Page 378 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of inrerlaminar fracture toughness uvirh interface control 359

Fig. 8.29. Delamination patterns of a carbon fiber-epoxy matrix composite with varying stitch densities.
Aftcr Liu (1 990).

the CAI test is controlled largely by the damage induced by the impact itself rather
than by the failure process during compressive loading. Furthermore, the compres-
sive strength of stitched composites is sometimes lower than the unstitched
counterpart even in the undamaged state (Farley et al., 1992; Reeder, 1995). This is
attributed to the damage of the in-plane reinforcement fibers and the high stress
concentration around the stitch holes, as mentioned before. Although the removal
of the surface loops of the stitch strands shows no direct effect on the prominent
failure mechanisms, it increases significantly the compressive strength both before
and after impact loading (Farley and Dickinson, 1992).
The anomalous behavior of stitched carbon fiber composites with respect to
impact damage response mentioned in the above paragraph deserves more attention.
Similar observations on Kevlar stitched glass fiber composites show no beneficial
effects of stitching in terms of impact damage resistance for thin composite
laminates (less than a few mm). However, for thick laminates (-larger than 20 mm)
through-the-thickness stitching produces much better damage control than un-
stitched laminates (Mouritz, 1997). This brings up a “size” effect on stitched

400
h
3
)r
300
C
a
-0
e 200
$
n
- 100
rd
+
e
unstitched 51mm 25mm 13mm
Stitch spacing

Fig. 8.30. Effect of stitch spacing on total absorbed energy after impact for different stitched composites.
After Kang and Lee (1 994).