《纺织复合材料》课程参考文献（3-D textile reinforcements in composite materials）07 Braided structures

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 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, 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 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 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 (±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 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]. 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 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 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 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. 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 through￾out 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 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 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

222 3-D textile reinforcements in composite materials 3-iagid 7.4 Solid braid fabrication and geometry. woo'ssaudmau'peaupoom//:dny Aq paiaanad WV IZ:IE:ZI I 1OZ 'ZZ Aenur 'Aupines CARR【E @】 CORE YARM ROTOR- 一F0:47tH 7.5 Method of advanced 3-D solid braiding (compliments of Toyoda Automatic Loom Works,Ltd)

222 3-D textile reinforcements in composite materials 7.4 Solid braid fabrication and geometry. 7.5 Method of advanced 3-D solid braiding (compliments of Toyoda Automatic Loom Works, Ltd). RIC7 7/10/99 8:21 PM Page 222 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 223 step zero step one step two step three step four path of carrier"a" 巧巧巧巧西 7.6 The Cartesian braiding process. other.The next step involves the alternate shifting of the columns (or rows) a prescribed distance.The third and fourth steps are simply the reverse shifting sequence of the first and second steps,respectively.A complete set of four steps is called a machine cycle (Fig.7.6).It should be noted that after one machine cycle the rows and columns have returned to their original positions.The braid pattern shown is of the 1 x 1 variety,so-called iannun because the relation between the shifting distance of rows and columns is one-to-one.Braid patterns involving multiple steps are possible but they require different machine bed configurations and specialized machines.This unique'multi-step'braiding technique is what renders Cartesian braiding a versatile process.Track and column braiders of the type depicted in Fig.7.6 may be used to fabricate preforms of rectangular cross-section such as T- beam,I-beam and box beam if each column and row may be independently displaced.Cartesian braided composites offer excellent shear resistance and quasi-isotropic elastic behavior due to their symmetric,intertwined structure.However,the lack of unidirectional reinforcement results in low stiffness and strength,and high Poisson effect.To help eliminate this,some advanced machines allow for axial yarns to be fed into the structure during fabrication. 7.3.2 Braid architecture,yarn grouping and shapes If one allows for multiple steps in a machine cycle,independent displace- ment of tracks and columns,and non-braider yarn insertion,the Cartesian braiding process is capable of producing a variety of yarn architectures, hybrids and structures.Consider the eight-step braid cycle shown in Fig.7.7, which also shows the phenomenon of yarn grouping. Yarn groups are sets of yarn tows that travel the same path.A multistep braiding process may have multiple yarn groups and a varying number of

other. The next step involves the alternate shifting of the columns (or rows) a prescribed distance. The third and fourth steps are simply the reverse shifting sequence of the first and second steps, respectively. A complete set of four steps is called a machine cycle (Fig. 7.6). It should be noted that after one machine cycle the rows and columns have returned to their original positions. The braid pattern shown is of the 1 ¥ 1 variety, so-called because the relation between the shifting distance of rows and columns is one-to-one. Braid patterns involving multiple steps are possible but they require different machine bed configurations and specialized machines.This unique ‘multi-step’ braiding technique is what renders Cartesian braiding a versatile process. Track and column braiders of the type depicted in Fig. 7.6 may be used to fabricate preforms of rectangular cross-section such as T￾beam, I-beam and box beam if each column and row may be independently displaced. Cartesian braided composites offer excellent shear resistance and quasi-isotropic elastic behavior due to their symmetric, intertwined structure. However, the lack of unidirectional reinforcement results in low stiffness and strength, and high Poisson effect. To help eliminate this, some advanced machines allow for axial yarns to be fed into the structure during fabrication. 7.3.2 Braid architecture, yarn grouping and shapes If one allows for multiple steps in a machine cycle, independent displace￾ment of tracks and columns, and non-braider yarn insertion, the Cartesian braiding process is capable of producing a variety of yarn architectures, hybrids and structures. Consider the eight-step braid cycle shown in Fig. 7.7, which also shows the phenomenon of yarn grouping. Yarn groups are sets of yarn tows that travel the same path. A multistep braiding process may have multiple yarn groups and a varying number of Braided structures 223 7.6 The Cartesian braiding process. RIC7 7/10/99 8:21 PM Page 223 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

224 3-D textile reinforcements in composite materials (steps 1 2) (steps 3&4) (steps 5&6) (steps 7&8) step 1 step 2 step 3 step 4 Architecture b 8 step 5 step 6 step 7 step 8 Group "a has 5 yarns Group"b" has 9 yarns cccb c Group "c" has 9 yarns dd ccc Group "d"has 5 yarns Eight-step 1x1 cycle d d Yarn grouping 7.7 Sample multiple step cycle showing idealized architecture (repeat geometry)and yarn grouping. yarns per group.It is possible to tailor the location of the yarn groups within the preform cross-section.That is to say,the braid cycle (i.e.shifting 9 sequence of tracks and columns)that will yield the desired grouping of yarns may be determined and different fibrous material utilized for the tows that make up a given group.In this way,unique hybrid composite materials may be formed which benefit both from the 3-D integrated nature of the 分 braid and from the hybrid effect and select yarn placement. The existence of yarn groups implies that sets of yarns trace the same path on the machine bed.After one complete machine cycle,each yarn in a group has moved to its leading yarn's location.This in turn implies that the braid geometry produced during one machine cycle(repeat)is the repeating geometry for the entire structure.That is to say,a cross-sectional slab of preform with the length produced during one repeat may be 'stacked-up'on top of one another to reproduce the entire preform(Fig. 7.7).It is possible,within Cartesian braiding process limits,to specify this braid architecture and determine the braid cycle which will yield it.It may be seen that knowledge of this repeat braid geometry is essential for future prediction of braided composite properties. One way of producing a braided preform with a complex cross-sectional shape is through implementation of the universal method (UM)of braid- ing [10].The basic concept behind the UM is to cut the complex cross- section of the preform into finite rectangular elements and then to braid these elements in groups.Since any shape may be estimated through a suit- able number of rectangular elements,the UM provides a plausible means

yarns per group. It is possible to tailor the location of the yarn groups within the preform cross-section. That is to say, the braid cycle (i.e. shifting sequence of tracks and columns) that will yield the desired grouping of yarns may be determined and different fibrous material utilized for the tows that make up a given group. In this way, unique hybrid composite materials may be formed which benefit both from the 3-D integrated nature of the braid and from the hybrid effect and select yarn placement. The existence of yarn groups implies that sets of yarns trace the same path on the machine bed. After one complete machine cycle, each yarn in a group has moved to its leading yarn’s location. This in turn implies that the braid geometry produced during one machine cycle (repeat) is the repeating geometry for the entire structure. That is to say, a cross-sectional slab of preform with the length produced during one repeat may be ‘stacked-up’ on top of one another to reproduce the entire preform (Fig. 7.7). It is possible, within Cartesian braiding process limits, to specify this braid architecture and determine the braid cycle which will yield it. It may be seen that knowledge of this repeat braid geometry is essential for future prediction of braided composite properties. One way of producing a braided preform with a complex cross-sectional shape is through implementation of the universal method (UM) of braid￾ing [10]. The basic concept behind the UM is to cut the complex cross￾section of the preform into finite rectangular elements and then to braid these elements in groups. Since any shape may be estimated through a suit￾able number of rectangular elements, the UM provides a plausible means 224 3-D textile reinforcements in composite materials 7.7 Sample multiple step cycle showing idealized architecture (repeat geometry) and yarn grouping. RIC7 7/10/99 8:21 PM Page 224 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 225 工 Specify: calculate erase pitch length inscribing exterior yarn diameter rectangle inscribing width and draw rectangles thickness cover grid step I step 2 step3 start track group identify 图■■ determine braiding location of elements peripheral (2 shown) yarns step 4 step 5 wos'ssaudmau'peaypoo/ 7.8 Five steps involved in implementing the universal method of braiding a complex shape. to determine an appropriate braid plan.Additionally,yarns may be added to or removed from the braiding process in order to vary the cross-section along the length of the braid.The UM utilizes only one braiding pattern for a preform.It is essentially a series of four-step 1 x 1 braid cycles which isolate the 'rectangles'of the complex cross-section and braid them in sequence.This method is demonstrated in Fig.7.8 using an I-beam as an example.Since any shape may be estimated through a suitable number of rectangular elements,the UM is applicable to curved shapes as well.Addi- tionally,the approach may be readily implemented through appropriate computer code and is piece-wise applicable to variations of the cross- section along the length of the braid. 7.3.3 Fabrication of braided structures The equipment used in the fabrication of 3-D Cartesian braided structures possesses five basic components.These are the machine bed,the actuating system,the take-up and braid compaction mechanism,the yarn carriers and the interface/control system. Inherent in the process of 3-D braiding is a limiting ratio of machine bed size to preform cross-sectional dimensions.The larger the spacing between yarn carriers on the machine bed (the spacing directly determines the

to determine an appropriate braid plan. Additionally, yarns may be added to or removed from the braiding process in order to vary the cross-section along the length of the braid. The UM utilizes only one braiding pattern for a preform. It is essentially a series of four-step 1 ¥ 1 braid cycles which isolate the ‘rectangles’ of the complex cross-section and braid them in sequence. This method is demonstrated in Fig. 7.8 using an I-beam as an example. Since any shape may be estimated through a suitable number of rectangular elements, the UM is applicable to curved shapes as well. Addi￾tionally, the approach may be readily implemented through appropriate computer code and is piece-wise applicable to variations of the cross￾section along the length of the braid. 7.3.3 Fabrication of braided structures The equipment used in the fabrication of 3-D Cartesian braided structures possesses five basic components. These are the machine bed, the actuating system, the take-up and braid compaction mechanism, the yarn carriers and the interface/control system. Inherent in the process of 3-D braiding is a limiting ratio of machine bed size to preform cross-sectional dimensions. The larger the spacing between yarn carriers on the machine bed (the spacing directly determines the Braided structures 225 7.8 Five steps involved in implementing the universal method of braiding a complex shape. RIC7 7/10/99 8:21 PM Page 225 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

226 3-D textile reinforcements in composite materials amount of yarn a carrier can hold),the more difficult it becomes for the braid to be formed owing to the 'pulling apart'action of the yarns them- selves.Some ingenious methods have been devised to overcome this limit to braidable cross-sectional size of preform [11].However,as a rule,there is a trade-off between the length of preform and the cross-sectional size of preform which may be fabricated from a single machine set-up.With this aside,the number of tracks and columns and the resulting yarn carrier spacing on a Cartesian braider's bed are important specifications.Figure 7.9 shows a 10 track by 24 column Cartesian braider that integrates stationary spacer tracks for the sole purpose of inserting axial (longitudinal)yarns. The transverse insertion (as seen in Fig.7.9)is carried out manually. However,some advanced machines allow for this step of the process to be automated. The actuating system of choice for the Cartesian braiding machines is pneumatic.When one considers the required displacement forces,precision of displacement and number of actuators involved,a pneumatic drive system becomes an attractive option.Figure 7.10 shows a 20 track by 20 column Cartesian braider that is capable of displacing each track and column independently.To accomplish this,small pneumatic cylinders are utilized in series for each track and column.As previously mentioned,this 2-0 results in the ability to fabricate complexly shaped or hybrid(yarn group- ing)preforms for specialized applications.Figure 7.10 shows some samples of the types of braids that may be formed on a machine with this capability. Take-up and compaction of the braid is a critical part of the process.For a continuous fabrication process,the braid must be drawn or taken up. Take-up is carried out after a complete machine cycle and before com- paction.As a result,the take-up distance directly determines the braid pitch length(i.e.the length of braid formed during one machine cycle)and result- ing architecture.It is therefore essential to have precise control of the amount of take-up.This is most commonly accomplished by utilizing a motor in conjunction with a worm gear assembly.Without interyarn friction,the yarn orientation angle within the braid would be determined solely by the angle that the not-yet braided yarn makes with the braid axis.In reality,interyarn friction does exist and allows braider yarns to remain in place once compacted.As a result,a much greater orientation angle may be obtained.The idea behind the braid compaction is to pack the yarns up to the desired orientation and then allow interyarn friction and interlacing to hold the yarn in place.To the authors'knowledge,this is commonly accomplished by manually inserting a rod in the braid conver- gence zone and gently compacting the braid after each complete machine cycle.It is suggested that the next generation of Cartesian braiding machines incorporate an automated version of this critical step.As

amount of yarn a carrier can hold), the more difficult it becomes for the braid to be formed owing to the ‘pulling apart’ action of the yarns them￾selves. Some ingenious methods have been devised to overcome this limit to braidable cross-sectional size of preform [11]. However, as a rule, there is a trade-off between the length of preform and the cross-sectional size of preform which may be fabricated from a single machine set-up. With this aside, the number of tracks and columns and the resulting yarn carrier spacing on a Cartesian braider’s bed are important specifications. Figure 7.9 shows a 10 track by 24 column Cartesian braider that integrates stationary spacer tracks for the sole purpose of inserting axial (longitudinal) yarns. The transverse insertion (as seen in Fig. 7.9) is carried out manually. However, some advanced machines allow for this step of the process to be automated. The actuating system of choice for the Cartesian braiding machines is pneumatic. When one considers the required displacement forces, precision of displacement and number of actuators involved, a pneumatic drive system becomes an attractive option. Figure 7.10 shows a 20 track by 20 column Cartesian braider that is capable of displacing each track and column independently. To accomplish this, small pneumatic cylinders are utilized in series for each track and column. As previously mentioned, this results in the ability to fabricate complexly shaped or hybrid (yarn group￾ing) preforms for specialized applications. Figure 7.10 shows some samples of the types of braids that may be formed on a machine with this capability. Take-up and compaction of the braid is a critical part of the process. For a continuous fabrication process, the braid must be drawn or taken up. Take-up is carried out after a complete machine cycle and before com￾paction.As a result, the take-up distance directly determines the braid pitch length (i.e. the length of braid formed during one machine cycle) and result￾ing architecture. It is therefore essential to have precise control of the amount of take-up. This is most commonly accomplished by utilizing a motor in conjunction with a worm gear assembly. Without interyarn friction, the yarn orientation angle within the braid would be determined solely by the angle that the not-yet braided yarn makes with the braid axis. In reality, interyarn friction does exist and allows braider yarns to remain in place once compacted. As a result, a much greater orientation angle may be obtained. The idea behind the braid compaction is to pack the yarns up to the desired orientation and then allow interyarn friction and interlacing to hold the yarn in place. To the authors’ knowledge, this is commonly accomplished by manually inserting a rod in the braid conver￾gence zone and gently compacting the braid after each complete machine cycle. It is suggested that the next generation of Cartesian braiding machines incorporate an automated version of this critical step. As 226 3-D textile reinforcements in composite materials RIC7 7/10/99 8:21 PM Page 226 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