Applied Composite Materials 8: 279–295, 2001.
`© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
`
`279
`
`Tensile Behaviour of Multilayer Knitted Fabric
`Composites with Different Stacking Configuration
`
`YANZHONG ZHANG, ZHENG-MING HUANG and S. RAMAKRISHNA
`Polymer and Textile Composites Laboratory, Department of Mechanical & Production Engineering,
`National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
`
`(Received 21 January 2000; accepted 10 May 2000)
`
`Abstract.
`In this paper, multilayer plain weft knitted glass fabric reinforced epoxy composite
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`/ ± 45
`laminates with different stacking configurations, i.e., [0
`/0
`], [0
`/90
`/90
`/0
`] and
`]4, [0
`◦
`[90
`]4, were investigated experimentally. The laminates were uniaxially tensile loaded until final
`fractures occurred. The experimental results show that with the change in layer stacking structure, a
`corresponding variation in composite strength and stiffness was achieved. The tensile strength and
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`/± 45
`/0
`] > [0
`/90
`/90
`/0
`] > [90
`]4 > [0
`]4, which implicates
`modulus rank as follows: [0
`a potential desiguability of Knitted Fabric Composites (KFC) for engineering applications. Failure
`behaviours of the fractured laminate specimens were examined using a ‘matrix digestion and layer
`peeling’ method, based on which the behaviour of each lamina in the laminate can be clearly shown.
`It was found that an angle-plied lamina in the laminate when subjected to a uniaxial tensile load
`has a different fracture mode from that of a single ply composite under an off-axial tensile load.
`This means that the lamina in the laminate is subjected to a more complicated load combination.
`By comparing the fractured mode of the latter lamina with that of the single ply composite, the
`load direction sustained by the lamina in the laminate can be identified, which provides a qualitative
`benchmark for verifying a theoretical simulation.
`
`Key words: knitted fabric composites, multilayer laminate, stacking configuration, mechanical prop-
`erties, fracture mode, internal load direction, experimentation.
`
`1. Introduction
`
`Knitting techniques are traditionally employed in clothing and apparel industry. By
`interlocking of loops of fibre bundles, various knitted fabrics can be produced. As
`one kind of textile materials, knitted fabrics possess some unique characteristics
`over other kinds of textile fabrics such as woven and braided fabrics, particularly
`in conformability and drapability which make them irreplaceable in conforming
`complicated contours or desired shapes. Due to these attractive features and also
`due to the feasibility of knitting high performance engineering fibres such as car-
`bon/graphite, glass and aramid in current technology, there is an increasing interest
`in the research and development of knitted fabrics for use in composite indus-
`try [1–5]. Compared with some other kinds of fiber reinforced composites, knitted
`fabric composites generally have inferior in-plane stiffness and strength, as indi-
`cated in Figures 1 and 2. Hence, they are not likely to be used as primary structural
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`
`Figure 1. Normalized strength (with respect to the fibre volume fraction) of different fibrous
`composites in their main directions.
`
`Figure 2. Normalized modulus (with respect to the fibre volume fraction) of different fibrous
`composites in their main directions.
`
`materials in high performance structure components. However, it is possible to
`make them secondary load-sharing structures, due to their feasibility of net or near
`to net shape contours [6] and some other superior mechanical performances such as
`high resistance to impact [7, 8]. A “flexible composite” with a much larger useable
`range of deformation than conventional laminated composites could be achieved
`by combining tougher resins with knitted fabrics [9].
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`TENSILE BEHAVIOUR OF MULTILAYER KNITTED FABRIC COMPOSITES
`
`281
`
`The mechanical properties of knitted fabric composites (KFC) varied with vari-
`ables such as knit architecture, stitch density, pre-stretching percentage on fabrics,
`inlay fiber bundles, tow size of fibers etc. have been experimentally investigated
`by previous researchers [10–18]. The anisotropy feature of KFC has already been
`recognized. These investigations were mainly carried out based on single layer or
`equivalent single layer knitted fabric reinforced composites. On the other hand,
`limited work has been done so far on the mechanical behaviour of knitted fabric
`composites with varied stacking structures. The present paper focuses on investiga-
`tion of the tensile behaviours of multilayer plain weft knitted glass fabric reinforced
`epoxy composites with different stacking configurations. Following laminates have
`/±45
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`◦
`/0
`], [0
`/90
`/90
`/0
`] and [90
`refers to
`]4, [0
`]4, where 0
`been considered: [0
`◦
`to the fabric course direction. With the variation
`the fabric wale direction and 90
`in the stacking arrangements, different tensile moduli and ultimate strengths have
`◦]4 > [0
`/±
`been found. The tensile strength and modulus rank as follows: [0
`◦
`◦] > [0/0
`
`◦] > [90/0 ◦]4.
`◦
`◦
`◦
`◦
`
`/90
`/90
`
`45
`A special attention has been focused on the fracture mode of each lamina layer
`in the laminate. This was achieved in the present paper by using a special technique,
`called “matrix digestion and layer peeling” method. The resin matrix in the frac-
`tured laminate specimen was digested and each fabric layer was peeled off. The
`fracture surface of each layer was thus clearly shown. A comparative study was
`also carried out using a single layer fabric reinforced composite under an off-axial
`tensile load. It was found that the fracture mode of an angle-plied lamina in the
`multilayer laminate is different from that of single layer composite subjected to the
`off-axial tensile load. Hence, the angle-plied lamina in the laminate is subjected
`to a more complicated load combination. By comparing the two fracture modes,
`the load direction sustained by the angle-plied lamina can be determined. These
`experimental evidences are useful for verifying a theoretical simulation on the
`multilayer knitted fabric reinforced composites.
`
`2. Experimental
`
`2.1. PLAIN WEFT-KNIT FABRIC
`
`Plain weft-knit fabrics were produced on a Flying Tiger knitting machine manually,
`using CPR407 glass fiber yarns made by ACI Fibreglass, Australia. The linear
`density of the yarn is 600 Tex (grams per 1000 m in length). Schematic diagram of
`a plain weft knitted fabric is shown in Figure 3.
`The row of loops in the longitudinal direction of the fabric is called “wale”,
`whereas the row of knit loops in the width direction is named as “course”. In this
`◦
`◦
`) direction. Thus, 90
`direction
`study, the wale direction is taken as the reference (0
`is along the course direction. The fabric stitch density was characterized as 3.4
`loops/cm in the wale and 3.8 loops/cm in the course directions, respectively.
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`YANZHONG ZHANG ET AL.
`
`Figure 3. Schematic diagram of a plain weft-knit glass fabric.
`
`2.2. SPECIMENS
`
`Composite laminates were fabricated by a hand lay-up method. The matrix material
`used was a mixture of epoxy resin R-50 and hardener H-64 (Chemicrete Enter-
`prises Pte. Ltd, Singapore) in a ratio of 100 : 48 by weight. Prior to commencing
`◦
`C for 2 h
`composite fabrication, the knitted fabrics were dried in an oven at 100
`to remove moisture that may influence adhesion between the epoxy matrix and the
`◦
`C
`glass fibers. The epoxy resin and the hardener were separately preheated at 45
`for 30 min to reduce viscosity. Then, the resin and the hardener were thoroughly
`mixed together, followed by a vacuum treatment for 3 ∼ 5 minutes to remove en-
`trapped air bubbles. The composite fabrication was begun by attaching the fabrics
`with pushpins onto a wooden plank that was covered with a PP (Polypropylene)
`sheet. The PP sheet was used as a release agent to ensure the surface quality of the
`final composite laminate. Straight aluminum rods of 2 mm diameter were inserted
`into the fabric edges to make the curling fabrics flat during the fabrication. The
`resin mixture was then poured into the fabrics and another PP sheet was placed
`on the fabric surface. A delicate brush was used to push any air cavities to sides,
`before applying another wooden plank and heavy weights. The resin-impregnated
`fabrics were cured at room temperature for 24 hrs. Multilayer laminates of four
`◦]4, [0
`/±45
`◦], [0/0
`
`◦] and [90/0 ◦]4, were
`stacking configurations, i.e., [0
`◦
`◦
`◦
`◦
`◦
`
`/90
`/90
`
`prepared. To facilitate comparison, single layer knitted fabric composite panels
`were also fabricated. The fiber volume fractions, Vf, were 22.2% for the multilayer
`laminates and 17.1% for the single layer composites, which were determined with
`a combustion method. Tensile specimens were cut carefully using a water-cooled
`diamond saw in parallel to the wale or course direction. All specimens have the
`same width of 25 mm. Aluminum end tabs of 45 mm in length were glued to the
`both ends of the specimens, leaving a testing gauge length of 120 mm.
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`TENSILE BEHAVIOUR OF MULTILAYER KNITTED FABRIC COMPOSITES
`
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`
`2.3. TESTS
`
`Tensile tests were carried out on an Instron testing machine (Type 8516) at a cross-
`head speed of 1.5 mm/min. Tensile strains were calculated based on the cross-head
`displacements. Macrostructure of failed specimens was observed using optical mi-
`croscopy, whereas microstructure of the specimens was examined under a Jeol
`Scanning Electron Microscope (JSM-5800LV). To identify the fracture mode of
`an internal layer in the fractured laminate specimen, a resin digestion process was
`employed. Details will be described in the following section.
`
`3. Results and Discussions
`
`3.1. MECHANICAL PROPERTIES
`
`Figure 4 plots the stress–strain curves of single layer knitted fabric composites
`◦
`◦
`◦
`, 45
`and 90
`off-
`subjected to a uniaxial tensile load at loading directions of 0
`axis (with respect to the wale direction). Averaged properties of the single layered
`composites are summarized in Table I. The composite anisotropy in strength and
`
`Figure 4. Typical stress-strain curves of single layer GF/Epoxy KFCs.
`
`Table I. Tensile properties of single layer GF/Epoxy KFC.
`◦
`◦
`0
`45
`
`direction
`
`direction
`
`Ultimate strength, MPa
`Young’s modulus, GPa
`Failure strain, %
`
`59.1
`4.85
`2.2
`
`41.3
`4.44
`1.9
`
`◦
`90
`
`direction
`
`34.9
`3.63
`1.6
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`
`Figure 5. Typical stress-strain curves of multilayer KFC laminates with different stacking
`configurations.
`
`stiffness with respect to a particular loading direction is quite obvious. Figure 5
`shows typical tensile stress versus tensile strain curves of the multilayer KFCs
`with different stacking sequences. Both the stress–strain curves of single layer and
`multilayer KFCs exhibit a linear behaviour at the initial stage of load and then go
`to nonlinear. Grossly, the nonlinear region of the single layer KFC is relatively
`limited. However, the multilayer KFC exhibits a trend of more ductile character-
`◦]4 and [0
`◦], the ductile features/0
`istic. For the stacking structures of [0
`◦
`◦
`◦
`/90
`/90
`
`are quite apparent, as shown in Figure 5. These results indicate that a progressive
`failure process might have occurred in the laminates. Comparing the tensile stress-
`◦]4 composite with that of the single layer composite loaded in
`strain curve of the [0
`the wale direction, it might be concluded that a damage evolution is more serious
`in a multilayer than in a single layer knitted fabric composite. The ultimate strains
`of the laminates with different stacking arrangements are graphed in Figure 6.
`Figure 7 shows the changes in ultimate tensile strength of the four multilayer
`KFC laminates, giving a similar variation trend as that of the single layer KFC ma-
`◦]4 structure is higher than those of all the other stack-
`terials. The strength of the [0
`/+ 45
`/− 45
`◦] and [0/0
`◦],/0
`◦]4, [0
`ing structures. For the structures of [0
`◦
`◦
`◦
`◦
`◦
`◦
`
`/90
`/90
`
`their tensile strengths decrease gradually with the increase of inclined angles of the
`/+45
`◦]
`/− 45
`inside layers. However, the change is not apparent between the [0
`◦
`◦
`◦
`/0
`◦] laminates. In general, the strength of a KFC is proportional/0
`and [0
`◦
`◦
`◦
`/90
`/90
`
`◦
`) direction
`to the portion of fibers oriented in the loading direction; the wale (0
`specimen exhibits the highest strength when loaded in the wale direction. It is
`inferred that a variation in the loop architecture of knitted fabrics can also influence
`the sensitivity of the laminate tensile strength on stacking angles.
`Figure 8 shows the stiffness of the multilayer KFC versus different stacking con-
`figurations. It was found that an approximately linear relationship exists between
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`TENSILE BEHAVIOUR OF MULTILAYER KNITTED FABRIC COMPOSITES
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`
`Figure 6. Comparison on failure stain of multilayer KFC laminates.
`
`Figure 7. Comparison on the ultimate tensile strengths of KFC laminates with different
`stacking configurations.
`
`the tensile modulus and the stacking configuration. Based on these experimental
`evidences, it is possible to choose a stacking sequence so that an optimal strength
`and stiffness behavior can be achieved for the resulting multilayer knitted fabric
`composite when subjected to differently combined load conditions.
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`286
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`YANZHONG ZHANG ET AL.
`
`Figure 8. Comparison on the tensile modulus of KFC laminates with different stacking
`configurations.
`
`3.2. POST-FAILURE ANALYSIS
`
`3.2.1. Overall Failure Behavior
`
`Ramakrishna investigated the failure mechanism of single layer knitted glass fiber
`fabric reinforced epoxy composites in a previous work [18]. Matrix cracking, in-
`terface debonding, and fiber breaking were the main forms of a composite failure.
`These failure forms also occurred in the present multilayer knitted fabric reinforced
`laminates. Figure 9 shows the SEM photographs of the surface of a failed multi-
`layer knitted fabric laminate. The fiber breaking is distinct in Figure 9(a), whereas
`the interface debonding between fiber yarns and surrounding matrix is indicated in
`Figure 9(b). Furthermore, the smooth surface surrounding the fibre yarns in Figure
`9(b) suggests that the matrix cracking already happened there.
`
`3.2.2. Fracture Mode of Inside Layers
`
`For single ply composites, with the change in the load direction with respect to the
`fabric wale direction, the fractured surfaces occurred in different parts of the fabric,
`as shown in Figure 10. When the load direction is parallel to the wale direction, the
`fracture usually takes place over the head of loops or along the fabric side loops
`(referring to Figures 11(a) and 11(b)). When the load direction is perpendicular
`to the wale direction (i.e., in the course direction), the fracture occurs near the
`points where yarns cross over each other (Figure 10(c)). A similar fracture mode
`◦
`off-
`as that of a course direction loaded specimen has also been found for the 45
`axial loaded specimen (Figure 10(b)). Under the off-axial load, it was found that
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`TENSILE BEHAVIOUR OF MULTILAYER KNITTED FABRIC COMPOSITES
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`
`(a)
`
`(b)
`
`Figure 9. SEM photographs of a failed multilayer KFC.
`
`the angle between the fractured surface and the load direction was the same as
`◦
`. Thus, both the course directional loaded specimen and
`the off-axis angle, i.e., 45
`◦
`off-axial loaded specimen fractured along the needle loops of the fabric
`the 45
`(Figure 11(c)). These results are consistent with previous investigations [3, 19–21].
`On the other hand, only the fracture modes of the outside layers of a broken
`multilayer laminate specimen can be observed. It is generally difficult to know
`detail of the fracture behaviors of those lamina layers inside the laminate. Figure 12
`shows the photograph of a fractured multilayer knitted fabric laminate under uni-
`axial tensile load. Although the matrix used was somewhat transparent, it is still
`difficult to depict the failure mode of a fabric layer inside the laminate. However,
`information of the fracture details of all the laminae in the laminate is important for
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`YANZHONG ZHANG ET AL.
`
`◦
`Figure 10. Characteristic Fracture Status of single layer KFC: (a) 0
`◦
`loading.
`(c) 90
`
`◦
`loading; (b) 45
`
`loading;
`
`Figure 11. Schematic diagram of basic loop components and fracture types: (a) Loop com-
`◦
`◦
`◦
`ponents; (b) Fracture mode for 0
`loading; (c) Fracture mode for 90
`and 45
`off-axis
`loading.
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`TENSILE BEHAVIOUR OF MULTILAYER KNITTED FABRIC COMPOSITES
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`289
`
`Figure 12. A photograph of a fractured multiplayer KFC.
`
`the whole laminate failure analysis. It was recognized in this study that the matrix
`digestion by means of a combustion method is very efficient to achieve the desired
`purpose. It was found that after burning the broken specimen in a furnace using
`a specially designed procedure, the fracture detail of each layer in the laminate
`could be clearly shown. For the present GF/Epoxy material, the procedure began
`◦
`◦
`C furnace for 1 h, followed by 500
`C holding for
`by putting the specimen in a 300
`another 1 h. After cooling down at an ambient condition to room temperature, the
`multilayer laminate specimen was safely separated into distinct single layer fabrics.
`All the single layer fabrics could be peeled off and their fracture characteristics
`could be observed.
`Figure 13 through Figure 15 show the photographs of all the fabric layers de-
`composed from the fractured laminates with different stacking sequence. It is seen
`◦]4, [0
`◦]
`that the fracture form of each fabric layer decomposed from [0
`◦
`◦
`◦
`/90
`/90
`/0
`◦]4 laminate specimens is the same as that of a corresponding single layer
`or [90
`/+45
`◦]
`/−45
`KFC under uniaxial tensile load. However, after digestion for the [0
`◦
`◦
`◦
`/0
`and −45
`specimen, it was found that the fractured surfaces of the +45
`◦
`◦
`plied
`◦
`inclined angle with the loading direction. Instead,
`fabric laminae did not have a 45
`the fractured surfaces are nearly perpendicular to the loading direction. This is dif-
`ferent from the fracture form of the single layer knitted fabric composite subjected
`◦
`off axial tensile load. Therefore, an angle-plied lamina in the laminate
`to the 45
`is generally subjected to a more complicated load combination even though the
`whole laminate is under a uniaxial load condition.
`
`3.2.3. Load share of Angle-Plied Lamina
`
`The matrix digestion and layer peeling method can be further used to determine the
`load direction sustained by an angle-plied lamina in the laminate. This is achieved
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`YANZHONG ZHANG ET AL.
`
`◦
`◦
`Figure 13. Fracture modes of fabric layers in [0
`/±45
`
`
`◦] laminate./0
`
`again by comparing the fracture mode of the angle-plied lamina with that of the
`single layer composite. Details are described as follows.
`When a single layer composite is subjected to an off-axial tensile load (Fig-
`ure 16(a)),
`the resultant stress on an angle inclined surface is equal to the overall
`applied stress, as indicated in Figure 17(a). The two stress components on the
`surface, i.e., the stresses in perpendicular and parallel to the surface, are given
`respectively by
`σα = σ cos2 α,
`τα = σ
`sin 2α,
`2
`where α is the angle of the inclined surface and σ is the applied external stress.
`On the other hand, the angle-plied lamina in the laminate is generally subjected
`to combined normal and in-plane shear stresses, as shown in Figure 16(b) and
`Figure 16(c). If the overall applied stress on the laminate is a tensile load, then the
`normal stress sustained by the lamina should be tensile. However, the shear load
`direction is undetermined. Suppose that the shear stress sustained by the lamina
`
`(1)
`(2)
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`TENSILE BEHAVIOUR OF MULTILAYER KNITTED FABRIC COMPOSITES
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`291
`
`◦
`◦
`◦
`Figure 14. Fracture modes of fabric layers in [0
`/90
`/90
`
`
`◦] laminate./0
`
`has an assumed direction as shown in Figure 16(c). Then, the normal and the shear
`stress components on a similar inclined surface are derived as
`= σ cos2 α + τ (sin α + cos α),
`sin 2α + τ (sin α − cos α).
`= σ
`2
`In the case of α = 45
`◦
`, the single layer composite fractured along the inclined
`surface. As the shear strength of a composite is generally higher than its tensile
`strength, Equations (1) and (2) imply that the failure of the single layer composite
`subjected to the uniaxial tensile load is mainly caused by the normal (tensile) stress
`component on the inclined surface. On the other hand, the experiment has shown
`plied laminae in the [0
`/+45
`/−45
`◦] laminate did/0
`that the fracture of the ±45
`◦
`◦
`◦
`◦
`
`not occur along a similar inclined surface. Therefore, the resulting normal (tensile)
`stress component, given by Equation (3), must be smaller than the shear stress
`component that is given by Equation (4). Thus, the actual shear stress direction
`should be opposite to the assumed direction as indicated in Figure 16(c) and 17(b).
`
`(3)
`
`(4)
`
`(cid:10) α
`
`σ
`
`τ
`
`(cid:10) α
`
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`YANZHONG ZHANG ET AL.
`
`
`Figure 15. Fracture modes of fabric layers in [90
`
`◦]4 (a) and [0◦]4 (b) laminates.
`
`4. Conclusions
`
`An experimental program has been carried out in this paper to characterize the
`tensile behaviors of multilayer knitted fabric reinforced epoxy laminates under
`uniaxial tensile load. The tensile moduli and ultimate strengths of the laminates
`vary with different lay-up configurations. Due to stacking constraint, a progressive
`failure process generally occurs in the laminate, which makes the multilayer lami-
`nate more ductile than a corresponding single layer composite. The fracture mode
`of each lamina layer in the laminate can be clearly identified based on a “matrix
`digestion and layer peeling” method. It is found that an angle-plied lamina in the
`laminate subjected to uniaxial tensile load has a different fracture mode than that of
`a single layer composite under an off-axial tensile load. A more complicated load
`
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`TENSILE BEHAVIOUR OF MULTILAYER KNITTED FABRIC COMPOSITES
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`
`◦
`Figure 16. A schematic diagram of (a) single layer KFC under 45
`off-axis tensile load;
`◦
`◦
`
`◦] laminate under uniaxial tensile load; and (c) load sharing of 45◦/0
`/±45
`(b) multilayer [0
`
`◦
`◦
`◦] laminate./0
`angle-plied lamina in the [0
`/±45
`
`
`◦
`Figure 17. Stress analysis of an isolated element from (a) single layer KFC under 45
`◦
`◦
`◦
`◦] laminate under uniaxial load./0
`angle-plied lamina in [0
`/±45
`
`load; (b) 45
`
`off-axis
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`
`combination must have been applied to the angle-plied lamina, and the determina-
`tion of the resulting load direction is described in the paper. Such information may
`be useful for laminate failure analysis. It is expected that the “matrix digestion
`and layer peeling” method can be equally well applied to other kinds of fibrous
`composite laminates.
`
`References
`
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`Amsterdam, 1989.
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`5. Gommers, B., Verpoest I., and Van Houtte, P., ‘Modeling the Elastic Properties of Knitted-
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`6. Epstein, M. and Nunni, S., ‘Near Net Shape Knitting of Fibre Glass and Carbon for
`Composites’, in 36th International SAMPE Symposium, 1991, p. 102.
`7. Ramakrishna S. and Hull, D., ‘Energy Absorption Capacity of Epoxy Composite Tubes with
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`Skechers v Nike
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`Skechers EX1041
`Skechers v Nike
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