Failure Analysis of Resistance Spot Welded Advanced High Strength Steel Sheets

Abstract

A method to analyze the failure (fracture) behavior of resistance spot welded joints was developed in this work, which was applied for the similar welds and the dissimilar welds of advanced high strength steel sheets. As for the similar welds, TRIP980 and DP980 sheets were analyzed along with a conventional mild steel sheet (GMW2) for comparison purpose. Based on the TRIP980 sheet, as for the dissimilar welds, one was fabricated with the DP980 sheet (DP980-TRIP980) and the other with a conventional mild steel sheet GMW2 (GMW2-TRIP980). The method included the characterization of base material sheets regarding hardening data and fracture criteria mainly based on the inverse numerical analysis of uniaxial tension tests. Besides, hardness measurement across the cross-section of the weld nugget along with optical microscopic observation was utilized to identify the shapes and dimensions of the base, heat-affected zone, and the fusion zone. Utilizing their geometric features, their fracture criteria as well as mechanical properties were characterized based on the inverse numerical analysis of a newly designed miniature tension test. Ultimately, the characterized properties of the base materials and the weld zones were applied to analyze the failure (mode and strength) in the coupon tests of lap-shear and U-shape tension tests for welded sheets (also, as an effort to validate the calibrated properties). All analysis was performed under the quasi-static condition in this work.