Association of Metallurgical Engineers of Serbia AMES Scientific paper UDC: 621.791.763 FAILURE BEHAVIOUR OF RESISTANCE SPOT WELDED LOW CARBON STEEL IN TENSILE-SHEAR AND COACH-PEEL TESTS: A COMPARATIVE STUDY Majid Pouranvari 1* , Pirooz Marashi 2 1 Young Researchers Club, Islamic Azad University, Dezful Branch, Dezful, Iran 2 Mining and Metallurgical Engineering Department, Amirkabir University of Technology, Tehran, Iran Received 09.07.2009 Accepted 28.09.2009. Abstract In this paper, behaviour of resistance spot welded joint was studied under tensile- shear and coach-peel loading condition. Failure modes of resistance spot welds, pullout and interfacial, were investigated based on experimental observation. Optical micrographs of the cross sections of spot welds in shear-tensile and coach peel specimens before and after failure are examined to understand the failure mechanism. Results showed that there is a critical fusion zone size to ensure pullout failure mode. The experimental results showed that in pullout failure mode during shear-tensile test, necking is initiated at nugget circumference in the base metal and then the failure propagates along the nugget circumference in the sheet to final fracture, while pull out failure during coach peel test occurred by crack initiation and propagation near the weld nugget/HAZ boundary. The critical fusion zone size required to ensure pullout failure mode during tensile shear test was larger than that of during coach-peel test. Key words: Resistance spot welding, coach-peel test, tensile-shear test, failure behaviour Introduction Resistance spot welding (RSW) is considered as the dominant process for joining sheet metals in automotive industry. Vehicle crashworthiness, which is defined as the capability of a car structure to provide adequate protection to its passengers against injuries in the event of a crash, largely depends on the integrity and mechanical performance of spot welds [1, 2]. Overload failure mode of spot welds is a qualitative measure of the weld reliability. Generally, spot welds fails in two modes: interfacial and * Corresponding author: M. Pouranvari mpouranvari@yahoo.com