Nonlinear ultrasonic characterizing online fatigue damage and
in situ microscopic observation
in situ microscopic observation
(1. College of Mechanical Engineering and Applied Electronics Technology, Beijing University of Technology,
Beijing 100124, China;
2. School of Mechanical and Electrical Engineering, Henan University of Technology, Zhengzhou 450007, China)
Beijing 100124, China;
2. School of Mechanical and Electrical Engineering, Henan University of Technology, Zhengzhou 450007, China)
Abstract: A robust experimental procedure was developed, by which the evolution of fatigue damage in AZ31 magnesium alloy was tracked online with the ultrasonic nonlinearity parameter β. β values of three sets of samples under different stress levels were measured. Microstructures of specimens at different fatigue stages were observed in situ by optical microscopy. The experimental results show that there is a significant increase in β linked to the accumulation of persistent slip bands (PSBs) and micro-cracks at the early stages of fatigue life and reaches the maximum, about 55% of fatigue life. Ultrasonic attenuation coefficient increases with the expanding of micro-cracks and leads to β decrease slightly after 55% of fatigue life. The variation of β with fatigue cycles is in good agreement with the growth of PSBs and micro-cracks. In addition, it has no significant effect on the experimental results for the changes of low- and high-cycle fatigue and the fatigue mode with tension−tension and tension−compression.
Key words: nonlinear ultrasonic; AZ31 magnesium alloy; fatigue damage; persistent slip bands