Optimising The Lamination Properties Of Textile Composites

A thesis submitted to

The University of Manchester

For the degree of Doctor of Philosophy

in the Faculty of

Engineering and Physical Sciences

By

Ali Hasan Mahmood

Textiles Science & Technology

School of Materials The University of Manchester

2011

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Table of Contents Table of Contents .............................................................................................................. 2

Table of Figures ................................................................................................................ 5

List of Tables ..................................................................................................................... 8

List of Equations ............................................................................................................... 8

Abstract ............................................................................................................................. 9

Declaration .......................................................................................................................10

Copyright Statement ........................................................................................................11

Acknowledgements ...........................................................................................................13

CHAPTER 1 INTRODUCTION ....................................................................................14

1.1. RESEARCH BACKGROUND ..................................................................................14 1.2. PROJECT AIM AND OBJECTIVES ..........................................................................15 1.3. BRIEF CONTENT OF REMAINING CHAPTERS .........................................................16

CHAPTER 2 LITERATURE REVIEW .........................................................................17

2.1. INTRODUCTION ..................................................................................................17 2.2. COMPOSITES ......................................................................................................17

2.2.1. Matrix ...........................................................................................................17 2.2.1.1. Thermoplastic resins .............................................................................18 2.2.1.2. Thermoset resins ...................................................................................18

2.2.2. Reinforcement fibres .....................................................................................19 2.2.2.1. Glass fibre ............................................................................................19

2.3. FIBRE REINFORCED COMPOSITES ........................................................................22 2.4. MANUFACTURING OF COMPOSITES ....................................................................22 2.5. COMPOSITE FAILURE .........................................................................................24

2.5.1. Delamination .................................................................................................25 2.5.2. Importance of filling yarn ..............................................................................26 2.5.3. Effect of thickness and number of laminated layers .......................................28 2.5.4. Effect of thermal conditioning on glass composite failure ..............................29 2.5.5. Effect of hygro-thermal exposure on glass composites ...................................29 2.5.6. Effect of water absorption .............................................................................30

2.6. THROUGH-THE-THICKNESS REINFORCEMENT .....................................................30 2.6.1. Through-the-thickness stitching .....................................................................31 2.6.2. Z-Fibre Pinning .............................................................................................33

2.7. YARN TEXTURING FOR INCREASING THE BONDING STRENGTH ............................35 2.7.1. Air-jet texturing ............................................................................................36

2.7.1.1. Types of operations in air-jet texturing process......................................37 2.7.1.2. Texturing nozzles..................................................................................38

2.7.2. Key considerations for the air-jet texturing process ........................................46 2.7.2.1. Wetting of the yarn before entering the jet .............................................46 2.7.2.2. Primary flow length ..............................................................................47 2.7.2.3. Filament fineness ..................................................................................48 2.7.2.4. Reduction in strength of textured yarn ...................................................48 2.7.2.5. Overfeeding ..........................................................................................48 2.7.2.6. Filament cross-section...........................................................................49

2.8. COMMINGLING PROCESS ....................................................................................49 2.8.1. Jet design for the commingling process .........................................................50 2.8.2. Commingled yarns for composites .................................................................51 2.8.3. Glass filament commingling process .............................................................52

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2.9. SELECTION CRITERIA FOR THE AIR-JET TEXTURING PROCESS ..............................53 2.10. SUMMARY .........................................................................................................54

CHAPTER 3 GLASS YARN TEXTURING, WEAVING AND COMPOSITE MANUFACTURING PROCESS .....................................................................................56

3.1. INTRODUCTION ..................................................................................................56 3.2. AIR-JET TEXTURING MACHINE ...........................................................................56

3.2.1. Texturing machine components .....................................................................56 3.2.2. Feeder yarn creel ...........................................................................................56 3.2.3. Rollers arrangement ......................................................................................57 3.2.4. The jet box ....................................................................................................58 3.2.5. Oil application device ....................................................................................59 3.2.6. Winding unit .................................................................................................59 3.2.7. Suction gun ...................................................................................................60 3.2.8. Gearing arrangement .....................................................................................61 3.2.9. Texturing machine set up for glass yarn .........................................................62 3.2.10. Alteration in the drawing zone...................................................................62 3.2.11. Alteration in the winding zone...................................................................63 3.2.12. Type of jet used ........................................................................................63 3.2.13. Selection of the overfeed value ..................................................................64 3.2.14. Selection of the air pressure value .............................................................65

3.3. WARPING PROCESS ............................................................................................67 3.4. GLASS FABRIC PRODUCTION ..............................................................................68

3.4.1. Problems during weaving process ..................................................................71 3.5. COMPOSITE MANUFACTURING ...........................................................................73

3.5.1. Vacuum bagging technique ...........................................................................73

CHAPTER 4 CHARACTERISATION, EQUIPMENT AND PROCEDURES ............77 4.1. INTRODUCTION ..................................................................................................77 4.2. BREAKING STRENGTH (TENACITY) TESTING OF GLASS YARNS ............................77 4.3. DENSITY, FIBRE VOLUME FRACTION AND VOID CONTENT .................................78 4.4. TENSILE TESTING ...............................................................................................80 4.5. FLEXURE TESTING (THREE POINT BENDING) .......................................................82 4.6. INTER-LAMINAR SHEAR STRENGTH (ILSS) .........................................................85 4.7. INTER-LAMINAR FRACTURE TOUGHNESS ............................................................86

4.7.1. Mode I Inter-laminar fracture toughness ........................................................87 4.8. SCANNING ELECTRON MICROSCOPE (SEM) ........................................................90

CHAPTER 5 EFFECT OF THE TEXTURING PROCESS ON GLASS YARN TENACITY ......................................................................................................................92

5.1. INTRODUCTION ..................................................................................................92 5.2. TENACITY OF THE FEED YARNS ..........................................................................92 5.3. TENACITY OF THE 300 TEX CATEGORY ...............................................................93 5.4. TENACITY OF THE 600 TEX CATEGORY ...............................................................97 5.5. TENACITY OF COMBINED CORE-AND-EFFECT FEED YARNS ................................ 100 5.6. BROKEN FILAMENTS AND LOSS IN LINEAR DENSITY ......................................... 101 5.7. SUMMARY ....................................................................................................... 103

CHAPTER 6 COMPOSITES MADE WITH TEXTURED YARNS: MECHANICAL TESTING, RESULTS AND DISCUSSION .................................................................. 104

6.1. INTRODUCTION ................................................................................................ 104 6.2. COMPOSITES NOMENCLATURE ......................................................................... 104 6.3. FIBRE VOLUME CONTENT ................................................................................. 105 6.4. TENSILE TESTING OF COMPOSITES .................................................................... 106

6.4.1. Tensile properties of 300 tex plain weave composites .................................. 107

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6.4.2. Tensile properties of 300 tex twill weave composites ................................... 108 6.4.3. Tensile properties of 600 tex plain and twill composites .............................. 109

6.5. FLEXURE TESTING OF COMPOSITES .................................................................. 113 6.5.1. Flexure properties of 300 tex plain weave composites .................................. 114 6.5.2. Flexure properties of 300 tex twill weave composites .................................. 115 6.5.3. Flexure properties of 600 tex composites ..................................................... 116

6.6. INTER-LAMINAR SHEAR STRENGTH (ILSS) TESTING ......................................... 118 6.6.1. ILSS of 300 tex plain and twill weave composites ....................................... 118 6.6.2. ILSS of 600 tex plain and twill composites .................................................. 120 6.6.3. Microscope and SEM Analysis .................................................................... 121

6.7. FRACTURE TOUGHNESS (MODE I) TESTING ...................................................... 125 6.8. SUMMARY ....................................................................................................... 131

CHAPTER 7 COMPOSITES WITH TEXTURED AND NON-TEXTURED CORE YARNS ........................................................................................................................... 133

7.1. INTRODUCTION ................................................................................................ 133 7.2. CORE TEXTURED YARN COMPOSITES ................................................................ 133

7.2.1. Fibre volume content of CT composites ....................................................... 133 7.3. MECHANICAL PROPERTIES OF CT COMPOSITES ................................................ 134

7.3.1. Tensile properties of 600 tex CT composites ............................................... 134 7.3.2. Flexure properties of 600 tex CT composites ............................................... 136 7.3.3. ILSS of 600 tex CT plain and twill composites ............................................ 137

7.4. MIXED YARN COMPOSITES ............................................................................... 138 7.4.1. Fibre volume content of WfW composites ................................................... 138

7.5. MECHANICAL PROPERTIES OF WFW COMPOSITES ............................................ 138 7.5.1. Tensile properties of 600 tex WfW composites ............................................ 138 7.5.2. Flexure properties of 600 tex WfW composites ........................................... 140 7.5.3. ILSS of 600 tex WfW composites................................................................ 141

7.6. COMPARISON OF MECHANICAL PROPERTIES ..................................................... 142 7.7. PRODUCTION OF MIXED YARN FABRIC ON A POWER LOOM................................ 145 7.8. SUMMARY ....................................................................................................... 147

CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ................................................................................................................................ 148

8.1. CONCLUSIONS ................................................................................................. 148 8.1.1. Tenacity of yarn after texturing ................................................................... 148 8.1.2. Tensile properties of composites .................................................................. 149 8.1.3. Flexure properties of composites ................................................................. 149 8.1.4. Inter-laminar shear strength and fracture toughness of composites ............... 149 8.1.5. Weave structure .......................................................................................... 150 8.1.6. Composites with combination of textured and non-textured yarns ................ 150

8.2. RECOMMENDATIONS FOR FUTURE WORK ......................................................... 150

REFERENCES............................................................................................................... 152

APPENDIX A: CALCULATIONS FOR DRAW RATIO AND OVERFEED ............. 162

APPENDIX B: MECHANICAL PROPERTIES .......................................................... 166

Word count: 38232 words

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Table of Figures Figure 2.1 Schematic diagram of the filament winding process [Mazumdar 2002] ..23 Figure 2.2 Schematic diagram of the sequence of delamination crack propagation between the layer in a woven-fabric laminate as viewed from the top [Kim and Sham 2000] ......................................................................................................................26 Figure 2.3 Resin rich areas in woven fabric composite ............................................27 Figure 2.4 Schematic diagram of the stitched preform [Nie et al 2008] ....................31 Figure 2.5 Schematic diagram of Z-pinning process Mouritz [2007] .......................34 Figure 2.6 Mechanism of air-jet texturing [Acar et al 2006] ....................................37 Figure 2.7 First Air-Jet Process Taslan by Du Pont ..............................................39 Figure 2.8 Taslan jets (a) Type 7 (b) Type 8 (c) Type 9 (d) Type 10 (e) Type 11 (f) Type 14...................................................................................................................41 Figure 2.9 Taslan Type 20.......................................................................................42 Figure 2.10 Standard-core Hemajet [Heberlein guide 1991] ....................................43 Figure 2.11 (Hemajet LB-02 Universal Housing with T-Series Jet Core) [Heberlein guide 1991] .............................................................................................................43 Figure 2.12 Heberlein Hemajet EO-52 [Oerlikon 2010] ..........................................44 Figure 2.13 Hemajet jet cores (a) A and T series, (b) A-2, S-2 and T-2 series [Oerlikon 2004a, 2007b] .........................................................................................45 Figure 2.14 Heberlein Jet Housing (a) Hemajet LB-04, (b) Hemajet LB-24 [Oerlikon 2007a, 2009b] .........................................................................................................46 Figure 2.15 Commingling process [Alagirusamy et al 2005] ...................................50 Figure 2.16 Air Inlet Configurations for Commingling Process [R. Alagirusamy et al 2005] ......................................................................................................................51 Figure 3.1 Creel Section ..........................................................................................57 Figure 3.2 Rollers Section .......................................................................................58 Figure 3.3 Jet box and components .........................................................................59 Figure 3.4 Oil application roller ..............................................................................59 Figure 3.5 Winding unit ..........................................................................................60 Figure 3.6 Suction gun ............................................................................................60 Figure 3.7 Gearing arrangement ..............................................................................61 Figure 3.8 Modified thread line diagram of Sthle RMT-D air-jet texturing machine for glass yarn ..........................................................................................................62 Figure 3.9 Jet housing (Heberlein hemajet LB-13) ..................................................63 Figure 3.10 Jet core (T-370) ....................................................................................63 Figure 3.11 Core-and-effect textured glass yarns .....................................................66 Figure 3.12 Single end warping machine (made by the Shirley Institute) .................67 Figure 3.13 Glass yarn warping in process ..............................................................68 Figure 3.14 Hand loom ...........................................................................................69 Figure 3.15 Dead weight for warp yarn tensioning ..................................................70 Figure 3.16 (1/1) Plain weave fabrics ......................................................................71 Figure 3.17 (1/3) Twill weave fabrics......................................................................71 Figure 3.18 Entanglements during the shedding process ..........................................72

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Figure 3.19 Entanglements in 300 + 34 tex 3 bars pressure textured warp yarns ......73 Figure 3.20 Configuration diagram of the vacuum bagging process .........................74 Figure 3.21 Vacuum bag .........................................................................................74 Figure 4.1 Glass yarn specimen undergoing breaking strength testing .....................78 Figure 4.2 Composite specimen undergoing tensile testing ......................................81 Figure 4.3 Flexure testing assembly (a) three point bending (b) four point testing [Hodgkinson 2000] .................................................................................................83 Figure 4.4 Potential failure modes for flexure testing [BSI 14125 1998]..................83 Figure 4.5 Composite specimen undergoing Inter-laminar shear strength (ILSS) testing .....................................................................................................................86 Figure 4.6 Schematic diagrams of the basic modes of fracture, mode I (opening), mode II (shear), mode III (tearing) [Robinson and Hodgkinson 2000] .....................87 Figure 4.7 Double cantilever beam (DCB) specimen geometry, (a) end-blocks, (b) piano hinges [Robinson and Hodgkinson 2000] ......................................................88 Figure 4.8 DCB test specimen undergoing fracture toughness testing ......................89 Figure 4.9 Section of DCB with piano hinges indicating t ....................................90 Figure 4.10 Prepared samples for scanning electron microscopy (SEM) ..................91 Figure 5.1 Tenacity of the feed yarns ......................................................................93 Figure 5.2 Tenacity of textured and non-textured glass yarns of 300 tex category ...94 Figure 5.3 photomicrographs of 300 + 34 tex 5 bars textured yarn structure ............95 Figure 5.4 Photomicrographs of 300 + 68 tex 5 bars textured yarn structure ............95 Figure 5.5 Tenacity of textured and non-textured glass yarns of 600 tex category ...97 Figure 5.6 Comparison of tenacity of 300 and 600 tex textured yarns ......................98 Figure 5.7 Photomicrographs images of 600 + 34 tex 5 bars textured yarn structure 99 Figure 5.8 Photomicrographs of 600 + 68 tex 5 bars textured yarn structure ............99 Figure 5.9 Comparison of tenacity of non-textured feed yarns ............................... 100 Figure 5.10 Linear density (tex) of textured glass yarns (a) 300 tex (b) 600 tex category ................................................................................................................ 102 Figure 6.1 Tensile strength of 300 tex plain weave composites.............................. 107 Figure 6.2 Tensile modulus of 300 tex plain weave composites ............................. 107 Figure 6.3 Tensile strength of 300 tex twill weave composites .............................. 108 Figure 6.4 Tensile modulus of 300 tex twill weave composites ............................. 109 Figure 6.5 Tensile strength of 600 tex plain & twill weave composites.................. 110 Figure 6.6 Tensile modulus of 600 tex plain & twill weave composites ................. 110 Figure 6.7 Tensile tested samples of 600 tex non-textured plain weave composites ............................................................................................................................. 112 Figure 6.8 Tensile tested samples of 600 + 68 tex 5 bars textured plain weave composites ............................................................................................................ 113 Figure 6.9 Flexure strength of 300 tex plain weave composites ............................. 114 Figure 6.10 Flexure modulus of 300 tex plain weave composites .......................... 114 Figure 6.11 Flexure strength of 300 tex twill weave composites ............................ 115 Figure 6.12 Flexure modulus of 300 tex twill weave composites ........................... 116 Figure 6.13 Flexure strength of 600 tex plain & twill weave composites ............... 117 Figure 6.14 Flexure modulus of 600 tex plain & twill weave composites .............. 117

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Figure 6.15 ILSS of 300 tex plain weave composites ............................................ 118 Figure 6.16 ILSS of 300 tex twill weave composites ............................................. 119 Figure 6.17 ILSS of 600 tex plain & twill weave composites ................................ 120 Figure 6.18 600 tex non-textured twill weave composite ....................................... 122 Figure 6.19 600 + 34 tex 5 bars twill weave composite ......................................... 123 Figure 6.20 SEM images 600 + 34 tex 5 bars twill weave composite ..................... 124 Figure 6.21 SEM image 600 + 34 tex 5 bars plain weave composites .................... 125 Figure 6.22 SEM image 600 without textured plain weave composite ................... 125 Figure 6.23 Typical load versus crosshead displacement curves for mode I specimens of the 600 non-textured twill weave and the 600 + 68 tex 5 bars twill weave composites ............................................................................................................ 127 Figure 6.24 Initiation and propagation values for mode I testing of 600 + 68 tex 5 bars textured and 600 non-textured twill weave composites .................................. 128 Figure 6.25 Comparison of the mean values of G1c (visual, 5 % offset and propagation) for mode I DCB testing of 600 + 68 tex 5 bars textured and 600 non-textured twill weave composites............................................................................ 129 Figure 6.26 SEM micrographs of fracture surfaces of 600 tex twill weave non-textured composite ................................................................................................ 130 Figure 6.27 SEM micrographs of fracture surfaces of 600 + 68 tex 5 bars twill weave textured composite ................................................................................................ 131 Figure 7.1 Tensile strength of 600 tex CT plain & twill weave composites ............ 135 Figure 7.2 Tensile modulus of 600 tex CT plain & twill weave composites ........... 135 Figure 7.3 Flexure strength of 600 tex CT plain & twill weave composites ........... 136 Figure 7.4 Flexure modulus of 600 tex CT plain & twill weave composites .......... 136 Figure 7.5 ILSS of 600 tex CT plain & twill weave composites ............................ 137 Figure 7.6 Tensile strength of 600 tex plain & twill weave WfW composites ........ 139 Figure 7.7 Tensile modulus of 600 tex plain & twill weave WfW composites ....... 139 Figure 7.8 Flexure strength of 600 tex plain & twill weave WfW composites ........ 140 Figure 7.9 Flexure modulus of 600 tex plain & twill weave WfW composites ....... 140 Figure 7.10 ILSS of 600 tex plain & twill weave WfW composites ....................... 141 Figure 7.11 Tensile strength of 600 tex plain & twill weave composites ................ 142 Figure 7.12 Tensile modulus of 600 tex plain & twill weave composites ............... 143 Figure 7.13 Flexure strength of 600 tex plain & twill weave composites ............... 143 Figure 7.14 Flexure modulus of 600 tex plain & twill weave composites .............. 144 Figure 7.15 Inter-laminar shear strength of 600 tex plain & twill weave composites ............................................................................................................................. 145 Figure 7.16 Production of mixed yarn fabric on a power loom .............................. 146

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List of Tables Table 2.1 Available glass types and their properties [Vaughan 1998] ......................20 Table 2.2 Fibre glass filament designations [Vaughan 1998] ...................................21 Table 3.1 Fabric specifications ................................................................................71 Table 3.2 Consumable materials required for the vacuum bagging [Cripps 2000] ....75 Table 5.1 Number of filaments in glass yarns ..........................................................96 Table 6.1 Fibre volume content of glass composites .............................................. 105 Table 7.1 Fibre volume content of CT composites................................................. 134 Table 7.2 Fibre volume content of WfW composites ............................................. 138

List of Equations

Density of specimen = S (g/cm3) = LSAS

WAS

mmm

,,

,

(4.1) [BS ISO 1183-1, 2004]79

f

Cff WV

(4.2) [Khan 2010]

.......................................................................79

10012

13

MMMMW f

(4.3) [BS ISO 1172, 1999] ........................................80

R

Cf

f

Cfo WWV

100100

(4.4) [Khan 2010].......................80

bhF

(4.5) [BS 2782-10: Method 1003 1977] ..............................................81

2

2

2 36123

Lsh

LS

bhFL

f (4.6) [BSI 14125 1998] ......................84

sF

bhLE f 3

3

4 (4.7) [BSI 14125 1998] .....................................................84

bh

FILSS

max

43

(4.8) [BS ISO 14130 1998] ....................................86

FPG c

a 2b

31

(4.9) [ASTM D 5528-01 2007] ............................................89

223

1031

2

at

aF

(4.10) [ASTM D 5528-01 2007] .........................90

9

Abstract Woven glass composites have been used for many years in commercial applications

due to their light weight, competitive price and good engineering properties.

Absorption of energy by laminated composite material results in damage in various

forms, the most common of which is delamination. Inter-laminar fracture causes the

layers of composite to separate, resulting in a reduction in stiffness and strength of

the composite structure, matrix cracking and in some cases fibre breakage takes

place. The aim of this project was to improve the inter-laminar bond strength

between woven glass fabric and resin. Air jet texturing was selected to provide a

small amount of bulk to the glass yarn. The purpose was to provide more surface

contact between the fibres and resin and also to increase the adhesion between the

neighbouring layers. These were expected to enhance the resistance to delamination

in the woven glass composites.

Glass yarns were textured by a Sthle air jet texturing machine. Core-and-effect yarn

was produced instead of a simple air textured yarn. Hand loom and vacuum bagging

techniques were used for making the fabric and composite panels from both textured

and non-textured yarns. Density and fibre volume content were established for

physical characterisation. Breaking strength (tenacity) of the yarns and tensile,

flexure, inter-laminar shear strength (ILSS) and fracture toughness (mode 1)

properties of the composites were determined. Projection microscopy and SEM

imaging techniques were used to assess the fractured surfaces of the composite

specimens. The yarn tenacity and the tensile properties of the composites were

significantly reduced after the texturing process, whereas flexure properties were

unchanged. However, significant improvement was observed in the ILSS and

fracture toughness of the composites after the texturing process. It was also observed

that the composites made from the fabrics with textured yarns in only the weft

direction are the most advantageous as they maintained the tensile and flexure

properties but have significantly higher inter-laminar shear strength.

10

Declaration

No portion of the work referred to in the thesis has been submitted in support of an

application for another degree or qualification of this or any other university or other

institute of learning.

Ali Hasan Mahmood

11

Copyright Statement

I. The author of this thesis (including any appendices and/or schedules to this

thesis) owns certain copyright or related rights in it (the Copyright) and he

has given The University of Manchester certain rights to use such Copyright,

including for administrative purposes.

II. Copies of this thesis, either in full or in extracts and whether in hard or

electronic copy, may be made only in accordance with the Copyright,

Designs and Patents Act 1988 (as amended) and regulations issued under it

or, where appropriate, in accordance with licensing agreements which the

University has from time to time. This page must form part of any such

copies made.

III. The ownership of certain Copyright, patents, designs, trade marks and other

intellectual property (the Intellectual Property) and any reproductions of

copyright works in the thesis, for example graphs and tables

(Reproductions), which may be described in this thesis, may not be owned

by the author and may be owned by third parties. Such Intellectual Property

and Reproductions cannot and must not be made available for use without the

prior written permission of the owner(s) of the relevant Intellectual Property

and/or Reproductions.

IV. Further information on the conditions under which disclosure, publication

and commercialisation of this thesis, the Copyright and any Intellectual

Property and/or Reproductions described in it may take place is available in

the University IP Policy (see

http://www.campus.manchester.ac.uk/medialibrary/policies/intellectual-

property.pdf), in any relevant Thesis restriction declarations deposited in the

University Library, The University Librarys regulations (see

http://www.manchester.ac.uk/library/aboutus/regulations) and in The

Universitys policy on presentation of Theses.

12

This thesis is dedicated to my (late) father (Mr. Jafar Mahmood), mother (Mrs.

Shahina Mahmood), my wife (Mrs. Sana Ali), my son (Master Saami Ali), my

brothers (Mr. Faiq Ali, Mr. Ammar Hasan, Mr. Hani Hasan), my sister (Mrs.

Aisha Faiq) and my nephew and niece (Master Hadi and Miss Manal).

13

Acknowledgements First and foremost, praises and thanks to Allah S.W.T who bestowed upon us all the

blessings and the faculties of thinking, learning and searching.

This study would not have been possible without the financial support of my

employer and sponsor, NED University of Engineering & Technology funded

through the Higher Education Commission (HEC) of Pakistan.

I would like to express my deepest gratitude for my supervisors, Prof. Porat and Dr.

Gong, whose encouragement, guidance and most importantly support from the initial

to the final level enabled me to think independently and to develop an understanding

of the subject.

I would also like to thank my parents, my brothers and sister, and my wife for

keeping up with me and my demands and their moral encouragement. They boosted

my ego, when it was needed and supported me in various ways but, all through their

unconditional love.

I would also like to sincerely thank Prof. Peter Foster, Dr. Sheraz Hussain Yousfani,

Dr. Laraib Alam Khan, Dr. Syed Naveed Rizvi, Dr. Alan Nesbitt, Dr. Chris Wilkins,

Dr. Chi Zhang, Mr. Steve Butt, Mr. Adrian Handley and Mr. Tom Kerr for their

valuable help, advice and technical assistance.

Many thanks go to PPG Industries for providing the glass filaments and Mr. Keith

Wilson for providing the best advices and support for texturing glass yarn.

Last but not least, I am indebted to any of my colleagues and staff members, and in

fact anyone else who has supported and assisted me in conducting this work.

14

Chapter 1 Introduction

1.1. Research background

Composite materials have gained substantial popularity for a wide range of

applications in structural components because of their high strength-to-weight and

stiffness-to-weight ratios. However, failure due to delamination (the separation of

laminate layers) is of great concern. Delamination, as indicated by various

researchers, is the most common cause of damage in glass composites. This happens

under the impact of load and results in fibre-matrix de-bonding. The purpose of this

research is to improve the bond strength between the glass and the matrix by using

textured yarns developed through the air jet texturing process. The concept was to

produce bulk in the yarn through texturing in order to provide more surface contact

between the fibre and resin, and between the neighbouring layers. The technique of

air jet texturing was utilised by Ma et al [2003] to improve the coated ratio and the

bond strength of glass/PVC fabrics. Koc et al [2008] found improvement in adhesion

of PET yarns to rubber by incorporating a very small amount of texturing. Langston

[2003] also found improvement in inter-laminar shear strength of composites by

texturing Aramid yarns and the reason was the anchoring and entanglement between

the layers due to the bulkier yarn structure.

One potential disadvantage of using textured yarns is the reduction in in-plane

mechanical strength due to the disorientation of filaments introduced in the texturing

process. Therefore, this study was based on the production of core-and-effect

textured reinforcement (glass) yarns. The intention was to keep the disorientation of

filaments as small as possible to minimise strength reduction while producing

sufficient texture to enhance the inter-laminar bonding strength. With the core-and-

effect yarn, the core yarn was processed with a minimum overfeed ratio to maintain

the strength of the final yarn. The effect yarn, however, was subjected to moderate

overfeed for the development of loops and bulk.

15

The yarns produced were then woven and a number of weave structures were

investigated and optimised, these fabrics were then used to produce composites

which were subjected to various tests.

1.2. Project aim and objectives

The aim of this project was to minimise the problem of delamination in composites

by increasing the bond strength between the reinforcement glass yarn fabric and the

resin and between the neighbouring layers.

In order to achieve the aim, the following tasks were planned:

review the literature in the fields of textile composites, delamination

behaviour of composites and the causes of delamination, other means for

improving the lamination strength, air jet texturing and commingling

processes;

manufacture the core-and-effect textured glass yarn through air jet texturing

and investigates the optimum texturing parameters;

investigate the effect of texturing parameters on the tenacity of glass yarns;

manufacture woven glass fabrics on a hand loom from both the textured and

non-textured glass yarns;

producing multi-layered thermoset composites by using a suitable technique

of composite manufacturing;

investigate the effect of texturing on the physical and mechanical properties

of these composites.

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1.3. Brief content of remaining chapters

Chapter 2 covers the literature review including a short introduction to composites, a

literature survey of delamination and the preventive measures that are commonly

used and the air jet texturing and commingling processes.

Chapter 3 describes the equipment and techniques employed for the production of

samples used in this study together with their merits and constraints. This includes

the study of air jet texturing machine, texturing of glass yarns, fabric development

and finally the fabrication of composite panels.

The physical and mechanical test methods and equipment used to characterise the

textured and non-textured glass composites and the scientific principles involved in

the techniques are described in detail in Chapter 4.

Chapters 5, 6 and 7 cover the experimental work, results and discussion parts of this

study. The comparison of the tenacities of textured and non-textured glass yarns and

the effect of texturing on their tenacity are investigated in Chapter 5.

Chapter 6 includes the results and discussion regarding the effect of texturing on the

mechanical properties of the fabric composites made from core-and-effect textured

glass yarns.

Chapter 7 concerns the effect of texturing on the mechanical properties of

composites made from textured and non-textured core yarns. These composites were

developed by changing the composition of fabrics on the basis of their constituent

yarns.

Chapter 8 presents the conclusions of this work and suggests future work.

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2. Chapter 2 Literature review

2.1. Introduction

This project is concerned with improving the lamination strength of glass reinforced

composites by modifying the fabric surface using air-jet textured yarn. The work is

based on a combination of textiles and composites technologies and relevant topics

to this work are reviewed below. This chapter includes a short introduction to

composites followed by a literature survey of delamination and the preventive

measures that are commonly used. It includes studies regarding the air-jet texturing

process, the commingling process and their importance for composites.

2.2. Composites

Composite materials are engineered, heterogeneous materials comprising two or

more constituent materials with a discrete and recognisable interface separating

them. These are macroscopic combinations and the most common naturally

occurring composite is wood. The two constituent materials are the matrix and the

reinforcement. Reinforcement fibres are usually of high strength/stiffness and are

generally orthotropic (having different properties in different directions depending

upon the direction of the applied load). The matrix material is ordinarily of a high

performance type. Moreover, both fibres and matrix may be organic or inorganic in

nature [Reinhart 1998, Peters 1998].

2.2.1. Matrix

The matrix acts as a binder for the fibres because it has adhesion and cohesion

characteristics. It helps in transferring of load to the fibres and between the fibres

and also guards them from environmental impacts. Orientation and location of the

fibres in the composite structure are maintained by the matrix. By distributing the

load evenly among the fibres, it resists damage and crack propagation. The matrix

contributes to the electrical and chemical properties of the composite [Reinhart 1998,

Peters 1998].

Most commercially produced composites use a polymer matrix material often called

a resin which is classified into two types, namely thermoplastic and thermoset resins.

18

2.2.1.1. Thermoplastic resins Thermoplastic resins are usually cheaper for fabrication. They can be stored safely

for long periods of time before moulding. They have the ability to be re-moulded by

application of temperature and pressure as the molecules are generally not cross-

linked. They are characterised by toughness and high impact strength. However, they

suffer thermal degradation with repetitive temperature cycling [Reinhart 1998].

The examples include Polyether ether ketone (PEEK), Polyphenylene sulfide (PPS),

Polyether ketone ketone (PEKK), Polyamide (PA or Nylon), Polybutylene

terephthalate (PBT), Polyethylene terephthalate (PET), Polyethylene (PE),

Polypropylene (PP), Polyvinyl chloride (PVC).

2.2.1.2. Thermoset resins Thermoset resins are generally available in liquid form and after mixing with other

ingredients they solidify. They form cross-linkages between the molecules during the

curing process and thus once cured, they cannot be remoulded. Thermosets are

relatively easy to process and usually do not require pressure or high temperature to

form. They normally possess a short workable shelf life [Peters 1998, Varma and

Gupta 2000].

Examples of thermosets resins include Epoxy, Polyester, Vinylester, Polyurethane,

Polyimide, Cyanate ester, Phenolic triazine.

Epoxy resins are relatively lower molecular weight polymers and are used as a

matrix for fibre composites in structural applications. They have a number of

advantages over the other types of polymers. They are inherently polar in nature

which provides excellent adhesion to a wide range of fibres. They have relatively

lower curing shrinkage and no volatile by-products which prevent undesirable void

formation. After curing, the epoxy resins possess high chemical and corrosion

resistance and good mechanical, thermal and electrical properties. However, they

have higher viscosity, are higher in cost and their major limitations are a longer

19

curing time and poor performance in hot-wet environments [Penn and Wang 1998,

Varma and Gupta 2000].

2.2.2. Reinforcement fibres

The purpose of fibre as reinforcement is to provide integrity and strength to the

structure by carrying the majority of the applied structural loads. Fibres are stronger

because while having smaller diameter, they have fewer defects and have the

possibility to align the crystal or molecular structure. Flaws or defect propagation

usually cause failure of the material. However, due to the presence of many fibres in

the composite structure, sudden damage does not usually occur. Most of the fibres

have to rupture before the complete failure of the composite and hence usually

warning signs are there before the collapse.

Fibre reinforcement, which is the discontinuous phase, is responsible for the primary

engineering properties of composites. The mechanical properties of composites

increase by increasing the fibre volume content up to a level where enough matrix

material is available to support the fibres and transfer the load within the composite

[Reinhart 1998].

Some examples of the reinforcement fibres are: glass, carbon, Kevlar (Aramid),

boron, polyethylene, silicon carbide, silicon nitrite, silica, etc.

Glass yarn was chosen for this project because it has a very wide appeal for

structured composites due to its low cost, easier handling and it is relatively easier to

process in the university research environment. Glass yarns possess a wide range of

properties and tailored performance for specific purposes which suited them for

many applications from small electrical products such as printed circuit boards to

boats and larger ships [Sims and Broughton 2000]. The next section describes the

types and properties of glass fibre.

2.2.2.1. Glass fibre Glass fibre is most widely used as a reinforcement for structural composites. Glass is

described as an amorphous material. It is made up of elements such as silicon, boron

and phosphorus which are transformed into glass by mixing with oxygen, sulphur,

20

tellurium and selenium. There are several glass compositions available (Table 2.1)

depending upon the desired properties for end use [Vaughan 1998]: Table 2.1 Available glass types and their properties [Vaughan 1998] Glass type Key features

A-glass High alkali or soda glass for good chemical resistance

E-glass Low alkali glass (aluminium borosilicate) for excellent electrical

insulation properties

C-glass Composed of soda borosilicate for excellent chemical resistance

S-2 glass Composed of magnesium, aluminium silicate and offers higher

physical strength (40% higher tensile strength than E-glass)

D-glass Superior dielectric constant than E-glass

R-glass Resistant to alkali and is used in reinforcing concrete

Low K An experimental fibre similar in properties to D-glass

Hollow

fibre

Tube-like or hollow fibre glass specific applications in light weight

reinforced aircraft parts

The properties of glass fibre depend on the composition of the original glass melt.

Some of the properties which glass fibre usually exhibits are:

High tensile strength In some applications the strength to weight ratio

exceeds steel wire.

Heat and fire resistance Due to its inorganic nature, glass fibre does not

support combustion.

Chemical resistance Not susceptible to fungal, bacterial or insect attack.

Moisture resistance Due to non-absorbency of water, glass fibre never

swells, rots, stretches or disintegrates in a moist atmosphere.

Thermal properties With having a low coefficient of thermal linear

expansion and a high coefficient of thermal conductivity, it performs well in

thermal functions.

Electrical properties As it has a non-conductive nature, it is efficiently used

for electrical insulation.

21

Glass yarns are created in many varieties so a particular system for yarn

classification is essential. Therefore, glass yarn nomenclature has been developed

based on both alphabetical and numerical designations.

For example ECG 150 4/2 s:

Where;

E Identifies the glass composition (E-glass).

C Recognizes filament type (C = continuous).

G Filament designation indicates filament diameter (from Table 2.2, G = 9 micron).

150 Stands for 1/100th of the single strand yield i.e. (15000 yards/pound).

4 Indicates the number of single strands twisted together i.e. Four strands of 150 1/0 are twisted together.

2 Shows the number of twisted yarns plied together. By multiplying the two figures (4 x 2), the total number of basic strands in a plied yarn is obtained.

Moreover, by dividing the basic strand yield with total number of strands in

the yarn, yarn yield can be obtained.

S Designation of twist. Either 'S' or 'Z'.

Table 2.2 Fibre glass filament designations [Vaughan 1998] Filament

designation

Filament diameter

in 10-4 m

B 1.5 3.8

C 1.8 4.5

D 2.1 5

DE 2.5 6

E 2.9 7

G 3.6 9

H 4.2 10

K 5.1 13

Therefore, the above yarn comprises type E-glass, having continuous filaments of 9

micron diameter. The yarn contains 8 (4 x 2) basic 150 strands, having a glass yield

of 1875 (15 000/8) yards/pound and using 'S' twist to create balance [Vaughan 1998].

22

2.3. Fibre reinforced composites

Fibre reinforced composites can be classified according to the form in which the

reinforcement fibre material is used. These are short discontinuous, long

discontinuous and continuous fibre reinforced composites. It can be further classified

according to the structure of the reinforcement such as woven, non-woven, braided,

knitted etc.

The parameters of fibres i.e. length, orientation and volume content dominate the

engineering properties of the composite. Among them, the length of the fibre is very

important and continuous and long discontinuous fibre composites are better in terms

of engineering properties [Reinhart 1998].

2.4. Manufacturing of composites

There are a number of processes used for manufacturing composites depending upon

the type of the end product and the performance required. A brief description of

some of the general composite manufacturing techniques is provided below:

The hand lay-up process is one of the oldest composite manufacturing techniques

and is still widely used for prototype part manufacturing and in the marine industry.

It is a labour intensive process in which the liquid resin is applied to the mould

followed by the placement of the reinforcement. The process of application of resin

and reinforcement layer continued until a suitable thickness is achieved. After fibre

wet-out, the laminate is allowed to cure. The spray-up process is also used as an

alternative to hand lay-up process in which the chopped fibres and resin are

deposited on to the mould by means of a spray gun [Mazumdar 2002, Khan 2010].

The filament winding process is used for making tubular parts and specialised

structures like pressure vessels. The process involves winding the resin impregnated

fibres at the desired angle over a rotating mandrel. Figure 2.1 shows the fibres

passage moving through the resin bath and after impregnation they move back and

forth by means of the guide while the mandrel rotates at a specified speed. The

desired angle is achieved by controlling the motion of the guide and the mandrel

[Mazumdar 2002].

23

Figure 2.1 Schematic diagram of the filament winding process [Mazumdar 2002]

Pultrusion is a low-cost and a high volume manufacturing process in which the fibre

reinforcement after impregnation with resin is pulled through a heated die to make

the part. Pultrusion is used for the fabrication of composite parts with constant cross-

section profile e.g. rods, beams, channels, tubes, walkways and bridges, handrails,

light poles, etc [Mazumdar 2002].

Resin transfer moulding (RTM) is a closed mould operation in which the

reinforcement material is placed and clamped between two matching mould surfaces.

The resin is injected into the mould cavity through a port or series of ports under

moderate pressure. After curing the part is removed from the mould. Sometimes, for

assisting the resin flow and to remove the air bubbles, a vacuum is also created inside

the mould. The advantages associated with the RTM process are: lower investment

and operating cost, dimensional accuracy, manufacturing of complex parts, good

surface finish, low volatile emission due to closed moulding process. However, the

limitations are complex tooling design and also substantial trial-and-error

experimentation or flow simulation modelling is required for manufacturing the

complex parts [Mazumdar 2002].

The resin infusion process is an alteration to RTM in which only vacuum is used to

drive the resin flow and the laminates are enclosed in a one sided mould covered

with a bag. The resin is introduced inside the bag by means of one set of pipe work

24

while the second set allows the vacuum to be drawn from the bag. This technique is

commonly known as vacuum bagging and is utilised for this project as described in

Section 3.5.1 [Mazumdar 2002, Khan 2010].

The resin infusion technique has several names. Some of them are Vacuum Infusion

(Crystic VI), Co-injection RTM (CIRTM), Liquid resin infusion (LRI), Modified

vacuum infusion (MVI), Vacuum assisted Injection moulding (VAIM), Vacuum

assisted resin injection moulding (VARIM), Vacuum assisted resin transfer moulding

(VARTM), Vacuum infusion moulding process (VIMP) [Summerscales 2010].

2.5. Composite failure

Composite materials have a wide range of applications in structural components

because of their high strength-to-weight and stiffness-to-weight ratios. However, the

problem of delamination is of great concern. Failure caused in laminated composites

is usually by the separation of two laminate layers. Normally impact, shock and

cyclic stresses are responsible for failure. The problem of delamination is due to the

weakness of the composite in the through-the-thickness direction and the reason is

the inherent low adhesion inter-laminar strength [Pekbey and Sayman 2006].

Damage of any composite as a reaction to impact usually appears in the form of one

or more combined failure mechanisms which are matrix cracking, fibre fracture,

fibre-matrix de-bonding and delamination. The most crucial and common life-

restricting crack growth mode in laminated composites is delamination. Apart from

load application, various material properties and geometric parameters also influence

the failure mechanisms. However, whatever the mechanism is, the damage always

causes reduction in the stiffness and strength of the composite structure [Jang et al.

1989, Gweon and Bascom 1992, Pavier and Clarke 1995, Zhou and Davies 1995,

Adanur and Onal 2001, Ray 2005].

Baucom et al [2005a, 2006b] tested the S2-glass and E-glass composites with various

fabric architectures under repeated drop load impact in order to find out the damage

effect. The 4-ply specimens were observed under reflected light photography and

Scanning Electron Microscopy for visualisation of internal damage. It was found that

25

the damage mechanism was dominated by matrix cracking, matrix de-bonding,

delamination of layers and tensile fracture of fibres.

Pekbey and Sayman [2006] indicated that delamination causes serious degradation to

the composite structure. They found experimentally that the compressive strength of

composite materials was reduced with the presence of delamination as it always

weakened the structure.

Kumar et al [2007] investigated the relationship between post-impact compression

strength and the delamination area by performing impact tests on woven E-

glass/epoxy composite laminates. They found an increase in the delamination area

with increasing impact energy levels, which resulted in a decrease of compression

strength after impact. The decrease in load carrying capacity was assumed to be a

response to the degraded cross-sectional area of the sample under the action of

impact damage.

2.5.1. Delamination

Ebeling et al [1997] and Kim and Sham [2000] studied the failure mechanism of

delamination during the double cantilever beam test by the examination of crack

front movement across the width of the woven fabric laminated composite. Figure

2.2 illustrates multiple crack fronts, one for each warp yarn and the progress of crack

propagation between the layers when viewed from the top. Figure 2.2(a) shows

stable crack propagation where the crack front was most advanced in the direction

parallel to the exposed yarn (i.e. warp). However, the crack front lagged where the

yarns were perpendicular to it (i.e. weft) and the overall crack front seemed

discontinuous. Figure 2.2(b) shows unstable crack growth with a sudden load drop.

The entire crack front jumped forward but arrested instantaneously at the next

undulation resulting in a continuous crack front. Figure 2.2(c) shows recurrence of

Figure 2.2(a) for the adjacent cell. The repetition of approximately the same

procedure happened with crack propagation before complete delamination of the

composite laminate. The orientation of the yarn at the crack tip during the stress state

resulted in the change of discontinuous and continuous crack fronts periodically and

hence is responsible for the inter-laminar fracture toughness.

26

Figure 2.2 Schematic diagram of the sequence of delamination crack propagation between the

layer in a woven-fabric laminate as viewed from the top [Kim and Sham 2000]

2.5.2. Importance of filling yarn

Woven fabric laminated composites have an advantage over the unidirectional

layered composites with having a non-planar interply structure which provides

resistance to the growth of the crack. This is because of the interaction of a

delamination crack with the matrix regions and the weave structure during its

propagation. Some other advantages of woven fabrics are easy handling for

automation and conformability for complex shapes [Kotaki and Hamada 1997, Kim

and Sham 2000, Suppakul and Bandyopadhyay 2002].

Sample Width

Direction of delamination propagation

27

The toughness of the matrix is very important in preventing delamination and the

resin-rich areas play a very vital role. Ebeling et al [1997] highlighted two types of

resin-rich areas in glass woven fabric composites and their importance in

delamination. According to them, the first one was a yarn undulation area, where two

yarns intersected each other. The depth of this resin-rich area was half the ply

thickness. The second area was called the interstitial area and was situated at the

junction of four intersecting yarns, having the depth of resin equal to the thickness of

ply as shown in Figure 2.3.

Figure 2.3 Resin rich areas in woven fabric composite

Ebeling et al [1997] experimentally proved that for a brittle matrix, these areas and

especially the interstitial areas, promoted cracking and fracture of composites by

fracturing ahead of the main matrix. However, for stiffer matrices, they acted as

points of increased toughness and momentarily arrested the growth of the crack. The

undulation of the fibres which were perpendicular to the crack direction usually

restricted the crack jump. According to Ebeling et al [1997], delamination started

from the fibre/matrix de-bonding which is the easier path to follow. However, the

presence of filling yarns in the woven fabric forced the crack path to follow the inter-

laminar path and the changing of the crack path caused an increase in the

delamination toughness. They further concluded that composite toughness definitely

increased by increasing the matrix toughness.

Kotaki and Hamada [1997] investigated the fracture toughness of laminated

composites of differently placed satin weave structures. Their experimental results

also showed the highest fracture toughness with the sample which had more

transverse fibre strands.

28

2.5.3. Effect of thickness and number of laminated layers

The thickness of the composite is an essential factor for estimating the structural

damage, absorption of energy and resistance to penetration. Delamination behaviour

was examined by Xiao et al [2007] by making composites of a varying number of

layers. Plain woven S2 glass/SC-15 epoxy composites were manufactured and tested

under quasi-static punch shear apparatus. It was observed that thin laminated

structures had linear failure behaviour, while the thick laminated structures had bi-

linear failure characteristics. The damage sequence reported under action of load was

based on the following steps:

Delamination initiation

Delamination propagation

Fibre compression and shear failure

Fibre tension and shear failure

While examining the bi-linear behaviour, it was observed that the commencement of

delamination took place as a result of transverse shear loading under the application

of punch load. During delamination propagation, a gentler slope of the load-

displacement curve was observed and the flexure and shear stiffness were dropped.

However, the composite continued to carry the load until complete delamination and

the initiation of fibre failure.

Improvement in the load bearing capability and decrease in the amount of deflection

during impact loading was also indicated by Adanur and Onal [2001] for the thick

composite laminates. Aslan et al [2002] performed impact testing on E-glass/epoxy

woven laminated composites to investigate the significance of thickness and

dimensional effects. It was concluded that the peak impact force and the duration of

contact of load were vital factors. Thick composite laminates proved to be stiffer and

possessed high peak forces and smaller contact durations as compared to the thinner

composite laminates. The reason suggested was the increase in flexure and contact

stiffness with the increase in thickness. Therefore, thickness was found to be a

significant and governing factor for dynamic response and damage mechanism under

impact loading.

29

Sutherland and Soares [2004] indicated the difference of delamination damage modes for thinner and thicker composite laminates of E-glass Polyester/epoxy

composites when subjected to high incident energies. According to them, the thinner

laminates suffered bending and fibre damage whereas indentation damage was found

for the thicker laminates followed by the internal delamination. They also found that

the energy at which the delamination starts increased with the increase in laminate

thickness.

2.5.4. Effect of thermal conditioning on glass composite failure

The exposure to severe thermal conditions of the environment and the effect of

thermal shock on the damage behaviour of glass composites were characterised by

Ray [2005]. The glass-polyester and glass-epoxy woven composites were treated by

varying the holding durations and by altering the number of cycles of high and low

temperatures. It was found that in comparison to glass-polyester, glass-epoxy

composites showed more resistance to thermal shocks because of more cross-linking

and greater adhesion properties. Moreover, improvement was found in inter-laminar

shear strength values with exposure to short holding times and fewer thermal fatigue

cycles. The reason suggested was an improvement in adhesion at the fibre-matrix

interface as an outcome of the surface chemistry mechanism and the post-curing

effect. However, interfacial de-bonding, crack initiation, and reduction in shear

strength values were observed with increasing exposure time to higher and lower

temperature extreme conditions and also with increasing number of cycles. This was

because of the increased residual stresses generated as a result of the difference in

thermal coefficients between the fibre and resin. This was a consequence of the

weakening of the interface and the delamination.

2.5.5. Effect of hygro-thermal exposure on glass composites

Jana and Bhunia [2008] examined the influence of environmental conditions such as

humidity and elevated temperature on the properties of glass composites. S2

Glass/SC-15 epoxy composite was exposed to hygro-thermal ageing conditions and

it was found that the matrix was affected and deteriorated. Inter-laminar shear stress

(ILSS) and delamination damage tolerance (DDT) were used as the tools for

evaluation and DDT was taken as the measure of stress on the onset of delamination.

It was observed that both ILSS and DDT reduced with the increasing exposure cycles

30

of humidity and temperature. It was suggested that hygro-thermal ageing caused

leaching of soluble degradation products which was also indicated by Gu and

Hongxia [2008] and there was a loss of weight. The matrix degradation weakened

the bond between the fibre and matrix and ultimately the failure occurred. The modes

of failure after the hygro-thermal ageing which resulted in delamination were matrix

cracking, fibre breakage to a certain extent and fibre matrix de-bonding.

Studies by Haque and Hossain [2003] also revealed that moisture absorption caused

hydrolysis and leaching effects resulting in diffusion of water into the matrix

materials. They observed micro-structural damage like fibre de-bonding and matrix

cracking due to swelling of the polymer matrix. They also observed that mechanical

properties deteriorated at elevated temperature beyond the glass transition

temperature which was probably due to the increased visco-elastic nature of the

resin. Their study showed that the degradation in strength at elevated temperatures

was more severe than that resulting from moisture absorption.

2.5.6. Effect of water absorption

The effect of water absorption on glass/polyester composites was investigated by Gu

and Hongxia [2008]. They combined two layers of E-glass plain woven fabric with

unsaturated polyester by using the vacuum resin infusion technique. Deterioration of

the composite matrix, reinforcing material, and interface was observed after

prolonged exposure to water (over 21 days) and the peeling strength was decreased.

The reason suggested by the researchers was the dissolution of some matrix elements

with water which percolate out and resulted in weight loss. However, peeling

strength seemed to increase with the exposure to water for 1-14 days. It was assumed

that during a short exposure, water molecules covered the voids of the matrix and

acted as a plasticiser and hence, an increase in weight was also observed. Moreover,

the hydroxyl group developed between the fibres and the matrix provided resistance

to the peeling action.

2.6. Through-the-thickness reinforcement

This project is concerned with improving the lamination strength between the fabric

and resin by modifying the individual fabric surface with the help of the air-jet

texturing process. However, to increase the delamination resistance of composite

31

structures, a common approach is through-the-thickness reinforcement. Growth of

delamination is restricted by bridging the cracks through stitching the laminate layers

in the thickness direction. The Z-fibre pinning process is also an attempt in which

transverse reinforcement is achieved, in the form of small diameter pins. A brief

account of these techniques with their merits and demerits is stated below.

2.6.1. Through-the-thickness stitching

The stitching process consists of sewing a high strength yarn, usually made of

carbon, aramid or glass, through the fabric composite preforms as shown in Figure

2.4. This process, in spite of having a number of advantages in terms of increasing

the laminate strength and resistance to delamination, also causes degradation of the

in-plane mechanical performance. Some of the critical factors are as follows:

Figure 2.4 Schematic diagram of the stitched preform [Nie et al 2008]

Improvement in impact damage resistance through stitching is sensitive to the type of

yarn used for stitching and also to the type and density of the stitching. According to

Kang and Lee [1994], chain stitching caused reduction of in-plane tensile strength

and modulus of S-2 glass/polyester composites with increasing stitching density of

Kevlar fibre. The reason suggested was the damage of some of the reinforcement

fibres during the penetration of the sewing needles.

Velmurugan and Solaimurugan [2007] introduced a number of modifications to the

stitching process of glass/polyester composites stitched with Kevlar. They used

manual plain stitching in place of chain or lock stitch in order to reduce fibre damage

Stitching Yarn

32

during the stitching process. The selection of plain stitch was also to avoid the

formation of thread cross and resin-rich pockets as in the case with lock stitch.

Moreover, instead of using twisted yarns, they utilised untwisted fibre roving and the

reason suggested was the uniform distribution of fibres in the stitches which

consequently increased the absorption of energy. The twisted fibre yarns in contrast,

acted as a whole and resulted in single step de-bonding. With the above

modifications, improved tensile, shear and impact strengths were achieved.

An examination of E-glass plain woven preforms and composites stitched with

Kevlar, using scanning electron microscopy, was carried out by Mouritz [2004] to

identify micro structural damage. Breakage of fibres by the stroke of the sewing

needle and distortion of woven fibres due to the sliding action of the sewing thread

was observed. It was also found that the surface of the preforms suffered from

crimping of the woven fibres as a result of pressing against the stitches which

became a source of distortion. Mouritz [2004] concluded that stitching caused

degradation of tensile fatigue properties in the form of early initiation and growth of

cracks, which happened as a result of crimping and distortion of load bearing fibres.

According to Nie et al [2008] the in-plane tensile strength of stitched composites is

sensitive to the stitch spacing. Small stitch spacing with a higher number of stitches

would effectively suppress the delamination and enhance the load bearing capability

of the composite. However, a higher number of stitches caused more fibre damage

and ultimately reduced the in-plane tensile strength. Nie et al [2008] found 5 mm to

be the optimum stitch spacing for composites of plain weave T300 1 K carbon fibres

with improved inter-laminar in-plane and tensile strengths.

Stitching is more helpful for providing resistance to the crack propagation through

fibre bridging rather than the crack initiation. According to Parlapalli et al [2007],

stitching is effective when the delamination length goes beyond 0.5L where, L is the

length of the specimen of glass/epoxy laminate composite stitched with Kevlar and

Twaron threads. The reason suggested was the possible reduction of composite

stiffness due to stitching. Above the 0.5L delamination length, the stitching started to

become effective.

33

Mouritz [2003] also indicated that improvement in delamination resistance occurred

when crack length grew above 15mm. According to Mouritz [2003], the stitch

bridging zone is not fully developed before the 15mm crack length. Moreover,

because of having very few stitches in a 15 mm length, an insignificant suppression

of the crack growth took place.

According to Yoshimura et al [2008], reinforcement of laminated composites by

using the through-the-thickness stitching technique seemed more promising with

larger impact energy. Yoshimura et al [2008] suggested that with a larger impact

energy level and a larger delamination area, there was an increase in the number of

stitched threads to be strained. The applied energy was then spent more for

increasing the strain energy of threads than spent on crack extensions. However, with

smaller delamination under the impact of low energy, because of the lower number

of available strained threads, the applied work was largely spent on crack growth.

It can be summarised that the in-plane properties of composites may be improved,

degraded or unaffected by the stitching process depending on a large number of

interacting factors. These include the type of laminate, the lamination technique,

stitching conditions i.e. stitch type, density, yarn diameter, orientation and also the

type of loading (Mouritz et al 1997). The major advantage of the stitching process is

that it improves the inter-laminar fracture resistance by resisting the crack growth as

it moves from stitch to stitch [Mouritz et al 1997, Yoshimura 2008, Velmurugan and

Solaimurugan 2007]. However, the drawback of localised damage zones around the

stitches due to needle action, misalignment of fibres by the stitches, formation of

resin rich areas due to spreading of fibres around the stitches and also weak interface

between the stitched yarns and matrix are reported as the major detrimental concerns

[Kang and Lee 1994, Mouritz et al (1996a, 1997b), Beier 2008].

2.6.2. Z-Fibre Pinning

Z-fibre pinning is an alternative technique to the stitching of composite laminates in

the Z-direction. Z-fibres are small diameter rods made up of carbon, titanium,

aluminium, stainless steel, glass etc embedded in resin. The diameter ranges from

0.15 to 1 mm. Insertion of the pins takes place through a specialised ultrasonic

34

insertion gun from a collapsible foam sandwich in which the Z-pins are held as

shown in Figure 2.5. Usually Z-pins are inserted into the prepregs before the resin

curing process [Cartie et al 2004, Partridge and Cartie 2005].

Figure 2.5 Schematic diagram of Z-pinning process Mouritz [2007]

Z-pinning is advantageous in improving the damage tolerance of the laminated

composites by offering resistance to delamination but it has limitations as well.

Zhang et al [2006] demonstrated that Z-pinning was quite effective for delaying the

delamination propagation rather than the damage initiation. The reason suggested

was the weak bond between the pins and the base laminate due to the presence of

resin pockets around the pins. Moreover, the pins were initially placed vertically to

the mode II crack plane and resist less the damage initiation. The pin traction force

increased with the change of angle of the pins during the crack growth and hence

reduction of the delamination area was achieved during the crack growth.

According to Zhang et al [2006], Z-pinning is more effective for thicker laminates

due to the difference in failure mode. For thinner laminates, the dominant failure

mode during transverse impact load is bending which causes matrix cracking.

However, delamination due to inter-laminar shear stresses took place in the thicker

laminates and the Z-pins were found to be helpful in arresting the delamination

cracks for propagation.

35

Allegri and Zhang [2007] stated that Z-fibres were beneficial for improving the

resistance to de-bonding and provided hindrance in delamination growth but the

diameter of the inserted pins was critical. According to them, increasing the pin

diameter would be helpful in increasing the frictional sliding shear and was

advantageous for the joint strength. However, at the same time, it had a detrimental

effect on the in-plane mechanical properties because of the local misalignment of the

in-plane laminates which increased by using the larger diameter pins. Mouritz [2007]

also indicated that the development of resin zones was associated with the amount

and the diameter of Z-pins. Isolation of resin zones from each other took place when

the pins were spaced wide apart. However, with closely spaced or large Z-pins,

continuous resin channels extending in the fibre direction would form which resulted

in decreasing the mechanical properties.

Another problem which is more prominent in Z-pinning is that Z-pinning causes

swelling of laminates. Chang et al [2006] as cited by Mouritz [2007] explained that

the problem of swelling was due to the spreading out of laminates to provide room

for the pins and also by the resistance of Z-pins against the compaction of prepreg

during curing. Swelling causes reduction of the fibre volume content and ultimately

deteriorates the mechanical properties. Stitching, however, raises the fibre volume

content by compacting the laminate preforms [Mouritz 2004a, 2007b].

2.7. Yarn texturing for increasing the bonding strength

The aim of this project is to increase the inter-laminar bond strength between woven

fabric of glass and resin, and between the neighbouring layers. Texturing increases

the bulk of glass yarns and this is expected to improve the adhesion between the

glass yarn and the resin and the resistance to delamination. Although a number of

techniques for producing textured filament yarns have been developed such as gear-

crimping, edge-crimping, stuffer-box, knit-de-knit, false twist and air-jet texturing,

the main techniques used are false twist, stuffer-box and air jet texturing processes.

The stuffer-box method caused buckling of the yarn in a wave form followed by the

heat setting in the crimped state. False twist is the process of twisting, setting and de-

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twisting thermoplastic filament yarns. Due to the setting, the deformation is

permanently set in the yarn [Hearle et al 2001].

However, for texturing of glass yarn, the false twist and stuffer-box processes are not

practically possible because of the stiff nature of the yarn. In addition, yarns textured

through these processes are very stretchy and only show the texture in the relaxed

state. Therefore, a purely mechanical texturing process by means of an air-jet was

considered the only option for texturing the glass yarn for composite reinforcements.

2.7.1. Air-jet texturing

Air-jet texturing does not require thermoplastic yarn as it works on a purely

mechanical basis. Textured yarns, having an appearance just like spun yarns, can be

produced from thermoplastic, cellulosic or nonorganic filament yarns by the action

of a highly turbulent, non-uniform, supersonic jet of air. Formation of loops takes

place on the surface of the filament yarn, giving it a voluminous character. The

feeding of the yarn leads the delivery or take-up process. A pressurised air jet causes

the filaments of the constituent yarn to texture and blend together as shown in Figure

2.6. The supply yarn is usually wetted by a wetting unit just before feeding into the

texturing nozzle. A wide range of filament yarns can be textured by the air-jet

process [Demir and Behery 1997].

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Where, L1 = The starting points of the separation of filaments inside the nozzle.

L2 = The starting points of the loop formation process.

L3 = The furthest point of the loops reached outside the nozzle.

Figure 2.6 Mechanism of air-jet texturing [Acar et al 2006] 2.7.1.1. Types of operations in air-jet texturing process There are three types of operations for producing a wide variety of textured yarns

namely

Single-end texturing

Parallel texturing

Core-and-effect texturing

In the single-end process, as the name suggests, a single end of yarn is introduced to

a nozzle with overfeed to produce the resultant yarn. In the parallel texturing process,

two or more yarns are usually fed to the nozzle for blending but have the same

amount of overfeed. The supply yarn may differ in terms of raw material, linear

densities, number of constituent filaments, etc. However, the versatility and

uniqueness of the air-jet process is found in the core-and-effect texturing process. In

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this process, one or more yarns are supplied to the nozzle with relatively lower

overfeed to form the core and the other group is fed at the same time to the nozzle at

a higher overfeed percentage to create the desired bulk and where relevant a

voluminous effect. For example, a wide variety of fancy yarns is produced through

the core-and-effect process [Demir and Behery 1997].

2.7.1.2. Texturing nozzles The nozzle is the most important component in the line of air-jet texturing and is the

heart of the process. Since the 1950s, lots of research work has been done to develop

an efficient air texturing nozzle and a number of different designs and shapes have

come into being. However, the purpose of the jet is always to create a supersonic,

turbulent and non-uniform flow to entangle filaments for creating loops and

producing textured yarn [Acar 1989].

Among the number of jets available in the market for producing a variety of textured

yarns, Taslan jets by Du Pont and Hemajet jets by Heberlein have made the most

significant commercial contribution to the field.

The first British patent [Du Pont 1952] and US patent [Du Pont 1957] was believed

to be the first process of air-jet texturing and was licensed under the brand name

Taslan by Du Pont as shown in Figure 2.7. A turbulent region was produced by

passing compressed air through a narrow space. The yarn was fed through the

turbulent zone and the formation of loops took place.

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Figure 2.7 First Air-Jet Process Taslan by Du Pont

According to Demir and Wray [1989], the early jets were developed and modified on

a trial and error basis and there was no understanding of using wet yarn. In the next

modification, as per Figure 2.8a, a venturi, (a short tube with a tapered construction

in the middle that causes an increase in the velocity of flow of a fluid) was used to

speed up the compressed air.

Moreover, the jet was modified by adding a baffle plate and by introducing a screw-

type air channel to produce a spin in the air (shown in Figure 2.8b).

In 1954, Du Pont introduced Taslan Type 9 (Figure 2.8c) as a further amendment of

the texturing nozzle and which stayed longer in the industry. A longitudinal airflow

channel with a venturi was used as a modification and a pre-twisted supplied yarn

was fed at an angle of 45 through a stepped, tubular needle [Du Pont 1954, Du Pont

1960].

The major drawback of this jet was the crucial setting of the needle which had to be

done by specially trained operators for a reasonable texturing effect through the

nozzle [Demir and Wray 1989]. Further developments by Du Pont in the field of jet

design came in the form of the Taslan 10 Jet (Figure 2.8d) patented in 1960 [Du Pont

40

1960]. The design concept was altered by using the straight (axial) path for yarn flow

and the air entered at a right angle to the yarn channel. The negative aspect of this

design was the uncontrollable acceleration of the air stream due to the straight exit

tube. The Taslan Type 11 nozzle [Du Pont 1970, Du Pont 1972] (Figure 2.8e) was

the modified version through which this defect was overcome by using a venturi type

channel configuration.

Several versions of the Taslan 11 Jet were also designed by modifying the

compressed air inlet into the turbulence chamber. An advanced development

appeared as Taslan 14 Jet [Du Pont 1976] with a baffle element as shown in Figure

2.8f to deflect the air-jet at the exit of the nozzle. Initially, flat plate-type impact

elements were used but cylindrical bars, conical elements and spherical bodies were

utilised later on [Wickramasinghe 2003].

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Figure 2.8 Taslan jets (a) Type 7 (b) Type 8 (c) Type 9 (d) Type 10 (e) Type 11 (f) Type 14

With all the previous Taslan Jets, the problem found was the difficulties of setting up

and also inconsistency of product variation among nozzles. This was claimed to be

overcome with the introduction of Taslan 20 Jet as shown in Figure 2.9 [Du Pont

1981].

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Figure 2.9 Taslan Type 20

The attractive fea