Experimental Research On The Fatigue Degradation Of Bonding Interface Using Nonlinear Ultrasonics

Ultrasonic nonlinear phenomenon will appear because of the nonlinear characteristics for bonding interface under fatigue loads, and there are secondary and high-order harmonics for the ultrasonic waves propagating through the bonding interface. This ultrasonic nonlinear phenomenon can be characterized by a defined nonlinear parameter. With the means of measuring the nonlinear parameter in an experimental way, we can obtain the degrees of damage accumulation for bonding interface under fatigue load, the degradation of mechanical properties for the bonding interface can thus been interrogated.In this thesis, we studied the ultrasonic nonlinear characteristics of AZ31magnesium alloy bonded specimen under tensile fatigue loads, and established the relationship between mechanical degradation of the bonding interface and ultrasonic nonlinear parameter in an experimental way. By applying fatigue load in different degrees for AZ31magnesium alloy adhesive specimens, and by using the nonlinear ultrasonic test system built in this thesis, ultrasonic longitudinal waves were excited and received; Making Fast Fourier Transform (FFT) to the received ultrasonic longitudinal signal and we can thus get the magnitudes of fundamental and secondary harmonics of the received signal. According to the relative nonlinear parameter based on fundamental and secondary harmonics, we finally established the relationship between relative nonlinear parameter and cycles of fatigue loads. Furthermore, we also compared the nonlinear ultrasonic test results based on different excitation frequency and different fatigue load amplitude. The experimental research result shows that:1. With the increase of the cycle of the fatigue loads, the nonlinear parameters of the adhesive surface obtained with the nonlinear ultrasonic test also increases gradually. Especially in the early stage of fatigue, nonlinear parameters increase rapidly with the growth trend of the cycle of fatigue load. With the further increase of the cycle of fatigue load, the growth trend of the nonlinear parameters slow down;2. For the nonlinear ultrasonic test under different excitation frequencies, it shows that the frequency have little influence on the test results, which means that the nonlinear coefficient of different frequency is basically the same trend with the increase of the cycle of fatigue load;3. For different magnitude of fatigue load, the experimental result shows that the nonlinear parameters are bigger when the magnitude of fatigue load is bigger, even though the number of fatigue cycle is the same, which indicates that there is higher degree of damage.