A novel technique to measure the local mechanical properties of third generation advanced high strength steel resistance spot welds

Distinct microstructures and mechanical properties are produced in resistance spot welds (RSW) of advanced high-strength steels (AHSS). The local mechanical properties within the fusion zone (FZ) and heat-affected zones (HAZ) govern the global failure response of RSW automotive structural assemblies in crash events. In this study, a novel experimental procedure was developed to characterize the local properties of RSW sub-regions using sub-size tensile and simple shear specimens extracted from the spot weld, with local strain measurement utilizing digital image correlation (DIC) techniques. The FZ properties were obtained using a simple shear test, while the HAZ properties were characterized using a tensile test containing a central RSW. A range of AHSS grades were considered to evaluate the proposed methodology, including 3rd Gen-980, 3rd Gen-1180, and press-hardened steel (PHS1500) with respective minimum ultimate tensile strengths of 980 MPa, 1180 MPa, and 1500 MPa. Strain localization occurred wherever the hardness was lowest such as in the subcritical heat-affected zone, with limited deformation in the harder regions, such as the upper-critical HAZ and FZ. The local stress-strain responses were then compared to a hardness scaling approach to estimate the tensile properties of RSW based on the local hardness in each zone. The hardness scaling method only appears to be a viable method if the microstructures across the weld have similar microconstituents in all sub-zones across the weld.