A mechanical properties and failure mechanism study for resistance spot welded AHSSs under coach-peel and lap-shear loads

Based on mechanical tests, microstructure characterizations and finite element simulations, the mechanical properties and failure mechanisms of resistance spot welds (RSWs) were investigated in the study. Different types (B1500HS, 340/590DP, 420/780DP and 820/1180DP) and thicknesses (1.2-1.8 mm) of steel plates were welded as coach-peel (CP) samples. To quantify the effect of plate thicknesses on the failure bending moment, MCP, of CP samples, a new parameter, standardized failure bending moment, MSCP = MCP /T1 /T2, was introduced. Based on the new revealed relationship between the MSCP and low strength metal plate tensile strengths (LSMP ab), a MCP prediction model was provided. Moreover, the lap-shear (LS) test data (standardized shear strength, aSS) published in a previous work were also analyzed. With the increment of ab values (<800 MPa), aSS and MSCP parameters increased linearly. When the ab values > 800 MPa, the slopes of aSS and MSCP fitting lines were decreased. The similar variation trends of aSS and MSCP parameters implied similar RSW failure mechanisms. The new revealed relationship between the RSW minimum hardness values (Hmin, measured from base metals or heat affected zones) and LSMP ab values can explain the trends. Finally, by simulations analyses, the effect of different loading conditions on the mechanical performance of RSWs was also studied. CP loads induced the most severe stress concentration near the RSWs, which can severely weaken RSW mechanical performance.