In order to investigate the fatigue properties under varying loading conditions in a heat welding joint by friction stir spot welding of aluminum alloy and CFRP, this study was carried out repeated two-step loading test. Moreover, this study was established a fatigue damage evaluation method with safety and high accuracy under two-step loading conditions. As a result, a fatigue life of joints under constant force amplitude conditions was 2.85 times the difference between the regression line by the least-squares methods and the fatigue life having the failure probability from 10 to 90 %. Moreover, the cumulative damage of the joints based on the Miner’s law was much lower than 1, the effect of load interaction on fatigue life of the joints was confirmed under varying loading conditions. Therefore, the fatigue damage evaluation of the joints with the safe side and high accuracy under varying loading conditions could be achieved by considering a variation of the fatigue life of the joints using confidence intervals and correcting the effect of load interaction using Corten-Dolan method.
