Characterization of fracture toughness for different zones of resistance spot welded dual phase steel using micro-cantilever testing

Resistance spot welding is the most predominant joining method in automotive industries. Typically a car body structure contains 4000~5000 spot welds and the safety of vehicles is to a large extent determined by the property of resistance spot welds that assemble all steel components together. Unfortunately, advanced high strength steels are known to be more susceptible to weld metal failure of resistance spot welds than other conventional mild steels. A resistance spot weld exhibits a complex and non-homogenous microstructure with a very strong gradient of mechanical response and microstructural characteristics from one zone to another. Ultra-fast heating and cooling rate imposed by spot welding process make it nearly impossible to ex-situ simulation of the thermal cycle using thermal-mechanical testing methods such as Gleeble to evaluate the local mechanical properties of different zones. This work focuses on the application of micro-scaled fracture tests to measure the fracture toughness of different welding zones for spot welded dual phase steels. Focused ion beam milling was used to fabricate notched micro-sized cantilevers. Due to large plastic deformation associated with the micro-cantilever bending, linear elastic fracture mechanics was inapplicable. J-integral testing was performed at the microscale to apply elastic-plastic fracture mechanics that require a detailed knowledge of crack extension. Therefore, cyclic loading was applied to determine the crack length and critical J value. It was found that the method is useful for the comparative study of the fracture toughness at different welding zones to the accurate correlation between welding parameters, microstructure and mechanical performance.