Page 371 - Engineered Interfaces in Fiber Reinforced Composites

3 52 Engineered interfaces in fiber reinforced composites

plate

Fig. 8.20. Schematic presentation of three dimensional braiding. After Du et al. (1991).

The process of stitching uses the conventional technology to convert two-
dimensional preforms to three-dimensional ones. The types of stitch strand
materials, stitch density, the size of the stitch strand, and the types of stitching
method determine the final stitch preform. Kevlar fiber strands are among the most
popular due to their flexibility which is required to bend into a small curvature in the
needle hole. There are two types of stitching methods, namely lock stitch and chain
stitch (Fig. 8.22). A lock stitch tends to become unbalanced because of the high
tension in the bobbin thread or the needle thread. Fig. 8.23 shows a lock-stitching
proccss for bonding woven fabric layers.
The unlimited variability of the geometric forms which can be obtained using the
knitting technique is especially useful for producing preforms with complex shapes.

Fig. 8.21. Schematic presentation of an orthogonal woven fabric. After Chou et al. (1986).