A high resolution approach for nonlinear sub-harmonic imaging

• Resolution of nonlinear sub-harmonic imaging is enhanced by MUSIC.
• Sub-harmonic imaging with MUSIC is formulated for frequency selectivity.
• Closed crack location is verified on cross section after tearing specimen apart.

Nonlinear sub-harmonic phased array imaging is used to visualize closed cracks. Sub-harmonic imaging has broader beam width than that of input frequency imaging due to its longer wavelength. If the broad beam width results in low image resolution, it can be hard to determine the locations of cracks. Various methods have been developed to enhance the resolution, and MUltiple SIgnal Classification (MUSIC), in particular, is an old but well-established way to do so. A form of sub-harmonic imaging that implements MUSIC is presented here. A numerical simulation was used to compare the proposed method with a conventional one, and MUSIC gave narrower beam width. Experiments also demonstrated that MUSIC could improve the resolution of sub-harmonic imaging.