Influence of impact dents on the fatigue strength of aluminium alloy friction stir welds

The objective of this paper is to present an investigation on the effect of low velocity impact damage on the fatigue strength of friction stir welds in aluminium alloys 2198-T851 and 2024-T351. Three parts of work have been carried out. First, laboratory coupons representing thin sheet butt welds were tested under low velocity impacts at various incident energy levels and two impact locations. Performance of the butt welds was evaluated in terms of the impact force, indentation and local bending stresses. Failure mechanisms were established. Second, dynamic transit analysis of impact loading was performed using the commercial FEA package ABAQUS. Residual stresses due to impact induced plastic deformation were obtained, which was found to play a crucial role on fatigue crack initiation. Third, to model the post-impact fatigue testing, a quasi-static load analysis was performed on plate containing an impact dent subjected to in-plane cyclic stresses. Geometrical and material nonlinear modelling was used to simulate the impact residual stresses. This research has shown that, due to the impact dent and local damage, high cycle fatigue strength of the welded plates is reduced by 32% compared to the un-impacted welded plates of 2198-T851. Fatigue strength reduction is not found to be sensitive to the impact location.