Micro-defect imaging with an improved resolution using nonlinear ultrasonic Lamb waves

Ultrasonic inspection is a widely used nondestructive testing approach in industrial fields for more accurate life prediction and efficient management strategies of critical structural components. However, it is quite challenging to detect a kind of micro-defect, whose size is much smaller than the ultrasonic wavelength but larger than the ultrasonic amplitude (namely, there is no appearance of non-classical acoustic nonlinearity). In this article, identification and imaging of a micro-defect of this size range with an improved resolution is conducted by the combination of the second-harmonic generation (SHG) of ultrasonic Lamb waves and the reconstruction algorithm for the probability inspection of damage. An intuitive model is first developed to explore the physical mechanism of a micro-defect-induced variation of generated second harmonic of a primary Lamb wave in a plate. Variations of amplitudes of second harmonics generated in propagation paths are used to construct the micro-defect image. A phase-reversal technique is employed to enhance the signal-to-noise ratio of the SHG. Comparisons between images constructed by linear and nonlinear acoustic features of Lamb wave propagation are presented. Results show that the image of the micro-defect with an improved resolution is successfully obtained by the proposed approach, while there is no visualized result obtained by the conventional linear ultrasonic one.