A change in crack-tip plastic zone/rubber particle interactions induces a transition in the fatigue crack propagation (FCP) behaviour of rubber-modified epoxy polymers. The transition occurs at a specific K level, K-T which corresponds to the condition where the size of the plastic zone is of the order of the size of the rubber particles. At Delta K>Delta K-T, rubber-modified epoxies exhibit improved FCP resistance compared to the unmodified epoxy,This is because the size of the plastic zone becomes large compared to the size of the rubber particles and, consequently, rubber cavitation/shear banding and plastic void growth mechanisms become active. At Delta K

Keywords:ELASTOMER-MODIFIED EPOXIES;TOUGHENING MECHANISMS;CRACK-PROPAGATION;PROPERTY RELATIONSHIPS;COMPOSITES;POLYMERS;TOUGHNESS;BEHAVIOR;FAILURE;MODEL