Page 358 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interlaminar fracture toughness with interface control 339

8.2.3. Impact resistance and tolerance of fiber composites with tough matrices

Interest in matrix resin fracture toughness in relation to interlaminar fracture
toughness of fiber composites is due to their predominant effect on the post-impact
residual mechanical properties, particularly the compressive strength-after-impact
(CAI), stiffness and fatigue strength. A number of researchers have studied the
impact damage resistance and damage tolerance of various thermosets with and
without modifications and of thermoplastic resin systems (Williams and Rhodes,
1982; Hirschbuehler, 1987; Evans and Masters, 1987; Sohi et al., 1987; Bowles, 1988;
Poon et al., 1990; Recker et al., 1990; Bradley, 1990; Kim et ai., 1993; Srinivasan,
et al., 1992; Ishikawa et al., 1995). Rubber-modified epoxies in general have better
resistance to impact damage than their unmodified counterparts. The CAI test is a
standardized test in aerospace industry, which has been developed to characterize
the damage tolerance of composite materials. This test has two steps: an impact
damaged composite panel is loaded in compression to measure the residual
properties. The impact test is largely a mode I1 high shear rate crack propagation
test leading to multiple delamination, while the compression test causes further
growth of delamination cracks by macrobuckling in a dominant opening mode I
fracture.
Low velocity drop-weight impact tests on laminated panels in the thickness
direction have shown that the modified matrix composite system absorbs inelastic
energy by a damage process involving delamination and intralaminar transverse
shear cracks which produce barely visible impact damage. The characteristic load-
displacement records obtained from the test also show that the toughened resin
systems absorb much higher elastic energy than unmodified resin systems (Poon et
al., 1990). In contrast, for the same input impact energy, unmodified matrix
composite systems fails by fiber and matrix fractures which coalesce to form a major
through-the-thickness crack with extensive delamination in every ply of the
laminate. The damage area is shown to be significantly smaller for the modified
resin systems than the baseline epoxy resin system. (Recker et al., 1990; Sohi et al.,
1987; Srinivasan, et al., 1992). Fig. 8.7 shows representative C-scan damage area
data plotted as a function of impact energy for several different composite systems
containing thermosets and thermoplastic resins, clearly indicating the advantages of
tough resin systems. Further, the residual CAI strength is also shown to be much
higher for the composites with modified epoxies and thermoplastic matrices than the
unmodified epoxy system (Hirschbuehler, 1987; Recker et al., 1990; Kim et al.,
1993).
One of the most important properties which control the damage tolerance under
impact loading and the CAI is the failure strain of the matrix resin (see Fig. 8.8).
The matrix failure strain influences the critical transverse strain level at which
transverse cracks initiate in shear mode under impact loading, and the resistance to
further delamination in predominantly opening mode under subsequent compressive
loading (Hirschbuehler, 1987; Evans and Masters, 1987; Masters, 1987a, b; Recker
et al., 1990). The CAI of near quasi-isotropic composite laminates which are
reinforced with AS-4 carbon fibers of volume fractions in the range of 65-69% has