JCOMA 623
`
`Composites: Part A 31 (2000) 197–220
`
`www.elsevier.com/locate/compositesa
`
`The potential of knitting for engineering composites—a review
`K.H. Leonga,*, S. Ramakrishnab, Z.M. Huangb, G.A. Biboa
`aCooperative Research Centre for Advanced Composite Structures Ltd (CRC-ACS), 506 Lorimer Street, Fishermens Bend, VIC 3207, Australia
`bDepartment of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
`
`Received 24 November 1998; received in revised form 20 July 1999; accepted 5 August 1999
`
`Abstract
`
`Current literature on knitted composites tends to address the aspects of manufacture and characterisation separately. This paper aims to
`bring together these two sets of literature to provide the reader with a comprehensive understanding of the subject of knitted composites.
`Consequently, this paper contains a detailed outline of the current state of knitting technology for manufacturing advanced composite
`reinforcements. Selected mechanical properties of knitted composites, and some of the predictive models available for determining them are
`also reviewed. To conclude, a number of current and potential applications of knitting for engineering composites are highlighted. With a
`comprehensive review of the subject, it is believed that textile engineers would be able to better understand the requirements of advanced
`composites for knitting, and, by the same token, composites engineers can have a better appreciation of the capability and limitations of
`knitting for composite reinforcement. This should lead to more efficient usage and expanded application of knitted composites. q 2000
`Elsevier Science Ltd. All rights reserved.
`
`Keywords: Knitted fabrics; B. Mechanical properties
`
`1. Introduction
`
`The textile industry has developed the ability to produce
`net-shape/near-net-shape fabrics using highly automated
`techniques such as stitching, weaving, braiding and knitting.
`In view of the potential for cost savings and enhanced
`mechanical performance, some of these traditional textile
`technologies have been adopted for manufacturing fabric
`reinforcement for advanced polymer composites. Knitting
`is particularly well suited to the rapid manufacture of
`components with complex shapes due to the low resistance
`to deformation of knitted fabrics [1]. Furthermore, existing
`knitting machines have been successfully adapted to use
`various types of high-performance fibres, including glass,
`carbon, aramid and even ceramics, to produce both flat and
`net-shape/near-net-shape fabrics. The fabric preform is then
`shaped, as required, and consolidated into composite
`components using an appropriate liquid moulding tech-
`nique, e.g. resin transfer moulding (RTM) or resin film
`infusion (RFI).
`is
`The use of net-shape/near-net-shape preforms
`obviously advantageous for minimum material wastage
`and reduced production time (see, for example, Nurmi and
`
`* Corresponding author. Tel.: 161-3-9646-6544; fax: 161-3-9646-8352.
`E-mail address: khlcrcas@ozemail.com.au (K.H. Leong).
`
`Epstein [2]). However, the development of a fully fashioned
`knitted preform can prove time consuming and expensive so
`that this option could still be economically inefficient over-
`all. In such instances, flat knitted fabrics with a high amount
`of formability/drapability should be used to form over a
`shaped tool for subsequent consolidation to produce the
`required composite component (see, for example, Ho¨hfeld
`et al. [3]).
`Notwithstanding the exceptional formability, there are
`serious concerns over the generally poorer in-plane mechan-
`ical performance of knitted composites compared with more
`conventional composites and materials [4–8]. This relative
`inferiority in properties of knitted composites results predo-
`minantly from the limited utilisation of fibre stiffness and
`strength of the severely bent fibres in the knit structure that
`afford the fabric to be highly deformable. In addition,
`damage inflicted on the fibres during the knitting process
`could also degrade mechanical properties [6].
`This paper aims to provide to the reader a general appre-
`ciation of the knitting process and the many opportunities it
`provides for producing efficient fibre reinforcement for
`advanced composites. Within this objective, the paper first
`outlines some of the more common types of knitting tech-
`niques and machines, and discusses some of the recent inno-
`vations to facilitate the manufacture of knitted composites
`with improved mechanical performance. In this context, the
`
`1359-835X/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
`PII: S1359-835X(99)00067-6
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`Fig. 1. Schematic diagrams showing the wale and course components of a
`knitted fabric, and the principles of (a) weft and (b) warp knitting.
`
`Fig. 2. Schematic diagrams showing the (a) tuck and (b) float stitches.
`
`performance of advanced knitted composites with respect to
`mechanical properties such as tension, compression, energy
`absorption, impact and bearing are reviewed. Analytical and
`numerical models currently available for predicting stiffness
`and strength of knitted composites are also presented.
`Finally, some current and potential applications of knitting
`for engineering composites are highlighted.
`
`2. The knitting process
`
`Literature on the basics of knitting is widely available,
`including one by Gohl and Vilensky [9], upon which most of
`this section of the paper is based.
`Knitting refers to a technique for producing textile fabrics
`by intermeshing loops of yarns using knitting needles. A
`continuous series of knitting stitches or intermeshed loops
`
`is formed by the needle catching the yarn and drawing it
`through a previously formed loop to form a new loop. In a
`knit structure, rows, known in the textile industry as
`courses, run across the width of the fabric, and columns,
`known as wales, run along the length of the fabric. The
`loops in the courses and wales are supported by, and inter-
`connected with, each other to form the final fabric (Fig. 1).
`A wale of loops is produced by a single knitting needle
`during consecutive knitting cycles of the machine. The
`number of wales per unit width of fabric is dependent on
`inter alia the size and density of the needles1 used as well as
`the knit structure, yarn size, yarn type, and the applied yarn
`tension. A course of loops, on the other hand, is produced by
`a set of needles during one knitting cycle of the machine.
`The number of courses per unit length of fabric is controlled
`
`1 The density of needles is more commonly represented by the term
`‘gauge’, which is a measure of needles per unit length in inches.
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`199
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`Fig. 3. Illustrations of (a) flat-bed and (b) circular weft knitting machines.
`
`by manipulating the needle (knockover) motion and yarn
`feed. Standardised tests for measuring and quantifying the
`number of wales and courses in a unit length of knitted
`fabric are well documented in the literature [10].
`Depending on the direction in which the loops are
`formed, knitting can be broadly categorised into one of
`two types—weft knitting and warp knitting (Fig. 1). Weft
`knitting is characterised by loops forming through the feed-
`ing of the weft yarn at right angles to the direction in which
`the fabric is produced (Fig. 1(a)). Warp knitting, on the other
`
`hand, is characterised by loops forming through the feeding of
`the warp yarns, usually from warp beams, parallel to the direc-
`tion in which the fabric is produced (Fig. 1(b)). More precisely,
`warp knitting is effected by interlooping each yarn into
`adjacent columns of wales as knitting progresses. Fig. 1
`shows the basic structure of the weft (i.e. plain knit) and
`warp (i.e. single tricot) knitted fabrics. Generally, weft-knit
`structures are less stable and, hence, stretch and distort more
`easily than warp-knit structures so that they are also more
`formable. It is noteworthy that an obvious advantage of
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`Fig. 4. Schematic diagrams of the (a) cylinder, and (b) dial, needles of a circular knitting machine, and (c) the manner in which they interact to effect the
`knitting process.
`
`warp over weft knitting is that the former tends to have a
`significantly higher production rate since many yarns are
`knitted at any one time. The ease with which weft-knitted
`fabrics unravel and the cost associated with warping beams
`are also important considerations in choosing between weft
`and warp knitting. Clearly, weft knitting is preferred for
`developmental work whereas warp knitting would be
`more favourable in large-scale production.
`In knitting, float and tuck stitches/loops (Fig. 2) represent
`the main routes for modifying knit structures to achieve
`specific macroscopic properties in the fabric. In general a
`tuck stitch makes a knitted fabric wider, thicker and slightly
`less extensible. A float stitch, on the other hand, creates the
`opposite effect, as well as increases the proportion of
`
`straight yarns in the structure, which is an important consid-
`eration for many composites applications.
`
`3. Knitting machines
`
`According to Gohl and Vilensky [9], weft knitting
`machines may be broadly classified into two types, namely
`flat-bed and circular, whilst the two most common warp
`knitting machines are the Tricot and the Raschel.
`
`3.1. Weft knitting
`
`3.1.1. Flat-bed machines
`Flat-bed, or flat-bar, machines are characterised by the
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`201
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`yarn feeders to effect knitting. As with the flat-bed
`machines, the motion of the needles are controlled by cams.
`Since with a circular machine the yarn is knitted in a
`continuous fashion, significantly higher production rates
`are achieved compared with flat-bed machines. This contin-
`uous knitting also means that fabrics produced on circular
`machines are tubular and contain no seams. Circular
`machines have gauges ranging from 5 to 40, and therefore
`their fabrics normally consist of small loops with relatively
`high-stitch densities.
`
`3.2. Warp knitting
`
`3.2.1. Tricot machines
`Tricot machines have only a single needle bar and up to
`four yarn guide bars to a needle (Fig. 5). The needle bed is
`straight and occupies the width of the machine. The guide
`bars essentially move relative to the needles to facilitate
`interlooping of yarns with adjacent loops as the fabric is
`knitted. Being typically fine gauge machines, the tolerance
`between the needles and yarn guides is very fine and there-
`fore Tricot machines are commonly used with multifilament
`yarns. With the smoothness and regularity in fibre diameter,
`speedier and relatively problem-free knitting is achieved
`with these machines. It is noteworthy that the non-stretch
`characteristics of Tricot knits and thus their relative stability
`of structure often render them substitutes for woven fabrics.
`
`3.2.2. Raschel machines
`Raschel knitting machines may have one or two straight
`needle beds that occupy the width of the machine. Depend-
`ing on the knit structure more than 20 guide bars can be
`used, although the usual number is between four and 10.
`Due to the greater number of guide bars that a Raschel
`machine can accept, it is possible to knit an immense variety
`of structures on these machines. Nevertheless, the basic
`stitch formation of Raschel knits is the same as for Tricot
`knits.
`Since Raschel machines usually have more guides fitted
`to them than Tricot machines,
`they are coarser gauge
`machines too. The coarser tolerance between the needle
`and yarn guides means that spun yarns can be knitted. It is
`noteworthy that Raschel has become the generic name for
`describing fabrics knitted on a warp knitting machine with
`two needle bars. Further, Raschel fabrics generally tend to
`be characterised by their open mesh, net or lace-like struc-
`ture, that are usually knitted from spun, rather than multi-
`filament, yarns.
`The myriad of knit architectures that are possible with
`either weft or warp knitting are highlighted by Ramakrishna
`[11].
`
`4. Fibre damage during knitting
`
`During knitting, fibres are required to bend over sharp
`radii and manoeuvre sharp corners in order to form the
`
`Fig. 5. Schematic diagram showing the relative positions of the guide bars
`to the knitting needle in warp knitting machines.
`
`arrangement of their needles on a horizontal or flat needle
`bed (i.e. linear needle arrangement) (Fig. 3(a)). Most flat-
`bed machines have two needle beds which are located oppo-
`site to each other. The motion of the needles during knitting
`is controlled by cams in the yarn carrier which act upon the
`butt of the needles as they travel back and forth along the
`needle bed. This action causes each of the needles to rise
`and fall in turn to facilitate loop formation of the yarn along
`the length of the needle bed. It is from this action that the
`term ‘weft knitting’ is derived. It is noteworthy that flat-bed
`knitting machines have low production rates since the yarn
`is knitted back and forth across the needle bed. This results
`in slight time delays with each direction change that would
`become significant over an extended period. Flat-bed
`machines have gauges ranging from 3 to 15 and therefore
`their fabrics are normally of large loops with low stitch
`densities.
`
`3.1.2. Circular machines
`Circular weft knitting machines may be single- or double-
`bed and their needles, as the name suggests, are arranged in
`a circular needle bed (i.e. circular needle arrangement) (Fig.
`3(b)).
`Single-bed machines have their needles arranged verti-
`cally along the perimeter of the circular knitting bed. This
`set of needles are called cylinder needles (Fig. 4(a)).
`Double-bed machines have an additional set of needles,
`called dial needles, mounted horizontally along the circum-
`ference of a dial which in turn sits above and perpendicular
`to the cylinder needle bed (Fig. 4(b)). The relative positions
`of the dial needles are so that they are sandwiched between a
`pair of cylinder needles, and vice versa. In both types of
`machines, the needles are normally rotated past stationary
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`Fig. 6. Typical stress–strain curves of rib-knit composites under (a) tension and (b) compression, loadings [23].
`
`knitted loops of the structure. However, most load-bearing
`fibres suitable for engineering composites exhibit high
`elastic stiffness and high dimensional stability [2,12], thus
`making it difficult to fulfil this requirement without causing
`significant damage to the fibres. In fact, Lau and Dias [13]
`found that the loop strength of glass yarns increases almost
`exponentially with knitting needle diameter. As a result, this
`limits the choice of structures to relative simple ones and
`modifications to conventional machines are sometimes also
`necessary. Concessions such as using ceramic guides with
`an extension force spring [6] and employing more flexible
`fibres have proved successful in alleviating the problem of
`fibre damage, although the latter option tends to compro-
`mise the final properties of the composite [14,15]. With
`advanced fibres, may they be glass [5], carbon [16], or
`aramid [17], more flexible fibres normally means using
`spun yarns consisting short fibres (50–100 mm) that are
`twisted together. In this way, some of the superior properties
`of the advanced fibres are retained whilst improving on
`knittability. Incidentally, spun yarns have also been shown
`to be advantageous
`for
`improved wetting properties
`compared with monofilament yarns [14,15].
`Lau and Dias [13] pointed out that when yarns come into
`contact with knitting elements of the machine, due to fric-
`tion, the tension in the yarn, T, would build up according to
`Euler’s capstan equation:
`(cid:133)1(cid:134)
`T (cid:136) Ti emq
`where Ti is the yarn input tension, m the mean coefficient of
`friction between the yarn and the knitting elements, and q
`the sum of the angles between the yarn, needles and other
`knitting elements in contact with the yarn. Whilst, on the
`one hand, the superior tensile properties of advanced fibres
`suggest good knittability,
`their generally low-rupture
`strains, on the other, tend to mean that quite large tension
`build-up in the yarn, which would otherwise be relieved by
`
`more stretchable yarns, such as wool, are also created. As a
`result, fibre damage due to premature tension failure is also
`a significant impediment to the knittability of advanced
`fibres.
`Damage to the fibres also arises from abrasion with knit-
`ting elements of the machine. Usually only surface filaments
`in a fibre yarn are fractured in the process which makes the
`fibres appear hairy or frayed [13]. Through dust emission
`measurements, Andersson et al. [18,19] showed that the
`knittability of a fibre or yarn is related to its toughness
`and surface characteristics, the latter of which can be modi-
`fied through sizing or lubrication [13]. It should be noted
`that for advanced composites, the compatibility between the
`size and the matrix resin warrant serious attention. Chou and
`Wu [20] showed that fraying increases with the amount of
`tension exerted on the fibres during knitting and claimed
`that some degree of fraying, which promotes fibre bridging,
`could actually enhance composite properties such as tensile
`strength and impact resistance, albeit only marginally.
`
`5. Mechanical properties
`
`The in-plane mechanical properties of knitted composites
`are usually anisotropic [6,7,16,21–29] (Fig. 6(a)). This is
`due to a difference in the relative proportion of fibres
`oriented in the knitted fabric [16,24], and is therefore a
`function of the knit structure [6,22,23] as well as knitting
`parameters, such as stitch density [22,23,30]. The knit struc-
`ture is not only controlled by the choice of knit architecture
`but also by the amount and manner to which the fabric is
`deformed, and thereby modifying the relative fibre orienta-
`tion prior to consolidation [21,25–28,31,32] (Fig. 7(a)).
`Similarly, knitted composite properties are also controlled
`by manipulating parameters such as loop lengths or stitch
`density of a particular knit architecture [22,23,33] (Fig.
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`203
`
`Fig. 7. For a particular knit architecture, the in-plane properties of knitted composites are affected by (a) the amount of deformation in the fabric [31], and (b)
`the knit parameters of the fabric [33]. (a) Composites with fabric deformed along the wale axis and tested in the wale and course directions. (b) Three different
`architectures, each knitted to several loop lengths and stitch densities, and tested in the wale and course directions.
`
`7(b)). Leong and colleagues [31], for instance, reported that
`the tensile stiffness and strength of composites reinforced
`with Milano-rib knit are enhanced with deformation in the
`fabric. They [33] also showed that the tensile properties of
`Milano-rib, plain- and rib-knit composites improve and
`degrade with loop length and stitch density, respectively
`(Fig. 7(b)). It is noteworthy that knitted composites are
`nevertheless much more isotropic under compression than
`under tension since their compression properties are domi-
`nated by those of the matrix [7,21,23,31,33] (Fig. 6(b)). It
`will be noted from Fig. 6 that, also due to the dominance of
`the matrix, knitted composites are generally superior in
`compression than in tension.
`Verpoest and colleagues [29] inferred that the in-plane
`strength and stiffness of knitted composites are inferior to
`woven, braided, non-crimp and unidirectional materials
`
`with an equivalent proportion of in-plane fibres due to the
`limited utilisation of fibre stiffness and strength resulting
`from the severely bent fibres in knit structures. Similarly,
`knitted composites are also expected to have in-plane prop-
`erties that are close to those of random fibre mats compo-
`sites. (Later in the paper, some data are provided in Tables 2
`and 3 which illustrate the above statements). Interestingly,
`there is some evidence which suggests that a knitted compo-
`site built up of multiple layers of fabric can exhibit better
`tension [16,27,28] and compression [7] strengths, strain-to-
`failure [7,27,28], fracture toughness [34], and impact pene-
`tration resistance [35], compared to laminates with only a
`single layer of fabric. This has been attributed to increased
`fibre content and/or mechanical interlocking between neigh-
`bouring fabric layers through nesting.
`The complex nature of knit structures is mirrored in the
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`Fig. 9. Example of a failed compression knitted composite sample [31].
`
`macroscopically as parallel rows of matrix cracks running
`along the loading axis [7] (Fig. 9).
`Mechanical properties aside, the curved nature of the
`knitted loops has its advantage. The highly looped fibre
`architecture ensures that knitted fabrics are able to easily
`undergo significant amounts of deformation when subjected
`to an external force. Their formability raises the potential of
`knitted fabric for cost-effective composite fabrication of
`complex and intricate shapes. This advantage extends to
`permit holes in a composite to be formed or knitted in,
`instead of drilled. With continuous fibres diffusing stresses
`away from the hole, the strength in the knitted/formed hole
`region is increased, thus leading to notch strength [17] and
`bearing properties [7,8] that are higher than for composites
`with a drilled hole (see Table 1). Knitted composites have
`been shown to be generally notch-insensitive where notched
`strengths are either higher or similar to their unnotched
`counterparts [17] (see Table 1).
`The three-dimensional (3D) nature of knitted fabrics are
`also effective in promoting fibre bridging to enhance open-
`ing mode fracture toughness where improvements of up to
`10 £ and 5 £ over those of glass prepreg and woven ther-
`moset composites, respectively, have been reported [38,39]
`(Fig. 10). It is noteworthy that the difference in fracture
`toughness between a knitted and a woven carbon/thermo-
`plastic composite appears to be less significant [40]. As
`pointed out earlier, the fracture toughness also improves
`with the number of fabric layers used in the composite
`[34]. These superior Mode I fracture toughness values are
`reflected in the energy absorption capabilities [7,24,41] (as
`exemplified in Table 2) and impact penetration resistance
`[42] of knitted composites. It is noteworthy that impact
`damage appears as a region of dense and complex array of
`cracks on the impacted surface, whilst on the unimpacted
`surface, it is characterised by a myriad of matrix micro-
`cracks that generate radially from a densely damaged zone
`(Fig. 11(a)). Consequently, the damage zone takes on a
`trapezoidal shape (Fig. 11(b)) that is typically observed in
`
`Fig. 8. Representative micrographs showing the fracture modes in knitted
`composites subjected to (a) tensile and (b) compressive loadings [31]. (a)
`Fracture of load bearing fibre tows at yarn cross-over points and legs of
`knitted loops. (b) Euler buckling of a load bearing fibre tow.
`
`failure behaviour of these materials. Under tensile loading,
`failure usually results from fibre fractures at yarn cross-over
`points and/or at the side legs of knitted loops, which, respec-
`tively, correspond to regions of high stress concentration
`and planes with minimum fibre content [7,16,22,26,31]
`(Fig. 8(a)). For a multilayer laminate, ultimate tensile failure
`is usually preceded by multiple cracking of the matrix [7].
`The cracks, which initiate from yarn–matrix debonding [36]
`and so correspond to the rows and columns of knitted loops in
`the fabric, develop progressively with loading until a satura-
`tion density is achieved before final failure occurs [37].
`Under compressive loading, failure is dictated by Euler
`buckling in regions of minimum lateral support which
`mainly occur in the plane of the legs of the knitted
`loops (Fig. 8(b)). The fact that the legs are very often
`curved rather than straight further promotes buckling,
`thereby causing the fibres
`to fracture prematurely
`[7,21,31]. This buckling, which subsequently causes
`debonding of the fibres from the matrix,
`is observed
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`205
`
`Table 1
`Typical notched [17] and bearing [8] properties of knitted and woven composites (in the wale/warp direction)
`Property (cid:133)W =D (cid:136) 3(cid:134)
`
`Notched
`
`Bearing
`
`Knitted aramid/epoxy
`
`Knitted glass/epoxy
`
`Woven glass/epoxy
`
`Unnotched
`
`Formed
`
`Drilled
`
`Formed
`
`Strength (MPa)
`Strain-to-failure (%)
`
`63
`–
`
`100
`–
`
`61
`–
`
`338
`4.4
`
`Drilled
`
`275
`3.7
`
`Drilled
`
`369
`5.1
`
`the post-impact
`[43]. Predictably,
`prepreg laminates
`compression strength of knitted composite laminates
`decreases with the size of the damage zone, which in turn
`increases with impact energy [7].
`
`6. Modifications and innovations
`
`6.1. In-lay yarns and float stitches
`
`As mentioned earlier in this paper, the looped nature of
`the knit structure renders knitted composites inferior as
`structural materials. Moderate improvements to the strength
`and stiffness of knitted composites are achievable with the
`incorporation of float stitches in to basic architectures [5].
`Table 3 reveals that tensile properties, for example, are not
`significantly enhanced with this method since the float
`stitches carry with them an inevitable amount of crimp.
`
`Fig. 10. Comparison of Mode I fracture toughness values for thermoset
`composites reinforced using different types of textile fabric [39].
`
`A more effective way of enhancing the in-plane proper-
`ties of knitted composites is by introducing virtually
`straight, uncrimped fibres
`into the knitted structure
`[16,24,41,44–46]. These straight fibres are introduced by
`insertion into either a weft- and/or warp-knit structure
`during the knitting process. By so marrying the weaving
`and knitting processes, the hybrid fabric guarantees an opti-
`mum combination of improved mechanical properties (due
`to the straight fibres) and good forming characteristics (due
`to the knitting component of the fabric) [12,44]. Further,
`with inserted yarns, the anisotropy of a knitted composite
`can also be manipulated to suit a particular requirement (see
`Table 2). Whilst the tensile strength and stiffness and the
`energy absorption capabilities of knitted composites are
`highly dependent on fibre content, Ramakrishna and Hull
`[16,24,41,46] showed that, at a constant fibre volume frac-
`tion,
`the introduction of in-lay yarns can significantly
`improve the properties, provided the uncrimped yarns are
`preferentially oriented.
`Weft-insert, weft-knit fabrics (Fig. 12) are produced on
`flat-bed machines that have the capability of continuously
`and progressively feeding a straight yarn in to the needle
`bed just ahead of each knitting action so that the yarn is
`locked inside the loops (Fig. 13).
`More recently, work at Dresden [47,48] has produced a
`version of multilayer multiaxial weft- and warp-insert weft-
`knit fabrics. These fabrics were produced using a modified
`V-bed flat knitting machine which incorporates warp and
`weft guides/feeders (Fig. 14), in addition to the standard
`knitting needles, through which uncrimped yarns are intro-
`duced into the fabric. Whilst the insertion of off-axis yarns
`are not yet possible,
`they are nonetheless theoretically
`possible to achieve. These multilayer weft-knit fabrics are
`therefore,
`in principle, very similar to their warp-knit
`counterparts (i.e. non-crimp fabrics) (see Section 6.3), and
`so they are expected to have similar performance. Further,
`Offermann [48] claimed that these fabrics have the potential
`of minimising damage to the uncrimped yarns, and produ-
`cing fully fashioned preforms (see Section 6.5). The cost
`and quality implications of this technique as compared with
`the non-crimp fabrics is however unclear at this stage.
`Alternative to weft-insert, weft-knit fabrics, straight, in-
`lay yarns can also be introduced into warp-knit structures
`using Raschel machines [9,45]. A warp-insert, warp-knit
`fabric has a typical warp-knit structure but between these
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`
`Table 2
`Comparison of selected mechanical properties for carbon/epoxy composites based on a weft-knit fabric with and without weft inserted in-lay yarns, and a
`woven fabric [111,112]
`
`Property
`
`Tensile strength (MPa)
`
`Tensile stiffness (GPa)
`
`Specific absorption energy (kJ/kg)
`
`Course/transverse Wale/longitudinal Course/transverse Wale/longitudinal Course/transverse Wale/longitudinal
`
`Knitted without inlay (cid:133)Vf (cid:136)
`20%(cid:134)
`Knitted with inlay (cid:133)Vf (cid:136) 20%(cid:134)
`[02/903]ns crossply (cid:133)Vf (cid:136) 55%(cid:134)
`0/90 Woven (cid:133)Vf (cid:136) 50%(cid:134)f
`
`29a
`
`260a
`1216
`625
`
`60a
`
`42a
`839d
`625
`
`11a
`
`32a
`82d
`17
`
`15a
`
`10b
`52d
`17
`
`17b,c
`
`70b
`43e
`–
`
`26b,c
`
`50b
`43e
`–
`
`a After Ramakrishna and Hull [16].
`b After Ramakrishna and Hull [24].
`c Values projected from tests conducted on composite tubes having Vf’s of up to 15% (after Ramakrishna and Hull [16]).
`d Values estimated from Rule-of-Mixtures, using unidirectional data from Eckold [111].
`e After Hull [112].
`f After Eckold [111].
`
`wales are laid unlooped yarn with minimum crimp in them.
`The knitted wales and the straight yarns are connected to
`each other by means of yarns passing from wale to wale
`whilst interlacing with the straight yarns, as in weaving,
`along the way (Fig. 15).
`Weft-insert, warp-knit fabrics (Fig. 16) are in principle
`produced in a way similar to that employed for weft-insert,
`weft-knit fabrics, except
`in this case a warp knitting
`machine is used instead of a weft [12]. These fabrics offer
`greater flexibility for the type and amount of in-lay fibres
`that can be used for obtaining an optimum preform in terms
`of cost and performance [45].
`
`6.2. Split-warpknits
`
`More recently, using the weft-insert, warp-knit technique,
`strips of thermoplastic film have been co-knitted with load-
`bearing fibres to produce what are known as split-warpknits
`(Fig. 17) [12,49–52]. The development of these fabrics is
`aimed at high speed, high volume production of composite
`components. Strips of polypropylene (PP) and polyethylene
`teraphthalate (PET) films are used instead of fibres to keep
`
`the cost low and to minimise the amount of induced micro-
`waviness in the in-lay yarns that arises due to a mismatch in
`thermal expansion coefficients between the thermoplastic
`and glass. Consolidation of the fabrics is accomplished by
`either heating and cooling in one common mould (i.e.
`single-mould technique), or by preheating in a press and
`then transferring to a separate cooler tool for forming (i.e.
`press-mould technique). Depending on the degree of
`preheating, amongst other things [51],
`the lower cost
`press-mould technique could produce composites of inferior
`mechanical properties (refer to Table 4). The relatively
`poorer properties are attributed to higher amounts of poros-
`ity and resin-rich regions, and less uniform fibre distribution
`[49]. On the whole, nonetheless, split-warpknit composites
`have comparable tensile and bending properties to equiva-
`lent commingled woven composites, but at only a fraction of
`the manufacturing cost [49].
`The split films were found to create rather large gaps
`between the straight glass rovings, particularly in biaxially
`reinforced composites. The size of the gaps is related to the
`size of the film, which has to be thick enough to ensure
`sufficient resin for complete impregnation and wet-out of
`
`Table 3
`Comparison of selected mechanical properties for glass/epoxy composites based on a weft-knit fabric with and without float stitches, continuous fibre random
`mat and woven fabric [5,111]
`Property (cid:133)Wf (cid:136) 45%(cid:134)
`
`Tensile strength (MPa)
`
`Tensile stiffness (GPa)
`
`Course/transverse
`
`Wale/longitudinal
`
`Course/transverse
`
`Wale/longitudinal
`
`Knitted without float stitchesa
`Knitted with float stitches
`(cid:133)1 £ 1(cid:134)a
`Knitted with float stitches
`(cid:133)2 £ 1(cid:134)a
`CFRMa
`0/90 wovenb
`
`32.3
`29.0
`
`67.0
`
`177.4
`330.3
`
`138.7
`70.5
`
`101.1
`
`191.9
`330.3
`
`7.7
`3.4
`
`7.4
`
`10.2
`15.6
`
`a After Rudd et al. [5].
`b Values estimated from tests conducted on laminates having a Vf of 33% (after Eckold [111]).
`
`11.8
`6.7
`
`9.8
`
`10.8
`15.6
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`
`207
`
`Fig. 11. Representative fractographs of the impact damage zone of a knitted composite [7]. (a) Plan view. (b) Cross-s