《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

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 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￾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￾mance of the composite, in particular overcoming some of the weaknesses of textiles. 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- 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 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 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

Resin impregnation and prediction of fabric properties 287 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 早口88号将导导8品88只品品8 fibre orientation (degrees) 9.1 Typical variation of modulus with fibre orientation. 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 environ- mental conditions. n 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 air- craft,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 perfor- mance 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

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 environ￾mental 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 air￾craft, 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 perfor￾mance 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

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 x 10-C 0℃ 0.31 90C 35.8 Quasi-isotropic laminate(0°，t45°，90) mnuT Tensile strength(MPa) 537 305 Tensile modulus (GPa) 50 73 Ultimate strain (% 1.2 0.42 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 inten- sive with long cure cycles and also there are significant hazards owing to the volatile products of reaction released into the atmosphere during the

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 inten￾sive 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 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 components. 9.3 Matched-die moulding To achieve a more uniform distribution of resin throughout the reinforce￾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 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 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 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

290 3-D textile reinforcements in composite materials respect to time [2,3].These approaches,applied at the design stage,have permitted fill procedures to be developed by which resin-starved areas are eliminated through the use of accurately positioned vents and/or different resin injection points.These predictive approaches require greater knowl- edge of the properties of the reinforcements,particularly their permeabil- ity,which,depending upon the nature of the reinforcement constriction, may be different in the longitudinal and transverse directions.Modelling of isothermal flow of resin of constant viscosity through textile reinforcements with isotropic permeability is based on D'Arcy's equation(9.3): Q=-KA.ip u 8x [9.3] where K is the permeability,u the resin viscosity and op/ox the pressure drop per unit length. 5 For random mat non-woven reinforcements permeability is isotropic in- pooM plane while for other textile structures the permeability will be different in different directions depending upon the nature of the textile structure.This differential permeability will result in complex flow patterns in the tool, making flow prediction even more important,although the use of D'Arcy's equation then becomes an over-simplification. 着之5 The vast majority of the tows employed in woven,braided or knitted rein- forcements comprise low twist or untwisted continuous filament yarns.The pressure flow of the low viscosity resins can be assisted by capillary flow in the parallel channels between the filaments and control of the filling oper- ation must be exercised to ensure resin'racing'or'tracking'does not occur. If this is not controlled the resin flow front will race ahead (or fall behind) before rejoining the pressure flow front,leading to unimpregnated enclosed dry regions.Hence variations in fibre volume fraction will result. A variation of this process is to vacuum assist the resin into the tool.The cavity,with the reinforcement in situ,is evacuated and the resin is forced under pressure into the tool,thus wetting out the fibre.This approach is known as vacuum assisted resin injection (VARI)[4]. 9.4 Degassing One of the major difficulties associated with composite manufacture is that of void formation during impregnation and cure [5].When these become entrapped within the matrix,stress concentrations can be established within the matrix.These may originate: during mixing of the resin formulation; during the complex chemical reactions that take place during the cure of thermosetting resins,when volatile gases are released and become encapsulated in the crosslinked resin;

respect to time [2,3]. These approaches, applied at the design stage, have permitted fill procedures to be developed by which resin-starved areas are eliminated through the use of accurately positioned vents and/or different resin injection points. These predictive approaches require greater knowl￾edge of the properties of the reinforcements, particularly their permeabil￾ity, which, depending upon the nature of the reinforcement constriction, may be different in the longitudinal and transverse directions. Modelling of isothermal flow of resin of constant viscosity through textile reinforcements with isotropic permeability is based on D’Arcy’s equation (9.3): [9.3] where K is the permeability, m the resin viscosity and dp/dx the pressure drop per unit length. For random mat non-woven reinforcements permeability is isotropic in￾plane while for other textile structures the permeability will be different in different directions depending upon the nature of the textile structure. This differential permeability will result in complex flow patterns in the tool, making flow prediction even more important, although the use of D’Arcy’s equation then becomes an over-simplification. The vast majority of the tows employed in woven, braided or knitted rein￾forcements comprise low twist or untwisted continuous filament yarns. The pressure flow of the low viscosity resins can be assisted by capillary flow in the parallel channels between the filaments and control of the filling oper￾ation must be exercised to ensure resin ‘racing’ or ‘tracking’ does not occur. If this is not controlled the resin flow front will race ahead (or fall behind) before rejoining the pressure flow front, leading to unimpregnated enclosed dry regions. Hence variations in fibre volume fraction will result. A variation of this process is to vacuum assist the resin into the tool. The cavity, with the reinforcement in situ, is evacuated and the resin is forced under pressure into the tool, thus wetting out the fibre. This approach is known as vacuum assisted resin injection (VARI) [4]. 9.4 Degassing One of the major difficulties associated with composite manufacture is that of void formation during impregnation and cure [5]. When these become entrapped within the matrix, stress concentrations can be established within the matrix. These may originate: • during mixing of the resin formulation; • during the complex chemical reactions that take place during the cure of thermosetting resins, when volatile gases are released and become encapsulated in the crosslinked resin; Q KA p x =- ◊ m d d 290 3-D textile reinforcements in composite materials RIC9 7/10/99 8:32 PM Page 290 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 291 during filling of the cavity as described above; owing to the complex nature of the textile reinforcement,since air can become entrapped in the interstices of the fabric structure.This can be particularly evident when coarse yarns(or tows)are used or in complex 3-D braided or woven structures and may be most prevalent at the solid tool/composite interface. During the formulation stage of the resin system,mixing is necessary to ensure that the hardeners,the crosslinking agents or any other additives are uniformly distributed and dispersed.The agitation during this formulation draws air into the uncured polymer along with the air already absorbed within the low viscosity fluid.As indicated above,these 'volatiles'are potential problem areas and must be eliminated in high-performance composites. After rigorous mixing,the resin mixture is degassed,under full vacuum, giving a deaerated fluid ready for application to the reinforcement.This deaeration can be assisted by heating the resin,to reduce its viscosity, although great care must be exercised to ensure that crosslinking is not initiated. 9.5 Preimpregnation One of the limitations of producing high-performance composite materials lies in the difficulty of achieving uniformity of fibre/resin distribution with low void content.Instead of relying on the pressure flow to force the resin throughout the reinforcement,dip coating and lick roll technology are used to apply a controlled and uniform amount of uncured resin to the rein- 8 forcement.The resin bath contains both the base matrix resin and the hard- eners in a partially cured resin system.The rolls of prepreg'are wrapped in release film and can be stored under refrigerated conditions for a period of time before the shelf-life of the product expires (normally 90 days at -18C for aerospace quality materials).Adoption of this route ensures uni- formity of resin distribution in the reinforcement and eliminates the need for the processor to handle resin systems but does require that low- temperature storage facilities are available on the production site. 9.6 Vacuum bagging The vacuum bagging system is used for producing non-critical components. Plies of thawed out and conditioned prepreg are cut into the appropriate shape either by hand or by an automated process such as a Gerber cutter system.Plies are placed in a precise order and orientation on a tool surface. The lay-up sequence and orientation of the plies is critical to the perfor- mance of the composite.A layer of release film is laid on top of the ply lay-

• during filling of the cavity as described above; • owing to the complex nature of the textile reinforcement, since air can become entrapped in the interstices of the fabric structure. This can be particularly evident when coarse yarns (or tows) are used or in complex 3-D braided or woven structures and may be most prevalent at the solid tool/composite interface. During the formulation stage of the resin system, mixing is necessary to ensure that the hardeners, the crosslinking agents or any other additives are uniformly distributed and dispersed. The agitation during this formulation draws air into the uncured polymer along with the air already absorbed within the low viscosity fluid. As indicated above, these ‘volatiles’ are potential problem areas and must be eliminated in high-performance composites. After rigorous mixing, the resin mixture is degassed, under full vacuum, giving a deaerated fluid ready for application to the reinforcement. This deaeration can be assisted by heating the resin, to reduce its viscosity, although great care must be exercised to ensure that crosslinking is not initiated. 9.5 Preimpregnation One of the limitations of producing high-performance composite materials lies in the difficulty of achieving uniformity of fibre/resin distribution with low void content. Instead of relying on the pressure flow to force the resin throughout the reinforcement, dip coating and lick roll technology are used to apply a controlled and uniform amount of uncured resin to the rein￾forcement. The resin bath contains both the base matrix resin and the hard￾eners in a partially cured resin system. The rolls of ‘prepreg’ are wrapped in release film and can be stored under refrigerated conditions for a period of time before the shelf-life of the product expires (normally 90 days at -18 °C for aerospace quality materials). Adoption of this route ensures uni￾formity of resin distribution in the reinforcement and eliminates the need for the processor to handle resin systems but does require that low￾temperature storage facilities are available on the production site. 9.6 Vacuum bagging The vacuum bagging system is used for producing non-critical components. Plies of thawed out and conditioned prepreg are cut into the appropriate shape either by hand or by an automated process such as a Gerber® cutter system. Plies are placed in a precise order and orientation on a tool surface. The lay-up sequence and orientation of the plies is critical to the perfor￾mance of the composite. A layer of release film is laid on top of the ply lay￾Resin impregnation and prediction of fabric properties 291 RIC9 7/10/99 8:32 PM Page 291 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

292 3-D textile reinforcements in composite materials Ply lay-up Breather cloth Bagging film Release film Vacuum port Seal Seal Tool base plate 9.2 Vacuum bagging process for the production of composites. up to prevent the resinous stack of plies from adhering to the fibrous breather cloth.This cloth is used to absorb any excess resin and distributes the applied pressure evenly over the lay-up.The complete assembly is enclosed in a sealed bag or the bagging layer is sealed to the surface of the tool surround beyond the boundaries of the component as shown in Fig. 9.2.A vacuum connector is inserted into this bagging film so that the ply stack can be consolidated under approximately one atmosphere of vacuum. This complete assembly,while still under vacuum,is placed in an oven at an elevated temperature to cure the resin system. 周网 While this route uses prepreg material,which should ensure an even dis- tribution of resin throughout the reinforcement,it is only operated at a maximum pressure of approximately 1 bar to consolidate the plies into a homogeneous'layer.This low pressure is insufficient to compact the layers adequately to produce a high performance component with high fibre volume fraction and low void content. 9.7 Autoclave For high-performance composites,high fibre volume and low void contents are essential.It is also important that distribution of both fibre and resin is uniform throughout the component.This is achieved by taking the vacuum bagging process one stage further.As previously described,prepregs in the form of unidirectional tows or woven fabrics impregnated with a partially cured resin system are used.The process follows the stages outlined in Fig. 9.3. A number of the steps in this process are similar to those used in the vacuum bagging process.In these steps care must be exercised both from the point of view of health and safety and to ensure that the lay-up is con- tamination free.Such contamination can seriously impair the performance of the composite component.A clean room is required and protective cloth-

up to prevent the resinous stack of plies from adhering to the fibrous breather cloth. This cloth is used to absorb any excess resin and distributes the applied pressure evenly over the lay-up. The complete assembly is enclosed in a sealed bag or the bagging layer is sealed to the surface of the tool surround beyond the boundaries of the component as shown in Fig. 9.2. A vacuum connector is inserted into this bagging film so that the ply stack can be consolidated under approximately one atmosphere of vacuum. This complete assembly, while still under vacuum, is placed in an oven at an elevated temperature to cure the resin system. While this route uses prepreg material, which should ensure an even dis￾tribution of resin throughout the reinforcement, it is only operated at a maximum pressure of approximately 1 bar to consolidate the plies into a ‘homogeneous’ layer. This low pressure is insufficient to compact the layers adequately to produce a high performance component with high fibre volume fraction and low void content. 9.7 Autoclave For high-performance composites, high fibre volume and low void contents are essential. It is also important that distribution of both fibre and resin is uniform throughout the component. This is achieved by taking the vacuum bagging process one stage further. As previously described, prepregs in the form of unidirectional tows or woven fabrics impregnated with a partially cured resin system are used. The process follows the stages outlined in Fig. 9.3. A number of the steps in this process are similar to those used in the vacuum bagging process. In these steps care must be exercised both from the point of view of health and safety and to ensure that the lay-up is con￾tamination free. Such contamination can seriously impair the performance of the composite component.A clean room is required and protective cloth- 292 3-D textile reinforcements in composite materials 9.2 Vacuum bagging process for the production of composites. RIC9 7/10/99 8:32 PM Page 292 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 293 Prepreg stored Gerber Cutting Plies hand-laid at-18℃ of plies into tool Final Trimming of Consolidation Assembly excess material in autoclave 9.3 Route for composites production using the autoclave process. ing should be worn at all times for the production of both defect-free com- ponents and health and safety reasons. The various steps in the manufacturing route are as follows. 1 Prepreg material is stored under refrigerated conditions at-18C.Prior to processing,rolls are removed and allowed to thaw and condition. 9 After reaching room temperature,the fabric is cut into shaped plies, poo taking fibre orientation into account.Computer-based nesting is used to optimize fabric utilization.These plies are labelled. 2 The plies are hand laid into the thoroughly degreased and clean mould- ing tool in the correct sequence and orientation.Constant inspection and signing-off of the lay-up at each stage is necessary to ensure per- formance and quality.Where the component comprises a large number A of plies,frequent debulking is required,i.e.the lay-up is compressed under vacuum,after which a further series of plies are laid-in.A bal- 9 anced lay-up,i.e.symmetry of lay-up about the neutral axis,minimizes the extent of springback.Once the lay-up is completed,a layer of release film is placed on top of the plies,the breather cloth placed on top of the release film and the whole assembly is bagged and sealed.A vacuum nozzle is attached to the complete assembly.These steps are identical to those shown in Fig.9.2 for vacuum bagging.Vacuum is then applied to the assembly. 3 After confirming the integrity of the seal,the bagged assembly,while still under vacuum,is placed in a computer-controlled autoclave which is programmed to follow a particular processing cycle of both tempera- ture and pressure.A typical cycle is as shown in Fig.9.4. The cycle is designed so that the maximum flow is achieved up to and including the hold period so that the fabric can be completely wetted- out and the interstices and the interfilament regions in the fabric struc- ture completely filled with resin.The application of pressure,through inert nitrogen gas at a very early stage in the cycle,consolidates the com- posite structure and the nitrogen minimizes the risk of fire and explo- sion.On ramping up the temperature to the maximum,the resin commences to crosslink through an exothermic chemical reaction.Heat-

ing should be worn at all times for the production of both defect-free com￾ponents and health and safety reasons. The various steps in the manufacturing route are as follows. 1 Prepreg material is stored under refrigerated conditions at -18 °C. Prior to processing, rolls are removed and allowed to thaw and condition. After reaching room temperature, the fabric is cut into shaped plies, taking fibre orientation into account. Computer-based nesting is used to optimize fabric utilization. These plies are labelled. 2 The plies are hand laid into the thoroughly degreased and clean mould￾ing tool in the correct sequence and orientation. Constant inspection and signing-off of the lay-up at each stage is necessary to ensure per￾formance and quality. Where the component comprises a large number of plies, frequent debulking is required, i.e. the lay-up is compressed under vacuum, after which a further series of plies are laid-in. A bal￾anced lay-up, i.e. symmetry of lay-up about the neutral axis, minimizes the extent of springback. Once the lay-up is completed, a layer of release film is placed on top of the plies, the breather cloth placed on top of the release film and the whole assembly is bagged and sealed. A vacuum nozzle is attached to the complete assembly. These steps are identical to those shown in Fig. 9.2 for vacuum bagging. Vacuum is then applied to the assembly. 3 After confirming the integrity of the seal, the bagged assembly, while still under vacuum, is placed in a computer-controlled autoclave which is programmed to follow a particular processing cycle of both tempera￾ture and pressure. A typical cycle is as shown in Fig. 9.4. The cycle is designed so that the maximum flow is achieved up to and including the hold period so that the fabric can be completely wetted￾out and the interstices and the interfilament regions in the fabric struc￾ture completely filled with resin. The application of pressure, through inert nitrogen gas at a very early stage in the cycle, consolidates the com￾posite structure and the nitrogen minimizes the risk of fire and explo￾sion. On ramping up the temperature to the maximum, the resin commences to crosslink through an exothermic chemical reaction. Heat￾Resin impregnation and prediction of fabric properties 293 9.3 Route for composites production using the autoclave process. RIC9 7/10/99 8:32 PM Page 293 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

294 3-D textile reinforcements in composite materials 200 hold 120C/cure 150 pressure 6 MPa 150 E 100 60 30 90 120 160 120 time (min) 9.4 Autoclave cure cycle. 195650t1 up rates,typically at 2-5C/min,are slow,to ensure that the exothermic 心， reactions are kept under control. 4 Once the cure cycle has been completed and the component cooled,also 20 at a slow rate,the excess resin around the periphery is trimmed off and holes drilled,etc.,where necessary. 5 Finally,the various components are put together to form the final 日 assembly. This route is used to manufacture high-quality composites mainly for aero- space applications.The fibre volume fraction for carbon fibre composites should be in the region of 60%and the void content <1%.Non-destructive quality control is performed using ultrasound scans or X-ray micrographs to confirm this.The autoclave can often form a bottleneck for composites manufacturing since commercial autoclaves are large and to be viable,full loads have to be assembled.Components of similar thickness are processed under the same cure cycle and hence production scheduling is crucial to the success of this operation. Since a single tool surface is utilized,a good finish is only secured on one surface of the component,the other surface being in contact with the release film.However,caul plates can be used to produce a good surface finish on both sides of the component,even for reasonably complex shapes. The example in Fig.9.5 shows how T-pieces can be manufactured.These caul plates solve the additional problem of consolidating both the web and the flanges simultaneously once the pressure is applied during the cycle. While autoclave processing provides high-performance composites,the operation of the autoclave has associated high running costs.This route can

up rates, typically at 2–5 °C/min, are slow, to ensure that the exothermic reactions are kept under control. 4 Once the cure cycle has been completed and the component cooled, also at a slow rate, the excess resin around the periphery is trimmed off and holes drilled, etc., where necessary. 5 Finally, the various components are put together to form the final assembly. This route is used to manufacture high-quality composites mainly for aero￾space applications. The fibre volume fraction for carbon fibre composites should be in the region of 60% and the void content <1%. Non-destructive quality control is performed using ultrasound scans or X-ray micrographs to confirm this. The autoclave can often form a bottleneck for composites manufacturing since commercial autoclaves are large and to be viable, full loads have to be assembled. Components of similar thickness are processed under the same cure cycle and hence production scheduling is crucial to the success of this operation. Since a single tool surface is utilized, a good finish is only secured on one surface of the component, the other surface being in contact with the release film. However, caul plates can be used to produce a good surface finish on both sides of the component, even for reasonably complex shapes. The example in Fig. 9.5 shows how T-pieces can be manufactured. These caul plates solve the additional problem of consolidating both the web and the flanges simultaneously once the pressure is applied during the cycle. While autoclave processing provides high-performance composites, the operation of the autoclave has associated high running costs. This route can 294 3-D textile reinforcements in composite materials 9.4 Autoclave cure cycle. RIC9 7/10/99 8:32 PM Page 294 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