《纺织复合材料》课程参考文献（3-D textile reinforcements in composite materials）09 Resin impregnation and prediction of fabric properties.pdf

9 Resin impregnation and prediction of fabric properties B.J.HILL AND R.MCILHAGGER 9.1 Introduction It is apparent from the previous chapter that there is a very wide range of textile products that can be used as reinforcements for composite materi- als and components.Such a wide choice provides the designer with great difficulty since an appropriate reinforcement must be selected for a specific application.There is no hard and fast rule for this selection and,in many instances,factors such as ease of manufacture become dominant and rein- 2102 forcements are often selected on the basis of this rather than for perfor- mance enhancement.In general,textile reinforcements for composites show good tensile strength but have poor performance in terms of com- pression or stiffness.This necessitates the use of a matrix to encapsulate the fibres,thus protecting them from damage but also enhancing the perfor- 豆 mance of the composite,in particular overcoming some of the weaknesses of textiles. 8 Structural composites can be defined as products that use fibre rein- forcements(50-70%by weight)of very high strength and stiffness in com- bination with polymeric,metal or other matrices.This class of composite has extremely unusual properties in which the matrix binds the reinforcing fibres together,forming a cohesive structure,providing a medium to trans- fer applied stresses from one filament through the matrix to the adjacent filaments.When polymeric matrices are used,composite structures with rel- atively low densities are produced which have very high specific properties, i.e.high strength/weight and high stiffness/weight ratios. Thus it is necessary to use a means of impregnating the reinforcements with a matrix system which can be polymeric or metallic,although the emphasis as far as this book is concerned is directed towards polymer matrix composites (PMC).The distribution of the matrix throughout the reinforcement is critical to the overall performance of the composite.Small variations in fibre volume fraction throughout the composite give rise to significant variations in properties.The simple rule of mixtures(9.1)for uni- 285

9.1 Introduction It is apparent from the previous chapter that there is a very wide range of textile products that can be used as reinforcements for composite materials and components. Such a wide choice provides the designer with great difficulty since an appropriate reinforcement must be selected for a specific application. There is no hard and fast rule for this selection and, in many instances, factors such as ease of manufacture become dominant and reinforcements are often selected on the basis of this rather than for performance enhancement. In general, textile reinforcements for composites show good tensile strength but have poor performance in terms of compression or stiffness. This necessitates the use of a matrix to encapsulate the fibres, thus protecting them from damage but also enhancing the performance of the composite, in particular overcoming some of the weaknesses of textiles. Structural composites can be defined as products that use fibre reinforcements (50–70% by weight) of very high strength and stiffness in combination with polymeric, metal or other matrices. This class of composite has extremely unusual properties in which the matrix binds the reinforcing fibres together, forming a cohesive structure, providing a medium to transfer applied stresses from one filament through the matrix to the adjacent filaments.When polymeric matrices are used, composite structures with relatively low densities are produced which have very high specific properties, i.e. high strength/weight and high stiffness/weight ratios. Thus it is necessary to use a means of impregnating the reinforcements with a matrix system which can be polymeric or metallic, although the emphasis as far as this book is concerned is directed towards polymer matrix composites (PMC). The distribution of the matrix throughout the reinforcement is critical to the overall performance of the composite. Small variations in fibre volume fraction throughout the composite give rise to significant variations in properties.The simple rule of mixtures (9.1) for uni- 9 Resin impregnation and prediction of fabric properties B.J. HILL AND R. McILHAGGER 285 RIC9 7/10/99 8:32 PM Page 285 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9

286 3-D textile reinforcements in composite materials directional tape demonstrates the influence of fibre volume fraction (v)on stiffness (E): E11=v·Er+(1-v)Em [9.1] where the subscripts f and m refer to fibre and matrix respectively. If there is more than one type of fibre then this relationship can be modified: E1=(1-)Em+i·E1+2·E2+3·E3.. [9.2] where vr is the overall fibre volume fraction and va,vp and ve the fibre volume fraction of the different fibre types. In order to achieve uniformity of properties,the resin must completely fill the interstices within the fabric and also,significantly,the spaces between the filaments making up the tows.When optimum packing is achieved,the spaces between the fibres account for 9.9%but in reality this is more likely 9 to be in the order of 20-25%since ideal packing of the fibre filament bundle is unlikely to occur under normal circumstances.Further,when more complex textile structures are employed as reinforcements,the efficiency of fibre packing will decrease even further,making fibre volume fractions in 毒 2 excess of 60%very difficult to realize.The filaments have to be completely encapsulated in the matrix in order to ensure effective and efficient load transfer between fibres and matrix and also to protect the filaments from damage. To achieve this effective load transfer it is important that complete wet- out of the fibrous mass is achieved.This implies that low-viscosity resins must be used which,in turn,suggests that thermosetting resins are employed and that the performance is developed and enhanced through the crosslinking of the resin system.In general,thermoplastic resins are of higher molecular weight,and hence of higher viscosity during processing, making complete wet-out,in particular in the interfilament spaces,very dif- ficult to achieve satisfactorily. While the fibres dominate the tensile and stiffness properties,the matrix material influences high-temperature performance,transverse strength and moisture resistance of the composite.The resin is also a key factor in tough- ness,shear strength and in particular interlaminar shear stress(ILSS)resis- tance and oxidation and radiation resistance.Figure 9.1 demonstrates the significance of small deviations in fibre orientation on tensile modulus; these deviations also significantly reduce the tensile strength.Hence in- plane misalignment or indeed reinforcement crimp can result in significant losses in mechanical performance. The matrix system has a significant influence on the fabrication process and associated parameters for forming the composite materials into inter- mediate and final components.Most carbon fibre composites are based on

directional tape demonstrates the influence of fibre volume fraction (vf) on stiffness (E): [9.1] where the subscripts f and m refer to fibre and matrix respectively. If there is more than one type of fibre then this relationship can be modified: [9.2] where vf is the overall fibre volume fraction and vf1, vf2 and vf3 the fibre volume fraction of the different fibre types. In order to achieve uniformity of properties, the resin must completely fill the interstices within the fabric and also, significantly, the spaces between the filaments making up the tows. When optimum packing is achieved, the spaces between the fibres account for 9.9% but in reality this is more likely to be in the order of 20–25% since ideal packing of the fibre filament bundle is unlikely to occur under normal circumstances. Further, when more complex textile structures are employed as reinforcements, the efficiency of fibre packing will decrease even further, making fibre volume fractions in excess of 60% very difficult to realize. The filaments have to be completely encapsulated in the matrix in order to ensure effective and efficient load transfer between fibres and matrix and also to protect the filaments from damage. To achieve this effective load transfer it is important that complete wetout of the fibrous mass is achieved. This implies that low-viscosity resins must be used which, in turn, suggests that thermosetting resins are employed and that the performance is developed and enhanced through the crosslinking of the resin system. In general, thermoplastic resins are of higher molecular weight, and hence of higher viscosity during processing, making complete wet-out, in particular in the interfilament spaces, very dif- ficult to achieve satisfactorily. While the fibres dominate the tensile and stiffness properties, the matrix material influences high-temperature performance, transverse strength and moisture resistance of the composite.The resin is also a key factor in toughness, shear strength and in particular interlaminar shear stress (ILSS) resistance and oxidation and radiation resistance. Figure 9.1 demonstrates the significance of small deviations in fibre orientation on tensile modulus; these deviations also significantly reduce the tensile strength. Hence inplane misalignment or indeed reinforcement crimp can result in significant losses in mechanical performance. The matrix system has a significant influence on the fabrication process and associated parameters for forming the composite materials into intermediate and final components. Most carbon fibre composites are based on E vE vE v E vE 11 1 1 2 2 3 3 = - ( ) 1 ◊ +◊ + ◊ +◊ f mf f f f f f . . . E vE v E 11 =◊ +- ( ) 1 ◊ ff f m 286 3-D textile reinforcements in composite materials RIC9 7/10/99 8:32 PM Page 286 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9

thermosetting epoxy matrices which offer low shrinkage during processing, excellent adhesion to the fibres, good property balance, particularly mechanical to electrical performance, and ease of fabrication. They also have a good heat resistance and stability over a wide range of environmental conditions. Typical fibre loading in high-performance composite materials is 60–65% by volume (65–70% by weight). Carbon fibres have a coefficient of thermal expansion which is a slightly negative sequence. Production of composites from fibres with a fairly broad range of coefficients of thermal expansion values permits the manufacture of components with an almost zero coeffi- cient of thermal expansion.This feature can be exploited, particularly in aircraft, to hold critical instrumentation in a precise position as the composite properties of the supporting component can be tailored specifically at the design stage. For particular components this demonstrates the potential to design or engineer specific properties into materials to meet the performance requirements and hence optimize the structural design. In comparison with steel and aluminium, carbon fibre composites are lighter, have lower thermal conductivity, are stiffer and stronger and have superior fatigue resistance. A summary of the typical properties of high strength and high modulus carbon fibre composite materials in an epoxy resin is shown in Table 9.1. The marked differences in properties between uni-directional (0°), transverse (90°) and the quasi-isotropic (0°, ±45°, 90°) fibre orientations should be noted. The high-modulus fibre composite data Resin impregnation and prediction of fabric properties 287 9.1 Typical variation of modulus with fibre orientation. RIC9 7/10/99 8:32 PM Page 287 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9

reflect the lower-strength, higher-modulus properties and lower shear strengths inherent in high modulus composites associated with the much higher thermal treatments that these fibres undergo during their manufacture. 9.2 Hand impregnation There are a number of ways in which fibres or reinforcements can be impregnated with resin. Initially, when composite materials were used for leisure goods such as sports canoes, the resin system was hand mixed and then applied by brush to each layer and consolidated using pressure applied through a hand-held roller. Chemical reaction proceeded in the presence of air to produce a crosslinked matrix. Such systems are highly labour intensive with long cure cycles and also there are significant hazards owing to the volatile products of reaction released into the atmosphere during the 288 3-D textile reinforcements in composite materials Table 9.1. Typical properties of carbon fibre composite materials Property High strength High modulus Unidirectional laminate Longitudinal (0°) Tensile strength (MPa) 1785 1165 Tensile modulus (GPa) 145 215 Ultimate strain (%) 1.2 0.55 Compressive strength (MPa) 120 840 Compressive modulus (GPa) 140 190 Ultimate strain (%) 1.1 0.45 Flexural strength (4pt) (MPa) 1995 1335 Flexural modulus (GPa) 135 190 Interlaminar SS (short beam) (MPa) 95 80 Transverse (90°) Tensile strength (MPa) 49 36 Tensile modulus (GPa) 9.5 7.0 Ultimate strain (%) 0.52 0.49 Additional properties Density (kg/m3 ) 1550 1610 Shear strength (in plane) (MPa) 72 59 Shear modulus (in plane) (GPa) 4.8 4.1 Poisson ratio (0 coupon) 0.30 0.24 Coeff. thermal expansion ¥ 10-6 /°C 0°C 0.31 — 90°C 35.8 — Quasi-isotropic laminate (0°, ±45°, 90°) Tensile strength (MPa) 537 305 Tensile modulus (GPa) 50 73 Ultimate strain (%) 1.2 0.42 RIC9 7/10/99 8:32 PM Page 288 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9

Resin impregnation and prediction of fabric properties 289 cure.Properties of materials produced in this way tend to be variable because of the lack of process control,in particular local variations in amount of resin applied.In addition,it is extremely difficult to occlude all the air entrapped between the plies since compaction of the layers is by hand only.With no direct escape route for this air,stress concentrations are set up during the exothermic cure reaction,creating large voids within the structure.In these applications,glass fibre,often in chopped strand mat form,and polyester resins were used and under these conditions,it is extremely difficult to achieve high fibre volume fractions and hence high performance. Hand lay-up techniques are used with open moulds to produce compo- nents with good surface finish characteristics.This is only possible on one surface.A gel coat is applied to the tool surface and allowed to cure.Plies of textile reinforcement are laid in on top of this hard gel coat finish,each being coated with resin and compacted.In this way the composite compo- nent is assembled and allowed to cure at room temperature.This labour- intensive hand lay-up operation in open tools is used to produce large 2 components. 9.3 Matched-die moulding To achieve a more uniform distribution of resin throughout the reinforce- oo/ ment,more automated systems came into use [1].Pre-mixed resin and hard- ener are injected,under pressure from a pressure pot,into the reinforcement placed in closed matched cavity tools.The resin spreads out 豆 radially from the point of injection,permeating through the reinforcement until the cavity is completely filled with resin.Under such conditions the 8 flow paths must be fully understood and predictable,otherwise resin- starved areas are created even in very simple geometric configurations in which the resin front impinges on the cavity boundary wall when the flow front can no longer expand in the radial direction.Two such fronts on adja- cent walls will result in the flow converging on a point within the rein- forcement.Unless high pressures are used,this region will remain dry,i.e. not impregnated with resin.If high pressure is used,then compression of the enclosed air will occur,which will cause an increase in the air temper- ature.At best this temperature rise will accelerate the crosslinking reaction prematurely and at worst the temperature will rise to such a degree that thermal degradation of the resin will occur.The outcome of this will be burn marks on the component and significant loss of mechanical properties. Hence air vents must be accurately positioned in these areas to assist the removal of entrapped air and provide a quality composite. Such problems have led to a considerable amount of effort being made to model and predict the precise position of the molten resin front with

cure. Properties of materials produced in this way tend to be variable because of the lack of process control, in particular local variations in amount of resin applied. In addition, it is extremely difficult to occlude all the air entrapped between the plies since compaction of the layers is by hand only. With no direct escape route for this air, stress concentrations are set up during the exothermic cure reaction, creating large voids within the structure. In these applications, glass fibre, often in chopped strand mat form, and polyester resins were used and under these conditions, it is extremely difficult to achieve high fibre volume fractions and hence high performance. Hand lay-up techniques are used with open moulds to produce components with good surface finish characteristics. This is only possible on one surface. A gel coat is applied to the tool surface and allowed to cure. Plies of textile reinforcement are laid in on top of this hard gel coat finish, each being coated with resin and compacted. In this way the composite component is assembled and allowed to cure at room temperature. This labourintensive hand lay-up operation in open tools is used to produce large components. 9.3 Matched-die moulding To achieve a more uniform distribution of resin throughout the reinforcement, more automated systems came into use [1]. Pre-mixed resin and hardener are injected, under pressure from a pressure pot, into the reinforcement placed in closed matched cavity tools. The resin spreads out radially from the point of injection, permeating through the reinforcement until the cavity is completely filled with resin. Under such conditions the flow paths must be fully understood and predictable, otherwise resinstarved areas are created even in very simple geometric configurations in which the resin front impinges on the cavity boundary wall when the flow front can no longer expand in the radial direction. Two such fronts on adjacent walls will result in the flow converging on a point within the reinforcement. Unless high pressures are used, this region will remain dry, i.e. not impregnated with resin. If high pressure is used, then compression of the enclosed air will occur, which will cause an increase in the air temperature. At best this temperature rise will accelerate the crosslinking reaction prematurely and at worst the temperature will rise to such a degree that thermal degradation of the resin will occur.The outcome of this will be burn marks on the component and significant loss of mechanical properties. Hence air vents must be accurately positioned in these areas to assist the removal of entrapped air and provide a quality composite. Such problems have led to a considerable amount of effort being made to model and predict the precise position of the molten resin front with Resin impregnation and prediction of fabric properties 289 RIC9 7/10/99 8:32 PM Page 289 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:57 AM IP Address: 158.132.122.9