Fatigue Limit Estimation Based on Dissipated Energy for Al/Al and Al/Steel Friction Stir Welding Joints

Abstract Background Friction stir welding was developed as a joining technique for lightweight alloys, such as aluminum, and has been increasingly recognized for its capability to join dissimilar materials in multi-material structures. The practical application of friction stir welding requires high levels of safety and reliability. Objective This study introduces a rapid method for estimating the fatigue limit of friction stir welding using infrared thermography. It specifically focuses on energy dissipation due to localized plastic deformation and evaluates the fatigue properties of friction stir welding joints based on this dissipated energy. Methods Temperature variations by dissipated energy and their phase difference were recorded during a staircase stress level test, where stress amplitude increments occurred in short cycle steps on the specimen. In the estimation of the fatigue limit of joint, the temperature variation by dissipated energy is categorized into two groups by the domain decomposition method using the least-squares approximation. Results The joints were observed to fracture at locations where temperature variations due to dissipated energy were locally elevated. The estimated fatigue limits calculated from these temperature variations were found to closely align with the actual fatigue limits. Thus, this study demonstrates that the fatigue limit of friction stir welding joints can be effectively estimated by monitoring the increases in temperature variation caused by dissipated energy. Conclusions The fatigue limits of aluminum alloy-to-aluminum alloy and aluminum alloy-to-steel friction stir welding joints can be estimated by observing the increase in temperature variation by dissipated energy.