Damage-based simulation of ductile failure process of spot welded joints

Abstract

Spot welding is an efficient method of joining sheet metals in automobile body structure. The large number of spot-welded joints in the structure influence fatigue reliability and crashworthiness through localized failure of the joint. Consequently, thorough understanding of deformation and the failure process of the spot-welded joint is essential with respect to design safety and reliability. Complex metallurgical variations of spot-welded joint render deformation and failure characterization a challenging task. In this respect, this paper is aimed to describe the mechanics of the failure process of spot-welded joint under quasi-static loading conditions. Assessment of ductile failure of the joint is performed using finite element (FE) simulation, validated by experimental measurements made on spot-welded specimen. For this purpose, spot-welded cross-tension specimens made of dual-phase steel sheets (DP600) are fabricated and tested to failure at machine crosshead speed of 5 mm/min.

Characteristic load-displacement curve of the joint is then established. A 3D FE model of the test set- up is developed with the model geometry discretized using 8-node continuum elements. Strength

properties and hardening parameters of the heat affected zone (HAZ) and fusion zone of the spot- welded joint is established based on hardness test measurements relative to the base sheet material.

Ductile-like failure of the joint is predicted using damage-based model that incorporates Rice-Tracey

ductile damage initiation criterion and linear energy-based damage propagation criterion. The FE- predicted pull-out button failure of the joint compares well with the observed ductile deformation mode

that is accompanied by localized necking at fracture. FE results also indicate that the calculated damaged region is small and confined to the peripheral of interfaces between HAZ/fusion zone and HAZ/base sheet metal. In this region, severe triaxial stress state occurs in association with localized necking of the sheet metal. In the vicinity of the HAZ/fusion zone interphase, the predicted von Mises stress gradient is likely due to mismatch of the stiffness of adjacent regions and the inherent geometrical stress concentration factor.