Understanding interfacial failure mechanisms of advanced high strength automotive steels resistance spot welds under opening-mode loading

This paper addresses interfacial failure (IF) of automotive steel resistance spot welds under opening mode loading (i.e., Mode I), simulated through the cross-tension test. Two modeling approaches were developed to predict cross-tension strength (CTS) during IF: one based on structural stress and the other on fracture mechanics. The structural stress model correlates CTS with the hardness of the fusion zone, while the fracture mechanics model links CTS to the toughness of the fusion zone. Our evaluation of these models across various automotive steels reveals that the hardness-based model is suitable for steels with fusion zone hardness below 350 HV, where IF typically results in ductile fracture. In contrast, for higher-grade steels with fusion zone hardness exceeding 400 HV, the toughness-based model is more appropriate, as IF occurs through brittle crack propagation. The transition in the interfacial failure mechanism of spot welds under Mode I loading-from ductile tensile failure to brittle crack propagation-is governed by the ratio of toughness and hardness of the fusion zone. These insights not only refine the prediction of CTS but also inform the design of effective post-weld heat treatments aimed at enhancing CTS without sacrificing strength under shear loading mode, particularly through martensite refining strategy.