frp strengthening of metallic structures

Delivered by ICEVirtualLibrary.com to:IP: 79.130.107.190

On: Tue, 15 Nov 2011 08:26:59

FRP composites life extension and strengthening of metallic structures


On: Tue, 15 Nov 2011 08:26:59

I C E d e s i g n a n d p r a c t i c e g u i d e s One of the major aims of the Institution of Civil Engineers is to provide its members with opportunities for continuing professional development. One method by which the Institution is achieving this is the production of design and practice guides on topics relevant to the professional activities of its members. The purpose of the guides is to provide an introduction to the main principles and important aspects of the particular subject, and to offer guidance as to appropriate sources of more detailed information.

The Institution has targeted as its principal audience practising civil engineers who are not expert in or familiar with the subject matter. This group includes recently graduated engineers who are undergoing their professional training and more experienced engineers whose work experience has not previously led them into the subject area in any detail. Those professionals who are more familiar with the subject may also find the guides of value as a handy overview or summary of the principal issues.

Where appropriate, the guides will feature checklists to be used as an aide-memoire on major aspects of the subject and will provide, through references and bibliographies, guidance on authoritative, relevant and up-to-date published documents to which reference should be made for reliable and more detailed guidance.


On: Tue, 15 Nov 2011 08:26:59

ICE design and practice guide

FRP composi tes life extension and strengthening of metallic structures

E d i t e d b y SSj M o y

"l1 Thomas Telford


On: Tue, 15 Nov 2011 08:26:59

Published by Thomas Telford Publishing, Thomas Telford Ltd, 1 Heron Quay, London E14 4JD. URL: http://www.thomastelford.com

Distributors for Thomas Telford books are USA: ASCE Press, 1801 Alexander Bell Drive, Reston, VA 201914400, USA Japan: Maruzen Co. Ltd, Book Department, 3-10 Nihonbashi 2-chome, Chuo-ku, Tokyo 103

Australia: DA Books and Journals, 648 Whitehorse Road, Mitcham 3132, Victoria

First published 2001

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

ISBN: 0 7277 3009 6

Institution of Civil Engineers, 2001.

All rights, including translation, reserved. Except as permitted by the Copyright, Designs and Patents Act 1988, no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying or otherwise, without the prior written permission of the Publishing Director, Thomas Telford Publishing, Thomas Telford Ltd, 1 Heron Quay, London E14 4JD.

This book is published on the understanding that the editor(s) /author (s) is/are solely responsible for the statements made and opinions expressed in it and that its publication does not necessarily imply that such statements and/or opinions are or reflect the views or opinions of the publishers. While every effort has been made to ensure that the statements made and the opinions expressed in this publication provide a safe and accurate guide, no liability or responsibility can be accepted in this respect by the editor(s)/author(s) or publishers.

Produced by Gray Publishing, Tunbridge Wells, Kent Printed and bound in Great Britain by Bell & Bain Ltd, Glasgow


On: Tue, 15 Nov 2011 08:26:59

Contributors

Dr Stuart Moy, Department of Civil and Environmental Engineering, University of Southampton, Southampton SO 17 1BJ

Dr Paul Hill, DML Composites, Devonport Royal Dockyard, Plymouth PL1 4SG

Mr Jim Moriarty, London Underground Limited, 30 The South Colonnade, London E14 5EU

Adrian Dier, MSL Engineering Limited, MSL House, 5-7 High Street, Sunninghill, Ascot SL5 9NQ

Alan Kenchington, Structural Statics, Burntwood, Kings Worthy, Winchester S021 IAD

Brett Iverson, Defence Research Agency, Structural Materials Centre, R178 Building, DERA Farnborough, Farnborough GU14 6TD

Acknowledgements The research projects that provided the technical information on which this guide is based were funded by the Department for the Environment and the Regions and the Engineering and Physical Sciences Research Council under the Partners in Technology and Link Inland Surface Transport Schemes.


On: Tue, 15 Nov 2011 08:26:59

Contents

Overview of fibre reinforced polymer composites for strengthening 1 Introduction and experience of composites in other industries 1 Composites in construction 2 Materials 3 Manufacturing techniques 5 Performance 7 Adhesive bonding 9

1. General introduction 11 General 11 Scope 11 Applications of technique 11 Definitions 12

2. Design specification 13 Design loads 13 Load factors 13 Environment 14

3. Design process overview 17 Repair and strengthening 17 General design principles 17 Structural design 19 Materials selection 19

4. Composite design 23 Selection 23 Strain compatibility 23 Thermal compatibility 24 Ultimate strength capacity 25 Choice of manufacturing process 25 Determination of basic material properties 26 Accounting for long-term degradation 27 Design allowable strength and stiffness 29 Laminate design 30 Attachments and joints 33 Galvanic corrosion 35


On: Tue, 15 Nov 2011 08:26:59

5. Structural design 36 Design from first principles (hand calculations) 37 Design by finite element analysis 45

6. Implementation 48 Manufacturing methods 48 Health and safety/COSHH 51 Storage of materials 52 Quality assurance 52 Non-destructive examination 53 Repair 54

7. Tests 57 Materials testing 57 Structural tests 57

8. Verification 59

9. Monitoring 60

10. References 61

Index 63


On: Tue, 15 Nov 2011 08:29:49

Overview of fibre reinforced polymer composites for strengthening

Introduction and experience of composites in other industries

The history of composite materials has been discussed widely in standard textbooks. ~ Glass fibre reinforced materials were first used in aircraft radar covers at the end of the 1930s. Their use spread rapidly through marine craft to the point today where they are utilized in demanding services such as pressure vessels, pipes and blast panels on offshore oil and gas platforms. Carbon fibres were developed by the Royal Aircraft Establishment at the end of the 1960s and were seized upon for the high strength and stiffness per unit mass they offered. They are now used in applications as diverse as sports goods, stealth aircraft and pipework repairs and are being used for structural upgrade and life extension in construction (Figure 1).

In all these applications, composites have been selected because they offer a performance or cost-benefit over traditional solutions the saving can usually be traced back to the generic benefits of using composites, namely:

low density and high mechanical properties, giving low mass components; high durability, composite materials are extremely resistant to most common

environments; ease of installation, derived from low mass and use of adhesive bonding

techniques.


On: Tue, 15 Nov 2011 08:29:49

FRP c o m p o s i t e s

Composites, in the sense used here, are composed of fibres in a resin matrix. The fibres are very strong and stiff, and the matrix enables them to work together and be utilized in engineering components. There are a variety of materials and manufacturing methods, which all lead to final components with different mechanical properties. The selection of materials is therefore coupled with the selection of manufacturing process and these must be completed concurrently with the structural design.

The aim of this introduction is to give a brief overview of the constituents of composite materials, manufacturing methods and to familiarize the reader with some of the common terminologies and generic properties.

Composites in There is considerable interest in the use of fibre reinforced polymer (FRP) composites construction in construction. There are some examples of all-composite structures such as the

Aberfeldy Bridge in Scotland and the Bonds Mill Bridge in England (see Figure 2). The most common application has been in structural strengthening and upgrade and it has been reported4 that approximately 150 bridges and buildings in the UK have been strengthened using FRP. Figure 3 shows two examples.

Strengthening is usually carried out by bonding (gluing) FRP plates or strips to the existing structure. The strips can be pre-stressed if necessary. In concrete structures the FRP can be kept thin because of the very favourable stiffness of FRP compared to concrete and also because the bond strength between FRP and concrete is limited by the concrete rather than the adhesive.

In metallic structures the FRP strips have to be thicker because the stiffness of FRP is less than double that of steel, for example. The forces developed in the FRP will be high and these have to be transmitted across the adhesive. Old metal will often be in

f i g u r e 2 A view of Bonds MM Bridge


On: Tue, 15 Nov 2011 08:29:49

O v e r v i e w o f fibre r e i n f o r c e d p o l y m e r c o m p o s i t e s f o r s t r e n g t h e n i n g

poor condition, because of manufacturing imperfections, corrosion or the presence of paint or other surface finishes. Consequently surface preparation of the metal substrate is very important if a good bond is to be achieved between the metal and the FRP. The choice of bonding method is also important. The obvious approach is to use a suitable adhesive applied to one or both of the bonding surfaces. However the choice of glue is critical to ensure that full cure is achieved on site. There are other methods that produce the FRP strip at the same time as bonding it to the metal.

In certain circumstances it may be necessary to provide an alternative load path to that provided by the existing structure. The new structure could be manufactured entirely from FRP composite.

It can be seen that the choice of materials and the manufacturing process are interrelated and also affect the structural design. The purpose of this design and practice guide is to set out guidelines for the design and execution of FRP strengthening schemes for metallic structures. It is a Code of Practice based on experience gained from two major research programmes and from various strengthening schemes. There are no official standards for this type of work, so this represents a first attempt at producing design recommendations.

Construction professionals considering the use of FRP composites, have to gain an understanding of the physical make-up and material properties of FRP as well as knowledge of the manufacturing techniques used. This introduction seeks to provide this and also to outline the jargon of FRP.

It must be emphasized at the outset that FRP is more expensive than conventional construction materials. However there are other factors which need to be taken into consideration. FRP can be connected simply; because of its light weight, strengthening schemes require very little falsework so that disruption is minimal. Thus in costing a strengthening scheme it is construction rather than material cost which needs to be considered. Indeed the durability of FRP is such that whole-life costing might be even more representative of relative value.

Materials Engineering composites are commonly formed by the intimate mixing of two distinct phases, a fibrous reinforcement, and a continuous medium (resin) termed the matrix, which encapsulates the reinforcement. The fibre reinforcement generally has high specific properties, i.e. high strength and stiffness at a relatively low density. The matrix in comparison has lower strength and stiffness.

In simple terms, the fibres and their arrangement define the material's mechanical properties and act to resist primary loads. The matrix acts to transmit the loads into the fibres, protects the surface of the fibres from damage and inhibits brittle fracture associated with the 'brittle' fibres.

Fibres There are three types of fibre in general use, aramid (better known by the trade names Kevlar^ and Twaron), glass and carbon. General properties of the different fibre types are shown in Table 1.

Aramid fibres. Aramid fibres are manufactured from a synthetic organic compound and are produced in low and high modulus grades. The main characteristics of aramid fibres are their high strengths, moderate Young's moduli and low densities. Laminates

3


On: Tue, 15 Nov 2011 08:29:49

FRP composites

Table I Typical properties of common reinforcing fibres

Fibre Tensile strength (MPa) Young's modulus (GPa) Density (kg/m 3 ) C o s t (/kg)

Aram id 3 1 5 0 - 3 6 0 0 5 8 - 1 6 0 1 3 9 0 - 1 4 7 0 20 Carbon 2 1 0 0 - 7 1 0 0 2 2 0 - 9 0 0 1 7 4 0 - 2 2 0 0 1 0 - 2 0 0 Glass 3 4 4 5 - 4 8 9 0 7 2 - 8 7 2 4 6 0 - 2 5 8 0 2.5

formed from aramid fibres are known for their low compressive and shear strengths. The fibres themselves are susceptible to degradation from ultraviolet (UV) light and moisture. Aramid fibres are therefore used for structures requiring high tensile strength (TS) and impact resistance but not shear or compressive strength (CS) and are often used in combination with other fibres (hybridized) to provide improved impact resistance.

Carbon fibres. Carbon fibres are produced from three different organic precursor materials: - rayon, polyacrylonitrile (PAN) and pitch. The manufacturing techniques vary from precursor to precursor and are complex. They produce fibres with different properties and in many grades. The main characteristics of carbon fibres are their high strengths and Young's moduli, and their low densities and thermal expansivities. Generally there is a play-off between strength and modulus; the higher the strength the lower the modulus and vice versa. The wide range of fibres and properties that are available provide the maximum possibility for optimization of the material to provide properties specifically matched to a particular application. Carbon fibres are used for structures that are weight sensitive, or which have high stiffness requirements. Carbon fibres are generally the most appropriate for the strengthening of metallic structures.

Glass fibres. Glass fibres are used for the majority of composite applications. The main forms are E-glass, most frequently used, and S2- or R-glass (trade names from different manufacturers), which is a more expensive high strength version. The main characteristics of glass fibres are their high strengths, moderate Young's moduli and density, and their low thermal conductivity. Glass fibres are used for structures that are not weight critical and which can be designed to accommodate their lower Young's moduli.

Resins The main structural resins are unsaturated polyesters and epoxies, with phenolics used where there is a requirement for fire resistance. The main properties of these resins are summarized in Table 2.

Unsaturated polyester resins. Unsaturated polyester resins are used for the majority of composite structures. They consist of a relatively low molecular weight, unsaturated

Table 2 Typical properties of common resin systems

Tensile strength Young's modulus Strain at Density C o s t Resins (MPa) (GPa) failure (%) (kg/m 3 ) (/kg)

Polyester 5 0 - 7 5 3 . 1 - 4 . 6 1 .0 -2 .5 I I 1 0 - 1 2 5 0 - 2 . 5 Epoxy 6 0 - 8 5 2 . 6 - 3 . 8 1 . 5 - 8 . 0 1 1 1 0 - 1 2 0 0 - 5 - 1 0 Phenolic 6 0 - 8 0 3 . 0 - 4 . 0 1 . 0 - 1 . 8 1 0 0 0 - 1 2 5 0 -

4


On: Tue, 15 Nov 2011 08:29:49

O v e r v i e w o f fibre r e i n f o r c e d p o l y m e r c o m p o s i t e s f o r s t r e n g t h e n i n g

polyester dissolved in styrene. Curing occurs by the polymerization of the styrene, which forms cross-links across unsaturated sites in the polyester.

Polyester resins are relatively inexpensive, easy to process, allow room temperature cure and have a good balance of mechanical properties and environmental or chemical resistance.

The main issues relating to the use of polyester resins are:

moderate adhesive properties; styrene vapour release during cure, now considered a health risk and subject to

emission regulations; curing is strongly exothermic and can cause damage if processing rates are too

high; shrinkage on cure of up to 8%.

Epoxy resins. Epoxy resins are used for the majority of high performance composite structures. They are generally two-part systems consisting of an epoxy resin and a hardener that is either an amine or anhydride. A wide variety of formulations are available giving a broad spectrum of properties. The higher performance epoxies require the application of heat during a controlled curing cycle to achieve the best properties.

Epoxies have excellent environmental and chemical resistance. Compared to polyesters, epoxies require more careful processing and are more expensive by a factor of 1.53. However, epoxies demonstrate better mechanical properties, give better performance at elevated temperature and exhibit a much lower degree of shrinkage (23%). Their use incurs less waste and permits faster production rates; they can therefore be competitive with polyester in terms of cost.

Phenolic resins. Phenolic resins are of particular interest in structural applications owing to their flame-retardant properties, low smoke generation and high heat resistance (up to 316C). Phenolic resins are produced by a condensation process that involves reacting phenol with formaldehyde.

Phenolics exhibit good dimensional stability and resistance to acids. Undesirable features of phenolics are their relatively low toughness and generation of water during curing. This latter point is important, as a phenolic that is not fully cured will give off water, in the form of steam, during a fire, which can cause failure of the laminate.

Manufacturing Apart from the raw materials, the way in which a composite material is manufactured techniques will also affect its properties. This is because the manufacturing route dictates the

proportion of fibres to resin (called the fibre volume fraction), the amount of voids in the material, and the overall consistency.

Hand lay-up This process is also commonly known as contact moulding and wet lay-up. A rigid mould is coated with liquid resin, dry reinforcement is laid onto the resin, and a roller is used to press the reinforcement into the resin until the reinforcement is soaked with the resin. This layer is called a ply. The process is repeated until the desired number of plies has been added.

5


On: Tue, 15 Nov 2011 08:29:49


V a c u u m bag mould ing This process is similar to hand lay-up, but final laminate consolidation is achieved by covering the component with a plastic film (vacuum bag) and drawing a vacuum over the component. The pressure differential compresses the uncured composite, forcing out excess resin and drawing out entrapped air. This results in a laminate of higher fibre volume fraction and better consistency when compared to the hand lay-up process.

For resin infusion under flexible tooling (RIFT), dry fabric reinforcement is first laid into a mould. The reinforcement is covered with a vacuum bag, the edges sealed and a vacuum drawn using a pump. The vacuum is used to suck resin into the reinforcement thus forming the composite component. This process results in a laminate of low void content and high volume fraction. The resin infusion process is well suited to larger components or structures and where a closed mould process is required to reduce emissions. It is used in the strengthening of metallic structures, particularly when the metal surface is uneven or pitted. The resin takes up the surface imperfections while bonding the composite to the metal. In manufacturing it is ideal for producing small numbers of components.

Where large numbers of components are required resin transfer moulding (RTM) is a similar, but more suitable technique. Closed metal moulds are normally used (which tend to be more expensive to make, but are more durable than the cheaper tooling for RIFT), and the dry fibre preform is laid up within it. Resin is then injected under positive pressure (sometimes with vacuum assistance), to form the final component. The mould can also be heated to accelerate the cure of the resin and so reduce cycle times.

Pre-preg This method is used in both aerospace and motor racing industries because it gives the most controlled form of component. Unidirectional fibres are pre-impregnated with a resin (usually an epoxy) and these sheets are laid up individually in whatever direction is desired to give fibres in the orientations required. The whole component is cured in an oven under vacuum or in an autoclave. High control over fibre alignment, fibre volume fraction and void content can be achieved. This approach is not practical for construction applications.

Pultrusion The above techniques are all labour driven. Pultrusion is an automated technique that was designed to reduce the end-cost of components. It is similar to extrusion in principle, in that the material is taken through a die to produce a component of constant cross-section. Fibres are arranged to feed into a resin bath, and then into a heated die, where they cure. The solid product emerges from the die and is pulled, driving the whole process and giving it its name. There is no freedom to control the fibre alignment as with other methods (fibres must be aligned predominantly to the axis of the machine), or the volume fraction (the faster the running speed, the higher the pull force and the higher the volume fraction of the composite).

Fi lament winding Filament winding is another automated technique used to produce objects with rotational symmetry. Pipes, tanks and pressure vessels are commonly produced in this way. The fibres are impregnated with resin on line and wound on to a rotating mandrel. The fibre placement is achieved using a computer controlled (CNC type) arm, which can move along the length of the mandrel as the mandrel rotates. The

Resin infusion under flexible tooling/Resin transfer mould ing

6


On: Tue, 15 Nov 2011 08:29:49

O v e r v i e w o f f i b r e r e i n f o r c e d p o l y m e r c o m p o s i t e s f o r s t r e n g t h e n i n g

fibre angle can therefore be controlled to a certain extent, however geometry of the part most commonly dictates the angles that can be achieved (fibres naturally follow the high points on the surface profile).

Performance The fibres give the final composite its strength and stiffness, so the main objective of the design is to place the fibres in line with the loads to be carried. Fibres aligned in

Design one direction produce highly anisotropic materials with very high stiffness and strength in the direction of the reinforcement. Away from this direction, properties tend to fall away rapidly until at the orientation perpendicular to the fibres they become similar to those of the matrix.

It is common for individual plies of unidirectional material to be combined to form a more complex construction. Such a laminate may contain many individual layers, each at different orientations with respect to one another, the sequence of plies being determined by design considerations.

Reinforcements are available where tows or ravings (bundles of fibres) are stitched or woven together to form a fabric containing fibres at set orientations. Such reinforcements are characterized by the arrangement of the tows; common forms are:

unidirectional (UD), at least 95% of fibres in the 0 direction; biaxial fabrics (BX), fibres in the 0 and 90 directions; bias fabrics (XF), fibres in the +45 and -45 directions; quadraxial fabrics (QF), fibres in the 0, +45, -45 and 90 directions. Especially in woven fabrics, the 0 direction is called the warp and the 90 direction the weft. Reinforcements with fibres in different directions are usually balanced, i.e. they have equal number of plies in the plus and minus angle directions. Unidirectional fabrics are the most highly anisotropic, whereas balanced, quadraxial, reinforcements possess quasi-isotropic in-plane properties. Figure 4 shows some examples of reinforcing fabrics.

Typical properties of resin-infused laminated epoxy composites are shown in Table 3.

General rules for designing laminates. These are based on good practice that has evolved through experience:

Align fibres in the directions of all applied loads. Even if a design requires only 0 plies, include at least 10% of the laminate weight

as 90 or 45 plies to accommodate unexpected loads.


On: Tue, 15 Nov 2011 08:29:49

F R P c o m p o s i t e s

Table 3 Typical properties of composite materials produced using the RIFT method

Longitudinal (0) Transverse (90) TS CS Modulus TS CS Modulus

Fibre Reinforcement (MPa) (MPa) (GPa) (MPa) (MPa) (GPa) I / | 2 p (kg/m 3 )

Aramid Unidirectional 1280 2 9 0 70 39 150 6 0.34 1370 E-glass Unidirectional 7 0 0 5 8 0 39 72 85 9 0.26 1920 E-glass Quadraxial 380 312 23 383 324 23 0.28 1920 HS-carbon Unidirectional 2003 7 9 7 1 12 5 0 85 7 0.26 1510 HS-carbon Quadraxial 596 4 2 0 4 8 6 3 2 4 0 4 4 9 0.31 1510 HM-carbon Unidirectional 1 157 346 310 2 4 75 6 0.30 1660

TS, tensile strength; CS, compressive strength; HS, high strength; H M , high modulus.

Alternate ply orientations to prevent large matrix cracks forming. Where possible, keep the difference in angular orientation between adjacent plies

to below 60. Balance the laminate about the mid-plane to prevent bending effects reducing

in-plane properties. Optimize the stacking sequence to maximize properties for a particular load case. Taper thickness of laminates at edges to reduce peel stresses.

Codes. There are few codes for design of composite components. Certain products have been standardized, such as tanks, vessels and pipework (e.g. BS4994 for GRP tanks5). Given the variety of materials which can be used it is hard to know what mechanical properties to use in design calculations. The aerospace industry has completed thousands of materials tests to qualify specific materials but this then prevents adoption of newer and better developments until more testing has been carried out. Work is also underway in Europe to define a standard pultrusion that will have certain specified mechanical properties, but the end markets for these materials will be limited (though still significant). Recently the Concrete Society published design guidance for strengthening concrete structures using composites.4

The rest of the design and practice guide aims to fill the gap by covering strengthening of metallic structures and also by considering the design of all-composite structures. In the main, traditional hand-calculation methods will give a reasonable starting point, but more detailed analysis may then be required, perhaps using finite element (FE) type approaches. Most common FE packages now have composite modules, enabling more accurate modelling. It may be that for critical components the designs will then need to be validated by full-scale testing.

Durability. Durability can be defined as the ability of the material to continue to meet the performance specification with time. The environments most commonly of interest are water or chemicals, dynamic loading and fire.

Chemical resistance. In general composites are not greatly affected by common chemicals. Strong oxidizing agents and bases are the usual exceptions, but their effect varies with the resin matrix. Of the more common chemicals, water is usually the most aggressive, and the one that needs to be considered. Epoxies are the most resistant of the common matrices. Polyesters and phenolics are usually adequate.

Typically, glass reinforced composites show a loss in strength on exposure to water (of up to 50% in some circumstances). The rate of degradation depends on the rate of

8


On: Tue, 15 Nov 2011 08:29:49

O v e r v i e w o f f i b r e r e i n f o r c e d p o l y m e r c o m p o s i t e s f o r s t r e n g t h e n i n g

moisture absorption, and so increases with temperature, concentration (relative humidity) and area of material exposed. Carbon and aramid reinforcements show excellent retention of properties during service (usually above 80% of short-term strengths).

Fatigue performance. In general, fibre reinforced composites are more resistant to dynamic loads than metals. The fatigue mechanism involves the accumulation of cracks within the material, which in themselves are not critical. As the general level of damage increases the strength and stiffness of the material reduces (properties away from the fibre axes show a greater change). Carbon fibres show the greatest resistance to fatigue; glass and aramid show a greater reduction. Aramids in particular are susceptible to tension-compression loading. As a rough indication, if the loads in carbon reinforced composites are below half of the long-term strength then fatigue is unlikely to be a problem.

Fire performance. Epoxy and polyester-based materials are flammable. However glass-polyester composites are now commonly used in the offshore industry for fire protection. The degradation of the material actually occurs at the surface first, and then slowly proceeds through the thickness of the material. This gives excellent protection to the rear face of the composite.

Phenolic resins are used primarily because they do have good performance in fires. While polyesters protect the substrate underneath, they emit noxious fumes. The smoke and toxicity of the smoke emitted by phenolics are much reduced compared to other resins. Further, the rate of flame spread is also slower. Therefore in fire critical applications phenolic resins are commonly selected, and it is possible to satisfy various design standards, as set out in BS476.6

Adhesive bonding While there are a number of adhesive types available (epoxies, acrylics, poly-urethanes, cyanoacrylates, etc.) the ones most suitable for connecting composite components are epoxies. There is a wide range of epoxies available, and selection must be made to suit the curing conditions. Those with the best properties usually have to be cured at elevated temperature but systems are available that cure at ambient temperature. Formulations suitable for structural strengthening are readily available.

Adhesive bonds perform best in shear, and are not suitable for carrying tensile, peel or cleavage loads. Presuming that shear joints can be achieved the first step in making the bond is to ensure the surfaces to be joined are adequately prepared. The surfaces must be of a suitable cleanliness and surface roughness. Any contaminants left on the surface (oils from handling, etc.) will reduce the bond strength, and the surfaces need to have a degree of roughness to ensure a good key is achieved. It is common to prepare substrates to painting standards (e.g. SA2^), and while this gives repeatability to the process it may not always guarantee the highest strength.

Once the surface is prepared the adhesive needs to be mixed. Manufacturers can provide pre-weighed packs and automatic dispensers, or adhesives and hardeners can be measured out as specified by the manufacturer. Careful batching and mixing is essential to ensure the adhesive will cure.

The adhesive must be applied to the bonding surfaces. It is not always necessary to apply to both surfaces, but the application must ensure no air is trapped when the

9


On: Tue, 15 Nov 2011 08:29:49


surfaces are brought together. Some temporary support may be required, depending on the components being bonded.

The development of strength in the bond will depend on the specific adhesive and curing conditions. However, it is possible to get working strengths at ambient temperature within a matter of hours.

10


On: Tue, 15 Nov 2011 08:30:13

I. General introduction

General This design and practice guide has been prepared by the participants in the LINK Inland Surface Transport and the Partners in Technology (PIT) Programmes entitled Carbon Fibre Composites for Structural Upgrade and Life Extension, Validation and Design Guidance. The Participants were Devonport Royal Dockyard Limited, London Underground Limited, MSL Engineering Limited, Defence Evaluation and Research Agency (DERA), Southampton University, and Structural Statics Limited. This guide constitutes part of the final report for the project and is based on the findings from the projects and the experience of the participants in carrying out strengthening of metallic structures.

Scope The recommendations contained in this guide mainly cover the structural strengthening or repair of metallic components of onshore structures, using carbon fibre reinforced plastics (CFRP). The recommendations are concerned with strengthening and repair schemes in which either the CFRP is bonded to an existing structure, such that the existing structure and CFRP act together, or the new composite structure provides an alternative load path to the existing structure. The recommendations may be of limited application when applied to substrates other than metallic substrates, as each may require unique bonding materials and surface preparation techniques, and consideration of other potential failure modes.

The recommendations give advice on the selection of laminate materials, design of strengthening and repair schemes, FE analysis of strengthening schemes, manufacture of laminate, implementation of strengthening or repair, and remedial action to be undertaken where there is damage to the composite material used for the strengthening or repair.

Applications of The recommendations are applicable to the strengthening or repair of metallic technique components using CFRP, and to the analysis and design of composite sections made

entirely from advanced composite materials.

These recommendations apply to both serviceability and ultimate limit state requirements.

I I


On: Tue, 15 Nov 2011 08:30:13


Definitions For the purposes of this guide the following definitions shall apply: CFRP: Acronym for carbon fibre reinforced plastic a composite laminate

formed from carbon fibres embedded in a resin matrix. RIFT: Acronym for resin infusion under flexible tooling a manufacturing

process involving (vacuum) differential pressure to compress the fibre mats and to draw the resin through the fibres.

Reference material: The transformed properties (e.g. area, second moment of area, etc.) of a steel or CFRP section will be evaluated in the units of one of the materials, the so-called reference material.

CLT: Acronym for classical laminate theory.

12


On: Tue, 15 Nov 2011 08:30:32

2. Design specification

A design specification should be prepared at the outset of a project. This should include, as a minimum, a description of the structure or component to be repaired or strengthened and a consideration of design loads, load factors, design life and the environment. All limit states shall be defined within the specification. Design loads, load factors and design life for the CFRP reinforced structure should be agreed with the asset owner and the asset manager.

Design loads Where CFRP is used for strengthening and repair, the material is only subject to stresses due to changes in load effects following the application of CFRP.

Load effect changes may be due to the following:

changes in dead, superimposed dead load and live load; temperature effects due to temperature changes after the application of CFRP

caused by external restraints on the reinforced member or by; accidental load; changes in geometry due to load effects (possibly non-linear); partial failure of CFRP (which may leave a load bearing and possibly serviceable

structure); time effects on the substrate (e.g. aerobic or anaerobic corrosion, fatigue cracking,

gas embrittlement, creep, chemical conversion, chemical expansion, wear); time effects on CFRP (e.g. moisture uptake in matrix, UV radiation, fatigue

cracking, creep, corrosion at interface, electrolytic action between CFRP and substrate, abrasion and wear, moisture attack on non-carbon fibres, chemical attack by overcoating;

CFRP and substrate material property changes caused by elevated temperatures during a fire.

Load factors Partial load factors should take account of the following: inaccurate load assessment; poor structural or geotechnical data; incomplete or inaccessible structural survey information; imperfect structural modelling.

The magnitude of the factors should take account of the probability of occurrence, the design life of the strengthening or repair and the consequences of failure.

13


On: Tue, 15 Nov 2011 08:30:32


Environment As with all materials, the environment to which a composite component is to be exposed throughout its life must be considered at the design stage. The effects of exposure on the mechanical properties of a composite material will vary according to the nature of the environment and the constituents of the composite. The environmental conditions to be expected must therefore form an integral part of the design specification.

Environmental conditions are generally considered with respect to their effect on the degradation of the mechanical properties of the composite material. However, the environment may also impose additional indirect loads on the component that also need to be considered. Thermal stresses due to variation between installation and operational temperatures, or due to diurnal ambient temperature variation, can be significant and must be considered.

The response of laminate to a given set of environmental conditions will depend on the fibres, resin and manufacturing route. The effects of the following environmental conditions are considered:

temperature; moisture; chemicals including animal excreta; UV radiation; fire.

T e m p e r a t u r e It is usual to limit the operating temperature to which a composite material is subjected to 20C less than the glass transition temperature (tg) for an epoxy resin or the heat distortion temperature (HDT) for a vinylester or polyester resin. At this temperature there is an increase in rate of change of mechanical properties with respect to temperature, meaning that the performance degrades more rapidly. For cases where the composite material is also subjected to an aggressive environment, e.g. for repairs to process pipework, the operating temperature is limited to tg or HDT less 30C.

The operating temperature will have very important indirect effects on the composite component, and as such, must be included as a part of any design specification. At the design stage, consideration should be given to the following temperature effects as a minimum:

A higher operating temperature will increase the rate of creep or stress relaxation. Higher temperatures increase the rate at which environmental degradation due to

moisture uptake will occur. The coefficient of thermal expansion (CTE) of the composite material may not

match that of the structure to which it is applied. In this case, variation in temperature away from that at which the composite was applied will lead to thermal stresses. Thought should be given to the effect that this has on the design allowable stresses in both the composite and the structure.

Thermal cycling can cause degradation of the laminate through matrix cracking. This has been an area of research on supersonic aircraft where temperature differences often range from +120 to 50C. Matrix cracking is more common when there are a large number of rapid cycles and the structure experiences extreme negative temperatures during each cycle.

14


On: Tue, 15 Nov 2011 08:30:32

D e s i g n s p e c i f i c a t i o n

Moisture Any polymeric material in proximity to moisture, whether in the form of humidity or complete immersion, will absorb a certain amount of water. Polymers will absorb moisture until an equilibrium point is reached, this is known as saturation. In theory, the polymer being used and level of moisture in the environment will determine the saturation point. This saturation point will be different for a polymer and a composite produced from the same material due to the effect of the fibres. This generally reduces the amount of moisture absorbed. However, polymeric fibres such as aramid and polyethylene, will also absorb moisture. This leads to additional problems because the fibres swell as the moisture is absorbed, which can lead to internal stresses and fibre or matrix disbonds.

It can be seen therefore that it is very important to specify ambient moisture level, in order that the design allowable stress is reduced in line with the expected long-term degradation of the composite. However, the mechanical degradation due to moisture should not be viewed in the same way as moisture corrosion in metals. Once saturation is reached there will be no further reduction in mechanical performance. This is unlike steel, which will continue to corrode.

Chemicals There are a whole host of chemicals that could come into contact with a composite over its life-span, but it is important to characterize which chemicals it will see for periods that could produce mechanical degradation. Characterizing a composite's response to a chemical is often time-consuming and costly. It is therefore important to do as much research as possible before testing. Materials suppliers will often be able to provide information on what chemicals are likely to degrade the composite.

Organic solvents are known to attack the polymer matrix of a composite and degrade its mechanical properties. However, as these solvents often evaporate quickly the time it spends in contact with the composite would generally be small. This may not give any significant degradation, but this would be highly dependent on the solvent and the exposure time.

The molecular size of the chemical relative to the matrix will give an indication to whether it will be absorbed by the composite. If it is too large no absorption will take place and degradation, if any, will only be on the exposed surface. Ionic fluids such as acid and alkalis can be absorbed by the composite and this could degrade the material. The concern with acids and alkalis is that although the material will reach equilibrium for fluid uptake, the ionic components of the fluids will continue to attack the fibres. This means that the assumption that at equilibrium the ultimate mechanical property degradation has been reached does not apply. For this reason it is very important to specify the potential exposure to acids and alkalis so that account may be taken of this when determining the design allowable stress for the composite material. In addition to this, glass fibre reinforced plastics can show a catastrophic reduction in strength when exposed to acidic solutions while the composite is under load (a phenomenon known as stress corrosion). Principally this is due to the fibres cracking when attacked by the acid.

Ultraviolet radiat ion Glass and carbon fibres are very resistant to UV degradation. Aramid fibres are susceptible to UV degradation. However, light is not transmitted through an aramid laminate and so only the surface plies will be affected. It is also possible to eliminate this by the addition of a protective layer. Aramid composites are successfully used on satellites, where UV radiation is much greater, through this process. The resin matrix

15


On: Tue, 15 Nov 2011 08:30:32

FRP composites

may degrade slightly as a result of UV exposure, although this may be designed out through choice of appropriate resins and/or the inclusion of a pigmented gel coat. Where a composite is to be subject to direct sunlight it is important to specify this so that the appropriate steps may be taken to guard against UV degradation.

Fire Polymer composites are susceptible to degradation through fire. The organic polymer matrix will, once a fire has taken hold, act as a fuel and promote fire growth. Along side this, the by-products given off from the polymer as it burns are often highly toxic. There are polymers that do not easily burn and composites containing phenolic resin are often used in areas where there is a potential threat of fire. However, current polymers that have good fire performance or contain fire retarding additives tend to have reduced mechanical performance. There are materials being developed that will solve this dilemma but they are several years away from full production. Currently, where high mechanical performance is required it is possible to protect the composite through the use of intumescent and fire retardant coatings. These react to the presence of fire and either provide an insulating barrier for the composite or produce by-products that reduce the energy of the fire.

16


On: Tue, 15 Nov 2011 08:30:55

3. Design process overview

Following strengthening or repair, a component is likely to be stiffer than before and may consequently attract a greater proportion of load than hitherto, if the component is part of a redundant structure. This may be dependent on the level of pre-strain at the time of strengthening and the stress-strain curve of the parent component or substrate. Furthermore, the nature of CFRP is such that total failure may be imminent once first ply failure has occurred which may reduce the deformation capacity of the component. These aspects must be considered during design, either explicitly or by increasing factors of safety against CFRP failure.

When laminate is not placed symmetrically about the neutral axis of an unreinforced section, it should be recognized that there will be a shift in the position of the neutral axis. The stresses due to additional loads carried after the placement should be calculated by reference to the shifted neutral axis. The total stresses may then be found by superposition of the stresses due to pre-load and the stresses due to the additional loads.

Although laminates will generally enhance the performance of existing structures, the placement of the reinforcement may induce a different failure mode. For buckling considerations, the placement of the laminate should be such as to ensure that there is an increase in flexural stiffness, without a proportionate increase in the slenderness of the reinforced component. Thus, the position of the CFRP should be such that the radius of gyration is increased.

When laminates are bonded to existing structures subject to changes in loads (with no stress relief), the bond strength of the adhesive needs to be considered before full cure, to ensure the composite action of the metallic substrate and laminate is achieved. It may thus be necessary to allow some time for the resin to cure, so that the full shear strength is developed before imposing additional loads.

General design Composites can be used in structures designed plastically in which a significant principles amount of hinge rotation is allowed leading to redistribution of load. However, in

either elastic or plastic global analysis, the laminate shall always remain elastic. Thus, when composites are used in combination with metallic alloys which may undergo significant plastic deformations, reaching plastic strain values several times their initial yield strain, the failure stress (strain) of the laminate may be the limiting criterion to determine the load carrying capacity of the structure.

Repair and strengthening

17


On: Tue, 15 Nov 2011 08:30:55


When a metallic structure or structural component is reinforced with a CFRP laminate and then subject to loading, the differences between the mechanical properties of the metal and the laminate will need to be considered, as this will determine the load share between the laminate and the substrate. Furthermore, the failure modes of the materials may differ significantly.

Figure 5 illustrates, in flowchart form, the logical steps involved in the design or analysis process. Generally, the existing and additional loads will be known, as will be the existing structural form and mechanical properties of the substrate material. The only variable is the design of the laminate itself (fibre, type, resin, fibre architecture or orientations and thickness). The designer is free to select and orient the fibres to optimize the laminate properties in any direction (i.e. the properties can be anisotropic) and hence the performance of the strengthening or repair scheme. In many cases, such as beam flanges, the direction of the principal stress will be fairly obvious and clearly the optimum laminate will have very nearly all the fibres so oriented. There will be a small percentage of fibres laying in the orthogonal direction; these hold the fibre mats together. However, in other instances, the principal stress direction will not be known a priori and numerical analysis (i.e. FE analysis) may be required. Unfortunately, the computed stresses in the laminate are themselves a function of the laminate properties and it may be necessary to consider a number of alternative laminate designs to establish the optimum one. This design loop is as indicated by the broken lines in Figure 5.

It is necessary to ensure that no premature failure can occur in any component part of a composite strengthening or repair scheme. This entails a consideration of the original structure (the substrate), the CFRP laminate and the adhesive bond between them. The stresses in each component are dependent on the structural interaction between the substrate and the CFRP laminate and therefore, in turn, on the mechanical properties of the component materials. Furthermore, recognition has to be given to the different stress distributions arising from loads already existing in the structure before implementing the scheme and those (additional) loads applied

Load in substrate before strengthening/

repair

Additional load applied after

strengthening/ repair

Substrate mechanical properties

Stresses in substrate

Figure 5 Overview of design or analysis process

Design checks

OK

Load share between

substrate and laminate

Stresses in laminate

Laminate mechanical properties

r

Design of laminate

Not OK

Design loop

18


On: Tue, 15 Nov 2011 08:30:55

D e s i g n p r o c e s s o v e r v i e w

afterwards. Note that sometimes it may be possible to reduce the existing loads before scheme implementation by jacking, propping or removal of superimposed dead load. Following the scheme implementation, the laminate will resist a proportion of the reinstated load (plus any new additional loads).

In the development of a design concept, various load conditions should be considered, as this will dictate possible failure modes and the choice of a suitable laminate. Various strengthening and repair schemes should be considered before deciding on the most appropriate solution. The concept design will normally be the result of a prolonged assessment process.

Stress concentrations between substrate and laminate should be mitigated, to prevent premature failure of the adhesive bond. This can be achieved by specifying a taper of the laminate of between 1:10 and 1:4 at the ends of laminate used to reinforce metallic sections, and also by avoiding sharp corners.

Structural design The design analysis techniques that can be used for strengthening and those for repair may differ. For instance, whereas it is possible to analyse an under-strength, though otherwise intact column using existing codes of practice, the codes are not directly applicable to a damaged (e.g. dented or out-of-tolerance bowed) column. Damaged members, whether reinforced or not, require a more fundamental assessment of their capacity. This may involve the use of advanced methods of analysis such as FE analysis.

The strength of members may be assessed in accordance with the relevant requirements given in Codes of Practice [e.g. Refs 711]. Several modes of failure may need to be considered. It should be noted that the use of Codes is restricted to intact components.

For the design of columns susceptible to flexural-torsional buckling modes of failure and more complex geometries, design of strengthening schemes using CFRP will of necessity be based on the results of FE analysis or other numerical methods.

The detailed design stages of a strengthening or repair scheme using composites should be based on laminate properties determined through testing. Testing allows for the effects of raw materials variability, processing conditions and process variability, and long-term effects. Although composite materials do not corrode, when using laminates for strengthening or repair, particular attention should be given to the effects of moisture, fatigue, creep and chemical exposure.

Consideration of the geometry of the component to be strengthened or repaired is important, as this will affect the placement of the laminate for maximum effective-ness and efficiency. The addition of a laminate will also always result in enhancing the stiffness of the component or structure. Since the weight-to-stiffness ratios of laminates are very low, the presence of the CFRP will also invariably improve the response of the structures to dynamic loads such as vibration.

Materials selection Material selection is an important part of the design process. It is vital that the materials selected are appropriate to the function that they will have to fulfil. The materials and attributes that have to be selected are:

fabric reinforcement (i.e. the arrangement of the fibres);

19


On: Tue, 15 Nov 2011 08:30:55


reinforcing fibre; resin system.

When selecting the material type, consideration must be given to the following factors:

structural and physical properties; suitability for use in the intended environment; suitability for use in the intended manufacturing process.

Fabric re in forcement There are five common types of fabric reinforcement, which can be used, these being:

Unidirectional fabrics: These fabrics have all the reinforcing fibres in the 0 direction. Unidirectional fabrics usually have the lowest weight of reinforcing fibre per ply and are usually narrower than other fabrics, therefore they require the most labour to lay up a laminate of a given thickness.

Biaxial fabrics: These fabrics have reinforcing fibres in the 0 and 90 directions. There are usually equal proportions of fibre in each direction (balanced). However, fabrics can be produced with different proportions of fibre in each direction.

Bias fabrics: These fabrics have reinforcing fibres in the +45 and 45 directions. There are usually equal proportions of fibre in each direction. These type of fabrics are usually specified where shear loads are predominant.

Triaxial fabrics: These fabrics have reinforcing fibres in the 0, +45 and 45 directions. Typical arrangements will have 50% of fibre at 0, 25% at +45 and -45, or 33% in each of the 0, +45 and -45 directions. Fabrics with different proportions of fibre in each direction can be produced.

Quadraxial fabrics: These fabrics have reinforcing fibres in the 0, +45, 45 and 90 directions. Typically, the arrangement will have 25% of fibre in each of those directions. Fabrics with different proportions of fibre in each direction can be produced. Quadraxial fabrics usually have the highest weight of reinforcing fibre per ply, therefore they require the least labour to produce a laminate of a given thickness.

The primary strength and stiffness of a composite laminate is in the directions of the fibres. This means that a composite component may be optimized by aligning fibres in the directions of the applied loads. For example, where a tensile flange is to be reinforced unidirectional fabric is appropriate for the direct stresses due to bending; where a web is to be reinforced, a 45 directional fabric is appropriate to carry the shear forces. It is common practice where unidirectional fabrics are used to include a small percentage (a value of 5% by laminate thickness is commonly adopted) of perpendicular fibres to accommodate unforeseen loading (such as may occur during installation).

Reinforcing f ibre The type of fibre selected will alter the laminate strength, stiffness and coefficient of thermal expansion (CTE).

The laminate must have sufficient strength (specific to the direction in which loading is applied) to allow the structural loads to be taken in a laminate of thickness compatible with the geometry, weight and process constraints. The laminate must also have sufficient stiffness (i.e. rigidity) to allow deflection and other serviceability

20


On: Tue, 15 Nov 2011 08:30:55

D e s i g n p r o c e s s o v e r v i e w

criteria to be met by the component. Note, the higher the rigidity of the laminate, the greater are the shear stresses in the bond, see Strain compatibility in Chapter 4.

The CTE of a laminate will vary widely depending on the type of fibre selected and the direction of the fibres. Where the CTE of the laminate differs from that of the material to which it is to be bonded then thermal stresses will be induced. In this instance, the thermal stresses arising from the worst credible temperature range must be accounted for in design.

Both the stiffness and the CTE of a laminate may be calculated using classical laminate theory (CLT). A brief overview of CLT is given in the section 'Accounting for long-term degradation' in Chapter 4.

Common types of reinforcing fibres used for structural upgrade and life extension are:

E-glass: This is the most commonly used type of glass reinforcement for structural applications. E-glass (E for electrical) draws well, and has good strength, stiffness, electrical and weathering properties. It is used for components that are only lightly loaded or for applications where low weight/high stiffness is not crucial. E-glass is also used in conjunction with carbon fibres where insulating properties are required, e.g. at the interface between a carbon fibre laminate and a metallic substrate. E-glass also provides good resistance against impact and abrasion damage; for this reason it is often used in surface plies to protect the main structural laminate.

High strength carbon: HS carbon offers higher strength and stiffness than E-glass at a lower density. HS carbon is used for components subject to high levels of stress, or where a high strength-to-weight or stiffness-to-weight ratio is required. Compared to E-glass, HS carbon offers superior resistance to environmental degradation.

High modulus and ultra-high modulus carbon: HM and ultra-high modulus (UHM) carbon offer higher stiffness than HS carbon but at a reduced strength. UHM carbon should be used where there is a requirement for very high stiffness or stiffness-to-weight ratio. UHM carbon is commonly used to reinforce structural members as it can remove the need for pre-stressing.

Aramid: Aramid fibres are produced in LM and HM grades. The main characteristics of aramid fibres are their high strengths, moderate Young's moduli and low densities. Laminates formed from aramid fibres are known for their low compressive and shear strengths. The fibres themselves are susceptible to degradation from UV light and moisture. Aramid fibres are therefore used for structures requiring high tensile strength and impact resistance, but not shear or compression strength. They are often used in combination with other fibres (hybridized) to provide improved impact resistance.

Table 4 gives typical material properties for various fibres.

Table 4 Typical material properties for fibres

Fibre Strength ( N / m m 2 ) Young's modulus ( k N / m m 2 ) Density (kg/m 3 )

Aramid 3 1 5 0 - 3 6 0 0 5 8 - 1 6 0 1 3 9 0 - 1 4 7 0 Carbon 2 1 0 0 - 7 1 0 0 2 2 0 - 9 0 0 1 7 4 0 - 2 2 0 0 Glass 3 4 4 5 - 4 8 9 0 7 2 - 8 7 2 4 6 0 - 2 5 8 0

21


On: Tue, 15 Nov 2011 08:30:55


Resin system The main functions of the resin system are:

to protect the fibres from environmental and mechanical damage; to transfer loads between the fibres.

The choice of a resin system will have only a small effect on the laminate mechanical properties, as these are fibre dominated. Thus the important characteristics of the resin system are its environmental stability and its adhesive properties (for transfer of load).

Common types of thermoset resins (thermoplastic resins are used in composite components although their use is not common in structural applications) are: Phenolics: Phenolic resins have good performance in fire (low smoke and toxic

fume emission, low inflammability, heat release and surface spread of flame), good retention of mechanical properties at elevated temperatures. Phenolic resins should be used where these properties are of prime importance. Care must be taken during manufacture to avoid problems due to residual acid catalyst, residual water and microvoids resulting in reduced long-term mechanical properties of laminates. In addition to this, the manufacture of phenolic laminates requires a higher level of personal protective equipment than for manufacture using other types of resin. Phenolic resins are generally only used to produce laminates with glass fibre reinforcement rather than carbon or aramid.

Polyester: Polyester resins have good mechanical properties. Manufacturing difficulties lead to problems when producing laminates with high fibre volume fractions. This leads to the use of polyester resins being limited to moderately loaded structural components. Polyester resin is generally only used to produce laminates with glass fibre reinforcement rather than carbon or aramid.

Vinylester: Vinylester resins have good mechanical properties and excellent chemical resistance. The drawbacks of vinylester resins are a high degree of shrinkage during cure and a high styrene content, leading to concerns for use in open mould manufacturing processes. Vinylester resins are normally selected where good chemical resistance is required.

Epoxies: Epoxy resins have excellent mechanical properties and excellent chemical resistance except to strong acids. Epoxies have low shrinkage during cure, meaning that laminates produced with epoxies have low internal residual stress levels. Precise control of the resin mixing operation is required in order to ensure that no uncured material remains in the laminate. Uncured material would reduce both initial laminate properties and affect the long-term durability of the structure. Epoxy resins are selected where high structural performance is required.

In some instances it is possible to use a variety of resins within the same laminate, e.g. a phenolic surface layer over an epoxy laminate for fire protection. Once a generic type of resin has been selected, a specific resin system must be chosen. Consideration must be given to the mechanical and physical properties, as well as the environmental resistance and processing characteristics (viscosity, gel time, cure schedule and shrinkage) of the resin.

22


On: Tue, 15 Nov 2011 08:31:16

4. Composite design

Selection Selection of the type of laminate to be used (fabric type, fibre type and resin type) should be a concurrent activity with the structural design. Careful consideration needs to be given to exactly what function the composite component is to perform within the structure. Key considerations are strain and thermal expansion compatibility between the laminate and the substrate material, and ultimate strength of the laminate. Composite materials can be produced with a wide range of Young's moduli, CTE and ultimate strengths, these properties may be arranged so that they are direction specific or quasi-isotropic. This stage in the design process, tailoring material properties to requirements, is the key difference between designing in composites and designing in metals.

Strain When bonding a composite component to an existing structural member it is compatibility important to consider the effects of strain compatibility; the shear stress in the

adhesive bond will place an upper limit on the stiffness or thickness of the composite that can be applied. Table 5 indicates appropriate laminate types for given applications. 7 - 1 2 For example, the use of UHM carbon fibre reinforcement on a concrete or timber structure would not be appropriate. The shear stress generated at

Table 5 Recommended laminate types for typical constructional materials

Substrate Appropriate Reinforcement Substrate rigidity ( k N / m m 2 ) reinforcement rigidity ( k N / m m 2 )

Timber 4 - 2 2 E-glass/HS carbon 2 3 - 1 1 2 C o n c r e t e 1 8 - 3 8 E-glass/HS carbon 2 3 - 1 1 2 Cast iron, wrought iron, steel 9 0 - 2 0 5 UHM carbon 3 1 0 - 3 6 0

Table 6 Effect of fibre type on laminate stiffness

Fibre Fabric Fibre angles Stiffness in 0 ( k N / m m 2 )

HS carbon Unidirectional 0 112 HS carbon Quadraxial 0 , + 4 5 , - 4 5 , 90 4 8

Values calculated using CLT and manufacturers' data for the fibre and resin.

23


On: Tue, 15 Nov 2011 08:31:16


the interface between the substrate and the reinforcement would exceed the shear strength of the substrate material.

The choice of fibre type is not the only factor that will influence the stiffness of the produced laminate. As has been covered in the section on Fabric reinforcement, a composite material possesses strength and stiffness primarily in the direction in which the fibres are oriented. Table 6 gives an example of the effect of fabric type on the composite stiffness.13

Therma l When bonding a composite component to an existing structural member, it is also compatibil ity important to consider the effects of differential thermal expansion. The CTE of a

composite material will vary according to the choice of fibre and resin system. Furthermore, the CTE will also change dramatically according to the fibre orientation within the laminate. All of the reinforcement fibres commonly in use exhibit markedly different thermal expansion characteristics in the longitudinal and transverse directions. Many carbon fibres exhibit the unusual characteristic of contracting, in the longitudinal direction on heating.

Table 7 indicates typical CTE for a variety of fibre types laminated with epoxy resin.

The typical CTE for an epoxy resin is 60 x 1 0 - 6 K - 1 which, by the rule of mixtures, means that in most cases the CTE of a laminate is positive even if the reinforcing fibres contract on heating.

The testing required in order to determine the CTE is a specialized task. It is common practice to calculate the CTE of a composite material from the data supplied for the component materials using the following equation:

_ q m ( l -f)Em + affEf a c

~ (1-nEn+fEf ' ( i j where ac is the CTE for the composite material, am is the CTE of the resin matrix, ctf is the CTE of the fibre reinforcement, / is the fibre volume fraction, Em is the Young's modulus of the resin matrix and Ey is the Young's modulus of the fibre reinforcement.

The above equation relies on the fact that since there is no applied stress, the internal stresses due to thermal expansion must counter balance each other. This method supplies reliable results, although it is not entirely rigorous as the strains due to differential Poisson's contraction are neglected.

Table 7 CTE for fibres

Fibre Fabric Fibre angles

CTE in 0 ( x I 0 - 6 K - ' )

CTE in 90 ( x l O ^ K T 1 )

E-glass Unidirectional 0 8.4 20.91

Quadraxial 0 , + 4 5 , --45, 90 11.0 11.0

HS carbon Unidirectional 0 0.52 25.5 Quadraxial 0 , + 4 5 , --45, 90 9.76 9.76

UHM carbon Unidirectional 0 - 0 . 0 7 25 .68 Quadraxial 0 , + 4 5 , - 45 , 90 9.6 9.6

24


On: Tue, 15 Nov 2011 08:31:16

C o m p o s i t e d e s i g n

Given that the value of CTE for mild steel 12 x 1 0 _ 6 K _ 1 , the importance of allowing for differential thermal expansion is clear. For example, consider an U H M CFRP plate, which has been bonded to the tensile flange of a steel beam during a cold day. During hot weather, the CFRP plate will be subject to additional tensile stress and the adhesive bond subject to additional shear stress.

Ult imate Strength The ultimate tensile strength of a CFRP laminate used for rehabilitation must be capacity appropriate to the structural application. The reinforcement of structural members

using composite materials is often stiffness driven (e.g. where cast iron is reinforced on a permissible stress basis). In this instance, the reinforcing laminate should be tailored such that at the design stress an adequate material reserve factor exists.

It is also possible that in some structural upgrade schemes the design may be strength driven. For example, if the structural member to be strengthened was designed such that the formation of a plastic hinge were permitted (i.e. a blast wall support), then the ultimate strength capacity of the reinforcing laminate would govern design.

Another case where a reinforcement scheme would be strength rather than stiffness driven is if the composite plates are to be pre-stressed. Pre-stressing of composite reinforcement may be used where the member to be strengthened is close to its capacity due to dead load. In this instance the effects of dead load are overcome by transferring existing stress from the member into the reinforcement, thus it is this strength rather than the stiffness of the laminate which is important.

The following guidelines may assist in the selection of suitable laminate material properties for the strengthening, modification or repair of an existing component:

High strength (HS) carbon laminates are usually the most cost-effective for structural components.

Intermediate modulus (IM) carbon laminates should be used where there are very high tensile strength requirements, which cannot be met by thicker, HS carbon laminates because of geometry or weight constraints.

High modulus (HM) composites should be used where there are very high stiffness requirements, such as when limiting bending deflections.

A layer of E-glass should be used between the carbon fibre and the steel to prevent galvanic corrosion.

Choice of The various manufacturing processes (see Chapter 6) for composite materials produce manufacturing laminates of different qualities, which will affect the mechanical properties of the process laminate produced. The key process controlled parameters affecting the mechanical

properties of the laminate are the fibre volume fraction and the void content.

Table 8 gives an example of the effects of different manufacturing processes on the Young's modulus of a typical U H M carbon laminate.

Table 8 Effect of manufacturing process on elastic modulus

Process Fibre vo lume f rac t ion V o i d c o n t e n t Stiffness in 0 ( k N / m m 2 )

Hand lay-up 0.4 U p t o 5% 230 RIFT 0.54 < l % 310 Pre-preg 0.60 1 - 2 % 360

25


On: Tue, 15 Nov 2011 08:31:16


The process related costs increase from simple hand lay-up to the more complicated pre-preg process. The choice of manufacturing process needs to be made with careful consideration as to the end use. For example, where E-glass has been chosen for a laminate, as high strength or stiffness is not a requirement, it would be inappropriate to manufacture the laminate using the pre-preg process.

Determinat ion of The determination of material properties for design purposes will be based on data basic material obtained through testing of the specific laminate to be used. Laminate testing properties characterises the response of the composite material in a given laminate design. The

material properties' tests undertaken will be determined by the end-use of the component that is required. For designers unfamiliar with the use of composites, it should be pointed out that composites may not be treated as isotropic. Strengths and stiffnesses can vary widely according to the direction of load application and whether they are loaded in tension or compression. Even quadraxial fabrics, that are termed quasi-isotropic, will respond very differently if loaded through thickness compared with in-plane - as there are no fibres oriented in this direction.

When the material properties required have been identified, with careful consideration to the manner in which load will be applied to the composite, a testing matrix to determine these properties should be defined.

Materials test ing Coupon testing provides an empirical means of determining intrinsic material properties. These material properties are then modified to account for variability and long term degradation before being used for the design of composite components. Test methods developed specifically for composite materials must be used at all times, as methods developed for metals or plastics are not applicable. Test methods must account for the following requirements:

The need to evaluate properties in multiple directions. The need to condition specimens to quantify and control moisture absorption and

de-absorption. The increased importance of specimen alignment and load introduction method. The heightened sensitivity to specimen preparation practices.

British Standards, ASTM and a number of other standards agencies cover composite specific test methods. The applicable test standard used should be stated when quoting the results of the testing.

It is assumed that laminates will contain representative defects such as

voids; fibre misalignment; local fibre volume fraction variations; resin micro-cracks due to curing stresses.

Test laminates should be manufactured using representative processes, under representative environmental conditions, thus providing a test laminate representative of that specified by design.

Design allowable material properties need to account for the inherent variability of the composite material. A commonly used method of accounting for this variability

Al lowance for variabil i ty

26


On: Tue, 15 Nov 2011 08:31:16


is to apply a factor that characterises the material variability arising from a given manufacturing process. This approach provides a qualitative assessment of variability and should be used only where sufficient test data is not available. A more accurate method of accounting for material variability is to apply a statistical analysis to the raw test data. This approach is preferable to the earlier as it provides a quantitative assessment of variability.

It is recommended that a statistical analysis of the test results be conducted in order to determine strengths that account for material variability. There are a number of different statistical analyses that may be applied. The simplest approach is given by the following equation:

o~k = crm- 2s, (2)

where 0^ is the characteristic strength, am is the mean strength given by the test results and s is the standard deviation.

The statistical approach detailed above assumes that sufficient samples were tested to ensure statistical relevance. The number of specimens required for statistical relevance is not inherently specified. It is therefore recommended that this approach be used only for non-critical components.

A slight variation on this technique is the B-basis analysis. This analysis assumes that the test specimens belong to a common probability distribution. The probability distribution is termed a population', of which the test specimens are a random sample. A B-basis value is that which will be exceeded by 90% of results with 95% confidence. The B-basis value is given by equation:

o~k = o~m - hs, (3)

where is the characteristic strength, o~m is the mean strength given by the test results, Icb is the tolerance limit factor for the normal distribution and s is the standard deviation.

The value of kg can be obtained from statistical tables. The advantage of the B-basis approach is that the number of samples required is not fixed. The greater the number of test specimens which go to make up a sample, the lower the tolerance limit factor. Seventeen test specimens would be required in order to achieve a kg value of 2, which would give the same result as the simple approach described above.

Account of the variability in stiffness of a composite material can also be made using this approach.

Accounting for The importance of specifying the environment to which the composite material will long-term be exposed throughout its design life was covered in Chapter 2. The effect of long-degradation term degradation on composite material strength is accounted for by the use of partial

factors. The overall partial factor applied to the material strength is comprised of sub-factors defined for a number of degradation mechanisms. In this manner, the material strength can be modified according to a detailed appraisal of the degradation mechanisms to which the material will be exposed. Limits for the value of individual degradation factors for epoxy resins reinforced with glass, carbon or aramid fibres are given in Tables 914.

27


On: Tue, 15 Nov 2011 08:31:16


Table 9 Degradation factors for laminates with E-glass reinforcement

Degradation mechanism Suggested maximum Suggested minimum

Moisture 0.9 0.5 Chemical exposure 1.0 0.75 UV exposure 1.0 0.95 Fatigue 1.0* 0.25f Creep 1.0 0.4 Impact 1.0 0.75

Overall degradation \d 0.4 0.15

* T h e factor of unity applies only where there is no fatigue loading.

f Lower bound value due to high amplitude, high frequency load cycles.

Table 10 Degradation factors for laminates with aramid reinforcement


Moisture 0.9 0.8 Chemical exposure 1.0 0.85 UV exposure 0.9 0.75 Fatigue 1.0* 0.4f Creep 1.0 0.6 Impact 1.0 0.8


* f Refer footnote of Table 9.

Table 11 Degradation factors for laminates with carbon reinforcement


Moisture 1.0 0.85 Chemical exposure 1.0 0.85 UV exposure 1.0 0.95 Fatigue 1.0* 0.5f C r e e p 1.0 0.8 Impact 1.0 0.5


*, f Refer footnote of Table 9.

Table 12 Typical partial factor using E-glass

Degradation mechanism Factor

Long-term environmental exposure 0.4

Creep/s tress corros ion 0.4

Overall degradation 0.16

28


On: Tue, 15 Nov 2011 08:31:16


Table 13 Typical partial factor using CFRP

Degradation mechanism Factor

Moisture 0.85 Chemical exposure 0.85 UV exposure 0.99 Fatigue 0.62 Creep 0.9

Overall degradation 0.4

Table 14 Recommended stiffness degradation factor

Material Suggested degradation factor

Carbon FRP 0.91 Aramid FRP 0.91 Glass FRP 0.55

The 'impact' factor accounts only for accidental loading. If a component is to be designed specifically to resist impact loading a more stringent approach should be adopted. The 'overall degradation' row in the above tables sets limits on the partial factor applied.

The following examples are taken from real applications:

(1) E-glass structure, installed sub-sea and subject to a high static load. (2) CFRP plate used to reinforce bridge beam soffit subject to live loading and

indirect sunlight only.

The selection of suitable material partial factors is largely a question of experience and engineering judgement, the factors proposed in this document are presented as a guideline only. Current selection of material partial factors is based on 3040 years of limited experience and is, of necessity, conservative. As experience in the use of composites for structural applications grows, these material factors are likely to be increased.

The stiffness of a composite material is a fibre-dominated property where the fibres are oriented in the direction of the loading. In this instance, the environmental degradation of the stiffness of a composite material is negligible unless the environment is permitted to attack the fibre itself. Where multi-axial fabrics are to be used, or the direction of loading does not coincide with the fibre direction, then degradation of the stiffness of the matrix must be accounted for. It is good practice to apply an environmental degradation factor to the stiffness values of all FRP components to be used for structural rehabilitation. This gives an added degree of conservatism in the case of on-axis loaded unidirectional laminates. The recommended stiffness degradation factors are shown in Table 14 [Ref. 4, Table 5.4].

Design allowable The design allowable strength including variability and environmental degradation is strength and given by stiffness

n v A / ( A \

29


On: Tue, 15 Nov 2011 08:31:16


where is the characteristic material strength from equations (2) and (3), and Xd is the environmental degradation factor from Tables 914.

The design allowable stiffness may be calculated using the same approach, taking the characteristic material stiffness (which allows for variability) and applying the suggested degradation factor appropriate to the reinforcing fibre being used.

Laminate design It is not practical to undertake material test programmes prior to the design of every composite component. It is common practice to validate a range of composite materials, based on different fibre, resin and fabric types, and to use the material properties from these as validation for new designs. This approach may not however provide the most cost-effective solution, due to the fact that composite materials can be tailored specifically to the applied loads. For conceptual design work, it is common practice to calculate the laminate properties to be used for design (the calculated values would be confirmed by a test programme, prior to completion of the detailed design). This concept of designing a material to give the desired set of properties is one of the ways in which composite materials can deliver optimized designs.

The method used to determine the stiffness properties of laminates is known as CLT. The laminates are usually considered to be formed from multiple layers of unidirectional plies, arranged in the specified orientations. For example, a quadraxial fabric is modelled as unidirectional plies at 0, + 4 5 , 45 and 90.

The properties of the unidirectional plies are calculated from knowledge about the properties of the constituent materials and the process used to produce the laminate. The principal elastic constants of a unidirectional ply are:

E i , composite elastic modulus in the fibre direction; Eiy composite elastic modulus transverse to the fibre direction in the plane of the

reinforcement; E3, composite elastic modulus transverse to the fibre direction, normal to the

plane of the reinforcement, i.e. through thickness; G12, composite shear modulus in the plane of the reinforcement; 1/12, major Poisson's ratio; ui\y minor Poisson's ratio.

The following physical properties for the ply are also calculated:

a i , longitudinal CTE; Oiiy transverse CTE; t, thickness; p, density.

Equations for the calculation of these properties are beyond the scope of this document, but can be found in textbooks on the subject such as Ref. [14].

The properties of the unidirectional plies listed above, form the building blocks from which more complex laminates may be constructed. The unidirectional plies have three orthogonal planes of symmetry, the 1-2, 2-3 and 1-3 planes, which are illustrated in Figure 6.

For the 'building block' unidirectional ply shown above, a simple two-dimensional stress state is usually assumed for which the stress-strain relationship can be

30


On: Tue, 15 Nov 2011 08:31:16


Figure 6 Coordinate system for a unidirectional laminate (direction I - parallel to the direction of the fibres, direction 2 - normal to the direction of the fibres, parallel to the plane of the laminae, direction 3 -normal to the direction of the fibres, normal to the plane of the laminae)

G l o b a l a x i s

Figure 7 Coordinate system for a multidirectional laminate

L o c a l a x i s

denned as

'Sn S12 0 "

Sn S22 0

712 J 0 0 ^ 6 6 . [ T12 ( 5 )

where

Sn >22 $ 6 6 = 7 ^

31


On: Tue, 15 Nov 2011 08:31:16

FRP c o m

frp strengthening of metallic structures

Documents

thomas telford publishing

thomas telford books

c e d e s i g n

institution of civil

c t i c e g u i d e

o y l1 thomas telford

practice guides

experienced engineers