The Generation Mechanism Of The Nonlinear Lamb Wave For Early-stage Damage In Materials

The identification of micro-defects in engineering materials is very valuable in understanding and accessing the safety and integrity of materials under complex environmental conditions.The use of nonlinear ultrasonic testing technology to quantitatively identify and locate the early-stage degradation of mechanical properties of materials under fatigue load has been a hot topic in the field of non-destructive testing.Numerous studies have shown that early-stage material degradation can be characterized by weak material nonlinearities and material microstructures?micro-cracks,dislocations,etc.?.In this thesis,adopting the numerical simulation method,generation mechanism of nonlinear Lamb waves in randomly distributed micro-cracks and the generation mechanism of Lamb hybrid waves based on S₀ and A₀modes were investigated.Based on the wave theory,the second harmonic effect of Lamb wave propagating in randomly distributed micro-crack aluminum plate was simulated.The results showed that the influence of the randomly distributed micro-cracks on the phase velocity of the low frequency S₀ fundamental waves could be neglected and the time domain waveform almost unchanged,while the second harmonic of the fundamental wave can be clearly found in the frequency domain.By using a Monte Carlo simulation method,it was found that the acoustic nonlinear parameter increased linearly with the micro-crack density and the size of micro-crack zone,and it was also related to the excitation frequency,but it was hardly influenced by friction coefficient of the micro-crack surfaces.In addition,we found that the nonlinear effect of waves reflected by the micro-cracks was more noticeable than that of the transmitted waves.The propagation of Lamb waves in thin plates with quadratic nonlinearity by one-way mixing method using numerical simulations was investigated.It is shown that an A₀-mode wave can be generated by a pair of S₀ and A₀ mode waves only when mixing condition is satisfied,and mixing wave signals are capable of locating the damage zone.Additionally,it is manifested that the acoustic nonlinear parameter increases linearly with quadratic nonlinearity,meanwhile increases monotonously with the size of mixing zone.Furthermore,because of frequency deviation,the waveform of the mixing wave changes significantly from a regular diamond shape to toneburst trains.The results of this research are expected to provide theoretical basis and effective technical approaches for the accurate evaluation of early fatigue damage of thin plate materials.