-
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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2
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
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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.
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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).
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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.
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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.
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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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16
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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17
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.
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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
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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,
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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.
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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].
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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].
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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
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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
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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.
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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
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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.
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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.
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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
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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
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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
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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.
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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
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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.
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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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36
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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37
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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38
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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39
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
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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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41
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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42
Figure 2.9 Taslan Type 20
The attractive fea