3-Dimensional observation of the interior fracture mechanism and establishment of cumulative fatigue damage evaluation on spot welded joints using 590 MPa-class steel

To investigate the fatigue fracture mechanism in spot welded joints using a 590 MPa-class base metal, fatigue tests were conducted under constant loading conditions. In this study, three dimensional observation was made on the propagation behavior of fatigue cracks initiating at the edge of the slit between sheets. Moreover, an evaluation method of the fatigue life was proposed for random loading conditions. Since the mean load obviously affected the fatigue life of the spot welded joints, the proposed evaluation method was applied to account for this load effect. The proposed evaluation method satisfied the application range in the automobile industry. Therefore, it is thought that the proposed method is suitable for practical applications.

► Fatigue limit was very low compared with the tensile strength on spot welded joints.
► 3-D observation clarified the fatigue fracture mechanism in the spot welded joint.
► The fatigue limit of the joint was vanished under variable loading conditions.
► The mean load effect on the joint could be evaluated by the modified Goodman’s law.
► The proposed method can estimate the fatigue life under random loading conditions.