Fatigue Fracture Mechanism on Friction Stir Spot Welded Joints Using 300 MPa-class Automobile Steel Sheets under Constant and Variable Force Amplitude

The authors have shown an approach to evaluate the fatigue fracture mechanism of cross-tension specimen welded using low carbon steel by friction stir spot welding (FSSW) with the focus on fatigue crack behaviour. Results show that, the fatigue limit of FSSW specimen was very low comparing to the tensile force of base metal and FSSW joint itself. The fatigue cracks initiated near the interface between two thin steel sheets under constant force amplitude conditions were observed in detail by the 3- dimensional observation method. The typical morphologies of 3-dimension were observed with constant low force amplitude level at 0.19 kN and high force amplitude level at 0.49 kN of each fatigue life. It is clear that a main crack initiated at the end of slit and fracture modes were independent on force amplitude level. And, FSSW joint used in this study requires the large and few number of cyclic loading to initiate the fatigue crack under low and high force amplitude level, respectively. Moreover, an evaluation method for the cumulative fatigue damage under variable force amplitude conditions was observed. The results show that, this method can be used to evaluate FSSW joint efficiency in automotive industry and other practical applications.