Crack retardation mechanism due to overload in base material and laser welds of Al alloys

To determine the retardation mechanisms due to overload and to predict the subsequent evolution of crack growth rate, investigations are conducted on crack retardation caused by single tensile overloads in base material and laser-welded sheets of AA6056-T6 Al alloy. The effect of the overload ratio on the fatigue crack propagation behaviour of the C(T) 100 specimens was analysed by using experimental and Finite Element (FE) methods. The crack growth rate and fracture surface features were investigated for both base material and laser-welded sheets. The retardation due to overload is described in terms of the affected regions in front of the crack tip. The size and shape of the crack-tip plastic zone and the damage profile induced during the application of the overload in the base material are predicted by FE analysis in conjunction with a porous-metal plasticity model. The results show that the mechanisms of retardation in under-matched welds are substantially different from that of the homogenous base material. More significant crack retardation due to overload has been observed in the laser weld of AA6056-T6. Based on SEM observations of the fracture surfaces and the damage profiles predicted by the proposed FE model, the shape of the crack front formed during the overload application can be predicted. During the overload, the crack front extends into a new shape, which can be predicted by the ductile damage model; a higher load results in a more curved crack front. These outcomes are used to determine the dominant retardation mechanisms and the significance of retardation observed in each region ahead of the crack tip and finally to define the minimum crack growth rate after overload. (C) 2011 Published by Elsevier Ltd.