Residual stress effects on near-threshold fatigue crack growth in friction stir welds in aerospace alloys

In situ SEM observations [1] have revealed that fatigue crack propagation is caused by the shear band decohesion around the crack tip and the formation and cracking of the shear band is mainly caused by the plasticity generated in the loading part of a load cycle. Based on the crack tip plasticity a theoretical analysis has been performed and a model has been set up to consider the effect of the applied stress ratio on fatigue threshold. The obtained results have been compared with a wide range of published test data and good agreement has been achieved. This method is very easy to use and no fatigue crack closure measurement is needed, therefore this model is significant in engineering application.