Remaining fatigue life estimation under varying amplitude cyclic loading conditions combining ultrasonic waves and relative strain measurements for aluminum 6061-T6 plates with a notch

In this study, a method for estimating the remaining fatigue life of notched aluminum plate structures under varying amplitude loading conditions was proposed using ultrasonic waves and relative strain measurements. To generate and measure ultrasonic waves, three macro-fiber composite (MFC) transducers were installed on the target structure under an unknown static uniaxial loading level. The unknown static-loading-induced strain at the time of MFC transducer installation was estimated using the acoustoelastic effect without any reference data obtained from the zero-loading condition. In addition, a strain gauge was used to measure the relative strain induced by various amplitude cyclic loading conditions after estimating an unknown static strain. Absolute cyclic strain monitoring was performed by combining the estimated unknown static strain (mean strain) and the measured relative strain (strain range). Subsequently, using the Walker equation, the monitored absolute cyclic strains under various mean strains and strain ranges were converted into an equivalent uniform cyclic strain with a constant mean strain and strain range. Finally, the remaining fatigue life was estimated from the equivalent uniform cyclic strain and the pre-obtained strain-life curve. The performance of the proposed method was experimentally evaluated using notched aluminum 6061-T6 plate specimens under varying amplitude cyclic loading conditions.