Decoupling and precise imaging of multiple microcracks smaller than 1 mm

The detection of microcracks and decoupling of multiple cracks are crucial for ensuring the safe operation of equipment. Unfortunately, microcracks below 1 mm scale cannot be accurately imaged yet, and decoupling multiple microcracks is even more difficult. Therefore, a nonlinear phased array based on second harmonic was developed for imaging of multiple micro-cracks by experiments and numerical simulation with Total focus method (TFM) by Full Matrix Capture (FMC), the innovation of which is to evaluate the microcrack with small size by low frequency. The nonlinear ultrasonic phased array imaged experimentally successfully a facilitate micro-crack with 0.47 mm measured by the optical microscope. Furthermore, the numerical investigation on the mechanism of nonlinear phased array found that micro-cracks could generate the second harmonic, which follows the superposition principle and can be used for imaging micro-cracks. The minimum identification accuracy of nonlinear ultrasonic phased array was 0.04 mm for 1 MHz fundamental frequency. It overcomes the detection size limitation of linear ultrasonic array with the same fundamental frequency, which is half of the wavelength of fundamental wave as 3.063 mm. In addition, the spatial recognition of double micro-cracks in the horizontal and vertical direction were obtained by 10.00 mm and 5.00 mm, respectively. The nonlinear ultrasonic phased array shows high detection accuracy for multiple micro-cracks, which provides an experimental and theoretical basis for early damage detection and additive manufacturing defects imaging. © 2025 Published by Elsevier Ltd.