Experiment And Study On Method Of Aluminum Fatigue Crack Detection Based On Nonlinear Acoustics

Metal fatigue is the main reason for the failure of the metal structure. Since the aluminum alloy sheet is widely used in aircraft and aerospace equipment structure which, once spent, would inevitably result in huge losses and even casualties. In this paper, carried a detailed study on the features of aluminum alloy sheet fatigue crack,based on the principle of nonlinear acoustics, and applied phase conjugation method extended from the optical field to the acoustic signal processing.The solid medium nonlinear ultrasonic equation is analyzed, it proved the possibility of using non-linear coefficient b to characterize solid metal fatigue of materials. The principle of the time reversal technique was analyzed, and derived the theoretical time reversal method expressions. Lastly, describes the application of time reversal in fatigue crack detection.Constructed a circular piezoelectric crystal(PZT) as the core of metal fatigue evaluation system, which composed by the signal transmitter module, signals receiver module, signal storage display module, a number of 2024 aerospace aluminum alloy plate and computer data acquisition system. Conducted experiments between frequency 50 k Hz to 1MHz, and found second harmonic in fatigue cracks model, suggesting that the non-linear coefficient b can be used to characterize the degree of metal fatigue.The time reversal of acoustics can improve the signal/noise ratio(SNR) and realize adaptive focus, which can effectively solve the ultrasonic attenuation during propagation. By studying the self-focusing properties of acoustic wave in aluminum alloy plates, burst Lamb wave is taken as the excitation wave of time reversal process.Experiments of time reversal self-focusing by single-array and multi-array show that time reversal method can improve the SNR and has an excellent focus gain. Inaddition, some experiments of acoustic self-focusing are carried out on aluminum alloy plates with different size fatigue crack, and the experimental results show that the size of the fatigue crack has little effect on the peak amplitude of self-focusing wave. When macro-crack appears in aluminum alloy plates, uniform amplitude square wave will appear in focusing peak area of time domain waveform. And in the corresponding spectrum, the new frequency component-third order harmonic occurs.