Composites Science and Technology 70 (2010) 363–370

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Thermoplastic composite from innovative flat knitted 3D multi-layer spacer fabric using hybrid yarn and the study of 2D mechanical properties Md. Abounaim *, Gerald Hoffmann, Olaf Diestel, Chokri Cherif Institute of Textile Machinery and High Performance Material Technology (ITM), Technische Universität Dresden, Hohe Strasse 6, 01069 Dresden, Germany

a r t i c l e

i n f o

Article history: Received 24 July 2009 Received in revised form 2 November 2009 Accepted 6 November 2009 Available online 13 November 2009 Keywords: A. Hybrid yarn 3D spacer fabrics A. Textile composites B. Mechanical property E. Knitting

a b s t r a c t Due to their improved mechanical properties, 3D multi-layer spacer fabrics could be used for lightweight applications such as textile-based sandwich preforms. Modern flat knitting machines using high performance yarns are able to knit complex 3D multi-layer spacer fabrics consisting of individual surface and connecting layers. This paper reports on the development of 3D flat knitted spacer fabric for 3D thermoplastic composites using hybrid yarns made of glass (GF) and polypropylene (PP) filaments. Moreover, mechanical properties of reinforcement yarns, 2D knit fabrics and 2D composites manufactured using various integration methods of reinforcement yarns were also studied. The integration of reinforcement yarns as biaxial inlays (warp and weft yarns) is found to be the best solution for knitting, whereas the tuck stitches show optimal results. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Spacer fabrics are complex 3D constructions made of two separate fabric layers connected vertically with pile yarns or fabric layers. The conventional spacer fabrics composed of two surface layers bound with pile yarns are generally manufactured using weaving and knitting technologies. However, due to inferior mechanical properties, such as elasticity and deformability under applied loads, conventional spacer fabrics are not suitable for high performance composite applications. Moreover, the restricted distance between the plane layers contribute to the drawbacks of such spacer fabrics. One solution is to connect the planes by means of vertical fabric layers instead of pile yarns. When done on a flat knitting machine waste can be reduced and faster production times can be achieved. This type of 3D spacer fabric with multilayer reinforcement inlays in the fabric structures manufactured with flat knitting techniques is expected to show superior mechanical properties and be especially suitable for lightweight applications. Tensile and compression characteristics, flexural properties and energy absorption are just a few qualities which can be improved [1–12]. Future applications of composites made from 3D multi-layer spacer fabrics involve the replacement of conventional panel structures that are being used for aircraft, transport vehicles, marine applications and infrastructures, lift cabins, and ballistic protection for buildings and combat vehicles, etc. * Corresponding author. Tel.: +49 17622151327. E-mail addresses: [email protected], md_abounaimtex@yahoo. com (Md. Abounaim). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.11.008

Textiles generally produced by braiding, weaving and other unidirectional techniques are used for composites because of their excellent mechanical properties, such as high strength, stiffness and damage tolerance in impact loading. However, knitted composites exhibit greater drapability and higher impact resistance as compared to the above-mentioned textile based composites [1–12]. Modern electronic flat knitting machines are capable of manufacturing 3D complex shaped engineering structures. Unique technical features which allow rapid and complex production include individual needle selection capability, the presence of holding down sinkers, presser-foots, racking, transfer, adapted feeding devices combined with CAD system and modern programming installations. Furthermore, the flexibility of the knitting process in combination with the possibility of integration of reinforcement yarns into fabric structures is capturing the attention of many researchers [1–11]. In previous works, we reported the analysis and the manufacturing of 3D spacer fabrics on the basis of surface and connecting layers [6] and the development of 3D spacer fabrics possessing only the weft inlays [1–3]. The research works documented in this direction includes the basic production principles of some flat knitted spacer fabrics without reinforcements [4] and the theoretical presentation of knitted sandwich spacer fabrics [7]. Nevertheless, the developments of 3D multi-layer spacer fabrics produced with flat knitting are essential for high performance, complexly shaped composite applications [4,8–11]. Thermoplastic composites are comprised of at least one reinforcement material and a thermoplastic polymer as matrix. These composites show distinct advantages as compared to thermoset

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composites. Due to their high fracture toughness, easy recycling, elongation, short processing time, various forming possibilities, weld-ability, low cost and resistance to medias and corrosion, they appear to be more promising for industrial applications [1,2,6,9– 11,14]. Commingled hybrid yarns consisting of reinforcement and matrix filaments are soft, flexible, drapeable and are available at low cost, which makes them a forerunner for thermoplastic composite applications. The reinforcement component of the hybrid yarn is generally high performance fibers such as glass, carbon and aramid fibers. Glass fibers are used extensively due to low material cost and higher mechanical properties. The thermoplastic matrix is used to fix the reinforcement components in a defined order for better bearing of applied forces by, to ensure good adhesion between the fibers and matrix material and to develop low cost products, especially for the automobile industries [1,2,6,9–11,14–15]. The use of glass (GF) and polypropylene (PP) filaments in hybrid yarn in a volume combination of 52% and 48%, respectively, is reported to optimize the mechanical properties of textile reinforced thermoplastic composites. Based on these parameters, the commingled hybrid yarn composed of glass (GF) and polypropylene (PP) filaments is preferred for the development of 3D spacer fabrics [1–3,6,8–11]. The mechanical properties of thermoplastic knit composites are not only dependent on the hybrid yarns used, but are also greatly affected by the knit structures as well as the orientation of the reinforcement yarns [15–18]. Moreover, knit fabrics made from GF–PP hybrid yarns are prone to some difficulties during knitting because of high rigidity, a high friction coefficient and the brittleness of the glass filaments [19–21]. However, the flexible yarn feeding system and the improved yarn tensioning together with the optimized knitting process enable effortless knitting of glass filaments [1–3,6,9,11]. One goal of the research program ‘‘Textile-reinforced composite components for function-integrating multi-material design in complex lightweight applications” funded by the German Research Foundation (SFB 639) at the Technische Universität Dresden, is to develop reinforced U-shaped 3D spacer fabric using hybrid yarns on a flat knitting machine for high performance composite application. In order to achieve this objective, 3D spacer fabric was produced U-shaped on a flat knitting machine using GF-PP hybrid yarns. The reinforcement yarns were integrated individually in the spacer fabric structures as knit loops, tuck stitches, weft yarns and as biaxial inlays (warp and weft yarns aligned as multi-layer structure). These spacer fabrics were then used for 3D thermoplastic composites. Further analysis was performed on 2D fabrics based on the fact that each surface of a 3D spacer fabric is actually a 2D fabric. The analysis was intended to predict the mechanical behaviours of 3D spacer fabric composites. Therefore, 2D knit fabrics were manufactured separately with reinforcement yarns but compatible to the manufacturing process of knitted 3D multi-layer spacer fabrics. Subsequently, the reinforcement yarns were pulled out of the fabrics to study the effect of fiber arrangements on the tensile properties. The tensile properties of various 2D knit fabrics were measured and analysed. In the next stage, 2D composites were produced and mechanically investigated.

2. Experimental study Hybrid yarns consisting of glass (volume: 52%) and polypropylene filaments (volume: 48%) were used as high performance and thermoplastic materials, respectively. These hybrid yarns were developed as well as manufactured at the Institute of Textile Machinery and High Performance Material Technology (ITM) of Technische Universität Dresden (Germany) using the modified polypropylene filaments ‘‘Prolen H” by the CHEMOSVIT FIBRO-

CHEM a.s. Company of Slovakia and the glass filaments by the P–D Glasseiden GmbH Oschatz Company of Germany. However, hybrid yarns of 139 tex were selected as the knitting yarns for base fabrics (base loop yarn) and 1200 tex (three ply of 400 tex) as reinforcement yarns. 2.1. U-shaped 3D spacer fabric U-shaped 3D spacer fabrics were developed by creating individual surface and connecting layers made with the single jersey knit pattern. The fineness (gauge) of needles for fabric layers was determined considering the integration method of reinforcement yarns into the spacer fabric structures. To knit individual surface layers, needles of both needle beds were first divided accordingly. And the construction was carried out by knitting both surface layers and the connecting layers on the selected needles of both needle beds, respectively. 2.1.1. Reinforcement yarns as knit loops, tuck stitches and weft yarns In this case, two individual surface layers, a and b (Fig. 1), were knitted first on the selected needles of both front and rear needle beds of the flat knitting machine (CMS 320 TC E5 of company H. Stoll GmbH & Co. KG, Reutlingen, Germany). Both surface layers were knitted to the length of 45 mm (H). The connecting layer c was then knitted on the selected needles of a single needle bed being initially joined with the existing surface layer (a) on that needle bed keeping the take down system inactive. While knitting occurred, the holding down sinkers pressed this connecting layer downwards in the gap between both needle beds. Finally, the connecting layer (c) was shifted to the other needle bed using the loop transfer technique. Since the knitting of both surface layers (a and b) remained inactive while knitting the connecting layer (c), a vertical joint between the surface layers could be achieved with the connecting layer. By repeating these knitting steps, the produced 3D spacer fabric was shaped like an U made from two separate fabric layers vertically connected by individual fabric layers. These spacer fabrics were produced using the reinforcement yarns as knit loops, tuck stitches and as weft yarns, which could be seen by Fig. 5K, T, and W. In the case of knit loops, all surface and connecting layers were produced using the reinforcement yarns only as knit loops. But for tuck stitches and weft yarns, first the base loop yarn (finer yarn) was used to construct the base fabrics (for both surface and connecting layers) using a single jersey pattern. The reinforcement yarns were then integrated into the base fabrics as tuck stitches using selected needles. However, reinforcement yarns were integrated as weft yarns into the base fabrics using the loop transfer technique. The length (L) of the connecting layer could be knitted to a maximum of 35 mm in this case and is formed mainly by the combined effect of the type of machine, type of holding down sinker, the distance between the needle beds, the type of materials, yarn fineness, etc. The knitting techniques of individual fabric layers with reinforcement yarns as knit loops, tuck stitches and as weft yarns are shown in Fig. 4. 2.1.2. Reinforcement yarns as biaxial inlays (warp and weft yarns) The U-shaped 3D spacer fabric consisting of multi-layer (biaxial inlays) surface layers and tuck-stitched connecting layers was produced on the flat knitting machine Steiger Aries 3 by the Steiger Company of Switzerland. This machine offers an open carriage to guide warp yarns for multi-layer knitting. According to the new manufacturing principal shown in Fig. 2, two sets of warp yarns (C-1, C-2) were delivered individually through the open carriage of the flat knitting machine and guided separately to both needle beds using specially designed warp guides. Two weft yarns (B-1, B-2) were separately supplied laterally by yarn feeders. These two weft yarns (B-1, B-2) were joined together accordingly with

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Fig. 1. Knitting direction and schematic view of U-shaped 3D spacer fabric.

Fig. 2. Manufacturing of U-shaped 3D multi-layer spacer fabric.

two warp sets (C-1, C-2) using two respective base loop yarns (A-1, A-2) following the knitting technique K-1 to construct two separate biaxial reinforced surface layers (a and b). Subsequently, two tuck stitch layers (D-1, D-2) were knitted separately on both needle beds using the knitting technique K-2. Both multi-layer surface layers were knitted until a predesignated length of 45 mm was reached. The tuck stitch layers (D-1, D-2) were knitted to half the length of the predesignated connecting layer, then joined together with the base loop yarn (phases 2–4) and knitted to completion. When the warp yarns were drawn back to the length of the predesignated connecting layer (phase 5), the tuck stitch shaped layers could perpendicularly connect both multi-layer surface layers. The above mentioned manufacturing sequences were repeated to develop a U-shaped 3D spacer fabric consisting of multi-layer reinforced surface layers and tuck stitch formed connecting layers. This fabric is documented in Fig. 5B.

2.2. Thermoplastic composite using 3D spacer fabric To fabricate the 3D composite, the laboratory hot-pressing machine (COLLIN P300 PV, Dr. Collin GmbH, Germany) was used to process the U-shaped 3D multi-layer spacer fabric. Recently developed and specially designed mechanical tools [11,13] were used for the 3D thermoplastic molding process. High temperature and high pressure were used to consolidate the 3D spacer fabric into the composite. Temperature and pressure flowcharts, along with the used mechanical tools for 3D molding are shown in Fig. 3. 2.3. 2D knit fabrics with reinforcement yarns For further analysis, 2D fabrics were knitted separately with the reinforcement yarns similar to the manner in which the U-shaped 3D spacer fabrics were knitted: knit fabrics with reinforcement

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Fig. 3. Manufacturing of thermoplastic composite from 3D multi-layer spacer fabric.

yarns as knit loops, tuck stitches, weft yarns and as multi-layer biaxial inlays. Moreover, the specifications for the 2D knit fabrics were adjusted to reflect the respective layers of produced 3D spacer fabrics. The manufacturing details of these 2D knit structures are presented in Fig. 4. Additionally, fabric details are presented in Table 1. 2.4. Tensile testing of reinforcement yarns and 2D knit fabrics The tensile strengths of yarns and 2D knit fabrics were measured on the Zwick Z100 yarn testing machine (Zwick GmbH & Co. KG, Germany). The method used for assessing the degradation of tensile strength of reinforcement GF–PP hybrid yarns with different integrations was based on measuring the strength before (Tb) and after knitting (Ta), and calculating their strength loss (L) as a percentage according to the following formula:

L ¼ fðT b  T a Þ=T b g  100%

221 °C a pressure of 52 bars was applied (oscillating) and this temperature was kept for 10 min. Finally, the temperature was dropped down to room temperature keeping the pressure constant. Table 1 and Fig. 4 present details of knit fabrics that were used to fabricate composites. The tensile and flexural testing was also performed on the Zwick Z100 strength testing machine. The machine uses the four point loading method for testing the flexural strengths of the specimens. The testing was performed according to German test standards DIN EN ISO 527-4 for tensile and DIN EN ISO 14125 for flexural strength. The impact tests were carried out with the aid of a pendulum arm type impact tester, which functions based on the principle of Charpy Impact Test technique. Testing was carried out at the Institute of Lightweight Engineering and Polymer Technology (ILK) at the TU Dresden (Germany). Standard methods of sampling and testing were applied as stated in DIN EN ISO 179-2.

ð1Þ

The measurement of the yarn breaking force after knitting could be measured after being carefully unravelled from the knit fabrics. The tensile strengths of the yarns were measured using test standard EN ISO 2062. 2D knit fabrics were used to measure the tensile properties following the test standards EN ISO 13934-1. 2.5. Measuring the mechanical properties of 2D composites 2D composites were also produced from 2D knit fabrics on the laboratory hot-pressing machine (COLLIN P300 PV, Dr. Collin GmbH, Germany). At first, knit fabrics were put into the press at room temperature with a pressure of 9 bars. The temperature was then raised at the rate of 10 °C per minute until 221 °C was reached for the melting of the polypropylene. After 6 min at

3. Results and discussion 3.1. U-shaped 3D spacer fabrics and 3D thermoplastic composite Fig. 5 shows the flat knitted U-shaped 3D spacer fabrics manufactured using GF–PP hybrid yarns, along with an example of thermoplastic composite made from U-shaped weft reinforced 3D spacer fabric (type W). The fabric specifications of individual layers of these spacer fabrics are given in Table 1. 3.2. Tensile properties of reinforcement yarns and 2D knit fabrics Different methods of reinforcement yarn integration into 2D knit structures have a clear effect on the tensile properties of the

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Fig. 4. Arrangements of reinforcement yarns in 2D knit structures (binding technique, knit architecture, knit fabric and composite).

Fig. 5. 3D textile preforms and consolidated 3D composite.

hybrid yarns. The tensile strength of original reinforcement GF–PP hybrid yarn (1200 tex) was measured to be 307.6 N per mm2. The stress–strain curves of reinforcement yarns are presented in

Fig. 6a, whereas Fig. 6b shows the loss of tensile strength of reinforcement yarns due to different integration methods. The maximum loss of yarn strength was recorded as 65% for knit loops

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Table 1 Details of 2D knit fabric (layer of spacer fabric), yarn strength and composite. No.

K T W B

Reinforcement yarns used as

Knit loops Tuck stitches Weft yarns Warp and weft yarns

Loss of strength (reinforcement yarn)

Fabric specifications

Composite thickness (mm)

Percent

No. of wales per cm

No. of courses per cm

Specific weight (kg per m2)

Fabric thickness (mm)

56 38 16 13 (warp) 14 (weft)

1.9 1 1 2.8

2.5 4.1 3.8 2.7

1.2 1.3 0.9 0.8

1.8 2 1.5 1.2

0.75 0.8 0.6 0.5

Fig. 6. Influence of yarn arrangements on the tensile strengths of GF–PP reinforcement yarns (a and b) and knit fabrics (c and d).

and 45% for that of tuck stitches, but no more than 5–10% for biaxial inlays (warp and weft yarns). Consequently, knit fabric with reinforcement yarns as biaxial inlays shows improved tensile properties with equally reduced elongation in both course and wales directions. However, these properties are at their maximum in the course direction for weft yarns (c and d of Fig. 6). Nevertheless, knit fabrics with tuck stitches also have improved tensile strength in the course direction similar to biaxial inlays. However such tensile strength was recorded after a considerable amount of elongation before breaking. In the case of reinforcement hybrid yarns, about 45% less tensile strength was recorded when they were used as tuck stitches rather than as biaxial inlays. Even though the reinforcement yarns are orientated as tuck stitches (curved) into the fabric, the tensile strength of the knit fabric in the wales direction was found to be very inferior, approximately 10% that of the course direction. It can be inferred, that glass filaments of reinforcing

yarns, which are brittle upon bending, are damaged mostly when used for knit looping. On the other hand, such damage of glass filament is moderate with tuck stitching, where the curvilinear shapes of reinforcement yarns are not so high. However, warp and weft yarns as biaxial inlays (multi-layer) are not involved in the loop forming operation and are only placed as floated yarns into the knit structures. This result is also supported by the effect of the maximum orientation of reinforcing yarns in course and wales directions as biaxial inlays (none crimp yarns). 3.3. Mechanical properties of 2D composites The volume of glass filaments in all composites is measured at about 50%. The composites produced from knit fabrics with different arrangements of reinforcement yarns exhibit different mechanical properties, which can be seen in Fig. 7. The tensile

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Fig. 7. Mechanical properties of 2D composites (a and b: tensile properties; c and d: flexural properties; e and f: impact properties).

behaviour of the composites has, to a great extent, reflected the properties of the reinforcement hybrid yarns and knit fabrics. It is already seen, starting from knit loops to tuck stitches and then warp and weft yarns, that the degree of damage to the reinforcement yarn was decreased. It can also be seen from Fig. 7a and b that the tensile properties in course direction improve from composites with knit loops to tuck stitches, and then to weft yarns. But these properties are improved in both directions when they were used as biaxial inlays in the knit structures. Along with the reduction in glass filament breakage when knitted, the full orientation of reinforcement yarns as weft yarns and as biaxial inlays (non-crimp yarns) are cause for such superior tensile properties

in course direction of composites with weft yarns and in both directions of the biaxial inlays. For composites with tuck stitches and knit loops, the effect of inferior tensile properties can be surmised as the product of the already mentioned curvilinear shapes which seem to cause damage to the reinforcing glass filaments and a lack of proper fiber orientation. Keeping in view the analysis of flexural properties from Fig. 7c and d, the overall comparison of the knit structures can be ranked as most advantageous in the course direction and in both directions for composites with reinforcement weft yarns and biaxial inlays, respectively, and modestly advantageous for the tuck stitches. The flexural properties in course direction of composites with knit

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loops differ by 65%, 130% and 200%, respectively, as compared to tuck stitches, biaxial inlays and weft yarns. However, very inferior flexural properties were recorded in the wales direction for knit loops, tuck stitches and the weft yarns, whereas these properties are found improved in the wales direction for biaxial inlays. These effects are endorsed by the already mentioned combined effect of orientation and damage of reinforcement glass filaments. On the other hand, the results pertaining to the impact testing do not exhibit the same trends. The Fig. 7e and f elaborates the measured trends. The maximum impact strength and energy absorption were recorded for weft yarns in fiber orientated course direction as usual, whereas tuck stitches in course direction show better impact properties nearly equal to the weft yarns. However, for the composites with biaxial inlays these properties are not as significant as they were in tensile and flexural testing. From the above results a view point could be forwarded, with the impact of impactor, the specimens experience slow displacements and contact forces reach their maximum levels in course direction due to good toughness of composites with reinforcement yarns as weft yarns and as tucks stitches. Generally, composites absorb energy during fracture mechanisms like delamination, shear cracking and filaments breakage. The presence of reinforcement filaments resists the deformation of the specimens leading to improved impact strength and energy absorption in filaments direction. Since the reinforcement yarns were integrated as higher courses per centimetre in the knit structures with weft yarns and tuck stitches, superior impact properties of their composites were expected in course direction. Considering the analysis of the mechanical properties of 2D composites, the integration of reinforcement yarns as multi-layer – biaxial inlays into the 3D spacer fabric could be ranked as the most effective method. However, the integration of reinforcement yarns as tuck stitches could be the substitute method for the medium level of composite properties if the biaxial reinforcements are not possible. Since 2D composites are considered to be the reflectors of the mechanical properties of 3D composites, superior mechanical properties are expected from 3D composites using 3D flat knitted spacer fabrics including multi-layer (biaxial inlays) reinforcements using GF–PP hybrid yarns. 4. Conclusion U-shaped 3D spacer fabrics were developed by flat knitting using GF–PP hybrid yarns in a single stage manufacturing process for lightweight application. Reinforcing yarns were successfully integrated into the 3D spacer fabric structures as knit loops, tuck stitches, weft yarns and as multi-layers (warp and weft yarns). 3D thermoplastic composites were also effectively produced from U-shaped 3D spacer fabrics. In order to investigate the most competent fiber arrangements, individual 2D knit fabrics were manufactured integrating the reinforcing yarns as they were in the spacer fabrics. The tensile strengths of the reinforcement yarns after being pulled out from 2D knit fabrics and of the 2D knit fabrics were found to be better for warp and weft inlays, whereas these properties were only moderately improved for tuck stitches. The mechanical properties of 2D composites from 2D knit fabrics were also recorded and seemed to be greatly affected by different arrangements of reinforcement yarns. Tensile and flexural properties were measured and were superior in the course direction for weft yarns and in both course and wales directions for biaxial inlays. In contrast, greatly improved impact properties were documented only for weft yarns and tuck stitches in the fiber oriented course direction. Hence, the integration of reinforcement

yarns as biaxial inlays into the 3D spacer fabric seems to be the most effective method. However, the integration of reinforcement yarns as tuck stitches could be an alternative method for composites requiring only moderately improved mechanical properties. Nevertheless, the thermoplastic composites using the flat knitted multi-layer 3D spacer fabrics as preforms are expected to be promising in high performance lightweight applications. Acknowledgement This article presents a portion of the results of the research program ‘‘Textile-reinforced composite components for function-integrating multi-material design in complex lightweight applications” of the German Research Foundation (SFB 639, TP A2 & A3) at the Technische Universität Dresden. The authors would like to thank the Foundation for their financial support. References [1] Abounaim M, Hoffmann G, Diestel O, Cherif C. Development of flat knitted spacer fabrics for composites using hybrid yarns and investigation of 2D mechanical properties. Text Res J 2009;79(7):596–610. [2] Abounaim M, Hoffmann G, Diestel O, Cherif C. 3D spacer fabric as sandwich structure by flat knitting for composites using hybrid yarn, AUTEX world textile conference, Izmir, Turkey; 2009. p. 675–81. [3] Abounaim M, Hoffmann G, Diestel O, Cherif C. Flat-knitted ‘‘spacer fabrics” with hybrid yarns for composite materials. Melliand Textileb 2008;3–4:87–9. E30–31. [4] Hong H, Araujo M, Fangueiro R. 3D technical fabrics. Knit Int 1996;1232:55–7. [5] Araujo MD, Hong H, Fangueiro R, Ciobanu O, Ciobanu L. Developments in weftknitting technical textiles. In: 1st Autex conference: TECHNITEX, vol. 1. Portugal; 2001. p. 253–62. [6] Abounaim M. Modelling of technical bindings and manufacturing of flat knitted and woven ‘‘spacer fabrics” with hybrid (GF/PP) yarn as sandwich structure. Master thesis no. 1310, Technische Universität Dresden, Department of Mechanical Engineering; 2006. [7] Ciobanu L. SANDTEX – developments on knitted sandwich fabrics with complex shapes. In: 1st Autex conference: TECHNITEX, vol. 1. Portugal; 2001. p. 490–96. [8] Ünal A, Hoffmann G, Cherif C. Development of weft knitted spacer fabrics for composite materials. Melliand Textileb 2006;4:224–6. E49–50. [9] Torun AR, Paul C, Hanusch J, Diestel O, Hoffmann G, Cherif C. Reinforced weft knitted preforms and spacer fabrics as well as woven spacer fabrics made of commingled hybrid yarns for RP. In: Techtextil symposium, Frankfurt, Germany; 12–14.06.2007. [10] Cherif C, Rödel H, Hoffmannn G, Diestel O, Herzberg C, Paul C, et al. Textile Verarbeitungstechnologien für hybridgarnbasierte komplexe Preformstrukturen (textile manufacturing technologies for hybrid based complex preform structures). Kunststofftechnik (J Plast Technol) 2009;2:103–29. [11] Collaborative Research Centre SFB 639. Textile–reinforced composite components for function-integrating multi-material design in complex lightweight applications, Technische Universität Dresden, Germany. [retrieved 30.06.09. [12] Badawi SSAM. Development of the weaving machine and 3D woven spacer fabric structures for lightweight composites materials. PhD thesis, Technische Universität Dresden, Department of Mechanical Engineering; 2007. [13] Lin S, Modler KH, Hanke U. The application of mechanisms in producing textile–reinforced thermoplastic composite. Mach Des Res 2008;24:380–4. [14] Alagirusamy R, Ogale V. Commingled and air jet-textured hybrid yarns for thermoplastic composites. J Ind Text 2004;33(4):223–43. [15] Fujita A, Maekawa Z, Hamada H. Mechanical behaviour and fracture mechanism of thermoplastic composites with commingling yarn. J Reinf Plast Compos 1993;12:156–72. [16] Dev VRG, Swarna A, Madhusoothanan M. Mechanical properties of knitted composites using glass ply yarns. J Reinf Plast Compos 2005;25:1425–35. [17] Gommers B, Verpoest I, Houtte V. Analysis of knitted fabric reinforced composites: Part 1. Fibre orientation distribution. Composites Part A 1998;29A:1579–88. [18] Padaki NV, Alagirusamy R, Sugun BS. Knitted preforms for composite application. J Ind Text 2006;35(4):295–321. [19] HU H, Zhu M. A study of the degree of breakage of glass filament yarns during the weft knitting process. AUTEX Res J 2005;5(3):141–8. [20] Lau K, Dias T. Knittability of high-modulus yarns. J Text Inst 1994;85(2):173–90. [21] Savci S, Curiskis JI, Pailthorpe M. Knittability of glass fiber weft-knitted preforms for composites. TexT Res J 2001;71(1):15–21.