The Fatigue Test Of Aluminum Alloy Sheet Based On Nonlinear Ultrasonic

When affected by external load or changes of application environment, the internal stress of the metal in aluminum alloy sheet will have some changes. These changes for a long period of time will cause material fatigue and cracks, which will cause metal failure or even a serious damage accident. If we can make an accurate detection of metal fatigue extent in the beginning stage of metel failure will reduce the loss to a great extent. Compared with the traditional ultrasonic testing method which can only detect macroscopic defects, nonlinear ultrasonic testing method has advantages such as high measurement resolution and Sensitivity to material fatigue.Ultrasonic Lamb wave is suitable for fatigue detection of aluminum alloy sheet based on nonlinear ultrasonic technology, as Ultrasonic Lamb wave has many advantages such as rich signal information, strong detection ability, long transmission distance and so on. This paper elaborates propagation modes and dispersion characteristics of Lamb wave, and draw the phase velocity dispersion curve and group velocity dispersion curve of aluminum alloy sheet using MATLAB numerical methods when knowing the velocity of transverse wave and longitudinal wave. Then this paper studies the one-dimensional nonlinear wave theory model and analyzes theoretically the nonlinear effects of Lamb wave. This paper also derives the necessary condition to have a obvious nonlinear effect of Lamb wave, during which introduces STFT time-frequency analysis method to determine Lamb wave modes in the received signals. This paper has carried on the detailed experimental studies on the key factors which can influence the detection results, for example, the adjustment of the emission signal, filter using, the second harmonic extraction algorithm, the frequency response of the probe, the selection standards of coupling agent, the adjustment of experiment system parameters and so on.The main conclusions of this paper are listed below. Selecting an appropriate fundamental frequency of Lamb wave whose phase velocity equal or approximately equal the phase velocity of one second harmonic mode of Lamb wave. Adjusting the angle of transmitter probe and receiver probe can inspire and receive specific mode of Lamb wave. This paper calculates the amplitude of the second harmonic filtered using superheterodyne phase-sensitive detection technique, and sets the amplitude of the second harmonic filtered as A₂ as well as sets the amplitude of the fundamental frequency before filtering as A1. Measuring the same point many times and making the slope of the fitted straight line as its true nonlinear coefficient is the method of the highest reliability. Compared with butter and medical ultrasonic coupling agent, using glycerin as coupling agent can obtain the best experimental repeatability. In terms of macroscopic damage detection, ultrasonic Lamb wave detection based on difference of linear sweep method is doing well, but in terms of fatigue damage detection, method based on nonlinear ultrasonic testing has more advantages than the traditional method based on difference of linear sweep.