Nonlinear ultrasonic techniques used in nondestructive testing: A review

It is important to conduct nondestructive testing and evaluation on structures and materials during their working life cycles. Ultrasonic testing (UT) is a key method, but conventional UT methods observe the received signal in time domain and are not suitable for detecting micro-defects, micro-cracks, and interface defects such as bonding and delamination, nor are they efficient at detecting metal fatigue and degradation defects because of the limitation of their wavelength. In recent years, based on nonlinear ultrasonic effects, such as high-order harmonic generation, sub-harmonics, beam mixing and shift of resonant frequency, nonlinear ultrasonic testing and evaluation techniques which analyze the signal characteristics mainly from frequency domain are developed. The nonlinear ultrasonic technique has a promising future to overcome the shortages of conventional UT techniques. The different nonlinear ultrasonic techniques based on different nonlinear acoustic effects are reviewed, the key techniques of nonlinear ultrasonic testing are summarized, and their future development is predicted.