Crack extension resistance of unidirectional fiber-reinforced composites and a cohesive force model of crack surface interactions.

Abstract

Crack extension resistance in fiber-reinforced composites is considered from the view point of crack surface interactions, which may be induced by some micromechanisms such as fiber bridging behind the crack tip. Using a concept of interactive stress, equivalent cohesive stress is evaluated as a function of the relative displacement between upper and lower crack surfaces. With some experimental results of crack extension parallel to fibers in a unidirectional continuous fiber-reinforced epoxy composite, we conclude that this inherent relationship between cohesive stress and relative crack surface displacement completely determines the crack extension behaviour of the composite. We also compare opening mode crack extension with in-plane-shear mode crack extension, and find that in-plane-shear mode cohesive stress is much larger than that of opening mode. Furthermore, in-plane-shear mode cohesive stress is shown to decline sharply under large relative crack surface displacement, while opening mode cohesive stress tends to remain a constant value. This difference of cohesive stresses accounts for the characteristic behaviour of crack extension in each mode.