Fatigue crack propagation of rubber‐toughened epoxies

Epoxies containing epoxy‐terminated butadiene acrylonitrile rubber (ETBN) or amino‐terminated butadiene acrylonitrile rubber (ATBN) were prepared and studied in terms of fatigue crack propagation (FCP) resistance and toughening mechanisms. Rubber incorporation improves both impact and FCP resistance, but results in slightly lower Young's modulus and T g As T g increases, the degree of toughening decreases. Rubber‐induced shear yielding of the epoxy matrix is believed to be the dominant toughening mechanism. Decreasing fatigue resistance with increasing cyclic frequency is observed for both neat and rubber‐toughened epoxies. This result may be explained by the inability of these materials to undergo possible beneficial effects of hysteretic heating. FCP resistance is linearly proportional to M c 1/2 , where M c is the apparent molecular weight between crosslinks determined on the rubber‐toughened material. FCP resistance also increases with increasing static fracture toughness K IC . ATBN‐toughened epoxies demonstrated better fatigue resistance than ETBN‐toughened systems.