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One of the primary concern during assessment of structural integrity of vehicle bodies is the quality of joints used therein. The increased trend in vehicle weight reduction (through use of thin sheet panels) calls for more critical evaluation of the quality of such joints. The electric Resistance Spot Welding (RSW) is one of the largely used joining process for vehicle body structure. Besides the quality of sheet metal, the quality of RSW joints is largely governed by the process parameters e.g. weld current, hold time, squeeze time etc. The quality of RSW joints for durability can be assessed by mechanical strength and fatigue life. The paper presents a methodology for durability assessment of such joints using laboratory specimens and arriving at optimum values of process parameters to maximise the fatigue life. The methodology presented makes extensive use of Design of Experiment (DOE) techniques for better reliability of the results.

Strain controlled tests were performed on 35 C8 steel specimens (IS 3930-1979) to determine cyclic stress-strain curve (CSSC) by Companion Specimens Test (CST), Multiple Step Test (MST) and Incremental Step Test (IST) procedures. A good agreement was found between the CSSCs obtained by CST and MST procedures. The CSSCs obtained by IST procedure using Incremental Step Loading Blocks (ISLB) of different maximum strain amplitude indicate that the parameters of CSSC depend on the maximum strain amplitude in the ISLB and resulted in a family of CSSCs. The dependence of CSSC parameters on the maximum strain amplitude in the ISLB are approximated by polynomials. Fatigue lives estimated based on IST results for irregular strain-time histories were found to be comparable with the experimentally evaluated fatigue lives.