Toward an ultra-high resolution phased-array system for 3D ultrasonic imaging of solids

Ultrasonic phased-array (PA) systems have been widely adopted in the field of nondestructive evaluation for material characterization and imaging of internal defects. Whereas many defects exhibit complex three-dimensional structures, most PA systems provide only two-dimensional images. In this Letter, we demonstrate the ability to create high-resolution 3D images of internal defects using a PA system based on a piezoelectric and laser ultrasonic system (PLUS). The PLUS combines a piezoelectric transmitter to insonify the structure to be inspected with a laser Doppler vibrometer to create a matrix array of receiver points without contact. The small size of the laser beam results in an ultra-multiple number of elements on the order of thousands, which is impossible to achieve with a conventional piezoelectric matrix array transducer. An emission from a piezoelectric transmitter compensates for the intrinsically low sensitivity of a laser Doppler vibrometer. After formulating the 3D imaging algorithm of the PLUS, we demonstrate that the PLUS with 4096 receiving points (i.e., 64x64 points) achieves high-resolution 3D imaging in a specimen with a flat bottom hole. We also visualize the complex structure of stress corrosion cracking. We believe that the 3D imaging capability of the PLUS may open up new avenues to the accurate evaluation of material strength, the identification of the types of defects, and the elucidation of the mechanisms of defect initiation.