Effect of split sleeve cold expansion on fatigue and fracture of rolled AZ31B magnesium alloy

The split sleeve cold expansion process was successfully introduced to rolled AZ31B. The effect of such a process on fatigue and fracture behavior of the cold expanded samples was experimentally investigated and correlated to the associated microstructural changes.Cold expanded samples were prepared using several cold expansion levels, which were then fatigue tested to find the optimum degree of expansion that results in the greatest improvement in fatigue performance. This study revealed that 6 % cold expansion is the optimum expansion level for the investigated material, showing sub-stantial plastic deformation around the hole, without causing any macro-or micro-structural damage under this processing condition. Comparing fatigue performance between cold expanded and untreated samples showed a significant enhancement in fatigue life. This improvement is more profound in the high cycle regime, where it increased the endurance limit from 80 MPa for the untreated samples to 150 MPa for the cold expanded ones. Digital Image Correlation (DIC) technique was adopted for crack detection and monitoring to study the effect of the cold expansion process on crack growth behavior. It was found that cold expansion not only postponed the crack initiation but also considerably decreased the crack growth rate in cold expanded samples. The delay in crack initiation and slower crack growth is believed to be due to the residual stresses induced by the cold expansion process. This was corroborated by considering the effect of induced residual stress in the calculation of the stress intensity factor. Fractography investigation of a cold expanded sample revealed sub-surface crack initiation, which was attributed to the extensive texture evolution near the cold expanded hole.