REVIEW OF THE MICROSCOPIC AND MACROSCOPIC ASPECTS OF FRACTURE OF UNMODIFIED AND MODIFIED EPOXY-RESINS

Epoxy resins are useful as adhesives and as matrix materials in advanced composites. However, unmodified (or neat) epoxies are brittle and their low fracture energy causes serious concern in some applications. For this reason, modified epoxies have been developed by the addition of a rubbery or a rigid dispersion phase, or both, and considerable improvement in fracture properties has been achieved. In this paper, the deformation and failure processes involved in the fracture of unmodified epoxies have been discussed, and a review of the fracture behaviour of the diglycidyl ether of bisphenol A polymers modified with carboxyl-terminated butadiene acrylonitrile is presented together with illustrations of the microscopic aspects of localized deformation and their relationship to the macroscopic fracture behaviour. The degree of improvement in fracture properties in modified materials depends to a great extent on the unmodified epoxy. If the latter is capable of even small-scale plastic deformation at the crack tip, then this induces in the modified system a number of additional microscopic failure mechanisms such as cavitation of rubber particles, enhanced shear deformation of the matrix, debonding and tearing of rubber, crack pinning, and debonding and pull-out of fibres. © 1990.