Development of a simplified finite element model for Ultra High Strength Steel Resistance Spot Weld fracture 2020-01-0220
This paper describes the development of a simplified fracture finite element (FE) model for resistance spot welding (RSW) composed of Ultra high strength Steel (UHSS), which can be implemented for large scale vehicle FE model. As a result of requirements for improved fuel economy and emissions, use of UHSS is expanding. It is known that RSW which is composed of UHSS has two types of fracture mode, heat affect zone(HAZ) fracture and nugget zone fracture. Although simplified RSW fracture model composed of conventional grade steel was developed by Toyota Motor Corporation on 2006, the simplified model which is applicable to fracture propagation both of HAZ and nugget fracture has not been developed. It is observed that bifurcation of RSW fracture mode between different nugget diameter, as the result of lap shear and peel experimental coupon test which were composed of UHSS sheets. In order to analyse phenomena of UHSS RSW fracture mode bifurcation, detailed coupon test FE simulation composed of hexahedra solid element were conducted. The analytical results revealed that fracture of RSW is defined not only by plastic strain on the element, but also stress triaxiality state on element. The detailed model with new fracture criteria has represented the mode bifurcation of UHSS RSW fracture and correlated well with experimental coupon test result. Based on these result, simplified new fracture model to be implemented for large scale vehicle FE model was developed. New model consists of hexahedra solid element for nugget zone and shell element for HAZ zone. Result of coupon test simulation with new model has also represented mode bifurcation of UHSS RSW fracture and correlated well with experimental coupon test result.