Page 176 - 3D Fibre Reinforced Polymer Composites

Stitched Composites 165
fibres as it pierces the preform. Sewing prepreg tape can be more difficult because the
inherent tackiness of the uncured resin matrix impedes the needle action. The materials
most often used as the reinforcing threads for stitching are carbon, glass and Kevlar
yarns, although it is possible to sew with other types of fibrous materials including
Spectra@ and high strength thermoplastics. The yarns can be sewn into the preform in a
variety of patterns, with the most common types being the lock stitch, modified lock
stitch and chain stitch. These three stitch types are shown in Figure 8.2 (Morales,
1990). The standard lock and chain stitches are used occasionally, but the most popular
stitch style is the modified lock stitch in which the loops crossing the needle and bobbin
threads are formed at one surface of the composite to minimise in-plane fibre distortions
inside the material.

Figure 8.1 Large stitching machine used to stitch composite wing panels (From
Beckwoth & Hyland, 1998).

When composites are stitched the through-thickness threads can be inserted in any
number of patterns. Examples of stitch patterns used to reinforce composites are shown
in Figure 8.3, and of these the most popular pattern is horizontal stitching (Dransfield et
al., 1994). During the stitching process the threads are usually placed close together to
ensure high damage tolerance, and most composites are reinforced with 1 to 25 stitches
per cm2. This is equivalent to a fibre volume content of stitched threads of about 1% to
5%. This is a similar volume content for the through-thickness reinforcement in many
3D woven, braided and knitted composites. It is often difficult to stitch composites at
higher densities because of the excessive amount of damage caused to the preform.