Applied research on pulse-inversion technique in acoustic nonlinear parameter measurement of fatigued metals

Material degradation is usually preceded by some kind of nonlinear mechanical behavior.When a single sinusoidal ultrasonic wave is launched to a nonlinear medium,the wave will be distorted as it propagates,so higher harmonics will be generated.The paper studies the measurement method of nonlinear parameters and signal processing algorithms with pulse-inversion technique,and develops a robust experimental system.There is linear relationship between amplitudes of the second-harmonic waves and the squared fundamental waves as a function of increasing input voltage amplitude,the experimental system is reliable.Using this method,ultrasonic nonlinear parameters of a group of LY12 aluminum samples fatigued are measured.The experimental results show that the acoustic nonlinear parameter can characterize the fatigue degree of LY12 aluminum,the pulse-inversion technique is very efficient in extracting the second-harmonic amplitude by canceling out the odd harmonics which are mainly due to the instrumentation.This technique can provide a practical way in the nondestructive characterization of material and structure degradation in early fatigue life.