XVI Introduction continued in Chapter 3,including a detailed description of finite element simulation techniques for forming of dry fabric preforms.The methodology here can be considered similar to that used for sheet metal forming,albeit with a more complex material model.Chapter 4 continues the modelling theme,with a description of 'virtual testing',whereby materials input data for forming simulation are predicted from the material structure.This topic is of particular interest,as it may offer the opportunity to select materials that are fit-for- forming,or even to design new materials with a specific component in mind. Chapter 5 details the use of modern simulation techniques for composite forming within an optimisation scheme,with the aim of selecting materials and process parameters to eliminate such defects as wrinkling or undesirable fibre orientations.Chapter 6 describes the methodology and current status of simulation tools for compression moulding,including applications to sheet moulding compound(SMC)and glass mat thermoplastic(GMT).The following chapter completes the initial treatment of simulation and modelling,with a description of composite distortion -notably the common phenomenon of spring-in'-caused by manufacturing induced stresses. The second half of the book begins with four chapters describing forming technologies for a range of materials.This begins with a relatively new family of materials-composite/metal hybrids-which have recently found applications in the aerospace sector(notably as fuselage panels for the Airbus A380).Another new family is covered next,referred to as self-reinforced polymers'.These materials include fibre and matrix from the same polymer material,addressing one of the current concerns for polymer composites-recycling.The next two chapters cover more conventional materials-thermoset prepreg and thermoplastic composite sheet.Prepreg forming technologies are described in detail,from the traditional hand lay-up and autoclave cure approach to current 00 developments in automated tape placement and diaphragm forming.The 、%y thermoplastics chapter includes a detailed description of the range of material forms,along with their appropriate forming and consolidation techniques. Chapter 12 describes the current state-of-the-art in simulation software for composite forming within an industrial context,detailing the use of modern software tools to design the material lay-up,and describing how these tools can be integrated within the manufacturing environment.Finally Chapter 13 covers the issue of benchmarking of composite forming.This topic is particularly timely,drawing on current worldwide efforts to compare both formability characterisation tests and forming simulation tools for benchmark materials.It is hoped that this will lead to standardisation of formability testing -a key requirement for more widespread use of analysis tools-and guidelines on the accuracy of the range of simulation approaches that are currently available
continued in Chapter 3, including a detailed description of finite element simulation techniques for forming of dry fabric preforms. The methodology here can be considered similar to that used for sheet metal forming, albeit with a more complex material model. Chapter 4 continues the modelling theme, with a description of `virtual testing', whereby materials input data for forming simulation are predicted from the material structure. This topic is of particular interest, as it may offer the opportunity to select materials that are fit-forforming, or even to design new materials with a specific component in mind. Chapter 5 details the use of modern simulation techniques for composite forming within an optimisation scheme, with the aim of selecting materials and process parameters to eliminate such defects as wrinkling or undesirable fibre orientations. Chapter 6 describes the methodology and current status of simulation tools for compression moulding, including applications to sheet moulding compound (SMC) and glass mat thermoplastic (GMT). The following chapter completes the initial treatment of simulation and modelling, with a description of composite distortion ± notably the common phenomenon of `spring-in' ± caused by manufacturing induced stresses. The second half of the book begins with four chapters describing forming technologies for a range of materials. This begins with a relatively new family of materials ± composite/metal hybrids ± which have recently found applications in the aerospace sector (notably as fuselage panels for the Airbus A380). Another new family is covered next, referred to as `self-reinforced polymers'. These materials include fibre and matrix from the same polymer material, addressing one of the current concerns for polymer composites ± recycling. The next two chapters cover more conventional materials ± thermoset prepreg and thermoplastic composite sheet. Prepreg forming technologies are described in detail, from the traditional hand lay-up and autoclave cure approach to current developments in automated tape placement and diaphragm forming. The thermoplastics chapter includes a detailed description of the range of material forms, along with their appropriate forming and consolidation techniques. Chapter 12 describes the current state-of-the-art in simulation software for composite forming within an industrial context, detailing the use of modern software tools to design the material lay-up, and describing how these tools can be integrated within the manufacturing environment. Finally Chapter 13 covers the issue of benchmarking of composite forming. This topic is particularly timely, drawing on current worldwide efforts to compare both formability characterisation tests and forming simulation tools for benchmark materials. It is hoped that this will lead to standardisation of formability testing ± a key requirement for more widespread use of analysis tools ± and guidelines on the accuracy of the range of simulation approaches that are currently available. xvi Introduction Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 6:57:44 PM IP Address: 158.132.122.4
1 Composite forming mechanisms and materials characterisation A C LONG and M J CLIFFORD,University of Nottingham,UK 1.1 Introduction This chapter describes the primary deformation mechanisms that occur during composites forming.Experimental procedures to measure material behaviour are described,and typical material behaviour is discussed.The scope of this poo M description is reasonably broad,and is relevant to a variety of manufacturing processes.While other materials will be mentioned,the focus here is on forming materials based on continuous,aligned reinforcing fibres.Specifically,materials of interest here include: .Dry fabrics,formed to produce preforms for liquid composite moulding. .Prepregs,comprising aligned fibres(unidirectional or interlaced as a textile) within a polymeric (thermoset or thermoplastic)matrix. While other materials are also formed during composites processing,the above have received by far the most attention amongst the research community.The techniques described here can also be applied to polymer/polymer composites, although these materials present a number of challenges (see Chapter 9). Moulding compounds such as glass-mat thermoplastics (GMTs)and thermoset sheet moulding compounds (SMCs)are formed by a compression (flow) moulding process;here formability is usually characterised by rheometry(see Chapter 6). Focusing on continuous,aligned fibre materials,a number of deformation mechanisms during forming can be identified(Table 1.1).The remainder of this chapter will focus on methods for characterising materials behaviour.Materials testing typically has a number of objectives.Often the primary motivation is simply to understand materials behaviour during forming,and in particular to rank materials in terms of formability.If this can be related to the material structure,then this understanding may facilitate design of new materials or optimisation of manufacturing process conditions.Another aim may be to obtain materials data for forming simulation.For the most advanced codes,this may
1.1 Introduction This chapter describes the primary deformation mechanisms that occur during composites forming. Experimental procedures to measure material behaviour are described, and typical material behaviour is discussed. The scope of this description is reasonably broad, and is relevant to a variety of manufacturing processes. While other materials will be mentioned, the focus here is on forming materials based on continuous, aligned reinforcing fibres. Specifically, materials of interest here include: · Dry fabrics, formed to produce preforms for liquid composite moulding. · Prepregs, comprising aligned fibres (unidirectional or interlaced as a textile) within a polymeric (thermoset or thermoplastic) matrix. While other materials are also formed during composites processing, the above have received by far the most attention amongst the research community. The techniques described here can also be applied to polymer/polymer composites, although these materials present a number of challenges (see Chapter 9). Moulding compounds such as glass-mat thermoplastics (GMTs) and thermoset sheet moulding compounds (SMCs) are formed by a compression (flow) moulding process; here formability is usually characterised by rheometry (see Chapter 6). Focusing on continuous, aligned fibre materials, a number of deformation mechanisms during forming can be identified (Table 1.1). The remainder of this chapter will focus on methods for characterising materials behaviour. Materials testing typically has a number of objectives. Often the primary motivation is simply to understand materials behaviour during forming, and in particular to rank materials in terms of formability. If this can be related to the material structure, then this understanding may facilitate design of new materials or optimisation of manufacturing process conditions. Another aim may be to obtain materials data for forming simulation. For the most advanced codes, this may 1 Composite forming mechanisms and materials characterisation A C L O N G and M J C L I F F O R D , University of Nottingham, UK Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 6:57:44 PM IP Address: 158.132.122.4
2 Composites forming technologies Table 7.7 Deformation mechanisms for continuous,aligned fibre based materials during forming Mechanism Schematic Characteristics Intra-ply shear Rotation of between parallel tows and at tow crossovers,followed by inter- tow compaction Rate and temperature dependent for prepreg Key deformation mode (along with bending)for biaxial reinforcements to form 3D shapes Intra-ply tensile Extension parallel to tow direction(s) loading Sunysijqnd For woven materials initial stiffness low until tows straighten;biaxial response governed by level of crimp and tow 2 compressibility 86-0 Accounts for relatively small strains but represents primary source for energy dissipation during forming wo'ssaudmaur'peaypoowy/:dy Ply/tool or Relative movement between individual ply/ply shear layers and tools Not generally possible to define single friction coefficient;behaviour is pressure and(for prepreg)rate and temperature dependent 名 Ply bending Bending of individual layers Stiffness significantly lower than in- plane stiffness as fibres within tows can slide relative to each other;rate and temperature dependent for prepreg Only mode required for forming of single curvature and critical requirement for double curvature Compaction/ Thickness reduction resulting in consolidation increase in fibre volume fraction and (for prepreg)void reduction For prepreg behaviour is rate and temperature dependent
Table 1.1 Deformation mechanisms for continuous, aligned fibre based materials during forming Mechanism Schematic Characteristics Intra-ply shear · Rotation of between parallel tows and at tow crossovers, followed by intertow compaction · Rate and temperature dependent for prepreg · Key deformation mode (along with bending) for biaxial reinforcements to form 3D shapes Intra-ply tensile loading · Extension parallel to tow direction(s) · For woven materials initial stiffness low until tows straighten; biaxial response governed by level of crimp and tow compressibility · Accounts for relatively small strains but represents primary source for energy dissipation during forming Ply/tool or ply/ply shear · Relative movement between individual layers and tools · Not generally possible to define single friction coefficient; behaviour is pressure and (for prepreg) rate and temperature dependent Ply bending · Bending of individual layers · Stiffness significantly lower than inplane stiffness as fibres within tows can slide relative to each other; rate and temperature dependent for prepreg · Only mode required for forming of single curvature and critical requirement for double curvature Compaction/ consolidation · Thickness reduction resulting in increase in fibre volume fraction and (for prepreg) void reduction · For prepreg behaviour is rate and temperature dependent. 2 Composites forming technologies Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 6:57:44 PM IP Address: 158.132.122.4
Composite forming mechanisms and materials characterisation 3 require a full mechanical characterisation of the material under axial,shear and bending loads.The use of such data is described in detail in Chapter 3. In almost all cases,test methods are non-standardised and have been developed by designers or researchers with a particular material and process in mind.This means that test methods,specimen dimensions,data treatment and presentation differ between practitioners.Here we will give a description of what we believe to be 'best practice',although this is clearly a subjective assessment.Benchmarking and comparison of results between laboratories is being addressed within an international exercise;this is discussed in detail in Chapter 13. 1.2 Intra-ply shear This mechanism occurs when the material is subjected to in-plane shear.This essentially corresponds to relative sliding of parallel tows within a fabric layer or composite ply,and (for textile-based materials)rotation of tows at their crossovers.Intra-ply shear is usually considered to be the primary deformation mechanism for aligned fibre-based materials.Coupled with low bending resistance,the ability of materials to shear in this way allows them to be formed to three dimensional shapes without forming folds or wrinkles.A good analogy here is to compare a woven fabric to a sheet of paper.Both may have a similar bending stiffness,but unlike paper the ability of the fabric to shear allows it to be formed over shapes with double curvature. Various experimental methods exist to characterise the shear resistance of dry textiles and aligned or woven composite materials.Early developments here were for apparel fabrics;of particular relevance is the 'Kawabata Evaluation System for Fabrics (KES-F),a series of test methods and associated testing equipment for textile mechanical behaviour including tensile,shear,bending, compression and friction.However whilst this system has been used widely for clothing textiles,its application to reinforcement fabrics has been limited.2 This is probably due to the fact that KES-F provides single point data at relatively low levels of deformation,coupled with the limited availability of the (expensive)testing equipment. Amongst the composites forming community,two widely used test methods are the picture frame test and the bias extension test.-12 In this section we present a guide to the use of these test methods and how to make good use of the output data. 1.2.1 Picture frame test The picture frame (or rhombus)test can be used to measure the force generated by shearing technical textiles and textile composites,including thermoplastic and thermoset based materials.Cross-shaped test samples can be cut or stamped
require a full mechanical characterisation of the material under axial, shear and bending loads. The use of such data is described in detail in Chapter 3. In almost all cases, test methods are non-standardised and have been developed by designers or researchers with a particular material and process in mind. This means that test methods, specimen dimensions, data treatment and presentation differ between practitioners. Here we will give a description of what we believe to be `best practice', although this is clearly a subjective assessment. Benchmarking and comparison of results between laboratories is being addressed within an international exercise; this is discussed in detail in Chapter 13. 1.2 Intra-ply shear This mechanism occurs when the material is subjected to in-plane shear. This essentially corresponds to relative sliding of parallel tows within a fabric layer or composite ply, and (for textile-based materials) rotation of tows at their crossovers. Intra-ply shear is usually considered to be the primary deformation mechanism for aligned fibre-based materials. Coupled with low bending resistance, the ability of materials to shear in this way allows them to be formed to three dimensional shapes without forming folds or wrinkles. A good analogy here is to compare a woven fabric to a sheet of paper. Both may have a similar bending stiffness, but unlike paper the ability of the fabric to shear allows it to be formed over shapes with double curvature. Various experimental methods exist to characterise the shear resistance of dry textiles and aligned or woven composite materials. Early developments here were for apparel fabrics; of particular relevance is the `Kawabata Evaluation System for Fabrics (KES-F)', a series of test methods and associated testing equipment for textile mechanical behaviour including tensile, shear, bending, compression and friction.1 However whilst this system has been used widely for clothing textiles, its application to reinforcement fabrics has been limited.2 This is probably due to the fact that KES-F provides single point data at relatively low levels of deformation, coupled with the limited availability of the (expensive) testing equipment. Amongst the composites forming community, two widely used test methods are the picture frame test3±9 and the bias extension test. 8±12 In this section we present a guide to the use of these test methods and how to make good use of the output data. 1.2.1 Picture frame test The picture frame (or rhombus) test can be used to measure the force generated by shearing technical textiles and textile composites, including thermoplastic and thermoset based materials. Cross-shaped test samples can be cut or stamped Composite forming mechanisms and materials characterisation 3 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 6:57:44 PM IP Address: 158.132.122.4
4 Composites forming technologies Crosshead mounting 回 Clamping plate O L pf ⊙ Clamping pins Bearings 876-70 1.7 Schematic of picture frame shear rig.Lpr is the side length measured between the centres of the bearings,Fpr is the axial force measured by the load cell,dpr is the rate of crosshead displacement and is the frame angle. 2212E1 from rolls or sheets of material using a template.Great care should be taken to ensure that the fibres are perfectly aligned with the edges of the template.Test samples are held in a purpose-built square frame,hinged at each corner(see Fig. 1.1).The frame is loaded into a tensile test machine,and two diagonally opposite corners are extended,imparting pure and uniform shear in the test specimen on a macroscopic scale.There is no uniformly applied standard test procedure.Depending on the material to be tested,various methods can be used to restrain test specimens.For dry fabric,impaling samples on a number of pins may improve repeatability.This approach avoids imparting tensile strain in the fibres and reduces bending of tows.Other materials,such as thermoplastic composites,may need to be tightly clamped to prevent fibres from slipping. During the test,the axial force required to deform the sample is recorded. Since many impregnated composite materials are based on polymers with viscosities that depend on shear rate,it may be useful to perform tests at different cross-head displacement rates.It is important to observe the surface of the test samples during the test,as misaligned or poorly clamped samples can wrinkle almost from the start.These results should be discarded.It is usual for samples to wrinkle towards the end of the test (between 50 and 70 of shear deformation)and by careful observation the test can be used to estimate the
from rolls or sheets of material using a template. Great care should be taken to ensure that the fibres are perfectly aligned with the edges of the template. Test samples are held in a purpose-built square frame, hinged at each corner (see Fig. 1.1). The frame is loaded into a tensile test machine, and two diagonally opposite corners are extended, imparting pure and uniform shear in the test specimen on a macroscopic scale. There is no uniformly applied standard test procedure. Depending on the material to be tested, various methods can be used to restrain test specimens. For dry fabric, impaling samples on a number of pins may improve repeatability. This approach avoids imparting tensile strain in the fibres and reduces bending of tows.9 Other materials, such as thermoplastic composites, may need to be tightly clamped to prevent fibres from slipping.7 During the test, the axial force required to deform the sample is recorded. Since many impregnated composite materials are based on polymers with viscosities that depend on shear rate, it may be useful to perform tests at different cross-head displacement rates. It is important to observe the surface of the test samples during the test, as misaligned or poorly clamped samples can wrinkle almost from the start. These results should be discarded. It is usual for samples to wrinkle towards the end of the test (between 50ë and 70ë of shear deformation) and by careful observation the test can be used to estimate the 1.1 Schematic of picture frame shear rig. Lpf is the side length measured between the centres of the bearings, Fpf is the axial force measured by the load cell, d_ pf is the rate of crosshead displacement and is the frame angle. 4 Composites forming technologies Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 6:57:44 PM IP Address: 158.132.122.4