FATIGUE CRACK-GROWTH THROUGH ARALL-4 AT AMBIENT-TEMPERATURE

Fatigue cracks were grown in the 5 layer aluminum alloy-Aramid fiber laminate composite ARALL-4 over the range of cyclic stress intensity factors (DELTA-K) from 3.5 to 91 MPa square-root-m. Near the threshold, crack growth rate was about the same as for unreinforced aluminum alloys, but at high DELTA-K, crack growth rates were significantly lower. Crack closure was measured over this range of growth rates and found to be different than for unreinforced aluminum alloys. The magnitude of closure was also dependent on crack length. Cracks opened progressively towards the tip with increasing load in much the same way as for unreinforced aluminum alloys. Removal of the aluminum outer layer and some of the epoxy revealed that fibers were intact close to the crack tip, but heavily damaged further away. By adjusting the fatigue crack growth curve of an unreinforced aluminum alloy for the closure exhibited by the composite, it was possible to approximate the crack growth rate for the composite over the lower to mid range of DELTA-K, but at higher values of DELTA-K, this model seriously overestimated measured crack growth rates. Therefore, fiber bridging affects both closure and maximum stress intensity factor at the crack tip. Standard fracture mechanics cannot be applied to describe these effects.