Page 357 - Engineered Interfaces in Fiber Reinforced Composites

338 Engineered interfaces in fiber reinforced composites

shown to play an important role for stress distribution at the crack tip which
controls the composite qc (Lee, 1984, 1987).
Addition of rigid fillers along with rubber modification of epoxy resins produce a
synergic effect to enhance the interlaminar fracture resistance of carbon fiber
composites (Kim et al., 1992). Fig. 8.6 shows the crack growth resistance curves (R-
curves) of mode I interlaminar fracture toughness, GYc, for composites containing
several different modified matrices, taking into account the residual displacement
effect caused by non-elastic deformation. Epoxy resins are modified with combi-
nations of CTBN rubber, A1203 particles and A1203 short fibers before being cured
with piperidine. The rubber-modified matrix improves the crack growth resistance
of the composite about 100% compared to the control. Simultaneous presence of
rigid fillers and rubber phase increase the crack growth resistance even further,
outperforming the composites with rubber phase only, particularly at large crack
extensions. These results have been explained in terms of the compensating effects of
rigid fillers (which decrease) and rubber phase (which increases) on the plastic
deformation of the matrix material. Toughening mechanisms which occurs in the
presence of rigid fillers, such as interfacial debonding and fiber bridging by short
A1203 fibers, pinning by A1203 particles and increase in fracture surface area due to
the irregular crack path, effectively improve the gross crack growth resistance. At
the same time, the rigid fillers reduce the matrix non-linear failure strain thus
limiting its plastic deformation. However, this loss in toughness is more than
compensated by the other toughening mechanisms of the fillers. It is suggested that
when both rubber and rigid fillers are present synergism of toughening mechanisms
by both these modifiers takes place.

2.
x

1
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Fig. 8.6. Mode I potential energy release rate, OR, plotted as a function of crack extension, h, for carbon
fiber composites containing different matrices: E (pure epoxy); ER (rubber-modified epoxy); ERF (short
fiber-modified epoxy); ERP (rubber-and particle-modified epoxy). After Kim et al. (1992).