Deformation and fracture of miniature tensile bars with resistance-spot-weld microstructures

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 strain-mapping 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.