7 Braided structures TIMOTHY D.KOSTAR AND TSU-WEI CHOU 7.1 Introduction The design and fabrication of preforms for advanced composites has gained considerable attention in light of the recently developed textile preforming techniques.It is within this realm of preforming technology that the full advantage of the knowledge of process-structure-property relations may be realized.The fabrication history of these preforms directly determines composite microstructure and resulting mechanical properties.Textile pre- forms may be loosely classified into two-dimensional (2-D)and three- dimensional(3-D)structures,depending on the degree of reinforcement between layers [1]. 7.1.1 2-D fabrics 2-D fabrics woven on a loom generally contain two sets of yarns.These yarn groups are interlaced at right angles,with the longitudinal yarns being referred to as warp yarns and the cross yarns as weft.A basic loom consists of two harnesses that control warp yarn separation,a shuttle that passes the weft yarn through the separated warp yarns,and a beat-up mechanism that compacts the fabric.By controlling the separation sequence of the warp yarns,different fabrics may be formed.Two-dimensional woven fabrics offer a high degree of yarn packing,enhanced impact resistance and cost- effective fabrication.However,some in-plane elastic properties,notably resistance to shear,and strength are sacrificed. Knitted (2-D)fabrics contain chains of interlaced loops.Depending on the orientation of the looping yarn,knits may be classified as either warp or weft.In warp knitting,the looping yarns run in the warp or longitudinal direction and in weft knitting the yarns travel in the weft or horizontal direction.Both fabrics are formed using similar fabrication schemes.The most common mechanism used is the latch needle.Many such needles are employed simultaneously in fabricating the knit.As the process is repeated, 217
7.1 Introduction The design and fabrication of preforms for advanced composites has gained considerable attention in light of the recently developed textile preforming techniques. It is within this realm of preforming technology that the full advantage of the knowledge of process–structure–property relations may be realized. The fabrication history of these preforms directly determines composite microstructure and resulting mechanical properties. Textile preforms may be loosely classified into two-dimensional (2-D) and threedimensional (3-D) structures, depending on the degree of reinforcement between layers [1]. 7.1.1 2-D fabrics 2-D fabrics woven on a loom generally contain two sets of yarns.These yarn groups are interlaced at right angles, with the longitudinal yarns being referred to as warp yarns and the cross yarns as weft. A basic loom consists of two harnesses that control warp yarn separation, a shuttle that passes the weft yarn through the separated warp yarns, and a beat-up mechanism that compacts the fabric. By controlling the separation sequence of the warp yarns, different fabrics may be formed. Two-dimensional woven fabrics offer a high degree of yarn packing, enhanced impact resistance and costeffective fabrication. However, some in-plane elastic properties, notably resistance to shear, and strength are sacrificed. Knitted (2-D) fabrics contain chains of interlaced loops. Depending on the orientation of the looping yarn, knits may be classified as either warp or weft. In warp knitting, the looping yarns run in the warp or longitudinal direction and in weft knitting the yarns travel in the weft or horizontal direction. Both fabrics are formed using similar fabrication schemes. The most common mechanism used is the latch needle. Many such needles are employed simultaneously in fabricating the knit.As the process is repeated, 7 Braided structures TIMOTHY D. KOSTAR AND TSU-WEI CHOU 217 RIC7 7/10/99 8:20 PM Page 217 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9
218 3-D textile reinforcements in composite materials the series of interlaced loops that are formed constitute the fabric.Knitted fabrics provide a high degree of formability and enhanced in-plane shear resistance.As a result,their application to high strain composites,such as inflatable skins,is readily apparent.Finally,increased directional stability can be obtained by adding laid-in yarns in the desired directions. Perhaps the most simple way of adding through-the-thickness reinforce- ment is the stitching process.An industrial size sewing machine is usually employed whereby a needle is used to penetrate the layers of fabric and pull the stitching yarn through the preform.Though cost-effective,con- siderable fiber damage occurs through needle penetration.The resulting reduction in composite strength can be appreciable,making stitching an unattractive option.In recent years,novel stitching techniques have been developed where the fibers are effectively spaced to reduce breakage greatly during needle penetration. 7.1.2 3-D fabrics 3-D knitted fabrics are akin to their 2-D brothers.They may be produced 业 by either weft knitting or warp knitting process.Additional strengthening is accomplished by the use of laid-in yarns in the mutually orthogonal direc- tion.The knitted preform which deserves the most attention is the multi- axial warp knit.The knit consists of longitudinal,latitudinal and bias (t) yarns held together by a through-the-thickness tricot stitch.These 3-D knits possess the characteristics of unidirectional laminates while enjoying enhanced stiffness and strength in the thickness direction. 3-D weaving is achieved through a modification of the traditional 2-D weaving process.The two main types of 3-D woven fabrics are angle- interlock and orthogonal structure.Angle-interlock weaving is carried out by utilizing multiple harnesses on a conventional loom.The shifting sequence of the harnesses determines the undulation of the warp yarns. Many geometric variations are possible owing to the unlimited combina- tions of loom configuration and harness sequencing.These multilayer interlocked structures are ideal for thick-section composites.The rein- forcement in the thickness direction may be tailor designed to enhance composite impact resistance.However,the low shear performance and limit to shape geometry make woven fabrics an undesirable option in many applications. Orthogonal woven fabrics possess three sets of mutually perpendicular yarns.Inherent in such a structure are matrix-rich regions between the intersections of the three sets of yarns.Fabrication of these preforms is accomplished by inserting alternating,in-plane yarns between the station- ary thickness direction yarns.In this fashion,both Cartesian and cylindri- cal geometries are possible [2]
the series of interlaced loops that are formed constitute the fabric. Knitted fabrics provide a high degree of formability and enhanced in-plane shear resistance. As a result, their application to high strain composites, such as inflatable skins, is readily apparent. Finally, increased directional stability can be obtained by adding laid-in yarns in the desired directions. Perhaps the most simple way of adding through-the-thickness reinforcement is the stitching process. An industrial size sewing machine is usually employed whereby a needle is used to penetrate the layers of fabric and pull the stitching yarn through the preform. Though cost-effective, considerable fiber damage occurs through needle penetration. The resulting reduction in composite strength can be appreciable, making stitching an unattractive option. In recent years, novel stitching techniques have been developed where the fibers are effectively spaced to reduce breakage greatly during needle penetration. 7.1.2 3-D fabrics 3-D knitted fabrics are akin to their 2-D brothers. They may be produced by either weft knitting or warp knitting process. Additional strengthening is accomplished by the use of laid-in yarns in the mutually orthogonal direction. The knitted preform which deserves the most attention is the multiaxial warp knit. The knit consists of longitudinal, latitudinal and bias (±q) yarns held together by a through-the-thickness tricot stitch.These 3-D knits possess the characteristics of unidirectional laminates while enjoying enhanced stiffness and strength in the thickness direction. 3-D weaving is achieved through a modification of the traditional 2-D weaving process. The two main types of 3-D woven fabrics are angleinterlock and orthogonal structure. Angle-interlock weaving is carried out by utilizing multiple harnesses on a conventional loom. The shifting sequence of the harnesses determines the undulation of the warp yarns. Many geometric variations are possible owing to the unlimited combinations of loom configuration and harness sequencing. These multilayer interlocked structures are ideal for thick-section composites. The reinforcement in the thickness direction may be tailor designed to enhance composite impact resistance. However, the low shear performance and limit to shape geometry make woven fabrics an undesirable option in many applications. Orthogonal woven fabrics possess three sets of mutually perpendicular yarns. Inherent in such a structure are matrix-rich regions between the intersections of the three sets of yarns. Fabrication of these preforms is accomplished by inserting alternating, in-plane yarns between the stationary thickness direction yarns. In this fashion, both Cartesian and cylindrical geometries are possible [2]. 218 3-D textile reinforcements in composite materials RIC7 7/10/99 8:20 PM Page 218 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9
Braided structures 219 In summary,the limitations of the weaving,knitting and stitching processes include poor shear resistance,limited strength in the primary loading direction,and the inability to produce complexly shaped parts. These shortcomings,as will be seen,are largely overcome with the adapta- tion of braiding. 7.2 2-D braiding Braided fabrics(2-D)may be either circular or flat,where the flat braid is a special case of the more common circular braid.The similarity between the machines used to form these fabrics suggests a starting point to further explain their structure. Traditional circular braiders utilize a horngear arrangement as shown in Fig.7.1(a).The gear train is covered by a track plate which has intertwin- ing tracks used to guide the yarn carriers. The horngears 'pass'the yarn carriers to and from each other in an alter- nating fashion as shown in Fig.7.1(b).For the case of flat braiders,the track- ing system does not form a complete circle (Fig.7.1c).In this configuration, the end horngears have an uneven number of slots which allow the yarn dmau peaupoo//:dny 2T10e Base plate A)circular :ssauppv dl Horngears Track plate D)regular B)carrier transfer Track plate Horngears E)basket C)flat Base plate F)in-laid 7.1 Mechanisms and samples of 2-D braids after [1]
In summary, the limitations of the weaving, knitting and stitching processes include poor shear resistance, limited strength in the primary loading direction, and the inability to produce complexly shaped parts. These shortcomings, as will be seen, are largely overcome with the adaptation of braiding. 7.2 2-D braiding Braided fabrics (2-D) may be either circular or flat, where the flat braid is a special case of the more common circular braid. The similarity between the machines used to form these fabrics suggests a starting point to further explain their structure. Traditional circular braiders utilize a horngear arrangement as shown in Fig. 7.1(a). The gear train is covered by a track plate which has intertwining tracks used to guide the yarn carriers. The horngears ‘pass’ the yarn carriers to and from each other in an alternating fashion as shown in Fig. 7.1(b). For the case of flat braiders, the tracking system does not form a complete circle (Fig. 7.1c). In this configuration, the end horngears have an uneven number of slots which allow the yarn Braided structures 219 7.1 Mechanisms and samples of 2-D braids after [1]. RIC7 7/10/99 8:20 PM Page 219 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9
220 3-D textile reinforcements in composite materials 7.2 A bank of flat braiders (compliments of Foster-Miller,Inc.). carriers to reverse their paths and form a flat braid.Recently,flat braiding machines have been developed where a series of straight braider 'banks' 20 are used to form thin-walled,structural shapes [3](Fig.7.2).Circular braids are usually formed over an axisymmetric mandrel which determines the 月网 final shape of preform.In addition,axial laid-in yarns may be used to increase longitudinal stiffness.Figure 7.1 also shows some common 2-D braids.By specifying the location of yarn carriers on the machine,different braiding patterns may be accomplished.The pattern of Fig.7.1(d)may be loosely compared to a twill weave and that of Fig.7.1(e)to a plain weave. Figure 7.1(f)shows a regular braid with axial in-laid yarns.Owing to the symmetric machine arrangement,braider yarns are oriented at equal and opposite angles about the longitudinal axis.This angle may be directly determined by machine operating conditions.A Wardwell 72 carrier circu- lar braider is shown in Fig.7.3.Finally,while 2-D braids offer cost-effective fabrication,the limitation in available braid geometry and their 2-D nature has restricted their use. 7.3 3-D braiding 3-D braids are formed on two basic types of machines.These are the horngear and Cartesian machines which differ only in their method of yarn carrier displacement.While the horngear type machines offer improved braid speed over the Cartesian machines,the Cartesian machines offer compact machine size,comparatively low development cost and braid archi- tectural versatility
carriers to reverse their paths and form a flat braid. Recently, flat braiding machines have been developed where a series of straight braider ‘banks’ are used to form thin-walled, structural shapes [3] (Fig. 7.2). Circular braids are usually formed over an axisymmetric mandrel which determines the final shape of preform. In addition, axial laid-in yarns may be used to increase longitudinal stiffness. Figure 7.1 also shows some common 2-D braids. By specifying the location of yarn carriers on the machine, different braiding patterns may be accomplished. The pattern of Fig. 7.1(d) may be loosely compared to a twill weave and that of Fig. 7.1(e) to a plain weave. Figure 7.1(f) shows a regular braid with axial in-laid yarns. Owing to the symmetric machine arrangement, braider yarns are oriented at equal and opposite angles about the longitudinal axis. This angle may be directly determined by machine operating conditions. A Wardwell 72 carrier circular braider is shown in Fig. 7.3. Finally, while 2-D braids offer cost-effective fabrication, the limitation in available braid geometry and their 2-D nature has restricted their use. 7.3 3-D braiding 3-D braids are formed on two basic types of machines. These are the horngear and Cartesian machines which differ only in their method of yarn carrier displacement. While the horngear type machines offer improved braid speed over the Cartesian machines, the Cartesian machines offer compact machine size, comparatively low development cost and braid architectural versatility. 220 3-D textile reinforcements in composite materials 7.2 A bank of flat braiders (compliments of Foster-Miller, Inc.). RIC7 7/10/99 8:20 PM Page 220 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9
Braided structures 221 w4 tT￥n 7.3 Wardwell 72 carrier circular braider (compliments of the Center for WV IZ Composite Materials,University of Delaware). 2502 Horngear machines with square or circular arrangement are employed in the fabrication of solid braids (Fig.7.4a).Present-day machines are limited to 24 yarn carriers and therefore limit the size and shape of preform. The micro-geometry of braid is also restricted and is shown in Fig.7.4(b). As can be seen,the braider yarns form intertwined helical paths through- rm心eti n refo心dr out the structure. new braiding processes have been introduced.These include AYPEX [4], interlock twiner [5,6],2-step [7],3-D solid (Fig.7.5)and Cartesian [8]which is more commonly referred to as four-step or track and column in the literature.An excellent recent review of textile preforming methods is supplied by Chou and Popper [9].Of all the 3-D braiding processes,the 3-D solid and Cartesian methods represent the apex of braiding technol- ogy.Since they differ mainly in approach to yarn carrier displacement (horngear vs.track and column),we need only understand a single process and the structures that may be formed. 7.3.1 Cartesian braiding process The basic Cartesian braiding process involves four distinct Cartesian motions of groups of yarns termed rows and columns.For a given step,alter- nate rows (or columns)are shifted a prescribed distance relative to each
Horngear machines with square or circular arrangement are employed in the fabrication of solid braids (Fig. 7.4a). Present-day machines are limited to 24 yarn carriers and therefore limit the size and shape of preform. The micro-geometry of braid is also restricted and is shown in Fig. 7.4(b). As can be seen, the braider yarns form intertwined helical paths throughout the structure. To allow for more flexibility in preform size, shape and microstructure, new braiding processes have been introduced. These include AYPEX [4], interlock twiner [5,6], 2-step [7], 3-D solid (Fig. 7.5) and Cartesian [8] which is more commonly referred to as four-step or track and column in the literature. An excellent recent review of textile preforming methods is supplied by Chou and Popper [9]. Of all the 3-D braiding processes, the 3-D solid and Cartesian methods represent the apex of braiding technology. Since they differ mainly in approach to yarn carrier displacement (horngear vs. track and column), we need only understand a single process and the structures that may be formed. 7.3.1 Cartesian braiding process The basic Cartesian braiding process involves four distinct Cartesian motions of groups of yarns termed rows and columns. For a given step, alternate rows (or columns) are shifted a prescribed distance relative to each Braided structures 221 7.3 Wardwell 72 carrier circular braider (compliments of the Center for Composite Materials, University of Delaware). RIC7 7/10/99 8:20 PM Page 221 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:31:21 AM IP Address: 158.132.122.9