Abstract
The high strength steel (HSS) sheet is widely used in many kinds of automotive component, in order to reduce the body weight and improve crashworthiness. Crashworthiness of a spot-welded component is strongly affected by the strength of spot welded joints, and the fracture within a weld lowers he mechanical performance. Especially as for HSS, the strength of spot welded joint does not necessarily increase with the base metal strength and spot weld fracture possibly occurs during crash deformation. Therefore it is important to estimate the spot weld fracture precisely, to know the overall performance of the component. In the present study, the FE analysis method for the crash of automotive structural parts is discussed, which can predict the fracture of spot welded joint throughout the simulation by conducting the appropriate criteria of the fracture.
A fracture prediction during FE simulation is performed by using equivalent plastic strain as a fracture criterion, which is determined as follows; (1) fracture location and fracture load are experimentally investigated by tension test with the single spot weld joints, (2) static FE analyses simulating the tension tests are carried out, where the corresponding measured stress-strain relations are defined for the nugget, HAZ and base metal, and (3) the equivalent plastic strain when the analysis load comes to the experimental fracture load is determined as a fracture criterion. In the tension tests, both shear mode and peel mode are examined for various sheet combinations. For both loading mode, observed fracture mode can be categorized into three types; base metal fracture, HAZ fracture or nugget fracture.
This FE analysis method is applied to crash analyses of hat shaped automotive component of the ultra high strength steel (UHSS), and the accuracy of the analysis result coupled with the fracture prediction is verified. These analyses results show good agreement with experimental results. And, the influence of the component details, such as sheet thickness, nugget diameter and spot weld pitch, on the energy absorption is evaluated by this analysis method. These results will be very useful for optimizing component design for crash performance.
Keywords: Weld, Fracture, Analysis, FE, HSS