Through-the-thickness stitch

This section examines the advantages and disadvantages of using three-dimensional textile preforms, especially through-the-thickness stitches, as the reinforcements for composites. Their major mechanical properties are compared with those of conventional two-dimensional composites, such as strength, stiffness, interlaminar properties, impact resistance and tolerance, etc. Dransfield et al. (1994) have recently given a useful review on the improvement of interlaminar fracture toughness of stitched composites. 

Effect of through-the-thickness stitches on flexural strength and Mode I interlaminar fracture toughness of carbon fiber-epoxy matrix composites manufactured using unidirectional prepregs". 

Farley (1992) has made an in-depth study of the negative effect of fiber misalignment. Fig. 8.26 shows the gross in-plane waviness created by through-the-thickness stitches. It is also reported that many microcracks are created around the stitch strands, although the microeracks appear not to have propagated under combined temperature and humidity cycles (Furrow et al., 1996). 

Fig. S.26. In-plane fiber waviness created by through-the-thickness stitch strands. After Farley (1992).

The increases in through-the-thickness reinforcement achieved by NCFs have been demonstrated by a number of authors. For example. Backhouse et aV compared the ease of delaminating polyester stitched 0/ 45 carbon fibre NCF with equivalent carbon fibre/epoxy UD laminates. There were large increases, some 140%, in the measured... 

Three dimensional structures of multiaxial warp knitted fabrics have been recently developed for multidirectional reinforcement of composites. Multilayers of linear yams are assembled in warp (0°), weft (90°) and bias ( 0) directions to provide in-plane reinforcement in specific directions and they are stitched together by knitting yams to provide structural integrity and through the thickness reinforcement [1,2,3,4,5]. 

The most important innovation is the introduction of a preparatory production step, called tailored reinforcement (TR). All of the required elements are integrated into the preform by means of a special stitching type, e.g., structural 3D or TT (through the thickness) reinforcement, load-bearing elements, etc ... 

Stitching. The benefit of through-the-laminate-thickness stitching has already been demonstrated to lock the laminate plies together and to reduce the propensity for transverse tensile failure... 

The RTM process can be spht into four steps as depicted in Fig. 7.1. First a stack of dry reinforcements is assembled into a preform, often formed into the shape of the final part and held together with some form of binder or stitching. This preform is placed into the mould (Fig. 7.1(a)) and the mould is then closed to the desired final cavity thickness (Fig. 7.1(b)). After closing the mould, a liquid thermoset resin is injected into the preform through one or more inlet ports (Fig. 7.1(c)). Resin cute is then initiated and completed either by implementing a thermal cure cycle or by waiting the desired amount of time before demoulding the manufactured part (Fig. 7.1(d)). 

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