Page 351 - Engineered Interfaces in Fiber Reinforced Composites

332 Engineered interfaces in jiber reinforced composites

8.2.2. Correlations between matrix properties and composite interlaminar properties

Since the development of rubber-toughened epoxy reins, a large volume of
information has appeared in the literature, addressing the advantages and
drawbacks of these materials as matrices in composites. Carbon fibers of various
types have been used as principal reinforcements for composites in aerospace
applications. Modification of matrix materials allows the aforementioned failure
mechanisms to occur more extensively, which are not present or insignificant in
unmodified matrix composites, along with some indirect influences on the fiber-
matrix interfacial properties. A comprehensive summary has been presented on the
relationship between matrix toughness and composite interlaminar fracture tough-
ness (Bradley, 1989a, b, 1990).
The general observation is that improvement in interlaminar fracture toughness
of carbon fiber composites containing such toughened resins has been rather
disappointing. Although rubber-modified epoxy shows up to twentyfold increase in
fracture toughness of bulk resins, G;",, it imparts only a moderate eightfold
improvement in mode I interlaminar fracture toughness of composites, Gf,, with
fiber volume fraction yf greater than 55% (Hunston et al., 1987; Jordan and
Bradley, 1987, 1988). This result is in sharp contrast to the fact that for brittle
matrices the composite qc is somewhat larger than the resin ct", (Hunston et al.,
1987). A compilation of the data published for the relationship between the
composite mode I interlaminar fracture toughness, G;,, and the neat resin fracture
toughness, GK, is presented in Fig. 8.2. (Hunston et al., 1987; Russell and Street,
1987; Jordan and Bradley, 1988; Bradley, 1989a; Kim et al., 1992). The composite
GFC values represent those obtained for the steady-state crack growth rather than for

"0 2 4 6 0 10
Neat Resin GIE (kJ/rn2)

Fig. 8.2. Composite mode I interlaminar fracture toughness, GF,, as a function of respective neat resin
toughness, C;.,: (0) Kim et al. (1992); (0) from Russell and Street (1987); (0) toughened thermosets and
(m) thermoplastics from Hunston et al. (1987); (A) from Bradley (1989a); (*) from Jordan and Bradley
(1988).