Improvement of Fatigue Properties of Resistance Spot Welded Joints in High Strength Steel Sheets by Shot Blast Processing

摘要

This paper discusses the effect of shot blast processing on the fatigue strength of resistance spot welded joints of 980 MPa steel sheets and the mechanism by which the fatigue strength is improved. The fatigue limit load of shot blasted spot welded joints was extended to approximately twice that of the non-blasted types. While high compressive residual stress was conferred on the outer surface of the shot blasted steel sheets, there was little variation in the residual stress on the side of the overlapped surface. Shot blasting on the outer sheet surface increased the initiation life of the fatigue cracks that occurred on the overlapped surface and reduced the propagating speed of the cracks that grow from the overlapped surface toward the outer surface. The initiation and propagation of fatigue cracks were affected in the region where compressive residual stress was not conferred. An FE-analysis suggests that the compressive residual stress on the outer sheet surface reduces the opening of the sheet separation of the spot welded joints under the fatigue test load and reduces the maximum principal stress around the tip of the corona bond.