Review of applications for advanced three-dimensional fibre textilecomposites

A.P. Mouritza,*, M.K. Bannisterb, P.J. Falzonb, K.H. LeongbaDefence Science and Technology Organisation, Aeronautical and Maritime Research Laboratory, P.O. Box 4331, Melbourne, Victoria 3001, AustraliabCooperative Research Centre for Advanced Composite Structures Ltd. (CRC-ACS), 506 Lorimer Street, Fishermens Bend, Victoria 3027, Australia

Received 10 November 1998; accepted 28 April 1999

Abstract

Current and future potential applications for three-dimensional (3D) fibre reinforced polymer composites made by the textile processes ofweaving, braiding, stitching and knitting are reviewed. 3D textile composites have a vast range of properties that are superior to traditional2D laminates, however to date these properties have not been exploited for many applications. The scientific, technical and economic issuesimpeding the more widespread use of 3D textile composites are identified. Structures that have been made to demonstrate the possible uses of3D composites are described, and these include applications in aircraft, marine craft, automobiles, civil infrastructure and medical prosthesis.! 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Three-dimensional composites; E. Weaving; E. Braiding; E. Knitting; E. Stitching

1. Introduction

Polymer laminates reinforced with a two-dimensional(2D) layered fibre structure have been used with outstandingsuccess for over 50 years in maritime craft [1], for aboutthirty years in aircraft [2,3], and for nearly twenty years inhigh performance automobiles [4] and civil infrastructuresuch as buildings and bridges [5]. Despite the use of 2Dlaminates over a long period, their use in many structuralapplications has been limited by manufacturing problemsand by some inferior mechanical properties. The manufac-turing of laminates can be expensive because of the highlabour requirement in the manual lay-up of plies. The needby some industries (particularly the aircraft industry) tofabricate laminates from prepreg tape adds to the productioncost, because expensive refrigeration facilities are needed toprolong the shelf lives of the prepreg before the resin beginsto cure. Added to these costs is the poor drape of manyprepreg and fabric plies, which makes them difficult tomould into complex shapes. As a result, many complexcomponents need to be built from a number of machinedlaminate parts that must then be joined by co-curing, adhe-sive bonding or mechanical fastening. This is a majorproblem in the aircraft industry, where structures such as

wings need to be made from a large number of smallercomposite parts such as skin panels, stiffeners and stringers,rather than being fabricated as a single integral structure.Fabrication problems such as these have impeded the wide-spread use of laminates in aircraft structures because theycan be significantly more expensive than many aerospacealloys [2].The application of 2D laminates in some critical struc-

tures in aircraft and automobiles has also been restricted bytheir inferior impact damage resistance and low through-thickness mechanical properties when compared againstthe traditional aerospace and automotive materials such asaluminium alloys and steel. The low through-thicknessproperties, such as stiffness, strength and fatigue resistance,have impeded the use of 2D laminates in thick structuressubjected to high through-thickness and interlaminar shearstresses. The problem added to this is that many 2D lami-nates have low resistance to delamination cracking under animpact loading because of their poor interlaminar fracturetoughness. As a consequence of this, their post-impact in-plane mechanical properties can be severely degraded,particularly their compression strength and fatigue perfor-mance. While these properties can be improved to a certainextent by the use of toughened resins or fibre interleaves,these solutions usually are expensive and do not overcomemany of the problems associated with the manufacturing oflaminates.In an attempt to overcome many of the problems with the

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1359-835X/99/$ - see front matter ! 1999 Elsevier Science Ltd. All rights reserved.PII: S1359-835X(99)00034-2

* Corresponding author. Tel.: ! 61-396268276; fax: ! 61-396268999.E-mail address: adrian.mouritz@dsto.defence.gov.au (A.P. Mouritz)

manufacturing and mechanical properties of laminates,considerable attention has been given over the past 30years to the development of advanced polymer compositesreinforced with 3D fibre architectures. 3D composites canbe made in a number of ways using techniques as diverse asembroidery and z-rods", in which short composite rods areinserted through-the-thickness of traditional 2D laminates.However, most attention has been given to 3D compositesmanufactured by the textile techniques of weaving, braid-ing, stitching and knitting.In this paper, it will be shown that composite structures

made with 3D textile fabrics are potentially less expensiveto manufacture and provide better through-thicknessmechanical properties than composites made with the tradi-tional 2D fabrics. However, the ability of 3D composites toreplace 2D laminates in many structural applications hasbeen largely unsuccessful. The aim of this paper is to

examine this situation by reviewing present and futureapplications for 3D textile composites made by weaving,braiding, stitching and knitting. Because some of theseapplications are commercially confidential, only thosecurrent applications and potential uses that have beenreported in literature can be reviewed. This paper will deter-mine the main factors impeding the use of 3D composites aswell as identify the key technology issues that need to beresolved before the applications can be expanded. Thispaper is not intended to be a review of the textile processesused to manufacture 3D composites nor a review of theirmechanical properties. However, in some cases it is neces-sary to briefly describe the production and properties of 3Dcomposites in order to identify the issues impeding their use.In this paper, the 3D composites made by weaving, braiding,stitching and knitting will be treated separately, althoughsome of the issues are common.

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Fig. 1. A Jacquard loom used in the manufacture of 3D woven preforms.

2. 3D woven composites

The machinery and processes for integrally weavingmulti-layer fabrics of 3D woven composites have beenoutlined in numerous papers, including those by Mohamedet al. [6] and Bannister and Herszberg [7], and thereforeonly the basic weaving process is described here. Warpyarns are fed into the weaving loom from a source, whichcan consist of a framework containing individual packagesof yarn (known as a creel), or a number of cylindrical beamsonto which the necessary amount of yarn has been pre-wound (warp beams). The warp yarns are then fed througha lifting mechanism, which selects and lifts the requiredyarns and creates a space (the shed) into which the weftyarns are inserted at right angles to the warp. This liftingmechanism can be mechanically controlled or, in moreadvanced looms, electronically controlled. The sequencein which the warp yarns are lifted controls the interlinkingof the warp and weft yarns, and thus the pattern is created inthe fabric. A comb-like device (reed) is used to correctlyspace the warp yarns across the width of the fabric and tocompress the fabric after the weft yarns are inserted. Thebinder yarns can be aligned in the warp direction or insertedin the weft direction and their path through-the-thickness ofthe preform is controlled by the lifting sequence. Fig. 1shows a computer-controlled Jacquard loom capable ofweaving 3D preforms for composites, although weavingcan also be performed using less sophisticated machinerysuch as manual hand looms.3D woven composites were first developed nearly 30

years ago in an attempt to replace expensive high tempera-ture metal alloys in aircraft brakes [8]. The 3D weaving toproduce the preform for the brake component wasperformed by the Avco Corporation. A specialised loomwas developed to allow the weaving of hollow cylindricalpreforms in which carbon fibres were aligned in the radial,circumferential and axial directions. The preform wasprocessed into a carbon–carbon composite displayingsome desirable properties for aircraft brakes, namely highspecific strength and specific stiffness properties as well asexcellent resistance to thermal deterioration.Research and development of 3D woven composites

remained at a low level until the mid 1980s, when interest

was renewed because of problems being encountered withthe traditional 2D laminates used in some aircraft structures.Two examples of problems being experienced were, firstly,that aircraft manufacturers were finding it expensive toproduce complex components from laminates and,secondly, aircraft maintenance engineers were finding thatlaminates were highly susceptible to impact damage fromdropped tools. These types of production and maintenanceproblems were the main forces behind the effort to assess thepotential benefits of using advanced 3D woven compositesin aircraft structures and components. Research into the 3Dweaving process and the properties of 3D woven compositesover the past 10–15 years has revealed a number of advan-tages over traditional laminates, and these are listed in Table1.One important advantage of 3D weaving is that preforms

for a composite component with a complicated geometrycan be made to the near-net-shape. In contrast, prepregmaterials used in 2D laminates can only be easily processedinto relatively simple shapes such as flat and slightly curvedpanels. This ability of 3D weaving to produce near-net-shape preforms can greatly reduce the cost of a componentby reducing material wastage, the need for machining andjoining, and the amount of material handled during lay-up.Examples of the complexity and variety of componentsmade using 3D woven composites are presented in Fig. 2.An advantage of 3D weaving is that preforms can be

made on standard industrial weaving looms used for produ-cing 2D fabrics by making minor modifications to themachinery. This minimises the capital costs incurred bycomposite manufacturers because they do not requireexpensive custom-built looms to produce 3D wovenpreforms. However, a range of specialised looms havebeen developed over the past ten years that have higherweaving speeds and are capable of weaving more complexshapes than traditional looms which have been modified [9].Another benefit of 3D weaving is that fabrics with a wide

variety of fibre architectures can be produced withcontrolled amounts of binder yarns for the through-thick-ness reinforcement. Two of the most common architecturesare the orthogonal and layer interlock weaves, which areillustrated in Fig. 3. The important difference betweenthese two architectures is the weave pattern of thethrough-thickness binder yarn. These preforms can bewoven from almost any type of yarn, including carbon,glass, Kevlar# and ceramic fibres (SiC, A12O3). In addition,the amounts and types of warp, weft and binder yarns can becontrolled to tailor the properties of the composite for aspecific application, although the proportion of binderyarns in most preforms is usually less than 5%. It is alsopossible to produce hybrid woven preforms using more thanone type of yarn material, however the mechanical proper-ties of hybrid composites as well as their use in structuralapplications has not been examined in detail.Another important advantage of 3D woven composites

is their high ballistic impact damage resistance [10] and

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Table 1Advantages of 3D woven composites over 2D laminates

3D weaving can produce complex near-net-shape preforms3D woven composites with a complex shape can be less expensive andsimpler to manufacture3D weaving can tailor the through-thickness properties for a particularapplication3D woven composites have higher delamination resistance, ballisticdamage resistance and impact damage tolerance3D woven composites can have higher tensile strain-to-failure values3D woven composites have higher interlaminar fracture toughnessproperties

low-velocity impact damage tolerance [11–15], which havebeen a major problem with the use of 2D laminates in mili-tary aircraft structures. For example, Chou et al. [13] reportsthat the impact energy needed to initiate damage in 3Dwoven carbon–bismaleimide composites is up to !60%higher than in a 2D carbon–bismaleimide laminate. Theimproved impact damage resistance usually results in 3Dcomposites experiencing a lower reduction to their in-planemechanical properties than that suffered by the 2D counter-part laminates [14,16–19]. The superior damage toleranceof 3D composites occurs because the through-thicknessbinder yarns are able to arrest or slow the growth of dela-mination cracks formed under an impact loading. Thebinder yarns are also largely responsible for some 3Dwoven composites having greatly increased tensilestrain-to-failure values [20] and for their mode I inter-laminar fracture toughness values being !6–20 times

higher than the unidirectional carbon fibre reinforcedepoxy laminates [21].Despite the advantages and potential benefits of 3D

woven composites, these materials have failed to findmany commercial applications. They have been used ortested in only a few specialised structures by the building,aircraft and marine industries, where the cost and/or perfor-mance of traditional laminates and metals have been unac-ceptable. The building industry has used 3D woven glasscomposites in a couple of niche applications. Muller et al.[22] report that I-beams made with a 3D woven compositeare used in the roof of a ski chair-lift building in Germany.Due to the steep terrain, it was difficult to transport and liftheavy steel beams at the building site, and therefore lighter3D woven composite beams were used which demonstratedcost savings and improved performance over steel andconventional composite beams. The only other civil

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Fig. 2. Examples of 3D woven preforms: (a) cylinder and flange; (b) egg crate structures; (c) turbine rotors woven by Techniweave Inc.; and (d) variouscomplex shapes woven by Shikishima Canvas Co., Ltd. (Courtesy of the Techniweave Inc. and Shikishima Canvas Co. Ltd.).

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Fig. 3. (a) Orthogonal and (b) layer-interlock interlock woven fibre architectures commonly used in 3D woven composites.

infrastructure application for a 3D woven composite hasbeen for manhole covers in some petrol station forecourts.3D woven composites are currently used for only one

reported structural application in aircraft. Woven ‘H-joint’connectors were used for joining honeycomb sandwichwing panels on the Beech starship. Wong [23] reports thiswoven connector was critical to the cost-effective manufac-ture of the wing and improved the stress transfer at the joint,thus reducing the peeling stresses. Apart from this applica-tion, 3D woven composites have been used in a variety ofdemonstration structures for aircraft that are listed in Table2. 3D woven composites have also been used to improve thestrength of repairs to damaged boat hulls [24]. While 3Dwoven composites are not presently used as biomedicalmaterials, Limmer et al. [25] have considered their use inleg prosthesis.Despite the current applications and many demonstra-

tions of the potential uses of 3D woven composites, theyhave failed to replace laminates in most aircraft structures orfind many niche applications. Some of the possible causesfor this are summarised in Table 3. One of the main draw-backs of 3D weaving is the inability of current looms toproduce fabric that contain in-plane yarns aligned at anglesother than 0 and 90". This results in 3D woven compositeshaving highly anisotropic properties and low shear andtorsion properties, which thereby renders them unsuitablein many aircraft structures where materials with isotropicproperties are required. There are new weaving techniquesto produce fabrics containing 0 and 90" yarns sandwichedbetween an outer layer of ^ 45" yarns. However, the fabrics

can only be made with highly specialised and expensivelooms.Another problem is the in-plane properties and failure

mechanisms of 3D woven composites that have not beenextensively characterised. 3D weaving offers the capabilityto produce composites with a wide range of architectureswoven from a variety of yarn materials. However, studies ofthe compression, tension, flexure and fatigue propertieshave been confined largely to carbon–epoxy compositeswith an orthogonal or layer interlock architecture [11–19,21,26]. Only a few studies have characterised themechanical properties of 3D woven composites reinforcedwith other architectures or fibre types (e.g. glass, Kevlar#)[11,12,27,28] while their durability under harsh environ-mental conditions has not been examined. As a conse-quence, there does not exist a large database of materialproperties that can be used in the certification of 3Dwoven composite structures, particularly in trying to provethe lifetime performance of these materials with regard tofatigue and environmental effects. The lack of a large data-base has also made it difficult to determine the optimumweave architecture and yarn material required to providethe desired in-plane and through-thickness (impact damagetolerance) properties for a specific structural design.Furthermore, due to the complex 3D fibre architecture, ithas proven difficult to develop analytical or computationalmodels for predicting their mechanical properties except forYoung’s modulus [29–31]. This combination of factors hascaused scepticism for many composite designers aboutusing this material in weight-bearing structures or criticalengine/machinery components, particularly in aircraft.An additional problem with 3D woven composites is that

many of their in-plane properties are generally inferior to2D laminates with an equivalent amount of fibres aligned inthe load direction. While the stiffness values of 3D wovencomposites are similar to 2D laminates, their tension andcompression strengths are generally lower by !15–20%[16–19,27,32,33]. This reduction in strength is attributedlargely to crimping and distortion of in-plane fibres by thebinder yarns [20,28]. In fact, the amount of distortionexperienced by the fabric as it is woven is usually so severe,that the preform architecture rarely corresponds to its idea-lised structure [34]. At the moment there is no clear under-standing of the level of control needed over the weavingparameters to produce preforms of the required quality.Parameters such as yarn tensions, take-off mechanisms,yarn surface condition and binder density all affect the qual-ity of the preform, but it is unclear what precision is neededto control these parameters to produce an optimum weavearchitecture.Obviously, there are a number of technical, scientific and

economic challenges that must be overcome before 3Dwoven composites are generally accepted by the designers,manufacturers and users of composites. While the future ischallenging, it is still possible to envisage a range of appli-cations over the next decade for these materials. In the

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Table 2Aerospace demonstrator components made with 3D woven composites

Turbine engine thrust reversers, rotors, rotor blades, insulation, structuralreinforcement and heat exchangersRocket motors, nozzles and fastenersEngine mountsT-section elements for primary fuselage frame structuresRib, cross-blade and multi-blade stiffened panelsT- and X-shape elements for filling the gap at the base of stiffeners whenmanufacturing stiffened panelsLeading edges to wings

Table 3Issues impeding the use of 3D woven composites

Difficult and expensive to manufacture quasi-isotropic 3D wovencomposites3D woven composites generally have lower tension, compression, shearand torsion propertiesIn-plane mechanical properties and failure mechanisms of 3D wovencomposites are not well characterisedValidated methods are not available for predicting many of the propertiesand long-term durability of 3D woven compositesPoor understanding of the influence of weaving parameters on the preformarchitecture and composite properties

aerospace industry, the use of 3D woven composites ishampered by the current inability to economically producecommercial quantities of fabric that contain ^ 45" yarns.However, there are few specific aerospace components thatare completely manufactured with 0/90" fabric or have aminimal requirement for shear or torsion performance,such as some leading and trailing edge components thatexperience mainly bending forces. 3D woven compositesare candidate materials for these types of componentsbecause their impact performance, which is superior to 2Dlaminates, will be an important design consideration.The potential applications for 3D woven composites

appear to be more promising in non-aerospace industriessuch as maritime, civil infrastructure and land transporta-tion. In these industries, the need to save weight and theconservative design philosophies are not as demanding asthey are in the aerospace industry, so there is more scope foruse of 3D woven composites. Possible applications arefloors and floor beams in trains and fast ferries, and flatload trays in trucks. The improved impact energy absorptioncapacity of 3D woven composites also makes them suitableas crash members in cars, buses and trucks. Impact perfor-mance is also critical for shipping containers and othercontainer transport applications (e.g. coal trucks, chemicaltransport), where the critical design consideration, otherthan impact, is the ability to carry as heavy a dead load aspossible. The reduction in the weight of a container, ascompared to a standard metal container, will allow heavierpayloads to be carried, thus making the container more costefficient.

A specialised sub-group of 3D woven fabric that iscurrently used commercially in composites is ‘DistanceFabric’. This material consists of two parallel skins of 2Dglass fabric integrally connected by a low density of thethrough-thickness glass yarns. Distance fabric compositesare an alternative to honeycomb or foam material to makesandwich structures because they generally have better(although still low) mechanical properties. These compo-sites are primarily used to manufacture double-walledtanks or the wall lining for chemical storage tanks, carand truck spoilers/fairings, lightweight walls, dome struc-tures and composite tooling.

3. 3D braided composites

Braiding was the first textile process used to manufacturea 3D fibre preform for a composite. This process was devel-oped in the late 1960s to produce 3D carbon–carboncomposites to replace high temperature metal alloys inrocket motor components in order to achieve weight savingsof 30–50% [35]. While only a few of these rocket motorcomponents were made, it did demonstrate the ability ofbraiding to produce a light-weight composite componentwith an intricate shape.The process for manufacturing the motor components is

now known as four-step (or row-and-column) braiding, aterm given because of the four distinct operations in thebraid cycle. A complete description of this braiding processis given by Ko [36] and Brown and Crow [37]. The fibrearchitecture produced by the four-step braiding is illustratedin Fig. 4, and it is characterised by almost all the braideryarns being offset at different angles between the in-planeand through-thickness directions. Since the mid-1980s,variations to the original four-step process have beenproposed, such as circular versions of four-step braiding[38], six-step braiding [39] and multi-step braiding [40].In addition, two other distinctive styles of 3D braidingthat have also gained popularity are two-step braiding andmulti-layer interlock braiding. Two-step braiding, whichwas first described by Popper and McConnell [41] in1987, produces braids with the axial yarns interconnectedby a small amount of through-thickness reinforcing yarns(Fig. 4(b)). Utilising the conventional maypole approach of2D braiding, Brookstein [42,43] developed a braid inter-locking process for making preforms consisting of multipletriaxial braided layers that are interlaced by a parallel seriesof through-thickness braider yarns (Fig. 4(c)). A similartechnique was recently developed by Murata MachineryLtd. in Japan [44]. These different processes offer the abilityto manufacture a range of braided architectures which canbe tailored for specific structural applications by providingthe optimum amounts of axial and through-thickness yarns.The major developments in 3D braiding over recent years

have been driven by the superior manufacturing and mechan-ical properties of braided composites over traditional 2D

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Fig. 4. Comparison of fibre architectures of three distinctive 3D braids: (a)4-step braid (b) 2-step braid; and (c) multi-layer interlocked braid.

laminates, and these properties are listed in Table 4. Braidedpreforms have higher levels of conformability, drapability,torsional stability and structural integrity, which makes itpossible to produce composite structures with intricategeometries to the near-net-shape. This can lower the manu-facturing cost considerably because the amount of fabrichandling and material scrap is reduced, as is the need forextensive machining and joining. 3D braided compositesalso have higher delamination resistance, better impactdamage tolerance and lower notch sensitivity than 2D lami-nates because of the through-thickness reinforcement[11,44–49].Despite these advantages, the applications for 3D braided

composites have been limited for reasons that aresummarised in Table 5. One major limitation is that themaximum preform size is determined by the braidingmachine size, and most industrial machines are only ableto braid preforms with a small cross-section (under 100 mmin width). Extremely big and expensive machines areneeded to produce preforms large enough for typical aircraftstructures. Another problem is that many 3D braidingmachines are still in a research and development stage,and only a few machines are presently able to commerciallymanufacture preforms. 3D braiding machines are also slow,and as a result 3D braids cannot presently compete with 2Dbraids and laminates on a cost-saving basis [50].Compounding the difficulties in using 3D braided compo-

sites in structural applications is that their mechanical prop-erties are generally lower than for 2D laminates with anequivalent weight fraction of in-plane fibres. Crane andCamponeschi [46] and Macander et al. [51] found that thetensile and compressive properties of 3D braided compo-sites are low because most braided axial tows are off-axisfrom the loading direction and are heavily crimped. Theyhave also shown that the Young’s modulus and strength ofthe 3D braided composites are sensitive to factors such asbraid angle, braid pattern and tow size [46,51]. This sensi-tivity increases the amount of mechanical testing needed todetermine the optimum braid fibre architecture for a compo-site structure, which is a further impediment to their use.While the tensile and compressive properties of 3D braidedcomposites have been studied in some detail for compositesreinforced by fabrics with low axial braid angles (12–20"),the properties of 3D composites with fibre orientations that

are commonly required for aircraft structures, such as 0/ ^45" or quasi-isotropic configurations, have not been studied.Properties of 3D braided composites such as flexural

strength [46,51], fracture toughness [52] and fatigue perfor-mance [45] have only received scant attention, while manyother properties, including translaminar and interlaminarshear strengths, interlaminar fracture toughness and creepresistance, have not been investigated at all. The perfor-mance and durability of these composites after environmen-tal ageing have also not been examined. It is known that thefailure mechanisms of 3D braided composites are morecomplex than for 2D laminates because they are dependenton the braid pattern, braid angle, tow material, tow size andedge effects, however a systematic study of these para-meters on the failure mechanisms has not been performed.Until the mechanical properties and failure mechanismshave been thoroughly investigated, the braided compositescannot be used with sufficient confidence, particularly inaircraft components, where a large amount of structuraland environmental testing is required for new materials tobe certified for use. Added to the problem that there is alimited amount of mechanical property data is the problemthat models for predicting the mechanical performance of3D braided composites are at present only able to accuratelydetermine the Young’s modulus in tension or compression.Models for calculating the strength and fatigue life have notbeen developed, but these are necessary for optimising thedesign of braided composite structures.Arguably, the most important finding from studies on

small specimens is that the mechanical properties arestrongly influenced by the edge condition of the braid.Cutting the edges from braided composites adversely affects

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Table 4Advantages of 3D braided composites over 2D laminates

3D braiding has the ability to produce complex near-net-shape preforms3D braiding processes can be automatically controlled, which increasesproduction and preform quality3D braided composites with a complex shape can be inexpensive andsimple to manufacture3D braided composites have higher delamination resistance and impactdamage tolerance3D braided composites have greatly superior crashworthiness properties3D braided composites are less sensitive to notches

Table 5Issues impeding the use of 3D braided composites

Almost all 3D braiding machines are still under developmentMost 3D braiding machines are only capable of producing narrowpreforms3D braiding machines have long set-up times and are slow and expensiveThe spools in 3D braiding machines are small because they arecontinuously moving in the production of the preform, consequentlyproduction runs between machine set-ups are briefMany of the mechanical properties of 3D braided composites have notbeen investigatedStiffness and strength of 3D braided composites are generally lower than2D laminatesScaling the results from mechanical tests performed on small 3D braidedcomposite specimens to large braided structures is difficult because ofedge effectsThe mechanical performance of large 3D braided composite structureshave not been extensively studiedLarge amount of scatter in the mechanical properties of braided compositestructuresPredictive models for determining strength and fatigue performance havenot been developedDurability and long-term environmental ageing tests on 3D braidedcomposites have not been performed

their properties because the yarns are no longer continuousaround the edges of the specimen [46,51]. This is signifi-cant, because it reveals that small coupon tests cannotprovide a reliable measure of the properties of large compo-site structures, and as a consequence there is a requirementto undertake a comprehensive full-scale structural testprogram before a 3D braided composite can be used. Thistype of testing is usually slow and expensive, and willimpede the application of 3D braided composites in manyprimary aircraft structures.The performance of some 3D braided structures have

been determined by large-scale mechanical testing[55,56,59]. Gause and Alper [59] performed compressiontests on channel sections, hat-sections and cruciformsections, to investigate their buckling, post-buckling and

crippling behaviour. They also conducted pull-off tests onT-sections to measure the tensile failure strength of stif-fener-to-flange joints. It was found that the 3D braidedstructures have on average only two-thirds the bucklingand crippling performance of the same structures madewith 2D laminate. This poor performance was attributedto the unbalanced stiffness of the composite through thebraided sections. There was a large amount of scatter inthe test results that Gause and Alper believe may be a char-acteristic of these composites. Any significant scatter inproperties will almost certainly contribute to the difficultyin using these materials in load-supporting structures.The applications for 3D braided composites will remain

limited until the issues outlined in Table 5 are largelyresolved. Despite these problems, a variety of demonstratorcomponents have been made that clearly illustrate the versa-tility of this textile process. 3D braiding has been used tomanufacture biomedical devices [53], C-, J- and T-sectionpanels [41–53], I-beams [38,54–58], bifurcated beams [41],connecting rods [54], rib stiffened panels, airframe spars[38], F-section fuselage frames [38], fuselage barrels [50]and rocket engine nozzles. Fig. 5 shows two examples of thedegree of complexity that can be achieved in the 3D braid-ing of aircraft structures and components. However, any useof these materials is likely to be confined to long slenderstructural elements where weight saving is critical. NASA iscurrently leading a detailed study into applications foradvanced composites, and the potential use of 3D braidedcomposites in stiffeners and stringers for aircraft wings aswell as circumferential frames for aircraft fuselages arebeing assessed [50]. Other possible applications includestructural booms, tail shafts on aircraft, and propeller shaftson marine craft. 3D braided composites are also strongcandidates for use in the beams and shells for automobilebodies and chassis as well as in drive shafts. These compo-sites offer weight savings of up to 50% compared to steel[60] with similar damage tolerance and crashworthiness.However, it is unlikely that braided composites will beused in automobiles, until the braiding and resin infusiontechnologies are developed to a stage where the cycle timefor component production is less than two or three minutes,so that production rates of 10,000 to 200,000 componentsper year can be achieved [4].

4. 3D stitched composites

The stitching of composites has been reviewed in somedetail by Morales [61] and Dransfield et al. [62], and will beonly briefly described here. Fig. 6 illustrates the stitchingprocess, which basically involves sewing high tensilestrength yarn (e.g. glass, carbon or Kevlar#), through anuncured prepreg laminate or dry fabric plies using an indus-trial sewing machine. Stitching has also been performedusing polyester thread, although Kevlar# is the most popu-lar yarn material because of its high strength and flexibility.

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Fig. 5. (a) A rib-stiffened panel and (b) rocket nozzle fabricated by ARCusing 3D braiding. (Courtesy of the Atlantic Research Corporation).

Through-thickness yarns have been stitched into compositesto densities ranging from 0.4 to 25 stitches/cm2, howevermost stitching is performed between 3 and 10 stitches/cm2.A variety of sewing machines can be used to stitch compo-sites, although they can usually be classified as single-needle or multi-needle machines.Stitching is occasionally used to reinforce prepreg lami-

nates, however the tackiness of the uncured resin makessewing difficult with some of the in-plane fibres beingbroken and distorted. This damage can adversely affectthe mechanical properties, with reductions in Young’smodulus, strength and fatigue resistance of 10–20% beingcommon, although much larger reductions have beenreported as pointed out by Mouritz et al. [63] and Mouritzand Cox [64]. Because of this problem, stitching is usedmostly to sew dry fabric preforms before they are consoli-dated with a resin into a composite.The stitching of composites was first assessed in the early

1980s by Holt [65] and Cacho-Negrete [66] as a method forjoining uncured carbon fibre–epoxy prepreg laminates toobtain high lap joint strengths. These early studies wereaimed at determining whether stitching would be a suitablereplacement to adhesive bonding and riveting for joiningcomposite structures in advanced fighter aircraft. Holt [65]showed that stitching has considerable promise as a joiningmethod, with the tensile strength of stitched panel-to-stif-fener joints being up to 72% higher than joints withoutstitching. In some cases, the strength of stitched joints was

even higher than joints reinforced with metal rivets. Soonafter these first studies were reported, stitching was used toreinforce flat laminate panels in the through-thickness direc-tion to improve properties such as impact damage toleranceand, to a lesser extent, through-thickness strength.The advantages of stitched composites over 2D laminates

are summarised in Table 6. Improvements in impact damageresistance and post-impact mechanical properties have beenmajor reasons for the burgeoning amount of research intostitched composites, since the mid 1980s. Most researchwork has been aimed at determining the effect of stitchingon the in-plane and interlaminar properties of flat compositecoupons and small panels. A large amount of mechanicalproperty data now exists on the effect of stitching on thetensile, compressive, flexure, fatigue, interlaminar shear andinterlaminar fracture properties, and some data is also avail-able on creep and translaminar fracture toughness.Currently, the amount of mechanical property data forstitched composites is more than for 3D woven compositesand substantially more than for 3D braided and 3D knittedcomposites. The interlaminar fracture toughness propertiesof stitched composites have been reviewed by Dransfield etal. [62] and Mouritz and Jain [67], while the in-planemechanical properties have been extensively reviewed byMouritz et al. [63] and Mouritz and Cox [64].In contrast to the large amount of work performed on

stitched coupons and small flat panels, comparatively littleresearch has been reported on stitched composite structures.Most of the published research on stitched structures hasbeen performed on aircraft joints, because of the desire ofaircraft manufacturers to reduce the number of mechanicalfasteners used on primary structures, particularly on thewings and fuselage of supersonic fighter aircraft whereenvironmentally-assisted fretting fatigue around rivets canbe a problem. Stitches are also lighter than mechanicalfasteners and provide a more uniform stress state over thejoint area compared to fasteners where the stress is concen-trated around the rivets [68]. The composite structures thathave been stitched are lap joints [69–74], angle joints [70],wing-to-spar joints [67,75], T- and J-section stiffeners[67,69,76,77]. These structures have been made to demon-strate to the aircraft industry that stitching is developing intoa viable alternative to the traditional joining methods of co-curing, adhesive bonding or riveting.While stitching has the potential to be used for joining

laminates, a number of scientific and technical issues stillneed to be resolved and these are summarised in Table 7.Mechanical tests on stitched lap joints show that the failurestrength is determined by a number of factors such as lami-nate thickness, joint overlap length, stitch yarn material,density of stitching, and the distance of stitching from theoverlap edge [69–73]. The effect these parameters have onthe joint properties has not been fully characterised, andwhile experimental and computational studies are inprogress, much more research is needed. The lack of alarge database on the performance of stitched joints

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Fig. 6. An illustration of stitching composite material (from Dransfield etal. [62]).

Table 6Advantages of stitched composites over 2D laminates

Can be inexpensive and simple to manufactureImproved handling of preforms (plies prevented from moving)Improved impact damage tolerance, particularly to barely visible impactdamageImproved delamination resistance to ballistic impact and blast loadingImproved modes I and II interlaminar fracture toughnessImproved interlaminar fatigue resistanceImproved joint strength under monotonic and cyclic loadingSlight improvement in through-thickness tensile modulus and strength

combined with the inability of computational methods asyet to accurately predict the strength and fatigue properties,has made it difficult to certify stitched joints for use inaircraft. Until these problems are overcome, it can beexpected that the use of stitching in joining aircraft struc-tures will remain limited.Another factor which has impeded the use of stitching on

aircraft is that some structures such as wing-to-spar jointsthat have been stitched are prone to accelerated environ-mental degradation in hot-moist conditions [66,75]. White-side et al. [75] measured the moisture content in stitchedwing-to-spar joints to be several percentage points higheraround the stitches than in the bulk composite, because thestitching yarns provided a pathway for the rapid ingress ofwater. Cacho-Negrete [66] reports that this moisture causedthe stitched joints to fail prematurely under shear loading.This type of environmental degradation will not onlyimpede the use of stitching in aircraft structures, but willprobably be an impediment to its use in high performancemarine craft, such as racing yachts.While environmental degradation appears to be a

problem for stitched composites, comparatively littleresearch has been performed to understand and control theproblem. Furrow et al. [78] examined the effect of tempera-ture and humidity cycling on the compression strength andfatigue life of a stitched carbon fibre–epoxy laminate panel,but found that stitching had only a small affect. Qi et al. [79]studied the effect of hydrothermal cycling on the impactdamage tolerance of stitched and unstitched carbon fibre–epoxy laminate panels. They showed that environmentalcycling caused cracking around the stitches as well ascaused the stitches to debond from the surrounding compo-site material. Despite this damage, the impact damage toler-ance of the stitched composite remained the same as theunstitched laminate. Clearly, much more research into thedurability of stitched composites in harsh environments isneeded, before they can be certified for use as aerospace andmarine structures.On another topic, most current stitching machines are

limited to a vertical stitching plane in which the stitch isinserted normal to the laminate surface. These machineshave difficulty sewing curved shapes (such as T-joints),

because of the limited access for the needle head. Dexter[80] believes that complex composite structures require off-axis stitching using robotically controlled multi-needlemachines. The problem with these machines is that theyare still largely in the development stage and are expectedto be expensive when they become commercially available.Another problem with current sewing machines is their diffi-culty in stitching large and thick structures. The size of thesewn composite is limited by the width of the machine whenperforming multi-needle stitching or by the reach of theneedle head on single-needle machines. Most industrial-grade sewing machines that have been adapted to stitchcomposites can only handle preforms less than !1 m wideand!5 mm thick. Preforms of this size are usually too smallfor many aircraft structures. However, Brown [81] reportsthat NASA has a 28 m long sewing machine (shown in Fig.7) capable of stitching fabric over 15 m long, nearly 3 mwide and about 40 mm thick. The capital cost of largepurpose-built sewing machines are extremely high andbeyond the budget of most composite fabricators, which isanother major factor impeding the wider use of stitching inlarge composite structures.Until the problems outlined in Table 7 are resolved, it is

likely that most applications for stitched composites will berestricted to relatively simple structures that are not used inharsh environments. However, a considerable amount ofwork has been undertaken by NASA in association withsome large aircraft manufacturers such as Northrop Grum-man and the Douglas Aircraft Co. to produce a stitched wing[82–89]. The project aims to manufacture impact-tolerantcomposite aircraft wing cover panels that are 25% lighterand 20% cheaper than conventional aluminium wings [81].Fig. 8 shows a stitched wing cover panel manufactured bythe Douglas Aircraft Co. that is 2.4 × 3.0 m2. As well as thiswork, stitching is also being assessed for fabricating struc-tures in aircraft fuselages [83,84,86–88,90].

5. 3D knitted composites

3D knitted composites are arguably the least understoodof the four classes of 3D textile composites reviewed in thispaper. While a considerable amount of research has beenperformed on composites reinforced with 2D knitted fabrics[91,92], by comparison little is known about the mechanicalproperties and applications of 3D knitted composites. Thissection provides a brief description of the three types of 3Dknitted composite currently available, which are broadlycategorised as sandwich, non-crimp, and near-net-shapecomposites. The production, properties and applications ofthese different composites are described separately,although they have many advantages and disadvantages incommon that are listed in Tables 8 and 9, respectively.

5.1. 3D knitted sandwich composites

3D knitted sandwich fabrics were developed as

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Table 7Issues impeding the use of stitched composites

Most sewing machines cannot stitch large and thick composite structuresSewing machines require access to both sides of the preformMost sewing machines cannot stitch curved composite structures with acomplex shapeThe effects of stitching parameters (eg. stitch density, yarn materials, yarndenier) on joint strengths is not fully understoodStitching usually degrades the in-plane mechanical propertiesThe environmental ageing and durability of stitched composites is notfully understoodPredictive models for determining strength and fatigue performance havenot been satisfactorily developed

reinforcement for polymer composites by Verpoest andcolleagues in the early 1990s. Sandwich preforms areproduced on double-bed Raschel machines by knitting thetop and bottom skins simultaneously on each needle bed.During the knitting process, yarns are intermittentlyswapped between the two sets of needles to create a coreof through-thickness yarns, called pile, which are intercon-nected to the skins. The density and relative orientation ofthe pile yarns are easily manipulated by controlling the levelof yarn crossover between the two skins, and preforms havebeen made with piles aligned in the vertical direction orinclined at 45". The two needle beds can be independentlyprogrammed to produce skins with different structures and,hence, different mechanical properties [93–98]. So far, 3Dknitted sandwich preforms have only been produced withpolyester and glass yarns [94,95].Since 3D knitted sandwich composites are a recent devel-

opment, there is only a small amount of published informa-tion on their mechanical properties and potentialapplications. Verpoest et al. [94] and Philips et al. [95]report that these composites have a higher energy absorptioncapacity, but lower flexural stiffness and specific

compressive strength compared with several more conven-tional sandwich polymer composites containing polymer(PMI) foam or Nomex# cores. The 3D composites areexpected to be cheaper to manufacture and have betterskin-to-core peel strengths compared with, say, honeycombsandwich composites, although this has not yet been demon-strated.In view of the limited understanding of the mechanical

properties and long-term durability of 3D knitted sandwichcomposites, they have not yet been made into structuralcomponents. However, they show considerable promisefor use in bicycle helmets (see Fig. 9), because they arelight-weight and the preform drapes more easily over thehelmet mould than prepreg tape or 2D woven fabric.Furthermore, the knit architecture of the skin can becontrolled to provide optimum air-flow for heat dissipationfrom the wearer’s head, which is a major benefit duringendurance cycling.

5.2. 3D warp knitted non-crimp composites

3D warp knitted non-crimp preforms are produced using

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Fig. 7. A stitching machine used to stitch composite materials to make wing panels (from Brown [81]).

Fig. 8. A stitched composite wing panel (from Smith et al. [89]).

a combination of fibre tow placement and warp knitting.Preforms are made from layers of non-crimp fabric (madewith unidirectional tows) stacked in the required orienta-tions and then bound together with binder yarns insertedin the through-thickness direction by warp knitting needles.Bi-, tri- and quad-axial fabrics of glass, carbon and Kevlar#have been produced using polyester and Kevlar# binders[99–102]. Non-crimp fabrics with interleaved layers ofrandom fibre mat have also been warp knitted [103]. Theamount of binder used is normally below 5% of the totalfibre fraction to minimise the amount of damage to the non-crimp fabric. Dexter and Hasko [100] report that where theknitting needles pierce the non-crimp fabric, they causedistortions and fractures in the in-plane fibres.Three main advantages provide the impetus for the devel-

opment of 3D knitted non-crimp composites. Firstly, unlike3D weaving, the preforms can be made cost-effectively withoff-axis reinforcement. Secondly, like 3D weaving, the knit-ting process has the potential to greatly lower productioncosts by reducing the time needed to produce the preform[102]. Finally, this material has superior impact damageresistance and marginally better damage tolerancecompared with traditional 2D prepreg tape laminates[102,104]. These factors have been instrumental in havingthe composite demonstrated for some aircraft structures,such as wing stringers [105] and wing panels [80], whereseveral warp-knitted non-crimp fabrics are bound togetherby through-the-thickness stitching. Apart from these aircraft

structures, the only other applications for warp knittedcomposites are being assessed is in automobiles. Hamiltonand Schinske [106] briefly reported that composites rein-forced with stitched multi-axial glass fabrics were beingconsidered for use in car bumper bars, floor panels anddoor members, however further details have not beenpublished.The tensile, compression, flexural, interlaminar shear,

shear, impact and post-impact compression properties of3D knitted non-crimp composites have been studied insome detail [100,102,103,105,107]. In most cases the prop-erties have not been compared directly against traditionaltape laminates made with an equivalent amount of in-planefibres, and for this reason it is difficult to assess the relativeperformance of the 3D knitted composites. It does appear,however, that these composites have inferior or at best simi-lar tensile properties compared with prepreg tape laminatesof similar lay-up [101]. It is believed the tensile propertiesare degraded by crimp and fracture to the in-plane fibresduring knitting, although the adverse effects of this damageon the properties has not been fully investigated. The crimpdamage also appears responsible for 3D knitted compositeshaving lower compressive strengths than prepreg tape lami-nates with an equivalent amount of in-plane fibres [101].

5.3. 3D near-net-shape knitted composites

Near-net-shape or fully-fashion 3D knitted compositeswere first made during the early 1990s, but since then littleinformation has been published about the knitting techniqueor their mechanical properties. Near-net-shape knitting canbe performed by a two-bed weft knitting machine, howeveradditional needle beds are required for producing 3D (multi-layer) fully-fashion fabrics [108,109]. The additionalneedles and yarn guides are needed both to create the differ-ent layers of knits and to facilitate the transfer of yarnsbetween the layers. The final near-net-shape fabric is predo-minantly a result of careful stitch control during the knittingprocess. Several demonstration components have beenreported in literature including jet engine vanes [108,109],T-shape connectors [110], I-beams [109], a rudder tip fair-ing for a mid-size jet engine aircraft, and even medicalprosthesis [91]. Despite these successful trials, the develop-ment of 3D knitted near-net-shape composites is still in anearly state, and the high cost of machine and software devel-opment stands in the way of a more rapid progress.

6. Conclusions

The development of advanced 3D textile composites forspecialised aircraft components began in the late 1960s, andsince then these materials have attracted increasing attentionbecause of their potential uses in aircraft, marine vessels,civil infrastructure and medical prosthesis. This review hasshown that the potential uses of composites made with3D woven or 3D braided fabrics range from I-beams and

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Table 8Advantages of 3D knitted composites over 2D laminates

3D knitted preforms have better formability because they are moredrapable3D knitting can produce more complex near-net-shape preformsSome types of 3D knitting can be done on existing automatic machineswith little modification3D knitted sandwich composites have a lower specific densitySome types of 3D knitted composites have higher impact damagetolerance and energy absorption (crash) properties

Table 9Issues impeding the use of 3D knitted composites

Many 3D knitting machines are still under development, and cannotproduce commercial quantities of fabricMost conventional knitting machines cannot make thick preformsWeft knitting of non-crimp fabrics causes breakages and distortions to thein-plane fibres3D knitted composites generally have lower stiffness and strengthpropertiesIn-plane properties and failure mechanisms of 3D knitted composites arenot well characterisedValidated methods are not available for predicting many of the propertiesand long-term durability of 3D knitted compositesPoor understanding of the influence of the knitting process parameters onthe composite propertiesKnitted composite components usually contain ‘soft spots’ and ‘hardspots’ caused by a change in the knit structure due to stretching of thefabric during prefoming

T-joints through to rocket motor nozzles and rib-stiffenedpanels. Stitching has considerable potential for joiningcomposites and for improving the damage tolerance ofstructures such as aircraft wing panels. Currently, the leastdeveloped composites are made from 3D knitted fabrics,although they have potential applications in niche areaswhere the components are a complex shape, such as bicyclehelmets and engine vanes.Despite the wide variety of demonstration components

made from 3D textile composites, these materials currentlyhave few commercial applications. 3D woven composites,for example, have only been used in a few niche applica-tions such as manhole covers and highly specialised jointson advanced aircraft. The reason for the low usage of 3Dwoven, braided, stitched and knitted composites is compli-cated, and is due to a combination of economic, manufac-turing, mechanical property and durability issues. Many ofthese issues have been identified in this paper. In addition tothese impediments, another hindrance to the use of 3Dcomposites is not strictly a technical issue, but more oneof perception. Even though 2D textile fabrics are currentlyused within the composites industry, the advanced textileproduction techniques described in this paper are new tomost designers and manufacturers of composite structures,and are not seriously treated as alternative productionmethods due to their origins within the traditional textileindustry. A change in attitude will be needed beforeadvanced 3D textile composites can begin to makegains within the composite industry. As the impedi-ments outlined in this paper are overcome and thecost of 3D composites falls, it is expected that theirapplications will increase gradually. In the short-term,however, it is expected in most cases the compositeswill continue to be used in niche applications, despitethe construction of stitched composite wing panels.

Acknowledgements

The authors thank Techiweave Inc., Shikishima CanvasCo. Ltd., Atlantic Research Corporation and the editor ofAerospace America for permission to use photographspresented in the paper.

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