THE FLEXURAL BEHAVIOR OF COMPOSITE SANDWICH STRUCTURE

MOHD HAKIMI BIN OMAR

A report submitted in partial fulfillment of the requirements for the award of the degree of

Bachelor of Mechanical Engineering

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

OCTOBER 2008

SUPERVISOR’S DECLARATION

We hereby declare that we have checked this project and in our opinion this project is

satisfactory in terms of scope and quality for the award of the degree of Bachelor of

Mechanical Engineering.

Signature

Name of Supervisor :

Position :

Date :

Signature

Name of Panel :

Position :

Date :

STUDENT’S DECLARATION

I hereby declare that the work in this thesis is my own except for quotations and

summaries which have been duly acknowledged. The thesis has not been accepted for

any degree and is not concurrently submitted for award of other degree.

Signature

Name : Mohd Hakimi Bin Omar

ID Number : MA05037

Date :

DEDICATION

I humbly dedicate this thesis to

my lovely mom and father, Che Jamilah Binti Hasan and Omar Bin Muhamad

my dearest sister, Nur Hanani Binti Omar, Qistina Binti Omar, Hafizah Binti Omar,

Izzati Binti Omar and Nur Bazla Binti Omar

my dearest cousin, Nik Norma Binti Nik Idris

and to all my friends.

who always trust me, love me and had been a great source of support and motivation.

ACKNOWLEDGEMENTS

First of all, I want to thank The Almighty Allah SWT for the beautiful life that

has been given to me in the past 22 years and the present. I am very thankful to be given

the time and chances to finally complete this research.

I would like to express my sincere gratitude to my supervisor Mr. Mohd Ruzaimi

Bin Mat Rejab for his invaluable guidance, continuous encouragement and constant

support in making this research possible. He has always support me in times when I

faced difficulties during completing this research and constantly giving the best advice

to help me. I am always impressed with his effort in putting up with my attitude and still

treated me well as his student after giving him such a difficult time. I apologize for the

hard times. I also want to thank Dr. Ahmad Syahrizan Bin Sulaiman who have given his

best in making me understand how this research has to be done and have guided me

throughout the research.

My sincere thanks go to all my lab mates and members of the staff of the

Mechanical Engineering Department, UMP, who helped me in many ways whenever I

needed. Thanks for always putting up the best effort in helping me learn and familiarized

myself with the equipments in the lab so that I can finish this research.

The best thanks goes to my family especially to my parent. I am very thankful to

have them as my father and mother because they never gave up on me and constantly

support me morally and financially which are things that I needed the most in order to

complete this research. But most of all, thanks for the love and attention that they gave

to me which I will cherish until the end of time. Thanks for never stop believing in me

although I have let them down so many times. Thanks for always pray for my success

and happiness in the past, present and the future. Thanks for everything.

ABSTRACT

Sandwich construction is widely used because of its ability to provide high bending

stiffness coupled with light weight. The main objective of this study is to investigate the

effects of core under flexural test and to investigate performance of sandwich panels

from the energy absorption point of view. The fibre used is woven roving glass fibre

where the matrix used is based on saturated polyester resin. The composite skins of the

sandwich panels were prepared by hand lay-up technique. Three point bending test was

carried out to obtain flexural strength and flexural strain of the composite sandwich

structures. Two different cores of sandwich panels were tested in this project. Sandwich

panels with polyurethane core was found had higher flexural strength and higher energy

absorption compared with sandwich panels with polyethylene core. From the

observation, increasing the debonding strength of the core-face interface, the failure

mode changed from debonding of the core-face interface to the failure of the face. The

experimental results show that mechanical properties were improved when span

decreased and also depends on core materials. Thus, the influence of core materials and

span length of the sandwich panels has been evaluated.

ABSTRAK

Pembangunan struktur berlapis (sandwich structure) telah digunakan secara meluas

kerana kemampuan untuk menyediakan kekuatan lenturan yang tinggi dan struktur yang

lebih ringan. Objectif utama dalam kajian ini adalah untuk menyelidik kesan terhadap

perbezaan teras di bawah ujian lenturan dan mengkaji kemampuan struktur berlapis dari

sudut tenaga serapan. Bahan gentian yang digunakan adalah jenis gentian woven roving

dan polyester tak tepu sebagai bahan matrik. Permukaan komposit disediakan dengan

menggunakan teknik bengkalai tangan (Hand lay-up). Ujian 3 titik lenturan dijalankan

untuk mendapatkan kekuatan lenturan dan ketegangan lenturan struktur berlapis.

Struktur berlapis dengan menggunakan dua teras yang berbeza telah diujikaji dalan

projek ini. Struktur berlapis dengan teras polyurethane didapati menunjukkan kekuatan

lenturan dan penyerapan yang lebih tinggi dibandingkan dengan struktur berlapis yang

menggunakan polyethylene sebagai teras. Daripada pemerhatian, kekuatan antara muka

di antara teras dan permukaan meningkat, ragam kegagalan bertukar kepada kegagalan

berlaku di permukaan struktur. Keputusan ujikaji menunjukkan bahawa sifat mekanikal

dapat dipertingkatkan dengan mengurangkan panjang span dan juga bergantung kepada

bahan teras yang digunakan. Dengan demikian, pengaruh bahan teras dan panjang span

struktur berlapis telah dibuktikan.

TABLE OF CONTENTS

Page

SUPERVISOR’S DECLARATION ii

STUDENT’S DECLARATION iii

DEDICATION iv

ACKNOWLEDGEMENTS v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS viii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xiv

LIST OF ABBREVIATIONS xv

CHAPTER 1 INTRODUCTION

1.1 Project Background 1

1.2 Problem Statement 2

1.3 Project Objective 3

1.4 Scope of Study 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 4

2.2 History 4

2.3 Structure In Sandwich Panels 5

2.3.1 Faces/Skin 5 2.3.2 Principle of Skin 6

2.3.3 Fiberglass2.3.4 Properties of Fiberglass2.3.5 Core2.3.6 Principle of Core

788

11

2.4 Mechanical Testing 12

CHAPTER 3 METHODOLOGY

3.1 Introduction 16

3.2 Overview of Methodology 17

3.3 Define Problem 18

3.4 Literature Review 18

3.5 Sandwich Beam Geometry 18

3.6 Sandwich Panels Fabrication 19

3.6.1 Skin 193.6.2 Preparation of Mould 203.6.3 Preparation of Matrix Material 203.6.4 Preparation of Laminate 213.6.5 Preparation of Skin Specimens 213.6.6 Core 223.6.7 Bonding The Sandwich Panel 22

3.7 Mechanical Test 22

3.7.1 Three-point Bending Test 23

CHAPTER 4 RESULTS AND DISCUSSION

4.1 Introduction 26

4.2 Test Specimen 26

4.3 Load-Displacement Behavior 28

4.3.1 Polyethylene Core 284.3.2 Polyurethane Core 29

4.4 3-point Bending Result 31

4.4.1 Result for Polyethylene Core 314.4.2 Result for Polyurethane Core 324.4.3 Energy Absorption Result for Polyethylene Core 344.4.4 Energy Absorption Result for Polyurethane Core 35

4.5 Comparison Between Polyethylene and Polyurethane Core 38

4.6 Failure Mode of Sandwich Panels 40

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS

5.1 Conclusions 42

5.2 Recommendations 44

REFERENCES45

APPENDICESA Sample Calculation 48

B1 Stress-Strain Curve for Polyethylene Core 50

B2 Stress-Strain Curve for Polyurethane Core 54

C Comparison of Stress-Strain Curve Among Specimens 58

LIST OF TABLES

Table No. Page

4.1 Density of core material 27

4.2 Dimensions of Test Specimens 27

4.3 Experimental data of sandwich panels 29

4.4 Experimental data of sandwich panels 30

4.5 Experimental data of sandwich panels 32

4.6 Experimental data of sandwich panels 33

4.7 Energy absorption for polyethylene core 35

4.8 Energy absorption for polyethylene core 37

LIST OF FIGURES

Figure No. Page

2.1 The skin of the sandwich panels 5

2.2 How the stress act on the skin 6

2.3 Fiberglass 7

2.4 Types of core 9

2.5 Shear stiffness of typical core materials as a function of density 10

2.6 Sandwich construction with laminated-reinforced foam core 11

2.7 Core weak in shear 11

2.8 Core strong in shear 12

2.9 3-point and 4-point flexural tests 12

2.10 3-point bending test 13

2.11 3-point bending test apparatus 14

3.1 Flow chart of the project 17

3.2 Sandwich beam dimension used for three-point bend test 19

3.3 Woven roving glass fibre 20

3.4 The process of cutting the specimens using vertical T-jaw machine

21

3.5 Schematics of three point bend test fixture setup 23

3.6 Universal Testing Machine 3639 (INSTRON) 24

4.1 Load-displacement curve plots for polyethylene core 284.2 Load-displacement curve plots for polyurethane core 30

4.3 Stress-strain curves for polyethylene core 31

4.4 Stress-strain curves for polyurethane core 33

4.5 Energy absorption for polyethylene core at deflection 15mm 34

4.6 Energy absorption for polyurethane core at deflection 15mm 36

4.7 Stress-strain curves for polyethylene and polyurethane core at span 50mm

38

4.8 Stress-strain curves for polyethylene and polyurethane core at span 250mm

39

4.9 Energy absorption for polyethylene and polyurethane core 39

4.10 Core-face interface debonding 40

4.11 Failure of face by fiber breakage 41

4.12 Failure of the core 41

LIST OF SYMBOLS

Strain

Stress

f Flexural Strength

fE Modulus of Elasticity

P Load

L Support Span

b Width of Test Beam

d Depth of Test Beam

D Deflection

m Slope

Density

v Volume

m Mass

LIST OF ABBREVIATIONS

FRP Fiber reinforced plastic

GRP Glass reinforced plastic

PUR Polyurethane

PIR Polyisocyanurate

PF Phenolic Foam

EPS Expanded Polystyrene

XPS Extruded Polystyrene

ASTM American Society for Testing and Material

MEKP Methyl ethyl ketone peroxide

CHAPTER 1

INTRODUCTION

1.1 PROJECT BACKGROUND

A sandwich structure composite is a special class of composite materials that is

fabricated by attaching two thin but stiff skins to lightweight but thick core. The core

material is normally low strength material but its higher thickness provides the sandwich

composite with high bending stiffness with overall low density. As a result sandwich

components achieve the same structural performance as conventional materials with less

weight. Sandwich structures are used in many applications ranging from buildings to

aerospace systems because of their high specific bending stiffness and strength and good

acoustical insulation. Sandwich structure also ideal for large ship hulls because of their

good shock resistance combined with reasonable affordability. This project is to study

the effect of core sandwich structure under flexural test that is to determine the flexural

behavior based on core material and to identify the span length of different core

material. The other is to investigate performance analysis of sandwich structure in term

of energy absorption using different core material.

1.2 PROBLEM STATEMENT

Sandwich composite is highly subject to damage and crack. This damage and

defect is very difficult to detect in composite materials. For example in wind turbine, the

damage can occur from events such as dropped tools during maintenance or debris

impacting during periods of high winds. This will cause delaminations, core disbonds,

and crack. The defect will reduce the stiffness of the structures and will damage the

fatigue life of the component. Usually, the damage cannot see with naked eye. Besides

that, defects can also arise during manufacture, particularly where pre-pregs are not

used. This defect can take the form of resin dry spots, skin-core disbonds and such.

While the cause and nature of defects are wide ranging the effect is similar. Composite

sandwich structure such as wind turbines and wings are subjected to large flexural

loadings. Consequently, the flexural behavior of these structures is critical to their use.

To overcome this problem, testing methodology that can simulate the flexural loading is

needed so that the suitability of composite sandwich constructions can evaluate to the

application. In addition, the ability to determine the effect of defects on stiffness and

fatigue performance will allow for better service life prediction.

1.3 PROJECT OBJECTIVE

1.3.1 To investigate the effects of core sandwich structure under flexural test.

1.3.2 To investigate the performance of sandwich structure from the energy absorption

point of view.

1.4 SCOPE OF STUDY

1.4.1 Determination of flexural behavior based on different core material.

1.4.2 Identification the span length of different core material.

1.4.3 Performance analysis of energy absorption using different core material.

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

International research effort continuously looking for new, better, and efficient

construction materials. The main goal is to improve the structural efficiency,

performance, and durability of civil engineering and transportation applications. In the

past decades various sandwich panels have been utilized in the construction of

aerospace, marine, architectural, and transportation industry [1]. Lightweight, excellent

corrosion characteristics and rapid installation capabilities created tremendous

opportunities for these sandwich panels in the industry [1]. This structure provides an

efficient use of the materials and utilization to each component to its ultimate limit. The

structures offers also very high stiffness-to-weight ratio [10]. It enhances the flexural

rigidity of a structure without adding a substantial weight therefore it provides

significant advantages in comparison to the use of the material alone for structural

system [1]. Sandwich construction has superior fatigue strength and excellent acoustical

and thermal insulation [9]. Sandwich structures also ideal for large ship hull because of

their good shock resistance combine with reasonable affordability [9].

2.2 HISTORY

Historically, the principle of using two cooperating faces separated by a distance

in between was introduced in 1820 by Delau [1]. The first extensive use of sandwich

panel was during the World War II. In the “Mosquito” aircraft, sandwich structure was

used, mainly because of the shortage of other materials in England during the war [3].

The faces were made of veneer while the core consisted of balsa wood. One of the early

uses of sandwich structure in an aeroplane application was in 1937 where balsa wood

core and cedar plywood face sheets was used in the construction of De Havilland

albatross airplane [1]. During World War II the first theoretical analysis of sandwich

theory was published [1]. By the completion of World War II and in the late 1940’s,

some of the first theoretical works on sandwich constructions documented [1].

2.3 STRUCTURE IN SANDWICH PANELS

Generally, single layer sandwich structure consists of three main parts that is two

face sheets and a core. With an additional sheet, called internal sheet, inserted into the

core, a two-layer sandwich panel is then formed.

2.3.1 Faces/Skin

The faces adhesively bonded to the core to obtain a load transfer between the

components. This way the properties of each separate component is utilized to the

structural advantages of the whole assembly leading to a very high stiffness-to-weight

and high bending strength-to-weight ratio. Typically, the facing layer realized by

aluminum plates, high pressure laminates, and glass fiber reinforced plastics. Basically,

the skins are thin, stiff, and very strong. Figure 1 below show the example of skin part in

sandwich structures.

Figure 2.1 The skin of the sandwich panels [5]

facingadhesive

adhesiveTricel honeycom

2.3.2 Principle of Skin

Figure 2.2 How the stress acts on the skin [4]

From the Figure 2.2, it shows how the stress acts on the skin of the sandwich

structures when load is applied. If the panels bend, the core and the skin elongate and

shrink linearly from the neutral axis. The thick black line represents the new section of

the panel after bending. Because the skin are glue to the core, both of them will stretch

the same amount where they bond together but physical properties are completely

different and therefore will react differently to this elongation. Equation below will show

the relation between stress and strain.

L

(2.1)

Strain = amount of stretch (in)/original length (in)

E (2.2)

Stress (psi) = Modulus of elasticity (psi) x strain

Logically, the force acting on the skin is larger than the core because of the large

modulus of elasticity for the skin material. Equal strain at the boundary multiplied by

larger modulus will produce larger stress in the skin.

2.3.3 Fiberglass

Figure 2.3 Fiberglass [6]

Fiberglass or also called glass fiber (Figure 2.3) is a material made from

extremely fine fiber of glass. It is use as a reinforcing agent for many polymer products.

The basis of textile grade glass fiber is silica, 2SiO and in pure form it’s exists as a

polymer nSiO )( 2 . It has no true melting point but softens up to 2000 Co where it starts to

degrade. Glass fibers are useful because of their high ratio of surface area to weight. The

resulting composite material, properly known as fiber-reinforced polymer (FRP) or

glass-reinforced plastic (GRP), is called "fiberglass" in popular usage in the industries.

Glassmakers throughout history have experimented with glass fibers, but mass

manufacture of fiberglass was only made possible with the advent of finer machine-

tooling. In 1893, Edward Drummond Libbey exhibited a dress at the World's Columbian

Exposition incorporating glass fibers with the diameter and texture of silk fibers. What is

commonly known as "fiberglass" today, however, was invented in 1938 by Russell

Games Slayter of Owens-Corning as a material to be used as insulation. It is marketed

under the trade name Fiberglas, ® which has become a genericized trademark.

2.3.4 Properties of Fiberglass

Glass fibers are useful because of their high ratio of surface area to weight.

However, the increased surface area makes them much more susceptible to chemical

attack. By trapping air within them, blocks of glass fiber make good thermal insulation,

with a thermal conductivity of 0.05 W/m-K.

Glass strengths are usually tested and reported for "virgin" fibers: those which

have just been manufactured. The freshest, thinnest fibers are the strongest because the

thinner fibers are more ductile. The more the surface is scratched, the less the resulting

tenacity [13]. Because glass has an amorphous structure, its properties are the same

along the fiber and across the fiber [12]. Humidity is an important factor in the tensile

strength. Moisture is easily adsorbed, and can worsen microscopic cracks and surface

defects, and lessen tenacity.

In contrast to carbon fiber, glass can undergo more elongation before it breaks

[12]. The viscosity of the molten glass is very important for manufacturing success.

During drawing (pulling of the glass to reduce fiber circumference) the viscosity should

be relatively low. If it is too high the fiber will break during drawing, however if it is too

low the glass will form droplets rather than drawing out into fiber.

2.3.5 Core

The core is the structure that placed between two thin faces of the sandwich

structure. The core material is normally low strength material but its higher thickness

provides the sandwich composite with high bending stiffness with overall low density

[7]. The function of the core is to support the thin skin so that they don’t deform and stay

fixed relative to each other. The core of a sandwich structure can be almost any material

Energy Absorbtion at deflection 15mm

7.356962

4.516049

1.6800741.4848270.915761

0

1

2

3

4

5

6

7

8

Polyurethane

En

erg

y A

bso

rbti

on

(J)

50mm

100mm

150mm

200mm

250mm

Figure 4.6 Energy absorption for polyurethane core at deflection 15mm

From Figure 4.6, the maximum energy values were obtained at 50mm span

which is 7.356962J and then values were started to decreasing. The value of energy

absorption decreased at 100mm span and this decrement continues at span 150mm,

200mm and 250mm. The lowest energy absorption occurred at 250mm span which is

0.915761J. The others specimens started from 100mm to 200mm span, the energy

absorption are located in the range of 5.0J to 1.0J. The specific energy absorption for

polyurethane core are summarize in Table 4.8.