Page 377 - Engineered Interfaces in Fiber Reinforced Composites

358 Engineered interfaces in fiber reinforced composites

Wang, 1995; Herszberg et al., 1996; Leong et al., 1996; Furrow et al., 1996). The
varying roles of reinforcement architecture including fiber stitching has been
reviewed (Bibo and Hogg, 1996; Kim, 1997) on the impact response of the
composites. Laminates containing carbon fiber woven fabrics have also shown to
provide higher impact damage resistance compared to those made with prepreg
cross-plies (Kim et al., 1996). Numerous impact data of stitched and unstitched fiber
composites of various constituent combinations consistently show that the extent of
damage as measured from the damage area is less and the CAI strength is higher for
the stitched fiber composites (Liu, 1990; Kang and Lee, 1994; Adanur et al., 1995;
Wu and Wang, 1995). This result is particularly true when the major damage mode
during impact and the fracture mode in the subsequent CAI test are induced mainly
by delamination. Fig. 8.28 clearly indicates that the decrease of damage area is
proportional to the increase in stitch density (Liu, 1990). It appears that the
optimum stitch density has not been reached in this particular study. Close
examination of the damage surface reveals that some stitching points coincided with
the delamination boundary which is indicative of such stitch strands acting as
delamination arresters, as shown in Fig. 8.29. Fig. 8.30 also displays the strong
dependence of total energy absorbed by stitched laminates on stitch spacing, type of
stitches and matrix material.
In some isolated cases, the stitching technique provides no beneficial effects on the
impact resistance of carbon fiber-epoxy matrix composites (Herszberg et al., 1996;
Leong et al., 1996). When orthotropic laminates are subjected to drop weight impact
or projectile impact under tension, the damage area and the CAI are very similar
between the composites with and without stitches. This disappointing result is
thought to be associated with the excessive stitch density and the unfavorable failure
modes, such as transverse shear, of the stitched specimens during impact. The
stitched composites containing such transverse shear cracks tend to fail by shear
under compression, resulting in a lower CAI strength than the unstitched
composites. It should be mentioned here that the residual compressive strength in

Stitch density (cm-2)

Fig. 8.28. Normalized delamination area due to impact as a function of stitch density for a carbon fiber-
epoxy matrix composite. After Liu (1990).