Constituent and composite quasi-static and fatigue fracture experiments

Quasi-static and fatigue Mode I fracture experiments were performed at a variety of loading rates and temperatures using a polymer-matrix composite (PMC) (IM7/977-3) and its neat resin (977-3). Neat resin quasi-static toughness showed slight increases in toughness with increasing loading rate and no dependence on temperature until the test temperature neared the glass transition temperature, where the toughness decreased. Composite quasi-static delamination initiation toughness showed no clear trends with respect to temperature or loading rate, but toughness values were generally higher at higher temperatures. Composite fatigue delamination onset and crack propagation rates were strongly dependent on temperature, with delamination onset occurring at a lower number of cycles and crack propagation rates being higher for a given maximum applied strain energy release rate. The neat resin fracture mechanisms were a key factor in explaining the composite quasi-static and fatigue behavior, but no quantitative predictions of composite fracture behavior could be made since the crack tip mechanisms apparently changed.