A resistance spot welding process has become indispensable in the joining of auto-body components in the automobile industry since the 1950’s. It is extremely important to understand the strength of the spot weld under quasi-static, impact and fatigue loading conditions in order to predict durability and crashworthiness. Because a modern vehicle typically contains about 7000 to 13000 spot welds. Failure of the spot weld is prone to occur prior to failure of the base metal when a large amount of load is applied to the structure due to the stress concentration at the heat-affected zone (HAZ). It is necessary to consider the material properties of a HAZ, in order to improve the accuracy for calculating the failure load of spot-welded specimens using the finite element analysis. However, in many researches, material properties of the HAZ are not considered since there are little reliable data to describe the stress-strain curves of the HAZ at various strain rates.
In this paper, a methodology to identify the dynamic material properties of the HAZ near a base metal at various strain rates is proposed. In order to obtain the mechanical properties of a HAZ of SPRC340R, thermal simulated HAZ specimens which have similar characteristics, such as hardness values and microstructures representative of the actual weld HAZ near a base metal, were fabricated. At first, the thermal simulated HAZ is fabricated by the Material Thermal Cycle Simulator (MTCS). And then dynamic tensile tests are performed at various strain rates of 0.001 /sec to 100 /sec. The properties were applied to the finite element analysis of tensile-shear specimens under the static and dynamic loading conditions to verify its validity and effectiveness. Load and displacement curves were calculated with and without considering HAZ. The results are compared to the experimental results. The comparison induces that the amount of errors was reduced when the dynamic material properties of thermal simulated HAZ were applied to the HAZ in the analysis. The comparison also demonstrates that the dynamic material properties obtained from thermal simulated HAZ specimens are valid in the analysis of spot welded specimens.