Ultrasonic non-destructive testing method based on a flexible microchannel array acoustic metasurface lens

In conventional ultrasonic non-destructive testing (NDT) systems, acoustic lenses (wedges) are often used to redirect incident beams, enhancing sensitivity in specific directions. However, such lenses add considerable rigidity and volume at the macroscopic level and cause significant energy attenuation. Their fixed geometries also hinder acoustic coupling and transmission on surfaces with varying topographies. This work proposes a phase-gradient acoustic metasurface based on flexible microchannel array. This design enables ultrasonic fields modulation in the MHz range and allows conformal coupling to irregular surfaces. Furthermore, the beam redirection properties of metasurfaces with various materials and geometries was investigated, along with wavefront control robustness under mechanical deformation. Experimental results confirmed the method's effectiveness in ultrasonic NDT. Compared to angular wedge lenses, the proposed metasurface lens increased the amplitude of deflected signals by over 40% and eliminated the need for water immersion. Even on inclined surfaces up to ± 15°, the flexible lens conformed well to the target, enabling detection of subsurface defects with a minimum crack resolution of 0.25 mm. This work provides a new paradigm that reconciles rigid contact mismatch and dynamic wavefront control, laying theoretical and experimental foundations for intelligent NDT systems in complex environments.