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Stitched Composites 183

stitches are not noticeably damaged or deformed, and when a crack reaches the stitches
it passes around without causing the threads to break. Thereby, a zone is developed
behind the crack tip where unbroken stitches bridge across the delamination. Within
this so-called ‘stitch bridging zone’ the stitches are able to support a large amount of the
applied stress by elastic stretching because of the high modulus and strength of the
thread material. As a result, the tensile stress acting on the crack tip is reduced, and this
slows the crack growth rate. A photograph of a stitch bridging a delamination crack is
shown in Figure 8.17.

delamination needle
crack thread

bobbin
thread

Figure 8.16 Schematic diagram of the mode I interlaminar toughening process in
stitched composites (From He and Cox, 1998).

Figure 8.17 Scanning electron micrograph showing a stitch bridging a delamination
crack (from Watt et al., 1997).