Modeling of Plastic Deformation and Failure Near Spot Welds in Lap-Shear Specimens

The failure mechanism of resistance spot welds in dual-phase steel lap-shear specimens is first investigated based on experimental observations. Optical micrographs of the cross sections of spot welds in lap-shear specimens of a dual-phase steel before and after failure are examined to understand the failure mechanism. The experimental results suggest that under lap-shear loading conditions, the necking failure is initiated in the sheet near the middle part of the nugget circumference under tension and then the failure propagates in the sheet along the nugget circumference to final fracture. Based on a two-dimensional elasticity theory, an analytic solution for an infinite plate containing a rigid circular inclusion subjected to a resultant shear force is used to investigate the stress and strain distributions near the nugget in lap-shear specimens. The elastic analytic solutions and the results of a two-dimensional elastic finite element analysis indicate that the initial yielding starts near the two sides of the nugget in the sheet. As the applied load increases, the locations of the maximum equivalent plastic strain along the inclusion circumference shift from the locations near the two sides of the inclusion to the locations near the middle part of the inclusion. According to the forming limit diagram (FLD), the location of the initial necking failure should occur in the sheet near the middle part of the nugget circumference as observed in experiments.