Failure Mode and Fatigue Behavior of Dissimilar Friction Stir Spot Welds in Lap Shear Specimens of Transformation Induced Plasticity Steel and Hot Stamped Boron Steel Sheets

Abstract

Failure mode and fatigue behavior of dissimilar friction stir spot welds in lapshear specimens of transformationinduced plasticity steel (TRIP780) and hotstamped boron steel (HSBS) sheets are examined in this paper. Optical micrographs of the dissimilar TRIP780/HSBS friction stir spot welds made by a concave silicon nitride tool before and after testing are obtained and examined. These micrographs indicate that subject to quasistatic and cyclic loading conditions, the TRIP780/HSBS welds fail from cracks growing through the TRIP780 sheets where the tool was plunged into and the thickness was reduced. The bending moments and the transverse shear force near the welds are derived with consideration of the load offset, the weld gap, and the bend distance for calculation of analytical global stress intensity factor solutions for the welds in lapshear specimens. A fatigue model of kinked crack growth is used to estimate fatigue lives based on the local stress intensity factor solutions for kinked cracks. The estimated fatigue lives with consideration of the weld gap and the bend distance are in agreement with the fatigue test results under lowcycle loading conditions and lower than the fatigue test results under highcycle loading conditions. The estimated fatigue lives suggest that the weld gap and the bend distance can significantly affect fatigue lives of the friction stir spot welds in lapshear specimens under cyclic loading conditions.