Page 195 - 3D Fibre Reinforced Polymer Composites

184 30 Fibre Reinforced Polymer Composites
The length of a stitch bridging zone is dependent on a number of factors, such as the
stitch density, stitch thickness, elastic modulus and tensile strength of the stitch yarn.
For most types of stitched composites the stitch bridging zone is between 10 and 50 mm
long. In the case of the stitched glass/vinyl ester composite with the R-curve shown in
Figure 8.15, the length of the stitch bridging zone is about 20 mm. The initial rapid rise
of this R-curve over the first 20 mm of crack extension is due to the formation of the
stitch bridging zone, and once it is fully developed the R-curve remains constant. At the
rear of the stitch bridging zone the crack opening displacement becomes large enough to
cause the stitches to fail. Failure occurs by stitches breaking in the plane of the crack
(as shown in Figure 8.18a) or breaking away from the crack plane and then being pulled
out of the composite (Figure 8.18b).

Figure 8.18 Scanning electron micrographs showing (a) the tensile failure of a stitch
and (b) a broken stitch that has been pulled out of the composite (from Watt et al, 1997).