关键词: 非线性超声, 解调, 耦合特征, 损伤评估, 损伤指标, 振动声调制

Abstract: Vibro-acoustic modulation (VAM) is a nonlinear non-destructive testing technique, which detects damages by utilizing the modulation effects between high-frequency ultrasonic wave and low-frequency vibration. However, the coupling mechanism between the two excitations, and the relationship between damage severity and modulation intensity are still not well understood. VAM experiments are performed on an aeronautic aluminum plate under various crack lengths. The corresponding signals are AM-FM demodulated in time-frequency domain, aiming to investigate how the vibration and ultrasonic wave interact. The dependences of modulation intensity on excitation parameters and crack lengths are analyzed. The results show that, both amplitude and frequency modulations are present in the VAM signals when there is damage in the aluminum plate, and the modulation signal is the fundamental and higher-order harmonics of the vibration. The modulation intensity does not increase monotonically with increasing crack length due to the influences of excitation voltages and vibration modes of the plate. Thus it is not reliable to estimate the damage severity. According to the nonlinear spring model and the factors that affect the amplitude modulation of VAM signal, a damage index to estimate the relative damage severity is derived and verified.

Key words: Coupling characteristics, Damage evaluation, Damage index, Demodulation, Nonlinear acoustics, Vibro-acoustic modulation