Composite forming mechanisms and materials characterisation 15 0.6 0.55 0-0000000000-口 ◆ awnjoA 0.5 +3L 0.45 量6L 012L 0.4 20 40 60 80 100 (a) Compaction pressure kPa 0.45 0.4 0.35 +3L 0.3 ◆6L +12L 0.25 0 20 40 60 80 100 (b) Compaction pressure kPa 0.551 0.5 0.45 +3L 0.4 +6L +-12L 0.35 0 20 40 60 80 100 (c) Compaction pressure kPa 7.9 Typical dry glass fabric compaction behaviour at low pressures (up to 1 bar)for 3,6 and 12 material layers:(a)triaxial non-crimp,(b)unidirectional non-crimp,(c)plain weave
1.9 Typical dry glass fabric compaction behaviour at low pressures (up to 1 bar) for 3, 6 and 12 material layers: (a) triaxial non-crimp, (b) unidirectional non-crimp, (c) plain weave. Composite forming mechanisms and materials characterisation 15 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
16 Composites forming technologies that fibre volume fraction initially builds up rapidly with pressure,tending towards a plateau defining the maximum practical fibre content.Each graph shows that for low pressure compaction (as illustrated here),increasing the number of layers within the stack can ease compaction (i.e.lower pressure to attain the required fibre volume fraction).This applies particularly to woven fabrics (e.g.Fig.1.9c)and may be explained by nesting between the layers. Other effects observed experimentally include: Number of compaction cycles-if the platens are moved apart,the unloading curve does not superimpose on the loading curve and the unloaded material will generally have a higher fibre volume fraction than before compaction due to unrecovered compaction within the tows.Subsequent loading cycles will each attain a higher fibre volume fraction for the same applied pressure, converging on a maximum value after a number of cycles.This is relevant for example in liquid composite moulding,where two compaction cycles may be applied -firstly during preform manufacture and again on mould tool closure. Saturation-at low rates (where fluid flow effects are negligible),the compaction curve is usually shifted to the right if the material is lubricated, i.e.the material is more compressible,so that lower pressures are required to achieve a given fibre volume fraction.This is relevant in vacuum infusion (a.k.a.VARTM),where the material is compacted under atmospheric ￥759 pressure as the resin front advances,with the reinforcement behind the flow front lubricated. 到 Several authors have presented models for compaction behaviour of fabric reinforcements.These fall into two groups:phenomenological models based on solid mechanics principles,and empirical models to provide a simple representation of the data.Phenomenological models are based typically on representation of reinforcement fibres as a series of beams contacting at a finite number of points along their length.The number of contacts typically increases during compaction,so that the bridging fibre sections gradually stiffen.For example,Cai and Gutowski3o have proposed a series of models,providing valuable insight into compaction behaviour.However,the models are not strictly predictive,as whilst they consist of physically meaningful parameters,the values of these must be adjusted to fit to experimental data. For convenience simple empirical models may be preferred.Power law relationships have been proposed by several authors,for example: =n·ps 1.6 Here B is an empirical factor often referred to as the stiffening index,and Vro is equivalent to the fibre volume fraction at a compaction pressure of 1 Pa (although Vro is also usually determined empirically).This type of equation has been found to fit well to experimental data for a wide range of materials.27.29
that fibre volume fraction initially builds up rapidly with pressure, tending towards a plateau defining the maximum practical fibre content. Each graph shows that for low pressure compaction (as illustrated here), increasing the number of layers within the stack can ease compaction (i.e. lower pressure to attain the required fibre volume fraction). This applies particularly to woven fabrics (e.g. Fig. 1.9c) and may be explained by nesting between the layers. Other effects observed experimentally include: · Number of compaction cycles ± if the platens are moved apart, the unloading curve does not superimpose on the loading curve and the unloaded material will generally have a higher fibre volume fraction than before compaction due to unrecovered compaction within the tows. Subsequent loading cycles will each attain a higher fibre volume fraction for the same applied pressure, converging on a maximum value after a number of cycles. This is relevant for example in liquid composite moulding, where two compaction cycles may be applied ± firstly during preform manufacture and again on mould tool closure. · Saturation ± at low rates (where fluid flow effects are negligible), the compaction curve is usually shifted to the right if the material is lubricated, i.e. the material is more compressible, so that lower pressures are required to achieve a given fibre volume fraction. This is relevant in vacuum infusion (a.k.a. VARTM), where the material is compacted under atmospheric pressure as the resin front advances, with the reinforcement behind the flow front lubricated. Several authors have presented models for compaction behaviour of fabric reinforcements. These fall into two groups: phenomenological models based on solid mechanics principles, and empirical models to provide a simple representation of the data. Phenomenological models are based typically on representation of reinforcement fibres as a series of beams contacting at a finite number of points along their length. The number of contacts typically increases during compaction, so that the bridging fibre sections gradually stiffen. For example, Cai and Gutowski30 have proposed a series of models, providing valuable insight into compaction behaviour. However, the models are not strictly predictive, as whilst they consist of physically meaningful parameters, the values of these must be adjusted to fit to experimental data. For convenience simple empirical models may be preferred. Power law relationships have been proposed by several authors, for example: Vf Vf 0 PB 1:6 Here B is an empirical factor often referred to as the stiffening index, and Vf 0 is equivalent to the fibre volume fraction at a compaction pressure of 1 Pa (although Vf 0 is also usually determined empirically). This type of equation has been found to fit well to experimental data for a wide range of materials.27,29 16 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 17 0.4 0.35 Increasing number of layers and cycles 0.3 Random fibres 0.25 0.2 0.15 Oriented fibres 0.1 0.05 0 0 0.05 0.10.15 0.20.250.30.35 0.4 0.45 V和 1.10 Compaction master curve,showing the relationship between stiffening index(B)and initial fibre volume fraction (Vo)from equation(1.6)for a range of dry reinforcements. Correia29 recently showed that the two parameters in equation(1.6)appear to be related,as indicated in Fig.1.10.This also provides a good way of characterising material behaviour,with highly compressible materials(such as random mats) towards the left of the curve and less compressible materials(based on highly aligned fibres,e.g.non-crimp fabrics)towards the right. A 1.6.2 Consolidation behaviour of prepreg The majority of thermoset and thermoplastic prepreg materials undergo limited reductions in thickness during consolidation (typically <20%).This is because materials in semi-finished (i.e.as supplied)form are usually relatively well consolidated,and all that is required is the reduction of void content to an acceptable level (typically <1%for aerospace applications).Unfortunately as the materials have no clear paths for entrapped voids to escape,very high pressures may be required to achieve the desired degree of consolidation.Hence reasonably powerful hydraulic presses or high-pressure autoclaves are usually employed to consolidate these materials.A number of authors have conducted consolidation experiments for both thermoset and thermoplastic materials,31-33 using a similar approach as described above for dry fabric.As would be expected behaviour is highly dependent on temperature and rate,with increasing time at pressure resulting in a reduction in void content to a limiting value. In an attempt to reduce the pressure levels required for consolidation,various partially impregnated or 'semi-preg'materials have been developed in recent
Correia29 recently showed that the two parameters in equation (1.6) appear to be related, as indicated in Fig. 1.10. This also provides a good way of characterising material behaviour, with highly compressible materials (such as random mats) towards the left of the curve and less compressible materials (based on highly aligned fibres, e.g. non-crimp fabrics) towards the right. 1.6.2 Consolidation behaviour of prepreg The majority of thermoset and thermoplastic prepreg materials undergo limited reductions in thickness during consolidation (typically <20%). This is because materials in semi-finished (i.e. as supplied) form are usually relatively well consolidated, and all that is required is the reduction of void content to an acceptable level (typically <1% for aerospace applications). Unfortunately as the materials have no clear paths for entrapped voids to escape, very high pressures may be required to achieve the desired degree of consolidation. Hence reasonably powerful hydraulic presses or high-pressure autoclaves are usually employed to consolidate these materials. A number of authors have conducted consolidation experiments for both thermoset and thermoplastic materials,31±33 using a similar approach as described above for dry fabric. As would be expected behaviour is highly dependent on temperature and rate, with increasing time at pressure resulting in a reduction in void content to a limiting value. In an attempt to reduce the pressure levels required for consolidation, various partially impregnated or `semi-preg' materials have been developed in recent 1.10 Compaction master curve, showing the relationship between stiffening index (B) and initial fibre volume fraction (Vf0) from equation (1.6) for a range of dry reinforcements. Composite forming mechanisms and materials characterisation 17 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
18 Composites forming technologies years.The general idea is to minimise the required polymer flow distances and provide continuous channels for air removal within the mould.For example, materials can be assembled as layers of resin film and dry fibre,with the resulting process often referred to as resin film infusion.One popular family of thermoplastic materials is based on co-mingled yarns,where reinforcement fibres are intimately mixed with polymeric fibres.Such materials can be processed by heating,pressure application and cooling.Pressure application can be undertaken within a hydraulic press at modest pressures or using a vacuum bag. A number of studies have been published on consolidation behaviour of co- mingled fabrics,following initial work by Van West.32 Wilks20 analysed con- solidation behaviour of co-mingled glass/polypropylene between parallel platens at various rates and temperatures.Typical results are included in Fig.1.11. Consolidation pressure increases with reducing thickness(increasing normalised thickness)and this increase becomes steeper as the material approaches full consolidation.The pressure to achieve a given thickness increases approxi- mately linearly with compaction rate.In the tests shown here,voids could not be eliminated completely as the compaction pressure was limited to 0.7 MPa. Garcia Gil28 performed a similar analysis for vacuum consolidation,demon- strating that void contents of <2%could be achieved under atmospheric compaction pressure within 300 seconds at 180C.Here the vacuum-based process is advantageous as it removes most of the air from the material prior to significant polymer flow.A matched mould process in contrast may allow voids to become entrapped within the material before the target thickness is reached. 0.7 量-0.2mm/min 0.6 ◆-1.5mm/min 200C -2.5 mm/min (edW) 0.5 章-5mm/min 日-10mm/min 0.4 -e15 mm/min --25 mm/min 0.3 ×-35mm/min -e-40 mm/min 0.2 0.1 0 0.6 0.65 0.7 0.75 0.8 0.85 Normalised thickness 7.77 Typical consolidation behaviour for a commingled glass/polypropylene material,showing required pressure as a function of normalised thickness (or degree of consolidation)at 200C for various consolidation rates.Normalised thickness is the fully consolidated thickness (zero voidage)divided by the current thickness
years. The general idea is to minimise the required polymer flow distances and provide continuous channels for air removal within the mould. For example, materials can be assembled as layers of resin film and dry fibre, with the resulting process often referred to as resin film infusion. One popular family of thermoplastic materials is based on co-mingled yarns, where reinforcement fibres are intimately mixed with polymeric fibres. Such materials can be processed by heating, pressure application and cooling. Pressure application can be undertaken within a hydraulic press at modest pressures or using a vacuum bag. A number of studies have been published on consolidation behaviour of comingled fabrics, following initial work by Van West.32 Wilks20 analysed consolidation behaviour of co-mingled glass/polypropylene between parallel platens at various rates and temperatures. Typical results are included in Fig. 1.11. Consolidation pressure increases with reducing thickness (increasing normalised thickness) and this increase becomes steeper as the material approaches full consolidation. The pressure to achieve a given thickness increases approximately linearly with compaction rate. In the tests shown here, voids could not be eliminated completely as the compaction pressure was limited to 0.7 MPa. Garcia Gil28 performed a similar analysis for vacuum consolidation, demonstrating that void contents of <2% could be achieved under atmospheric compaction pressure within 300 seconds at 180ëC. Here the vacuum-based process is advantageous as it removes most of the air from the material prior to significant polymer flow. A matched mould process in contrast may allow voids to become entrapped within the material before the target thickness is reached. 1.11 Typical consolidation behaviour for a commingled glass/polypropylene material, showing required pressure as a function of normalised thickness (or degree of consolidation) at 200ëC for various consolidation rates. Normalised thickness is the fully consolidated thickness (zero voidage) divided by the current thickness. 18 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 19 Various models have been proposed for prepreg consolidation,based typically on a combination of fabric compaction and fluid flow.28.31-33 Where voids are entrapped within the material prior to consolidation(e.g.for traditional prepregs or co-mingled fabrics consolidated without a vacuum)it is important to consider the pressure generated within entrapped voids,which can be done conveniently using the ideal gas law. 1.7 Discussion This chapter has described the deformation behaviour of reinforcements and composites,focusing on the key deformation mechanisms.Clearly the majority of attention amongst the research community has been on intra-ply shear and compaction behaviour,and here a wealth of data are available in the literature. Less attention has been applied to tensile and bending loads for materials used within composites,although much may be learned here from the large body of work on conventional textiles.The majority of the tests used are non-standard, and as materials data are required increasingly for manufacturing process simulations,this issue must be addressed as a matter of urgency.Some initial efforts here are discussed in Chapter 13. Almost all of the effort in the field of materials characterisation for com- posites forming has involved analysis of a single deformation mechanism in isolation.In practice of course several modes will occur simultaneously,for 155 example intra-ply shear and in-plane tension.Coupling between these mechanisms is not clear at present,and may provide insights allowing increased A accuracy from forming simulations.Given the wide variety of materials avail- able,predictive modelling to determine material behaviour from constituent properties becomes highly desirable.Suchvirtual testing'tools would allow a wide range of materials to be analysed prior to component manufacture, facilitating selection and design of new materials with formability in mind.This is the subject of Chapter 4. 1.8 References 1.Saville B P,Physical testing of textiles,Woodhead Publishing Ltd,Cambridge, 1999. 2.Lomov S V,Verpoest I,Barburski M and Laperre J,'Carbon composites based on multiaxial multiply stitched preforms.Part 2.KES-F characterisation of the deformability of the preforms at low loads',Composites Part A,2003 34(4)359- 370. 3.McGuiness G B and O'Bradaigh C M,'Development of rheological models for forming flows and picture frame testing of fabric reinforced thermoplastic sheets', Journal of Non-Newtonian Fluid Mechanics,1997 73 1-28. 4.Long A C,Rudd C D,Blagdon M and Johnson M S,'Experimental analysis of fabric deformation mechanisms during preform manufacture',Proc.IIth International
Various models have been proposed for prepreg consolidation, based typically on a combination of fabric compaction and fluid flow.28,31±33 Where voids are entrapped within the material prior to consolidation (e.g. for traditional prepregs or co-mingled fabrics consolidated without a vacuum) it is important to consider the pressure generated within entrapped voids, which can be done conveniently using the ideal gas law. 1.7 Discussion This chapter has described the deformation behaviour of reinforcements and composites, focusing on the key deformation mechanisms. Clearly the majority of attention amongst the research community has been on intra-ply shear and compaction behaviour, and here a wealth of data are available in the literature. Less attention has been applied to tensile and bending loads for materials used within composites, although much may be learned here from the large body of work on conventional textiles. The majority of the tests used are non-standard, and as materials data are required increasingly for manufacturing process simulations, this issue must be addressed as a matter of urgency. Some initial efforts here are discussed in Chapter 13. Almost all of the effort in the field of materials characterisation for composites forming has involved analysis of a single deformation mechanism in isolation. In practice of course several modes will occur simultaneously, for example intra-ply shear and in-plane tension. Coupling between these mechanisms is not clear at present, and may provide insights allowing increased accuracy from forming simulations. Given the wide variety of materials available, predictive modelling to determine material behaviour from constituent properties becomes highly desirable. Such `virtual testing' tools would allow a wide range of materials to be analysed prior to component manufacture, facilitating selection and design of new materials with formability in mind. This is the subject of Chapter 4. 1.8 References 1. Saville B P, Physical testing of textiles, Woodhead Publishing Ltd, Cambridge, 1999. 2. Lomov S V, Verpoest I, Barburski M and Laperre J, `Carbon composites based on multiaxial multiply stitched preforms. Part 2. KES-F characterisation of the deformability of the preforms at low loads', Composites Part A, 2003 34(4) 359± 370. 3. McGuiness G B and O'Bradaigh C M, `Development of rheological models for forming flows and picture frame testing of fabric reinforced thermoplastic sheets', Journal of Non-Newtonian Fluid Mechanics, 1997 73 1±28. 4. Long A C, Rudd C D, Blagdon M and Johnson M S, `Experimental analysis of fabric deformation mechanisms during preform manufacture', Proc. 11th International Composite forming mechanisms and materials characterisation 19 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