Dark-field ultrasonic imaging method using mode-converted longitudinal evanescent field

We propose a dark-field evanescent imaging method to visualize surface/subsurface micro defects with a high signal-to-noise ratio (SNR). This method utilizes the mode-converted longitudinal evanescent field (MCLEF) generated at defects by the incidence of a shear (S) wave. When an incident S wave only has the in-plane displacement on the top surface of a specimen, the 2D scan of a laser Doppler vibrometer, that can only measure out-of-plane displacements, can selectively probe the MCLEF with out-of-plane displacements. Note that the MCLEF can be generated even at a defect that is much smaller than the diffraction limit. In this paper, after describing the principle of the proposed method, we prove the concept in a specimen with a hole by finite element (FE) simulation and experiments. Further FE simulations demonstrate its super-resolution imaging capability for holes of different sizes and higher SNR than a conventional method for various defect geometries.