Fatigue of rubber-modified epoxies: Effect of particle size and volume fraction

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<Delta K-T, both neat and rubber-modified epoxies exhibit similar FCP resistance because the plastic zone size is smaller than the size of the rubber particles and hence, the rubber cavitation/shear banding and plastic void growth mechanisms are not operating. As a result of these interactions, the use of smaller 0.2 mu m rubber particles in place of 1.5 mu m rubber particles results in about one order of magnitude improvement in FCP resistance of the rubber-modified system, particularly near the threshold regime. Such mechanistic understanding of FCP behaviour was employed to model the FCP behaviour of rubber-modified epoxies. It is shown that the near threshold FCP behaviour is affected by the rubber particle size and blend morphology but not by the volume fraction of the modifiers. On the other hand, the slope of the Paris-Erdogan power law depends on the volume fraction of the modifiers and not on the particle size or blend morphology.