Page 54 - 3D Fibre Reinforced Polymer Composites

Manufacture of 30 Fibre Preforms 43

Figure 2.33 Illustration of complex preform manufacture via stitching

There are disadvantages with the stitching process, the main one of which is a reduction
of the in-plane properties of the resultant composite component (i.e. tension,
compression, shear, etc.). As the needle penetrates the fabric it can cause localised in-
plane fibre damage and fabric distortion which has been found to reduce the mechanical
performance of the composite (Mouritz et al., 1997; Mouritz and Cox, 2000). This
reduction in performance can be aggravated by the surface loop of the stitch, which can
also crimp the fabric in the thickness direction if the tension in the stitch thread is high.
The presence of the stitch thread and the distortion in the fabric that it creates also
causes a resin-rich pocket to be formed within the composite. This pocket can act as a
potential crack initiator, which can possibly affect the long-term environmental
behaviour of the material. More detail on the damage caused during stitching and the
mechanical performance of stitched composites can be found in Chapter 8.

2.5.2 Technical Embroidery

A version of stitching which can be used to provide localised in-plane reinforcement
together with through-thickness reinforcement is technical embroidery. In this process a
reinforcement yarn is fed into the path of the stitching head and is stitched onto the
surface of the preform (see Figure 2.34). With current computer controlled embroidery
heads it is possible to accurately place this in-plane yarn in quite complex paths, which
allows high stress regions of a component to be reinforced by fibres laid in the
maximum stress direction.
Although this technology appears best suited for the placement of localised
reinforcement, the technical embroidery technique can also be used to construct