Analysis of GFRP Crash Boxes…………..………………………… 46

3.5.1 Different types of cross sectional geometries with no triggers……………. 47

3.5.2. Notch Triggers for Different Cross Sectional Crash Boxes………………. 50

3.5.2.1. Square crash boxes with notch triggers ……………………………... 50

3.5.2.2. Cylindrical crash boxes with notch triggers …………….……..……. 52

3.5.2.3. Hexagonal crash boxes with notch triggers …………………………. 54

3.5.2.4 Decagonal crash boxes with notch triggers ………………………….. 57

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3.5.3. Slot Triggers for Different Cross Sectional Crash Boxes ………………... 59

3.5.3.1 Square crash boxes with different types of slot triggers …………….. 59

3.5.3.2 Cylindrical crash boxes with different types of slot triggers ………... 61

3.5.3.3 Hexagonal crash boxes with different types of slot triggers ………… 63

3.5.3.4 Decagonal crash boxes with different types of slot triggers…………. 65

3.5.4. Thickness Variation (Front End) Triggers for Different Cross Sectional

Crash Boxes…....................................................................................................... 67

3.5.4.1 Square crash boxes with thickness variation triggers ………….….…. 68

3.5.4.2 Cylindrical crash boxes with thickness variation triggers ………..….. 70

3.5.4.3 Hexagonal crash boxes with thickness variation triggers …………… 72

3.5.4.4 Decagonal crash boxes with thickness variation triggers ……………. 74

3.6 Comparative Analysis of Crashworthiness of GFRP crash boxes……………… 76

3.6.1 Consolidated results for each type of geometry used for crash boxes…….. 76

3.7 Observations from the Chapter………………………………………….…........ 80

3.8 Summary of the Chapter….…………………………………………………….. 81

Chapter – 4: Manufacturing of the Experimental GFRP Crash Box

Specimens………………………………………………………………………...... 83

4.1 Introduction………………………………………………………………….…. 83

4.2 Description of the Hand Lay-Up Process………………………………………. 83

4.3 Step by Step Procedure for Making Specimens……………..…………………. 84

4.3.1 Precautions while dealing with glass fibers, hardening agents and resins… 84

4.3.2 Making of Mould for the Specimen……………………………………….. 85

4.3.3 Step by Step Hand Lay-Up Process for Composite Crash Box…………… 88

4.3.3.1 Application of Releasing Agent on the Mould Surface……………… 89

4.3.3.2 Wrapping of Mylar Film on the Mould Surface……………………... 90

4.3.3.3 Preparation of the Resin……………………………………………… 91

4.3.4.4 Application of resin and hardener mixture to fiber mat……………… 93

4.3.4.5 Wrapping of fiber mat on the mould…………………………………. 94

4.4 Summary of the Chapter………………………………...………….…...……… 100

Chapter – 5: Experimental Impact Testing of GFRP Crash Box Specimens…. 101

5.1 Introduction…………………..…………………………………….……....... 101

5.2 Drop Weight Impact Testing…….…………………..………………………. 101

5.2.1 Drop weight impact testing machine…………..……………………….. 101

5.2.2 Preparation for Drop Weight Impact Test……………………………… 105

5.2.2.1 Making of Clamp for Specimen………………..…………………. 105

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5.2.2.2 Safety Precautions during Drop Weight Impact Testing…...……... 112

5.2.2.3 Drop Weight Impact Testing Procedure…………..………………. 116

5.3. Results and Discussion………………..……………………………………….. 120

5.3.1 Results of experiments on GFRP crash boxes without trigger…………….

120

5.3.2 Results of experiments on GFRP crash boxes with front end trigger

(thickness variation 1 trigger)…………………………………………………… 123

5.3.3 Results of experiments on GFRP crash boxes with slot trigger (type-1 slot

trigger)……………………………………………………………………………

..

126

5.4. Effect of Triggers on Various Cross Sectional Crash boxes…………………... 128

5.4.1 Effect of triggers on square geometry………………………..……..……... 128

5.4.2 Effect of triggers on cylindrical geometry………………………………… 130

5.4.3 Effect of triggers on hexagonal geometry…………………..….....……….. 131

5.4.4 Effect of triggers on decagonal geometry………….……………...………. 133

5.5 Correlation of Experimental Test and Numerical Simulation………………….. 134

5.5.1 Need for Correlation of Experimental Test and Numerical Simulation…... 135

5.5.2 Numerical Simulation of Drop Weight Impact Testing of GFRP Crash

Boxes……………………………………………………………………………..

.….

136

5.5.2.1 Calibration of Simulation Parameters in LS-DYNA…………………. 139

5.5.3 Correlation of Drop Weight Impact Test and Numerical Simulation for

GFRP Crash Boxes………………………………....…………………………… 141

5.5.3.1 Correlation of square crash boxes ……………………….…………... 141

5.5.3.2 Correlation of cylindrical crash boxes……………………………….. 146

5.5.3.3 Correlation of hexagonal crash boxes……………………………...… 150

5.5.3.4 Correlation of decagonal crash boxes……….……………………….. 155

5.6 Key Points from Experimentation and Numerical Simulation of Drop Weight

Impact Test ……………………….…....................................................................... 159

5.7 Summary of the Chapter……………………………………………………….. 160

Chapter-6: Summary, Conclusions and Future Scope……….…………………. 161

6.1 Summary of the Research …………………………………………………... 161

6.2 Conclusions…………………………………………………………………. 163

6.3 Specific Contributions of the Study…………………………………………. 165

6.4 Usefulness of the Present Research…………………………………………. 166

6.5 Recommendations for Future Scope of the Study…..….…………………… 166

References………………………………………………………………….............. 168

List of Publications and Presentations…………………………………………… 173

Brief Biography of the Candidate……...………………..….……………………. 176

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Brief Biography of the Supervisor (Guide)………...………………..................... 176

Brief Biography of the Supervisor (Co-Guide)………………………………….. 177

xv

List of Tables

Table

No. Description

Page

No.

3.1 Material properties of GFRP Composite 41

3.2 Force Comparision of experiment and present numerical simulation 44

3.3 Comparison of energy absorbed and peak force for geometries without

trigger

48

3.4 Comparison of energy absorbed and peak force for square crash boxes with

notch triggers

52

3.5 Comparison of energy absorbed and peak force for cylindrical crash boxes

with notch triggers

54

3.6 Comparison of energy absorbed and peak force for hexagonal crash boxes

with notch triggers

56

3.7 Comparison of energy absorbed and peak force for decagonal crash boxes

with notch triggers

58


different slot triggers

61


with different slot triggers

63


with different slot triggers

65


with different slot triggers.

67


different thickness variation triggers

69


with different thickness variation triggers

71



73



75

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various types of triggers

76


with various types of triggers

77



78



79

3.20 Comparison of S.E.A consolidated from Numerical Analysis for all the

cases

80

5.1 Test energy absorbed and peak force comparision for crash boxes with

different geometries

122

5.2 Test energy absorbed and peak force comparision for crash boxes with front

end trigger

125

5.3 Test energy absorbed and peak force comparision for crash boxes with slot

trigger

127

5.4 Test energy absorbed and peak force comparision for square crash boxes

with various triggers

129

5.5 Test energy absorbed and peak force comparision for cylindrical crash boxes


131

5.6 Test energy absorbed and peak force comparision for hexagonal crash boxes


132

5.7 Test energy absorbed and peak force comparision for decagonal crash boxes


134

5.8 Details of parameters used in LS-DYNA simulation 140

5.9 Energy absorbed and force level comparision of test and simulation for

square crash box without trigger

142


square crash boxes with front end trigger

144


square crash boxes with slot trigger

145


cylindrical crash boxes without trigger

147

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cylindrical crash box with front end trigger

148


cylindrical crash boxes with slot trigger

150


hexagonal crash boxes without trigger

151


hexagonal crash boxes with Front End Trigger

153


hexagonal crash boxes with slot trigger

154


decagonal crash boxes without trigger

156


decagonal crash boxes with front end trigger

157

5.20

Energy absorbed and force level comparision of test and simulation for

decagonal crash boxes with slot trigger

158

xviii

List of Figures

Figure

No. Description

Page

No.

1.1 Classification of vehicle safety 1

1.2 Typical cabin region in the center along with the crumple zones in a

passenger car

3

1.3 Typical vehicle collision in urban traffic 3

1.4 Typical RCAR Test Collision 4

1.5 Damage to the radiator due to low speed impact 5

1.6 Automobiles made of composites 6

1.7 Composite structure inside BMW I3 electric car 8

1.8 Various high speed electric cars made from composites 8

1.9 Front structural components in a vehicle, with crash boxes 9

1.10 Crash Box in a vehicle 10

1.11 The BIW 11

1.12 Representation of long-fiber reinforced composites for various cases 12

1.13 Fiber failure modes 13

1.14 Matrix failures 13

1.15 A typical fiber debonding and fiber pullout failure 14

1.16 A typical delamination failure due to bending of composite 15

1.17 Axial Impact Testing of Composite 16

1.18 Typical desirable force versus displacement plot for component subject to

impact

16

3.1 Example of stress-strain curve used for material model-58 40

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3.2 Crash box meshed model for sample 40

3.3 Crash box simulation setup 42

3.4 Comparison of experimental result and present numerical simulation 43

3.5 Ideal force versus displacement curve 45

3.6 Typical practical force versus displacement curve 45

3.7 Various cross sections of the crash box used for the study 47

3.8 Various crash boxes used for the study before deformation 47

3.9 Deformation of the crash boxes after the impact 48

3.10 Force versus displacement curves for geometries without trigger 48

Square crash boxes with notch triggers

3.11 The square crash boxes before impact 50

3.12 The square crash boxes after impact 51

3.13 The force versus displacement curves for square crash boxes 51

Cylindrical crash boxes with notch triggers

3.14 The cylindrical crash boxes before impact 53

3.15 The cylindrical crash boxes after impact 53

3.16 The force versus displacement curves for cylindrical crash boxes 53

Hexagonal crash boxes with notch triggers

3.17 The hexagonal crash boxes before impact 55

3.18 The hexagonal crash boxes after impact 55

3.19 The force versus displacement curves for hexagonal crash boxes 55

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Decagonal crash boxes with notch pattern triggers

3.20 The decagonal crash boxes before impact 57

3.21 The decagonal crash boxes after impact 57

3.22 The force versus displacement curves for decagonal crash boxes 58

Square crash boxes with different types of slot triggers

3.23 The Square crash boxes before impact 60

3.24 The Square crash boxes after impact 60

3.25 The force versus displacement curves for square crash boxes with different

slot triggers

60

Cylindrical crash boxes with different types of slot triggers

3.26 The Cylindrical crash box before impact 62

3.27 The Cylindrical crash box after impact 62

3.28 The force versus displacement curves for cylindrical crash boxes with


62

Hexagonal crash boxes with different types of slot triggers

3.29 The Hexagonal crash box before impact 64

3.30 The Hexagonal crash box after impact 64

3.31 The force versus displacement curves for hexagonal crash boxes with


64

Decagonal crash boxes with different types of slot triggers

3.32 The Decagonal crash box before impact 66

3.33 The Decagonal crash box after impact 66

3.34

The force versus displacement curves for decagonal crash boxes with


66

xxi

Square crash boxes with thickness variation triggers

3.35 The Square crash boxes before impact 68

3.36 The Square crash boxes after impact 68

3.37 The force versus displacement curves for square crash boxes with different

thickness variation triggers

69

Cylindrical crash boxes with thickness variation triggers

3.38 The Cylindrical crash boxes before impact 70

3.39 The Cylindrical crash boxes after impact 70



71

Hexagonal crash boxes with thickness variation triggers

3.41 The hexagonal crash boxes before impact 72

3.42 The hexagonal crash boxes after impact 72



73

Decagonal crash boxes with thickness variation triggers

3.44 The decagonal crash boxes before impact 74

3.45 The decagonal crash boxes after impact 74

3.46 The force versus displacement curves for decagonal crash boxes with


75

4.1 The hand lay-up process 84

4.2 The cross-sections of all specimens were maintained to be within a circle of

same radius R

85

4.3 The various shapes of the crash boxes 86

4.4 The different moulds for each cross section of the crash box 88

xxii

4.5 The glass fiber mat 89

4.6 The releasing agent 90

4.7 The Mylar film roll 90

4.8 Wrapping of Mylar film on the mould 91

4.9 Application of releasing agent on Mylar film 91

4.10 Resin and Hardener 92

4.11 Measuring quantity of Resin and Hardener 92

4.12 Mixing of Resin and Hardener 93

4.13 Application of Resin mixture on mat by brush 93

4.14 Spreading of Resin and Hardener mixture by roller 94

4.15 Beginning of wrapping of fiber mat on the mould 94

4.16 Wrapping of fiber mat on the mould 95

4.17 Applying finishing touches to the specimen 95

4.18 Square specimen after curing period 96

4.19 Cylindrical specimen after curing period 97

4.20 Hexagonal specimen after curing period 97

4.21 Decagonal specimen after curing period 98

4.22 Cutting of the specimen 98

4.23 Sample for square specimens 99

4.24 Sample for cylindrical specimens 99

4.25 Sample for hexagonal specimens 99

4.26 Sample for decagonal specimens 100

xxiii

5.1 The schematic diagram of drop weight impact testing machine 102

5.2a Drop weight Impact Testing Setup 103

5.2b Drop weight Impact Testing Machine 104

5.3 Data acquisition system setup 105

5.4 Schematic diagram of specimen with clamp and base plate 106

5.5 Dimensions of the base plate and the specimen clamp for square specimen 107

5.6 3-dimensional image of the base plate and the specimen clamp for square

specimen (top)

108

5.7 3-dimensional image of the base plate and the specimen clamp for square

specimen (bottom)

108

5.8 Dimensions of the base plate and the specimen clamp for cylindrical

specimen

109

5.9 3-dimensional image of the base plate and the specimen clamp for

cylindrical specimen (top)

110

5.10 3-dimensional image of the base plate and the specimen clamp for

cylindrical specimen (bottom)

110

5.11 (a) Machined clamps in their final shape 111

5.11 (b) Intermittent checking of fitting of the crash box onto the clamp 112

5.12 The testing is protected with a locked up cage to prevent inadvertent entry

of any person

112

5.13 The safety clamp for impactor 113

5.14 Safety clamp for impactor with safety pin installed 114

5.15 Wooden frame for impactor 114

5.16 Protective gear used during testing 115

5.17 Weight plates attached to the impactor 115

5.18 Motor mounted on the main frame 116

xxiv

5.19 Laser equipped distance measuring device 117

5.20 Data acquisition system 117

5.21 High speed camera setup 118

5.22 The different cross sections of geometries used in the experimental study 119

5.23 The different types of geometries used in the experimental study 119

5.24 The crash boxes with various geometries after impact 121

5.25 The force versus displacement curves for test of crash boxes with different

geometries

121

5.26 The crash boxes with front end trigger after impact 123

5.27 The force versus displacement curves for test of crash boxes with front end

trigger

124

5.28 The crash boxes with slot trigger after impact 126

5.29 The force versus displacement curves for test of crash boxes with slot

trigger

126

5.30 The deformation for square crash boxes with various triggers 128

5.31 The force versus displacement curves for square crash boxes with various

triggers

129

5.32 The deformation for cylindrical crash boxes with various triggers 130


various triggers

130

5.34 The deformation for hexagonal crash boxes with various triggers 131


various triggers

132

5.36 The deformation for decagonal crash boxes with various triggers 133

5.37 The force versus displacement curves for decagonal crash boxes with

various triggers

133

5.38 The simulation setup for drop weight impact test 137

xxv

5.39 PART_COMPOSITE configuration in HyperMesh software 138

5.40 The deformation of square crash boxes without trigger in test and

simulation

141

5.41 The force versus displacement curves for square crash box without trigger

in test and simulation

142

5.42 The deformation of square crash boxes with front end trigger in test and

simulation

143

5.43 The force versus displacement curves for square crash boxes with front end

trigger in test and simulation

143

5.44 The deformation of square crash boxes with slot trigger in test and

simulation

144

5.45 The force versus displacement curves for square crash boxes with

slot trigger in test and simulation

145

5.46 The deformation of cylindrical crash boxes without trigger in test and

simulation

146

5.47 The force versus displacement curves for cylindrical crash boxes without


146

5.48 The deformation of cylindrical crash boxes with front end trigger in test and

simulation

147

5.49 The force versus displacement curves for cylindrical crash boxes with front

end trigger in test and simulation

148

5.50 The deformation of cylindrical crash boxes with slot trigger in test and

simulation

149

5.51 The force versus displacement curves for cylindrical crash boxes with slot


149

5.52 The deformation of hexagonal crash boxes without trigger in test and

simulation

150

5.53 The force versus displacement curves for hexagonal crash boxes without


151

5.54 The deformation of hexagonal crash boxes with Front End Trigger in test

and simulation

152

5.55 The force versus displacement curves for hexagonal crash boxes with Front

End Trigger in test and simulation

152

xxvi

5.56 The deformation of hexagonal crash boxes with slot trigger in test and

simulation

153

5.57 The force versus displacement curves for hexagonal crash boxes with slot


154

5.58 The deformation of decagonal crash boxes without trigger in test and

simulation

155

5.59 The force versus displacement curves for decagonal crash boxes without


155

5.60 The deformation of decagonal crash boxes with front end trigger in test and

simulation

156

5.61 The force versus displacement curves for decagonal crash boxes with front

end trigger in test and simulation

157

5.62 The deformation of decagonal crash boxes with slot trigger in test and

simulation

158

5.63 The force versus displacement curves for decagonal crash boxes with slot


158

xxvii

List of Abbreviations

ABS Anti-Lock Braking System

BIW Body in White

CFRP Carbon Fiber Reinforced Plastic

D Displacement

D.C.B Double Cantilever Beam

ELFORM Element Formulation

GFRP Glass Fiber Reinforced Plastic

E Energy Absorbed

F Crash Force

F mean Mean Force

F peak Peak Force

GFRP Glass Fiber Reinforced Plastic

Mat 58 Material 58

MDO Multi Layout Optimization

M.M.C Metal matrix composites

R Radius

RCAR Research Council for Automobile Repairs

S.E.A Specific Energy Absorption

xxviii

List of Symbols

Ea Young's modulus - longitudinal direction

Eb Young's modulus - transverse direction

Ec Young's modulus -normal direction

Es Specific Energy Absorption

Gab Shear modulus (along local plane-ab)

Gbc Shear modulus (along local plane bc)

Gca Shear modulus (along local plane-ca)

m Damage Exponent

M Damage Operator

Sc Shear strength, (ab) plane

ϵ11c Strain at longitudinal compressive strength

ϵ11t Strain at longitudinal tensile strength

ϵ22c Strain at transverse compressive strength

ϵ22t Strain at transverse tensile strength

ϵgms Strain at shear strength

ρ Mass Density

δ Crush-displacement

ˆij Stress Component

Damage Evolution Variable

Vba Poisson's ratio

xxix

X Longitudinal strength

Xt Longitudinal tensile strength.

Xc Longitudinal compressive strength

Y Transverse strength

Yt Transverse tensile strength

Yc Transverse compressive strength

1

Chapter-1: Introduction

In today’s world automobile sector is facing a continuous and ever increasing challenge

from consumers as well as from various regulating and certification authorities which want

the automobile to satisfy certain requirements. The most important regulation criterions are

related to the vehicle crash safety and emissions. But in urban driving conditions it is

common that the vehicle is subjected to low speed impacts which leads to damage to the

vehicle and repair for the same. These low speed impacts are more frequent and lead to

increase in repair cost which indirectly burdens the insurance organizations.

1.1. Automobile Safety and Crashworthiness

With increasing incidents of road accidents, there is a huge damage to passengers as

well as vehicles. The most common type of collision is the frontal collision in regular traffic

scenarios. Not only the damage caused to the vehicle, but also the injuries caused to the

occupant are important in the event of a crash. Hence safety and protection against severe

damage to vehicle play very important roles in vehicle design. In case the vehicle is damaged

more severely it may increase the cost required to repair it. Therefore, to reduce the damage

to human life and the vehicle, safety systems are to be designed according to the usage.

Fig.1.1: Classification of vehicle safety

Vehicle safety can be classified into two types, active safety and passive safety

(fig.1.1). Active systems for safety help the occupants (mainly driver) in preventing the

collision of the vehicle by various methods like assisting in handling and control of the

vehicle. Examples of active safety technology are anti-lock braking system (ABS), cornering

2

assist (to maintain stability in sharp turns), hill hold assist, hill descend assist, remote

pressure monitor for tires, driver sleep/nap warning system, laser sensors for detecting

obstacles/animals/pedestrians in advance etc., Passive systems for safety provide injury

control and minimization during a collision. Examples for it are three point seat-belts, driver

airbag, passenger airbag, side airbag, curtain airbag, knee airbag, seats equipped with

head/neck support. These are very vital for the survival of an occupant after the vehicle

accident/collision. Since the crash box also acts to minimize the effect of collision on the

passenger by absorbing the energy of collision, it is also an element of passive safety. In

addition, unlike other passive elements, it is integrated into the structure of the vehicle.

Automobile crashworthiness is the capability of the structure of the vehicle to absorb or

dissipate the energy of impact in the events of collision. In passenger vehicles, the concept of

crashworthiness also contains the degree of reducing the impact of the collision on the

occupants so that the injuries to occupants are less and there is enough space between the

vehicle structure and occupants for safety in case of collisions. The important thing in

crashworthiness is controlling the deceleration within the distance available during the crash

and preventing any intrusion into occupant space that can lead to injuries. Crash box is a

structural element of the automobile to specifically improve the crashworthiness in head-

on/front collisions. It functions along with the specifically designed crumple zones which are

useful in absorbing the crash energy by deformation.

In general the vehicle structure is divided into two parts, namely, (i) Safe occupant

cabin or cage region and (ii) crumple zones (fig.1.2). The crumple zones, which are a part of

passive safety, are provided in the vehicle structure. Crumple zones play a vital role in

improving the safety of the passengers. The protection of occupants located in the cabin

region depends on the proper functioning of the energy absorbing mechanisms in case of an

impact/accident. The crumple regions consist of specially designed structural components

useful for minimising the damage to the vehicle as well as the occupants by undergoing a

systematic crush/deformation during the impact. In general the crumple regions are provided

by the car manufacturers in the front and rear portions of the car, as in general the events of

front collisions are more, followed by rear impacts, compared to side impacts. In a car, the

crumple zone is made such that it is less stiff compared to the occupant region. So that in case

of an impact/accident the crumple region undergoes crush/deformation systematically thus

reduces the damage to the occupant cabin.

3

Fig.1.2: Typical cabin region in the center along with the crumple zones in a passenger

car

Improvements in automobile designs have steadily reduced the severity of damage to

the passenger cabin and also improved the safety of occupants. While the severe life-

threatening accidents at high speed, in which the complete vehicle is severely damaged, are

sporadic, more regular and chronic are the accidents occurring in the low speed urban traffic

conditions (fig. 1.3). In the latter type of accidents, the vehicle is subjected to impacts at

speeds ranging from low to medium range. Generally the urban vehicles are subjected to

more collisions in heavy traffic regions during rush hours. According to statistics, the loss

incurred due to the damage repair of the vehicle is more due to these regular urban traffic

collisions.

Fig.1.3: Typical vehicle collision in urban traffic

The increased cost for vehicle damage repairs leads to higher financial burden on

automobile insurance companies and increase in premium value to customer. Thus, damage

4

repair costs financially impact both the customer and the insuring companies. This lead to the

formation of Research Council for Automobile Repairs (RCAR) in Europe.

RCAR is an association of car manufacturers. Their objective is to minimize the

damage to the automobiles as well as reduce injury to human beings due to vehicular

accidents. The monetary losses incurred due to accident are due to hospitalization of injured

people as well as repair/service costs required for the damaged vehicle. RCAR is promoting

improvements in passenger car reparability and reduction in damage to vehicles by increasing

the crashworthiness of the vehicles under these speed conditions. RCAR provides score for

the cars, based on which the insurance premium is adjusted for vehicle insurance. Vehicles

with poor score have a higher insurance premium, thus promoting car manufacturers for

making the cars safer for urban traffic conditions. Typically RCAR council uses a low speed

impact testing procedure at 15 km/hr (+/- 1 km/hr) to determine the effectiveness of the front

energy absorbing mechanism used by the car manufacturer, known as the front structural

RCAR test (fig. 1.4 & 1.5).

Before Impact After Impact

Fig.1.4: Typical RCAR Test Collision

Low speed vehicle collision tests are performed to assess and classify the vehicle

insurance costs. For example, if the vehicle has more deformation then it may lead to more

vehicle insurance premium for the vehicle insurance and vice-versa. So the damage to

structural components of the vehicle should be minimal in order to achieve lower

repair/service charges and to get a good rating for vehicle insurance premium. The design of

a good structure in the frontal region of a vehicle plays an important role for absorption of

impact energy in the event of a head-on/frontal collision. But, there are many challenges for

completion of the front region design as many other factors are also related to it like the

aesthetic appeal, mass of the vehicle, front overhang of the vehicle, manufacturing cost etc.,

5

So the design of the front region is to be made keeping in mind the overall cost of the total

vehicle. Therefore low speed regulations pose a new challenge to the automobile

manufacturers as it requires cost-effective and mass-effective design of the front energy

absorption system.

Fig.1.5: Damage to the radiator due to low speed impact

1.2. Composites in Automobiles

Composite materials can be used for making light-weight and safe vehicle, which also

helps in reducing fuel-consumption of the vehicle. Composites are made from combination of

higher stiffness fibers (such as glass, kevlar, carbon etc.,) used in combination with a matrix

that is usually made of polymer resins (such as epoxy, polyester etc.,).The composite

materials provide greater flexibility for industrial manufacture and usage. In general the

material properties of a composite material are much better compared to the individual parent

material properties.

Glass fiber reinforced plastics (GFRP) have many advantages. They are economical,

have good strength to weight ratios, easy to manufacture, better corrosion resistance

compared to metals and good energy absorption capacity. Thus, GFRP can be an economical

alternative for making light weight cars which will finally help in reducing the fuel

consumption. Composite materials are also useful to achieve greater degree of

crashworthiness due to their higher strength to weight ratio, which increases importance of

composites in crash energy absorption mechanisms. Composites can be tailor-made by

6

changing their geometric parameters for better usage in crashworthiness applications which

will be discussed later in this study.

(a) (b) (c)

(d)

Fig.1.6: Automobiles made of composites, (a) Chevrolet made futuristic concept car

using composite material, (b) Lamborghini uses composite materials in its superfast

cars, (c) BMW used composite material for I3 electric vehicle and (d) Ford Soybean.

Due to their higher strength to weight ratios composites find large possibilities of usage

in manufacturing of aircrafts, spacecrafts etc. where the reduction in weight of the

components is highly important but their use in passenger cars has been limited, barring few

attempts previously (fig. 1.6), due to their overall higher cost compared to metals. The usage

of composites in passenger vehicles is low mainly because of the cost incurred for making

structural components. As the mass production methods and assembly lines were previously

established based on metals (mainly steel) it requires a complete new procedure in case the

composites are used, and moreover the overall cost of composite materials is higher when

compared to metals.

7

Nevertheless, composite materials provide a large scope in making of tailor-made

designs for safety along with light-weight structures which can be far better in

crashworthiness applications compared to metals. In general the crash energy is absorbed by

metal alloys by yielding of the material, but in case of composite material the crash energy is

absorbed by complex fiber-matrix failures, which are discussed in detail in latter sections. In

this study focus will be done on controlling as well as modifying the deformation of

composite materials subjected to axial impact.

1.3. Potential for Composites in Electric Vehicles

Today manufacturers of electric and hybrid electric vehicles are giving top priority in

design to structural changes to accommodate the new fuels, which are aimed to reduce

emissions. This new fuel, primarily electric power from batteries, requires substantial space

and proportion of weight to accommodate the batteries on the vehicle (fig.1.7). Therefore,

there is an impending need to compensate the weight by using further lighter materials for

structural parts, which is possible with the usage of composite materials. But electrical

insulation is one of the key factors to be considered while building electric cars (fig. 1.8). If

there is collision of the electric car there is high possibility that the insulation on wiring from

high voltage electric battery of the vehicle is teared due to collision, which can lead to a

dangerous state of electric shock to the occupants and also rescuers. All these developments

and future requirements of the automobile industry pave way for futuristic materials such as

composite materials which may satisfy multiple and ever growing needs of the future

automobiles.

Composite materials have many advantages over metal alloys that are particularly

attractive for automobile sector. Some of these advantages are mentioned below:

Thermal expansion co-efficient is less.

Tailor made complex shapes can be achieved at industrial level.

Stable retentions of complex shapes and stable dimensions after manufacturing.

Corrosion resistance helpful in usage for varied environmental conditions.

Higher strength/stiffness to weight ratios

Lesser inherent mass useful for making lighter weight components.

Higher surface finish quality can be achieved.

8

Better reliability and better shape retention

Manufacturing can be simplified for mass production helpful to reduce overall cost.

Fig.1.7: Composite structure inside BMW I3 electric car

Fig.1.8: Various high speed electric cars made from composites

Clockwise from top-left: 1. Genovation GXE 2. Rimac Concept One

3. Vanda Dendrobium 4. Tesla Roadster

9

1.4. Research Background

There is a high demand for automotive industry to continuously improve the performance

of the vehicle and reduce cost by means of weight reduction in the structure of the vehicle

(Fig 1.9). In present scenario it is also necessary to meet the standards set by crash safety

regulating authorities in various parts of the world. In such a case the BIW (body in white or

structure) of the vehicle is very important for vehicle crash performance in case of a collision.

Crash boxes, are considered as vital components in automobile BIW for the purpose of

absorbing energy in collisions. A properly designed crash box can offer 20% weight saving

with equivalent performance compared to other parts in the vehicle. In addition, it may also

offer reduction in the manufacturing cost up to 10% per unit with proper selection of the

material and geometry used to make it.

Fig.1.9: Front structural components in a vehicle, with crash boxes

(A2MAC1.com)

Crash boxes are in general made of metal alloys, but composites can also be used as

material for them (fig. 1.10). Crash box is placed just behind front bumper back beam of the

vehicle. Its main role is absorption of the crash energy when a head-on crash occurs, through

its large deformation and thereby to protect the vehicle and its occupants from severe

damage. When a head-on collision occurs, crash box should deform during absorption of

kinetic energy, and this is to be done before the deformation can occur in other vehicle

10

components placed behind it. Thus it is important in the design of the crash box to ensure

initialization of a proper deformation mode and systematic continuation of deformation in the

body of the crash box in the event of vehicle collision, thereby minimizing severity of

damage to passengers as well as the vehicle itself (fig. 1.11).

Fig.1.10: Crash Box in a vehicle (A2MAC1.com)

Improper design of crash box may lead to more deformation of the vehicle structure.

Successful design of an effective crash box is an essential criterion for certification of the

vehicle by crash regulating authority, in crash tests.

11

(a) (b)

Fig.1.11: The BIW, (a) standalone BIW with the crash box shown, (b) BIW and crash

box assembled into the full vehicle (A2MAC1.com)

1.5. Background of Deformation and Fracture in Composite Material Crash Boxes

In this section the theoretical and experimental background of deformation and failure of

composite materials having bearing on the behaviour of composite material crash boxes is

presented.

1.5.1. Types of Composite Materials

Composites can be defined as materials consisting of macroscopic combinations of

various types of constituent-materials having a significant difference in physical properties.

The combination of these base materials leads to a formation of completely new material with

new physical properties. In general the composite materials are far better in material

characterization when compared to the base/parent materials.

The constituent materials usually falls into two categories: reinforcement and matrix. The

reinforcement’s functions are to add strength, stiffness, and ductility. The matrix’s functions

are to protect the reinforcement, to transfer load, and to add temperature and chemical

resistance. There are several different reinforcement such as particle-reinforced and fiber-

reinforced. Fiber-reinforced composites are divided into continuous fiber and discontinuous

12

fiber composites. Our main focus is on continuous fiber-reinforced composite since it is most

commonly used in high strength structural applications.

Fig.1.12: Representation of long-fiber reinforced composites for various cases; (a) same-

orientation, (b) perpendicular to each-other (normal) orientation, and (c) random

orientation of fiber-reinforcements [Peters, 1998]

1.5.2. Failure Modes in Compressively Loaded Composite Material Structures

Generally, failures in a structure happen when it is unable to withstand the load or has

unsatisfactory behaviour due to deformations. For composites, internal material-failure starts

prior to visualization of damage at macroscopic level. The detailed material-failure may be

defined to occur in three different modes, namely, (i) Breakage of fiber, (ii) Matrix

deformation and (iii) Separation of fiber from the matrix. The separation of fiber from the

matrix may in turn occur in different ways, namely, (a) de-bonding (b) pull out and (c)

separating of laminas from each-other in multi-laminated composites (referred as de-

lamination). The effects of microscopic damages on macroscopic behaviour of composites

can be known only if the magnitudes of the damages are sufficiently high for visualization.

1.5.2.1. Breakage of fiber

When cracks propagate in the directions perpendicular to the fiber, fiber breaking occurs

due to which the laminates are separated completely. Fiber gets fractured whenever the

fracture-strain is attained during deformation. Even though the fiber is the main reason for

higher mechanical properties of composite, fractures in fiber is accounted for just a smaller

portion of the complete energy absorption.

13

Fig.1.13: Fiber failure modes: Compressive load causes shear type failure in fiber

[Mamalis et al., 1998].

1.5.2.2. Matrix-Deformation or Matrix-Cracking

A matrix-material bounding all the fibers in a composite, fractures in case of large

deformations. Thermosetting-resins, like epoxy and polyester, are less ductile (brittle) in

nature and will withstand less degree of dimensional changes before fracturing occurs, while

metal-matrices can withstand large amounts of changes by deforming plastically. The energy

absorbing mechanism of polymer matrices is characterized by cracking and small

deformations, so their contributions compared to metal matrices in terms of energy-

absorption by matrices are low.

Fig.1.14: Matrix failures: Compression in transverse direction to fiber orientation

causes shear mode failure in matrix [Mamalis et al., 1998].

1.5.2.3. Separation of Fibers from the Matrix

When fractures occur fiber gets separated from matrix-material due to number of crack

formations parallely formed to the fiber directions (de-bonding-cracks). During this the bond

between the fiber and matrices gets damaged. Generally this type of failure happens when the

fiber is stronger and the matrix-material is weak. As the strength of the matrix-material

reduces there are chances of more cracks developments between fiber-matrix interfaces

leading to an increment in the total absorption of impact energy.

14

Fiber pull-outs take place in cases where brittle/discontinuous fiber is used with a much

stronger matrix-material. The fiber fractures at weaker section and may not be in the plane

same as that of combined composite’s plane-of-failure. Due to the breaking of fibers stresses

are induced in the matrix material, which leads to matrix-yielding. This causes the damaged

fibers of-being pulled out of matrix-material, instead of fiber getting fractured repeatedly at

the location of composite-material fracture. These type of phenomenon can be witnessed

specially for those fibers having length lesser than half of the critical-fiber-length.

Note that, fiber debonding and fiber pull-out may appear to be similar phenomena,

because of failure taking place at the fiber matrix interface in both cases, however, fiber

debonding cracks takes place where a matrix crack is unable to propagate across a fiber,

whereas fiber pull-outs are a result of the inability of a crack, initiated at a fiber break, to

propagate into the tough matrix. The fiber pull-outs are usually accompanied by extensive

matrix deformation, which is absent in fiber debonding.

Fig.1.15: A typical fiber debonding and fiber pullout failure [Mamalis et al., 1998].

Cracks propagation in one of the ply/lamina in a laminated composite can be ceased if the

crack-tip touches the fiber in adjacent lamina/ply. This is similarly observed for matrix crack

at the fiber matrix interface. Because of higher shear-stress in the matrices adjacent to the

crack-tip, the cracks may be diverted and may change the path as, parallel to the plane of

interface of laminas. This type of crack is called delamination-crack and, is effective for large

15

amounts of energy absorptions. Delaminating crack generally happens when laminates are

exposed to flexural-loadings, as in the case of the Charpy and Izod impact tests.

Fig.1.16: A typical delamination failure due to bending of composite

[Mamalis et al., 1998]

1.5.3. Axial Crushing of Composites

In the crashworthiness applications, like vehicular safety, there is a very high

requirement for materials as well as designs capable of absorbing high levels of energies and

help in reducing the damage to vehicle as well as passengers. Composite materials can be

very useful for usage in crash energy absorbing mechanisms as the level of energy absorption

of composites is high. Usually composites undergo a complex pattern of deformation when

subjected to impact loads involving failure of matrix-material as well as fibers which are used

as reinforcements and this increases the total energy absorption values for composites

effectively. But there is always need for the structures to have deformation in a controlled or

systematic manner so that the intensity of damage can be reduced effectively. Numbers of

crush-tests have been conducted for composite materials to study deformation mode,

absorption of energy and crush force. Long thin wall components are generally avoided

during design to avoid unpredictable deformations like buckling of the components.

The exterior/outer layers experience both transverse shear cracking in the matrix

caused by compressive load as well as tensile stress in the direction of the fibers, as the

composite is pushed in the outward direction. This leads to fiber failure due to tensile

stresses.

16

Fig. 1.17: Axial Impact Testing of Composite.

(a) Component after impact. (b) Cross-section of the impacted component.

[Okano et. al., 2010]

Fig.1.17 (a) showed crushing of composite-tube with a different crash-test in which a circular

shaped composite-tube was impacted. The crash-fronds formed due to splaying of the

composite-material can be observed. Fig.1.17 (b) showed the cut-open cross-section of the

deformed tube. As can be seen in the figure on axial impact composites form inner and outer

fronds where delamination and fiber fracture takes place and the central crack propagates

between these fronds forming a wedge of deformation debris.

Fig.1.18: Typical desirable force versus displacement plot for component subject to

impact

Fpeak

Fmean

17

A typical desirable force versus displacement plot of axially impacted component is

shown in Fig.1.18. Generally during the beginning of the crash impact, the force level is high

leading to an initial peak in force. This is carried forward by a stable crush force in which the

force level does not vary much and is nearly a constant value. Force in the initial stage is high

because the deformation is yet to be initiated in the component. The region that lies between

the high peak force and the nearly stable force is known as the transition zone/region. The

transition region is controlled by the effectiveness of the triggers used for initiating the

deformation in the component. The less variation in force-level in the stable-zone leads to

decelerating effect on the impact of the component.

Effective absorption of crash impact energy is most characteristic of a component in a

crash impact event. The absorption of energy (total) E, can be defined mathematically as

follows

Eq. 1.1

Wherein δ is crush-displacement and F is crush-force and ds is the displacement. The

S.E.A. or specific energy absorption denoted by Es can be defined as the energy absorbed per

unit mass of the component under impact.

Eq. 1.2

Here ρ is mass density of the material used for component, A is the cross sectional

area, δ is crushed/deformed length. The S.E.A. or Es not only depends on physical properties

of the material used for the component but also dependents on type of geometric cross section

used for component. In automobiles the total mass/weight of the components used for

manufacture of structures plays a significant role in total cost as well as the fuel economy of

the vehicle.

Hence the S.E.A or Es is one of the most important parameter to know the

crashworthiness of various types of geometries that can be used for making the crash energy

absorbing systems. With the use of different geometric shapes, the cross section area (A)

changes and this has an effect on the S.E.A (Es).

18

Thus the research problem concerning the crash box is to find the optimum geometric

design and choice of material that can result in maximum possible absorption of energy of

frontal RCAR speed level of collisions. Several authors in the past have proposed a variety of

crash box geometries made of both metals and non-metals and studied their energy

absorption characteristics through numerical simulation as well as experimental work. The

details of this past work available in the literature are exhaustively reviewed in the next

chapter leading to the identification of the specific gaps in research and selection of

objectives of the present doctoral work.

19

Chapter-2: Literature Review

In this chapter, an exhaustive literature review is carried out on the work related to the central

theme of the present work. The review has been presented in a chronological order of

progress of work with suitable comments on salient aspects of geometric shape and material

of crash box and their influence on the deformation pattern and energy absorption

characteristics.

2.1. Review of the Past Work

Langseth et al., (1998) studied the crashworthiness of aluminum extrusion subjected

to impact loading and also validated numerical model using LS-DYNA with experimental

results. For increasing energy absorption ability of thin walled aluminum structures subjected

to impacts axially, an experimental evaluation was done to study combined effect of

extrusions as well as aluminum foams.

Miyazaki et al., (1999) performed axial impact test for rectangular tubes for obtaining

bending as well as impact resistance. The crush behaviour was captured using high speed

cameras and also simulated numerically for analysis. They found compressive strains reach a

top level in the tube-wall at the central portion of the tubes in the axial route; uneven surfaces

and ripple at the surfaces of the tubes are due to buckling-deformation.

Compressive and bending type loading of crash box considering it as a walled beam

structure representing a part of the vehicle structure with the use of finite-element process

was studied by Noma et al., (2002). According to them, area of component can be divided

into two different areas such as an effective area and ineffective area. The effective collapse

force was studied and improvement of the vehicle structure was done.

Carbon Fiber Reinforced Plastic (CFRP) composite which can be appreciably useful

due to their high-stiffness and light-weight was studied by Ando et al., (2003). CFRP also has

an advantage on impact protection as it gives the liberty for tailor made physical-properties.

In their evaluation, a multi layer shell was used to reproduce the impact characterization of

carbon fibers. Basic impact-test values of easy to make specimens were taken to evaluate

characterization of carbon fibers. Then this was used for modeling of crash-box of Nissan-

20

GT500 race car. Simulated results were in proper correlation compared to experiments.

Hence the optimized model was developed on the basis of numerical method.

Bernal et al., (2004) targeted on using braided composite materials for car frontal

structure in case of frontal crash. The front longitudinal beam of the vehicle structure was

studied by them; S.E.A values and absolute energy absorbed were compared for structures

made of CFRP and GFRP materials.

Maddever et al., (2005) targeted on the usage of aluminum-foams obtained by melting

technique of metal matrix composites (M.M.C) for use in crash-boxes. Efficiency of hollow

extruded aluminium was evaluated against similar crash-boxes with M.M.C foam with

different values of densities. Details regarding the deformation and crash-energy absorption

were explained. Outcome of the study indicates sizable upgrades, permitting a useful

decrease in crash-box dimensions.

Marcus et al., (2005) studied the optimisation of metallic crash box with an objective

of reducing its mass. They used LS-OPT for solving the optimization problem and they also

used Neural-Networks as meta-model. They carried out their research work based on the

finite element model provided by Saab Automobiles for their passenger cars.

Rossi et al., (2005) studied the post buckling deformations characteristic for

aluminium-alloy extrusions under longitudinal-axial impact. LS-DYNA was used in their

study. They focussed on studying post buckling deformations, often induced through an

axial-crumpling motion which generates cloth like folds as the crash energy is absorbed.

They determined that increment in flange had a major effect at the axial crush pressure and

deformation parameters.

Zhang et al., (2007) studied the effect of patterned surfaces for traditional thin walled

rectangular tube in order to increase absorbed energy values when compression is done

axially. They carried out quasi static crash of the tubes using numeric analysis by use of LS-

DYNA. They introduced different pattern types with the usage of pyramid shaped element.

One type was focused at initiating deformation in extension-mode and the other was aimed at

creating new deformation type for increasing absorption of crash-energy. Effectiveness of

different patterned types was studied for crashworthiness of tube.

21

Performance under crash for thermoplastic crash box made of composites was studied

by Hamidreza et al., (2008). To get more details LS-DYNA was used for numerical

simulation of the tubes. Tube wall was modelled using layer of shell elements to replicate the

delamination. Contacts were used between the different layers of the lamina. Optimization

technique was used to find the best suitable design for the crash box.

Rajendran et al., (2008) studied the closed-cell-aluminium foams equipped with AISI-

304L stainless-steel tubes to know crashworthiness. Drop weight experiments were carried

out with the usage of a free-flight drop-tower on aluminum-foam fitted to stainless-steel tubes

to find the acceleration time data of the impactor. They determined that the foam is useful for

increasing crashworthiness.

Extensive numerical analyses of dynamic buckling of a crash box, taking into

consideration both the elastic waves and plasticity consequences was studied by Rusineka et

al., (2008). It became evident that both elastic-wave propagation and the plasticity acted

during the first level of impact. This change in elastic-wave propagation and the plasticity

defined the disintegrate region. In order to approximate effectively the buckling, correct

description of the stress-rate sensitivity of materials were considered. The principal concept

was to guarantee a high plastic stress stage under excessive pressure to prevent any unwanted

buckling.

Impact crash on the empty and foam filled square tubular structures was studied by

Zarei et al., (2008). The multi layout optimization (MDO) technique was used to locate the

most appropriate empty tube that absorbed most energy with minimal self-weight. Square-

tube thickness, length and width, were decided as optimization parameters. On the basis of

this optimisation method the best square-tube that absorbed most energy with minimal self-

weight was found.

Hadavinia et al., (2009) studied the mode I and mode II inter-laminar crack

propagation and their effect on the deformation mode of the composites. The double-

cantilever-beam (D.C.B), 3 point end notched flexure ( 3 E.N.F ) and axial-crush boxes,

samples were made from carbon- epoxy twill weave fabric and these examined for quasi

static loading to decide the inter-laminar fracture-toughness in mode I ( G I C ), mode II (G II

C) and S.E.A of every lay-up. The deformation method of composites was additionally

22

studied via finite-element software LS-DYNA and result was validated with the applicable

experimentation.

Effect of fiber orientation at the interlaminar fracture toughness of GFRP composite

crash boxes was studied by Ghasemnejad et al., (2009). It was proven that the fiber

orientation at interface fracture plane influences the interlaminar fracture of GFRP composite

substances. The interlaminar fracture strength of interface fracture planes of 0 / 90, 90 / 90

and 0 / 45 had been close collectively whilst +60/-60 behaved pretty differently. The interface

plane of 0 /90 confirmed the most interlaminar fracture strength while it turned into minimum

for the +60/ -60 interface plane. It was revealed that energy absorbing capacity in axially

crushed composites depended on the interlaminar fracture toughness of laminates.

Hadavinia et al., (2009) focused on the effectiveness of fiber orientations at the inter-

lamina fracture-toughness for mode I and mode II, as well as absorbed energy for carbon-

epoxy twill weave composites. They proved that the lamina designs near interface-fracture

planes effects the inter-laminar fracture-toughness in woven-CFRP composites. The variation

of laminate bending and brittle fracturing deformation was determined for the lay ups of [0] 4

and [0/45] 2. In this situation a variation in mode I and mode II, effects the absorbed energy

of woven-CFRP composites. For laminates layout of [45] 4, the mode II crack progress was

witnessed in the facet wall, due to which buckling and transverse shear crush modes

occurred. Importance of inter-laminar fracture toughness for laminates was determined.

Lee et al., (2009) gave an analytical system for increasing the efficiency of tubes full

of granule, on the basis of effective-thickness principle from the previous studies. A goal was

picked to evaluate the analytical setup and display the crash of a tubes packed with granule.

They investigated the crashworthiness of a tube filled with granules and proposed modified

analytic expressions for the energy absorption in the tubes full of granules.

Xiao et al., (2009) studied the damage mechanics of composites, using mat-58, in LS-

DYNA for braided-composite materials using axially loaded tubes. Numerical simulation

analysis for the model was done with the use of single-element formulations.

Xiao et al., (2009) focused on higher S.E.A for tubes made of composites. They

observed that simulation model formed on the basis of continuum-damage-mechanics (C D

M) method was insufficient to replicate un-loading behaviour of deformed composite

23

materials. They conducted experimentation and found that, braided-composite tube forms

various continuous crush-fronds. Localized un-loading occurred in cases where material

portion moving-out of the crush-front became part of crush-frond.

Ghasemnejad et al., (2010) focused on results of delaminated failures in hybrid-

composite crash-boxes crashworthiness performances, in comparison with non hybrid types.

The combinations in twill-weave and uni-directional CFRP composites were considered.

Delaminating effect on mode I and mode II with the identical lay ups was done to analyze the

effects of delaminating cracks progress on absorption of energy for hybrid-composites. It was

proven that hybrid-laminate design displayed better fracture-toughness in mode I and mode II

delaminations and also better crashworthiness during crushing. Simulation of hybrid-

composite impacts was done with the aid of LS-DYNA software and the results were

validated with the applicable experiments.

Crash-energy absorption behaviour in Al (aluminium) closed-mobile foam crammed

commercial 1050H14 Al crash boxes in two sizes and three different thicknesses under quasi-

static and dynamic deformation velocities was studied by Toksoy et al., (2010). The quasi-

static and dynamic crushes of empty as well as foam stuffed crash boxes have been simulated

with usage of LS-DYNA. It was shown both experimentally and numerically that partial-

foam filling tended to alter the deformation modes of empty crash-boxes from a non-

sequential to a sequentially folding mode. The experiment as well as simulated results

showed comparable crushing values and deformation modes. In order to evaluate the

efficiency of partial-foam filling, the S.E.A values of empty, in-part and absolutely foam

filled crash-boxes were characterized using box-wall thickness and foam filler relative-

density. The consequences confirmed that empty crash-boxes were energetically more

efficient than absolutely and in part foam crammed crash-boxes, importance of foam relative-

density was also found. It was revealed that, on comparison with complete foam fill, the

crucial foam density for partial-foam filling decreases with increment of crash-box wall

thickness.

Crash-boxes fabricated from metallic alloys steel-aluminum, subjected to crash-

impact loading was studied by İnce et al., (2011). The evaluation of the crash-box was carried

based on impact-loading. Hybrid type crash-box, manufactured from steel-aluminium was

modelled and evaluated for impact load. The hybrid crash-boxes were optimal for weight

reduction, 17.5% weight-reduction was obtained with the aid of hybrid crash-boxes.

24

Moreover, it was essential to use customized techniques for the hybrid crash box to increase

the weld strength.

Ghamarian et al., (2011) studied the crumble mechanism and absorption of energy

through axial compressions of the end capped and thin walled circular aluminium tubular

structures, which were hollow and also filled with polyurethane-foam. Simulation model was

developed to evaluate deformation in tube. The experiment and simulation outcomes were

useful to decide absorption of energy due to the deformations of thin wall tube and

collapsible foam. The performances of end capped tube were evaluated with non capped

tubes, in order to manage preliminary top load.

Qureshi et al., (2012) studied various sinusoidal type patterns which had been applied

to the crash-box; the effect of these was evaluated for crash-impact. Possibilities to alter the

crash deformation-mode and buckling-frequency through introduction of pattern were

checked. It was found that in the pattern containing nodal-hinges, the pressure peaks might be

managed by method of manipulation of the longitudinal as well as horizontal wave-

amplitude.

Obradovic et al., (2012) developed analytic, numeric and experiment models to study

the crash energy absorption characteristics of composites. In spite of the complex nature of

fracture, a close result was obtained for both numerical and experimental methods, performed

for simpler structured tubes. The simplified finite element model was capable of simulating

the absorption of energies with an error of 10 %.

Deformation mechanisms of expanded-metallic tubular structures under impact crush

was studied using experiments by Graciano et al., (2012). They also calculated the

crashworthiness potential of metallic tubular structures. It was determined that the force-

deformation response and failure mechanism of the tubes relied upon the aspect-ratios of the

expanded-metallic tubular structures.

Tarlochan et al., (2012) focused on crushing behaviour in polymer composite

sandwich systems. They performed experiments for investigating behaviour of composite-

sandwich systems under quasi static compressions. The test samples were made of

glass/carbon fiber and epoxy resins. The optimized layout of their research had absorption of

energy and force-efficiency greater than traditional metal alloys.

25

Influence of adding a variety of patterned shapes on box beam crash absorbers was

studied by Qaiser et al., (2013). Addition of patterned shapes on beam crash absorbers lead to

higher energy absorption in case of pure-bending deformation. Energy efficiency increased

by 53.49 % for ideal case in comparision with original model. Moreover, patterned shapes on

box beam generally distributed energy uniformly.

Costas et al., (2013) studied 5 unique types of energy absorbers via experimentation

as well as numeric analysis. The outcomes showed improvements for energy absorbed of the

whole steel padding components when compared to base specimen. Moreover, in terms of

efficiency, the tube with corrugated CFRP insert and the cork-crammed tube required a

redesign, so that the results of the simulations with modified design showed a good gain in

absorbed energy.

Eshkoor et al., (2013) studied distinct trigger configurations and their variations in the

parameter related to crash and deformation styles of silk-epoxy composites. Plug-type

triggers gave a very exceptional sample of failure as well as vulnerable crash parameter in

comparison to different triggers. Moreover, 4-piece triggers were not good for epoxy

composite tubes, as it considerably decreased the quantity of peak force with a massive

reduction in energy absorbed values. Finally, the systematic and no catastrophical

deformations were observed in components.

Oshkovr et al., (2013) studied various types of quasi static compressive testing

considering rectangular tube made from different quantities of silk and epoxy laminates such

as 12, 24 and 30 laminates. It was observed that the results from the two approaches, finite

element analysis and experiments have a good agreement in terms of failure modes and

crashworthiness characteristics.

Gedikli et al., (2013) investigated the crashworthiness of tubes manufactured using

aluminium ( AL6061 T6 ), high-strength-steel ( AIS I1018, HSLA 350, DP 600, DP 800 )

and tailor welded tube (T.W.T), made from aluminium and high-strength-steel (AL 6061 -T6

& AIS I1018, AL 6061 - T6 & HSLA 350, AL 6061 - T6 & DP 600, AL 6061 - T6 & DP

800). Simulation analysis was done to find the effectiveness of materials type, thicknesses

and aspect- ratios (tube-length/diameter) of tubes.

26

Waimer et al., (2013) carried out experimentations for studying dynamic-failure

behaviours of generic CFRP specimens when axially loaded, useful as crash absorbers in

aerospace applications. Various materials and designs were parameterized to study the

behavioural effects. Effectiveness of all parameters was studied based on force displacement

characterization and absorption of energy.

Effect of corrugation on the crush behaviours, absorption of energy, and mode of

failure in round aluminum tubes was studied by Eyvazian et al., (2014). Experiments were

completed on 5 geometries for corrugate as well as simple-shaped tubular structures along

with corrugation varying in length and orientation, under axial-compressive load. It was

proven that tube with corrugation displayed a uniformly distributed load versus displacement

values without any preliminary high loads. After applying corrugation, there was also

improvement in mode of failure.

Dhatreyi et al., (2014) studied the characterizing of trans-laminar fracture in plain

weave, fabric reinforced composites subjected to mixed mode loadings. Using fracto-graphy

of the fractured specimens, the influence on the mechanism of failures was checked and also

the propagation of cracks was studied.

Gurusideswar et al., (2014) studied the effects of clay when mixed in epoxy and glass

/ epoxy composite materials at lower strain-rates. The clay is mixed in the epoxy-resin using

stirring and then by process of sonication. The glass / epoxy nano-composites were made by

glass fiber with epoxy–clay mixture using hand lay-up process and then following a

compression-molding.

2.2. Gaps in existing Research

It is clear from the literature review that very little work done in the past considered a

detailed study and comparison of the behaviour and energy absorption performance of crash

boxes made of composite materials. Moreover, only a few different cross sectional shapes

were experimented. Detailed crash box sub system level analysis such as the effect of various

geometric shapes and triggers (which act as deformation initiators) in the design was done

sparsely. The amount of work using composite material crash boxes is also limited. Very less

study is done on comparison of crashworthiness of composite material crash boxes under

27

axial impact loading with sufficient variety of cross sections being used. Research work on

using triggers as deformation initiators is also limited in the literature. Very less study was

done for the feasibility of trigger type features in combination with various cross sections of

the crash boxes. Comparative crashworthiness analysis of crash boxes with and without

geometrically intrinsic triggers subjected to RCAR speed level impact loading was not done.

Very less study is available on crashworthiness behaviour for GFRP composite crash boxes

made of different types of geometric shapes along with the application of various types of

triggers subjected to impact loading under RCAR speed conditions.

After careful assessment of these research gaps, the following research objectives

were identified for the present doctoral work.

2.3. Research Objectives of the Present Work

1. Finite element simulation of proposed crash box when subjected to impact.

Comparative analysis of crashworthiness behaviour of the crash boxes with different

geometries along with combination of various triggers.

2. Manufacturing of the required specimens of crash boxes for experimental analysis.

3. Experimental analysis of crashworthiness behaviour of the crash boxes with different

geometries along with combination of different triggers.

4. Suggestion of best combinations of geometric cross sections and trigger types for

improving crashworthiness behaviour of composite crash boxes based on force versus

displacement plots and specific energy absorption.

These research objectives are further detailed out in the following paragraphs.

The primary objectives of the present work is to, study crash energy absorbing

characteristics of a Glass fiber reinforced plastic (GFRP) crash boxes when subjected to an

axial impact load with various types of geometrical cross sections and different trigger types

extensively. GFRPs have good energy absorbing characteristics useful for crashworthiness

applications. They are also cheaper and can be easily manufactured. In the first phase of

work, finite element method based numerical analysis was done extensively to study the

effectiveness of various cross sectional shapes on crashworthiness of crash boxes without

application of triggers. Later different triggers were applied to various cross sections of

GFRP crash boxes and study was carried further in a comparative manner to know the

relative effectiveness of various combinations of trigger types and geometric cross section by

28

analyzing the F-D diagrams (force versus displacement curves), energy absorption values and

specific energy absorption (S.E.A).

Crash boxes made of composite material (GFRP) are used for this study. In present study

GFRP crash boxes made of four different types of cross sections: square, cylindrical,

hexagonal and decagonal are used. A comparative numerical simulation analysis is done to

study the effect of each type of cross section on the crashworthiness behaviour of crash boxes

when subjected to impact at low velocity, on the basis of automotive “RCAR Test”. For the

purpose of improvement in crashworthiness of the composite crash box, triggers are applied

to the composite crash boxes to obtain higher value of energy absorption and required

deformation pattern. Triggers are widely used in crash boxes made of metal alloys whereas

the usage of triggers for composite crash boxes is still under development in automobile

industry. Crashworthiness of GFRP composite crash box structures with novel geometrically

intrinsic triggers are studied extensively for various types of geometric cross sections. For

this purpose various types of novel triggers, such as; Notch triggers (different types of notch

triggers), Thickness variation triggers (different types of thickness variation/front end

triggers) and Slot triggers (different types of slot triggers) are used with combination of all

the geometric shapes considered for this research. Force- displacement diagrams and Specific

Energy Absorption (S.E.A) are studied for all combinations of geometry and trigger types for

better understanding of behaviour of crash boxes subjected to axial impact. Later an

experimental analysis is done to correlate the numerical simulation. Best combinations of

geometric cross sections and trigger types for improving crashworthiness behaviour of

composite crash boxes are suggested based on force versus displacement plots and specific

energy absorption.

2.4. Scope of Study

This research is done to study the crash energy absorbing characteristics of a crash box

subjected to impact load with various types of geometrical cross sections and trigger

configurations. Extensive literature review was done to understand the research work done

and also to find the gaps in the research in this area. For better understanding the details of

composites were presented based on the configuration of composites along with background.

Numerical simulation was discussed in detail including the special techniques for simulation

of composites, material model used in LS-DYNA software, parameters and simulation

29

methodology. Manufacturing of composites (specimens) was discussed in detailed including

the ingredients, mould / mandrel preparation, procedure and precautions. Experimental

testing was discussed in detail including the preparation of test, test setup, specimen clamp

preparation, safety design of drop weight holder, data acquisition system and safety

precautions. Numerical simulation and experimental analysis was done extensively and

correlated to compare various types of geometries of the composite crash box for specific

energy absorption and also to understand the effectiveness of different triggers on

crashworthiness behaviour of GFRP crash boxes made of various geometric cross sections.

This research successfully highlights the relative effectiveness of various trigger types on the

energy absorption level and peak force variations for all the combinations of geometric cross

sections of the GFRP crash box. Result discussion was done to elaborate the effect of each

geometric cross section and each trigger type along with combination of geometric cross

section and trigger types on the crashworthiness of the GFRP crash boxes.

2.5. Research Methodology

In the present study GFRP crash boxes are considered. Analysis setup is built based on

“RCAR test”. Assuming the vehicle is undergoing low velocity impact test, popularly known

as the “Front Structural RCAR Impact”. Purpose of RCAR testing is to validate the

effectiveness of frontal energy absorption mechanism of the vehicle in low velocity impact

(16kmph), which is common in day-to-day traffic conditions. This test helps in finding the

crashworthiness of crash box under impact. For the vehicle to pass the test it is required that

the components mounted behind the crash box do not get damaged during the impact, so that

there is minimal damage and repair cost. For this to happen the crash box must absorb the

maximum possible energy. Hence crash box plays a very vital role in this test. In present

study behaviour of crash box is analyzed at sub-system/component level, so as to suggest the

relatively better combination of geometrical cross section and trigger configuration for GFRP

crash boxes. Component/sub-system analysis is helpful in saving computing time, is less

complex for analyzing and can be solved in relatively less powerful computers.

The crash energy absorbing characteristics of a crash box subjected to impact load with

various geometric cross sections and trigger types is studied extensively. In present study

GFRP crash box made of four different types of cross sections: square, cylindrical, hexagonal

30

and decagonal is considered. Numerical simulation analysis and is done extensively to

highlight the effect of each geometric cross-section and also each type of trigger on crash

behaviour of GFRP crash boxes. Comparative analysis is done to understand the relative

effectiveness of each trigger type on the energy and force variation; with the variations of

cross section and triggers for the crash boxes based on the RCAR Test. Force- displacement

diagrams and Specific Energy Absorption (S.E.A) is studied for all combinations of geometry

and trigger types for better understanding of behaviour of crash boxes subjected to axial

impact.

Finite element pre-processor called HyperMesh is used for setting up the simulation

model and applying all the boundary conditions. The finite element model of the component

is generated by meshing the required geometric shape of the component in pre-processor

HyperMesh. MAT LAMINATED COMPOSITE FABRIC: material type 58, is widely used

to model the composites for finite element simulation. MAT LAMINATED COMPOSITE

FABRIC (MAT-58) material model provided by finite element solver LS-DYNA is used for

crash box model. This material model can be used with shell and thick shell elements. LS-

DYNA is used for solving the model. HyperView and HyperGraph are used as post-processor

for extraction and analysis of the simulation results. Furthermore, the experimental analysis is

carried out for the crash boxes with various geometric shapes and trigger configurations using

drop weight impact testing machine setup to correlate with numerical simulation analysis.

Numerical simulation analysis and experimental test analysis is done to study the effect of

each geometric cross-section along with each type of trigger configuration on

crashworthiness of GFRP crash boxes based on the energy and force variation. Specific

Energy Absorption (S.E.A) is considered as factor for determining the crashworthiness of the

crash box.

31

2.6. Organization of the Thesis Report

Chapter-1: Introduction

A brief introduction is given about automobile safety. Crashworthiness of a vehicle is

discussed. Low speed collisions which occur more often in urban traffic are discussed along

with RCAR (Research Council for Automobile Repairs) test. Scope of composite materials in

automobiles and their benefits when used in electric cars is discussed.

Study on composites is a complex subject due to the complexity involved in the

formulation of composites which involves combination of different materials (fibers and

matrix). Hence their behaviour is also different when compared to other materials. To better

understand the complex behaviour of composites, background of composites is discussed.

The behaviour of fiber reinforced composites when subjected to axial impact loading is also

discussed. Later discussion is done on research background.

Chapter-2: Literature Review

Extensive literature survey is done which is beneficial for understanding the work

done by previous researchers and the trend of the research done on crashworthiness of

materials specially composites. Based on the literature available from the work done by

previous researchers, gaps in the existing research work were found. Gaps in existing

Research are presented. Some noticeable gaps found in the existing research are as follows;

comparision of crashworthiness behaviour for crash box made of GFRP composite material

from different cross sections is done in sparse. Detailed crash box sub system level analysis is

done rarely to study the effect of various types of triggers. Very little work is done on

comparative crashworthiness analysis of crash boxes with and without application of

geometry intrinsic triggers subjected to impact loading. Research objectives of the present

work are presented and explained. Scope of study and research methodology is discussed.

32

Chapter –3: Numerical Simulation of GFRP Crash Boxes

Numerical simulation of composites is discussed in detail along with the procedure

for numerical simulation of glass fiber reinforced plastic (GFRP) crash boxes. The

significance of force versus displacement diagram is also discussed. Initially, a comparative

numerical simulation analysis of GFRP crash boxes made of various cross sections was done

to study the effect of each type of cross section on the energy absorption of crash boxes in

low velocity impact, based on the automobile standard testing known as “RCAR Test”.

GFRP crash boxes with four types of cross sections: square, cylindrical, hexagonal and

decagonal were considered for crashworthiness behaviour analysis. In order to improve the

crashworthiness of the composite crash boxes, triggers were implemented in the design of

crash boxes to help in achieving desired deformation pattern, energy absorption and peak

force value. The comparative numerical simulation analysis of GFRP composite crash box

structures was carried further; with novel geometrically intrinsic triggers studied extensively

for all the different types of geometric cross sections considered. Various types of novel

triggers for composites were introduced namely; Notch triggers (different types of notch

triggers), Thickness variation triggers (different types of thickness variation/front end

triggers) and Slot triggers (different types of slot triggers) and studied extensively, to

understand the effect of each type of trigger on the crashworthiness behaviour of GFRP crash

boxes made of square, cylindrical, hexagonal and decagonal geometrical cross sections. Force

versus displacement curves were plotted for each case providing detailed insights into the

force variation during deformation. Specific Energy Absorption (S.E.A) was compared for all

the combinations of the cross sections as well as the trigger types used for the crash boxes for

better understanding of the crashworthiness characteristics of each combination of geometric

cross section and trigger type. Later a comparative analysis of all the crash boxes considered

in this study with all combinations of geometries and trigger types is presented in a

consolidated manner for a better understanding of effectiveness of each type of geometry

shape and trigger type on the crashworthiness of GFRP crash boxes.

Chapter–4: Manufacturing of the Experimental GFRP Crash Box Specimens

In order to correlate the numerical simulation with experimental results crash box

specimens are required. Therefore, specimens of GFRP crash boxes were prepared using

hand lay-up process. The hand lay-up process is explained in detail. Discussion is also done

on the specimen mould required for making each type of crash box considered in the study.

33

Procedure used for hand lay-up process for crash box specimen making is explained step-

wise, including safety precautions and practical tips for manufacturing of composites.

Chapter–5: Experimental Impact Testing of GFRP Crash Box Specimens

Experimental analysis of the GFRP crash boxes was carried out using drop weight

impact testing in Indonesia. Details of drop weight impact testing were discussed including

preparation of specimen clamps required for holding the specimen during test. Drop weight

impact testing of GFRP composites involves many dangers so details of safety precautions

were explained elaborately. Experimental results were presented in a comparative manner.

Factors influencing the numerical simulation and experimental results were discussed along

with their limitations. Numerical simulation model replicating the drop weight impact testing

was developed. Correlation of experimental results and numerical simulation results was

done. A good correlation between the experimental test and numerical simulation was

achieved. Discussion on correlation between the experimental test and numerical simulation

was presented followed by summary of the chapter.

Chapter- 6: Summary, Conclusions and Future Scope

Summary of the work done in this research is presented in sequential manner. The

conclusions drawn from the work done are presented in a detailed manner so that a clear

picture of the effectiveness of geometrical cross section as well as the trigger configuration

can be obtained. Contributions of the study are discussed. Usefulness of the present research

is presented. Prospective future scope for continuation of the present work is discussed to

pave way for further study in this research area.

34

Chapter–3: Numerical Simulation of GFRP Crash Boxes

3.1. Introduction

This chapter mainly focuses on the numerical simulation and analysis of glass fiber

reinforced plastic (GFRP) crash boxes. The objective of this numerical simulation is two-

fold, firstly, to predict the force-displacement relationship and energy absorption, secondly,

to prediction the deformation pattern under impact loading. Composite materials have

complex deformation pattern under impact loading based on their constituent materials, their

nature of distribution and geometry. It is very important to understand this mechanical

behaviour and deformation pattern subjected to impact loading to correctly characterize the

performance of the crash box. In the present analysis, first, numerical analysis is done on

GFRP crash boxes made of various types of cross sectional geometries and subjected to axial

impact loading to study the effect of each cross sectional shape on the energy absorbing

characteristics of the crash boxes. Later, various types of triggers are applied to all the cross

sectional shapes considered in the study. Comparative analysis is done for each type of

trigger to understand the effect of each trigger on the crashworthiness of the crash box.

Crashworthiness characteristics like energy absorption, peak loads and specific energy

absorption are compared for better understanding of each type of geometry and trigger

combination.

3.2 Merits of Pre-Test Numerical Simulation in Crashworthiness

The numerical simulation done before the actual experimental testing known as pre-test

numerical simulation has the following advantages:

Numerical Simulation is a very cost effective way of analysing multiple case studies in

crashworthiness applications.

Simulation helps in identifying the load bearing capacity or force level the component

can withstand. It provides an approximate picture of the crash event before the actual

crash is done.

35

It gives the opportunity to check various possibilities or combinations and their effect on

the crashworthiness before actually implementing them, hence decreasing the prototype

testing and therefore reducing the cost of testing.

Even though numerical simulation may not exactly replicate the deformation of complex

materials such as composites it is a very useful tool which helps the engineers by

providing an approximate picture of weak locations in the structure.

Numerical simulation gives an opportunity to the engineer to carryout analysis of the

crash impact in a very detailed manner; this includes force analysis, energy analysis with

reference to time history of the crash event which is very difficult in experimental test.

Numerical simulation gives an opportunity to the engineer to optimise as well as improve

the design of the structure for crashworthiness and compare its effectiveness to the

base/original structure.

Therefore it is a good idea to carry out the numerical simulation analysis before the

actual experimental testing is done. These numerical simulation analysis models are generally

referred as pre-test simulation models, which provide the designer not only an overview of

the crash behaviour beforehand but also these are also helpful for optimising the actual

prototypes. For this purpose the numerical simulation is done with simplified models which

are easy to handle and can be simulated as well as analyzed in lesser time. At later stage

when the prototype/specimen testing is completed, the pre-test numerical analysis model can

be refined and correlated with the experimental test specimen for further analysis. This

method is cost and time effective for the designer to study a larger number of cases required

for the study.

3.3 Numerical simulation of composites in LS-DYNA

The crushing behaviour of composite crash box under axial impact loading is very

important for improving the crashworthiness of the energy absorbing structures in

applications such as automotive. Analysis of composite materials subjected to axial impact

loading is a complex problem which requires the usage of numerical simulation codes

available commercially for solving such problems. LS-DYNA is a widely used simulation

code for solving such non-linear problems. It provides support for solving composite

materials subjected to impact loading. It has material models which are suitable for composite

materials, one such material model is MAT_LAMINATED_COMPOSITE_FABRIC or

36

MAT_058 or *MAT 58. This material model is more suitable to be used for GFRP crash

boxes considered in this study. This model can be used for modeling of composite materials

which have unidirectional layers, woven fibers and laminates. The LS-DYNA keyword for

this model is *MAT_LAMINATED_COMPOSITE_FABRIC or *MAT_058. LS-DYNA

version 7.1 was used for this study.

In material 58, Hashin failure criteria is used with changes for different types of composites

(Hashin, 1980). The maximum effective strain is applicable for element layer failure for

different types of composites. The MAT58 model is used for shell elements.

MAT 58 is developed from the work of Matzenmiller et. al., this model is also called as

Matzenmiller, Lubliner and Taylor (MLT) composite model and is based on plane stress

continuum damage mechanic model (Matzenmiller et. al., 1995).

For the composites with unidirectional layer, the failure criteria are defined by:

Failure mode for tensile fiber ˆ11 0:

Eq. 3.1

Failure mode for compressive fiber ˆ11 0:

Eq. 3.2

Failure mode for tensile matrix ˆ 22 0:

Eq. 3.3

37

Failure mode for compressive matrix ˆ 22 0:

Eq. 3.4

Whereˆijeffective stress components given by

Eq. 3.5

The damage evolution variable, is defined by:

Eq. 3.6

Note that, the longitudinal and transverse damage variables assume different values

for compression and tension. From the above equation, we define as the current strain in the

respective direction of damage, f as the failure elastic strain which is calculated by dividing

the strength i.e., f . Here, m is the damage exponent; m and e are calculated in material-58

model by:

38

Eq. 3.7

Where q is the strain where strength is reached.

The stresses ˆ11, ˆ 22 and ˆ12are the components of effective stress tensor. Therefore, ˆ

can be written as

Eq. 3.8

Where M is the damage operator and is the nominal stress. M can be written as

Eq. 3.9

Where 11,22and12are the damage variables that distinguish matrix, fiber and

shear damage.

When the damage occurs in at least one mode at any point, then the damage operator

becomes substantial for damage initiation in other modes. The material response after

damage initiation is defined as

Eq. 3.10

Where C is damaged elasticity matrix which is written as

and

39

Eq. 3.11

Where D11111221221 0, and 11, 22and12 reflects the current state of fiber,

matrix and shear damages respectively. where u12 and u21 are Poisson’s ratios respectively.

The stress components in the failure criteria are given by,

Eq. 3.12

Eq. 3.13

Where c, t are for compression and tension and p, n are parallel (11-direction) and

perpendicular (22-direction), X and Y are the longitudinal and transverse strengths, Sc is the

shear strength. The initial damage threshold, r = r0, in the criterion for loading in fiber

direction is determined by the initial damage variable. Example of stress-strain curve for

material model-58 is shown in Fig. 3.1.

Eq. 3.14

40

Fig. 3.1: Example of stress-strain curve for material model-58

3.4. Analysis Procedure of Composite Crash Boxes

In the present study crash boxes made of GFRP material are considered. GFRP crash boxes

can be manufactured by hand layup process. In this study the crash box is modeled with

Belystcho-Tsay shell elements with element formulation 2 (ELFORM-2formulation).

Unidirectional long fiber single laminate GFRP material shell-structure model is applied to

all the crash boxes considered in the study. Sufficiently small element size (5mm x 5 mm)

has been used throughout the crash box. The meshed structure is shown in Fig. 3.2.

Fig. 3.2: Crash box meshed model for sample

Since a variety of cross-sectional shapes and trigger shapes have been simulated in the

present work, for better quality of finite element mesh, the modeling, mesh generation,

application of displacement and load-boundary conditions for all numerical studies were done

in a separate high-end mesh generation and pre-processing software called as HyperMesh

(version 13.0). The pre-processed models are then are imported into the LS-DYNA for

41

solving. The material property data were taken from work done by Tabiei et al. (2005). GFRP

composite crash box is considered with thickness1.8 mm in the all studies.The

MAT_LAMINATED_ COMPOSITE_FABRIC material model (material type 58) provided

by finite element solver of LS-DYNA is used for crash box model with material properties as

given in Table 3.1.

*Consistency of units is very important in numerical simulation and hence all the units were

modified according to Ton, mm, sec units system followed by LS-DYNA.

MAT_LAMINATED_ COMPOSITE_FABRIC (material type 58) is widely used to model

the composites for finite element simulation (Schweizerhof et al., 1998). This material model

can be used with shell and thick shell elements. LS-DYNA explicit finite element solver is

Table 3.1: Material properties of GFRP Composite ( Tabiei et al. ,2005)

Property Symbol Value

Mass Density ρ 1.8 gm/cc

Young's modulus - longitudinal direction Ea 41400 MPa

Young's modulus - transverse direction Eb 3381 MPa

Young's modulus – normal direction Ec 3381 MPa

Poisson's ratio Vba 0.0244

Longitudinal tensile strength. Xt 786.6 MPa

Longitudinal compressive strength Xc 786.6 MPa

Shear modulus (ab) Gab 5244 MPa

Shear modulus (bc) Gbc 5244 MPa

Shear modulus (ca) Gca 5244 MPa

Transverse tensile strength Yt 191.1 MPa

Transverse compressive strength Yc 191.1 MPa

Shear strength, (ab) plane. Sc 53.82 MPa

Strain at longitudinal compressive strength ϵ11c 0.019

Strain at longitudinal tensile strength ϵ11t 0.019

Strain at transverse compressive strength ϵ22c 0.056

Strain at transverse tensile strength ϵ22t 0.056

Strain at shear strength ϵgms 0.011

42

used for solving the model and HyperView and HyperGraph are used as post-processor for

extraction and analysis of the simulation results.

In real time the crash box is joined to thick plate at the rear end (bottom end) and this thick

plate is attached to a much stronger vehicle structure (front rails). The front end (top end) is

subjected to impact in the event of a collision. (Refer Fig 1.9 & Fig 1.10)

For the component level analysis of the crash box, the setup is simplified for ease of study

on crashworthiness. The crash box is considered to be fixed at the bottom, (considered to be

attached to a rigid surface at the bottom for simplification of the problem). The top end of the

crash box is impacted by the impactor. The impactor (which represents the rigid barrier) is

considered to be a rigid plate. The impactor was given a velocity of 16kmph similar to RCAR

test as shown in Fig. 3.3.

Fig. 3.3: Crash box simulation setup

Contact was defined between the impactor and the crash box by treating the former as the

master and the latter as the slave for proper interaction between the two. An additional

contact interface was defined for the crash box walls, as the walls of crash box would come

into contact with each other during the process of deformation under the impact.

The analysis setup is done based on “RCAR test”, as if the vehicle is undergoing the low

velocity impact test popularly known as the “Front Structural RCAR Test”. The purpose of

this test is to validate the crashworthiness of front structure of the vehicle in a low velocity

impact (16kmph), which is common in day-to-day traffic conditions. This test helps in

finding the crashworthiness of crash box under impact. For the vehicle to pass the test it is

43

required that the components mounted behind the crash box do not get damaged during the

impact, so that there is minimal damage and repair cost. For this to happen the crash box

must absorb the maximum possible energy. Hence crash box plays a very vital role in this

test. In this study the component level analysis of the crash box is done, so as to suggest the

relatively best combination of geometric cross section and trigger configuration for the GFRP

crash box. Component level analysis is helpful in saving computing time, is less complex for

analyzing and can be solved in relatively less powerful computers.

Initial validation of GFRP crash box is carried out by comparing reported

experimental data and simulation results obtained from the present numerical simulation

analysis. The simulation is carried out under the same conditions as those of the experimental

work by Tabiei et.al. (2005). Comparision of test and simulation based axial force-

displacement curves are displayed in Fig 3.4.

Fig 3.4: Comparison of experimental result and present numerical simulation

Objective comparisons of relevant quantities extracted from these curves are given in

Table 3.2.

44

A good correlation is obtained between results of current simulation and those of

experimental test done by Tabiei et.al. (2005). The validated simulation model is used for

setting up the simulation for crash box for the cases required in this study. The material is

kept unchanged whereas only the geometry is updated according to the different cases of the

crash boxes explored in the present work.

The force versus displacement (F-D) diagram is used to study the behaviour of component

under crash. It basically shows the variation of force level with respect to displacement or

deformation. In general the F-D diagram is very useful in determining the parameters such as

primary peak force, secondary peak force and energy absorption. Primary peak force or

simply the peak force is the maximum force achieved during the crushing of specimen. The

peak force is generally high in the initial stage of crushing as the specimen resists the

deformation and thus the force level increases. It can be noted for better understanding

therefore that the peak force is the maximum reaction force offered by the specimen during

the impact. Energy absorption is another important parameter which is considered for

specimens subjected to impact or crash loads. Energy absorbed can be calculated from F-D

diagram by considering the area under the force versus displacement curve. The more the

area under the F-D curve, the more the energy absorbed. In addition to the above parameters,

another important parameter known as specific energy absorption is required to be taken into

consideration. Specific energy absorption (S.E.A) is defined as the energy absorbed by the

specimen per unit mass. In particular S.E.A is very important in automobile industry as the

performance of the specimen is considered with respect to the mass. Generally, higher values

of peak force, energy absorption and S.E.A are desirable for crash boxes undergoing impact.

Table 3.2: Force Comparision of experiment (Tabiei et al.,2005) and present numerical simulation

Parameter Experiment Test

(Tabiei et al., 2005) Present

Numerical Simulation

Peak Crush Force (kN) 31.56 36.21

Mean Crush force (kN) 9.357 8.90

45

In general, the crashworthiness of any component is characterized by the force versus

displacement diagram of the component under the crash loading. The ideal force versus

displacement diagram is shown in Fig. 3.5.

Fig.3.5: Ideal force versus displacement curve

In the ideal case, the force absorbed by the component is constant throughout the crushing

process and becomes zero when the component is fully crushed. The area under the curve

represents the energy absorbed by the component. The energy absorbed is maximum in the

ideal case whereas in practical cases the force absorbed by the component starts from zero

value and varies as the component is crushed. Example of a typical real force-displacement

curve is shown in Fig.3.6.

Fig.3.6: Typical practical force versus displacement curve

46

In the graph the deformation is represented by x-axis and the force by y-axis. Here the

energy absorbed (the area under the curve) is less than the ideal case. At some point during

the crushing, the force value reaches maximum value which is called the primary peak force.

If the force value subsequently in further deformation of the crash box reaches any additional

peak value, then it is called as secondary peak. In general when the component resists the

deformation, there is rise in force level. The force value mainly depends on the deformation

pattern of the component, and it varies as the component is crushed. With the intention of

controlling the deformation pattern various types of triggers can be introduced in the

geometry of the component. Triggers can be defined as geometric features or irregularities

applied intentionally to the component for the purpose of achieving the desired force, energy

and deformation pattern. Thus there is a wide scope for use of triggers for achieving required

level of energy absorption in crashworthiness, but the effect of triggers varies significantly

based on the type of material and geometry of the component. Specific Energy Absorption

(S.E.A) is an important parameter used to study the energy absorption of a component and is

defined as the energy absorbed per unit mass, so higher S.E.A means the component can

absorb more energy for less mass which is desirable in automobile industry.

The main purpose of using triggers is to increase the energy absorption level of the

component. Triggers are geometrical features which help in initiating the deformation in

components used for crashworthiness applications. Triggers are provided near the region of

impact so that the deformation can be initiated in the component near the impact region and

the deformation is carried on through the component in a sequential manner. In cases where

triggering is not used the component can deform in a random manner, which is not desirable

for crashworthiness applications

For calculation of the energy absorbed during the deformation process (that is the area

under the curve) HyperGraph software is utilised. Where the area covered under the graph

can be calculated during the processing of Force versus Displacement curves in the

HyperGraph software.

3.5 Numerical Analysis of GFRP Crash Boxes

In this section the finite element simulation of impact of GFRP crash boxes of different

geometrical cross-sections and types of triggers is presented. All the four types of crash boxes

are impacted with a rigid barrier at speed of 16 kmph as shown in Fig. 3.3 earlier. The

selection of dimensions for crash box for this study is based mainly on the dimensions of

47

crash boxes used in passenger cars. Uni-directional fiber orientation is considered along

longitudinal axis of specimen.

3.5.1 Different types of cross sectional geometries with no triggers

In this section the behaviour of the crash boxes is studied with four different types of

geometries viz., square, cylindrical, hexagonal and decagonal. The length of the crash box is

maintained as 120 mm and the characteristic radius of different cross sectional shapes is

maintained to be the same and equal to the radius ‘R’ of the circumscribed circle, which was

kept equal to 36 mm. All crash box specimens were considered with thickness of 1.8mm. The

cross sections of geometries are shown in Fig.3.7.

Square Cylindrical Hexagonal Decagonal

Fig. 3.7: Various cross sections of the crash box used for the study

All the cross sections are made such that they are circumscribed with in a circle of

radius (R) 36 mm in order to maintain uniformity (fig 3.7 & 3.8). Finite element analysis was

carried out for crash boxes without any trigger in this case as shown in fig.3.9.


Fig. 3.8: Various crash boxes used for the study before deformation

48


Fig. 3.9 Deformation of the crash boxes after the impact

Fig. 3.10: Force versus displacement curves for geometries without trigger

Table 3.3: Comparison of energy absorbed and peak force for geometries

without trigger.

S.NO. Geometry Energy

Absorbed ( J )

Peak Force (kN)

S.E.A (J/kg)

1 SQUARE 313.29 31.35 3935.82

2 CYLINDRICAL 368.46 36.67 4168.11

3 HEXAGONAL 353.22 81.87 4265.97

4 DECAGONAL 638.29 146.89 7345.12

49

From the Force versus displacement (F-D) diagram (fig.3.10) and table 3.3 it can be

observed that the decagonal has the highest peak force among all the specimens with 146.89

kN, followed by hexagonal and cylindrical specimens with 81.87 kN and 36.67 kN

respectively. The force level is lowest for square specimen with 31.35 kN. The deformation

modes of the crash boxes were significantly different with the variation of cross section.

While energy absorption level is having some interesting behaviour, the energy absorption is

highest for the decagonal with 638.29 J followed by cylindrical with 368.461 J and hexagonal

with 353.22 J. The energy absorption level is slightly less for hexagonal specimen compared

to cylindrical type even though hexagonal type has higher peak than cylindrical because of

the reason that hexagonal type is undergoing a non sequential deformation mode, that is the

deformation is initiating at the middle region for hexagonal type with buckling of the walls

rather than deformation occurring at the top region where the impactor is hitting the crash

box. It can be noted that non sequential deformation leads to reduced energy absorption

which indeed reduces the crash performance of the specimen. The lowest energy absorption

was observed for square type with 313.29 J. The S.E.A value is highest for decagonal with

7345.12 J/kg and is lowest for square with 3935.82 J/kg. Hexagonal has a S.E.A value of

4265.96 J/kg and S.E.A of cylindrical is 4168.11 J/kg. Hexagonal has slightly higher S.E.A

compared to cylindrical specimen as the mass of hexagonal is slightly less than the

cylindrical type. From the above table 3.3 it can be observed that decagonal crash box is

having the best performance in all the parameters and square crash box is having the least

performance.

It can be observed from the above results (fig.3.10) that there is a sudden drop in the force

level for all the specimens and the deformation pattern is non sequential with hexagonal

specimen having highest degree of non sequential deformation (fig. 3.9). Due to which the

energy absorbed is reduced as the sudden decrease in force level reduces the area under the F-

D curve dramatically. More over there is no significant second peak for any of the specimen

which can be helpful in increasing the amount of energy absorbed. Energy absorption and

deformation pattern can be altered to achieve the required level by induction of geometrical

features in the form of triggers.

50

3.5.2. Notch Triggers for Different Cross Sectional Crash Boxes

In this section the behaviour of the crash box is studied with Notch triggers applied to all

the cross sections considered in the study. Triggers are applied near the impacted region of

the crash box. Trigger-A (Edge notch): 1 mm wide notches are made on the side walls of

crash box starting from the edges and up to a length of 20mm, Trigger-B (Corner notch):

Corner notch is made on the crash box such that the edge is bent outwards forming an arc of

radius 5mm at the impacted edge and Trigger-C (Combination of Edge and Corner Notch):

Notch combination of triggers A and B. The length of the crash box is maintained as 120 mm

and thickness 1.8mm. The cross sections of geometries are kept unchanged.

3.5.2.1. Square crash boxes with notch triggers

The square crash boxes after application of notch triggers are shown in fig. 3.11. Four

types of square crash box models which are considered to investigate the effect of triggers are

as follows, a) Without trigger (No trigger), b) With trigger-A, c) With trigger-B and d) With

trigger-C. The remaining parameters are kept unchanged for all the crash boxes. Finite

element analysis is carried out for all the above mentioned cases of crash boxes (fig. 3.12).

No Trigger Trigger-A Trigger-B Trigger-C

Fig.3.11: The square crash boxes before impact

51


Fig.3.12: The square crash boxes after impact

Fig.3.13: The force versus displacement curves for square crash boxes

52

From the force versus displacement data obtained (fig.3.13 and table. 3.4) it can be

observed that lowest S.E.A value is for trigger-A type with a value of 3437.59 (J/kg). For

trigger-A there is a sudden drop in force level after the primary peak due to which the amount

of energy absorption has reduced. Whereas the S.E.A improvement (41.2%) is highest for

Trigger-C type with a value of 5584.65(J/kg). Trigger-C type has a primary peak force of

16.78 kN and a secondary peak force of 10.22 kN and the area covered under the curve after

the primary peak is more, due to this the amount of energy absorbed during this time period is

more and hence the total energy absorbed has increased. For no-trigger, even though the

primary peak force value is 31.35 kN, there is no significant rise in force level for secondary

peak and there is a sharp fall in the force level after the primary peak due to which the overall

energy level is low. Trigger-B type has the primary and secondary peak force values as 20.38

kN and 8.60 kN, it can be observed that there is no significant rise in force level after the

primary peak due to which the amount of energy absorbed is low, as the overall area under

the curve is less. There is also a significant change in the deformation modes of the specimens

by the use of various triggers (fig.3.12.).

3.5.2.2. Cylindrical crash boxes with notch triggers

The cylindrical crash boxes after application of notch triggers are shown in fig. 3.14. Four

types of cylindrical crash box models which are considered to investigate the effect of triggers

are as follows a) Without trigger (No trigger), b) With trigger-A, c) With trigger-B and d)

With trigger-C. The remaining parameters are kept unchanged for all the crash boxes. Finite


Table 3.4: Comparison of energy absorbed and peak force for square crash boxes with notch triggers

S.NO. Square Energy

Absorbed ( J )

Primary Peak Force

(kN)

Secondary Peak Force (kN)

S.E.A (J/kg)

Percentage Improvement

of S.E.A (%)

1 No trigger 313.29 31.35 7.12 3935.82 0

2 Trigger-A 272.12 22.75 8.59 3437.59 - 5.1

3 Trigger-B 415.91 20.38 8.60 5109.45 29.8

4 Trigger-C 451.24 16.78 10.22 5584.65 41.2

53

Fig.3.16: The force versus displacement curves for cylindrical crash boxes


Fig.3.14: The cylindrical crash boxes before impact


Fig.3.15: The cylindrical crash boxes after impact

54

From the force versus displacement data obtained (fig.3.16 and table. 3.5) it can be

observed that lowest S.E.A value is for No trigger type with a value of 4168.11 (J/kg). With

the use of Trigger-A the S.E.A improved by 11.51 percentage for a value of 4647.58 (J/kg).

With the use of Trigger-C the S.E.A value improved by 22.95 percentage and value of

5125.02 (J/kg) was obtained. The S.E.A improvement is highest for Trigger-B type with

35.45 percentage improvement for a value of 5646.12 (J/kg). Trigger-B type has a primary

peak force of 23.46 kN and a secondary peak force of 26.87 due to this the amount of energy

absorbed during this time period is more and hence the total energy absorbed is increased, as

the area under the curve has increased. For Trigger-C both the primary and secondary peak

values are less compared to Trigger –B, so the energy absorbed for Trigger-C is also less

compared to Trigger-B. For No trigger and Trigger-A types even though the primary peak is

high there is sudden drop in the force level which reduced the amount of energy absorbed as

the overall area under the respective curves was decreased due to this fall in force level. The

change of force level between the primary and secondary peaks was less for Trigger-B which

is desirable in crash worthiness. There is also a significant change in the deformation modes

of the specimens by the use of triggers (fig.3.15).

3.5.2.3. Hexagonal crash boxes with notch triggers

The hexagonal crash boxes after application of notch triggers are shown in fig. 3.17. Four

types of hexagonal crash box models which are considered to investigate the effect of triggers

are as follows, a) Without trigger (No trigger), b) With trigger-A, c) With trigger-B and d)

Table 3.5: Comparison of energy absorbed and peak force for cylindrical crash boxes

with notch triggers

S.NO. Cylindrical Energy

Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 368.46 36.67 9.61 4168.11 0

2 Trigger-A 408.52 39.65 11.36 4647.58 11.51

3 Trigger-B 512.66 23.46 26.87 5646.12 35.45

4 Trigger-C 462.27 15.65 16.29 5125.02 22.95

55




Fig.3.17: The hexagonal crash boxes before impact


Fig.3.18: The hexagonal crash boxes after impact

Fig.3.19: The force versus displacement curves for hexagonal crash boxes

56

Table 3.6: Comparison of energy absorbed and peak force for hexagonal crash boxes

with notch triggers

S.NO. Hexagonal Energy

Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 353.22 81.87 8.64 4265.97 0

2 Trigger-A 485.32 80.57 11.44 5884.13 37.93

3 Trigger-B 443.31 15.62 14.4 5268.79 23.51

4 Trigger-C 463.09 12.16 12.23 5529.52 29.62

From the force versus displacement data obtained (see Fig. 3.19 and Table. 3.6) it can be

observed that lowest S.E.A value is for no-trigger type with a value of 4265.97 (J/kg) as there

is a non-sequential deformation (deformation occurs at the mid of the crash box, see Fig.

3.18) due to which the energy absorption is poor. Therefore it is very important to have a

sequential deformation pattern for the crash box, in which the deformation occurs first at the

impact region and then the deformation is propagated to the rest of the specimen in a

sequential manner. For trigger-A the S.E.A improvement is highest with 37.93% with S.E.A

value of 5884.13 (J/kg). For trigger-B even though the primary and secondary peak forces are

low with values of 15.62 kN & 14.4 kN respectively, the change in the force level is minimal

due to which the energy absorbed (443.31J) is more hence the S.E.A improvement is

23.51 %. Similar behaviour is shown by trigger-C; here also the primary and secondary peak

forces are low with values of 12.16 kN & 12.23 kN respectively and the change in the force

level is minimal due to which the energy absorbed (463.09 J) is more hence the S.E.A

improvement is 29.62 %. There is also a significant change in the deformation modes of the

specimens by the use of various triggers (see Fig. 3.18). By the use of triggers the

deformation mode changed from non-sequential in no-trigger type to sequential in all the

specimens with triggers, this an important desirable change which is achieved by the use of

triggers.

57

3.5.2.4 Decagonal crash boxes with notch triggers

The decagonal crash boxes after application of notch triggers are shown in Fig. 3.20. Four

types of decagonal crash box models which are considered to investigate the effect of triggers

are as follows, a) Without trigger (No trigger), b) With trigger-A, c) With trigger-B and d)




Fig.3.20: The decagonal crash boxes before impact


Fig.3.21: The decagonal crash boxes after impact

58

Fig.3.22: The force versus displacement curves for decagonal crash boxes

From the force versus displacement data obtained (see fig. 3.22 and table. 3.7) it can

be observed that highest S.E.A is observed for no-trigger type with a value of 7345.12 (J/kg)

due to significant high primary peak force of 146.89 kN which is very high compared to other

types, this increases the area under the curve which in turn increases the S.E.A. Lowest

S.E.A value is for trigger-C type with a value of 5409.06 (J/kg) it also has the lowest primary

peak value and low secondary peak force with 23.44 kN and 15.15 kN respectively, this

means that the component cannot resist the deformation and is too easy for getting deformed

Table 3.7: Comparison of energy absorbed and peak force for decagonal crash boxes with notch triggers

S.NO. Decagonal Energy

Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 638.29 146.89 12.65 7345.12 0

2 Trigger-A 553.59 60.96 25.98 6404.32 - 12.80

3 Trigger-B 569.57 25.58 18.51 6403.26 -12.82

4 Trigger-C 477.62 23.44 15.15 5409.06 -26.35

59

under impact load due to which the force and energy levels are low. For trigger-A the primary

peak force is good with a value of 60.96 kN, but there is a sudden drop in the force level due

to which the overall energy is low even though the primary peak force is good. For trigger-B

the primary peak force and secondary peak force is 25.58 kN and 18.51 kN respectively.

Interestingly, the no-trigger type has the highest force, energy and SE.A values which means

that after application of the trigger the crash box is becoming too weak to withstand the

impact load hence the force as well as the energy levels are low. Therefore it is very

important to understand the behaviour of each type of geometry with various types of triggers

as trigger which is best for one type of geometry may not be the best for other type of

geometry.

3.5.3. Slot Triggers for Different Cross Sectional Crash Boxes

In this section the behaviour of the crash box is studied with slot triggers applied to all the

cross sections considered in the study. Slot triggers are applied near the impacted region of

the crash box at a distance of 5mm from the edge. Slots of size 5mmx5mm are made on the

crash box. Based on the array of the slots provided, the triggers are defined as type-1 slot,

type-2 slot and type-3 slot. The gap between each row of slot trigger is maintained as 20mm

along the length of the specimen. The length of the crash box is maintained as 120 mm and

thickness 1.8mm. The cross sections of geometries are kept unchanged.

3.5.3.1 Square crash boxes with different types of slot triggers

The square crash boxes after application of slot triggers are shown in (fig.3.23). Four types

of square crash box models which are considered to investigate the effect of triggers are as

follows, a) Without trigger (No trigger), b) With type-1 slot, c) With type-2 slot and d) With

type-3 slot. The remaining parameters are kept unchanged for all the crash boxes. Finite


60

No Trigger Type-1 Slot Type-2 Slot Type-3 Slot

Fig.3.23: The Square crash boxes before impact


Fig.3.24: The Square crash boxes after impact

Fig.3.25: The force versus displacement curves for square crash boxes with different

slot triggers.

61

From the force versus displacement diagram (see fig. 3.25 and table 3.8) it can be observed

that the type-1 slot has the maximum S.E.A among all the crash boxes with 4375.36 (J/kg),

and with energy absorption of 341.71 (J). The energy absorption is lowest for type-2 slot

crash box with S.E.A value of 3790.34 (J/kg). Specific energy absorption is a key parameter

to measure the energy absorption with respect to mass. The higher the S.E.A the more

efficient the energy absorption is for the part. It can be observed from the above results that

there is a slight increase in force level at secondary peak for type-1 slot, due to which the

energy absorbed is increased, as the force level increases the region under the force versus

displacement curve which denotes the energy absorbed also increases. For type-2 slot and

type-3 slot the S.E.A is less with values of 3790.34 (J/kg) and 3856.94 (J/kg) which is less

compared to no-trigger type, which means that the specimen is getting weak after application

of these triggers, which is not desirable.

3.5.3.2 Cylindrical crash boxes with different types of slot triggers

The cylindrical crash boxes after application of slot triggers are shown in (fig.3.26). Four

types of cylindrical crash box models which are considered to investigate the effect of triggers

are as follows, a) Without trigger (No trigger), b) With type-1 slot, c) With type-2 slot and d)

With type-3 slot. The remaining parameters are kept unchanged for all the crash boxes. Finite


Table 3.8: Comparison of energy absorbed and peak force for square crash boxes with


S.NO. Square Energy

Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 313.29 31.35 7.12 3935.82 0

2 Type-1 Slot 341.71 24.71 8.05 4375.36 11.16

3 Type-2 Slot 296.82 24.32 7.82 3790.34 - 3.69

4 Type-3 Slot 299.41 24.23 7.41 3856.94 - 2.00

62


Fig.3.26: The Cylindrical crash boxes before impact


Fig.3.27: The Cylindrical crash boxes after impact

Fig.3.28: The force versus displacement curves for cylindrical crash boxes with different

slot triggers.

63

From the force versus displacement diagram (fig. 3.28 and table 3.9) it can be observed

that the type-1 slot has the lowest S.E.A of all the crash boxes with 4004.98 (J/kg), with

energy absorption of 351.63 (J). The energy level is highest for type-2 slot crash box with

S.E.A of 4239.82 (J/kg). Type-3 Slot has S.E.A value of 4015.99 (J/kg). It can be observed

from the above results (fig.3.27) that there is a significant change in the deformation pattern

of the crash boxes with the addition of slot triggers, this change in the deformation under

crushing is responsible for variations in the energy absorption levels.

3.5.3.3 Hexagonal crash boxes with different types of slot triggers

The hexagonal crash boxes after application of slot triggers are shown in (fig. 3.29). Four

types of hexagonal crash box models which are considered to investigate the effect of triggers




Table 3.9: Comparison of energy absorbed and peak force for cylindrical crash boxes with different slot triggers.


Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 368.46 36.67 9.6 4168.11 0

2 Type-1 Slot 351.63 39.4 12.94 4004.98 - 3.91

3 Type-2 Slot 369.71 37.8 12.43 4239.82 1.72

4 Type-3 Slot 347.70 37.2 12.31 4015.99 - 3.64

64


Fig.3.29: The Hexagonal crash boxes before impact


Fig.3.30: The Hexagonal crash boxes after impact

Fig.3.31: The force versus displacement curves for hexagonal crash boxes with


65

From the force versus displacement diagram (see fig. 3.31 and table. 3.10) it can be

observed that the no trigger type has the lowest S.E.A of all the specimens with 4265.96

(J/kg), with energy absorption of 353.22 (J) and the deformation pattern is also non-

sequential or random. The energy level is highest for type-1 slot crash box with S.E.A of

4932.84 (J/kg). It can be observed from the above results that there is a significant increase in

the force level with value of 11.34 kN at secondary peak for type-1 slot which increases the

energy absorption. The deformation pattern also changed from non-sequential to sequential

with the addition of triggers, therefore it can be noted that the amount of energy absorbed

increases for sequential deformation pattern.

3.5.3.4 Decagonal crash boxes with different types of slot triggers

The decagonal crash boxes after application of slot triggers are shown in (fig.3.32). Four

types of decagonal crash box models which are considered to investigate the effect of triggers





with different slot triggers.


Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 353.22 81.87 8.64 4265.97 0

2 Type-1 Slot 403.50 78.2 11.34 4932.84 15.63

3 Type-2 Slot 360.89 73.7 7.02 4463.18 4.62

4 Type-3 Slot 388.72 71.0 8.16 4866.94 14.08

66


Fig.3.32: The Decagonal crash boxes before impact


Fig.3.33: The Decagonal crash boxes after impact

Fig.3.34: The force versus displacement curves for decagonal crash boxes with different slot triggers.

67

From the force versus displacement diagram (fig.3.34 and table 3.11) it can be observed

that the no trigger type has the highest S.E.A among all the crash boxes with 7345.12 (J/kg),

with energy absorption of 638.29 (J). The energy level is lowest for type-2 slot crash box

with S.E.A of 5967.14 (J/kg). It can be observed from the above results that there is a

significant increase in the force level with value of 146.89 kN at primary peak for no trigger

type which increases the energy absorption. For type-1 slot the secondary peak is 18.59kN

which is higher than other types but as the primary force peak is lower (138.26 kN) compared

to no trigger type (146.89 kN), therefore overall energy absorption is slightly less for type-1

slot crash box compared to no trigger crash box.

3.5.4. Thickness Variation (Front End) Triggers for Different Cross Sectional Crash

Boxes

In this section the behaviour of the crash box is studied with variation in thickness applied to

all the cross sections considered in the study. Triggers are applied near the impacted region of

the crash box. Thickness variation triggers are applied for a length of 5mm each in the crash

boxes by reducing the thickness to half of the original thickness that is from thickness value

of 1.8 mm to 0.9 mm at the trigger location (represented by green colour). Based on the array

of thickness variations provided, the thickness variation triggers are defined as thickness

variation 1, thickness variation 2 and thickness variation 3 types. The gap between each row

of trigger is maintained as 20mm along the length of the specimen. In case thickness variation

Table 3.11: Comparison of energy absorbed and peak force for decagonal crash boxes with different slot triggers.


Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 638.29 146.89 12.65 7345.12 0

2 Type-1 Slot 624.42 138.26 18.59 7260.69 - 1.15

3 Type-2 Slot 508.04 136.36 9.10 5967.14 -18.76

4 Type-3 Slot 651.10 129.45 15.80 6031.61 - 17.88

68

is applied at only the impacted edge (with only one row of thickness variation) it is called as

Front End trigger.

3.5.4.1 Square crash boxes with thickness variation triggers

The square crash boxes after application of thickness variation triggers are shown in

(fig.3.35). Four types of square crash box models which are considered to investigate the

effect of triggers are as follows, a) Without trigger (No trigger), b) With thickness variation 1,

c) With thickness variation 2 and d) With thickness variation 3. The remaining parameters are

kept unchanged for all the crash boxes. Finite element analysis is carried out for all the above

mentioned cases of crash boxes (fig. 3.36).

No Trigger Thickness Thickness Thickness

. Variation 1 Variation 2 Variation 3

Fig.3.35: The Square crash boxes before impact



Fig.3.36: The Square crash boxes after impact

69

Fig.3.37: The force versus displacement curves for square crash boxes with

different thickness variation triggers.

From the force versus displacement diagram (fig.3.37 and table 3.12) it can be observed

that the thickness variation 1 has the highest S.E.A of all the specimens with 4230.95 (J/kg),

with energy absorption of 327.05 (J). The energy level is lowest for thickness variation 3

specimen with S.E.A of 2813.64 (J/kg). It can be observed from the above results, that even

though the primary peak for no trigger type is high with value of 31.35 kN there is sudden

drop in the force level, whereas in case of Thickness variation 1 the force value is slightly

higher than other types even after the primary peak which increases the energy absorption

value.

Table 3.12: Comparison of energy absorbed and peak force for square crash

boxes with different thickness variation triggers

S.NO. Square Energy

Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 313.29 31.35 7.12 3935.82 0

2 Thickness

Variation 1 327.05 20.40 10.64 4230.95 7.49

3 Thickness

Variation 2 226.15 19.54 7.53 3519.62 - 10.57

4 Thickness

Variation 3 207.92 18.11 7.41 2813.64 - 28.51

70

3.5.4.2 Cylindrical crash boxes with thickness variation triggers

The cylindrical crash boxes after application of thickness variation triggers are shown in

(fig.3.38). Four types of cylindrical crash box models which are considered to investigate the







Fig.3.38: The Cylindrical crash boxes before impact



Fig.3.39: The Cylindrical crash boxes after impact

71

From the F-D diagram (fig.3.40 and table 3.13) it can be observed that the No Trigger

type has the highest S.E.A with value of 4168.11 (J/kg) with energy absorption of 368.46 J

whereas the lowest is for Thickness Variation 2 with S.E.A value of 3275.41 (J/kg). From the

Force versus Displacement graph, it can be observed that the force level for No Trigger type

is maximum at primary peak with value of 36.67 kN and the force level drop is gradual

compared to other types which increases the area under the F-D curve hence the energy is

also increased.

Fig.3.40: The force versus displacement curves for cylindrical crash boxes with different

thickness variation triggers.

Table 3.13 Comparison of energy absorbed and peak force for cylindrical crash



Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 368.46 36.67 9.6 4168.11 0

2 Thickness

Variation 1 342.57 37.43 19.88 3960.40 - 4.98

3 Thickness

Variation 2 272.41 36.14 13.27 3215.84 - 22.84

4 Thickness

Variation 3 271.63 33.9 13.14 3275.41 - 21.41

72

3.5.4.3 Hexagonal crash boxes with thickness variation triggers

The hexagonal crash boxes after application of thickness variation triggers are shown in

(fig.3.41). Four types of hexagonal crash box models which are considered to investigate the







Fig.3.41: The hexagonal crash boxes before impact



Fig.3.42: The hexagonal crash boxes after impact

73



From the F-D diagram (fig.3.43 and table 3.14) it can be observed that the Thickness

Variation 1 has the highest S.E.A with value of 5340.51 (J/kg) with energy absorption of

432.58 J whereas the lowest is for Thickness Variation 2 with S.E.A value of 4056.04 (J/kg).

From the F vs. D graph, it can be observed that the force level for Thickness Variation 1 at

secondary peak is higher compared to other types with value of 17.17 kN and the force level

drop is gradual compared to other types which increases the area under the F-D curve hence

the energy is also increased. Thickness Variation 1 displays a unique behaviour where the

crash box side wall gets teared after the application of this trigger (fig.3.42).

Table 3.14: Comparison of energy absorbed and peak force for hexagonal crash



Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 353.222 81.87 8.64 4265.97 0

2 Thickness

Variation 1 432.58 60.15 17.70 5340.51 25.18

3 Thickness

Variation 2 321.81 53.74 14.96 4056.05 - 4.92

4 Thickness

Variation 3 326.65 57.34 14.41 4207.25 - 1.38

74

3.5.4.4 Decagonal crash boxes with thickness variation triggers

The decagonal crash boxes after application of thickness variation triggers are shown in

(fig.3.44). Four types of decagonal crash box models which are considered to investigate the







Fig.3.44: The decagonal crash boxes before impact



Fig.3.45: The decagonal crash boxes after impact

75

Fig.3.46: The force versus displacement curves for decagonal crash boxes with


Table 3.15: Comparison of energy absorbed and peak force for decagonal

crash boxes with different thickness variation triggers


Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 638.292 146.89 12.65 7345.12 0

2 Thickness

Variation 1 692.32 78.60 44.31 8135.39 10.75

3 Thickness

Variation 2 567.01 116.25 34.1 6347.43 - 13.58

4 Thickness

Variation 3 528.23 116.32 31.23 6479.84 - 11.78

From the F-D diagram (fig.3.46 and table 3.15) it can be observed that the Thickness

Variation 1 has the highest S.E.A with value of 8135.39 (J/kg) with energy absorption of

692.32 J whereas the lowest is for Thickness Variation 2 with S.E.A value of 6347.43 (J/kg).

Even though the primary peak for Thickness Variation 1 is 78.60 kN the secondary peak is

higher (44.31 kN) compared to other types and also the force level after secondary peak is

higher than other types, hence the overall energy absorption value is higher.

76

3.6 Comparative Analysis of Crashworthiness of GFRP crash boxes

In this section crashworthiness behaviour of the GFRP crash box made of various

geometries and trigger types is summarized and then discussed in a comparative manner so

that a clear picture of the effectiveness of geometrical cross section as well as the trigger

configuration can be obtained. The results of numerical simulation were used for presenting

the results in a comparative as well as consolidated manner for an overall view of the various

combinations of crash box geometries and trigger types used in this study.

3.6.1 Consolidated results for each type of geometry used for crash boxes

(a) Square Crash Boxes

From table 3.16 it is observed that for square cross-sectional crash boxes the trigger

types Trigger-B, Type-1 Slot, Thickness Variation 1 and trigger-C are helpful for increasing

the S.E.A values compared to the crash box without trigger. Whereas the trigger types

Trigger-A, Type-2 Slot, Type-3 Slot, Thickness Variation 2 and Thickness Variation 3 have

Table 3.16: Comparison of energy absorbed and peak force for square crash boxes


S.NO. Square Energy

Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 313.29 31.35 7.12 3935.82 0

2 Trigger-A 272.12 22.75 8.59 3437.59 - 5.1

3 Trigger-B 415.91 20.38 8.60 5109.45 29.8

4 Trigger-C 451.24 16.78 10.22 5584.65 41.20

5 Type-1 Slot 341.71 24.71 8.05 4375.36 11.16

6 Type-2 Slot 296.82 24.32 7.82 3790.34 - 3.69

7 Type-3 Slot 299.41 24.23 7.41 3856.94 - 2.00

8 Thickness

Variation 1 327.05 20.40 10.64 4230.95 7.49

9 Thickness

Variation 2 226.15 19.54 7.53 3519.62 - 10.57

10 Thickness

Variation 3 207.92 18.11 7.41 2813.64 - 28.51

77

a negative effect in which the S.E.A value is reduced compared to the crash box without

trigger. The most suitable trigger type for square crash boxes is trigger-C as it helps in

achieving a huge 41.20 percentage increase in the S.E.A with a value of 5584.65 (J/kg). It

can also be seen that the thickness variation 3 type of trigger is most unsuitable for square

crash boxes as it reduces the S.E.A by 28.51 percentage with a value of 2813.64 (J/kg).

(b) Cylindrical crash boxes

From table 3.17 it is observed that for cylindrical cross-sectional crash boxes the

trigger types Trigger-A, Trigger-B, Trigger-C and Type-2 Slot are helpful for increasing the

S.E.A values compared to the crash box without trigger. Whereas the trigger types Type-1

Slot, Type-3 Slot, Thickness Variation 1, Thickness Variation 2 and Thickness Variation 3

have a negative effect in which the S.E.A value is reduced compared to the crash box without

trigger. The most suitable trigger type for cylindrical crash boxes is trigger-B as it helps in

achieving a 35.45 percentage increase in the S.E.A with a value of 5646.12 (J/kg). It can also

be seen that the thickness variation 2 type of trigger is most unsuitable for cylindrical crash

Table 3.17: Comparison of energy absorbed and peak force for cylindrical crash boxes



Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 368.46 36.67 9.61 4168.11 0

2 Trigger-A 408.52 39.65 11.36 4647.58 11.51

3 Trigger-B 512.66 23.46 26.87 5646.12 35.45

4 Trigger-C 462.27 15.65 16.29 5125.02 22.95

5 Type-1 Slot 351.63 39.4 12.94 4004.98 - 3.91

6 Type-2 Slot 369.71 37.8 12.43 4239.82 1.72

7 Type-3 Slot 347.70 37.2 12.31 4015.99 - 3.64

8 Thickness

Variation 1 342.57 37.43 19.88 3960.40 - 4.98

9 Thickness

Variation 2 272.41 36.14 13.27 3215.84 - 22.84

10 Thickness

Variation 3 271.63 33.9 13.14 3275.41 - 21.41

78

boxes as it reduces the S.E.A by 22.84 percentage with a value of 3215.84 (J/kg).

(c) Hexagonal crash boxes

From table 3.18 it is observed that for hexagonal cross-sectional crash boxes the

trigger types Trigger-A, Trigger-B, Trigger-C, Type-1 Slot, Type-2 Slot, Type-3 Slot and

Thickness Variation 1are helpful for increasing the S.E.A values compared to the crash box

without trigger. Whereas the trigger types Thickness Variation 2 and Thickness Variation 3

have a negative effect in which the S.E.A value is reduced compared to the crash box without

trigger. It is observed that for hexagonal cross-sectional crash boxes the most suitable trigger

type is trigger-A as it helps in achieving a 37.93 percentage increase in the S.E.A with a value

of 5884.13 (J/kg). It can also be seen that the thickness variation 2 type of trigger is most

unsuitable for hexagonal crash boxes as it reduces the S.E.A by 4.92 percentage with a value

of 4056.05(J/kg).




Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 353.22 81.87 8.64 4265.97 0

2 Trigger-A 485.32 80.57 11.44 5884.13 37.93

3 Trigger-B 443.31 15.62 14.4 5268.79 23.51

4 Trigger-C 463.09 12.16 12.23 5529.52 29.62

5 Type-1 Slot 403.50 78.2 11.34 4932.84 15.63

6 Type-2 Slot 360.89 73.7 7.02 4463.18 4.62

7 Type-3 Slot 388.72 71.0 8.16 4866.94 14.08

8 Thickness

Variation 1 432.58 60.15 17.70 5340.51 25.18

9 Thickness

Variation 2 321.81 53.74 14.96 4056.05 - 4.92

10 Thickness

Variation 3 326.65 57.34 14.41 4207.25 - 1.38

79

(d) Decagonal crash boxes

From table 3.19 it is observed that for decagonal cross-sectional crash boxes only the

trigger type Thickness Variation 1 is helpful for increasing the S.E.A values compared to the

crash box without trigger, as it helps in achieving a 10.75 percentage increase in the S.E.A

with a value of 8135.39 (J/kg). Whereas all the remaining trigger types have a negative effect

in which the S.E.A value is reduced compared to the crash box without trigger. It can also be

seen that the trigger-C type of trigger is most unsuitable for decagonal crash boxes as it

reduces the S.E.A by 26.35 percentage with a value of 5409.06 (J/kg).

Table 3.19: Comparison of energy absorbed and peak force for decagonal crash boxes



Absorbed ( J )

Primary Peak Force

(kN)


S.E.A (J/kg)


of S.E.A (%)

1 No trigger 638.29 146.89 12.65 7345.12 0

2 Trigger-A 553.59 60.96 25.98 6404.32 - 12.80

3 Trigger-B 569.57 25.58 18.51 6403.26 -12.82

4 Trigger-C 477.62 23.44 15.15 5409.06 -26.35

5 Type-1 Slot 624.42 138.26 18.59 7260.69 - 1.15

6 Type-2 Slot 508.04 136.36 9.10 5967.14 -18.76

7 Type-3 Slot 651.10 129.45 15.80 6031.61 - 17.88

8 Thickness

Variation 1 692.32 78.60 44.31 8135.39 10.75

9 Thickness

Variation 2 567.01 116.25 34.1 6347.43 - 13.58

10 Thickness

Variation 3 528.23 116.32 31.23 6479.84 - 11.78

80

3.7 Observations from the Chapter

The effects of geometrical shapes as well as the effects of various trigger

configurations like Notch triggers (trigger-A, trigger-B and trigger-C), Slot triggers (type-1

slot, type-2 slot and type-3 slot) and Thickness variation trigger (thickness variation 1,

thickness variation 2 and thickness variation 3) were studied. It was observed that the energy

absorption characteristics varied significantly with the use of different types of triggers for

different types of cross sectional shapes of the GFRP crash boxes. Therefore it can be

concluded that different type of geometries give better energy absorption values with

different trigger types. The parameter focused in this study was S.E.A as it reveals the energy

absorption characteristic for a component considering its mass.

Table 3.20: Comparison of S.E.A consolidated from Numerical Analysis for all the cases.

S.NO. Trigger Square Cylinder Hexagonal Decagonal

1 No trigger 3935.82 4168.11 4265.97 7345.12

2

Notch

Trigger

Trigger-A 3437.59 4647.58 5884.13 6404.32

3 Trigger-B 5109.45 5646.12 5268.79 6403.26

4 Trigger-C 5584.65 5125.02 5529.52 5409.06

5

Slot

Trigger

Type-1 Slot 4375.36 4004.98 4932.84 7260.69

6 Type-2 Slot 3790.34 4239.82 4463.18 5967.14

7 Type-3 Slot 3856.94 4015.99 4866.94 6031.61

8 Thickness

Variation

Trigger

Variation -1 4230.95 3960.40 5340.51 8135.39

9 Variation - 2 3519.62 3215.84 4056.05 6347.43

10 Variation - 3 2813.64 3275.41 4207.25 6479.84

*Units of S.E.A in the above table are (J/kg)

81

The following key observations are made from the numerical simulation of the GFRP

crash boxes.

Without Trigger :

1. The S.E.A was highest for decagonal crash box with a value of 7345.12 (J/kg).

2. The S.E.A was lowest for square crash box with a value of 3935.82 (J/kg).

This means that geometric shape plays a vital role in the energy absorption.

With Trigger :

1. For square type crash box, trigger-C was most suitable trigger with 41.20 % increase

in the S.E.A value (5584.65 J/kg) compared to no-trigger type crash box.

2. For cylindrical type crash box, trigger-B was most suitable trigger with 35.45 %

increase in the S.E.A value (5646.12 J/kg) compared to no-trigger type crash box.

3. For hexagonal type crash box, trigger-A was most suitable trigger with 37.93 %

increase in the S.E.A value (5884.13 J/kg) compared to no-trigger type crash box.

4. For decagonal type crash box, thickness variation-1 was most suitable trigger with

10.75 % increase in the S.E.A value (8135.39 J/kg) compared to no-trigger type.

Out of all the cases considered in the numerical analysis, the highest S.E.A was

observed for decagonal crash box with thickness variation 1 trigger with 8135.39 (J/kg) and

the lowest S.E.A was observed for square crash box with thickness variation 3 trigger with

2813.64 (J/kg), (from table3.20).

3.8 Summary of the Chapter

In this chapter the effects of geometrical shape as well as the effects of various trigger

configurations like trigger-A, trigger-B, trigger-C, type-1 slot, type-2 slot, type-3 slot,

thickness variation 1, thickness variation 2 and thickness variation 3 were studied. The use of

82

proper combination of geometry and trigger type plays a vital role in achieving desired level

of force and energy. It can also be noted that the deformation mode also varied significantly

with the use of different types of triggers. But the use of triggers depends mainly on the

feasibility of practically implementing the triggers in the manufacturing process. Therefore,

main factor obstructing the usage of triggers is the ease of manufacturing them considering

the manufacturing technique, cost incurred in manufacturing, skill of labour, type of

machinery and damage caused to the specimen while trigger is being applied to the specimen.

Thus, even though some triggers are efficient, the difficulty in practically manufacturing or

implementing them limits their usage. One such example is that manufacturing of trigger-A,

trigger-B and trigger-C is complex and requires special manufacturing equipment. Whereas,

for the application of type-2 slot, type-3 slot, thickness variation 2 and thickness variation 3,

special methods are required as there is possibility of damage to specimen during the

application of these triggers. These difficulties can be overcome at industrial manufacturing

level where there is feasibility of procuring advanced machinery and implementing skilled

labour and manufacturing techniques.

As far as specimen manufacturing is considered hand layup is most preferred technique as

it is simple and cost effective. The specimens like crash boxes without trigger (all shapes –

square, cylindrical, hexagonal and decagonal) can be made using hand lay-up process. For

trigger configuration specimens, type-1 slot and thickness variation-1, type of triggers can be

manufactured for all shapes of crash boxes – square, cylindrical, hexagonal and decagonal

using hand layup process. Due to limitation of manufacturing complex triggers other type of

triggers are not feasible to manufacture by simple hand lay-up process.

Therefore, the crash boxes a) Without trigger (all shapes – square, cylindrical, hexagonal

and decagonal), b) With Type-1 slot trigger (all shapes – square, cylindrical, hexagonal and

decagonal) and c) With thickness variation-1 trigger (all shapes – square, cylindrical,

hexagonal and decagonal) are feasible to manufacture with simple hand lay-up technique for

specimen testing. The manufacturing of specimens required for experimental testing is

discussed in detail in the next chapter.

83

Chapter–4: Manufacturing of the Experimental GFRP Crash Box

Specimens

4.1 Introduction

This chapter provides the details of manufacturing process of GFRP composite crash

boxes. Manufacturing of composites is different from metals and requires special techniques.

Therefore, this chapter is dedicated to explaining in detail the manufacturing process for

preparing the experimental specimens. The various steps involved in the manufacturing

technique require design of moulds, estimation of curing time, techniques of curing and the

methods of removal of the specimens from the moulds. Hand lay-up technique is used for

making all the GFRP composite crash boxes specimens used in the present work.

4.2 Description of the Hand Lay-Up Process

Hand lay-up process is one of the most commonly used and cost effective methods

using open-moulding as it can be done with simple setup of equipment. In this method the

fibers or fiber mats are placed on the mould or mandrel and then resin and hardener are

applied to it, which acts as a matrix for the composite specimen. Spraying of release gel on

the mould area is required in order to prevent the sticking of composite specimen to the

mould. Mylar films or sheets made of polyester of thickness 0.1 mm are placed on the mould

in order to avoid the adhesion of resin to the mould area. Absence of such separators can lead

to difficulty in the release of the composite specimen from the mould after curing. The fibers

or woven fiber mats can be placed on the mould in the direction needed for the composite

specimen. After this, a suitable liquid resin (such as epoxy) along with hardener liquid in a

suitable proportion that helps in curing in reasonable time is applied to the fibers that are

present on the surface of the mould. The application of resin can be done using a roller or a

brush. This step can be done repeatedly for obtaining the required number of laminates or

layers of the composite.

The resins generally selected are thermosetting polymers which get cured on their

own after the completion of the process. The curing time may vary slightly depending on the

type of resin and type of hardener used and it also depends on the environmental factors like

84

temperature of the place where curing is done. Although curing for most of the composites

can be done at room temperatures, some composites for special purpose may require a

specific temperature range to be maintained during the curing time. In general the curing can

be expected to complete in 2-3 days time period depending on the number of layers of

composite and the climatic conditions. During the curing period the composites are to be

stored in a neat place as many environmental factors like dust and smoke can affect the

quality of the final composite product. This method is useful in manufacturing many products

related to automobile, aerospace, electrical appliances etc. In general the matrix materials

used are epoxy, polyester and polyurethane resins and the fiber reinforcement materials

generally used are glass fiber, carbon fiber, kevlar fiber and aramid fiber (synthetic fiber).

Fig. 4.1: The hand lay-up process

4.3 Step by Step Procedure for Making Specimens

4.3.1 Precautions while dealing with glass fibers, hardening agents and resins

1. Always use gloves while dealing with glass fibers.

2. Use safety goggles to avoid small glass fiber pieces.

3. Use face mask, to cover nose in order to avoid irritating smell of the resin and

hardener.

4. Never touch the skin with glass fibers as it may cause skin irritation and rashes.

5. Never touch the resin or hardener directly with hand as it may stick to skin, in case

there is accidental contact wash thoroughly using soap or use petro chemical products

like kerosene oil or turpentine to remove it.

85

6. Always use the designated place for making the composites and clean the debris or

tiny glass fiber pieces immediately.

7. Keep the tool, fiber layout and mould ready before hand for pouring in order to avoid

solidification of resin + hardener mixture.

8. Always use clean environment for curing the specimen as dust and tiny debris may

stick to wet resin during curing

9. Always apply the releasing agent on the mould, Mylar films (both surfaces) in order

to release the specimen from the mould easily.

10. Use pressure while applying the resin + hardener mixture to fiber in order to get

proper distribution of the mixture, removal of air entrapped and to get uniform

thickness.

4.3.2 Making of Mould for the Specimen

In order to make crash boxes with four different types of geometries viz., square,

cylindrical, hexagonal and decagonal we need to make a mould for each shape. The

dimensions of all the GFRP specimens were made similar to those used in the finite element

simulation. That is length of the crash box is 120 mm, cross section is maintained such that it

is circumscribed by a circle of radius (R) 36 mm (fig. 4.2) and thickness of 1.8mm.

Maintaining the same radius of the circumscribed circle for all cross-sections serves the

purpose of comparison of their performance.


Fig. 4.2: The cross-sections of all specimens were maintained to be within a circle of same radius R.

The geometric shapes of the specimens are given in Fig. 4.3.

86


Fig. 4.3: The various shapes of the crash boxes

The moulds required are made according to the shape required for the crash boxes.

Four different moulds with the required cross-sectional shapes were fabricated. Each mould

was manufactured using mild steel plates of length 1 m and suitable widths so that when they

were welded together at their edges, it resulted in a mould cross-section that was radially 0.1

mm smaller than the inner radial dimensions of the specimen (as mylar films of thickness

0.1mm will be wrapped on the moulds during the manufacturing process). The dimensions of

the mould were kept well within the tolerance range (+/- 0.1 mm) as it effects the final shape

and size of the specimen. The dimensions of the moulds were measured in the metrology lab

in the workshop to check and ensure that the mould shape represents the shape of the final

specimen required. The length of the mould was kept to be of 1 meter, which is more than the

length of the specimen (120 mm) so as to provide ease in handling while preparing the

specimen. The extra length of the mould also enabled fixing it to a fixture firmly while the

hand lay-up process was done. The Fig. 4.4 gives the geometric shapes and actual images of

the fabricated moulds.

i) Square Mould:

Mould design Actual Image

87

ii) Cylindrical Mould:


iii) Hexagonal Mould:


88

iv) Decagonal Mould:


Fig. 4.4: The different moulds for each cross section of the crash box

The manufacture of mould is an important step because the dimensional and

geometric form accuracy of the specimens directly depends on those accuracies of the

corresponding moulds. Therefore, it is essential to use standard manufacturing, fitting and

measurement processes to maintain the quality of moulds.

4.3.3 Step by Step Hand Lay-Up Process for Composite Crash Box

Fig. 4.5 shows the glass fiber mat used in manufacturing of the GFRP composite specimens.

The mat contains unidirectional glass fibers and is of thickness 0.9 mm. The glass fiber mat

comes in roll form or bundle form. The required length of the mat was cut from the fiber mat

roll depending on the specimen size. Uni-directional fiber orientation is considered along

longitudinal axis of specimen.

89

Fig. 4.5: The glass fiber mat

4.3.3.1 Application of Releasing Agent on the Mould Surface

Releasing agent is applied on the surface of the mould so that the specimen does not

get stuck to the mould after the curing process. Here releasing agents used are wax polish and

silicone mould release spray (fig.4.6).

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Wax Polish Mould release spray

Fig. 4.6: The releasing agents

4.3.3.2 Wrapping of Mylar Film on the Mould Surface

Mylar film used for separation of mould from the specimen in the present work are

thin sheets (0.1mm thickness) made of polyester (fig.4.7) and it was wrapped on the mould in

order to avoid the adhesion of resin to mould area to avoid the difficulty in release of the

composite specimen from the mould after curing. The wrapping of Mylar film is one of the

important steps in hand lay-up process as any gap between the Mylar film and mould can lead

to distortion of shape of the final product. Mylar films are wrapped tightly on the moulds.

Fig. 4.7: The Mylar film roll

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Releasing agent was applied again on the surface of the Mylar film wrapped (fig.4.8).

Then, as the final step in the mould preparation, pressing is applied on Mylar film to make it

stick to the mould thoroughly everywhere (fig.4.9).

Fig. 4.8: Wrapping of Mylar film on the mould

Fig. 4.9: Application of releasing agent on Mylar film

4.3.3.3 Preparation of the Resin

Proper mixing of resin and hardener is very important for the manufacture of

composite as it acts as the matrix material which holds together the fiber reinforcement in the

composite. The hardener acts as a curing agent and helps the resin solidify in a reasonable

period of time. In general the resins used are of thermosetting type. For making of the

composite specimens L12 was used as polymer resin and K6 was used as hardener, (fig.

4.10).

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Fig. 4.10: Resin and Hardener

With the help of a measuring jar for each 100 ml of resin 20 ml of hardener is added

for proper combination (fig. 4.11).

Fig. 4.11: Measuring quantity of Resin and Hardener

Then the mixture is mixed thoroughly in a bucket before applying to the fiber mat.

The mixture is to be applied immediately as soon as it is ready or else it may start getting

harder due to solidification (fig. 4.12).

93

Fig. 4.12: Mixing of Resin and Hardener

4.3.4.4 Application of resin and hardener mixture to fiber mat

The resin and hardener mixture is applied to the fiber mat using a roller or brush (fig.

4.13). Even though the application of resin and hardener mixture is easy when done by brush,

a roller is used to spread it uniformly for getting uniform thickness and it also helps in

removal of air gaps between fiber mat and the resin mixture (fig. 4.14).

Fig. 4.13: Application of Resin mixture on mat by brush

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Fig. 4.14: Spreading of Resin and Hardener mixture by roller

4.3.4.5 Wrapping of fiber mat on the mould

After the resin-hardener mixture is applied on the fiber mat, this fiber mat is to be

wrapped immediately on the mould to get the proper shape. The fiber mat is to be wrapped on

the mould such that there is no gap between the mat and the mould, mild pressure is to be

applied on the fiber mat so that any air trapped can be removed. After the fiber mat is soaked

in resin-hardener mixture and completely wrapped on the mould, further amount of the resin-

hardener mixture is applied by brush to attain the required fiber-matrix volume proportion. It

is also important to finish the surface of specimen at this stage to minimise the voids and

stress concentrations sites on the surface (fig. 4.15).

Fig. 4.15: Beginning of wrapping of fiber mat on the mould

95

The fiber mat is to be wrapped around the mould starting from one end and ending at

the other end as shown in fig. 4.16. The composite mat was wrapped around the mould for

complete two wraps to attain the thickness of 1.8mm for the composite specimen.

Fig. 4.16: Wrapping of fiber mat on the mould

Paint brush is used for careful application of resin to the wrapped specimen to get the

proper surface finish (fig. 4.17).

Fig. 4.17: Applying finishing touches to the specimen

The techniques used for controlling the casting defects in specimens are as follows:

Spreading of Resin and Hardener mixture by roller so that the resin is properly spread.

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Proper tight wrapping of the resin applied fiber mat, so that there is no air trapped

between the mould and the fiber mat and the fiber mat reflects the mould shape after

wrapping.

Applying finishing touches to the specimen this helps to improve surface finish and

also helps in filling tiny gaps in the composite specimen.

The wet specimen on the mould should be allowed for curing in a neat and dry

environment. Care should be taken that there is no dust or smoke in that area. The specimens

take around two to three days for curing to complete. The curing was done at room

temperature. After the curing is completed the specimens were pulled out of the mould such

that one end of the mould is held firmly using a machine vice (fig. 4.18, fig. 4.19, fig. 4.20 &

fig. 4.21)

Fig. 4.18: Square specimen after curing period

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Fig. 4.19: Cylindrical specimen after curing period

Fig. 4.20: Hexagonal specimen after curing period

98

Fig. 4.21: Decagonal specimen after curing period

The specimens were then cut for the required length of 120 mm. Proper care is to

taken while cutting so that there is no vibration or crack in the specimen. Unlike metals extra

care is to be taken when machining the composite specimens. A cotton cloth was inserted

inside the specimen just beside the cutting area to provide support for the specimen in order

to avoid any kind of cracks due to machining (fig 4.22).

Fig. 4.22: Cutting of the specimen

After the cutting process was completed, geometrical triggers were applied to the

specimens. Due to the complexity of application of triggers only a) Type-1 slot trigger (all

shapes – square, cylindrical, hexagonal and decagonal) and b) Thickness variation-1 trigger

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(all shapes – square, cylindrical, hexagonal and decagonal) were feasible to incorporate. The

dimensional accuracy of the specimens manufactured by the above process was checked in

metrology lab and was found to be within the tolerance value of ± 0.1mm. The samples of

specimens made of various cross sections are shown in the fig. 4.23, fig. 4.24, fig. 4.25 and

fig. 4.26.

Fig. 4.23: Sample for square specimens

Fig. 4.24: Sample for cylindrical specimens

Fig. 4.25: Sample for hexagonal specimens

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Fig. 4.26: Sample for decagonal specimens

After the specimens are prepared they should be stored in a neat environment so that there

is no effect of surroundings on them. Care should be taken that the samples do not come in

contact with any chemical agents. Specimens were stored in carton boxes for transportation to

the testing facility.

4.4 Summary of the Chapter

In this chapter details of hand lay-up process are explained. With the overall success of the

simple hand lay-up technique as manufacturing process the GFRP crash boxes which are

manufactured are, as follows a) Without trigger (all shapes – square, cylindrical, hexagonal

and decagonal), b) With Type-1 slot trigger (all shapes – square, cylindrical, hexagonal and

decagonal) and c) With thickness variation-1 trigger (all shapes – square, cylindrical,

hexagonal and decagonal).

For the purpose of experimental repetitions of each case, four specimens for each variant of

crash box were manufactured successfully with as low a rejection percentage less than 25%,

primarily resulting from some of the crash box specimens sticking to the mould and getting

damaged in the withdrawal, which were discarded.

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Chapter–5: Experimental Impact Testing of GFRP Crash Box

Specimens

5.1 Introduction

This chapter provides the details of experimental impact testing of GFRP composite

crash boxes. Many essential precautionary steps that were taken before and during the

experiment are discussed. The experience gained through these experimental works is also

incorporated. In this study, the main purpose of conducting experimental impact testing of

GFRP composite crash box specimens is to validate and correlate the finite element

numerical simulation model developed and presented in the chapter 3. Therefore, the number

of crash box specimens manufactured and actually tested is a subset of the total number of

cases analyzed by the numerical simulation. The subset choice of experimental cases were

carefully chosen and their choice is justified due to the fact that it is not only very expensive

but also highly time consuming to undertake the experimental investigation of the all the

cases in the parametric sweep. In the beginning of this chapter, the description and

arrangement of the impact testing machine used are presented, followed by the preparation

for experimental study, procedure for drop weight impact testing, correlation of the

simulation results with experimental results and finally discussion of results and summary.

Details for preparation for test are provided so that it may serve as a guide to the reader.

Many challenges faced in the experimental testing and factors affecting experimental testing

and numerical simulation are discussed. Limitations of experimental testing as well as

numerical simulation are also presented.

5.2 Drop Weight Impact Testing

This section first delineates the experimental test rig used for the impact testing of the

GFRP composite crash box specimens followed by the description of preparation for testing,

actual testing procedure and the results obtained in the experimental work.

5.2.1 Drop weight impact testing machine

The drop weight impact testing machine is especially designed for drop-weight test of

pipe and plate (including plastics, ceramic and building materials). The machine used in the

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present experimental work conforms to the international standards of drop-weight impact

testing machines and it contains all the necessary high precision data sampling and

measurement systems. The schematic diagram of drop weight impact testing machine is

shown in Fig. 5.1.The drop weight testing enables the determination of characteristic force,

energy, and displacement parameters. The drop weight impact testing was done at the

Bandung Institute of Technology and Research University located in Bandung, Indonesia.

Fig. 5.1: The schematic diagram of drop weight impact testing machine

Component nomenclature of the drop-weight impact testing machine shown in Fig. 5.1

1. Guide column

2. Steel plate

3. Concrete base

4. Impactor assembly

a. Impactor head

b. Weights

c. Wheels

d. Frame

5. Impactor Clamp

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6. Hoist

7. Speed sensor

8. Specimen

9. Load cell

With reference to the Fig. 5.1, the nomenclature of various component parts of the test

rig. The Fig. 5.2a & 5.2b shows the image of the actual setup with ancillary components of

the experimental test rig.

Fig. 5.2a: Drop weight Impact Testing Setup

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Fig. 5.2b: Drop weight Impact Testing Machine

The load cell and the velocity measuring devices were connected to two of the

channels independently. The signals sensed by the load cell were fed to a charge amplifier

that converts the charge signal to a voltage signal. This voltage signal was then sent to a

personal computer through a data acquisition system. The data acquired was then converted

to a compatible ASCII file that gives the force-time data in digital form. This fundamental

data was then mathematically processed in the HyperGraph software to derive force and

displacement. The force-displacement curve was then plotted and the area under such curve

gives the energy absorbed (fig.5.3).

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Fig 5.3: Data acquisition system setup

5.2.2 Preparation for Drop Weight Impact Test

Several steps are involved in the preparation for actual drop-weight impact testing. Some of

these steps are meant for operator safety while the remaining are to ensure the successful

capturing of the both the force-displacement data and the images of the progression of the

fracture of the specimen under impact loading.

5.2.2.1 Making of Clamp for Specimen

In order to hold the specimen during the drop weight impact testing there is a

need for a clamping device, which can hold the specimen. The specimen clamp holds the

specimen from inside (like a press fit from inside) such that there is minimal gap between the

two. The clamp is mounted on the base plate, and base plate is mounted on the mounting

provision provided on the massive steel plate which is placed over concrete foundation of the

drop weight impact testing machine. Clamp is useful not only for holding the specimen

during the drop weight impact testing but also for helping in securing the specimen in the

center of the base plate (fig. 5.4). Clamp helps in preventing the specimen from any undesired

motion (slipping or moving sideways) during the impact. Separate clamps were made for

each type of crash box viz. square, cylindrical, hexagonal and decagonal.

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Fig 5.4: Schematic diagram of specimen with clamp and base plate

The base plate and clamp are very essential components in the experimental setup.

They are designed and fabricated well before the experimental work. Improper design of base

plate and the specimen clamp can have substantial influence on the output result. Aluminium

alloy 6061 was used as material for manufacturing of base plate and the specimen clamp. The

dimensions of the base plate and the specimen clamp are required to be maintained according

to the dimensions of the mounting bolts and shape of the base foundation of the drop weight

impact testing machine. Fig. 5.5 shows the design of the base plate and the specimen clamp

for square cross-section crash box prepared in the present study and the Fig 5.6 and Fig 5.7

shows its CAD model. Similarly, the Fig. 5.8, Fig 5.9 and Fig. 5.10 show, respectively, the

design and CAD model of the specimen clamp for cylindrical crash box. Dimensions of base

plate and the specimen clamp are provided below for reference.

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Fig 5.5: Dimensions of the base plate and the specimen clamp for square specimen.

All dimensions are in mm.

108

Fig 5.6: 3-dimensional image of the base plate and the specimen clamp for square

specimen (top)

Fig 5.7: 3-dimensional image of the base plate and the specimen clamp for square

specimen (bottom)

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Fig 5.8: Dimensions of the base plate and the specimen clamp for cylindrical specimen.

All dimensions are in mm.

110

Fig 5.9: 3-dimensional image of the base plate and the specimen clamp for cylindrical

specimen (top)

Fig 5.10: 3-dimensional image of the base plate and the specimen clamp for cylindrical

specimen (bottom)

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The base plate was deliberately designed to be common for all different clamps meant

for different cross-sectional shapes of the crash boxes. This was to help reduce the weight of

the total material involved in the rig to facilitate the international transport as well as material

saving. The clamps were so designed as to permit their easy mounting on the base plate with

the help of a screw on the base plate so that the clamps could be interchanged depending on

the specimen type for the testing.

The specimen clamp was made by machining of solid cylindrical block made of

aluminium alloy. The Fig. 5.11 (a) shows clamps after machining and in their final shape.

Care was taken such that there was no play between the specimen and clamp in the sideways

and the clamp was machined till a proper fit for each specimen was achieved as shown in

fig.5.11 (b).

Fig 5.11 (a): Machined clamps in their final shape

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Fig 5.11 (b): Intermittent checking of fitting of the crash box onto the clamp

5.2.2.2 Safety Precautions during Drop Weight Impact Testing

Drop weight impact testing is a dangerous experimental test and injuries may occur if

proper safety precautions are not followed. A cage of iron mesh was installed around the drop

weight impact testing machine, which would prevent any person from entering the test area

during testing period. The cage would be closed during the test period for safety reason (fig.

5.12).

Fig 5.12: The testing is protected with a locked up cage to prevent inadvertent entry of

any person

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The impact weight can be altered by changing the weight plates mounted on the impactor.

But there is always a danger that the heavy weight impactor may fall any time in case of any

malfunction of the impactor holding device or safety clamp, which holds the heavy impactor

(fig. 5.13).

Fig 5.13: The safety clamp for impactor

To safeguard against any such accidents, a safety pin was installed in the safety

clamp, which prevents the safety clamp from accidental release leading to sudden fall of the

heavy impactor. The mechanism of the safety pin involves a facility that until it is pulled out,

the impactor is not released thereby increasing the operational safety (fig. 5.14). The safety

clamp and the safety pin are operated by independent wires attached to them.

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Fig 5.14: Safety clamp for impactor with safety pin installed

For operations like changing the specimen, changing the specimen clamp and

measuring height of the impactor etc., which are to be done very near to the impact region the

impactor is brought down to a minimum height for safety reason, in order to further increase

the safety a wooden frame is placed below the impactor to prevent the user from any injury

from heavy impactor (fig. 5.15).

Fig 5.15: Wooden frame for impactor

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Due to the risk of flying debris from the impact and from broken GFRP sharp edged

pieces protective glasses and plastic hand gloves were used. To avoid tiny particles flying in

the air after the impact face mask was used. To avoid the loud impact coming from

continuous tests, sound protective ear plugs were used, Fig. 5.16.

Fig 5.16: Protective gear used during testing

The weight of the impactor can be modified using the weight plates (see fig. 5.17), which

are attached to the impactor. The weight plates can be interchanged with the help of bolts

attached to the impactor.

Fig 5.17: Weight plates (blue colour) attached to the impactor

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5.2.2.3. Drop Weight Impact Testing Procedure

The glass fiber reinforced plastic material used for the crash box specimens is considered

to be laminated type with two layers of uni-axial fiber laminates. The crash box is mounted

with help of a specimen clamp at the bottom. The impactor is considered to be rigid. The

weight plates of the impactor were added such that the total mass of 117kg was dropped from

a height of 4.2 meter. The mass of the impactor and the height at which it is kept for the

impact were derived from the results of finite element simulation of the impact process

(impact energy value from numerical simulation, chapter 3). The mass and height were

selected so as to be equivalent to values constituting a total impact energy of 4820 J, which

was the average measured value of energy for a large variety of crash box specimens

investigated in the numerical simulation. The impactor can be raised or lowered using the

motor mounted on top of the main frame (Fig 5.18). The uniformity of impact energy for all

specimens is ensured by setting the dead weight at the same height in all experiments.

Fig 5.18: Motor mounted on the main frame

The height of the impactor from the specimen is measured using a laser equipped distance

measuring device (fig. 5.19).

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Fig 5.19: Laser equipped distance measuring device

Experimentation output data recording or data acquisition equipment mainly consisted

of the load cell connected through proper channel to a computer (Fig. 5.20) and also high-

speed camera setup useful in capturing deformations of specimens.

Fig 5.20: Data acquisition system

High speed cameras were installed for capturing the deformation images in a detailed

and sequential manner (fig. 5.21). A MEGA SPEED make HHC X7 PRO portable high speed

camera having a capacity of 600 fps (frames per second) at 1920x1080 pixel resolution was

used. Since the entire test setup was enclosed in a cage of iron mesh for operator safety during

the impact process, the deformation images were captured from outside the cage (hence the

iron mesh is slightly visible in the images captured). In order to capture the correct

deformation due to impact, images of the final deformed specimens were captured exactly at

the end of the first impact from the impactor (in order to avoid the additional deformation of

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specimen due to rebound of the impactor, which was observed in some cases).

Fig 5.21: High speed camera setup

The load cell is connected to a computer wherein the data of impact force as a function

of time is recorded and plotted with the help of the LabView software. The MEGA SPEED

make HHC X7 PRO portable high speed camera is equipped with re-moveable SSD for data

security. The camera consists of a memory card in which the images as well as video of entire

impact process is recorded.

The load cell used is made by TML, Japan and it essentially consists of F-series foil

type strain gauge. The load cell provides voltage as output which is processed in LabVIEW

signal express software (2011) for obtaining Force versus Time (F vs. T). HyperGraph

software (13.0) was used for further processing of the data obtained from the experimental

data acquisition system, for filtrations of the noise (disturbance) due to transient conditions of

testing. Society of Automobile Engineers’, S.A.E – 60 filter which has a filtering frequency of

60 hertz along with curve smoothing options were used to get a smoother curve for the

(F vs. T) plots. The Images from high speed camera are processed using VIC-2D (Digital

Image Correlation Software) for obtaining the Displacement versus Time curve (D vs. T).

The (F vs. T) and (D vs. T) curve data is further mathematically processed in the

HyperGraph software, for obtaining the Force versus Displacement curve (F vs. D). The

force-displacement (F-D) curve was then plotted and the area under such curve gives the

energy absorbed, which can be calculated in HyperGraph software.

The present experimental work is carried on using GFRP material for the

crashworthiness study of composite crash box; the material is kept unchanged whereas only

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the geometry of the crash boxes is updated according to the cases required for studying the

crashworthiness behaviour of GFRP crash boxes. The length of the crash boxes is kept as 120

mm; the cross section is maintained such that it is circumscribed by a circle of radius (R) 36

mm and the thickness as 1.8mm. The geometries are shown in Fig. 5.22 and 5.23. Four types

of crash box models which are considered for the study are as follows, a) Square, b)

Cylindrical, c) Hexagonal and d) Decagonal.


Fig. 5.22: The different cross sections of geometries used in the experimental study


Fig. 5.23: The different types of geometries used in the experimental study

After the required specimen-clamp and specimen are mounted properly on the base plate

the cage around the test rig is locked-up and all the persons around the test rig are moved to a

safe distance. Then the data acquisition system is switched on followed by the high speed

camera. To release the impactor, first the safety pin is pulled out with the help of the wire

attached to it, which is immediately followed by releasing of the impactor by operating the

safety clamp. As soon as the safety clamp is released the impactor slides down the frame and

impacts the crash box specimen. During the impact, the GFRP crash box undergoes

deformation and fracture, the amount of deformation and the nature of fracture being

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dependent on the cross-sectional shape and the type of trigger employed on the crash box.

The fracture of the crash box results in debris of sharp tiny pieces flying at high speed around

the test area. After the impactor as well as the flying debris comes to rest, the cage is now

required to be opened for inspection and collection of the fractured crash box specimen.

Before this activity, it is essential to complete two more steps of safety. Firstly, the safety

clamp along with the safety pin is attached to impactor and the impactor is lifted upwards to

the designated height. Secondly, the wooden frame is placed back underneath the impactor to

prevent accidental drop of the impactor. Then the deformed crash box specimen is removed

carefully. It is necessary to wear hand gloves while handling the removal of the fractured

crash box, as it contains sharp broken edges of glass fiber that can cause injury to the

operator. The deformed crash box specimen is then taken to the inspection table for

measurement, observation and other studies to be conducted.

When the impact is in progress the data acquisition system consisting of load cell,

velocity measuring device and the high speed camera enable recording of very crucial data

about the performance of the crash box.

5.3 Results and Discussion

In this section the results obtained from the experimental testing as well as the

analysis of the impacted GFRP crash boxes of different geometrical cross-sections and types

of triggers in the drop weight impact machine are presented. The results are discussed in

order to understand the effect of various cross-sectional shapes and triggers on the important

performance variables of the crash box. Observations and inferences are also drawn on the

nature of deformation of the crash box in the experimental work and the same is compared

with that observed in the numerical simulation work.

5.3.1 Results of experiments on GFRP crash boxes without trigger

Fig. 5.24 shows the crash box specimens of the four cases of square, cylindrical,

hexagonal and decagonal cross sections without any kind of triggers in fractured condition at

the instant of the drop weight having come to stationery position after the impact.

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Fig.5.24: The crash boxes with various geometries after impact

It is interesting to see in Fig. 5.24 that the square cross sectional crash box undergoes

fracture at the bottom end (clamped end) where it is mounted on to the specimen clamp.

Whereas, the other three cross-sectional shape crash boxes undergo fracture at their free end

(impacted end) rather than at the clamped end. It is clear to see that the extent of fractured

length is progressively lower from square to cylindrical, hexagonal and decagonal crash

boxes, in that order. It may also be observed that the spread of the fractured fronds is highest

in the square crash box and this spread progressively reduces in cylindrical, hexagonal and

decagonal crash boxes, in that order.

Fig.5.25: The force versus displacement curves for test of crash boxes with

different geometries.

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Table 5.1: Test energy absorbed and peak force comparision for crash boxes

with different geometries

S.No. Test

Geometry

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak Force

(kN)

S.E.A

(J/kg)

1

Square 275.70 29.04 7.87 3463.50

2 Cylindrical 327.90 36.10 9.10 3709.63

3 Hexagonal 307.30 75.21 11.20 3801.40

4 Decagonal 558.50 136.01 14.88 6427.10

Fig.5.25 shows the comparative force-displacement plots and the Table 5.1 gives the

comparative values of total energy absorbed, primary peak force, secondary peak force and

the S.E.A for the four crash boxes tested. From these force-displacement plots it can be

observed that the decagonal crash box has the highest primary peak force of 136.01 kN in

comparison to hexagonal crash box (75.21 kN), cylindrical crash box (36.10 kN), and square

crash box (29.04 kN). It is seen that the trend of secondary peak force values is the same with

the decagonal crash box having the maximum value (14.88 kN) while the hexagonal,

cylindrical and square having lesser values in that order.

From the table 5.1 it can be observed that the S.E.A values of the four crash boxes

tested follow a consistent trend with those for the primary peak force and secondary peak

force. The S.E.A is highest for decagonal crash box (6427.10 J/kg), whereas the other three

crash boxes showed relatively lesser S.E.A values in the order of hexagonal (3801.40 J/kg),

cylindrical (3709.63 J/kg) and square (3463.5 J/kg).

In general the deformation of a crash box is preferred to occur at the end of impact

and progress inwards. Since it is observed in case of square crash box, that the initiation of

the fracture in it occurs at the clamped end, it is essential to work on the geometry to modify

it in such a way that the fracture initiates at the impacted end. Hence triggers which are

discussed later in this study are a point of interest in these types of cases. Triggers are placed

near the region which is impacted by the impactor, so that due to presence of trigger there is a

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local deformation in that region and then the crash box can deform in a sequential manner

rather than deforming catastrophically. In some cases use of triggers may also help in

generating a secondary peak force, which increases the total energy absorbed and is very

desirable in crashworthiness analysis. Triggers can also help modify the deformation pattern

and thus useful in modifying the force, energy and S.E.A levels.

5.3.2 Results of experiments on GFRP crash boxes with front end trigger (thickness

variation 1 trigger)

Fig. 5.26 shows the four types of crash boxes with front end trigger, having been subjected

to drop weight impact testing. For Front End Trigger (Thickness Variation 1), the thickness is

reduced from 1.8 mm to 0.9 mm up to a length of 5mm near the edge of the specimen which

gets impacted by the impactor. The remaining parameters are kept unchanged for all the crash

boxes.


Fig.5.26: The crash boxes with front end trigger after impact

It can be seen in Fig.5.26 that the same four crash boxes with front end trigger

performed differently under the impact in comparison to the case of no triggers discussed in

the previous sub-section. With front end trigger all the four crash boxes undergo initiation of

fracture at the impacted end. The extent of spread of the fronts follows a similar trend, where

the spread decreases progressively from square to cylindrical, hexagonal and decagonal crash

boxes in that order.

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Fig.5.27: The force versus displacement curves for test of crash boxes with

front end trigger

It can be seen in Fig.5.27 that after employing the front end trigger in the crash boxes

there is a significant rise in the magnitude of secondary peak force, which was not observed

in the crash boxes without trigger. This increase in the magnitude of secondary peak force

increases the net area under force-displacement curve, indicating that the crash boxes have

absorbed more total impact energy compared to the case of no triggers. The primary peak

force as well as the secondary peak force is still the largest for the decagonal crash box.

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Table 5.2: Test energy absorbed and peak force comparision for crash boxes with

front end trigger

S.No.

Test

Front End

Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Square 287.80 20.24 10.01 3723.20 7.49

2 Cylindrical 304.80 35.01 18.41 3524.70 - 4.98

3 Hexagonal 376.40 56.73 17.32 4645.80 22.21

4 Decagonal 605.70 71.22 41.03 7118.50 10.75

On comparison of values from the experimentation (Table 5.2), it is evident that relative

S.E.A values of various crash boxes after application of front end triggers are changed when

observed for S.E.A values of crash boxes without trigger (Table 5.1), except that the

cylindrical crash box has reduction in S.E.A. Square crash box has a S.E.A (3723.2 J/kg)

more than the cylindrical crash box (3524.7 J/kg) whereas the hexagonal crash box has a

higher S.E.A of 4645.8 J/kg and the decagonal has the highest value for S.E.A (7118.5 J/kg).

The percentage change in S.E.A values from the case of without trigger to the case of front

end trigger for different crash boxes is far more interesting. The hexagonal crash box gains

the most with a change of 22.21% in comparison the change of 10.75% for decagonal and

7.49% for square crash box. The cylindrical crash box in fact has a negative gain of -4.98%

indicating that the front end trigger is not beneficial for this geometric shape of crash box.

It is also interesting to observe from Tables 5.2 and 5.1 that the primary peak force values

for all crash boxes is lower in the cases with front end trigger than that of the cases without

triggers. This is because of the fact that after provision of front end trigger, the crash boxes

more easily undergo initial deformation due to the presence of triggers at the impacted end,

which reduces the resistance to deformation. This phenomenon of improved initiation of

deformation helps in the crash boxes in further easily undergoing the subsequent progressive

deformation, resulting in an improved net total amount of energy absorbed.

126

Energy absorption is highest for decagonal with 605.7 J and square has the least energy

absorption of 287.8 J. It can be observed that there is a significant increase in the energy

absorption level for hexagonal type with the use of front end trigger. The energy absorption

has increased for other geometries also except for cylindrical type.

5.3.3 Results of experiments on GFRP crash boxes with slot trigger (type-1 slot trigger)

For drop weight impact testing of crash boxes with slot trigger (type-1 slot trigger),

the slots are made in the front portion of the crash box at a distance of 5mm from the top edge

of the crash box. The size of slot is maintained as 5mm for all slots (holes). The remaining

parameters are kept unchanged for all the crash boxes. Fig 5.28 shows the crash boxes with

slot trigger at the end of the impact process. As seen, the spread of fronds progressively

increases from square crash box to cylindrical, hexagonal and decagonal crash box.


Fig.5.28: The crash boxes with slot trigger after impact

Fig.5.29: The force versus displacement curves for test of crash boxes with slot

trigger

127

Table 5.3: Test energy absorbed and peak force comparision for crash boxes with

slot trigger

S.No. Test Slot

Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Square 300.70 24.21 7.56 3850.30 11.16

2 Cylindrical 313.30 35.69 11.12 3564.40 - 3.91

3 Hexagonal 351.10 72.13 9.89 4291.60 12.89

4 Decagonal 546.40 130.32 17.75 6353.10 - 1.15

The Fig. 5.29 shows the force-displacement curves and Table 5.3 shows the energy

absorption and S.E.A for crash boxes with slot trigger. It can be seen that in this case also the

decagonal crash box has the highest primary peak force (130.32 kN) in comparison to other

crash boxes. Interestingly, in the case of slot trigger the benefit that accrued in the previous

case of front end trigger is not realized as none of the crash boxes have significant secondary

peak force. The trend of S.E.A values is similar to that with front end trigger. With slot

trigger, the percentage change in S.E.A in comparison to the case of without trigger is

negative for cylindrical and decagonal crash boxes whereas there is a marginal increase in the

other two crash boxes. After the use of slot trigger the S.E.A for cylindrical and decagonal

geometries is 3564.4 J/kg and 6353.1 J/kg. The S.E.A values for square and hexagonal type

are 3850.3 J/kg and 4291.6 J/kg respectively.

For slot trigger the maximum peak is observed for decagonal with 130.32 kN followed by

hexagonal with 72.13 kN, cylindrical with 35.69 kN and the least is observed for square with

24.21 kN. The energy absorbed by decagonal type is highest with 546.4 J followed by

hexagonal with 351.1 J, cylindrical with 313.3 J and the least is observed for square type with

300.7 J. While S.E.A has increased for square and hexagonal type with the use of slot trigger

compared to no trigger geometry.

128

It is interesting to observe that the primary peak force for all the square, hexagonal and

decagonal crash boxes in all the cases of without trigger, front end trigger and slot trigger

occurs at the same deformation of approximately 2.5 mm. However, for only the cylindrical

crash box the primary peak force without as well as with triggers occurs at a larger

deformation equal to nearly 4 mm.

5.4. Effect of Triggers on Various Cross Sectional Crash boxes

In this section the effects of front end trigger and slot trigger on energy absorption, peak

forces and S.E.A for each of the cross-sectional shapes of crash boxes are compared.

5.4.1 Effect of triggers on square geometry

Without trigger Front End Trigger Slot trigger

Fig.5.30: The deformation for square crash boxes with various triggers

Fig. 5.30 compares the deformation pattern in square crash box without trigger and two

different types of triggers. After application of triggers near the impact region there is a local

deformation initiation in that region and the component has achieved a sequential deformation

rather than deforming catastrophically, which is desirable in crashworthiness applications. As

can be seen, the triggers not only shifted the initiation of deformation from the clamp end to

the impacted end but also they improved the spread of the fronds in comparison to that

without triggers. In case of crash box with front end trigger a finer frond formation takes

place with fibers clearly getting debonded from matrix whereas in case of slot trigger a larger

size frond formation occurs as compared to crash box with front end trigger.

129

Fig.5.31: The force versus displacement curves for square crash boxes with various

triggers

Table 5.4: Test energy absorbed and peak force comparision for square crash boxes


S.No.

Square -

Trigger Type

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

No Trigger 275.70 29.04 7.87 3463.50 0

2 Front end

trigger 287.80 20.24 10.01 3723.20 7.49

3 Slot

Trigger 300.70 24.21 7.56 3850.30 11.16

From fig. 5.30, fig. 5.31 and table 5.4 it can be observed that for square cross-sectional

crash boxes, incorporating triggers has increased the S.E.A by 7.49% with front end trigger

and 11.16% with slot trigger though there is a slight decrease in primary peak force and

marginal introduction of the secondary peak force.

130

5.4.2 Effect of triggers on cylindrical geometry


Fig.5.32: The deformation for cylindrical crash boxes with various triggers

Fig.5.33: The force versus displacement curves for cylindrical crash boxes with

various triggers

131

Table 5.5: Test energy absorbed and peak force comparision for cylindrical crash

boxes with various triggers

S.No.

Cylindrical

-

Trigger

Type

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

No Trigger 327.90 36.10 9.10 3709.63 0

2 Front end

trigger 304.80 35.01 18.41 3524.70 - 4.98

3 Slot Trigger 313.30 35.69 11.12 3564.40 - 3.91

From fig. 5.32, fig. 5.33 and table 5.5, for cylindrical cross-sectional crash boxes the crash

performance is best when no trigger is used while with the use of trigger the S.E.A is getting

reduced slightly. Cylindrical has a peculiar behaviour of F-D diagram for no trigger type in

which the force level does not fall suddenly after it reaches the peak but instead the force

level decreases in the form of a plateau due to which the energy absorption increases

significantly. Even though the front end trigger type has better secondary peak force, it is the

overall energy absorbed that is making the no-trigger type more efficient compared to other

types.

5.4.3 Effect of triggers on hexagonal geometry


Fig.5.34: The deformation for hexagonal crash boxes with various triggers

132

Fig.5.35: The force versus displacement curves for hexagonal crash boxes with various

triggers

Table 5.6: Test energy absorbed and peak force comparision for hexagonal crash


S.No.

Hexagonal

-

Trigger

Type

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

No Trigger 307.30 75.21 11.20 3801.40 0

2 Front end

trigger 376.40 56.73 17.32 4645.80 22.21

3 Slot Trigger 351.10 72.13 9.89 4291.60 12.89

133

From fig. 5.34, fig. 5.35 and table 5.6 for hexagonal cross sectional crash boxes the most

suitable trigger is the front end type as it increases the S.E.A by 22.2% compared to no

trigger type, which is a very significant increase for crash behaviour. Even though no trigger

type has a higher peak of 75.21 kN compared to that of 56.73 kN of front end type. The crash

box with front end trigger has more than one secondary peak achieved after the primary peak

that makes major difference in energy absorption capacity as compared to crash box with no

trigger.

5.4.4 Effect of triggers on decagonal geometry


Fig.5.36: The deformation for decagonal crash boxes with various triggers


various triggers

134

Table 5.7: Test energy absorbed and peak force comparision for decagonal crash


S.No.

Decagonal

-

Trigger

Type

Energy

Absorbed

( J )

Primary

Peak Force

(kN)

Secondary

Peak Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

No Trigger 558.50 136.01 14.88 6427.10 0

2 Front end

trigger 605.70 71.22 41.03 7118.50 10.75

3 Slot Trigger 546.40 130.32 17.75 6353.10 - 1.15

From fig. 5.36, fig. 5.37 and table 5.7 for decagonal cross sectional crash boxes, the

front end trigger is the best suited as it increases the S.E.A by 10.75%. Even though the

decagonal crash box with front end trigger has a substantially lower primary peak force

(71.22 kN) as compared to that of crash box without trigger (136.01 kN) and that of crash

box with slot trigger (130.32 kN), it is its second peak of 41.03 kN that contributes to the

additional energy absorption thereby leading to a net increase in the overall energy

absorption.

5.5. Correlation of Experimental Test and Numerical Simulation

Correlation is not only useful for validating the numerical simulation but also it helps

researchers in analysing and refinement of the numerical simulation so that it is well tuned to

the experimental test. As there is always a need to reduce the number of prototypes for testing

it is not necessary to perform the experimental test for all possible cases. A few carefully

chosen sample cases can be experimentally investigated to validate the simulation model,

upon which the simulation model can be used to conduct much wider investigative study

about the process for all possible important combinations of influencing factors.

Correlation of experimental test and numerical simulation is generally done to validate the

numerical simulation with test data, but both of them have their own limitations as follows:

135

Experimental tests are influenced by many factors like dissimilarities or imperfections

present in the samples being tested. Whereas simulation does not take into account the

imperfections present in the samples like for example voids or impurities in the samples.

Experimental tests are influenced by the frictional forces between the moving parts of the

equipment. Whereas simulation does not consider the frictional losses during the

movement of parts in the equipment.

In numerical simulation the component is considered to be uniform throughout the

geometry but this may not be the practical case as the component may not be uniform

due to imperfections and this may vary the strength of the component.

Simulation is based on theories which govern its behaviour. These theories may involve

many assumptions made for simplification of the problem solving.

Numerical simulations depend on the material model chosen for defining the material of

the specimen. And it is a well-known fact that many parameters are set as default for

stability purpose in the background of the commercial numerical solving codes available

in the market for example LS-DYNA, Abaqus explicit etc.,

Numerical simulation is also dependent on mesh flow and mesh size of the elements used

for the structure.

Experimental tests are dependent on the efficiency and accuracy of the instrumentation

used for data acquisition and recording.

Numerical simulation depends on assumptions made when applying boundary conditions

like for example fixed end, pinned support for simplification of the problem definition.

Experimental test depends on the design of experimental setup as well as the losses

incurred during the reading of measuring values.

Numerical simulation also depends on the computing capacity and the hardware used for

solving.

5.5.1 Need for Correlation of Experimental Test and Numerical Simulation

Pre-test simulations are useful for determining the behaviour of structure as they provide

an approximate picture of crash event beforehand. Later the numerical simulation is

correlated with the experimental test so that it is beneficial for analyzing the various

possibilities for optimizing the structure. Experimental test gives real time failure mode but

detailed analysis during the crash event is not possible, as we can analyze only after the test.

136

Numerical simulation provides not only approximate scenario of the crash event but it also

provides a time history based result which can be analyzed even during the crash event.

In this study earlier (chapter-3), component level analysis was performed in the pre-test

numerical simulation for the composite GFRP crash box, which was very helpful in

determining the force and energy analysis. Even though the pre-test numerical simulation of

component level analysis of GFRP crash box was good at predicting the force and energy

levels for the crashworthiness, it was not sufficient to exactly replicate the complex

deformation modes of the laminated GFRP crash box and needed to be modified as per drop

weight impact test. In order to correlate the drop weight impacting with numerical simulation,

few updates are required to be made to the pre-test numerical simulation model such as; the

GFRP crash box is to be modeled using two layers of laminates as the required thickness (1.8

mm) for the crash box was achieved by using two layers of glass fiber mat (0.9 mm each)

during manufacturing of specimens (chapter-4).The impactor needs to be modeled as per the

impactor used in the experimental test (drop-weight testing impactor). The boundary

conditions are to be modified as per the test scenario like the crash box is in contact with the

specimen clamp at the bottom end and the bottom edge of the specimen is in contact with the

base plate.

Numerical simulation of drop weight impact testing of the GFRP crash boxes is done

for the purpose of correlation between the experimental test and numerical simulation. The

details of the numerical simulation for drop weight impact testing are further explained in

detail in the next section.

5.5.2 Numerical Simulation of Drop Weight Impact Testing of GFRP Crash Boxes

For the purpose of correlation with the drop weight impact testing the numerical

simulation model was updated as follows. The rigid impactor is modelled similar to drop

weight test impactor. The specimen-clamp which holds the specimen is modelled such that it

touches the internal surface of the specimen at bottom, without any interference (as in the real

test). The specimen is placed on the specimen-clamp which is attached to the base plate. The

required contacts are defined between the specimen and specimen-clamp, using

*CONTACT_SURFACE_TO_SURFACE card, with friction coefficient equal to 0.4. The

specimen-clamp and the base plate are attached to each other. Using

137

*CONTACT_SURFACE_TO_SURFACE card, contact is defined between the impactor and

specimen with friction coefficient equal to 0.5. Using *CONTACT_NODES_TO_SURFACE

card, additional edge contact is defined between specimen bottom edge (using edge nodes of

specimen) and base plate top surface, with friction coefficient equal to 0.35. Internal contact

is defined for the specimen itself with friction coefficient equal to 0.3, as the side-walls of the

specimen may be coming into contact with each other while undergoing deformation, using

*CONTACT_SINGLE_SURFACE card. The impactor, specimen-clamp and base plate are

assigned rigid material for analysis using *MAT_RIGID (MAT-20) in LS-DYNA. With the

use of rigid materials for parts with very high rigidity or parts which do not go any

deformation, the computation time in numerical analysis can be reduced. The specimen

parameters like material, thickness and length are kept unchanged (fig. 5.38).

Actual impactor, specimen with clamp

and base plate

Finite element mode of impactor, specimen

with clamp and base plate

Fig.5.38: The simulation setup for drop weight impact test

Laminated structure is considered for the GFRP crash box, with two layers of lamina.

As the specimens were manufactured from two layers of laminates in hand lay-up process.

The thickness of each lamina was 0.9 mm, so the total thickness of the specimen with two

laminates was 1.8 mm. In order to replicate the laminated layers of the GFRP crash box

specimen *PART_COMPOSITE card was used for the specimen in HyperMesh software

along with LS-DYNA as working profile. In this study HyperMesh is used for pre-processing

including the modeling of the finite element model. *PART_COMPOSITE card is useful for

laminated composites in defining the number of layers, material, thickness and orientation for

each lamina independently. The crash box model was meshed using 5x5 mm sized fully

138

integrated shell elements (ELFORM 16). Advantage of using fully integrated elements is that

with increased number of integration points (NIP) these elements are capable of predicting

the deformation in detailed manner at all the nodal coordinates of the element including

element bending and also these elements are helpful in avoiding hour-glass effect which is

observed in elements with less number of integration points. The components were modelled

in the mid - surface of the thickness for each part and thickness applied to each part is equally

added and offset on either sides of the mid – surface. As the specimens were manufactured

from two layers of laminates in hand lay-up process, number of plies (lamina) were also used

as two in *PART_COMPOSITE. The thickness of each lamina/ply was 0.9 mm, so the total

thickness of the specimen made of two laminates was 1.8 mm. Shown in example of

*PART_COMPOSITE card (Fig.5.39). The LS-DYNA keywords *PART_COMPOSITE and

*MAT 58 (MAT_LAMINATED_COMPOSITE_FABRIC) are linked to each other using the

material id (MID), here material id for GFRP is ‘2’ (MID=2). In *PART_COMPOSITE it is

possible to arrange the composite’s ply material, stacks, and its orientation according to the

laminate configurations of the particular composite material. Here as the crash boxes were

made of uni-directional fiber layout, hence the angles for fiber orientation {B(1), B(2)} are

kept as zero (0).

Fig.5.39: PART_COMPOSITE configuration in HyperMesh software

The numerical simulation was carried out in similar conditions, as those of drop

weight impact testing of the crash boxes to replicate the experimental test through numerical

simulation, so that the numerical simulation model can be correlated with the experimental

test as close as possible. The specimen parameters like material, thickness and length are kept

unchanged for all the cases used in this investigation.

139

In LS-DYNA, MAT-58 (MAT_LAMINATED_COMPOSITE_FABRIC) is used for

composite components modelled with shell elements. LS-DYNA provides additional controls

for composite material models in *Control Shell Card, for controlling the shell elements with

the help of laminated theory which is beneficial for MAT-58

(MAT_LAMINATED_COMPOSITE_FABRIC). In order to activate the laminated theory for

composite in this analysis, LAMSHT is activated using a value equal to one (LAMSHT=1) in

*Control Shell Card. Lamination theory is applied to correct for the assumption of a uniform

constant shear strain through the thickness of the shell. Unless this correction is applied, the

stiffness of the shell can be grossly incorrect if there are drastic differences in the elastic

constants from ply to ply, especially for sandwich type shells. Generally, without this

correction the results are too stiff. For the discrete Kirchhoff shell elements (which do not

consider transverse shear) this option is ignored.

5.5.2.1 Calibration of Simulation Parameters in LS-DYNA

Finite element simulation softwares use various types of non-physical parameters,

which are mainly used for the purpose of stability and convergence of the problem for

explicit analysis such as crash impacts. LS-DYNA also uses many such parameters. These

parameters have a considerable influence on the numerical simulation result. Many of these

parameters use default values assigned automatically in the backend by the solver code, used

by the finite element simulation software in absence of specific values as input from the user.

But it is advisable that the user calibrates these values with the help of experiments for

attaining a close correlation between the numerical simulation and experimentation.

In numerical simulation of impacts using LS-DYNA, the force value depends on

parameters like longitudinal compression stress limiting factor (SLIMC1) and softness factor

(SOFT) for the crash-front elements (row of elements in the component present near the

impacted region and these elements undergo deformations prior to other elements in the

model during simulation). Providing higher values for the above mention parameters results

in higher force values and vice-versa. Therefore, a justified value was needed to be assigned

to these parameters which could help in getting closer results for simulations on comparison

with results of experiments. These parameters were adjusted/calibrated based on

experimental data using past experience and as well as trial-and-error technique so that a

close relevance can be achieved between numerical simulation and experimentation

140

(table 5.8). Calibration of the parameters is very important for tuning the simulation model

for proper prediction of the result, without this there is a large deviation in the results of

experiments and simulations. Hence, calibrated simulation models should be utilised for case

studies used to study the influence of changes in dimensions, materials, loading and boundary

conditions.

Table 5.8: Details of parameters used in LS-DYNA simulation

Parameter Value Description

TSIZE 1E-07 (s) Time step for automatic element deletion.

(Element is deleted when current time step is less 1e-7 s.)

ERODS -0.6

(mm/mm)

Maximum effective strain for element failure. If lower than zero, element fails

when effective strain calculated from the full strain tensor exceeds ERODS.

(Chosen as to be significantly higher than any directional strain at failure

initiation.)

SOFT 0.85 Softening reduction factor for material strength in crash front elements.

(A value based on calibration with experimental data.)

SLIMT1 0.01

Factor to determine the minimum stress limit after stress maximum (fiber

tension).

(Small but non-zero residual strength is assumed after tensile failure to avoid

numerical instabilities)

SLIMC1 0.75

Factor to determine the minimum stress limit after stress maximum (fiber

compression).


SLIMT2 0.10

Factor to determine the minimum stress limit after stress maximum (matrix

tension).

(recommended value)

SLIMC2 1.00

Factor to determine the minimum stress limit after stress maximum (matrix

compression).


SLIMS 1.00 Factor to determine the minimum stress limit after stress maximum (shear).

(recommended value)

The specimens subjected to experimental testing include manufacturing defects such

as voids, fiber-misalignments etc. Due to these manufacturing defects the material

distribution may not be exactly uniform throughout the component, which influences the

results. In real time it is very difficult to manufacture components without any defects

considering the limitations of the manufacturing processes. Whereas in the numerical

simulation the component is considered with material exactly distributed throughout the

dimensions of the component, without any defects. In this scenario there is possibility of

deviation between results of experimentation and simulation, to compensate these deviations

in results correlation factors are used. Correlation factors are used to scale the result from the

numerical analysis by a definite value so that it is as close as possible to result of experiment.

Choice of correlation factor is made in such a way that the value is kept as low as possible, to

141

avoid over prediction of result of numerical analysis, this is done based on the experience as

well as trial and error-technique. In this study a correlation factor of 0.15 was chosen to scale

the result of numerical simulation, for its correlation with experimental drop weight impact

testing.

5.5.3 Correlation of Drop Weight Impact Test and Numerical Simulation for GFRP

Crash Boxes

In this section correlation is done for numerical simulation of drop weight impact test

and the actual experimental test. Objective comparisons of relevant quantities extracted from

experimental test as well as numerical simulation are presented for all the specimens of crash

boxes which were subjected to experimental drop weight impact testing.

5.5.3.1 Correlation of square crash boxes

i) Square crash boxes without trigger

Test Simulation

Fig.5.40: The deformation of square crash boxes without trigger in test and simulation

142

Fig.5.41: The force versus displacement curves for square crash box without


Table 5.9: Energy absorbed and force level comparision of test and

simulation for square crash box without trigger

S.No

Square -

No Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 275.7 29.04 7.87 6.50 3463.5 0

2 Simulation 308.8 33.21 8.20 7.54 3879.1 12.2

From fig. 5.40 the deformation mode of the square crash box in numerical simulation

is similar to experimental test; that is the deformation occurs at the clamped end (non-

impacted end) of the specimen leading to spread of fronds at the clamped end of the

specimen. From fig. 5.41, the numerical simulation has a slightly higher primary peak force

when compared to experimental test because simulation models are stiffer in the initial stage

of analysis during solving as they do not consider the non-uniformity in the component, to

compensate this use of softness parameter (SOFT) is done so that the stiffness of crash-front

elements can be reduced but high values of SOFT leads to reduction in the overall force level,

hence a balanced value was assigned. It was observed that, longitudinal compression stress

limiting factor (SLIMC1) had a greater influence on the mean force level, and it was

calibrated in combination with softness parameter (SOFT) for a reasonably good resemblance

with the experimental result. Therefore, the overall force-displacement behaviour is close

143

between the numerical simulation and experimental test. This technique is further carried out

on all the remaining cases of crash boxes. From table 5.9, the mean force for test is 6.50 kN

whereas for simulation it is 7.54 kN, the energy absorption and force levels are close to each

other for numerical simulation and experimental test and a deviation of 12.2 percent is found

for S.E.A value.

ii) Square crash boxes with front end trigger:

Test Simulation

Fig.5.42: The deformation of square crash boxes with front end trigger in test

and simulation

Fig.5.43: The force versus displacement curves for square crash boxes with front end


144

Table 5.10: Energy absorbed and force level comparision of test and simulation for

square crash boxes with front end trigger

S.No.

Square -

Front End

Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1 Test 287.8 20.24 10.01 7.46 3723.2 0

2 Simulation 328.1 23.56 12.3 9.78 4244.4 14.1

From fig. 5.42 the deformation mode of the square crash box with front end trigger in

numerical simulation is similar to experimental test; that is the deformation occurs at the

impacted end of the specimen leading to spread of fronds towards outside. This change in

deformation pattern is achieved with the use of trigger, the deformation initiation changes

from non-impacted end (no-trigger) to impacted end (front end trigger). In general the

deformation initiation in a specimen is preferred to occur at the impacted end. From fig. 5.43,

the numerical simulation has a slightly higher primary peak force when compared to

experimental test because simulation models are stiffer in the initial stage of analysis during

solving, however the overall force-displacement behaviour is close between the numerical

simulation and experimental test. From table 5.10 the mean force for test is 7.46 kN whereas

for simulation it is 9.78 kN, the energy absorption and force levels are close to each other for

numerical simulation and experimental test and a deviation of 14.1 percent is found for S.E.A

value.

iii) Square crash boxes with slot trigger:

Test Simulation

Fig.5.44: The deformation of square crash boxes with slot trigger in test and

simulation

145

Fig.5.45: The force versus displacement curves for square crash boxes with


Table 5.11: Energy absorbed and force level comparision of test and simulation

for square crash boxes with slot trigger

From fig. 5.44 the deformation mode of the square crash box with slot trigger in

numerical simulation is similar to experimental test; that is the deformation occurs at the

impacted end of the specimen leading to creation of short fronds, bent towards outside. From

fig. 5.45, the overall force-displacement behaviour is close between the numerical simulation

and experimental test. From table 5.11 the mean force for test is 6.96 kN whereas for

simulation it is 7.75 kN, the energy absorption and force levels are close to each other for


value.

S.No.

Square -

Slot Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1 Test 300.7 24.21 7.56 6.96 3850.3 0

2 Simulation 327.8 27.43 8.2 7.75 4196.8 9.2

146

5.5.3.2 Correlation of cylindrical crash boxes

i) Cylindrical crash boxes without trigger:

Test Simulation

Fig.5.46: The deformation of cylindrical crash boxes without trigger in test and

simulation

Fig.5.47: The force versus displacement curves for cylindrical crash boxes

without trigger in test and simulation

147

Table 5.12: Energy absorbed and force level comparision of test and simulation for

cylindrical crash boxes without trigger

S.No

Cylindrical

-

No Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 327.9 36.10 9.10 8.03 3709.63 0

2 Simulation 367.2 41.03 11.56 9.24 4154.78 12.3

From fig. 5.46 the deformation mode of the cylindrical crash box in numerical

simulation is similar to experimental test; that is the deformation occurs such that long fronds

are formed with fibers bending outward. The numerical simulation has a slightly higher

primary peak force when compared to experimental test because simulation models are stiffer

in the initial stage of analysis during solving as discussed in square crash box without trigger.

From fig. 5.47, the overall force-displacement behaviour is close between the numerical


for simulation it is 9.24 kN, the energy absorption and force levels are close to each other for


value.

ii) Cylindrical crash boxes with front end trigger:

Test Simulation

Fig.5.48: The deformation of cylindrical crash boxes with front end trigger in

test and simulation

148


front end trigger in test and simulation


for cylindrical crash box with front end trigger

S.No

Cylindrical

-

Front End

Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 304.8 35.01 18.41 7.98 3524.71 0

2 Simulation 329.2 39.31 20.89 8.43 3806.70 8.1

From fig. 5.48 the deformation mode of the cylindrical crash box with front end

trigger in numerical simulation is similar to experimental test; that is the impacted end easily

deforms and outward bending of short fronds takes place at the impacted end. From fig. 5.49,

the overall force-displacement behaviour is close between the numerical simulation and

experimental test. From table 5.13 the mean force for test is 7.98 kN whereas for simulation it

is 8.43 kN, the energy absorption and force levels are close to each other for numerical

simulation and experimental test and a deviation of 8.1 percent is found for S.E.A value.

149

iii) Cylindrical crash boxes with slot trigger:

Test Simulation

Fig.5.50: The deformation of cylindrical crash boxes with slot trigger in test and

simulation



150


for cylindrical crash boxes with slot trigger

S.No

Cylindrical

-

Slot Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 313.3 35.69 11.12 8.33 3564.40 0

2 Simulation 346.8 41.13 14.1 9.87 3945.8 10.7

From fig. 5.50 the deformation mode of the cylindrical crash box with slot trigger in

numerical simulation is similar to experimental test; that is the impacted end breaks and

outward bending of short fronds takes place. From fig. 5.51, the overall force-displacement

behaviour is close between the numerical simulation and experimental test. From table 5.14

the mean force for test is 8.33 kN whereas for simulation it is 9.87 kN, the energy absorption

and force levels are close to each other for numerical simulation and experimental test and a

deviation of 10.7 percent is found for S.E.A value.

5.5.3.3 Correlation of hexagonal crash boxes

i) Hexagonal crash boxes without trigger:

Test Simulation

Fig.5.52: The deformation of hexagonal crash boxes without trigger in test and

simulation

151

Fig.5.53: The force versus displacement curves for hexagonal crash boxes



for hexagonal crash boxes without trigger

S.No

Hexagonal

-

No Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 307.3 75.21 11.20 7.97 3801.4 0

2 Simulation 330.6 81.12 9.7 9.22 4090.3 7.6

From fig. 5.52 the deformation mode of the hexagonal crash box in numerical

simulation is similar to experimental test; that is outward spreading of fronds can be

observed. From fig. 5.53, the overall force-displacement behaviour is close between the

numerical simulation and experimental test. From table 5.15 the mean force for test is 7.97

kN whereas for simulation it is 9.22 kN, the energy absorption and force levels are close to

each other for numerical simulation and experimental test and a deviation of 7.6 percent is

found for S.E.A value.

152

ii) Hexagonal crash boxes with front end trigger:

Test Simulation

Fig.5.54: The deformation of hexagonal crash boxes with Front End Trigger in

test and simulation


Front End Trigger in test and simulation

153


for hexagonal crash boxes with Front End Trigger

S.No

Hexagonal

-

Front End

Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force (kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 376.4 56.73 17.32 9.14 4645.8 0

2 Simulation 426.4 61.31 20.71 10.62 5263.6 13.3

From fig. 5.54 the deformation mode of the hexagonal crash box with front end

trigger in numerical simulation is similar to experimental test; that is the impactor easily

breaks the triggered front end as its thickness is half the specimen thickness. From fig. 5.55,

the overall force-displacement behaviour is close between the numerical simulation and

experimental test. From table 5.16 the mean force for test is 9.14 kN whereas for simulation it

is 10.62 kN, the energy absorption and force levels are close to each other for numerical

simulation and experimental test and a deviation of 13.3 percent is found for S.E.A value.

iii) Hexagonal crash boxes with slot trigger:

Test Simulation

Fig.5.56: The deformation of hexagonal crash boxes with slot trigger in test and

simulation

154




for hexagonal crash boxes with slot trigger

S.No

Hexagonal

-

Slot Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 351.1 72.13 9.89 9.29 4291.6 0

2 Simulation 377.4 78.64 11.60 10.73 4613.6 7.5

From fig. 5.56 the deformation mode of the hexagonal crash box with slot trigger in

numerical simulation is similar to experimental test; that is the specimen breaks at the

impacted end and bending of specimen slightly bends inward below the frond formation.

From fig. 5.57, the overall force-displacement behaviour is close between the numerical


for simulation it is 10.73 kN, the energy absorption and force levels are close to each other

155

for numerical simulation and experimental test and a deviation of 7.5 percent is found for

S.E.A value.

5.5.3.4 Correlation of decagonal crash boxes

i) Decagonal crash boxes without trigger:

Test Simulation

Fig.5.58: The deformation of decagonal crash boxes without trigger in test and

simulation

Fig.5.59: The force versus displacement curves for decagonal crash boxes


156


for decagonal crash boxes without trigger

S.No

Decagonal

-

No Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 558.5 136.01 14.88 12.09 6427.12 0

2 Simulation 603.7 143.57 16.10 14.69 6947.69 8.1

From fig. 5.58 the deformation mode of the decagonal crash box in numerical

simulation is similar to experimental test; bending of short fronds formed after the impact can

be seen. From fig. 5.59, the overall force-displacement behaviour is close between the

numerical simulation and experimental test. From table 5.18 the mean force for test is 12.09

kN whereas for simulation it is 14.69 kN, the energy absorption and force levels are close to

each other for numerical simulation and experimental test and a deviation of 8.1 percent is

found for S.E.A value.

ii) Decagonal crash boxes with front end trigger:

Test Simulation

Fig.5.60: The deformation of decagonal crash boxes with front end trigger in test

and simulation

157


front end trigger in test and simulation


for decagonal crash boxes with front end trigger

S.No

Decagonal

-

Front End

Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 605.7 71.22 41.03 13.15 7118.5 0

2 Simulation 674.7 77.36 47.6 14.61 7930.1 11.4

From fig. 5.60 the deformation mode of the decagonal crash box with front end

trigger in numerical simulation is similar to experimental test; formation of finer fronds at the

impacted end with bending of fibres can be seen. From fig. 5.61, the overall force-

displacement behaviour is close between the numerical simulation and experimental test.

From table 5.19 the mean force for test is 13.15 kN whereas for simulation it is 14.61 kN, the

energy absorption and force levels are close to each other for numerical simulation and

experimental test and a deviation of 11.4 percent is found for S.E.A value.

158

iii) Decagonal crash boxes with slot trigger:

Test Simulation

Fig.5.62: The deformation of decagonal crash boxes with slot trigger in test and

simulation




for decagonal crash boxes with slot trigger

S.No

Decagonal

-

Slot Trigger

Energy

Absorbed

( J )

Primary

Peak

Force

(kN)

Secondary

Peak

Force

(kN)

Mean

Crush

Force

(kN)

S.E.A

(J/kg)

Percentage

change in

S.E.A

1

Test 546.4 130.32 17.75 13.86 6353.1 0

2 Simulation 596.6 137.53 20.41 15.47 6937.5 9.2

159

From fig. 5.62 the deformation mode of the decagonal crash box with slot trigger in

numerical simulation is similar to experimental test; formation of fronds is less instead the

lamina bends outward allowing the impactor to penetrate inside. From fig. 5.63, the overall

force-displacement behaviour is close between the numerical simulation and experimental

test. From table 5.20 the mean force for test is 13.86 kN whereas for simulation it is 15.47

kN, the energy absorption and force levels are close to each other for numerical simulation

and experimental test and a deviation of 9.2 percent is found for S.E.A value.

5.6. Key Points from Experimentation and Numerical Simulation of Drop Weight

Impact Test

Details on drop weight impact testing setup, clamp preparation for specimen as well as

impactor, safety precautions during drop weight impact testing were discussed in detail.

Comparative experimental analysis was done for different cross sections of the GFRP crash

boxes, without and with triggers applied to each type of crash box geometry.

PART_COMPOSITE model with LAMSHT (laminated shell theory option) was used along

with MAT_58 (MAT_LAMINATED_COMPOSITE_FABRIC) for laminated GFRP crash

boxes. A detailed finite element model with impactor, specimen-clamp and base plate was

developed along with the use of various contact definitions between the parts as per the

experimental test for better correlation. Simulation factors influencing the numerical result

like longitudinal compression stress limiting factor (SLIMC1) and softness factor (SOFT) for

the crash-front elements were calibrated in accordance with the test data. It was revealed that

SLIMC1 influenced the average force level, whereas softness factor (SOFT) was having

significant influence on the peak-force levels. It was observed that deformation mode,

specific energy absorption, force with respect to displacement value (F versus D diagram) and

mean force were in good agreement with each other in test as well as in simulation. Whereas,

the peak force value was slightly higher for the simulation compared to the test. This was due

to the fact that, simulation models are stiffer in the initial stage of analysis during solving as

they do not consider the defects present in the specimen and material distribution is assumed

to be uniform throughout the specimen. Whereas in the test the specimen strength may be

slightly lower than the ideal strength value due to non-uniformity of material caused by

defects and imperfections present in the specimen. Usage of correlation factors for scaling

numerical simulation result to compensate the defects in the specimens, helped in achieving a

good correlation between experimental test and numerical simulation for drop weight impact

160

of GFRP crash boxes. Hence, it is vital that the parameters in the simulation model which

have a significant influence on the results are calibrated using experimental data. Thereafter,

the calibrated simulation model can be used for further investigations for optimizations with

multiple case studies in an efficient as well as economic manner with significant reduction in

prototype testing.

5.7. Summary of the Chapter

In this chapter detailed discussion was done on drop weight impact testing setup, test

preparation, clamp preparation for specimen as well as impactor. Safety precautions during

drop weight impact testing were explained. Comparative experimental analysis was done for

different cross sections of the GFRP crash boxes, without and with triggers applied to each

type of crash box geometry. Also a comparative analysis was done comparing the

effectiveness of different triggers for each type of geometry.

Detailed discussion was done on the factors influencing the numerical simulation and

experimental testing with their limitations. Correlation between test and simulation was

discussed. Importance of pre test simulation was presented. The pre test simulation model

was able to predict the force and energy levels but it was not sufficient to exactly replicate the

complex deformation modes of the laminated GFRP crash box as it was a simplified model.

Therefore, a numerical simulation model replicating the drop weight impact testing was

developed and correlated with the test data. A good agreement was achieved between the test

and simulation.

161

Chapter-6: Summary, Conclusions and Future Scope

6.1. Summary of the Research

The work done in this study can be summarized as follows:

Importance of crashworthiness in automobiles is discussed to understand the vehicle safety

in the event of a crash. Use of composites in automobiles is discussed. Advantages of

composites in future cars like the electric cars are discussed.

Background of composites is discussed to understand the complex behaviour of

composites under impact loading.

Literature survey is done related to the work to be carried out and gaps in the existing

research work were found. Comparision of crashworthiness of crash boxes made of GFRP

composite material from different cross sections, detailed crash box sub system level

analysis is not done to study the effect of various types of triggers in combination with

different cross-sectional geometries. Comparative crashworthiness analysis of crash boxes

with and without application of geometry intrinsic triggers subjected to impact loading is

not done as well.

Numerical simulation of composites is discussed in detail including the procedure for

numerical simulation and behaviour of composites subjected to impact loading using force

versus displacement diagrams.

Crash energy absorbing characteristics of a crash box made of GFRP composite material

subjected to impact load are studied initially with the help of numerical simulation for

square, cylindrical, hexagonal and decagonal types of geometrical cross sections with and

without application of different triggers.

162

Pre-test numerical simulation analysis is done to know the effect of different types of

novel triggers namely; Notch Triggers (trigger-A, trigger-B and trigger-C), Slot

Triggers (type-1 slot, type-2 slot and type-3 slot) and Thickness Variation Triggers

(thickness variation 1 / front end trigger, thickness variation 2 and thickness variation 3)

are studied extensively, to understand the effect of each type of trigger on the

crashworthiness behaviour of GFRP crash boxes made of square, cylindrical, hexagonal

and decagonal geometrical cross sections.

Results obtained from the numerical simulation analysis are discussed in detail. Later

consolidated results are presented in a comparative manner for all the combinations of

triggers applied to each type of crash box geometry to know the effectiveness of each

trigger type for all the combinations of cross sections used in the numerical analysis.

Specimen making is discussed in detail and step wise including the safety precautions,

mould making and tips involved in the manufacturing process.

Drop weight impact testing is discussed in detail including the working of drop weight

impact machine, specimen clamp making, safety precautions and procedure to conduct the

test.

Drop weight impact testing, experimental analysis is carried out on square, cylindrical,

hexagonal and decagonal cross sectional GFRP composite crash boxes to study the crash

energy absorbing characteristics when subjected to impact load with and without

application of different triggers.

A comparative experimental testing and analysis is done to study the variation of peak

force, energy absorbed, S.E.A (specific energy absorbed) with the variation of geometries

and triggers applied to GFRP crash boxes subjected to drop weight impact load.

163

A detailed finite element model with impactor, specimen-clamp and base plate was

developed along with the use of various contact definitions between the parts as per the

experimental test for better correlation of numerical simulation with experimental test.

PART_COMPOSITE model with LAMSHT (laminated shell theory option) was used

along with MAT_58 (MAT_LAMINATED_COMPOSITE_FABRIC) for laminated

GFRP crash boxes in LS-DYNA. Additionally calibration of simulation parameters like

longitudinal compression stress limiting factor (SLIMC1) and softness factor (SOFT) was

done in accordance with experimental data.

The numerical simulation was correlated with the experimental data using correlation

factors to compensate the defects present in the specimens and the values were found to be

close to each other, within the acceptable deviation range. Even the deformation modes,

the F-D (force versus displacement) curves and S.E.A (specific energy absorption) values

were found to have a close match between experiments and numerical simulations.

The objective of this study is to highlight the effect of cross sections and triggers on the

crashworthiness of GFRP crash boxes and also to showcase the relative effect of each

trigger configuration on the energy and force level achieved; with the variation of cross

sections of the GFRP crash boxes.

6.2. Conclusions

The conclusions drawn from this study can be mentioned as follows:

Considering only the geometrical cross-section without any trigger the most efficient

cross-section for GFRP crash box was decagonal cross-section with S.E.A value of

7345.12 (J/kg) and the least efficient cross-section without any trigger was square cross-

section with S.E.A value of 3935.82 (J/kg).

164

This means that geometric shape plays a vital role in the energy absorption.

Considering the trigger configurations for square crash box, trigger-C was the most

efficient trigger with 41.20 percentage increase in the S.E.A with a value of 5584.65 J/kg

compared to square crash box with no-trigger.

Considering the trigger configurations for cylindrical crash box, trigger-B was the most


compared to cylindrical crash box with no-trigger.

Considering the trigger configurations for hexagonal crash box, trigger-A was the most


compared to hexagonal crash box with no-trigger.

Considering the trigger configurations for decagonal crash box, thickness variation-1 was

the most efficient trigger with 10.75 percentage increase in the S.E.A with a value of

8135.39 J/kg compared decagonal crash box with no-trigger.

The crash behaviour of the GFRP crash boxes varied significantly with the usage of

different cross sectional geometries and trigger types.

It can be noted that the deformation mode also varied significantly with the use of

different types of triggers.

Interestingly the effectiveness of each type of trigger changed with the each geometrical

cross section used for the crash boxes. It is evident from the above study that, each type

of geometry had a peculiar behaviour with different types of triggers.

Therefore it can be noted that a component made of same material gives different energy

absorption values with the combination of different geometrical cross sections and trigger

types.

165

Hence this study has highlighted the change in crashworthiness behaviour of GFRP crash

boxes not only with respect to different types of cross sections but also with respect to

various triggers applied to different cross sectional shapes.

This study also showcased the relative effect of each trigger configuration on the energy

and force level achieved with the variation of cross sections of the GFRP crash boxes.

Thus it can be noted that use of proper combination of geometry and trigger type plays a

vital role in achieving desired level of force, deformation mode and energy absorption.

6.3. Specific Contributions of the Study

The objective of the study is to investigate optimal geometric design for automobile

crash box for better energy absorption through its large deformation in head-on collisions of

passenger cars. This research is focussed firstly, to understand the change in crashworthiness

behaviour of GFRP crash boxes made of different cross sectional shapes and secondly, to

study the effect of various types of triggers on the crashworthiness behaviour of the GFRP

crash boxes made of different cross sectional shapes. Initially, deformation behaviour of the

proposed GFRP crash boxes was studied with the help of finite element simulation, for

different novel geometric shapes, along with addition of various triggers on the geometry of

the crash boxes. A comparison between GFRP crash boxes with various types of cross-

sectional geometries along with application of different types of triggers was done for better

understanding of the significance of each with respect to energy absorption, peak force and

specific energy absorption.

Later the required crash box specimens were manufactured for experimental testing.

Drop weight impact testing was performed on the GFRP crash box specimens to study their

crashworthiness behaviour experimentally. Comparison of the drop weight impact testing and

numerical simulation was performed. A good correlation between the experimental test and

numerical simulation was achieved.

A comparative as well as detailed analysis and discussion was done to clearly

understand the effect of each type of geometry along with application of each type of trigger

on the crashworthiness of GFRP crash boxes. In this study it was found that the

166

crashworthiness behaviour of GFRP crash boxes varied with the use of different cross

sectional geometries. It was also found that the effectiveness of each trigger was different

when applied to a different cross sectional geometry. This study has shown that the force

level and energy absorption can be altered and the required energy absorption can be

achieved by use of proper type of trigger for the particular geometrical cross section.

This study has brought out a comparative significance for usage of different types of

geometries and different types triggers for GFRP composite crash boxes subjected to axial

impact loading. Thus this study serves as a guide for employing triggers in GFRP crash boxes

which is useful for automobile engineers working with composites subjected to impact where

the force level is very important for energy absorption and safety of the vehicle.

6.4. Usefulness of the Present Research

The present research will be helpful to the vehicle design engineers as well as

researchers in the following ways.

For better understanding of the complex behaviour of composites when subjected to

impact loading.

For better use of cost effective composites such as glass fiber reinforced plastics (GFRP)

in crashworthiness applications

In making light-weight as well as electrically insulated vehicle structures using composite

materials.

To understand the importance of cross sectional shapes for composites in crashworthiness

applications.

For making better use of triggers in improving the crashworthiness of composites in

impact loading.

Better understanding for the selection of cross-sectional geometry and trigger type

combinations based on the energy absorption level.

6.5. Recommendations for Future Scope of the Study

Even though the present work is extensively done there is always a prospective for

future scope of the work which motivates the researchers to carry forward the study in

167

different domains and directions. The future scope of this study would be to include

various other types of materials like carbon fiber reinforced plastics, kevlar fiber

reinforced plastics etc., To continue the study in different domains like aerospace

applications involving space craft where there is high need to increase the strength to

weight ratio of the structure. To study the crashworthiness of composites in naval

applications like ships and submarines where there is need for the composites to

withstand higher forces in a corrosive environment.

168

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List of Publications and Presentations

International Journal Publications:

1. N. Nasir Hussain, Srinivasa Prakash Regalla and Venkata Daseswara Rao Yendluri.

“Numerical investigation into the effect of various trigger configurations on

crashworthiness of GFRP crash boxes made of different types of cross sections”,

International Journal of Crashworthiness, 2017, 22(5), pp. 565-581. DOI:

10.1080/13588265.2017.1286964


“Comparative Study of Trigger Configuration for Enhancement of Crashworthiness of

Automobile Crash Box Subjected to Axial Impact Loading”. Procedia Engineering,

2017, 173(1), pp. 1390-1398. DOI: 10.1016/j.proeng.2016.12.198

3. N. Nasir Hussain, Srinivasa Prakash Regalla and Venkata Daseswara Rao Yendluri,

Tatacipta Dirgantara, Leonardo Gunawan, Annisa Jusuf. “Drop-weight Impact Testing

for the Study of Energy Absorption in Automobile Crash Boxes Made of Composite

Material”. Journal of Materials: Design and Applications. (Communicated-under

review).


“Techniques for Correlation of Drop Weight Impact Testing and Numerical Simulation

for Composite GFRP Crash Boxes Using Ls-Dyna”. Journal of Composite Structures.

(Communicated-under review).

174

International Conference Publications and Presentations:


“Low Velocity Impact Characterisation of Glass Fiber Reinforced Plastics for

Application of Crash Box.” Materials Today: Proceedings, 2017,4(2), Part A, pp. 3252-

3262. DOI: 10.1016/j.matpr.2017.02.211. In Proceedings of 5th

International Conference

of Materials Processing and Characterization (ICMPC 2016), GRIET, Hyderabad,

India, March, 2016.


“Comparative Study of Trigger Configuration for Enhancement of Crashworthiness of

Automobile Crash Box Subjected to Axial Impact Loading”. Presented in 11th

International Symposium on Plasticity and Impact Mechanics (Implast 2016). Indian

Institute of Technology, Delhi, India, December, 2016.


“Large Deformation in Composite Crash Box for Automobile under Impact Loading

With Various Trigger Configurations”. Presented in International Conference on

Composite Materials and Structures (ICCMS 2017). Indian Institute of Technology,

Hyderabad, India, December, 2017.


“Economical Method for Manufacturing of Advanced Light Weight Material Crash Box

for Automobiles”. Materials Today: Proceedings, 2020. In Press. DOI:

10.1016/j.matpr.2020.02.081. In Proceedings of 1st International Conference on

Advanced Light-weight Materials and Structures (ICALMS-2k20), CMR Technical

Campus, Hyderabad, India, March, 2020.


“Analysis on Crashworthiness of Light Weighted Automobile Composite Material

CrashBox with Advanced Triggers”. Presented in 1st International Conference on

Advanced Light-weight Materials and Structures (ICALMS-2k20), CMR Technical

Campus, Hyderabad, India, March, 2020.

175

National Conference Publications and Presentations:


“Design of Trigger Geometry and Location for Crashworthiness of Automobile

Crashbox under Low Velocity Impact.” Presented in 2nd National Conference on Design

and Manufacturing Technologies for Product Life Cycle (DPLC-2016), Birla Institute of

Technology & Science, Pilani, Hyderabad Campus, Hyderabad, India, March, 2016.

176

Brief Biography of the Candidate

Mr. Nasir Hussain N obtained his B.Tech. in Mechanical Engineering with merit from

Jawaharlal Nehru Technological University (JNTU), Hyderabad, Telangana and

Master of Engg. in CAD/CAM (Mechanical Engineering) with gold medal from

Osmania University (OU), Hyderabad, Telangana. He is presently pursuing Ph.D. at

BITS-Pilani, Hyderabad Campus and also working as Deputy Manager at Hyundai

Motors India Engineering Pvt. Ltd. (Hyundai R&D Center, Hyderabad). He has over ten

years of experience in CAE simulation of automobile crash and safety. His areas of

research interests are crash safety of passenger vehicles, occupant injury and vehicle

safety performance (vehicle structural crash analysis). He has also submitted one patent.

Brief Biography of the Supervisor (Guide)

Prof. Srinivasa Prakash Regalla obtained his PhD in Mechanical Engineering from IIT

Delhi in 1998, M. Tech. in Manufacturing Science (Mechanical Engineering) from IIT

Kanpur in 1992 and B.Tech. in Mechanical Engineering from Kakatiya Institute of

Technology and Science, Warangal in 1990. Presently, he is a professor in the

department of Mechanical Engineering and the coordinator for the product design and

realization laboratory at the BITS Pilani, Hyderabad Campus. Previously, he was the

Head of the department of mechanical engineering, associate dean of work-integrated

learning programmes, assistant dean of research & consultancy, professor in-charge of

faculty affairs and lead of the industry engagement imperative of mission-2015 at BITS

Pilani. He published 22 sci/scie, 12 scopus and 15 other peer-reviewed journal papers

and more than 30 international conference proceedings papers, 2 books and submitted 3

patents. He completed 3 funded research projects, including as the PI of a project on low-

cost and affordable additive manufacturing (AM) made below-knee prosthesis funded by

DBT/BIRAC/BIG. He is currently a co-PI in an industrial R&D project. He taught a

large variety of undergraduate and postgraduate courses at BITS Pilani some of which

are newly introduced elective courses and lead the design of several new on-campus and

work-integrated learning programmes. He is a member of ASME, ISPO, SAE-India, TSI

and IEI. He has been a delegate of the IMPLAST series of conferences on plasticity and

impact mechanics since its occurrence at IIT Delhi in 1996.

https://www.bits-pilani.ac.in/hyderabad/spregalla/Profile

https://www.bits-pilani.ac.in/hyderabad/spregalla/Profile

177

Brief Biography of the Supervisor (Co-Guide)

Prof.Yendluri Venkata Daseswara Rao obtained his M. Tech. in Design Engineering

(Mechanical Engineering) from Indian Institute of Technology, Delhi (IIT Delhi) and

Doctor of Philosophy (PhD) in Mechanical Engineering from National Institute of

Technology, Raipur (NIT Raipur). He is presently Associate Professor in the Department

of Mechanical Engineering, BITS Pilani Hyderabad Campus. He has been working with

BITS Pilani for last ten years in various capacities and has several years of experience in

academics. He has served at various capacities during his tenure at BITS-Pilani some

notable positions are Head Department of Mechanical Engineering, BITS Pilani

Hyderabad Campus till Sept 2016, Faculty In charge Maintenance Division till Sept

2016, Faculty In charge Engineering Services Division, BITS Pilani Hyderabad Campus,

Examination Committee member of Birla Institute of Technology and Science, Pilani,

from 1st August 2010 to 1

st April 2013.He has published several papers in various

national and international journals, and conferences.He is actively involved in teaching

and consultancy for industry. He is a fellow of professional societies like Member of

American Society for Mechanical Engineering, USA, Fellow of Institute of Engineers,

India and Life Member of Indian Society for Technical Education (ISTE).

https://www.bits-pilani.ac.in/hyderabad/daseswararaoyendluri/Profile

https://www.bits-pilani.ac.in/hyderabad/daseswararaoyendluri/Profile

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