3-Dimensional observation of the interior fatigue fracture mechanism on friction stir spot welded using 300 MPa-class automobile steel sheets

Abstract

The authors have shown an approach to evaluate the mechanical properties of cross-tension type specimen welded by friction stir spot welding (FSSW) with the focus on fatigue crack behaviour. In this study, cold-rolled 0.8-mm-thick low carbon steel sheet JIS G3141 SPCC was used base metal. Results shown that, the hardness test values within the stirred zone in welding joint were much higher than that of the base metal. The fatigue strength of FSSW specimen was very low to compare with the tensile force of base metal and FSSW joint itself. The fatigue crack initiated near the interface between two thin steels sheet was observed in detail by the three-dimensional observation method. This method was used to investigate the crack initiation and propagation behaviour. The typical morphologies of the three-dimensional were observed with constant low force amplitude level of each fatigue life. It shows that a main crack initiated at the end of slit and FSSW specimen used in this study requires the large number of cyclic loading to initiate the fatigue crack. Furthermore, the typical morphologies of the three-dimensional with constant high force amplitude level were observed. From this observation, it is cleared that a main crack initiated at the end of slit and FSSW specimen used in this study requires few number of cyclic loading to initiate the fatigue crack. Therefore, fatigue fracture modes were independent on force amplitude level.