Design of Composite Structures Against Fatigue

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DESIGN

OF

COMPOSITE

STRUCTURES

AGAINST

FATIGUE

A p p l i c a t i o n s

to

W in d Turb ine B lades

E d i t e d

by

R

M

M a y e r

Warns

ISP*


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Design of Composite Structures Against Fatigue


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>is* ^

CU\

.

Design of Composite Structures Against

Fatigue

Applications to Wind Turbine Blades

Edited by

Rayner M Mayer

BSc, MSc, PhD, CEng, MIMechE

B U S


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First published 1996

This publication is copyright under the Berne Convention and the Inter

national Copyright Convention. All rights reserved. Apart from any fair

dealing for the purpose of private study, research, criticism, or review, as

permitted under the Copyright Designs and Patents Act 1988, no part may be

reproduced , stored in a retrieval system, or transmitted in any form or by any

means, electronic, electrical, chemical, mechanical, photocopying, recording

or otherwise, without the prior permission of the copyright owners. Unlicensed

multiple copying of this publication is illegal. Inquiries should be addressed

to:

The Managing Editor, Mechanical Engineering Publications Limited,

Northgate Avenue, Bury St Edm unds, Suffolk, IP32 6BW, UK

ISBN 0 85298 957 1

R M M ayer

A CIP catalogue record for this book is available from the British Library.

EUR 16687

The data here are provided in good faith, but neither the au thors , the original

providers oftheda ta, nor the sponsors, are able to accept responsibility for the

accuracy of any of the information included, or any of the consequences that

may arise from the use of the data or designs or constructions based on any of

the information supplied or m aterials described. The inclusion or omission ofa

particular material in no way implies anything about its performance with

respect to other materials.

Neither the publishers, the European Commission nor anyone acting on their

behalf are responsible for any statement made in this publication. Data,

discussion and conclusions developed by the Author are for information only

and are not intended for use without independen t substantiating investigation

on the part of the potential users. Opinions expressed are those of the Author

and are not necessarily those of the Institution of Mechanical Engineers, its

publishers, or sponsors.

Printed in Great Britain by

Antony Rowe Ltd, C hippenham , Wiltshire

The bulk ofthefunding for this work was provided under contract JOUR -007 1

with the European Commission (Joule programm e).


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Contents

A bou t the autho r ix

Scope of the boo k

Ho w to use this boo k

Preface xi

Acknowledgements xi i

Notation xiii

Units xiv

Chapter 1 Fatigue considerations 1

1.1 Introd uction 1

1.2 Design and m anufac ture 2

1.3 Fatigue conside rations 4

1.4 Stru ctura l design 6

1.5 M ateria ls and eva luation 10

1.6 D at a collection and analysis 11

1.7 No n-destruc tive evaluation (N D E) 12

1.8 Co nclus ions 13

Chapter 2 Properties of aligned fibres 15

2.1 Introd uction 15

2.2 M ateria ls 16

2.3 M echanical characterization 18

2.4 Significance for design 30

2.5 Con clusions 31

Chap ter 3 Influence of matrix and fabric 33

3.1 Introd uction 33

3.2 M aterials and testing 33

3.3 Static pro pertie s 36

3.4 Fatigue pro pertie s 38

3.5 Effect of m atrix 43

3.6 Effect of fabrics 44

3.7 Discussion 44

3.8 Co nclusion s 49


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vi Contents

Chapter 4 Influence of spectral loading 51

4.1 Introduction 51

4.2 The spectrum 51

4.3 Materials and testing 52

4.4 Constant amplitude data 55

4.5 Tests with WISPER/WISPERX 59

4.6 Discussion 61

4.7 Conclusions 63

Chapter5 Effects of environment 65

5.1 Introduction 65

5.2 Materials and specimens 65

5.3 Exposure to humidity 68

5.4 Hailstone simulation 70

5.5 Testing procedure 72

5.6 Results 74

5.7 Statistical evaluation 80

5.8 Conclusions and recomm endations 86

Chapter 6 Glass and hybrid

fibre

performance 89

6.1 Introduction 89

6.2 Materials and static properties 90

6.3 Fatigue of glass fibre laminates 94

6.4 Fatigue of hybrid fibre laminates 100

6.5 Fatigue of glass/carbon bolted joints 103

6.6 Conclusions 103

Chapter 7 Fatigue properties ofwoodcomposites 107

7.1 Introduction 107

7.2 Manufacture 107

7.3 Advantages 108

7.4 Constant amplitude data 110

7.5 Life predictions and the W ISPER X spectrum 115

7.6 Fatigue properties of alternative species 116

7.7 Effect of joint configuration on fatigue performance 118

7.8 Infra red condition monitoring of joints 119

7.9 Conclusions 121

Chapter 8 Benchmark tests 123


8.2 Material selection 123

8.3 Flexural testing 125

8.4 Tensile testing 126

8.5 Significance of static results 127

8.6 Fatigue testing 129


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Contents vii

8.7 Analysis of fatigue data 131

8.8 Recomm endations 131

Chapter 9 Comparison ofcouponand spar tests 133


9.2 Materials and manufacture 134

9.3 Material characterization 136

9.4 Fatigue testing of coupons 137

9.5 Methods of monitoring damage 138

9.6 Com ponent testing 141

9.7 Discussion 145

9.8 Conclusions 147

Chapter 10 Response ofbladeroots to high bending moments 149


10.2 Experim ental details 150

10.3 Pin-hole flange 151

10.4 Trum pet flange 156

10.5 T-bolt flange 165

10.6 Discussion 167

10.7 Conclusions 170

Chapter11 Influence of moisture on GFRP bolted joints 171


11.2 Effect of moisture 171

11.3 Materials and methods 173

11.4 Results and discussion 176


Chapter 12 Influence of complex loading on blade-root joints 181


12.2 Description of tests 182

12.3 Pin-hole flange 188

12.4 Rectangular blade-root 189


Chapter 13 Evaluation ofT-boltroot attachment 195


13.2 Load attachment principle, FEA-simulation 195

13.3 Tests on EN ER CO N rotor blades 198

13.4 Discussion 203


Chapter 14 Comparison of fatigue curves for glass composite

laminates 209


14.2 Linear regression analysis 210


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viii Contents

14.3 Reverse loading 213

14.4 Tensile loading 218

14.5 Com pressive loading 220

14.6 Flexural loading 220

14.7 Other analysis methods 222

14.8 Com paring new materials against the standard fatigue curve 222


Chapter 15 Recommendations for good-working practices, norms

and standards 227


15.2 Material selection and characterization 227

15.3 Design 229

15.4 Manufacture 229

15.5 Coupon testing 231

15.6 Structural testing 235

15.7 Type approval 236


Glossary 239

Index 243

Principal authors and addresses 247


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About the author

Rayner M. Mayer is the principal consultant with Sciotech, a company

involved with product innovation and development. He obtained his PhD at

Cambridge University and is a member of the Institution of Mechanical

Engineers and a chartered engineer. He is widely published in technical and

scientific journals.

The consultancy specializes in the application of fibre reinforced plastics to

load bearing components and structures for energy and transport applications.

Heisresponsible for the scientific and co-ordination of collaborative Europ ean

energy research and development programmes, such as Joule and Thermie.


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Scope of the Book

The first chapter outlines the process for designing with composite materials

and how fatigue impacts on design. This follows the engineering design

standard, BS 7000,A Guide to

Managing Product

Design.

The principal considerations are discussed in the next six chapters (Chapters

2-7),

starting with the fibre orientation and considering, in turn , the influence

of: the fibres; matrix; fabrics; spectral loading; moisture; impact; hybrid

fabrics; and wood veneers.

The various coupon test methods are then com pared by way ofabenchmark

test using the same m ater ial. The effect ofscale isconsidered in Chap ter9when

results from coupons and components are contrasted.

Aspects of structural testing of full-size components are described in

Chapters 9-12 . Both the methodology and the ability to monitor the damage is

discussed.

The final two chapters summarize the results from two viewpoints - that of

design and of good working practice.

How to use this Book

Information can be sought at various levels.

For those not acquainted with the mechanisms of the fatigue process,

Chapter

1

should be consulted. The design strategy is outlined, together with

how to assess the type of tests needed for materials and struc tures.

Specific design aspects are then considered in Chapters 2-9, starting with the

orientation of the reinforcement and type of matrix and ending with the

influence oftheenvironment. In Chapters

913

aspects of structural testing and

verification of the design are discussed.

Whilst design information is derived throughout the text, the coupon data is

collated and analysed in Chapter

14.

The standard curves can form the starting

point for design purposes. The recommendations in Chapter15form a design

check list; no doubt designers will add to this list as their experience grows. If

the text is to be used as a source of information then the index, list of con tents,

or summary tables in Chapter1should be consulted.

A glossary is provided to define the technical term s used within the text and

the industry. It is consistent with that adopted by the American ASM Inter

national Handbook Committee being the most authoritative source available.


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Preface

The use of com posite materials for load-bearing structures is gradually gaining

acceptance as knowledge is gained about their performance and durability.

The idea is intrinsically simple - to utilize two (or more) constituents, which

together have more attractive properties than the individual constituents.

In this book, the use of two types of composite materials is considered;

namely fibre reinforced plastics and, to a lesser extent, wood-com posites. The

question of how these materials can be successfully incorporated into primary

load-bearing structures is also addressed .

The study is applied to the inclusion of such materials into wind turbine

blades, which experience both static and dynamic (fatigue) loading. It is not

merely the loads that must be withstood, but also the effects of the environm ent

over prolonged periods of time.

This work describes a systematic approach by engineers from seven Euro

pean countries over a period of five years to elucidate the material and

structural response to fatigue loading. M ethods of detecting the accumulation

of damage over long periods of time a re also discussed.

The information presented in this book will assist in the design of com posite

structures against fatigue, though it will be necessary to characterize the

material combination and manufacturing process used at the detailed design

stage.

With its low environmental impact, the generation of electricity from

renewal energy sources will increasingly provide for the needs for the next

millenium. This study will allow, inter alia, the next generation of wind

turbines to be developed based on new designs of rotor b lades.

Whilst this book is self-contained, the accompanying volumes by the editor

(Design with Reinforced Plasticsand Design D ata for Reinforced Plastics,

(Chapm an & Hall) may be consulted if readers are not familiar with the design

process.

As this work

is

on-going,

we

would hope that our experience and those of our

readers could be incorporated in any future editions.

Ray ner M Mayer

Yateley 1995


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Acknowledgements

Funding support was provided by:

European Commission Directorate General XII - Science, Research,

Development; contract number JO UR -0071-D K

Danish Energy Agency

Department of Trade and Industry, UK

Jotun Polymer A/S, Norway

Norges Vassdrags - og Energiverk, Norway

NOVEM, Netherlands

Research Council of Norway

Renewable Energy Centre, NE L, UK

Swedish National Board for Industrial and Technical Development

Vlaamse Gemeenschap, Dept. Economie, Belgium

Werkgelegenheid en Binnenlandse Aangelegenheden, Belgium

and the Institutes and C ompanies to whom the authors are affiliated.

We wish to acknowledge the support and assistance of the following:

Giancarlo Caratti and Komninos Diamantaras, EC.DG XII

Ragnar Arvesen, Jotun Polymer, Norway

Geoffrey Dutton, Rutherford Appleton Laboratory, UK

Mark Hancock, Wind Energy Group Limited, UK

Monica Jong, ECN , N etherlands

W. Kurz, Aerodynamik Consult Company, Germany

David Mayer, Sciotech, UK

Jens-Peter Molly, DEWI, Germany

Hans Reiter, DLR, Germany

David Richmond, Flemings Industrial Fabrics, UK

B.A .J. Schaap, ECN, N etherlands


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Notation

Fabric

and

laminate notat ion

Process

SB -

sti tch bonding

FW

-

filament winding

W

-

weaving

I

-

inlaid

CSM

-

chopped s t rand

mat

Nota t ion numbers

-

alignment angle

or

plies

(numbers)

-

mass

in g/m

2

letters

-

me thod

of

fabrication

+

-

combination fabric

s

-

symmetrical

[p]

-

n u m b e r

of

plies

Exam ple 1 Comb ination fabric.

0/90

SB

(800)

+ CSM (100)

/ \ \

alignment proc ess m ass combination p rocess m ass

Example2 Biased fabric.

0

(567) / 90 (35)

W

/

\ \ w

alignment mass alignment mass process

Example 3 Plied laminate.

[45 W (250) / 0 (150) [4p]]s

/

\ \ / \

alignment process mass plies symmetrical


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Units

Properties

Modulus

(a) - (d)

Stress

Stress

Strain (g)

Poissons ratio

Barcol hardness

tensile

flexural

compressive

torsional

tensile

flexural

compressive

torsional

inter laminar shear

minimum (e)

maximum

mean

ampli tude

minimum/maximum (f)

Toughness ( impact s t rength)

Fib re fraction -

-

by

by

Water absorpt ion

Glass transition

volume

weight

tempera ture

Units

GPa

GPa

GPa

GPa

MPa

MPa

MPa

MPa

MPa

MPa

MPa

MPa

MPa

%

J/m or

%

%

%

C

Designator

Ef

E

c

G

,

Of

Oc

-

Omin

Oman

Ornean

Oall

R

V

B

n

J/m

2

Vf

w,

T

g

Notes

(a) If a stress-strain curve is non-linear then one can measure the initial slope

(tangent, Fig. 0.1) or the secant slope. For the latter, the strain range

should be defined if it is not taken between zero and the failure strain.

(b) Subscripts t, c, and f are used to designate tensile, compressive and flexura l

loading, respectively.

(c) Subscript f may also be used to designate

thefatigue

value of the modulus

once the loading has been specified, e.g. E

t f

.

(d) Parameters may be normalized by dividing their current value by their

initial

value.

For exam ple, moduli may be normalizedbydividingE

lf

by ,.


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Units

y^

//'

< >

y ^^r

Tangent / ^ j

1

S^ T

^ < ^

> ^ ,

Secant

|

1

1 |

, ^Linear limit

y

Strain()

Fig. 0.1 Determination ofthemodulus from the tangent or secant slope ofastress-strain curve

(

Time

Fig. 0.2 Definition of the param eters used to define an alternating stress of constant amplitude .


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Units

/Strain amplitude /Strain range

Time

Fig.

0.3

Definition

of

parameters used

to

define strain.

As

with


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CHAPTER 1

Fatigue Considerations

ft M.Mayer*

Composite materials provide an optimum design solution to fabricating structures that move or

rota te. The design principles are discussed with reference to fatigue loading. The selection and

manufacture of such materialsisoutlined, together with the various aspects of fatigue tha t have

been investigated.

1.1 Introduction

The composite m aterials discussed in this volume refer to two principal classes

of materials, namely polymers reinforced by either fibres or wood veneers . In

wood, the reinforcement

is

the cellulose microfibrils

in

the

cell

walls.

The

use of

such reinforcements produces a range of properties which cannot be sustained

by the matrixitself,such as stiffness, strength, and fatigue resistance (1)(2).

The reinforcement provides the mechanical strength, whereas the matrix

provides the means of transferring the load into and out of the reinforcement

and protecting it against the environment. However, the choice of such

materials is restricted by design requirements and consideration of shape,

processing ability, and cost.

In gene ral, the stiffness and strength of a composite increase with reinforce

ment fraction up to some limiting value. For

glass

reinforced plastic (GR P) the

reinforcement is typically 40-50 percent by volume, whilst for wood compo

sites it is in excess of 80 percent as the glue line has negligible volume.

Nevertheless, the material properties of these two groups are much closer to

one another than to metals since they are both composite materials whose

properties only arise as a result of lay-up.

Common properties of wood andfibrecomposites include: high strength and

stiffness to density ra tios; good environmental resistance; suitability for use in

the rapid manufacture of large structures. Moreover, moulding direct to final

shape facilitates assembly and minimizes wastage.

The materials and structures discussed in this publication are typical of

general applications in mechanical engineering in which composites are only

slowly penetrating. As these have not yet been investigated to any significant

exten t, designers have hitherto been very conservative in their designs.

Sciotech, UK.


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2 Design of Composite Structures Against Fatigue

Laminated wooden beams have been used for some time as structural

members for large-span trusses and roofs such as those in sport halls or

swimming pools. These comprise wooden planks, typically 12-50 mm thick,

glued together to give the appropriately shaped beam. Their loading is

primarily static in na ture . For complex shaped structures such as wind turbine

blades, wood veneers are used, typically 3-5 mm thick, and the loading is

primarily dynamic in character.

1.2 Design and manufacture

Structures can be designed on the basis of British Standard BS 7000, in which

the design is advanced in successive stages from conceptual through embodi-

ment to detailing and issuing instructions for manufacture (3). The design

sequence for proceeding to the em bodiment (or layout) stage is shown in Fig.

1.1 (4).

The na ture of composite m aterials

is

such that the selection of manufacturing

processes, materials, and properties are interrelated; choice of any one may

define the othertwo This reduces the design options by restricting the num ber

of possible m aterial combinations.

The fundamental design rule is to lay the reinforcement in the direction of

the principal stresses and to check tha t the stresses in the other directions are

such that the matrix has sufficient strength to withstand the loading. If the

matrix a lone cannot w ithstand the subsidiary stresses then the reinforcement

also needs to be laid in those directions.

As the design evolves, one m ust check for

fitness

of purpose ; if this cannot be

achieved, the structure, materials, or manufacturing process may need to be

reassessed. As the sequence is iterated in successively greater depth, it is

necessary to ensure that thereisno fundamental difficulty which would prevent

the design from being released for manufacture.

Manufacture

The design of composite structures involves the interrelation of structural

considerations, material properties, and processing. The processes listed in

Table 1.1 are capable of producing large structures, though some of these have

restrictions on the geom etrical shape (4).

The most common process, and one that is also the most labour intensive, is

that of

contact

m oulding, in which the dry reinforcement (fabric or veneer) is

laid up in a mould and is then impregnated with resin and consolidated. In

prepreg m oulding the reinforcement and resin are already intimately mixed

and prepared in sheet form by the material supplier.

Either fibres or fabric in the form of tape are used in thewinding technique

and these are passed through a resin ba th before being laid on to a mandrel at

the appropriate angle. They have a low labour content as automation is


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Conceptual design

Materials, processes

l

_-rzr

Shape,

form

Load path, joints

Resin, reinforcementtype andlay-up

\

Strength, stiffness

Mass

Durability

Environmental impact

L

Embodiment design

Fig. 1.1 The steps in establishing the embodiment from the conceptual design; dotted

lines indicate stop and think before iterating the loop. The process needs to be

repeated for the detailed design stage (4)

Tab le 1.1 Principal man ufacturing processes

for large structures

Contact moulding

Filament winding

Tape winding

Prepreg moulding

Resin transfer moulding


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possible; spars for wind turbines of up to 40 m in length have been manufac

tured by this method.

Resintransferm ouldinginvolves laying up the reinforcement in the form of

fabrics inside a mould, closing the mould, and injecting the catalyzed resin

mixture. Once the resin has hardened, the com ponent can be demoulded with

good surfaces all round.

It is essential that components and structures are evolved which can be

moulded or m anufactured directly to final shape . In this way no machining is

required and the outer (moulded) surface can act as an environmental barrie r.

Jointing

The principal methods of jointing are glueing and bolting, and careful attention

to design detail is required in order to ensure adequate load transfer between

components. A key rule is that joints between dissimilar materials (such as

composites and metals) should be located in areas of low stressing if at all

possible.

1.3 Fatigue considerations

The fundamental response of composite materials to fluctuating loading

(fatigue) is known (5) and has been characterized for some specific material

combinations like carbon fibres in an epoxy matrix for aerospace applications.

As with me tals, the load bearing capability decreases as the num ber of fatigue

cycles increases.

How ever, the way

in

which the dam age nucleates and grows

is

very different.

In metals, a crack (or cracks) is nucleated and damage increases by crack

growth. Owing to the anisotropic nature of com posites, one tends to

find

areas

of damage arising, which can grow and eventually lead to failure.

Following Reifsnider (5), it is believed that damage accumulates in three

stages of varying time length (Fig. 1.2).

During the initial load period (stage 1), there is generally a small drop in

stiffness associated with the formation of

some

damage. This

is

followed (stage

2) by a much longer time period in which the damage seems to increase linearly

with time and the stiffness falls very gradually. If the stressissufficiently high, a

third stage (3) is observed which is characterized by an ever increasing amount

of damage which ultimately leads to failure.

Substantiation

Fatigue can be investigated in various ways:

(a) increasing the size of the test piece from coupon through to com ponent;

(b) increasing the complexity of loading from constant amplitude through to

loading encountered in service;

(c) or a com bination of (a) and (b) (Fig. 1.3).


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^

' Residual strength

.Stiffness

/

-+-3

10' 10 10* IV

Cycles

IV

IV

F i g . 1.2 Th e three chara cter i s t i c s tagesofd a m a g eofc o m p o s i t e s(5)

- Stages

The ultimate goalis toapply the service loadingto the complete structure

(6). Such tests arecomplex and time consuming, but provide information

which could only otherwise be ob tained by evaluation in service.

The majority of tests arealwaysperformed on coupons for one or more of the

following reasons:

- low cost;

- ability to use standard test machines and fixtures;

- easeoftesting;

- prospectoftestingathigh frequencies circa 5 Hzorhigher becauseofthe

lower loads;

- rapid investigation of various materials and lay-ups;

- responseofa material to various typesofloading;

- effectsofenvironment;

- ability to obtain design allowable values for fatigue.

All this information is necessaryinorder to be able to designastructure against

fatigue.


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Increasing complexity

of specimen

Structures

Components

Specimens

Coupons

Basic

fatigue

data

Goal

Constant

amplitude

Variable

amplitude

Service

loads

-

Increasing

complexity

of applied

loads

Fig. 1.3 Possibilities of fatigue testing in terms of increasing comp lexity of loads and com ponents

(6)

The inverse relationship between the number oftests,their complexity, and

cost (called the pyramid of substantiation) (Fig. 1.4) was originally developed

for aircraft components (7).

A similar strategy has been adopted in the current investigations, that is a

large number of tests on coupons and a much smaller number on components

and structures.

1.4 Structural design

The design sequence for a load-bearing structure such as a rotor blade is

illustratedin Fig. 1.5.

The aerodynamic, structural, and manufacturing considerations are inter

twined. Aerodynam ics will set the shape envelope from which the loads can be


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Numberof tests

40 60 80

Relative cost (%)

100

Fig. 1.4 Substantiation of the design indicating the number of tests and the related costs for each

stage of testing of a composite component for Airbus Industrie (7)

Concept

Regulation

Blade number

Location

Terrain

Wind conditions

Load Calculations

Simulations

Static strength

analysis

zz

Fatigue

spectrum

Fatigue life

prediction

Estimated life

Fig. 1.5 The steps in establishing the ability of a rotor blade to withstand the imposed loadings


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8 Design of Com posite Structures Against Fatigue

calculated for a specific wind regime. The blade structure has to withstand the

imposed loads and transfer these loads to the shaft. The manufacturing process

then has to ensure the production of the desired shape and the location of the

reinforcement in appropriate directions.

Material selection is dependent on the manufacturing process, and this in

turn enables both static and fatigue properties to be determined. From these

data, fatigue lives can be predicted from the knowledge of the load spectrum

(Chapter 4). The structure has to withstand these loads with an adequate

margin of safety (10). If the loading is too high, then the design has to be

iterated.

There are a number of codes governing the general principles of structural

design, such as ISO 2394 and the Eurocodes (9 ) -( ll ). These are based on limit

state analysis in which the designer has to identify the ways in which a structure

fails to fulfil its function in terms of either ultimate loads or service loads.

The uncertainties in loads and materials are considered by using partial

safety coefficients. The value of these coefficients depends upon aspects such

as:

- material variability.

- whether the damage is progressive or catastrophic.

- whether the design is verified by testing.

- inspection in service.

- repairability.

- lifetime reliability target.

The methodsbywhich some of these aspects can be determ inedisillustrated in

subsequent chapters.

For wind turbines, the safety requirements have been set out in a new

international design code IEC 1400-1 (12). It specifies inter alia the limits for

the statistical analysis of the fatigue strength, and also makes allowance for

whether a structure fails in a safe manner.

Rotor

blade design

Owing to the diffuseness ofair,wind turbine blades need to be large in order to

capture any appreciable am ount of energy from wind. For example, a turbine

fitted with blades some 17 m in length would typically generate a maximum

power output of500kW.

For energy costs to be com petitive with o ther sources, it

is

essential to design

the blade so that: (a) its mass is effectively used and (b) blades can be

manufactured in a cost-effective manner (8).

In the spar/shell design (Fig. 1.6), the spar is designed to be the prime load

bearing member whilst the shell provides the aerodynamic shape and torsional

stiffness (13). The spar is generally wound by a tape winding process with the

filaments transverse to the length of tape (Fig. 9.2 in Chap ter9),the shell being


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1

Shell

Spar

Glass/polyester

Gelcoat

Fig. 1.6 Spar/shell design developed originally for the blades of the wind turbine erected at Nibe

(13)

Table 1.2 Evaluation of components

Component

Spar

Blade root

Flange type

Trumpet, pin-hole

Pin/hole

T-bolt joint

Flexhat

Trumpet

Purpose oftest

Manufacturing evaluation

Fatigue response

Design verification

Fatigue response

Design verification

Fatigue response

Description

(chapter)

9

10,12

10

13

12

10

made by contact moulding in a mould (4). The advantage of this design is that

each component can be separately optimized.

The stiffened shell

is

more commonly used with one or m ore stiffeners in the

form of webs (Fig. 7.2 in Chapter7and F ig. 13.3 in Chapter 13). As buckling

can be a problem with such large structures, structural foam is often used

between the skins of the shell to provide sufficient rigidity.

Bladeroot

The design of the blade root is complex as the torq ue from the blade has to be

transferred onto the shaft, usually via a flange. There is also a change of

ma terial, from composite to steel and, less comm only, to aluminium. Types in

common use include the trumpet, pin-hole, pre-stressed T-bolt (IKEA type),

and stud (14).

The tests shown in Table 1.2 cover both existing and improved blade root


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Table 1.3 Ma terials and processes investigated

Reinforcement

Glass

Aligned fibres at various

angles

Transverse filament tape

Combination fabrics

Woven and stitch

bonded fabrics

Balanced woven fabrics

Glass/carbon

Discrete layers of fabrics

Wood

Khaya

Poplar

Birch

Beech

Baltic pine

Process

Filament winding

Tape winding

Contact moulding

Prepreg moulding

Contact moulding-

Contact moulding

+

Vacuum bagging

Resin

Ortho-Polyester

Epoxy

Iso-polyester

Iso-polyester

Iso-NPG polyester

Vinyl ester

Epoxy

Iso-polyester

Epoxy

Iso-polyester

Epoxy

Description

(chapter)

2 ,

4

9

3, 4

3

3

3

6

5

6

7

connections, and were evolved to validate designs as well as their structural

response to load. These tests require large rigs, great care in introducing the

load (s), and the imposition of loads greater than the maximum design load, in

order to accelerate the test.

1.5 Materials and evaluation

Materials have been evaluated in the form of test plates manufactured by the

appropriate process. They are typical of those in use in general mechanical

engineering applications. F abric types have included w oven, inlaid, and stitch-

bonded (4) whilst fibre orientations varied from unidirectional to balanced

fabrics in the warp and weft directions. These are summarized in Table 1.3.

The following design aspects outlined below have been considered.

- Fibre alignment through manufacturing laminate plates by the filament

winding process in a laboratory . Consequently these values will set an upper

limit to what could be achieved in a factory environment (Chapter 2) .

-

Matrix

and fabric

construction

using various types of matrices with the same

type of reinforcem ent and various fabrics with the same resin. This provides

a link with other matrix systems such as vinyl ester and epoxy, whose

structural app lications a re more widely known (Chapter 3).

-

Hum idity andmoisture

as laminates can take up w ater under high humidity

and conversely give up water under very dry conditions. The amount of

water absorbed or desorbed has been measured and the effect on properties

determ ined. Since these measurements were made on thin coupons, they set


31/272

Fatigue Considerations 11

an upper limit to what could occur with structures where the laminates are

thicker and have fewer edges (C hapters 5 , 11).

-

Impact damage

of ice particles has been investigated using an 'ice gun'

as well as the combined effect of moisture penetration and damage

(Chapter 5).

- Spectral loading on rotor blades is generally evaluated using a load set

averaged over a number of different types of turbines. It is designated

WISPER. The relationship of this spectrum to a truncated spectrum

(WISPERX) and constant amplitude loading has also been investigated

(Chapter 4).

- Hybridfabricscomprise m ixtures ofglassand carbon fibres, as carbon fibres

are appreciably stiffer than glassfibres.Thus a combination of thesefibresn

a resin matrix could provide advantages in terms of obtaining the best

prope rties of bothfibres,yet at lower cost than that of carbon alone (Chapter

6).

- Boltedjoints.Bolting introduces compressive and shear stresses, which are

superimposed on the other loads seen by the blade rotor. This has been

studied for both glass and hybrid fabrics (Chapter 6).

- Wood composites.Various design aspects are considered including veneer

type,

jointing, and spectral loading (C hapter 7 ).

- Effect of

scale

on going from coupons to components has been investigated

for tape wound coupons and spars. Difficulties can arise with the quality of

manufacture, the degree of alignment of the fabric or veneer, the m ethod of

load introduction, and the method of testing (Chapter 9).

1.6 Data collection and analysis

There

is

still incomplete agreem ent for the static testing of composites and even

less for fatigue testing. Consequently, each institute has tended to develop its

own test methods, which will be internally consistent and

valid.

For

this

reason,

a benchmark test has been undertaken to validate the consistency of tensile and

flexural test methods (Chapter 8).

Many engineers represent fatigue results by normalizing a property by its

initial value; this may be stress, modulus or the number of cycles to failure.

Normalized data need to be converted to engineering units in order to establish

permissible values for design purposes. This approach has been developed by

Sims (15) (amongst othe rs) to cover various aspects of fatigue loading, testing,

and component geometry. In this work, strain

is

generally used . This approach

also facilitates comparison between data sets and helps to establish trends and

design principles.

The scatter in the data can be evaluated by statistical methods, and this is

discussed in Chapter 14. The confidence in the data and the probability of

survival are designated by limits, which are generally prescribed by design

codes (9)-(12). Following IEC 1400-1, these limits have been calculated for


32/272


Table 1.4 PrincipalNDEtechniques

Description

Technique Cause (chapter)

Visual Multiple 2-13

Stiffness change Overall change in stiffness 3-9

Strain gauges Local change in stiffness 9, 10, 12,13

Hysteresis in area under damping Change in load-rdeflection curve 2 ,3 ,7 ,9

Resonant frequency Overall change in vibration of component 9

Tem perature rise Local increase in temperature 9

Infra-red emission Local areas which contain damage 7, 9, 10, 13

95 percent survival probability and 95 percent confidence limits and are

given for relevant data sets. Other limits could be used depending upon the

design philosophy. This is discussed further in Chapter 15.

1.7 Non-destructive evaluation (NDE)

A variety of NDE techniques can be used to monitor the accumulation of

damage (Table 1.4). Of these ,

visual inspectionis

the most straightforward and

is also the most important. Cracks and damage areas a re readily visible as most

of the coupons and com ponents are translucent to transmitted light.

Localstrains

can be determined by strain gauges and this has been exten

sively used for all structural testing where damage generally occurs on a local

scale rather than a global scale (C hapter10).Some institutes also use these for

coupon tests to determ ine extension ra ther than using extensometers or cross-

head deflection of an actuator.

Stiffness

has been m onitored throughout the study. This enables the damage

stages to be identified (Fig. 1.2), and any deflection limit to be maintained. For

coupons, changes in the modulus can be determined from the stress-strain

curve, either during fatigue testing or by static loading.

The load-deflection curve which characterizes a material or structure can be

used to determine the hysteresis damping (Fig. 1.7). For composites the

internal dam ping is much higher than metals and is an indicator of dam age in

the material.

Resonant frequency and natural frequency are also two methods of

measuring the damping. They provide information at a global level ofthestate

of the material or structure at micro-level.

Damage induced in composites will generate heat and as these materials

have poor conductivity it should be generally easy to detect. Infra-red

emissions

can be detected with a suitable camera and will locate the areas of

damage and how these will grow. Thermocouples are simple to install and have

been placed in locations where high strains have been detected, possibly by a

strain survey (Chapter 9).


33/272

Fatigue Con sideratio ns 13

Average dynamic

modulus

Hysteresis

damping

Strain

Fig. 1.7 A typical load-deflection curve showing the averag e dynam ic mod ulus and the hysteresis

damping

Such N DE techniques can also be used to decide a suitable failure criterion.

As composite structures are generally stiffness-limited com pared with metals,

a substantial drop in stiffness may be sufficient for the structure to have

effectively 'failed'.

For wind turbine blades, the majority of which are generally positioned

upwind of the tower to avoid the effect of the tower shadow on the loading, a

sufficient loss of stiffness could, for example, result in insufficient clearance

between the blade and the tower.

1.8 Conclusions

Com posite materials like wood-veneer laminates and fibre reinforced plastics

have similar manufacturing techniques and similar prop erties. The strategy for

identifying the influence of specific fatigue parameters is outlined and the

framework is sketched within which the testing and evaluation has been

undertaken.

Coupon data are analysed in individual chapters and are subsequently

gathered together in Chapter14to permit a more detailed statistical analysis.

The principal observations and

findings

are assessed in Chapter15to provide a

set of recom mendations for good working practices.

References

(1 ) R IC H A R D S O N , T . , 1987 ,

Composites: a design guide

(Industrial Press, New York).

(2 ) D IN W O O piE , J . M . , 1981 ,

Timber, its nature and behaviour

(Van Nost rand, New York) .

(3 )

Guide to managing product design,

BS 7000, 1991 (BSI, Milton Keynes).

(4) MAYER, R . M. , 1993,

Design with reinforced plastics

(Chapman & Hal l , London) .

(5) RE IFS NID ER , K. F . (Edi tor ) , 1991,

Fatigue of comp osite materials

(Elsevier , Am sterdam ).


34/272


(6) HAIBACH, E.

1981,

Fatigue data for design applications in materials, experimentation and

design in fatigue,

Fatigue

Conference.

(7) SCH NEIDER, K. and LAN G, R . W ., 1990, Secondary source qualification of carbon fibre

prepregs for primary and secondary Airbus structures, 11th SAMPE Conference.

(8) PRETLOVE, A. J. and MAYER, R. M., 1994, Rotor size and mass - the dilemma for

designers of

WECS,

Wind

Engineering

18,

317-28.

(9) General principles on reliability for structures, ISO 2394,1986 (ISO , Geneva).

(10) Basis of design, Eurocode 1, part

1,1995

(CEN , B russels).

(11) Design of wooden structures, Eurocode 5, 1995 (CE N, Brussels).

(12) Wind turbine generator systems. Part 1. Safety requirements, IEC 1400-1, 1994 (IEC,

Geneva).

(13) JOHANSEN, . S., LILHOLT, H, and LYSTRUP, Aa, 1980, Wingblades of glass fibre

reinforced polyester fora 630kW w indturbine,Third International Conference on Composite

Materials, (Elsevier, Amsterdam).

(14) SANDBERG, O., 1989,Blade rootdesign,a stateofthe artsurvey, (FFA, Stockholm).

(15) SIMS, G. D. and GLA DM AN, D . G., 1978,Effect o ftest conditions on the fatigue strength

of a glass-fabric laminate.Plasticsan dRubber 1978, p.122etseq.


35/272


36/272

16 Design of Compo site Structures Against Fatigue

Table 2.1 Materials

Glass fibres

Type

Designation

Supplier

Diameter

Density

Stiffness

Strength

Failure strain

Bundle characteristics

Surface treatment/coating

E-glass

RPA 38 20/21EC12-300

Skandinavisk Glasfiber

12/im

2.615g/cm

3

70-72 GPa

-2000 MPa

~3 percent

300tex; 1000filamentsper tow

For polyester

Polyester

Type

Designation

Supplier

Density

Modulus

Ultimate tensile strength

Failure strain

Characteristics

Unsaturated polyester

UP333 and A lpolit UPS294V

Hoechst/Polyplex

-1.213

g/cm

3

-4 GPa

-100

MPa

3-5 percent

General polyester for glassfibres

These straight-fibre configurations of nominally continuous fibres can all be

fabricated

by a

winding technique (described below)

; this

ensures

a

high degree

of similarity in fabrication of all composite materials, and comparisons can be

made more easily and with greater confidence.

The fatigue loadings on the wingblades are mainly caused by the wind

fluctuations, causing flap-w ise bending of the wingblades, and by the gravity

loading, causing edge-wise bending of the wingblades during rotation. The

type of fatigue loading seen by the blades is tension-tension fatigue (R = 0.1)

and compression-compression fatigue (R = 10), both due to wind loads; in

addition tension-compression fatigue

(R =

1) is caused by gravity.

During fatigue loading of materials in genera l, and com posites in particular,

the m icrostructure, i.e.,fibre-configuration,may undergo changes. The effect

of such damage is recorded and studied through changes in the material

stiffness and in the development and change in the hysteresis-loops (stress-

strain loops during cyclic loading).

2.2 Materials

Characteristics

The constituent m aterials used to fabricate the composites are listed in Table

2.1.

The composites for wingblades are represented by several series of glass/


37/272


38/272


39/272

Properties of Compos ites with Long Fibres

19

fcla'i ' ' '

it^jjjLiiiaiiiitliiaii

a I

(b)

'M't'iVrMVi'ri'iiVrrrrtVi'iVi'i'

Fig. 2.1(a) Specimen for static and fatigue testing of comp osites with moderate strength. Dimen-

sions in mm . (b) Specimen for static and fatigue testing of composites w ith high strength

(large fraction of fibres in the 0 degree direction). Dimensions in mm

The load application during mechanical charac terization is made by hydraulic

control, both for monotonic (static) and cyclic (fatigue) loading. The load is

monitored continuously during testing.

The strain is recorded via two extensometers mounted on either side,

respectively, of the specim en; they are electrically coupled to give the net axial

strain, and thus to eliminate any possible bending strain induced in the

specimen. The (net) strain is recorded continuously during testing.

The mechanical characterization of the composite materials is made accord-

ing to the plan presen ted in Table 2.3 .


40/272


41/272

Properties of Com posites with Long Fibres 21

Table 2.4 Monotonie (static) tests

Material

(volume fraction

of fibres

50

percent)

G/poly 0 degrees

10 degrees

45 degrees

60 degrees

0 7

10

degrees

030 degrees

045 degrees

G/poly (benchmark)

Stiffness

E

GPa

47.2

40.9

17.4

15.4

42.0

30.4

37.2

23.8

Tension

Strength

2 .6

1.9

Monotonic

(static) testing

Tensile and compressive testing is made in the following cases and with the

parameters:

- tempera ture 20C

- humidity 'natu ral'

- displacement rate 0.5 mm/min

- strain rate ~ 8 x 1 0

- 4

sec~ '

- stress-strain curve is recorded

- stiffness, E,is calculated as initial slope of the stress-strain curve

- strength at failure, o

u

, is calculated

- strain at failure, E

U

is calculated

The results of mechanical characterization are given as stiffness E,strength ,

and strain e

u

in Table 2.4.

The stress-strain curves for some of the composites are shown in Fig.2.3;in

general fibre orientation has a large effect, such that0degree o rientation gives

high values of stiffness and strength, while the off-axis orien tations 45degrees,

60 degrees) are responsible for very low values. The combination of orien

tations, such as 0 degrees and 45 degrees, gives values dominated by the

0 degree orientation.

The data under monotonic (static) loading are of the expected level, and

serve as quality-control of the glass/polyester composites. The stress-strain

curves, both in tension and in compression, are of the 'no rm al' type and shape.

Cyclic (fatigue) testing

Tests are m ade in the following cases and with the parameters:

- tension-tension R = o

mm

/o

mux

=0.1

- compression-compression R = o

mm

/o

m

.

M

= 10

- tempera ture 20C


42/272


43/272


Table 2.5 Cyc lic fatigue) tests

Glass/polyester, comparisons

Fatigue Com posites Fibre orientations Figure

ratio

R

0 .1 ;

10

0 .1 ;

10

0.1

0.1

10

0.1

0.1

G/poly

**

G/poly (benchmark)

All

0/30

0 ; 1 0 ; 4 5 ; 6 0

0 ; 0 / 1 0 ; 0 / 3 0 ; 0 / 4 5

0 / 1 0 ; 0 / 3 0 ; 0 / 4 5

0; 10; 0 /10

Wa rp biased

2.4(a)

2.4(b)

2.4(c)

2.4(d)

2.4(e)

2.4(f)

2.4(g)

maximum load; the minimum load is calculated via the prescribed R ratio.

These loads are used as load-control parameters during fatigue testing.

The initial maximum straine

max

is used in the diagrams (S-N curves). The

results of the mechanical characterization are presented in similar fatigue

diagrams. These are plots oftheinitial maximum strain versus the logarithm of

number of cycles; for compressive loading the numerical, maximum strain is

used. The results are presented such that comparisons are made easy, i.e.,

diagrams are plotted on the same scale, and a reference line is used; this

reference is the fatigue curve for glass/polyester composite with a fibre

orientation of 0 degrees, tested at R = 0.1 (tension-tension fatigue). Several

series and comparisons are listed in Table 2.5.

The data for cyclic (fatigue) loading are presented and compared in a series

of diagrams (Fig. 2.4). The glass/polyester lam inates form a large group of data

and allow several types of comparisons.

The overview of all data in Fig. 2.4(a) shows the general shape of the S-N

curves, with a possible fatigue limit at cycles beyond

IO

7

10

8

cycles. The curves

are approximately straight lines with a slope which (numerically) decreases at

increasing values for the fibre orientations. This behaviour allows for simple

analytical expressions to be established for the S-N curves, and this leads to

potential design values for fatigue up to about 10

7

cycles.

The comparison (Fig. 2.4(b)) of tension-fatigue (R

=

0.1) and com pression-

fatigue(R =

10)

shows, generally, that there

is

little difference between tension

and compression. The tendency is that at fibre orientations close to zero

composites are stronger in tension than in compression, while at fibre orien

tations near 90 degrees, the compressive fatigue strength is higher than the

tensile fatigue strength.

The individual comparisons for glass/polyester are made by comparisons of

'rela ted ' composites under the same loading type and are illustrated in the Figs

2.4(c)-2.4(f).

For angle ply laminates (Fig. 2.4(c)) the fatigue strength reduction is small


44/272


+ 0 degrees = 0.1

10

degrees

o45 degrees

x 60 degrees

0/45

0/45

R=

10

Ull i

10

(b)

_L

+

R

= 0.1

R

= 10

10

IV

IV

Cycles

IV

IV

10'

10

Fig. 2.4(a) Fatigue diagrams of maximum nom inal strain versus logarithm of number of cycles to

failure, for glass/polyester composites - all composites al

K

= 0.1 and

R

= 10. (b)

Fatigue diagrams of maximum nominal strain versus logarithm of number of cycles to

failure, for glass/polyester com posites-0 d egre e/3 0 degree composites at

R

= 0.1 and

R

= 10


45/272


46/272

26


2.5

( e )

os

(f)

o J -

10

10

degreesR= 0.1

0 degrees 10 degrees

ffl 0 degrees 30 degrees

=

10

A 0 degrees 45 degrees

10' 10' IV

Cycles

10

s

10' 10'

-0 degrees

o

0 10 degrees

O 10 degrees

= < u

10' IO

3

IO

4

Cycles

10'

10' 10'

10'

10

F i g . 2 . 4 ( e ) F a t i g ue d i a g ra m s o f m a x i m u m no m i na l s t ra in v ers us l o g a r i thm of num ber o f cy c l e s to

fa i l ure , f o r g l a s s / po l y es t er co m po s i t e s - 0 deg ree , 0 deg ree / 1 0 deg ree , 0 deg ree / 3 0

deg ree , a nd 0 de g r ee / 4 5 deg ree co m p o s i t e s a t R = 10 . ( f ) Fat igu e diag ram s of

m a x i m um no m i na l s t ra i n v ers us l o g a r i thm o f num ber o f cy c l e s t o fa i l ure , f o r g l a s s /

po l y es t er co m po s i t e s - 0 deg ree , 1 0 deg ree , a nd 0 deg ree / 1 0 deg ree co m po s i t e s a t

= 0.1


47/272

Properties of Com posites with Long Fibres

27

2.5

0.5

-i r

-

1 1 r

-0 degrees

'

I I ml

10

IV IV IV

Cycles

IV IV

10'

10

Fig. 2.4(g) Fatigue diagram s of maximum nom inal strain versus logarithm of number of cycles to

failure, for glass/polyester composites - combination fabric (benchmark) at

R =

0.1

for angles of about10deg rees, while itislarge for45degree and60degree fibre

orientations.

For combination laminates with a significant fraction of 0 degree oriented

fibres the fatigue strength for R = 0.1 is practically unaffected by the angje

oriented fibres, as seen in Fig. 2.4(d) for R = 0.1 (tension-fatigue) while a

clear reduction in fatigue strength is recorded in Fig. 2.4(e) for R= 10

(com pression-fatigue), when the angle orientation is increased.

The comparison of laminates with0degree

fibres

and angle

fibres

forR= 0.1

(tension-fatigue)isseen in Fig. 2.4(f) for angles of10degrees. The presence of

0 degree fibres ensures fatigue strength values close to those of 'pure'0degree

laminates.

The results for the glass/polyester (benchmark) atR0.1 (Fig. 2.4(g)) are

displaced below the reference line, which is probably caused by the imperfect

manufacturing of this practical m aterial, made under industrial conditions.

Stiffness reduction

In most tests the stiffness changes are recorded regularly during fatigue testing,

typically at every tenth of a decade of cycles. The results are displayed in

diagrams showing stiffness reduction,E/E

0

, versus normalized lifetime (num-

ber of cycles), both with a linear parameter (Fig. 2.5(a)) and logarithmic

parameter (Fig. 2.5(b)). This figure shows the same data plotted in two

different ways. The real-time plot (Fig. 2.5(a)) also shows that the steep

reduction in modulus starts at about 85 percent of the (normalized) lifetime,

while the logarithmic plot (Fig. 2.5(b)) obscures this situation, although such

plots are often used for presentation of fatigue data.

The reduction in stiffness during fatigue loading

is

an indirect measure of the


48/272

28

Desi g n

of

Com posi te Structures Against Fat igue

1.0

0.9

1

1

0.7

o

0.6

0.5

(a)

1 1 1 ~i ; 1 r

I

I I I I

I

I I

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Normalized lifetime (N/N

(

)

(b)

0.1

0.2

0.3 0.4 0.5 0.6 0.7

Normalized lifetime (logAVlogN

t

)

0.8

0.9 1.0

Fig. 2.5(a) Stiffness reduction diagram , with normalized stiffness

ElE

B

versus normalized lifetime

;V/,V

f

for 0 degree/30 degree composites, fatigue tested at

R

= 10 with a maximum

strain of 0.8 percent, (b) Stiffness reduction diagram, with normalized stiffness

/-. //',,

versus normalized logarithmic lifetime log '/log /V

r

for 0 degree/30 degree compo

sites,

fatigue tested at

R

= 10 with a maximum strain of 0.8 percent

damage (cracks, delaminations) produced in the material. The selected results

show that a slow reduction in stiffness occurs during most of the life-time, and

that a final, rather fast, reduction takes place before failure.

Hysteresisloopchange

In selected tests, the hysteresis loops (- loops) are recorded at intervals

during fatigue testing. An example of the results is shown in Fig. 2.6, typically


49/272

Properties of ompo sites with Long Fibres

29

0.75 1.0

Strain (%)

1.75

Fig. 2.6 Hysteresis loop diag ram , for com posite under fatigue testing of maxim um initial strain of

1.0 p ercent. For cycle 1, the modulus is 31.2 G Pa; after 74 ,935 cycles, 24 .9 GPa and after

290,925 cycles, 18.0 GPa

as the first hysteresis loop at the start of fatigue testing and the loop after, a

specific number of cycles.

It is noted that the following phenomena occur as the number of cycles

increases:

(a) the loop area increases;

(b) the loop slop e (equivalen t to the stiffness) de creas es;

(c) the minim um strain and the maxim um strain increase.

Similar results are found with other composites (see Chapter 9). The last

ph eno m eno n is a shift on th e loop in the

-

dia gra m , this is a result of the fact

that the fatigue testing is under

load

control and that the material stiffness is

reduced. For the numerical values of stress and strain the following relations

hold

R - o

m

Jo

maii

P .

=

min

rl

min

'm ax t ' max

m in

=

O

mm

L

= (P

mm

IA)IE

m ax

=

o

max

lb . = (r

max

/A)IL

Th e last two relations show that both

m in

and

m a x

increase when

E

is reduc ed,

and that the increase is largest for e

m a x

(because

P

m ; i x

> P

min

.


50/272

30


ss

f

Initial y ^

^ ^ After

N

cycles

Strain

Fig . 2.7 Schem atic hy steresis loo ps at (/:" and V =

1 )

and at cycle

' (E

and

N); E

is the elastic

modulus (approximately equal to 'slope' of loop), ' is cycle number; the Rratio is 0.1

The 'loop-slope'isexpected to ex trapolate to , = 0,0. (Fig. 2.7): thisisnot

the case for several of the hysteresis loop results. This could be caused by

experim ental e rror at the start of (fatigue) testing, by an initial state of internal

stressinthe composites, and by changesinthese internal stresses during fatigue

loading.

2.4 Significance for design

These results set upper limits for those data which can be obtained under

commercial manufac ture, asfilamentwinding produces highly orientated, high

quality laminates. The best way of comparing fatigue data is nominal strain

versus the logarithm of cycles.

Orientation

The effect of

fibre

orientationisclear -thehigher the angle to the loadaxis,the

lower the nominal strain for any given number of cycles.

Having somefibresying along the load axis and some

fibres

at an angleisthe

best design practice. The tensile fatigue data presented on the basis of strain

show the sam e fatigue curves for all laminates with a substantial proportion of

0 degree fibres (typically 50-100 percent). For the designer the load-carrying

capacity

is

of

importance,

and therefore the stiffness

is also

significant

;

this calls

for

a

modera te proportion of off-axisfibreso avoid

a low

modulus and thus low

fatigue strength. Ideally, the off-axis fibres should be orientated to resist the

off-axis loads.


51/272


Loading

Com posites are better able to withstand tensile than compressive loading, even

if the laminate contains fibres lying along the loading axis. In general,

composites in service will be subjected to a mixture of tensile and compressive

loading. The fatigue properties of reverse loading (tension/compression)

would thus be less than that of pure tensile loading, as can be seen in Fig. 8.5.

The effect of spectral loading, which is a combination of compressive and

tensile loading, is discussed in Chapter 4.

Damage accumulation

Stiffness is an important property for designers, because it governs the

(maximum) deflection during loading. The reduction in stiffness is thus an

important consideration for long-time design. The curves ofE IE

0

versus

log

N

f

(Fig. 2.5) can lead to design-allowables for cases where a maximum reduction

in stiffness can be accepted, and the curves can be used to establish the

corresponding number of cycles. Alternatively, the point of the curve where

the fast reduction starts may be used as a design-allowable, if the actual

stiffness at that point can be accepted.

The hysteresis loops can also be correlated to temperature changes (in

creases) during fatigue loading, and such recordings can serve as damage-

monitoring techniques. These may be developed into practical methods to be

used in service.

Both of these damage parameters are discussed further in subsequent

chapters (for example, Chapters 7 and 9).

2.5 Conclusions

The fatigue of well-orientated fibre com posites shows a characteristic decrease

in properties as a function of

time.

Some

fibres

n the primary loading direction

are beneficial in terms of fatigue propertie s. Such com posites are better able to

withstand tensile than compressive fatigue.


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34

Design of Com posite Structures Against Fatigue

Tab le 3.1 Description of laminates

Laminate

Code

Combi iso

Combi NPGiso

Combi vinyl

Combi rub-vinyl

Combi Ortho

CSM

SB-Combi

MA-O

MA-90

MA-0/90

M A - 4 5

Resin type

lso-polyester

NPG/iso-polyester

Vinyl ester

Rub.mod. vinyl ester

Ortho-polyester

NPG/iso-polyester

NPG/iso-polyester

NPG/iso-polyester

NPG/iso-polyester

NPG/iso-polyester

NPG/iso-polyester

1

Layup*

[[0(400),90(400)WR ],((100CSM)]SB].,

[[0(4O0),90(400)WR],[(100CSM)]SB]

5

[[0(400),90(400)WR],[(100CSM)]SB].,

[[0(400),90(400)WR],[(100CSM)]SB],

[[0(400),90(400)WR],[(100CSM))SB]

5

[(100CSM)],,

[0(400),90(400),(100CSM)]SB]

5

[0(578),90(14)I].,

S

[90(578),0(14)I],

5

[0(400).90(40)I].

[+45(400) , -45(400)K]. ,

s

Fibre

weight

fraction

(%)

53

51

53

52

53

31

64

54

54

53

53

Fraction of

fibres in

load

direction

(%)

45

45

45

45

45

Random

45

98

:

50

(1

'Notes:

For layup conventions refer to Table 0.1

SB fabric Rovimat 800/100 (Chomarat)

Multiaxial knitted fabrics (Devoid AMT)

Table 3.2 Resin properties from Jotun Polymer (5)

T y p e

T e n s i l e M o d u l u s ( G P a )

T ens i l e S treng th (MP a )

E l o ng a t i o n a t brea k (% )

F l ex ura l mo dul us (G P a )

Flexural s trength (MPa)

Ortho

polyester

N P 4 1 - 9 0

3.6

65

3.7

3 .3

125

so polyester

N P 7 2 - 9 0

3.7

7 6

3.5

3 .6

135

NPG/iso

polyester

N P 2 0 - 8 0

3.6

73

6.5

3

124

Vinyl ester

N P 9 2 - 2 0

3.3

8 0

5

3.1

130

Rubber

modified

vinyl ester

N P 9 2 - 4 0

2.9

6 8

9

2.8

125

All resins were supplied by Jotun Polymer

bonded together. Five different resins were used - three polyester based and

two vinyl ester based; their properties are listed in Table 3.2 (5). The resins

differ from the greatest extent in their elongation to break (3-9 pe rcent).

The other six laminates were m ade to study the influence of different fabric

layups with the same resin (NPG-iso polyester). These comprised a stitch

bonded combination mat (SB) and the chopped strand mat by itself and four

laminates of inlaid construction having straight glass fibres held together by

polyester yarns.

Laminates MA-0 have 98 percent of their fibres running parallel to and 2

percent normal to the load direction. M A-90 is the same laminate as M A -0,

but tested perpendicular to the m ain fibre direction. MA-0/90 is a cross plied

multi-axial laminate with the same amount of fibres running parallel and

normal to the load direction. Laminate M A - 45 is laminate M A-0/90 rotated


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Influence of Matrix and Fabric 35

Grips

Anti-buckling

Q t l supports

Support jig

Specimen

V\

Extensometer

m

Grips

Specimen

Fig. 3.1 Experimen tal set-up for coupon fatigue tests

by45degrees. The MA laminates were also used to obtain basic ply properties

for laminate theory calculations.

All tests were performed on a servo-hydraulic MTS testing machine.

Specimens were cut as straight 25 x 5 x 270 mm coupons from the lam inate,

according to ISO 3268. In order to prevent buckling in compression an

antibuckling device similar to the support jig of ASTM 695 was used. Dimen

sionsof the device had to be slightly changed toalength of140 mmand w idthof

25 mm to fit the specimens; there is no standard procedure for G RP tens ion-

compression fatigue tests (Fig. 3.1). The gap between the antibuckling device

and the grips

was

kept

as

small

as

possible (less than

2mm).

This set-up w orked

well for most specim ens, but thin specimens with highly oriented fib res had the

tendency to fail in the gap between the antibuckling device and

grips.

In those

cases an ex tra set of metal supports was inserted into the test fixture, as shown

in F ig.3.1.These supports closed the gap near thegrips.The slit in the middle

ofthesupports, withitsorientation a t45degrees to the specimen, did not seem

to cause any buckling of the specimen in compression.

Strains were measured with an MTS Model 632.12C-20 extensometer,

attached to the edge of the specimen. Stress-strain curves could be taken

continuously by a com puterized data acquisition system. Testing rates of quasi

static tests were 2 mm/min for tensile and compressive tests. Frequencies of

fatigue tests were varied between 2 and 5 Hz to keep the average load rate

about constant, avoiding viscoelastic effects. Low frequencies were chosen to

prevent the specimens from heatingup.All fatigue testingwasdone in the load

control mode.


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36


Table 3.3 Static properties of laminates

Laminate code

Combi iso

Combi NPG/iso

Com bi vinyl

Combi rub-vinyl

Combi ortho

CSM

SB-Combi

MA-0

MA-90

MA-0/90

M A - + 4 5

Linear

Young s

modulus

(GPa)

17.1 0.4

16/1 0.4

16.8 0.5

16.9 0.7

17.3 0.7

8.4 0.4

16.0 0.7

28.3 0.7

10.6 0.4

20.4 1.0

10.4 0.3

Strain at

linear

limit

(%)

0.28

0.34

0.38

0.33

0.30

-

0.32

*

0.13

0.13

-

Ultimate

tensile

strength

(MPa)

209 20

205 6

227 5

237 14

226 7

125 1

250 16

602 8

7 1 2

366 25

6 0 5

Strain at

tensile

failure

(%)

1.40 0.20

1.62 0.12

1.72 0.03

1.68 0.07

1.77 0.10

1.61 0.03

2.25 0.06

2.34 0.30

2.35 0.20

2.50 0.30

>12

Ultimate

compr.

strength

(MPa)

209 9

258 14

288 25

259 14

294 23

211 6

248 10

>450

8 3 2

285 30

9 0 8

*no linear limit

3.3 Static properties

Static properties of all laminates were m easured in tension and compression.

Tensile static curves can be characterized by an initially linear s lope, the linear

limit, and ultimate failure properties (Fig. 3.2). These param eters are listed for

all laminates in Table 3.3. Tensile elastic moduli , w ere taken as the slope of

the linear part of the stress-strain curve. The linear limit was defined as the

stress-strain level when data points drop significantly below the line defining

the linear modulusE

t

.This is taken as a10percent d rop of the secant modulus

from theE

t

value. Note that the MA-0 laminate with 98 percent of its fibres

running in the load direction does not show a linear limit based on this

definition. Typical tensile stress-stra in curves are shown in

Fig.

3.3for NPG/iso

laminates.

Different resinswith the same combimat reinforcement showed very similar

static characteristics. Ultimate strength and strain have similar values withinthe experimental scatter (Table 3.3), there fore, only the stress-strain curve of

the combi NPG /iso lam inate is shown in

Fig.

3.3. The main difference between

the five combi laminates is the onset of non-linearity. Although the exact

position of the onset is difficult to determine, a trend between the different

resins can be seen. The iso- and ortho-resins have a lower linear limit than the

vinylesters and the NPG/iso polyester. This reflects the relativeflexibilityof

the various resins (Table 3.2). Compressive tests show a fairly linear stress-

strain curve up to the point of failure. Com pressive properties are also listed in

Table 3.3.

Different reinforcements with the same resin have a large influence on the

static stress-strain behaviour (Fig. 3.3). As expected, modulus and strength

increase with the amount of fibres in the load direction (Table 3.1). How ever,

the combimat and the CSM fail already at a strain of about 1.5 percent. All


57/272


58/272

38


600

500

400

300

Vi

200

SB-combi

2.5

3.0

.0 1.5 2.0

Strain (%)

Fig. 3.3 Typical static tensile stress- strain curv es for laminates with NPG /iso-polyester resin

continually and parallel in one direction (100 percent unidirectional plies).

These properties cannot be derived from complicated laminates, but the simple

multiaxial laminates tested here (M A-0, M A-90, and MA -45 ) can be used to

deduce such ply properties (10)(11).

Ply

properties for the multiaxial laminates

are listed in Table 3.4. Note that the axial properties (parallel and normal to

fibres) of a 100 percent unidirectional ply are slightly different from the

properties of the laminates M A-0 and MA-90 (Table 3.3). The ply properties

can be used to predict properties of more complicated laminates. Predictions

for the M A-0/90 laminate were found to be verygood.Predictions for (0/90/45)

laminates were good in the linear range. U ltimate properties can be predicted

with progressive failure analysis, 'last ply failure' calculations determine the

first fibre failure (11)(12). In all these cases the ply properties have to be

adjusted to the fibre volume fractions of the laminates, using micromechanic

correction formulae (12).

3.4 Fatigue properties

Fatigue tests of

all

specimens were performed under reverse loading

7?

=

1),

because it was found to cause the most severe fatigue conditions in other


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60/272


showed the highest fatigue strength. The S-N curves for the combimat

laminates with different resins were very similar to the one for NPG/iso (14).

Such curves do not reveal any information about the damage processes taking

place in the specimens. However, damage development studies can help to

assess the change of properties during the fatigue life, and they can lead to a

more simplified analysis of ultimate fatigue failure.

Damage development

Stresses and strains were monitored continuously for all specimens during

fatigue testing. A change in the stress-strain curve results from some damage

development. A reduction of the slope of the curve corresponds to a drop in

Young's modulus. Initially, such a drop corresponds to the onset and increase

of matrix cracking in the lam inate. La ter, o ther damage forms, like delamina-

tion and fibre failure, can contribute to a m odulus drop too (14) (15).

Reverse loading fatigue shows more special characteristics in the change of

the stress-strain curve with cycle num ber. T he slope of the tensile part of the

curve drops to lower values due to an increase in damage (matrix cracking).

But the compressive slope of the curve remains at the same value through most

of the fatigue life (Fig. 3.5). Compression seems to close the cracks in the

material. This means that the damage introduced to the specimen during the

tensile part of the fatigue cycle does not influence the com pressive modulus of

the material. Once these cracks are closed under compression, the material

appears undamaged to a stress-strain measurement. Only shortly before

failure of the specimen do both the tensile and compressive modulus drop

rapidly. Itisvery likely that a t this point severe damage, including fibre failure,

develops in the specimen, which causes rapid degradation of the properties.

Stress-strain characteristics during fatigue, including hysteresis effects, are

described in more detail in (14) (15).

Change of the tensile fatigue modulus is a good way to characterize damage

development during fatigue. Figure 3.6 shows the change of the modulus with

cycle number for different fatigue strains for the combi-NPG/iso laminate.

Cycling belowthelinear limit

Specimens have the same initial tensile fatigue modulus E

tfi

as the primary

static modulus E

t

. The tensile fatigue modulus E

l{

remains constant for a

number of cycles before it starts to drop. This indicates that the specimen is

initially free of damage, i.e., no matrix cracking. Cracking sets in after an

initiation period, when the modulus starts to drop. The initiation period

increases if a specimen is cycled at very low amplitudes, i.e., far below the

linear limit.

Figure 3.6

shows two

specimens which were cycled at strains below the linear

limit. The modulus E

t{

of the specimen cycled at 0.32 percent strain drops after

about 1500 cycles, while the modulus of the specimen cycled at 0.25 percent


61/272

Strain

Fig. 3.5 Schematic of stress-strain curves of a fatigue test

strain drops after approximately 3000 cycles. This onset of matrix cracking

happens at much lower numbers of cycles than ultimate failure (Fig. 3.7).

Cycling abovethelinear limit

Specimens develop matrix cracking already when loaded in thefirstcycle. The

initial modulus E

t

depends on the fatigue strain (Fig. 3.6). High strain levels

cause much initial damage, giving low initial fatigue moduli

t fi

. The moduli

drop imm ediately with increasing cycle num ber, reflecting an instant growth of

damage.

Critical fatigue modulusE

cr

Once matrix cracking occurs, the crack density increases, causing the tensile

modulus to drop. The modulus E

lf

drops slowly and fairly linearly with the

logarithm of the cycle number up to a critical value

cr i t

(Fig. 3.6). This


62/272

42


1.1

l .o

s

I 0.9

y

s

3

0.8

I 0.7

0.6

0.5

' S

1 . 6 1 ^ ' ^

. 2 5

^ , 0 . 3 2

- ^ ^ 5 9 '

i

10.96

i

:

0.59

No

damage

Slight -

damage

>

: Severe

damage

:

1

10'

to

2

i o

3

io

4

10

5

Cycles to failure

10' 10'

Fig . 3 .6 Ch ang e of tens i l e fa t igue mo du lus

E

r

wi th cycle number for various ini t ia l s tra ins (%) of

the co m bi m a t N P G / i s o -po l y es t er l a m i na te ( l o a ded i n w e f t d i rec t i o n) . Th e upper tw o

curves show data for specimens cycled below the l inear l imi t

modulus appears to be independent of the strain amplitude. Since a linear

relationship between modulus drop and matrix crack density was found in

other composites (3), matrix crack density seems to accumulate linearly with

the logarithm of the cycle number.

Slightly different behaviour was observed for specimens cycled below the

linear limit (0.35 percent). After the linear drop of stiffness E

tf

with cycle

number the reduction slowed down and eventually halted, indicating a satu

ration of the damage. The modulus E

f

at saturation was slightly above the

critical modulus

cr i t

. Damage saturation was also found in crossplied (0/90

3

)s

GRP (3) (16). This means that all matrix cracks had been formed, but the

laminate was able to sustain the damage.

E

crit

reflects the stiffness of the laminate at a state of dam age, whichismostly

due to m atrix cracking. A t this level, thefibresseem to be still undamaged and

so able to sustain the applied load.

Failure

Once the modulus drops belowE

crit

all properties ofthelaminate degrade very

rapidly. A rapid drop in tensile stiffness coincides with a rapid drop of

compressive stiffness E

c{

,which stays constant at the previous static valueE

c

(14).

A different damage mechanism, probably damage of the load bearing

fibres, gets activated and rapidly

causes

ultimate failure. A further indication of


63/272

Influence of M atrix and Fabric

43

2.0

1.8

1.6

1-4

h 1.2

A

I 1.0

3

I 0.8

0.6

0.4

0.2

0

iso / combi

NPG iso

vinyl

rub-v inyl

ortho

NPG iso / SB combi

95/95

limit

So me

da ma g e

No

m l

O.Q.

J L

S. .

N.

1

10 IO

2

IO

3

IO

Cycles

10

s

IO

6

IO

7

Fig . 3 .7 /V cur ves for com bi lamin ates wi th variou s res ins and one SB-c om bi re info rcem ent . T he

stages of da m age in the Com bi-N PG/ iso -poly es ter laminat es are indic ated . Ini t ia l s tra in i s

p l o t t ed beca us e t e s t s were per fo rmed under l o a d co ntro l , whi ch mea ns s t ra i ns i ncrea s e

s l ight ly during fat igue

severe damage formation is the sudden appearance of and increase in the

hysteresis effect in the fatigue stress-strain curve (between loading and

unloading curve) when the fatigue modulus drops below

Design of Composite Structures Against Fatigue

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