In this article, weld fracture criteria based upon low strain rate (i.e., ε̇∼10−3-10−2s−1) tensile-shear tests of spot welds in dual-phase (DP) steels DP600, DP780, and DP980 are developed. Three empirical equations are inferred from least-squares root-fitting analyses of tensile-shear testing data. Building upon existing results in the literature, the first equation relates the tensile-shear force to the weld diameter. The second and third equations relate, respectively, a critical weld diameter and a critical tensile-shear force for interfacial fracture to the sheet thickness and hardness extrema in the heat-affected zone. These idealized equations can serve as the basis for further development of fracture criteria resembling material flow laws that account for higher strain rates and more complicated deformation paths. The effect of spot-weld placement in specific patterns or arrays on deformation and fracture behavior was also investigated to explore underlying effects from deformation field interactions between adjacent spot welds.
Plastic deformation of miniature tensile bars generated from dual-phase steel weld microstructures (i.e., fusion zone, heat-affected zone, and base material) was investigated up to final rupture failure. Uniaxial tensile true stress-strain curves beyond diffuse necking were obtained with a novel strainmapping technique based on digital image correlation (DIC). Key microstructural features (including defects) in each of these three metallurgical zones were examined to explore the material influence on the plastic deformation and failure behavior. For weld fusion zones with minimal defects, diffuse necking was found to begin at 6 pct strain and continue up to 55 to 80 pct strain. The flow stresses of the weld fusion zones were at least twice those of the base material, and fracture strains exceeded 100 pct for both materials. The heat-affected zones exhibited a range of complex deformation behaviors, as expected from their microstructural variety. Only those fusion zones with substantial defects (e.g., shrinkage voids, cracks, and contaminants) failed prematurely by edge cracking, as signaled by their highly irregular strain maps.
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