Abstract: Detection accuracy and maximum detection depth in large thickness components with high attenuation often pose a challenge for conventional linear ultrasonic arrays or planar ultrasonic arrays because of the serious loss of acoustic energy of phased array ultrasonic beams in the non-focused area. In this study, a three-dimensional acoustic field distribution model of synthetic beams is first established to analyze the characteristics of acoustic energy distribution of different ultrasonic array transducers. It is found that the annular ultrasonic transducer has a better acoustic field at the same focus depth and a fully symmetric beam focal spot with fewer elements. Then, a Total Focusing Method (TFM) applied to annular ultrasonic arrays and a group velocity optimization method of anisotropic material detection are proposed. In this method, the full matrix data is reconstructed along the axial line of the transducer to realize pointwise infinity focusing along the depth. Finally, experiments on a 3D printed titanium alloy specimen with prefabricated defects are conducted based on the developed ultrasonic array immersion C-scan system. The experimental results show that the annular array transducer based on the TFM algorithm can achieve accurate detection of the flat bottom hole and the transverse hole defects with a diameter of 0.8 mm and depth of 5 mm in the 55 mm thick specimen. Compared with the conventional dynamic depth focusing method, the C-scan images of the TFM have a better signal to noise ratio and quantitative accuracy.

Key words: total focusing methods, annular array transducers, ultrasonic arrays, detection of high attenuation material, three-dimensional imaging