Multi-modal ultrasonic TFM imaging inspection for micro-defects of thin-diameter rods

The highly curved surface of thin-diameter rods poses significant challenges for conventional ultrasonic testing technology. Ultrasonic beam will cause severe scattering and waveform transformation on the surface of thin-diameter rods, resulting in weak signals for micro-defects. Therefore, the multi-modal ultrasonic total focusing method (MTFM) is proposed for thin-diameter rods based on the acoustic field threshold. Firstly, the multi-modal ultrasonic propagation of array beams at the liquid-solid interface of the thin-diameter rod is analysed. Then, the acoustic field distribution within the rod is computed using a dual-medium phased array model. The effective inspection areas of longitudinal waves and transverse waves are divided, and the MTFM algorithm for a thin-diameter rod is established. Finally, a φ10 mm rod with micro-defects is machined, and the ultrasonic TFM inspection experiment of the thin-diameter rods is carried out. Compared with conventional TFM, for defects with a diameter of φ0.4 mm, the average signal-to-noise ratio (SNR) of the proposed MTFM algorithm increased by 18.48%, while the average array performance indicator (API) decreased by 0.68.