Page 338 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 7. Improvement of' transverse ,fracture toughness with interjace control 319

where A and h are parameters determined in the relationship between fiber strength
a; and fiber length e: or = A&-h. In theory, the fracture toughness should be at its
maximum when the shrinkage stress can just compensate the Poisson contraction
stress, which in turn allows the denominator of Eq. (7.15) to become close to zero.
The R,, versus qo plots for a CFRP and a GFRP given in Fig. 7.24 suggest that the
fracture toughness can be improved by reducing qo without deteriorating other
mechanical properties. Bailey et al. (1977) have been successful in developing special
expanding monomers for such applications. Epoxy resins are copolymerized with
approximately 5-10% dinorbornene (or tetramethyl) spiro ortho carbonates that
balance the shrinkage of resin by expansion through ring opening reactions. In a
series of experimental studies (Lim et al., 1984; Lam and Piggott, 1989a, b, 1990),
the residual thermal stresses in a CFRP have been reduced successfully with such
special expanding monomers. The composites made therefrom have one-third of the
usual residual stress with some 50% improvement of Izod impact toughness and a
slight reduction in ILSS, as shown in Fig. 7.25. The addition of the expanding
monomers also gives improved fatigue properties and better resistance to water
absorption of the CFRP.
Unlike carbon fiber composites, the expanding monomers demonstrate little
beneficial effect on GFRPs and KFRPs: it actually decreases the toughness by about
30% for GFRP (Lim et al., 1984). It appears that there are many unresolved issues
regarding the effects of important parameters, and there must be an optimum value
for the fiber clamping stress, 40, which would maximize the fiber pull-out toughness.
If qo is too high, interfacial debonding and subsequent fiber pull-out would be
inhibited; whereas if qo is too low, only a small frictional energy is dissipated,
because qo controls directly these failure mechanisms through its influence on the

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