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Stitched Composites 179

8.3.5 Fatigue Properties of Stitched Composites
An understanding of the fatigue endurance of stitched composites is essential because of
their potential applications in structures that are subject to long-term fluctuating loads,
such as aircraft wing panels and beams for civil infrastructure. The fatigue performance
of stitched composites has been determined for a variety of cyclic load conditions,
including repeated compression-compression and tension-tension loading.
The effect of stitching on the fatigue properties of composites under cyclic
compression loads has been examined in great detail, particularly the stitched quasi-
isotropic carbodepoxy laminates that may be used in future aircraft (Dow and Smith,
1989; Furrow et al., 1996; Lubowinski and Poe; 1987; Portanova et al., 1992;
Vandermey et al., 1991). Stitching is generally found to reduce the compression fatigue
life of composites, with no study yet to observe an improvement to fatigue performance.
A typical example of the effect of stitching on the compression fatigue-life (or S-N)
curve for a carbodepoxy composite is shown in Figure 8.12 (Portanova et al., 1992). It
is seen that stitching reduces the fatigue life of the composite, and the reduction is due
mainly to fibre distortion and crimping caused by stitching. Under cyclic compression
loading it is observed that fatigue damage initiates at the fibres close to those stitches
that have experienced the greatest out-of-plane distortion. It is believed that the fibre
distortion promotes the early formation of fatigue-induced kink bands, which gradually
rotate to even greater misalignment angles under continued fatigue loading until
catastrophic failure.

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Figure 8.12 S-N curves for stitched and unstitched carbodepoxy composites fatigued
under compression-compression loading ( Data from Portanova et al., 1992).