Page 368 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interlaminar fracture toughness with interface control 349

Laminates with interleaves also enhance, to a great extent, the damage resistance
and tolerance under impact loading in terms of both damage area and residual CAI
(Masters, 1989; Sun and Rechak, 1988; Rechak and Sun, 1990; Lu et al., 1995). The
role of the thin discrete ductile resin layer which is placed on one side of standard
prepreg tapes is to alter the failure mode by allowing the transverse cracks and
delamination to be arrested upon reaching the interleave. Fig. 8.17 shows the cross-
sections of AS4/0808 carbon-epoxy laminates with and without thermoplastic
interleaves which have been impacted at 3.56 kJ/m and 8.9 kJ/m per unit laminate
thickness, respectively. The corresponding plots of delamination size versus impact
energy for these laminates are shown in Fig. 8.18 (Masters, 1989). The micrographs
clearly indicate that in the laminates without interleaves, a series of transverse cracks
occur with extensive delamination, the number of these cracks increasing with
impact energy. Delamination appears to have initiated at the intersection of the
transverse crack and the laminar interface (Masters, 1987a). A triangular form of no

IMPACT SITE (3.56 Wlm)
f

IMPACT SITE (8.9 Wlm)
$.

+
IMPACT SITE (3.56 Wlm)

IMPACT SITE (8.9 kJlm)
f

Fig. 8.17. Microphotographs of interply cracking and delamination after impact in carbon fiber-epoxy
matrix composites (a) without and (b) with interleaving layers. After Masters (1989). Reproduced by
permission of Trans Tech Publications Ltd.