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Fatigue cracks, as one of the early performance deteriorations of plate-like structures, are critical to be detected in a timely manner to prevent further failures and disastrous accidents. But the intimate contact of crack interfaces is difficult to form a significant scattering field of traditional ultrasonic Lamb waves, resulting in the insensitivity of Lamb wave phased array-based imaging methods to crack imaging. To break through the detecting limitations of conventional Lamb wave phased array, the nonlinear Lamb wave-based phased array method is first proposed to realize fatigue crack imaging, which is achieved by a new form of phased array with hybrid elements, signal processing for harmonic enhancement and time-reversal based harmonic field focusing. The hybrid array is designed for efficient transmission and capture of fundamental wave and harmonics, and Pulse-inversion technology and continuous wavelet transform filter are used to extract and enhance second harmonic response, and the images are obtained by the time domain topological energy imaging algorithm. Finally, the imaging results of barely visible fatigue cracks and through-holes in metallic plate structures using linear and nonlinear Lamb wave-phased array are compared. The nonlinear ultrasonic Lamb wave-phased array demonstrates excellent capability of fatigue crack imaging.
Li et al. (Mon,) studied this question.