NUMERICAL FRACTURE PREDICTION OF LASER WELDED HIGH-STRENGTH STEEL FRAME FOR AUTOMOTIVE SEAT

摘要

Generally, the weld in high-strength steel includes a hard nugget and the heat-affected zone (HAZ), which is a transition segment. Due to the width of the laser beam used to make the weld, the weld is much smaller, i.e., narrower, than the entire structure; thus, it is not trivial to construct a finite element model for structural or crash simulations. To define the micromechanical fracture mechanism of a laser weld under a combined load, it is necessary to create a fracture model that considers stress triaxiality and Lode parameter dependency. This paper proposes a fracture analysis method that considers the combined load on an automotive seat frame joined using laser welding. This paper studied only lap joints for joining similar materials, although automotive seat frames include lap, butt, and T-joint welds. A T-shell element was used in this study to represent the laser welding zone and edge contact between metal sheets in the simulation. The GISSMO fracture model in LS-DYNA, a commercial finite element program, was used to predict the fracture behavior of the laser weld. GISSMO fracture model parameters were calculated. The analysis considered uniaxial tension, simple shear, notched tension, and biaxial tension test data. To evaluate the combined load condition, the relationship between stress triaxiality and the local fracture strain was examined. Material behavior was defined based on the relationship between the strength and hardness of the base metal and welding zone. Using the reverse analysis method, the GISSMO parameters were verified and applied to a C-type laser welding shape.