Development of Simplified Finite Element Model for Ultra-High-Strength Steel Resistance Spot Weld Fractures

This paper describes the development of a simplified fracture finite element (FE) model for resistance spot welds (RSW) of ultra-high-strength steel (UHSS) that can be incorporated into large-scale vehicle FE model. It is known that the RSW of UHSS generates two types of fracture modes: heat-affected zone (HAZ) and nugget zone fractures. Lap shear and peeling coupon tests using UHSS sheets found that the different RSW fracture modes occurred at different nugget diameters. To analyze this phenomenon, detailed simulated coupon tests were carried out using solid hexahedral elements. The analytical results revealed that RSW fractures are defined by both the application of plastic strain on the elements and the stress triaxiality state of the elements. A detailed model incorporating a new fracture criteria model recreated the different UHSS RSW fracture modes and achieved a close correlation with the coupon test results. Based on these results, a new simplified fracture model that can be incorporated into large-scale vehicle FE model was developed. This new model uses solid hexahedral elements to simulate the nugget zone and shell elements for the HAZ. Simulated coupon tests using the new model were also capable of recreating the different UHSS RSW fracture modes and correlated closely with the coupon test results.