MTR-MUSIC: A time-reversal based super-resolution ultrasonic imaging method for multilayer structure defects

Closely spaced defects, like porosity and microcracks, are common during material manufacturing and service. Due to the limitations of array aperture and ultrasonic frequency, traditional ultrasonic imaging methods cannot characterize defects under the Rayleigh limit. The time-reversal multiple signal classification (TR-MUSIC) method is effective for subwavelength defect imaging. However, TR-MUSIC is only suitable for isotropic medium, not including multilayer mediums. Furthermore, the axial localization ability of TR-MUSIC is influenced by severe axial extended artifacts. To deal with the above limitations, a modified time-reversal multiple signal classification (MTR-MUSIC) method is proposed. An improved Green's function is developed to correct the travel time errors caused by acoustic refraction in multilayer mediums, and the total focusing imaging index is utilized as a weighted factor to mitigate the axial artifacts associated with TR-MUSIC. Experiments are conducted on 2 mm spaced side-drilled holes in multilayer structure specimens. The results show that MTR-MUSIC can accurately image two holes with a distance of 0.43 Rayleigh limit in triple-layer mediums, and the full width at half maximum of the axial main lobe is below 1 mm, which shows better axial localization ability. It demonstrates promise for MTR-MUSIC in super-resolution imaging defects among anisotropic mediums, including immersion detection, weld structures, and composite materials.