Page 211 - 3D Fibre Reinforced Polymer Composites

200 30 Fibre Reinforced Polymer Composites

projectile, resulting in higher post-impact mechanical properties than the unstitched
laminate. The effect of the amount of stitching on the compression-after-ballistic
impact strength of a carbodepoxy composite is shown in Figure 8.34. The strength
values shown were determined after a tumbling 12.7 mm projectile travelling at high
speed had perforated the composite. The post-impact strength is seen to rise steadily
with the volume percent of stitching, and this clearly demonstrates that stitching is an
effective technique in improving the ballistic impact damage tolerance of composite
materials.

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300
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275
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250
225 - I
200 - 8 rc
175 -
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150 W
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125
100 1 # 1 . I . I . I . I

Figure 8.34 Effect of stitching content on the compression-after-ballistic impact
strength of a carbodepoxy composite (Data from Keith, 1999).

8.5.3 Blast Damage Tolerance
The potential use of stitched composites in military structures has led to an evaluation
of their damage tolerance to explosive blasts (Mouritz 1995a, 1995b, 2001). Blast
studies have revealed that stitching is highly effective in reducing the amount of
delamination damage caused by the shock wave from an explosion. For example,
Figure 8.35 shows the effect of stitch density on the amount of blast damage and the
flexure-after-blast strength of a composite (Mouritz, 2001). The results shown are for
the composite subject to a medium and high intensity explosive blast. It is seen that the
amount of delamination damage decreases with increasing stitch density, and this
results in the stitched composites having similar or higher post-blast flexural strengths
than the unstitched laminate. The superior ballistic and explosive blast damage
tolerance properties of stitched composites indicate that these materials are ideally
suited for use in military aircraft.