Page 187 - 3D Fibre Reinforced Polymer Composites

176 30 Fibre Reinforced Polymer Composites

crazing. This damage permits further rotation of the fibre bundle until it becomes
axially unstable and a kink band is formed. Mouritz and Cox (2000) suggest that a kink
band will initiate in a single region of a stitched composite that has suffered the highest
degree of fibre distortion. Since the spread of a kink band is usually unstable, a stitched
composite may fail before other kink bands form near stitches that have caused less
distortion. This process accounts for the observation in Figure 8.8b that the reduction to
compression strength is not affected by stitch density, because even the most lightly
stitched materials still have severely crimped fibres.
Stitching is expected to have a beneficial effect on the flexural properties of
composites because it suppresses the growth of delaminations formed under bending,
and thereby increases the strength. However, this is believed to be offset by damage
incurred with stitching, particularly the distortion and breakage of fibres, which lowers
the flexural properties. In some types of stitched composites, the distortion of fibres
close to the constrained laminate surface can cause bending-induced compression
failure at a reduced flexural stress. In other stitched laminates, the clusters of broken
fibres close to the stitches leads to fibre fracture on the tensile side of a flexural
specimen. From existing data and limited observations, it appears that there is
competition between the failure mechanisms within stitched composites. That is,
stitching suppresses delamination cracking which can raise the strength, but stitching
also facilitates compression failure and tensile rupture that can lower the strength. The
competition between these different mechanisms is probably a close one, and this would
account for the modest reduction to the flexural properties with stitching.

8.3.3 Interlaminar Shear Properties of Stitched Composites
The interlaminar shear properties of stitched composites have not been extensively
evaluated, and as yet the effect of stitching on these properties is not fully understood.
From published research it appears that the interlaminar shear strength, like the tension,
compression and flexure properties, can be improved and degraded by stitching
(Mouritz et al., 1997). For example, Figure 8.9 shows the effect of stitch density on the
interlaminar shear strengths for Kevladepoxy and carbodepoxy composites that have
been stitched using Kevlar yarn (Jain and Mai, 1997; Kang 8z Lee, 1994). The
interlaminar shear strength of the Kevlar/epoxy composite increases steadily with stitch
density, and this is attributed to the suppression of interlaminar cracking by the stitches.
The strength of the carbodepoxy composite, on the other hand, drops slightly when
stitched, although the strength does not appear to be affected significantly by the stitch
density.
Figure 8.10 presents interlaminar shear strength data for a variety of composites
plotted against stitch density. The data was collected from various published sources by
Mouritz and Cox (2000). The normalised interlaminar shear strength (z/zo) is defined as
the strength of the stitched composite (z) divided by the strength of the equivalent
unstitched laminate (a). The range of enhancement or degradation of interlaminar
shear strength is about 15-20%, which is similar to the improvement or reduction to the
tension, compression and flexure properties of stitched composites.
The improvement to the interlaminar shear strength is probably due to the stitches
inhibiting the delamination crack growth process (Cholarkara, 1989; Mouritz and Cox,
2000). It is well known that as an interlaminar crack grows through a laminate, a zone
forms behind the crack tip where stitches bridge the delamination. This is known as a