Failure mechanisms in toughened epoxy resins—A review

The subject of improving the fracture toughness of brittle epoxy resins is receiving significant attention in order to improve the design strain of fiber-reinforced composites for aerospace structural applications. Various rubber-modified and particle-filled epoxy resins have been considered as candidate materials. Such modified resins have been observed to yield a ten- to thirty-fold increase in fracture toughness compared to the unmodified material. In order fully to utilize the potential of such materials, it is necessary to understand the failure mechanisms leading to the improvement in toughness. This paper provides a critical review of the existing theories that have been proposed for the various toughening mechanisms related to modified epoxy resins.