Abstract: In order to monitor the early fatigue damage of AZ31 magnesium alloy by nonlinear ultrasonic basing on second harmonic, the measurement system of ultrasonic nonlinearity parameters is established. Thick plate fatigue specimens of magnesium alloy are designed, these specimens are pre-stress concentration area, and the effectiveness of stress concentration area is simulated by the finite element method. Ultrasonic nonlinearity parameters  of three magnesium specimens with different loading stress are measured, the experimental results show that there is a significant increase in  linked to fatigue degree in the early stages of fatigue life. Meanwhile,  of non-stress concentration area are measured to one specimen, the ultrasonic nonlinearity parameters magnitude is the 27% of stress concentration zone, and further show that the changes in  derive from the changes of micro-defect by magnesium alloy material fatigue, and also confirm the reliability of the experiment. In addition, the experimental results of the wave velocity and elastic constants show that the two linear parameters are not sensitive to early fatigue damage of the magnesium alloy material.

Key words: AZ31 magnesium alloy, Fatigue damage, Nondestructive testing, Nonlinear ultrasonic