Fatigue of woven
thermoplastic composites
The effect of the fibre matrix interface
Fatigue of woven thermoplastic
composites
The effect of the fibre matrix interface
Proefschriftter verkrijging van de graad van doctor ann de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op donderdag 09 april 2015 om 15:00 uur
door
Shafqat Rasool
Master of Science in Materials Science & Engineering geboren te Faisalabad, Pakistan
Copromotor: Dr. ir. H.E.N. Bersee
Samenstelling promotiecommissie:
Rector Magnificus, Voorzitter
Prof. dr. ir. R. Benedictus, Technische Universiteit Delft, promotor Dr. ir. H.E.N. Bersee, Technische Universiteit Delft, copromotor Prof. dr. R. Curran, Technische Universiteit Delft
Prof. dr. J. Thomason, Stratclyd University, Glasgow, UK Prof. dr. W. Van Paepegem, University of Gent, Belgium Prof. dr. Conchur O Braidagh, University College Cork, Ireland
ISBN:
Copyright © 2015 by S. Rasool
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without the prior written permission of the author. Cover design: S. Rasool
Photography: F. Oostrum, S.Rasool
Summary
Due to continuous increase in the global energy demand and increase in the price of fossil fuels has shifted the global focus on the renewable alternatives to volatile fuels. Wind energy has emerged as one of the significant and promising contributor to world’s renewable energy share. A significant growth of wind energy production has been observed in last few decades and further increase in the wind energy production is foreseen in the future. The unavailability of land area for more and larger wind farms has shifted the global focus on off-shore installation of wind turbines. One of the key design trend is to increase the size of the rotor blade as the turbine power output is directly related to the length of the blade. The increase in the length of the rotor blade causes significant increase in the dead weight of the blade and challenges like larger tip deflections and high bending moments are expected to be encountered once these larger turbines are operational. Processing of these large structures and recyclability after the blade useful life are also major concerns. The current materials are not expected to overcome these problems. Thermoplastic polymers and stiffer carbon fibre are potential solutions to these challenges. However development of new matrix and fibre systems is not sufficient, good fibre-matrix interfacial properties are important for obtaining good static and fatigue properties of the composite materials.
The major portion of the published work on the influence of the fibre to matrix interactions on the long term properties is limited to glass fibre and thermosetting resin systems. The limited amount of work on thermoplastic composites mostly covers the effect to fibre matrix adhesion on the properties and behaviour under static loading only. Furthermore, published research work at composite-level regarding the fibre matrix adhesion and its effects on composite’s fatigue properties is mostly limited to composites with unidirectional fibre, leaving the open wide scope for the fibre matrix interface research in the field of woven fabric composites. The research work is focused on the gaining the in-depth insight of the effect of fibre surface treatment on the tensile fatigue properties of woven fabric composite and characterization of damage and failure mechanisms as a result of difference in fibre matrix interfacial interactions in these composites.
The nature of fatigue phenomena is very complex in composite materials. This complexity rises in woven composites due to geometry of the reinforcement. In order to make understanding of fatigue behaviour of woven composite simple, first their behaviour is studied under static tensile loading. A non-contact surface strain measurement technique based on digital image correlation is employed to track the changes and transitions in the surface strains with the applied tensile load. A uniform distribution of surface stain fields is
obtained in composites with relatively good adhesion between fibre and the matrix. For composite with poor fibre matrix adhesion, local strain fields with high localized strains are obtained. The study of the strain field allows the identification and understanding of damage and failure modes in static as well as fatigue loading.
Samenvatting
Door de contante toename van de wereldwijde vraag naar energie en de prijsstijging van fossiele brandstoffen is focus op duurzame alternatieven voor vluchtige brandstoffen verschoven. Wind energie is veelbelovend en vormt een significant aandeel in de wereldwijde productie van duurzame energie. De afgelopen decennia is de productie van wind energie aanzienlijk gegroeid en voorspelt wordt dat het verder zal toenemen. Door het gebrek aan landoppervlak voor grote windmolenparken is de wereldwijde focus verschoven naar het offshore plaatsen van wind turbines. Een belangrijke ontwerp trend is het vergroten van de bladen wat direct gerelateerd is aan het uitgaande vermogen van de turbine. Het vergroten van het blad zorgt voor een significantie toename van het dode gewicht en uitdagingen zoals het doorbuigen van de uiteinden en hoge buig momenten zullen ontstaan. Het verwerken van deze grote constructies en de recyclebaarheid na de nuttige levensduur worden ook gezien als grote belemmeringen. Met de huidige materialen wordt niet verwacht dat deze problemen worden overwonnen. Thermoplastische polymeren en stijvere koolstofvezels zijn potentiele oplossingen voor deze uitdagingen. De ontwikkeling van nieuwe matrix en vezels systemen is echter niet voldoende, goede vezel-matrix eigenschappen zijn belangrijk voor het verkrijgen van goede statische en vermoeiings-eigenschappen van een composiet materiaal.
Het overgrote deel van het gepubliceerde werk over de invloed van de langtermijn eigenschappen van vezel-matrix interacties is gelimiteerd tot glasvezel en thermohardende hars systemen. De beperkte hoeveelheid werk over thermoplastische composieten gaat meestal alleen over het effect van de vezel-matrix-hechting met betrekking tot de eigenschappen en het gedrag onder statische belasting. Het gepubliceerde werk op composiet-niveau over vezel-matrix-hechtingen en de effecten op de vermoeiingseigenschappen is overigens meestal beperkt tot composieten met uni-directionele vezels, waardoor een groot gebied overblijft voor onderzoek naar vezel-matrix verbindingen op het gebied van geweven composieten. Het onderzoek is gefocust op het verkrijgen van een diepgaand inzicht in de effecten van vezel-oppervlak behandelingen op de vermoeiings-eigenschappen van geweven textiel composieten en de karakterisatie van schade en faalmechanismen als resultaat van verschillende vezel-matrix interacties in deze composieten.
Het fenomeen vermoeiing van composieten wordt als zeer complex beschouwd. De complexiteit neemt bij geweven composieten toe vanwege de geometrie. Om het vermoeiingsgedrag te begrijpen is eerst het gedrag onderzocht onder statische trekkracht. Een contact-loze oppervlakte meettechniek wordt gebruikt om de veranderdingen en
transities in de oppervlakte-rek in kaart te brengen onder de aangebrachte trekspanning. Er wordt een uniforme distributie van het oppervlakte-rek veld verkregen in composieten met relatief goede hechtingen tussen de vezels en matrix. Voor composieten met slechte vezel-matrix hechtingen worden lokale rekvelden met plaatselijk hoge rekken verkregen. Het bestuderen van het rek-veld zorgt ervoor dat de schade en faalmechanismen, in zowel statische als vermoeiingsbelasting, geïdentificeerd en begrepen kunnen worden.
Contents
Chapter 1
Introduction
...1
1.1 Rotor blade materials ... 4
1.1.1 Composites in wind energy ... 5
1.1.2 Thermoplastic composite in wind energy ... 5
1.1.3 Carbon fibre in wind energy ... 6
1.2 Focus on fatigue research on alternative blade materials ... 7
1.3 Fibre-matrix interfaces tailoring for optimal fatigue properties ... 9
1.4 Research objectives ... 10
1.5 Thesis outlines ... 10
Chapter 2 Fatigue in composites: An overview of literature
... 15
2.1 Introduction ... 15
2.2 Fatigue behaviour of unidirectional composites ... 16
2.3 Fatigue behaviour of woven fabric composites ... 17
2.3.1 Fatigue damage in cross ply composites ... 18
2.3.2 Fatigue damage in woven composites ... 19
2.4 Factors affecting the fatigue behaviour of composite materials ... 27
2.4.1 Effect of matrix properties: Thermosetting vs. thermoplastics ... 27
2.4.2 Effect of fibre ... 29
2.4.3 Effect of fibre matrix interfaces ... 31
2.4.3.1 Static behaviour ... 32
2.4.3.2 Fatigue behaviour ... 36
2.4.4 Effect of testing parameters ... 39
2.5.1 Linear SN formulations ... 42
2.5.2 Nonlinear SN formulations ... 44
2.5.3 Fatigue limit ... 47
2.6 Fatigue life prediction methodologies ... 48
2.6.1 Constant life diagram (Goodman’s diagram) ... 48
2.6.1.1 CLD formulations ... 49
2.6.1.2 Comparison of CLD formulations ... 50
2.6.2 Damage accumulation ... 51
2.6.2.1 Residual strength based models ... 51
2.6.2.2 Residual stiffness based models ... 53
2.6.3 Physics based approach ... 55
2.7 Scatter in fatigue-life data ... 57
2.7.1 Sources of scatter ... 57
2.7.2 Statistical aspects of scatter ... 59
Chapter 3 Fatigue of woven glass fibre composites: Effect of
matrix
... 73
3.1 Introduction ... 73
3.2 Experimental ... 74
3.2.1 Materials ... 74
3.2.2 Processing ... 75
3.2.3 Fibre volume fraction ... 76
3.2.4 Specimen preparation ... 77
3.2.5 Testing ... 78
3.3 Results and discussion ... 79
3.3.1 Static behaviour ... 79
3.3.2 Fatigue behaviour ... 86
Linear regression ... 87
Non-linear regression ... 92
3.3.2.2 Experimental vs. prediction ... 98
3.3.3 Comparison with selected data from wind energy fatigue data bases ... 104
3.4 Conclusions ... 109
Chapter 4 Effect of fibre matrix interfaces on static tensile
properties: Local strain behaviour
... 113
4.1 Introduction ... 113
4.2 Experimental ... 114
4.2.1 Materials and processing ... 114
4.2.2 Specimen preparation ... 115
4.2.3 Testing ... 116
4.2.3.1 Static tensile testing ... 116
4.2.3.2 Loading-unloading and reloading testing ... 116
4.2.3.3 Digital image correlation technique (for full field surface strain measurement) ... 117
4.2.3.4 Ultrasonic C-scan ... 120
4.2.3.5 Microscopy ... 120
4.3 Results & Discussions ... 120
4.3.1 Global static tensile behaviour ... 120
4.3.1.1 Stress-strain behaviour ... 120
4.3.1.2 Loading-unloading-reloading tests... 125
4.3.1.3 C-scan results ... 128
4.3.2 Local strain behaviour ... 129
4.3.2.1 Longitudinal local strain behaviour ... 129
4.3.2.2 Transverse local strain behaviour ... 148
4.3.2.3 Local shear strain behaviour ... 150
Chapter 5 On the scatter in fatigue strength- fatigue life data:
Effect of specimen geometry
... 157
5.1 Introduction ... 157
5.1.1 Choice of probability distribution ... 158
5.1.2 Theory of Probability Distribution ... 159
5.1.3 Scatter definition ... 161
5.1.4 Goodness of fit ... 161
5.1.5 Sample size ... 162
5.2 Experimental ... 162
5.3 Results & discussion ... 163
5.3.1 Procedure for application of probability distributions ... 164
5.3.2 Failure modes ... 173
5.4 Conclusions ... 177
Chapter 6 Effect of fibre matrix interfaces: Tensile fatigue
behaviour
... 181
6.1 Introduction ... 181
6.2 Experimental ... 182
6.3 Results & discussion ... 183
6.3.1 Round-I fatigue tests (low cycle fatigue) ... 183
6.3.1.1 Strain vs. fatigue cycles behaviour ... 183
Global strain behaviour ... 183
Local strain behaviour ... 184
6.3.1.2 Effect of peak fatigue stress and post fatigue analysis ... 192
6.3.2 Round II fatigue tests (high cycle fatigue) ... 201
6.3.3 Fractographic observations ... 206
6.4 Conclusions ... 216
Chapter 7 Conclusions and Recommendations
219
7.1 Matrix toughness and fatigue properties ... 220
7.2 Scatter in the fatigue-life data ... 222
7.3 Static and fatigue behaviour and fibre matrix interfacial properties ... 222
Chapter 1
Introduction
The global warming has emerged as one of the biggest environmental issue in last two decades. The global warming is caused by warming of earth caused by greenhouse gases such as carbon dioxide (CO2) and methane. The largest source of CO2 emission is from fossils
fuels like coal, natural gas and oil. The major use of fossil fuels is in heating, electricity, transportation and industrial sector. Electricity and heat generation account for about 2/3
rd of the total CO
2 emission (figure 1.1).
The current population of the word is over 7-billions and will increase to 9-billions by 2050 [Carl2 [2012]]. With continues population growth, there is an immense increase in the
demand of energy supply. Burning more and more fossils fuels to meet this demand will result in a drastic increase in the CO2 emission. Also these energy resources are limited and
they will reach the depletion in next centuries. Hence, there is a need to reduce the dependency on fossil fuels and increase drastically the productivity of electricity through renewable resources e.g. solar, geothermal, and wind energy.
Figure 1.1 Global CO2 emission from fossil fuel consumptions in 2013, IEA (international
Wind energy has emerged as one of the very prominent solutions to the global energy problem and environmental issues. According to IEA’s energy technology prospective (ETP) estimation, wind energy could contribute a 12% of CO2 emission reduction from the power
sector [Islam et al3 [2012]]. The possible contribution to reductions by different renewable
energy sources is shown in figure 1.2.
Figure 1.2 Contribution to CO2 emission reductions in various energy sectors [Islam et al3
[2013]]
In order to reduce the impact on environment and to further depletion of energy resources, focus is to increase the electricity productivity through wind power. Due to continuous increase in price of fossil fuels, and increase in efficiency of wind turbines in last decade, wind energy has become cost efficient. In order to achieve these objectives, more and more wind turbine are need to be installed which requires a large land area for very large wind farms. However the problem is being tackled by increasing the power output of a single wind turbine by increasing the size of the wind turbine since the power output of a wind turbine is proportional to it size. As illustrated in figure 1.3, the size of wind turbines increases steadily over last few decades and this trend is expected to continue in the future. Installation of these large turbine onshore creates the problems like noise pollution. Hence the focus of wind energy sector is to go off-shore for large wind turbines. In 2013, 418 new off-shore wind turbines in 13 wind farms in different countries of Europe has been installed with total capacity of 1567-MW which is 34% more that the installed capacity during year 2012. By the end of 2013, total 2080 wind turbine are installed and grid connected with a cumulative output of 6562-MW in 69 wind farms in eleven different countries of Europe. During 2014, the 12 offshore projects currently are under construction. Once completed, the installed capacity will increase further by 3-GW making total capacity in Europe to 9.4-GW. Average off-shore wind turbine size is 4-MW. The average offshore wind farm size was 485-MW in 2013, 78% more than the average wind farm size in year 2012. [4]
Figure 1.3 Increase in the size of wind turbine: past, present and future [source Gerrad Hassan]
The blades of the wind turbine is a very important part and account for the 10 to 14% of the total mass of the wind turbine and about 20 to 30% of total cost of the wind turbine [Ancona and McVeigh22 [2001]]. Currently, the blade length has reached 80 meters as in the Vestas’s
V164- 8.0 MW turbine with a rotor diameter of 164-meters [figure 1.4]. With the increase in wind turbine size, it is feared that current blade materials and technology is reaching their limit and alternative materials and technologies should be explored and investigated for rotor blade application.
Figure 1.4 Vestas’s First V164-8.0MW prototype wind turbine with 80-m blade installed at Danish National Centre for Large Wind Turbine in Osterild, Denmark [5]
1.1 Rotor blade materials
The blades of a wind turbine are exposed to the external wind and due aerodynamic design of blade the life forces are created which drives the rotor. The lift forces on the blade profile cause loads at right angle to the blade and cause the blade bending flap-wise.[Bronsted et al6 [2005]] These loads can be static due to the immense weight of rotor blade cause
gravitational forces acting on the turbine even the turbine is standstill or dynamic (fatigue) due to fluctuations in the wind speed. The increase in blade weight with the increase in blade length follows a power law as shown in figure 1.5. Therefore, the materials chosen for rotor blades must be stiff, strong, and light and must have very high resistance against the fluctuating loads. The density vs. stiffness diagram of all materials is shown in figure 1.6. Woods are looking very attractive as a rotor blade material due to their low density but are easy to rule-out due to their low stiffness which can cause a large blade deflections for very large rotor blades. Composites are considered as very attractive materials for large rotor blades due to their moderate density and moderate stiffness.
Figure 1.5 Overall rotor mass of several blade designs as the rotor radius and swept area increase with blade length [Fingersh et al24 [2006]]
Figure 1.6 Density vs. Stiffness diagram for all material classes [7]
1.1.1 Composites in wind energy
The most used material to manufacture the rotor blade of wind turbines is glass fibre reinforced plastics. The glass fibre possesses a density of 2.6-g/cm3 and moderate stiffness
ranging from 70 to 75-GPa. However, a new type of glass fibre (H-glass) is being introduced with a stiffness of 80-GPa [Brondsted and Nijssen23 [2013]]. The matrix materials used are
thermosetting polymer e.g., Polyesters, Epoxies and vinyl ester.
1.1.2 Thermoplastic composite in wind energy
Thermosetting composites are not fully recyclable. Most of the wind turbines installed few decades ago are approaching their end life (generally 20 years). Now the biggest challenge the wind industry is facing that how to handle this huge amount of waste. Currently, these blades are being dumped in the land which imposes a big environmental concern. Pyrolysis of blades is another method. On the application of heat, the matrix material is decomposed and evaporated and the glass fibre can be reused for other low performance application. However, this method requires a large amount of energy and the smoke and volatiles produced during the process will also have a huge negative impact on the environment. The wind turbines of present time and of the future with much larger rotor blade size made
from thermosetting materials will make the problem more serious after the end-of-life of these mega-structures. Thermoplastic materials which are considered as fully recyclable and is one of the answers to the blade material recyclability problem after end of blade life. Thermoplastic composites are processed by application of heat and subsequently to press to final shape. The thermoplastic part can be recycled upon reheating and pressing. This impose a serious limitation on the use of thermoplastic composites for the rotor blade since it is very difficult to manufacture such a large and thick structure through melt processing. Also the large amount of heat energy needed during melt processing of thermoplastic composites have a negative impact on the cost of the blade.
The most popular processing method for rotor blade in blade manufacturing is vacuum infusion which requires a resin viscosities (< 500 cps) for good wetting of fibres and impregnation [23]. On the contrary, the thermoplastic resins possess very high viscosities which make them unsuitable as a rotor blade materials. At Delft University of technology, a new resin system was developed for production of thermoplastic composites based on Anionically Polymerised Polyamide-6 (APA6) [Van Rijswijk13[2007], Teuwen 15 [2011]]. This
system allows the manufacturing of thick thermoplastic composite and is characterised by its low materials cost, short processing cycle and recyclability. The low viscosity of reactive APA6 mixture makes it possible to use state of the art method (vacuum infusion) for manufacturing of thick thermoplastic composites through in situ polymerization of APA6. At the end of service life, the mixing components (monomers: initiator, activator) of APA6 can be retrieved back upon heating that signifies the importance of APA6 composites as a fully recyclable blade material [Parlevleit16 [2010]]. The research efforts at Delft university
of technology in finding more suitable chemistry and developing experimental set-up for successful reactive processing was started by Van Rijswijk13 in 2003 which later was
continued by Teuwen15 [2011]. The goals of this on-going research is to have cost effective,
recyclable thick thermoplastic composites for the rotor blade application.
1.1.3 Carbon fibre in wind energy
Carbon fibre are highly stiff with low density reinforcing material for composites and have been used for decades in composites for aerospace industry. For large wind turbines, carbon fibre can be an attractive alternative to glass fibre for composites to be used for rotor blade. Large, stiffer and lighter blades are possible with carbon as a reinforcing material. Also with the increase in blade length, carbon fibre can reduce the tip deflection and tower with large diameter can be accommodated.
Replacement of glass fibre with the carbon will impose the challenge of processing and low fracture toughness and fibre alignment since compressive strength of carbon fibre is greatly
affected by the fibre misalignment. However, carbon fibre as a reinforcement at selective location in the blade has been used years ago by Vestas and Gamesa in their off-shore wind turbine. The carbon fibre was used in spar cap of blades of Vestas’s V90 3-MW and Gamesa’s G87-2.0-MW turbines [figure 1.7] Incorporation of carbon fibre in the spar caps of these blades reduced the weight by 20% as compared to the weight of blade fully made of glass fibre [10].
Figure 1.7 Carbon fibre use in the spar of Gamesa G87 and G90 blade design [Wood10
[2012]]
1.2 Focus on fatigue research on alternative
blade materials
Before the fatigue of composites for rotor blade is discussed, a brief introduction of general fatigue terminologies is given as shown in figure 1.7. The fatigue load is defined by the cyclic wave form mostly the sinusoidal and characteristics of fatigue wave form is given in figure
1.8(a). The fatigue test data is generally as a plot with fatigue life along the horizontal or x-axis and stress, strain or displacement along the vertical or Y-x-axis as shown in figure 1.8(b). The fatigue properties are generally characterized by the slope of the SN curve: the flatter is the better means superior fatigue properties and fatigue resistance over a fatigue-life curve with steeper slope. An account of other characteristics of SN curve; fatigue limit, scatter in fatigue-life, is given in chapter 2.
(a) Cyclic load characteristics (b) SN curve characteristics
Figure 1.8 General Fatigue terminology
Among the requirements of cost, recyclability and efficiency, the blade material should have excellent fatigue resistance. Since the blade is subjected to dynamic loads during the turbine operation due to gravitational changing loads due to rotor rotation and fluctuations in the wind. Among all the rotating and vibrating structures, blade of a wind turbine experiences the greatest number of the fatigue loading reversal during its useful life, figure [1.9]. A lot of attention has been paid to this subject during last decade. Intensive independent researcher programs have been carried out and comprehensive fatigue databases have been complied: SNL/DOE/MSU11 database in USA and Optdate12 database in Europe. A
comprehensive review of fatigue of rotor blade composites is given in Nijssen17 [2006].
Fatigue in composites in reference to wind turbine applications is addressed extensively by Kensehe20[1994], Mandell, and Sambroscky21 [1992].
During an intended blade life of 20-years, the blades experience between 108 to 109 fatigue
cycles [Nijssen17 [2006]]. Current blade materials are brittle in nature and have low fracture
toughness. The current life of blade can possibly be increased by using the materials with improved or higher fracture toughness. The thermoplastic matrices are very tough materials and possess relatively much higher fracture toughness as compared to their counterpart
thermosetting polymers. Therefore the fibre reinforced with thermoplastic resins is a potential alternative to brittle thermosetting composites as a blade material for the large turbine of the future.
Figure 1.9 Fatigue loading regimes for different structures
1.3 Fibre-matrix interfaces tailoring for optimal
fatigue properties
In order to make the rotor blade more cost-effective, the investigation on new resin systems and fibre is not sufficient. The interface between the resin and reinforcing fibre also plays an important role in defining the structural performance of composite materials. The role of interface is to transfer load between fibre and the matrix. The full potential of stiff and strong fibre can only be exploited through excellent load transfer between fibre and matrix brought by strong fibre-matrix interfacial adhesion [Van Rijswijk13 [2007]].
Research on thermoplastic composite has shown that the fatigue performance of thermoplastic composites is inferior to the fatigue resistance of rival material thermosetting epoxy composites. This inferiority in the fatigue properties of thermoplastic composites has been attributed to the poor fibre to matrix bond strength [Gamstedt and Talreja19 [1999]].
The interaction at fibre-matrix interfaces can be chemical or mechanical or a combination of chemical or mechanical interactions. In situ polymerised APA6 composites showed better fatigue properties as compared to melt processed HPA6 composites. The improved fatigue resistance of APA6 composites is due to improved adhesion between glass fibre and APA6
brought up by increased crystallinity due to anionic processing. However the fatigue properties of APA6 composites is still inferior to the epoxy composites. The superior fatigue properties of epoxy composites are due to excellent fibre matrix bonding due to chemical interaction at the fibre matrix interface [Van Rijswijk13 [2007]].
1.4 Research objectives
Since during this research work, a separate project on optimization of processing parameters and fibre matrix interface of APA6 composites was ongoing [Teuwen15 [2011]],
hence the melt processed PPS composite from TenCate is selected which has comparable mechanical properties to APA6 and the excellent bonding between glass fibre and PPS matrix. No prior knowledge of static and fatigue behaviour of TenCate 8-H stain weave E-glass fibre reinforced PPS is available.
Toray T300J 40B 3K Polyacrylonitrile (PAN) based, high strength, carbon fibre are woven by TenCate Advanced Composites into a 5-H stain weave. These fibres are already surface treated and sized by the carbon fibre manufacturer. Ten Cate applies a heat treatment to these woven fibres for further surface modification. A comprehensive work has been done by Carnevale18 [2014] in order to characterize the effect of surface modification through
heat treatment on TenCate materials. However no prior knowledge of effect of surface treatment on tensile fatigue behaviour these materials is available in open literature. The objectives of this research are listed as:
To gain the knowledge and understanding of the effect of matrix on fatigue properties and fatigue behaviour of glass fibre composites.
To gain the understanding and characterize the effect of fibre matrix interface interactions on static tensile and fatigue properties and fatigue behaviour of woven fabric reinforced PPS.
1.5 Thesis outlines
In order to attain these objectives the thesis is divided into 7 chapters.
First a review of fatigue in composites materials covering various aspects of fatigue is given in chapter 2. The fatigue behaviour and fatigue mechanisms of unidirectional (UD) and woven composites is discussed. An account of the effect of fibre matrix interface on static and fatigue behaviour of composites will be given.
Also a brief account of fatigue life prediction methodologies, fatigue limit will be given and issue of scatter in fatigue/life data will also be addressed.
An account of investigation of matrix type and matrix properties on static and tensile fatigue properties of glass fibre composites will be given in chapter 3
The effect of fibre matrix interfaces on tensile static properties and tensile static behaviour will be given in chapter 4.
The issue of scatter in fatigue data will be addressed in chapter 5 and effect of specimen geometry on fatigue scatter in carbon fibre composites will be given.
The investigation on effect of fibre matrix interfaces on tensile fatigue behaviour will be presented in chapter 6.
The thesis will be concluded in chapter 7 and recommendation for further research will be also be given in this chapter.
References
1. CO2 EMISSIONS FROM FUEL COMNUSTTION: HIGHLIGHTS, IEA (International Energy
Agency) statistics, 2013 Edition
2. Carl Haub, Fact sheet: World population trends 2012, Population reference Bureau, Washington DC, www.prb.org
3. Islam M..R.,.Mekhilef S., Saidur R.,(2013), Progress and recent trends of wind energy technology, Renewable and Sustainable Energy Reviews, 21(2013), 456–468. 4. The European offshore wind industry -key trends and statistics 2013, A report by the
European Wind Energy Association (EWEA), 2014
5. Industry news: Vestas 8-MW turbine begins operation, CompositesWorld posted on 2/3/2014, http://www.compositesworld.com/
6. Brondsted, P., Lilholt, H., Aage, L. Composite Materials for Wind Power Turbine Blades, Annual review of Materials Research, 2005, 35:505-538.
7. http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/stiffness-density/basic.html
8. Vestas Wind Systems A/S, Life cycle assessment of offshore and onshore sited wind power plants based on Vestas V90-3.0 MW turbines, June 2006.
9. Hayman, B., Wedel-Heinen, J., Brondsted, P., (2008), Materials Challenges in and Future Wind Energy, MRS BULLETIN • VOLUME 33, www.mrs.org/bulletin, Harnessing Materials for Energy
10. Karen wood, (2012), Wind turbine blades: Glass vs. carbon fiber, Composites Technology, CompositesWorld
11. Optidate fatigue database: http://www.wmc.eu/optimatblades_optidat.php
12. MSU/DOE fatigue data base: http://windpower.sandia.gov/other/973002upd0309.pdf 13. Van Rijswijk, K., (2007), Thermoplastic composite wind turbine rotor blades-Vacuum
infusion technology for anionic polyamide-6 composites. PhD dissertation, Delft University of technology, Delft, Delft
14. Joncas.S, (2010), Thermoplastic composite wind turbine rotor blades: An integrated design approach. PhD dissertation, Delft University of technology, Delft
15. Teuwen, J.J.E., (2011), Thermoplastic composite wind turbine rotor blades: Kinetics and processability. PhD dissertation, Delft University of technology, Delft
16. Parlevliet, P.P., (2010), Residual strain thermoplastic composites: an experimental approach. PhD dissertation, Delft University of technology, Delft
17. Nijssen, R.P.L., (2006), Fatigue Life Prediction and Strength Degradation of Wind Turbine rotor Blade Composites. PhD dissertation, Delft University of technology, Delft
18. P.Carnevale (2014), mechanical properties: chapter in fibre matrix interfaces in carbon fibre composites: a meso level approach; PhD thesis, Delft University of technology, Delft
19. Gamstedt, E. K., & Talreja, R. (1999). Fatigue damage mechanisms in unidirectional carbon-fibre-reinforced plastics. Journal of Materials Science, 34(11), 2535-2546 20. Kensche, C.W., ‘GFRP Fatigue Data for Certification’, proc. EWEA Conference and
Exhibition, 1994, pp. 738-742
21. Mandell, J. F., Reed, R.M., Samborsky, D.D., ‘Fatigue of Fiberglass Wind Turbine Blade Materials’, Sandia National Laboratory contractor report: AND92-7005, Montana State University, August 1992
22. Dan Ancona and Jim McVeigh, (2001), Wind Turbine – Materials and Manufacturing Fact Sheet, A report prepared for the Office of Industrial Technologies, US
Department of Energy, by Princeton Energy Resources International, LLC www.perihq.com/.../WindTurbine-MaterialsandManufacturing_FactSheet 23. Brøndsted, P., Nijssen, R.P.L, (2013), Advances in wind turbine blade design and
materials, Woodhead Pub.
24. Fingersh, L., Hand, M., and Laxson, 2006, A., Wind Turbine Design Cost and Scaling Model,” Technical report, NREL/TP-500-40566.
Chapter 2
Fatigue in composites:
An overview of
literature
2.1 Introduction
Fatigue mechanism in homogenous materials (such as metal) is generally characterized by the initiation and propagation of a single crack. In contrast, the fatigue behaviour in composite materials is more complex due to their heterogeneous nature and is often characterized by damage mechanisms like; matrix cracking, fibre breakage, fibre matrix debonding and delamination. A single damage mechanism or combination of two or more damage mechanisms is often responsible for the composite failure in fatigue. The nature of damage event, their sequence and interaction depend on the fibre type, matrix properties, fibre matrix interfacial properties, architecture of reinforcement, laminate layup and loading and testing conditions [Gamested and Talreja 51[1999], Harris62 [2002]].
In this chapter, static and fatigue damage mechanisms, their sequence and interactions, and various factors affecting the static and fatigue life, damage and failure behaviour of continuous fibre composites is described. Furthermore a comprehensive account of fatigue life prediction methodologies for composites materials especially in relevance to fatigue of composites for wind turbine rotor blade is presented. The issue of scatter in static strength and fatigue life data of composite materials is also addressed.
Since fatigue in woven composite is a complex phenomenon and the intension here is to commence the discussion with the simplest (yet fairly complex) case of tension-tension fatigue damage in unidirectional fibre composites under axial loading. Many studies have shown that the fatigue life of multidirectional and woven fibre composites under tension-tension loading is controlled by the fibres in the 0-degree direction.
2.2 Fatigue behaviour of unidirectional
composites
Unidirectional composites possess the strongest properties in the longitudinal direction with their transverse properties being comparatively less. Static and fatigue behaviour of unidirectional composites have been studied for many decades and is well understood. Gamstedt and Talreja51 [1999] characterized the tensile fatigue of unidirectional composites
with three different regions corresponding to different failure mechanisms or interaction of mechanisms as illustrated in figure 2.1. The first region is the horizontal scatter band which coincides with the scatter band of the static failure and is characterized by a non-progressive nature and fibre fracture. The second region is of a progressive nature with a slope band of scatter. The main competing damage mechanisms in this region are fibre bridged matrix cracking or fibre-matrix de-bonding depending on the properties of the composites constituents [see section 2.4]. The fibre bridging effectively shields the matrix crack tip by cohesive traction and no progressive fibre breaks occur. This indicates that bridges fibres stay intact until the final fatigue failure. The region III is at low strain levels and is characterized by no failure or very slow damage progression rates caused by the crack arrest mechanism.
Figure 2.1 Fatigue life schematic of a unidirectional composite loaded in tension-tension fatigue parallel to fibres (Reconstructed from Gamestedt and Talreja51 [1999])
Three competing fatigue damage mechanisms for unidirectional composites under tension-tension fatigue loading are presented schematically in figure 2.2. The fibre break distribution under fatigue is dependent upon the variability in the tensile strength among the fibres and along the fibre length [Gamstedt and Ostlund53 [2001]]. If the variability in
the strength of fibres is small, a planar fibre break pattern will appear from a single fibre break. If fibre failure strain is lower than the strain needed for matrix cracking and onset of fibre matrix de-bonding, the composite will fail only by successive fibre fracture as illustrated in figure 2.2(a) and the slope of the second region will tend to flatten. If the matrix yield strain is higher than the fibre failure strain, the matrix crack can originate from a single fibre break and when a crack bridging mechanism occurs provided that the interfacial properties at fibre matrix interface are favourable as shown in figure 2.2(b). In case the distribution in fibre strength is wide, then fibre break locations will be random and matrix shear cracks can form and join together giving a non-planar brush fracture failure on final failure figure 2.2(c).
(a) (b) (c)
Figure 2.2 Different fatigue damage mechanisms for unidirectional composites subjected to uni-axial tension-tension fatigue loading: a) fibre fracture, b) fibre fracture and crack bridging, and c) fibre fracture and delamination (Adopted from Greenhalg58 [2009])
2.3 Fatigue behaviour of woven fabric
composites
Unidirectional fibre composites are characterized by their excellent in-plane specific strength and in-plane specific stiffness and are vastly used in industrial applications ranging from automotive, wind energy and aerospace. Despite their excellent properties, composites with unidirectional reinforcement are very difficult to manufacture due to difficulty in handling during processing. It is extremely difficult to keel the fibre aligned and in-place.
To solve this issue, textile reinforcements are widely used as an alternative to unidirectional reinforcement for structural applications of composite materials that offer great ease of handling during manufacturing and conform to complex structural shapes through thermoforming process. In addition, woven composites offer excellent toughness, impact resistance and dimensional stability.
The most commonly used textile architectures are knitted, braided, woven and non-woven fabrics. 2D woven fabrics are obtained by interlacing tow fibre tows through weaving. The tows running along the weaving direction are called warp and tows transverse to weaving direction are named as fill or weft. 2D weaves are biaxial (warp and weft) and orthogonal (0 and 90). The points of contact of warp and weft yarns or roving are generally named as weave cross-over, yarn undulations or yarn crimp locations. The waviness due to weave structure results in complex strain-strain local fields and results in reduction of some of the mechanical properties and generation of new failure modes or sequence of failure modes compared to failure in unidirectional composite laminates [Naik et al95, [2002]].
Fundamental types of weaves types are plain; twill and satin weave as illustrated in figure 2.3.
Figure 2.3 Different weave architectures of woven reinforcements (Kelkar et al76 [2006])
Before explaining the fatigue damage behaviour of woven fabric composites, a short review of fatigue behaviour of cross ply composites is given here.
2.3.1 Fatigue damage in cross ply composites
Damage mechanism in cross ply laminates under tensile static or fatigue loading has been investigated by several researchers [Sjögren et al126 [2000], Olighara et al104 [1995], and
formation of parallel cracks in other plies. The crack density increased with an increase in stress. In next stage, the crack already formed joined together [crack coupling] and very few new crack are formed. This stage of damage is termed as characterized damage stage (CDS) and is characterized by equilibrium or saturation of crack density, crack spacing and crack size. During this stage fibre-matrix debonding can also occur depending on the fibre-matrix interfacial strength. The fibre-matrix debonding can lead to delamination near the free edges and at the 0/90 ply interface. Some fibre-breaks take place during the second stage as a secondary damage. The fibre break is significant in the early stage of fatigue life where the weak fibres break. Fibre break take place close to final failure due to stress concentration from matrix crack and fibre-matrix debonding. The schematic of damage sequence is shown in figure 2.4.
(a) (b) (c) (d) (e)
Figure 2.4 schematic of fatigue damage in unidirectional cross ply composites subjected to tension-tension fatigue (reconstructed from Harris et al62 [2002]) a)matrix cracking, fibre
break, b)crack coupling, de-bonding, c)delamination, d) delamination growth, e) fracture
2.3.2 Fatigue damage in woven composites
In order to understand the fatigue behaviour of textile woven composites, a good understanding of their behaviour under static tensile loading is needed. Unlike cross ply composites, first damage can be in warp, weft or matrix rich regions depending on the fabric geometry, laminate lay-up, and fibre-matrix interface. Generally first failure takes place in the weft bundles at the tip of the elliptical cross-section of the weft yarn at the interlacing region. On further loading, the damage will propagate towards the mid-section of the interlacing or weave cross over. Secondary damage mechanisms are shear failure in warp bundles and transverse failure in weft yarn and will reduce the stiffness of the composite. After that matrix cracks in the matrix region can originate. Failure of warp fibres in a unit cell controls the failure of an entire composite laminate whereas in cross ply or
multidirectional UD composites the nature of damage is progressive from ply to ply [Fujii et al50 [1993]].
The geometry of the fabric has a great influence on stress-strain and damage behaviour of the composite laminates under static loading. Osada et al106 [2003] compared the static
tensile behaviour of plain weave with 4-H satin weave for glass fibre/polyester composites. They observed the damage initiation as transverse cracks in the weft bundle at weave cross over points which were reflected as the first knee point on the stress-strain curve as shown in figure 2.5. Plain weave composites showed the knee point at relatively much lower strains when compared with the stress-strain behaviour of satin weave composites.
Figure 2.5 stress-strain comparison of single ply plain and satin weave composites (reconstructed from Osada et al106 [2003])
They explained that the weave parameters change with the change weave geometry and has an influence on the static stress-strain behaviour of woven composites. The main weave parameters are crimp ratio caused by the interlacing of the weft and warp yarns and aspect ratio of the yarn bundle as explained in figure 2.6.
They explained this behaviour by the extension-bending effect at the weave cross over points as the warp fibre bundles tends to extend in the loading direction. Due to the higher crimp ratio and larger yarn bundle aspect ratio in plain weave fabric, local bending at weave cross over point is significant at low applied strain. While the crimp ratio of the satin weave fabric is too small to cause bending at the weave cross-over points. Higher onset strain for the transverse cracking for the satin weave composites than that of the plain weave composites was explained by the smaller stress concentration in the weft yarn of low aspect ratio as compared to plain weave for which the stress concentration in the weft yarn was relatively higher due to the larger aspect ratio. Despite the same weave density, the modulus and tensile strength for satin weave composites were found to be 1.5 times more than those for the plain weave composites and a 20% higher crack density was observed in plain weave
as compared to satin weave composite upon failure. In order to eliminate the effects of nesting and difference in fibre volume fraction, they performed the similar tests on single ply composites. Results similar to multi-plie composites were found for strength ratio of satin and plain weave. However, onset stress and strain for transverse cracking were lower than those of 6-plies composites.
Figure 2.6 Definition of weave parameters: crimp ratio and aspect ratio (Osada et al [2003])
Figure 2.7 Schematic of fatigue damage development in woven fabric composites under static tension (reconstructed from Alif and Carlsson5 [1997])
Alif and Carlsson5 [1997] studied the failure mechanisms of 5-H weave carbon fabric and
4-H weave glass fabric composites with epoxy resin under tension, compression, and shear by using edge replica technique. They identified the damage sequence as weft transverse cracking, warp-weft debonding, longitudinal split in weft yarn and warp failure as final failure as illustrated in the figure 2.7.
The mechanism of longitudinal splitting was observed in the weft yarn nested between two warp yarns as illustrated in the schematic 2.8. As the warp yarn tends to extend on the application of a tensile load, stresses in the z direction (along the thickness direction of the composite) can develop in the weft yarn nested between two curved warp yarns.
Longitudinal splitting occurred close to final failure at around 92% of ultimate static tensile strength.
Figure 2.8 Mechanism of development of longitudinal split in weft yarn (reconstructed from Alif and Carlsson5 [1997])
Main damage mechanisms in woven composites under tension-tension fatigue loading are matrix micro cracking, fibre break, fibre-matrix debonding, crack coupling. Matrix cracking occurs in weft yarns as the first damage mechanism observed under fatigue. As the fatigue process continued the transverse crack density increases in the weft bundles. The transverse crack density, spacing and size will attain equilibrium and is regarded as the characteristic damage state (CDS) similar to UD-cross ply laminates. The dominant damage mechanisms during second stage are shear failure in warp yarn, matrix cracking in the matrix rich or gap areas, delamination between weft and warp and delamination between woven plies. As the fatigue process continues, these damages grow, resulting in stress concentrations causing the warp fibre to fail and fracture of composite. Final failure can take place in weave cross over location or matrix rich pockets depending on the weave type and geometry [Naik et al95 2001]. The fatigue damage and failure mechanism is illustrated in figure 2.9.
Takemura et al33 [1994] explained the high cycle fatigue (at low applied stresses) damage
behaviour of plain weave carbon fibre reinforced epoxy composites as a four stage process. In the first stage up to a few hundred cycles, no damage was observed. Only a single matrix crack was detected in SEM observations, therefore, no drop in stiffness was observed in the earlier stage of fatigue life for high cycle fatigue tests. The next stage, from few hundred to a few hundred thousand cycles, was characterized by initiation, propagation, and saturation of transverse cracks in the weft bundles. A stiffness drop was observed as the crack density and crack size increased. Only a few cracks are expected to be formed at the end of this
stage due to the damage saturation at the weave cross-over points. Once a crack occurs in the weft bundles, the weft bundles can partially withstand the axially applied load as long as interfacial debonding is restricted. Meta-delamination can reduce the stiffness of the composite at this stage but no stiffness reduction was observed at this stage due to no or very little fibre breakage. The third stage was ranged from a few hundred thousand cycles to a few hundred cycles before failure. In the third stage, quasi or meta-delamination at interlacing points between warp and weft bundles progressed and delamination between warp bundles commenced and no massive fibre breakage occurred. Delamination growth and debonding redistributed the stress within the warp bundles but no stiffness drop was observed. The last stage was ranged from a few hundred cycles before failure until the ultimate failure. The warp fibres at the weave cross over points has delaminated from the weft bundles and breakage of warp fibres resulted in final fracture. At low cycle fatigue (high stresses), the specimens failed catastrophically without showing any drop in stiffness.
(a) (b) (c) (d) (e)
Figure 2.9 Schematic of fatigue damage development in plain weave composites under tension-tension fatigue (reconstructed from Naik et al95 [2001]), a) undamaged, b)
transverse crack in weft yarn, c) matrix cracks and longitudinal crack within warp fibres, d) warp-weft delamination (meta-delamination), e) fracture
According to Fujii et al50 [1993], from the end of first stage to the start of second stage, a
meta-CDS exits in woven composites. The meta-CDS was described as the damage state corresponds to weave cross over points only. The stiffness reduction at the beginning of fatigue life was caused by debonding and matrix cracking in weft bundles. Transverse cracking and meta-delamination was observed by Patel et al110 [2000] in the radiograph of
5-H satin weave epoxy composites subjected to tensile fatigue loading as shown in figure 2.10. Fujii et al50 [1993] showed from the experiments that the meta-delamination is a
function of the weave pitch of the fabric because the debonding and delamination in weft bundles occurred at all weave cross over points.
A comparative study on the fatigue damage behaviour of composites with UD cross-ply and 8-H satin weave was made by Schulte et al122 [1987] using the same fibre type and matrix
system under similar fatigue conditions. They observed a similar matrix cracking behaviour for both type of reinforcements. They also compared the stiffness degradation behaviour during the fatigue process as shown in figure 2.11. There was a striking similarity in damage response of both composites until 50% of the fatigue life. After that woven composites showed more rapid degradation in comparison to cross-ply composites. This behaviour was attributed to the local delamination at weave cross over points in woven composites which reduced the composite stiffness more rapidly.
Ferriera et al44,45 [1999a, 1999b] established a relationship among loss stiffness, decrease of
strength range and temperature rise for plain woven glass fibre-polypropylene. They showed that these three parameters were interdependent during the fatigue life of polypropylene composites.
Figure 2.10 Radiograph of 5-H satin weave composites showing transverse matrix cracks and metal- delamination after subjected to tension-tension fatigue (Patel et al [2000])
Freire et al49 [2005] investigated effect of lay-up on fatigue damage behaviour of plain woven
glass reinforced polyester composites. They found earlier onset delamination for non-symmetric layup than that of non-symmetric layup for the same applied stress. Symmetric distribution allows better accommodation of internal stresses and reduces the number of stress concentrations and delays formation of transverse cracks. Delamination was observed only in inner plies for symmetric layup whereas composites with non-symmetric layup showed delamination of both outer and inner plies with the outer ply rupturing before the
final failure of the composite. The number of cycles for delamination initiation was found to be directly related to the fatigue resistance.
Khan et al78 [1998] investigated the effect of layup on tension-tension fatigue behaviour of
plain weave carbon fibre reinforced polyester composites. They found that the delamination is a dominant mechanism for composites with off axis plies. However for a 0°/90° layup, matrix cracking is the dominant mechanism and delamination plays a secondary role. Pandita et al107 [2001] observed that on axis fatigue properties were insensitive to testing
frequency. They found similar fatigue lives for glass fabric epoxy composites tested at 3-Hz and 10-Hz though the specimen temperature raised by 10°C at higher frequency. However, off-axis properties were found to very much sensitive to testing frequency.
Reis et al115 [2009] investigated the effect of stress ratio on fatigue behaviour of woven
carbon fibre epoxy composites. The stiffness drop was found to be similar for a fatigue life of 1- million cycles for all R-values at all stress amplitudes. For negative R-values, a drastic decay of fatigue strength range is observed due to the lower compressive strength of woven carbon fibre epoxy composite than the tensile strength.
Figure 2.11 comparison of residual stiffness behaviour with fatigue cycles of 8-H satin weave composites with UD-cross ply using similar fibre and matrix type (Reconstructed
from Schulte et al122 [1987])
Ectermeyer et al43 [1994] proposed a concept of critical fatigue modulus based on
experimental results. The tensile stiffness dropped above static linear range slowly and linearly with the log of fatigue life. Within the static linear range, there is a delayed initiation
Fatigue life (%) Stif fness redu ction, E/E 0 Matrix cracking Meta-delamination 8-H satin weave fabric
UD-cross ply 1.0 60 80 100 40 20 0.9 0.8 0.7
of stiffness drop and the stiffness drop became gradual and levelled out at high cycles. Once the stiffness was dropped to a critical level, beyond that the stiffness drop is rapid. They observed critical fatigue modulus at 25% stiffness drop for woven glass fibre combined with chopped strands reinforced polyester and phenolic composites. Comparing the fatigue life curve of both composites, it was found that the matrix had some influence on low cycle fatigue (high stress amplitudes) but at lower stresses fatigue properties were fibre dominant.
Takemura et al33 [1994] found that fatigue endurance limit is independent of the matrix
properties but is a function of fibre properties by comparing fatigue limit data of UD cross ply carbon fibre epoxy, plain weave carbon fibre epoxy and plain weave carbon fibre polyester composites for similar fibre volume fraction. They also found that the residual strength of woven carbon fibre reinforced epoxy composites was as high as their static tensile strength after 1-million cycles.
Curtis el al28 [1987] made an intensive investigation on the comparison of the fatigue
behaviour of woven and non-woven fibre composites under axial reverse loading. They studied the square symmetric [0,90,90,0]s ,two quasi-isotropic layups: [±45,90,0]s and
[0,90,45±]s and an angle ply layup [±45°,0°2]s for both woven and non-woven composites.
They found that that static tensile strength was higher than the static compressive strength for square symmetric and quasi-isotropic layup [0,90,45±]s. However for the quasi-isotropic
layup [±45,90,0]s, compressive strength was higher than static tensile strength. Static
properties of non-woven were found to be higher than those of woven composites. For square symmetric lay-up, non-woven composites outperformed the woven composites at all applied stress levels under axial tension-compression fatigue cycling. However for both quasi-isotropic layups, the fatigue performance of woven and non-woven composites was similar.
Nishikawa et al100 [2006] investigated fatigue crack constraint effect by using tow spread
effect. The fatigue lives of composites with spreaded tows were longer than those of conventional woven fabric carbon fibre reinforced epoxy composites. Fatigue crack initiation and growth were constrained by the tow spread. Tow spread reduced the stress concentration in the weft yarn at weave cross over point due to a small yarn bundle aspect ratio and low crimp ratio and hence better fatigue properties in comparison to woven composites with less tow spread.
Houshyar et al67 [2004] investigated the effect of fabric weave geometry on the static and
dynamic mechanical performance of glass fibre reinforced PP and PPE matrices. They found the strong influence of weave geometry on static tensile, flexural, storage modulus and glass
transition temperature. They reported the best performance by composites with 5-H satin weave fabric geometry when compared composites with plain, twill and basket weave geometries. Similar results were reported by Mariatti et al157 [2001] for the flexural
properties of woven composites. They also noticed that the resin flow within the tows was better for 5-H satin weave than for plain weave composites due to fewer cross over points and longer flow length. Hence, good impregnation also contributed to the better mechanical properties of satin weave composites over plain weave composites.
2.4 Factors affecting the fatigue behaviour of
composite materials
Fatigue properties and damage behaviour of composite materials can be greatly affected by the properties of constituents like; matrix toughness, fibre stiffness and failure strain, and properties of interphase region. Other factors that could have an influence on the fatigue behaviour of composites materials include: fibre orientation, fibre volume fraction, fatigue loading parameters, specimen size and geometry, and hot and wet condition.
2.4.1 Effect of matrix properties:
Thermosetting vs.
thermoplastics
Thermoplastic composites possess comparable basic mechanical (strength and stiffness) properties and superior fracture toughness compared to epoxies. Thermoplastic composites with this possibility present themselves as a potential alternative to thermosetting composites. One of the major drawbacks of thermoplastics is their higher viscosities compared to thermosets resulting in poor impregnation which makes it difficult to manufacture complex large structures via melt processing. At Delft University, researchers have made efforts in solving this issue by vacuum infusion of anionic polymerization of PA6. The main characteristics of this method are low cost, shorter production times and processing of thick composites [Van Rijswijk137 [2007]]. In addition to
recyclability, ease of joining and toughness are other potential advantages over epoxies. Donaldson41 [1985] compared fracture toughness properties of UD-carbon fibre-epoxy with
UD-carbon fibre-PEEK (polyetherehterketone) under mode-I and mode-II loading. He found that PEEK-carbon composites showed 14-times higher GIC values and 9.7-times GIIC values
than those for epoxy-carbon fibre composites. Makekawa et al89 [1994] performed tensile
with carbon fibre-PEEK. They performed the reliability evaluation using three parameter Weibull distribution to fit the static data. They found that the design allowables of carbon fibre-PEEK laminates are higher than those of carbon fibre-epoxy laminates for all stacking sequences. Dorey et al42 [1985] compared impact and residual properties after impact
between carbon fibre-epoxy and carbon fibre-PEEK composites. They reported that a high matrix toughness causes less extensive damage in carbon fibre-PEEK composites and significantly higher values of residual compressive strength over carbon/epoxy composites. Results similar to findings of Makekawa et al and Dorey et al were also reported by Sarah120
[1985].
From the outstanding performance of thermoplastic composites under static conditions, it is expected that their inherent toughness will also reflect into their superior behaviour over thermosetting composites when subjected to dynamic conditions. Gamstedt et al51 [1999]
showed that the tensile static properties of UD carbon fibre-PEEK were superior to those of UD carbon fibre-epoxy while the epoxy composite performed better under tension-tension fatigue. According to them, the difference in failure mechanism was responsible for this trend. Small localized fibre bridged cracks were observed in epoxy composites whereas fatigue damage of PEEK composite was characterized as more extensive and distributed damage with progressive fibre breakage. Static tensile and fatigue behaviour of cross-ply carbon fibre-epoxy and carbon fibre-PEEK was investigated by HENAFF-GARDIN et al64
[1992]. They found the similar static tensile properties for two materials but the fatigue lives of C-PEEK were significantly shorter than epoxy composite. Dickson et al40 [1985] showed
that fatigue properties of cross-ply C-epoxy and C-PEEK were similar but for angle ply lay-up (±45), PEEK composites performed better as compared with epoxy composites. Spearing et al129 [1992] compared the static tensile and tension-tension fatigue behaviour and
properties of notched cross-ply carbon fibre epoxy and carbon fibre PEEK laminates. They found no difference in static and fatigue properties of notched composites although extent of damage and cracking was reduced due to the inherent toughness of PEEK matrix. They explained the benefits of matrix toughness were offset by thermal residual stresses. Buggy and Dillon20 [1991] investigated flexural fatigue of graphite epoxy and graphite PEEK
composites and noted the superior fatigue properties of epoxy composite for unidirectional and cross-ply laminates but for angle ply layup properties were similar. Caprino and D’Amore21 [1998] conducted four point bending fatigue tests on specimens from random
continuous glass fibre PP (polypropylene) laminate and made a comparison of experimental results similar data for thermosets based composites from literature. They noticed no effect of matrix ductility on fatigue sensitivity of the composites and attributed this trend to fibre constraint. Also Zhag and Hartwig152 [2002] found that the fatigue life of unidirectional
composites with brittle epoxy matrix was better than thermoplastic (PEEK) composites using same reinforcement. They explained that the formation and growth of matrix cracks
in an epoxy matrix contributed to energy dissipation under fatigue load which improved the fatigue behaviour.
Song and Otani128 [1998], investigated the fatigue process of satin woven carbon fabric with
two different matrices (epoxy and PEEK) and reported that the fatigue properties of PEEK composites was better than those of epoxy composites. They characterized the damage progression of epoxy composites as extensive transverse matrix cracking followed by delamination, where as in PEEK composites transverse cracks were hardly observed until fatigue failure.
Van Rijswijk137 [2007] compared the static and fatigue properties of epoxy based composites
with melt processed Polyamide-6 (PA-6) and vacuum infused anionically polymerised polyamide (APA-6) composites with glass fabric reinforcement He found that static properties of APA-6 composites were slightly better than epoxy composites and much better as compared to melt processed (PA-6) composites. Fatigue properties of APA-6 composites were much better than PA-6 composite for laminate processed at 180°C. He noted that at these processing conditions, higher degree of conversion and crystallinity were obtained which better fibre matrix boding as compared to PA-6 composites. Fatigue properties of APA-6 composites were found to be poorer as compared to epoxy composites. Despite the superior static behaviour of thermoplastic composites over thermosetting composites under static conditions, their toughness do not always translate into their superior fatigue performance. Dedicated research efforts are needed to fully understand the factors and the mechanisms which impede or offset the fracture toughness of thermoplastic composites under fatigue conditions. These important factors include fibre-matrix interface, processing parameters, degree of crystallinity, and thermal residual stresses.
2.4.2 Effect of fibre
Wonderly et al143 [2005] found that the fibre dominant static mechanical properties are
sensitive to fibre properties whereas no effect of fibre was observed on matrix dominant properties for vinyl ester reinforced with carbon or glass fibres. The ratio of composites tensile strength was found to be equal to the ratio of tensile strength of the fibres. In carbon fibre composites, the damage was more localized as compared to glass fibre composites. Alif and Carlsson5 [1997] compared the static mechanical behaviour of epoxy matrix
reinforced with 5-H satin weave carbon fabric and 4-H satin weave glass fabric with carbon composites having 69% fiber volume fraction while that of glass-epoxy composite was reported to be 56%. Failure strains for glass composites were higher both in tension and compression than those of carbon fibre composites. Tensile strength of carbon composites
