《纺织复合材料》课程参考文献（3-D textile reinforcements in composite materials）06 Knitted fabric composites

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6 Knitted fabric composites H.HAMADA,S.RAMAKRISHNA AND Z.M.HUANG 6.1 Introduction In recent years,knitted fabric reinforcements have received great attention in the composites industry [1-10].This is attributed to the unique proper- ties of knitted fabrics compared with other reinforcement fabric structures such as woven and braid.Interlocking of loops of yarn makes knitted fabrics as shown in Fig.6.1.Here,the term 'yarns'represents individual filaments, untwisted fiber bundles,twisted fiber bundles or roving.These loops can 2110 glide over each other and thus give a high degree of deformability to knitted fabrics.This deformability provides drapeability,which makes knitted fabric reinforcement formable into the desired complex preform shapes for liquid molding to produce the composite component.Moreover,the use of advanced knitting machines allows the production of near net shape fabrics 豆 such as domes,cones,T-pipe junctions,flanged pipes and sandwich fabrics. The use of near net shape preforms has the advantage of minimum mate- rial wastage.A combination of net shape fiber preforms and conventional liquid molding techniques has the potential to mass produce and to reduce the production time,and thus lower the cost of composite material.This is important especially when the applications for composite materials are changing from high-cost and high-performance products of aerospace industry to low-cost and mass-producible products of the general engi- neering industry. Knitted fabrics are basically categorized into two types,namely warp knit fabrics and weft knit fabrics,based on the knitting direction.Schematic dia- grams of both the knitted fabrics are shown in Fig.6.1.Warp knitted fabric is produced by knitting in the lengthwise direction(wale direction)of the fabric,as shown by a solid line in Fig.6.1(a).Weft knitted fabric is produced by knitting in the widthwise direction(course direction)of the fabric (solid line in Fig.6.1b).Several types of knitted fabrics are used in the garment industry for fashion purposes [11].However,only a limited number of knit structures are being investigated for composites in engineering applications, 180

6.1 Introduction In recent years, knitted fabric reinforcements have received great attention in the composites industry [1–10]. This is attributed to the unique properties of knitted fabrics compared with other reinforcement fabric structures such as woven and braid. Interlocking of loops of yarn makes knitted fabrics as shown in Fig. 6.1. Here, the term ‘yarns’ represents individual filaments, untwisted fiber bundles, twisted fiber bundles or roving. These loops can glide over each other and thus give a high degree of deformability to knitted fabrics.This deformability provides drapeability, which makes knitted fabric reinforcement formable into the desired complex preform shapes for liquid molding to produce the composite component. Moreover, the use of advanced knitting machines allows the production of near net shape fabrics such as domes, cones, T-pipe junctions, flanged pipes and sandwich fabrics. The use of near net shape preforms has the advantage of minimum material wastage. A combination of net shape fiber preforms and conventional liquid molding techniques has the potential to mass produce and to reduce the production time, and thus lower the cost of composite material. This is important especially when the applications for composite materials are changing from high-cost and high-performance products of aerospace industry to low-cost and mass-producible products of the general engineering industry. Knitted fabrics are basically categorized into two types, namely warp knit fabrics and weft knit fabrics, based on the knitting direction. Schematic diagrams of both the knitted fabrics are shown in Fig. 6.1. Warp knitted fabric is produced by knitting in the lengthwise direction (wale direction) of the fabric, as shown by a solid line in Fig. 6.1(a). Weft knitted fabric is produced by knitting in the widthwise direction (course direction) of the fabric (solid line in Fig. 6.1b). Several types of knitted fabrics are used in the garment industry for fashion purposes [11]. However, only a limited number of knit structures are being investigated for composites in engineering applications, 6 Knitted fabric composites H. HAMADA, S. RAMAKRISHNA AND Z.M. HUANG 180 RIC6 7/10/99 8:11 PM Page 180 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:57 AM IP Address: 158.132.122.9

182 3-D textile reinforcements in composite materials Table 6.1.Classification of typical warp and weft knitted fabrics used in engineering applications Type Fabric classification Weft knitted fabric Warp knitted fabric structure structure 2-D fabric Plain,rib,Milano rib, Dembigh,Atlas inlaid fabrics 2-D fabric-3-D shape Plain,rib Dembigh,Atlas 川 3-D solid fabric Plain and rib fabrics Multiaxial warp knitted with inlay fiber yarns fabrics or noncrimp fabrics 3-D hollow fabric Single jersey face Single Dembigh face (sandwich fabric) structure structure since:(a)most engineering applications require only simple knit structures WV LS:OE and (b)unlike textile fibers (cotton and polyester),it is difficult to form stiff reinforcement fibers such as glass,carbon and aramid into complicated knit structures.Typical warp and weft knitted fabrics investigated for engineer- 2110 ing applications are summarized in Table 6.1. Both the warp and weft knitted fabrics can be further classified into four types based on the dimensional(D)arrangement of yarns.Type I fabrics are simple 2-D flat knitted fabrics shown in Fig.6.1.These fabrics can be cut to the required dimensions and laminated just as in conventional woven 豆 fabric composites.Using fully fashioned knitting machines it is possible to produce 2-D fabrics into the net shape of the components.Such 2-D fabrics with 3-D shapes may be categorized as Type II fabrics.As mentioned above, the combination of Type II fabrics with conventional composite molding techniques,makes it possible to cut down the fabrication costs.Type III fabrics are produced by stitching multiaxial layers of parallel yarn [12]. Because of minimum fiber crimp,they are also called non-crimp fabrics.A schematic diagram of a typical Type III fabric is shown in Fig.6.2.Owing to their superior properties and better drapeability than the woven fabric composites,they are being considered for building buses,trucks,ships and aircraft wings.Type IV fabrics,also known as sandwich fabrics or 3-D hollow fabrics,are produced by binding 2-D-face fabrics together using pile yarns [13].A schematic diagram of a typical Type IV fabric is shown in Fig. 6.3.These fabrics are sometimes referred to as 2.5-D fabrics,as the amount of fibers in the thickness direction is less than the fibers in the planar direc- tion of the fabric.They are considered to achieve the optimum design of high-performance and damage-tolerant composite structures. The objective of this chapter is to model the mechanical behavior of

since: (a) most engineering applications require only simple knit structures and (b) unlike textile fibers (cotton and polyester), it is difficult to form stiff reinforcement fibers such as glass, carbon and aramid into complicated knit structures. Typical warp and weft knitted fabrics investigated for engineering applications are summarized in Table 6.1. Both the warp and weft knitted fabrics can be further classified into four types based on the dimensional (D) arrangement of yarns. Type I fabrics are simple 2-D flat knitted fabrics shown in Fig. 6.1. These fabrics can be cut to the required dimensions and laminated just as in conventional woven fabric composites. Using fully fashioned knitting machines it is possible to produce 2-D fabrics into the net shape of the components. Such 2-D fabrics with 3-D shapes may be categorized as Type II fabrics.As mentioned above, the combination of Type II fabrics with conventional composite molding techniques, makes it possible to cut down the fabrication costs. Type III fabrics are produced by stitching multiaxial layers of parallel yarn [12]. Because of minimum fiber crimp, they are also called non-crimp fabrics. A schematic diagram of a typical Type III fabric is shown in Fig. 6.2. Owing to their superior properties and better drapeability than the woven fabric composites, they are being considered for building buses, trucks, ships and aircraft wings. Type IV fabrics, also known as sandwich fabrics or 3-D hollow fabrics, are produced by binding 2-D-face fabrics together using pile yarns [13]. A schematic diagram of a typical Type IV fabric is shown in Fig. 6.3. These fabrics are sometimes referred to as 2.5-D fabrics, as the amount of fibers in the thickness direction is less than the fibers in the planar direction of the fabric. They are considered to achieve the optimum design of high-performance and damage-tolerant composite structures. The objective of this chapter is to model the mechanical behavior of 182 3-D textile reinforcements in composite materials Table 6.1. Classification of typical warp and weft knitted fabrics used in engineering applications Type Fabric classification Weft knitted fabric Warp knitted fabric structure structure I 2-D fabric Plain, rib, Milano rib, Dembigh, Atlas inlaid fabrics II 2-D fabric – 3-D shape Plain, rib Dembigh, Atlas III 3-D solid fabric Plain and rib fabrics Multiaxial warp knitted with inlay fiber yarns fabrics or noncrimp fabrics IV 3-D hollow fabric Single jersey face Single Dembigh face (sandwich fabric) structure structure RIC6 7/10/99 8:11 PM Page 182 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:57 AM IP Address: 158.132.122.9

6.2 Description of knitted fabric Let us consider the plain weft knitted fabric shown in Fig. 6.1(b). The knit structure is formed by interlooping of one yarn system into continuously connecting vertical columns and horizontal rows of loops. This type of fabric can be produced using either flat bed or circular knitting machines. The vertical column of loops along the length of the fabric is called ‘wale’ and the horizontal row of loops along the width of the fabric is called ‘course’. The respective directions are called ‘wale direction’ and ‘course direction’. A single knit loop comprises a head loop, two side limbs and two sinker loops as shown in Fig. 6.4. Changing the structure of knit loops produces different knitted fabrics. Knitted fabrics are often specified using ‘wale density’ and ‘course density’. The wale density (W) is defined as the number of wales per unit length in the course direction. Similarly, the course density (C) is the number of courses per unit length in the wale direction of the fabric. Both the wale and course densities are mainly determined by the gauge of the knitting machine, i.e. the number of needles per unit length of the machine bed. The product of C and W gives the stitch density, N, of the fabric. N is defined as the number of knit loops per unit planar area of the fabric. 6.3 Tensile behavior of knitted fabric composites Composites are fabricated by impregnating knitted fabric of reinforcement fiber yarns with the matrix polymer. For a given knitted fabric structure, the mechanical behavior of composite material depends on the properties of 184 3-D textile reinforcements in composite materials 6.4 Schematic representation of various portions of a typical knit loop. RIC6 7/10/99 8:11 PM Page 184 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:57 AM IP Address: 158.132.122.9

a small ultimate failure strain, 1.3%. In the case of knitted glass fiber fabric reinforced polypropylene composite material, the stress–strain curve changes from an initial linearly elastic relationship to a significantly nonlinear relationship with an intermediate ultimate failure strain of 8.5%. The matrix polymer used in these composite materials mainly causes this difference. At the other end of the spectrum, a highly flexible stress–strain behavior could be achieved by reinforcing elastomeric material with a knitted fabric.A typical stress–strain curve of a knitted polyester fiber fabric reinforced polyurethane elastomer is shown in Fig. 6.5. The stress–strain behavior is characterized by a small initial linear elastic relationship, followed by nonlinear behavior with large ultimate failure strain of 60%. In other words, by selecting the type of matrix and reinforcement materials, the mechanical characteristics of a knitted fabric composite can be tailored from rigid to flexible. This chapter mainly concerns the mechanical behavior of the knitted glass fiber fabric reinforced epoxy composites, in which the stresses and strains are connected by fixed linear relationships. Hence, let us consider the tensile behavior of knitted glass fiber fabric reinforced epoxy composite in detail. The stress–strain curve is linear up to the knee point, which occurred at approximately 0.45% strain. Above the knee point, the material deformation and microfracture processes in the specimen cause the nonlinearity. A schematic representation of a typical fracture process in a knitted fabric composite is given in Fig. 6.6. At strain levels immediately 186 3-D textile reinforcements in composite materials 6.5 (cont.) RIC6 7/10/99 8:12 PM Page 186 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:57 AM IP Address: 158.132.122.9

188 3-D textile reinforcements in composite materials above the knee point,debonding of yarns oriented normal to the testing direction occurs.The cracks nucleated from the debonded sites propagate into resin-rich regions and coalesce into large transverse cracks.Unfrac- tured yarns bridge the fracture plane.The ultimate fracture of the tensile specimen occurs upon the fracture of bridging yarns.In other words,the tensile strength of composite material is determined mainly by the fracture strength of yarns bridging the fracture plane. 6.4 Analysis of 3-D elastic properties 6.4.1 Methodology of analysis The plain weft knitted fabric reinforced composite material investigated in this study is assumed to have only reinforcement fiber yarns and polymer matrix.For analysis purposes,a unit cell representing the complete knitted fabric composite is identified.A geometric model is proposed to determine the orientation of yarn in the composite (Section 6.4.2).Section 6.4.3 out- lines the procedure for estimating the fiber volume fraction of the com- posite.The unit cell is divided into four representative volumes,also called a 'crossover model'.The crossover model is further divided into sub- volumes,which are considered as transversely isotropic unidirectional fiber reinforced composites.A new micromechanical model is used to predict all the five independent elastic constants of the unidirectional fiber reinforced composites (Section 6.4.4).By considering the contributions of both the fibers and net matrix material,the compliance/stiffness matrix of each sub- volume in the material co-ordinate system is calculated using the new for- mulae.This compliance/stiffness matrix of each sub-volume is then transformed to the global co-ordinate system (see Section 6.4.5).A volume- averaging scheme has been applied to obtain the overall compliance/stiff- ness matrix of the knitted fabric composite (Section 6.4.6).The effects of fiber content and other parameters of knitted fabric on the elastic proper- ties of the composite material are identified (Section 6.4.7). 6.4.2 Geometric model A schematic diagram of an idealized unit cell of the plain weft knitted fabric is given in Fig.6.7.The basic assumption is that the projection of the central axis of the yarn loop on the fabric plane is composed of circular arcs.This assumption is reasonable as the knit loops are formed during knitting by bending the yarns round a series of equally spaced knitting needles and sinkers.The physical meanings of various symbols used below are also shown in the figure.The geometry of the unit cell can be described using

above the knee point, debonding of yarns oriented normal to the testing direction occurs. The cracks nucleated from the debonded sites propagate into resin-rich regions and coalesce into large transverse cracks. Unfractured yarns bridge the fracture plane. The ultimate fracture of the tensile specimen occurs upon the fracture of bridging yarns. In other words, the tensile strength of composite material is determined mainly by the fracture strength of yarns bridging the fracture plane. 6.4 Analysis of 3-D elastic properties 6.4.1 Methodology of analysis The plain weft knitted fabric reinforced composite material investigated in this study is assumed to have only reinforcement fiber yarns and polymer matrix. For analysis purposes, a unit cell representing the complete knitted fabric composite is identified. A geometric model is proposed to determine the orientation of yarn in the composite (Section 6.4.2). Section 6.4.3 outlines the procedure for estimating the fiber volume fraction of the composite. The unit cell is divided into four representative volumes, also called a ‘crossover model’. The crossover model is further divided into subvolumes, which are considered as transversely isotropic unidirectional fiber reinforced composites. A new micromechanical model is used to predict all the five independent elastic constants of the unidirectional fiber reinforced composites (Section 6.4.4). By considering the contributions of both the fibers and net matrix material, the compliance/stiffness matrix of each subvolume in the material co-ordinate system is calculated using the new formulae. This compliance/stiffness matrix of each sub-volume is then transformed to the global co-ordinate system (see Section 6.4.5).A volumeaveraging scheme has been applied to obtain the overall compliance/stiffness matrix of the knitted fabric composite (Section 6.4.6). The effects of fiber content and other parameters of knitted fabric on the elastic properties of the composite material are identified (Section 6.4.7). 6.4.2 Geometric model A schematic diagram of an idealized unit cell of the plain weft knitted fabric is given in Fig. 6.7. The basic assumption is that the projection of the central axis of the yarn loop on the fabric plane is composed of circular arcs. This assumption is reasonable as the knit loops are formed during knitting by bending the yarns round a series of equally spaced knitting needles and sinkers. The physical meanings of various symbols used below are also shown in the figure. The geometry of the unit cell can be described using 188 3-D textile reinforcements in composite materials RIC6 7/10/99 8:12 PM Page 188 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:57 AM IP Address: 158.132.122.9

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