High-speed and super-resolution ultrasonic imaging method for multilayer structures

Multilayer structures are necessary in aerospace and transportation. The ultrasonic imaging technique is widely used to detect these parts, ensuring their reliability in manufacturing and service. However, the most common imaging method, TFM (Total Focusing Method), fails to characterize defects when the defect spacing is less than the Rayleigh limit. Although the MTR-MUSIC (Modified Time-Reversal Multiple-Signal Classification) method can realize super-resolution imaging for closely spaced defects, its computational complexity increases exponentially with the number of layers, which is impossible for real-time scenarios. To overcome this problem, the F (fast)-MTR-MUSIC is proposed to realize high-speed and super-resolution defect imaging in multilayer structures. Firstly, the nonstationary wavefield extrapolation is used to migrate array data to the upper surface of the defect layer. Then, the wavenumber imaging index and the TR-MUSIC imaging index are calculated using migrated array data, and are multiplied together to form the F-MTR-MUSIC imaging index. Through this method, the computational complexity is proportional to the number of layers rather than an exponential relationship. Results demonstrate that the proposed F-MTR-MUSIC, which is the state-of-the-art imaging method in the super-resolution imaging of multilayer structures, exhibits a higher resolution than TFM, and faster speed than MTR-MUSIC in multilayer structures. For the defects spaced at 0.78 times of the Rayleigh limit in triple-layer structures, the PCID of the F-MTR-MUSIC is larger than 10.67 dB, and the computation time is <4.33 s, whereas the PCID of the TFM is smaller than 2.06 dB, and the imaging time of the MTR-MUSIC is longer than 172.40 s. Additional imaging results revealing inclined distributed holes and cracks further validate the practicality of F-MTR-MUSIC. This method shows promise for online monitoring of material manufacturing processes, including injection molding and additive manufacturing.