Indian Journal of Fibre & Textile Research Vol. 29, September 2004, pp. 366-375

Review Article

Textile preforms for advanced composites

Vinayak Ogale & R Alagirusamy" Department of Textile Tcchnology, Indian Institute of Technology, New De lhi 110016, India

Received 25 Jllly 2003; Revised received alld accepted 10 October 2003

Textile preforming operations playa key role in most of the composite manufac turing processes. The ba!;ic textile yarn and fabric forming processes have been modified and developed to a significant extent to meet the increasing demand from the composite manufacturing sector. Apart from being cost effective technology e mployed for obtaining different fibre orientations and near net shapes, textile preforms also lead to improved mechanical properties of the resulting composites in certain aspects. The developments in the field of textile performing along with their advantages and disadvanwges have been critically reviewed in thi s paper.

Keywords: Braided preform, Preform characteri zati on, Textile preform composite s

IPC Code: Int. CI.7 B32B 11 /00, D04H liDO

1 Introduction Composites consisting of fillers in the form of

inorganic/organic fibre or powder as well as metallic powders of relatively high strength and modulus embedded in or bonded to a matrix with distinct interface between them. Most usual fillers are fibres. Both fillers and matrix retain their physical and chemical identities, and they produce a combination of properties that cannot be achieved with either of the constituents acting alone.

I n the recent years, the use of texti Ie structures made from high performance fibres is finding increasing importance in composites applications. In textile process, there is direct control over fibre placements and ease of handling of fibres.

Besides economical ::tdvantages, textile preform technologies also provide homogenous distribution of matrix and reinforcing fibre . Thus, textile preforms are considered to be the structural backbone of composite structures I. This technology is of particular importance in the context of improving certain properties of composites like inter-laminar shear and damage tolerance apart from reducing the cost of manufacturing.

Textile industry has the necessary technology to weave high performance multifilament fibres, such as glass, aramid and carbon, which provide high tensile strength, modulus, and resistance to chemicals and

a To whom all the correspondence should be addressed. Phone: 26591419; Fax: +91-1 1-26851103; E-mail: rasamy61 @hotmail.com

heat to vario IS types of preforms. Depending upon textile preforming method , the range of fibre orientation and fibre volume fraction of preform vary, subsequently affecting the matrix infiltration and consolidation. As a route to mass production of textile composites, the production speed, material handling and materi al design flexibility are major factors responsible for the selection of textile reinforcement production. This opens a new field of technical applications with a new type of semi· finished material produced by textile industr/.

The basic element of reinforcement is the single fibre or filam _nl. In the case of short fibre composites, the continLloLls filaments are cut in to specific length and then mixed with the resin using a suitable process . For example, in the case of injection molding, the short fibres are mixed with polymers and then the mixture is injection molded. The short fibres are also used as chopped strand mats. The mat is used primarily in hand lay-up, continuOl s lamination and some closed molding applications. Fig. I shows the schematic diagrams of the chopped strand and continuous strand mal. Swirling of con tinuous strands of fibres onto a moving belt forms continuous strand mat. The mat is finished with a chemical binder that holds the fibre in place. Continuous strand mat is primarily llsed in compression molding, resin transfer molding and. pultrusion applications as well as to fabricate preforms and stampable thermoplastics3

•

Another form of reinforcement is known as roving. A roving refers to a collection of untwisted strands or

OGALE & ALAG IR USAMY: TEXTILE PR EFORMS FOR ADVANCED COMPOSITES 367

Chopped strand Fibre mat

Fig. 1- Forms of reinforcements

yarns. The glass strands are wound together to form a roving package suitable for internal or external unwinding. The roving is tail or made to suit specific applicati ons with polyester, epoxy and vinyl ester resi n systems. It can also be used fo r compounding with various thermoplasti cs and to produce a wide variety of reinforci ng materi als, including mats, woven fabrics, braids, knitted fabr ics and hybrid fabrics.

Prep regs are another class of reinforcement in which the fibres and resi n are already mi xed together. Advantages of prepregs include faste r curing, improved di stributi on uf resin throughout the part , cleaner manu fac turing fac iliti es, more preci se fibre ali gnment and placement, abili ty to use hi gher fibre contents and , therefore, less res in overall , and ability to lay- up the materi al into some shapes that would be di fficult with wet systems. Flex ible prepregs, like powder-impregnated tows, commingled yarns and core spun yarns are used fo r textile preform ing operations4

. The present paper reviews the developments made in the field of textile preforms along with their advantages and di sadvantages .

2 Textile Preforms for Composites 2.1 Woven Preforms

Developments in the field of preforming have led to the producti on of preforms with fibres orientated in di ffe rent directions with weaving, knitting and braiding indi viduall y or in combinations. Table I prov ides a comparati ve account of different preforming techniques with respect to yarn direction and fabric fo rmation principles. Whilc 20 woven tex tile structures are usually fo rmed into shape by molding or stitching, 3D texti Ie preforms are more suitable for the creation of net-shaped thi cker structures5

. Very good drapabil ity and complex shape formation with no gap and reduced manufacturing cost are the main features of woven fabrics. The plain woven fabric is sy mmetrical with good stability and reasonable porosity. However, it is the most difficult

Table I - A compari son among di fferent fabric formati on techniques

Parameter

Weav ing

Knitting

Braid ing

Nonwove n

Di rec tion of yarn int roducti on

Two (0°/90° ) (warp and weft)

One (0° or 90°) (warp or we ft )

One (machine direc tion)

Th ree or more (orthogonal)

Fabri c fo rmat ion principle

Interl ac ing (By selecti ve insertion of 90° yarns into 0° yarn system)

Interl ooping (By draw ing loops of yarn s over previous loops)

Intertwining (Position displ acement)

Mutual fi bre placement

of the weaves to drape, and the hi gh level of fibre crimp imparts relati vely low mechanical properti es as compared to the other weave styles. With thi ck fibres, this weave sty le gives excess ive crimp and , therefore, it tends not to be used for very heavy fabrics . In twill fabrics, the warp yarns alternately weave over and under two or more weft yarns in a regul ar repeated manner. Superi or drape is observed in the twill weave over the plain weave with only a small reduction in stability. With reduced crimp, the twill fabric also has a smoother surface and sli ghtly better mechanical properties. Satin weaves are fundamentally twill weaves modified to produce fewer inter-sec ti ons of warp and weft. Therefore, the low crimp gives good mechanical properties. Satin weaves allow fibres to be woven in the closest prox imity and are capable of producing ti ght and compact preforms5

.

Basket weave is fundamentall y the same as plain weave except that two or more warp fibres alternately interlace with two or more weft fibres. Basket weave is fl atter due to less crimp. Although the basket weave is stronger than a plain weave, it shows poor stabi li ty than plain woven fabrics. Basket weave can be used to develop heavy weight fabrics from thick fibres to avoid excessive crimping. Leno weave improves the stability in open fabrics which have a low yarn count. Leno woven fabrics are normally used in conjunction with other weave styles, because if used alone, their openness does not produce an effective composite component. Tri ax ial weaves show high levels of isotropy and dimensional stability even at low fibre

368 INDIAN J. FIBRE TEXT. RES., SEPTEMB ER 2004

Table 2 - A comparati ve properties of some woven pre forms

Property Woven preform Plai n Twill Satill Basket Lena Moek-

leno

Higher stability 4 3 2 2 I 3

Good drape 2 4 5 3 5 2

Low porosi ty 3 4 5 2 I 3

Smoothness 2 :\ 5 2 I 2 Balance 4 4 2 4 2 4

Low crimp 2 3 5 2 5 2

Rati ng scale: (5) Excellent (4) Very good (3) Good (2) Poor ( I) Very poor

vo lume fraction5.6

. T he characteri sti cs of so me woven preform s are compared in Table 2.

Weav ing is extensively used in the composi te industry, as it produces the vast majori ty of sing le layer, broadcl oth fab ric which can be used as reinforcement. The poor impact perfo rmance reduces in-plane shear properti es, and the poor de-lamination res istance of such structures has led to the use of sti tching techniques. In addition to weave crimp, sti tching is often considered as a factor which reduces the mechanical effi ciency of reinforcing f ibres7

. With some modifications, the standard industry 'machines can be used to manufacture fla t, multi-layer fabrics of wiele variety of structures which have highly improved impact performances. Multi-axial 3D weaving apparatus has also been reported . Bias yarns sandwiched between weft yarn s and the resulting assemblies bound together by warp yarns have produced unique structures. However, the main disadvantage of these multi -layer fabrics is that the standard looms cannot produce fabric that contains in­plane yarns at angles other than 0° and 90°. This results in structures having very low shear and torsion properties, thereby making them unsuitable in many aircraft structures where material s with anisotropic properties are required. To overcome this problem, a great deal of effort has been made for the development of looms that can produce fabric with ±45° fibres9

.

Several weaving techniques have been reported to produce multi-axial multi-layer 3D preforms. These include lappet weaving, tri-axial weaving and pile weaving7

.9

.t1 . Lappet weaving is a special technique in

which extra warp threads are introduced traditionally to develop isolated design motifs on open weave background. A standard weaving machine can be

modi fied to incorporate lappet system to develop integra lIy woven multi-axial mu lti··l ayer preform structures for composites. The ex tra warp yarns (bias yarns) can be made to run at any angle between wrap and weft di rections. To get the th rough-the-thickness reinfo rcements, the extra warp yarns can be interlaced with weft yarns at any posit ion withi n a multi -layer structure with the two extremes corresponding to weft yarns at th e same and opposi te fabric surfaces. However, the locati on of ex tra warp yarn which is limited to the outer surface of preform and the control of lappet bar movement in the reverse direction are the main limi tations to develop mul ti-layer preforms on lappet weav ing machi nes .

Tri axial multi- layer fab ri c structures for composi te rein forcements can be developed by inclusion of ax ia l warp yarns as a third warp set in the bas ic triaxiai process. However, the modi ficat ion of triaxial weavin a machine to incorporate such yarns would be

b 1"' a major engineering taskll . A patent - also describes the use of a j acquard shedding mechanism to manufacture multi-layer woven textile preforms. Greenwood el al. I ] described a method to develop stress oriented T -shaped woven preforms for composites. To deve lop T cross-sect ions, two sets of paralIel threads at right angles to each other and at 45° angle to warp are required. To achi eve COtTect orientation of threads, two sets of weft threads have been used to form thi s angle separately. This can be done by joining of two laye rs either along one side or side-by-side.

The typi cal examples of 3D woven fabrics are s imple interlock, orthogonal and complex shaped structures l4.15, Different multi-l ayer 3D woven structures and multi-axial pile fabrics are shown in Figs 2 and 3 respectively. Using the multi-warp weaving method, various fibre structures can be developed incl uding solid orthogonal panel, variable thickness solid panels and core or truss like structures. Orthogonal cross-lapped fabrics can be formed by the placement of yarns at right angle to each other, typically in either rectangular or cylindrical space. There is no interlacing or other form of entanglement to hold the structure. Yarn is alternately laid between the edges in alternating orthogonal direction to create thick structure. Table 3 shows comparison between some multi-a l(ial weaving techniques.

2.2 Knitted Preforms

Flat knit and shape knit products show the ability of the knitting process to manufacture complex shaped

OGALE & ALAGIRUSAMY: TEXTILE PREFORMS FOR ADVANCED COMPOSITES 369

(a) (c)

(b) (d)

ThrougIHhc-thicknc.:ss

binders ~

E"urrCfS r-~

Architecture changes in la yer configurat ion to form aperture.

(e)

Fig. 2-Multi-l ayer 3 D weave and its vari:lIlts (a) multi-layer 3D weave, (b) change of ::Ingle, (e) angle interlock , (d) variable thickness solid panel, and (e) near net shaped pre form

components. Jet engine vanes, T-shaped connectors, medical prosthesis, car wheel well s and aerospace fairings have all been successfu lly manufac tured. The structure of yarn in the preform is hi ghly curved and the interlooping of yarn permits a very elastic and flexib le structu re. Thi s is an advantage where components of complex shape without crimp are required and better formabl ity and drapability of preforms are critical. Because of good drapbal ity, a high degree of deformation can be possiblel 6

.

The yarn structure of the knit also tends to improve the impact performance of the composi tes; however, the structural performance of knitted compos ites is generall y low l7. Hong el al.18 described the various methods to develop 3D shaped knitted preforms, including tubul ar, spherical and box forms, by using SES 122FF flat knitting machines . 3D shapes can be developed by using a variety of structural combinations, such as different loop lengths and alteration in the number of knitting needles and courses. Wilde and Zigmann 19 compared woven fabric, outstretched glass/polyester(GF/PET) knit and 73% pre-stretched GF/PET knit. GF/PET co-knitted fabrics have been developed by simultaneous knitting of glass and polyester yarns. They also developed a weft knitted preform from GF/PET commingled yarns and observed that the stiffness of knitted fabric is

(a)

(b)

Fig. 3--Pile interlacing patterns (a) V-interlaci ng. and (b) W- interlacing

Table 3--Comparison among d iffere nt multi -axial weaving techniques9

Bias fi bre Uniformity o f T hrough-the- Multi ple placement bias fibre thickness layers

layers reinforcement

Rapier No Yes Yes Lappet Yes No No Screw shaft No Yes Yes Split reed Yes Yes Yes Guide Block Yes Yes Yes Bobbin Yes Yes Yes (polar )

more isotropic than that of woven fabrics . Further, they commented that the size of loops in kni tted fabrics would determine the strength and stiffness of composites.

The properties of flat knitted fabrics are different from those of other textile preforms. This area of knitting does overcome the problem of poor mechanical performance and it is the subject of intense interest within the aerospace industry . Woven and multi-axial fabrics are stiffer than conventional

• knitted fabrics and therefore more suitable for high performance applications where high tenacity and lower strains are required. With insertion of weft inlay threads, the stiffness of these fabrics can be increased.

370 INDIAN J . FIBRE TEXT. RES ., SEPTEMBER 2004

By using warp knitting techniques in conjunction with fibre placement concepts, multi-layer fabrics can be produced containing straight and relatively uncrimped fibres stacked in the required orientations. Karl Mayer Textilmachinenfabrik GmbH and LIBA Maschienenfabrik GmbH developed this technology . Fig. 4 shows multi warp knitted (MWK) structure. MWK structure is produced on special raschel machine and the structure consists of two diagonal weft yarns, followed by a warp yarn and a horizontal weft yarn. The layers are held together by pillar st itches from both sides . The diagonal weft insertions ensure parallel placement of yarns at constant distance. Weft insertion makes it possible to arrange the leas t part of the fibre in stretched form II. The diagonal angle of weft can be varied in the range of ±300-60° by suitably altering the course density .

By using LIBA technique, multi-axial fabrics can be produced on a special tricot machine using 5-7 weft inlaid yarn layers and a warp inlaid yarn layer. At each weft station, the yarn can be laid horizontally in the range ±300-60° and therefore easy interchangeability of layer positions is poss ible. LIBA technique gives Quadra-axial structures, where yarns

are arranged at 0°, +45°, -45° and 90°. Malimo technique is another method to develop multi-axial stitch bonded knitted structures by stitching of several layers of yarns together at various angles or piles of skewed fabric on modified st itch bonding machine. This will improve mechanical properties and

I . 20 structura consistency .

In knitted multi-axial structures, materials for inlays are normally high modulus or high temperature resistant polymer filaments such as polyester, nylon and PEEK, whereas glass, aramid or carbon threads

Carbon Yarns

Polyester Knitting Thread

Fig. 4-Multi-axial warp knit (MWK) structure

can be used as reinforcing fibre materi als. The use of these fabrics can lead to cost savings in the manufacture of composite components and the uncrimped nature of the yarn can also produce improved mechanical performance w en compared to traditional woven fabrics . These fabrics have excellent dimensional stability and outstanding in­plane shear resistance in all direction~ . The warp inlay multi-axial structures show higher elastic modulus compared to woven fabrics. Tear strength of warp inlay multi-axial structure is found to be higher than that of woven fabrics. This may be due to the shifting of yarn layers under force and bunch together to resist tearing7

.

2.3 Braided Preforms Two-dimens ional braided structures are intertwined

fibrous structures capable of having OOand ±8 fibre orientation. Thickness is built by over braiding on previously braided layers, similar to ply lay up process . Braiding can take place vertically or horizontally but the majority of composite braids is horizontal. The braiding process is capable of forming quite intricate preforms and has been successfully used with glass, aramid, carbon , ceramic and metal fibres. The braiding process can be varied during operation to produce changes in the cross-sectional shape as well as tapers, bends and bi furcations. Due to the high level of comformabali ty and damage resistance capability of braided structures, the composites industry has used the braided composites in various applications ranging from rocket launchers to automoti ve pars to aircraft structures '.

By simply changing the relative posi tion of carriers on track ring, different interlacing patterns can be produced. The pattern can be designed as 111 or diamond braid; however, the 2/2, 3/3 211 and 3/1 are the interlaci ng patterns of common use. Among all these patterns, 2/2 braid is the most popular and is referred as regular, standard, plain or fla t braid. The braid has the tightest structure when each yarn is in contact with all neighboring yarns. However, due to bulky fibre structure and crimp, it is very difficult to form tightest structure. The braided structure enables the composi te to endure shearing and impact better than woven preforms. Braids are continuously woven on the bias and have at least one axial yarn that is not crimped in the braiding process. This arrangement of yarns allows highly efficient load distribution throughout the braid. Either flat or tubular configurations are available as braids. Flat braids are

OGALE & ALAG IR USAMY: TEXTILE PREFORMS FOR ADVANCED COMPOSITES 37 1

used primaril y fo r selective reinforcement, such as strengthening specific areas in pultruded parts. Tubular braid can be pultruded over a mandrel to produce hollow cross-secti ons in a variety of parts such as windsurfer masts, lamp and utility poles 7.

Braiding was the first tex tile process used to manufacture a 3D fibre preform for a composite. 3D braiding is an extension of 20 braiding in which the fabric, constructed by intertwining or orthogonal interlacing of yarns to form an integrated structure through position displacement, provides through-the­thi ckness reinforcement and ready acceptability to the fab rication of wide range of complex shapes21

. 3D braids show comparatively hi gh level of fibre content and at the same time less than I % voids by interlocking conti nuous layers of braid. There are basically two types of braiding machines, one is rectangular and other is circular, to produce J or T beams and thi ck wall tubular structures respecti vely. Furthermore, multiply preforms can be developed by interlacing adjacen t layers on 3D braider7

. The key geometric parameters of 3D braids are fibre orientation , tota l fibre volume fraction, void and ax ial fibre percentage of total fibres. The speed ratio between braiding and take up as well as the linear density ratio of braider and axial yarn are the simple process parameters to adjust or control the microstructure of 3D braids.

Although 20 braided preforms offer numerous applications, the major limitation of 3D braiding is that the maximum preform size is determined by the braiding machine size and most available machines are only ab le to braid preforms with small dimensions only. Larger and more expensive machines will be needed to produce preforms suitable for a wider range of structural uses. Furthermore, many 3D braiding machines are still in a research and development stage and thus only a few machines are presently able to commercially manufacture preforms. 3D braid ing machines are also slow and have short production runs. As a result, 3D braids cannot compete with 20 braids and laminates on a cost saving basis 7.2 1.22.

2.4 Stitched Preforms Stitching of tex tile preforms is considered as one of

the important techniques to develop complex tex til e preforms. The process of stitching can be used in two ways: Firstl y, the stitching can be used simply as a way of assembling the single or multi-layered textile preforms together and ho lding them into the required shape during consolidation process. The second use of

stitching is to improve the impact performance of composite structures by the addition of through-the­thickness reinforcement23

.24

.

Stitching involves sewi ng high performance yarn (e.g. glass, carbon or kevl ar), through an uncured prep reg laminate or dry fabri c plies using an industrial sewi ng machine. Stitching has also been performed using po lyes ter thread, though kevlar is the most popular yarn because of its high strength and fl exibility . A variety of sewing machines can be used to stitch composites, although they can usually be classified as si ng le-needle o r multi-needle machines7

•

Fig 5 shows different types of stitches through-the­thickness of reinfo rcements. Stitching equipment currently used in the texti le industry is capable of manufacturing preforms from aerospace-grade materials and a significant amount of research effort has been devoted to the performance of these stitched composites. Special attention is provided particularly in manufacture of complex shaped structures using stitching l5

. Commonly avail ab le 20 fabrics are used as the main reinforcement in the structure. Thi s provides large amount of flexibility in eng ineering tex tile structures to get the desired composite properties.

For making near net shape preform and joining tex tile reinforce ment materi als, stitching is very important tool. It makes preform material ready to use and easy to handle. However, the stitching of preform creates faults in the plane of material which affects the structural properties of composites and some of the reinforcement fabric related properties. This damage adversely affects the mechanical properties of

Surface Loop \ /' Needle Thread

~" II :-- , ,', :;':;:;::'0;-:.:,'" .":";- L' :-.-:.:.:; .:.:. ~. :!::::-::: : ,:.:.:~:>:-":;:t'" ~-:' ":.;.

Laminate ' , . • • ri':: .~ .• . ".-~ ~ ; :;:-,;;;:.; ~ , •• • .;.,.~ :.: ... ';,"-~-~: :" LT - :~ "'-".

Knot (a) Bobbin Thread

_ ; 1. : _ J . i . . I..l \ • ~ " - ----~----

Pitch Surface Loop M

Laminate illilli (b) Knot

Fig. 5-- Different types of s titches for th rough the thickness reinforcements (a) lock stitch, and (b) chai n st itch

372 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2004

composites25-28. Weimer et 01.24 studied the various

parameters that affect the stitching process and observed that, in addition to the type of textile preforms and stacking sequence, the other parameters such as stitch direction, properties and appearance of thread material and stitching process strongly affect in-plane fibre distortion and compaction during stitching and impregnation process. Improved damage tolerance under impact and better post-impact mechanical properties have been major reasons for the burgeoning amount of research into stitched composites.

2.5 Nonwoven Preforms Nonwoven structures are fibre-to-fabric assemblies

produced by chemical, thermal or mechanical means or a combination of them. The thickness of sheet may vary from 25 Jlm to several centimeters and weight from 10 g/cm2 to 100 g/cm2

• Nonwovens have density less than usually demanded in structural composites. As nonwovens become more readily available with the range of properties, the market for composites is also expected to increase. Composites made from nonwoven glass with epoxy resins reduce exfoliation between layers at high temperature. Nonwoven-based composites are finding increased application in automotive, marine and other applications29

.

Two very popular types of composites used in medical applications are SMS (Spun bond, Melt blown and Spun bond) and MSM (Melt blown, Spun bond and Melt blown) which in actual sense are laminate structures. A method to develop composite material by coating nonwoven fabrics made of carbon fibre or a blend of carbon fibre and other organic fibre with a filler-containing liquid has been described in a patent30

. This low-priced pitch-based carbon fibre nonwoven can be used cost effectively in composites as friction materials, packaging and gasket applications. Composites developed from a nonwoven mat consisting of acrylic continuous fil aments and impregnated with an inorganic matrix offer a wide range of applications in the fields of construction, aerospace, filtration, industrial , marine, medical

. d . 31 protectIOn, sports an transportation .

3 Characterization of Textile Preforms The essential properties of textile preforms for

composites include high fiexibility , formability, stability and high axial rig idity. Table 4 shows some of the important properties of textile preforms . Consideration of geometrical properti es in designing

textile preforms will help to predict the resistance of preforms to mechanical deformation such as initial extension, bendrng and shear in terms of resistance to deformation of individual fibres. It will also provide the information regarding maximum achievable packing of a fabric.

Texti le preforms are subjected to a wide rage of complex deformations during manufacturing of composites. This includes intra- ply shear, inter-ply shear (if two or more plies are used), viscous friction between tool and material, inter-tow s lip and applied pressure during consolidation . Some other serious problems are also observed during composite formation, such as wrinkling (buckling of fibres) , and variations in fibre volume fraction due to spreading or bunching of fibres32

. The behaviour of textile preforms extended at any angle involves the consideration c f shear behaviour of fabrics . Tensile forces along the fibre axis may lead to fibre straightening and compressive fo rce can cause buckling.

Chen and Chou33 studied the mechanical properties of multilayer and angle interlock woven structures and observed that the mechanical properties of 3D woven structures heavily depend on the fabric structure. Straight yarns in orthogonal weaves provide high tensile streng th and stiffness and [his is directly proportional to the number of layers of orthogonal weave. However, the breaking elongation of orthogonal woven structure is independe nt of weave and number of layers, as breaking elongation primarily depends upon the type of yarn used. Further, they stated that the shear rig idity and shear hysteresis depend on binding weaves. Shear rigidity and hysteresis increase with the increase in number of layers. They also observed that the tighter binding weaves and more layers of orthogonal structure would produce higher bending stiffness and bending hysteresis and vice versa. Finally, it is concluded that the straight yarns in orthogonal weaves provide high tensile strength and stiffness and th is is directly proportional to the number of layers of orthogonal weave. However, the breaking elongation of orthogonal woven structure is independent of weave and number of layers.

Many researchers used kinematic (pin-joint) approach to study deformation behaviour of texti le fabrics. The acute angle between warp and weft can be Llsed as a measure of deformation caused due to shearing. Befo re any deformati on, this angle for

Textile preform

Low crimp uniweave

2-D Woven

3-D Woven

2- D Braid

3-D Braid

Multi -axial warp knit

Stitched fabrics

OGALE & ALAGIRUSAMY: TEXTILE PREFORMS FOR ADVANCED COMPOSITES 373

Table 4-Properties o f some tex tile performs

Advantage

High in-plane properties; good taliorability ; highly automated preform fabri catio n process

Good in-plane properties; good drapability; highly automated perform fabrication process; integrally woven shapes possible; suited for large area coverage and extensive data base

Moderate in-plane and o lit-oF-plane properties; automated preform fabrication process and limited woven shapes are possible

Good balance in o ff-axi s properties; automated preform fabrica tion process; well suited for complex curved shapes; good drapability

Good balance in in-plane and out-of-plane properties; well suited for complex shapes

Good taliorability for balanced in-plane properties; highly automated preform fabricati o n process; multi­layer hi gh throughput; material suited for large area coverage

Good in-plane properties; highly automated process; provides excellent damage tolerance and out-of-plane strength and excellent assembly aid

Limitation

Low transverse and out-of-plane properties; poor fabric stability; labor intensive ply lay-up

Limited taliorability for o ff-axis properties; low out-of­plane properties

Limited taliorability for o ff-axis properties and poor drapability

Size limitation due to machine availability and low out-of­plane properties

Slow preform fabrication process; size limitation due to machine availability

Low out-of-plane properti es

Small reduction in in-plane properties; poor accessibility to complex curved shapes

woven fabric is 90°. However, the angle decreases continuously with the increase in deformation until it reaches to critical shearing angle (called as locking angle) just before buckling of fibres. The traditional kinematic model cannot represent the later stages of deformation and locking angle. This model can be used only for single layer and critical experiments are necessary to determine locking angle. To overcome these problems, Prodromou and Chen34 modified the fabric geometrical parameters, such as tow width and tow spacing, and fibre properties like friction and buckling resistance which determine locking angle. Their study shows that besides geometric modifications in fabric parameters, the factors independent of fabric construction parameters, such as high friction coefficients, inter-ply shear interactions and reduction in global stiffness, may affect wrinkling behaviour.

bending properties of textile preforms. Formation of complex shaped composites involves bending of preforms in more than one direction and, therefore, the preforms are subjected to form double curvature, which leads shear deformation of preforms. Measurement of fabric buckling is relatively a good method to observe bending rigidity and frictional resistance to bending of fabric. The traditional approach to drape modeling for fabric reinforcements uses a geometric mapping which is based on fabric shear. Long et al.35 developed a geometrical model which describes shear force and shear strain energy as the function of fabric shear angle. They used warp knitted and woven preforms for the study. They found that the fibre structure has a significant effect on deformation characteristics of fabric. They also developed a mechanical model to determine shearing behaviour of woven and warp knitted preforms based on inter-tow friction. The obtained shear properties can be used for draping simulation.

Draping is particularly important in processes like Liquid Composites Molding (LCM) and thermoforming of prepregs. Drape with other preform properties such as wrinkling are also clearly related to

The drape simulation indicates the feasibility of draped form and gives undeformed fabric pattern as

374 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 2004

well as other important information such as distribution of yarn orientation, fibre volume fraction and thickness of the fabric stack. Wang et al. 36

predicted the draping behaviour of preforms by bias extension and simple shear test methods which are based on pin-jointed model. They also studied the effects of aspect ratio and boundary conditions on permeability. They observed considerable differences in preform permeability when different test methods, aspect ratios and boundaries were used. Page and Wang37 also studied yarn slippage in textile preforms by comparing carbon and glass fabrics and observed higher yarn slippage in carbon fabrics, whereas no such slippage of yarns was observed in glass fabric. The observed yarn slippage was found to be affected by the non-uniformity of deformation, boundary conditions and differences in fabric materials.

Although the various technologies are available for manufacturing composites, only liquid composite molding has the capability of manufacturing polymer composites with large size and complex shapes at low cost. In LCM, a fibre preform, which consists of several layers of dry continuous strand mat, woven roving or cloth, is placed into the mold cavity. The mold is then closed and resin is injected into the cavity to impregnate the preform. Therefore, the permeability of preforms is of prime importance in LCM. Fibre preform permeability is usually dependent on the fibre content, preform structure, binder, arrangement angle of fibre plies, etc. Therefore, the permeability of a given preform is a function of the fibre content only. There are two techniques (parallel flow and radial) for permeability measurements of fibre mats or preform. Han et al.38

used pressure transducers to measure the permeability of an individual layer in a multi-layered preform. Stoven et al.3

? determined the transfer permeability of planer textile reinforcements such as non··crimped stitched fabric by ultrasound transmission technique. They observed that the sound velocity inside the stack of fibres changes when it gets impregnated and the dry regions of the fibre stack depend on the dimensions of dry regions along the path of acoustic waves.

In the preparation of composites with various LCM techniques, the compaction of textile preforms during tool closure is another important parameter40

. The yarn bundles in preform get flattened during compaction and, therefore, reduce pores and gaps among fibres and yarns. This results in elastic

deformation, inter-layer packing and nesting. As the compressive force increases, the elastic deformation of fabric extends further and the thickness of preform reduces while fibre volume fraction increases. When the force reaches to a certain value, the fabric cannot be further compressed. Chen and Chou,,3 developed a 3D compaction model to study the compressive behavior of multi-layer woven preforms in terms of nesting and elastic deformation, which can be used to predict permea ility of preform. They found that the compressive force and reduction in preform thickness have empirical relationship and suggested that some further studies are required to understand nonlinear pressure-preform thickness relationship exhibited at the initial stages of experiments.

4 Conclusions With improved textile preforming techniques in

combination with developments in characterization of fibre structures, the production and processing of fibre and textile materials now helps to ach ieve outstanding composite properties. Better understanding of the functioning of reinforcing fibres in composite materials enables design and production of new textile-based composites for a wide range of applications. Optimization of traditional textile technologies and development of new textile production techniques will help to reduce manufacturing cost of advanced composites . With the advancement in geometrical modeling and predictive calculations of the physical and structural properties of textile preforms, desired textile preforms can be tailored with essential modifications in preform specifications as well as in structure and properties of fibres and yams . Although simple 2D textile preforms are finding. extensive usage in the commercial applications, the advanced 3D textile preforms are being used mostly in defense and aerospace applications only. As composites with 3D textile preforms can effectively replace conventional materials, it is necessary to develop cost effective ways of producing complicated 3D textile preforms and evaluating the properties relevant to commercial applications.

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