Effect of fatigue loading and residual stress on microscopic deformation mechanisms in a spot welded joint

The microstructure evolution in as welded and post-heated residual stress relived spot welded steel sheets during fatigue testing has been studied. The relationship between microstructure evolution and residual stress during fatigue loading has been considered. It has been found that under high fatigue load, dislocation density in spot nugget edge is much higher than that in nugget center area, which indicates significant plastic deformation occurred at the edge of spot nugget during fatigue testing. Under low fatigue load, dislocation density is quite low in both the edge and center area of spot nugget. The effect of post-heating is that more dislocations could be generated during fatigue testing for both high and low loading conditions. Post-heating results in lower strength of spot welded joint while it releases the residual stress in it, which reduces the fatigue life of the spot welded sheet.