Construction of Wide-Life-Region S-N Curves and Study on Fatigue Properties of Marine Aluminum Alloys

Objectives To meet the fatigue life design requirements of aluminum alloy hull structures under high-frequency loading, a wide-life-region S-N curve is constructed for a marine aluminum alloy. Methods Conventional and ultrasonic fatigue tests were conducted to acquire the stress and number of cycles in the high-cycle fatigue and long-life regimes. The maximum likelihood method and a double‑logarithmic linear model are employed to fit the median S-N curves for the two sets of tests separately. An equivalent stress amplitude correction factor is introduced to convert the ultrasonic fatigue stress data to the conventional frequency considering the frequency effect of high-frequency loading in ultrasonic tests. The model parameters are jointly estimated using the maximum likelihood method based on the combined dataset. Confidence intervals for the estimated parameters are derived from the profile likelihood theory, and the leave‑one‑out cross‑validation method was adopted to verify the robustness of the parameter estimates, thereby establishing a P-S-N curve covering a wide life regime. Results The results show that within the range of 1×104 to 1×108 cycles, the fatigue strength estimated directly from ultrasonic test data is 28% to 50% higher. Compared with the unified baseline P-S-N curve with a 97.72% survival probability, extrapolation based on the conventional fatigue test results with low dispersion would lead to an overestimation of fatigue life in the long-life regime by up to approximately 200 times. Conclusions The constructed wide-life-region S-N curve is based on the equivalent stress amplitudes from ultrasonic tests and the conventional fatigue test data, with the S-N curve parameters and the equivalent stress correction factor jointly estimated. It can provide an engineering reference for the very-high-cycle fatigue design and life prediction of aluminum alloy hull structures.