Fatigue crack growth in heat-treated aluminium alloys

Fatigue crack propagation tests have been performed in several heat-treated aluminium alloys under constant amplitude loading. All experiments were performed, in load control, in a servo-hydraulic closed-loop mechanical test machine. The tests were carried out using middle tension, M(T), specimens. The influence of stress ratio and thickness were analysed. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. A strong stress ratio and material dependence effects on the fatigue crack growth were observed. These effects are discussed in terms of the different dominant closure mechanism. The crack growth behaviour of heat-treated aluminium alloys depends mainly on whether the dominant closure mechanism is plasticity-induced or roughness-induced. The enhancement of roughness-induced closure promotes higher crack growth resistance in these alloys. Roughness-induced closure dominates crack closure in aluminium alloys age hardened by naturally ageing and also artificially aged alloys with higher contents of Mn and Cr elements. In alloys aged hardened by artificially ageing and simultaneously with a lower content of these alloying elements plasticity-induced closure is dominant.