Abstract: The fatigue testing and nonlinear ultrasonic testing are carried out on the complete aluminum plates, the aluminum plates containing the hole and the aluminum plates repaired with the composite patches. The amplitudes of fundamental ultrasonic and the second harmonic of the three sets of specimens under different fatigue cycles areextracted. The relative second-order ultrasonic nonlinear coefficients are obtained, and the graph of the relative second-order ultrasonic nonlinear coefficient with the fatigue cycles is fitted to study the corresponding relationship between the relative second-order ultrasonic nonlinear coefficients and the fatigue damage process. The results show that with the increase of fatigue cycles, the second-order ultrasonic nonlinear coefficients of the three sets of specimens show a slow rise first, then rise rapidly and reach the peak value, and finally decrease, which is consistent with the process of dislocation and crack generation and propagation during fatigue damage. At the same time, it is found that the pore damage will aggravate the transition of microcracks to macroscopic cracks, while repairing pore damage with composite patches can slow crack propagation.

Key words: nonlinear ultrasonic detection, composite patch, relative second-order ultrasonic nonlinear coefficient, fatigue damage