Page 209 - 3D Fibre Reinforced Polymer Composites

198 30 Fibre Reinforced Polymer Composites

The ability of stitching to reduce the amount of damage improves not only with the
incident impact energy. The effectiveness of stitching also improves dramatically with
stitching density, as shown in Figure 8.32 (Liu 1990). In the figure the normalised
delamination area defines the amount of impact damage to the stitched composite
divided by the amount of damage to the equivalent unstitched laminate. There is a rapid
reduction to the amount of impact damage with increasing stitch density, and in this
case it is seen that stitching reduced the delamination area by as much as 40% compared
with the unstitched laminate.

0.4 -

0.2 -

0.0 l . I , I . l , I , l , I
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Stitch Density (crn-*)

Figure 8.32 Effect of stitch density on the amount of impact damage to a glass/epoxy
composite. The composite was impacted at an energy of about 7.5 Jlmm (Data from
Liu, 1990).

The improved damage resistance provides stitched composites with higher post-impact
mechanical properties than the unstitched material. For example, Figure 8.33 (Rossi,
1989) compares the compression-after-impact strengths of a stitched and unstitched
carbodthermoplastic composite. It is seen the compression-after-impact strength of the
stitched composite is slightly higher. The higher post-impact strength is attributed to
two factors: firstly, the amount of delamination damage in lower in the stitched
material, and secondly, the stitches suppress the growth of the delaminations and inhibit
sublaminate buckling under compression loading.
Models for estimating the compression-after-impact strength of stitched composites
have not yet been formulated because of the complexity of modeling the growth of
multiple delaminations and the subsequent multiple sublaminate buckling processes that
can occur under compression. However, models have been developed for predicting the
compression strength of stitched laminates containing a single delamination (Shu and
Mai, 1993a, 1993b). These models provide insights into the effectiveness of stitching in