Fatigue crack growth in thin aluminium alloy sheets under loading sequences with periodic overloads

This study analyses the fatigue crack growth behaviour in thin AlMgSi1-T6 aluminium alloy sheets under several loading sequences containing periodic overloads. The observed fatigue crack growth behaviour is compared to constant amplitude loading and discussed in terms of type of loading sequence, intermediate baseline cycles and stress ratio. The crack closure parameter U was obtained for periodic single overload tests and compared with the crack growth transients. Crack retardation increases with overload periodicity and decreases with stress ratio increase. Higher crack growth retardation was observed under loading sequences with decreasing load levels in comparison to increasing ones as well as under loading sequences with extended periodicity. In spite of some discrepancy, attributed to the quick change of the closure levels, it is clear that plasticity-induced crack closure plays an important role on the load interaction effects observed in this aluminium alloy.