Volumetric imaging through a doubly-curved surface using a 2D phased array

Ultrasonic phased arrays have become widely used in recent years in non-destructive testing (NDT). However, most NDT arrays are 1-dimensional (1D), which generate 2-dimensional (2D) images from a single position and lack the ability to focus accurately through surfaces that are curved in multiple directions. In this paper, a 2D phased array is used to experimentally image artificial defects (represented by bottom-drilled holes and electrical discharge machined notches) within a test specimen with a doubly-curved surface profile in an immersion configuration. The array is mechanically scanned above the entire surface of the specimen and the 3-dimensional (3D), or volumetric, images generated at each position are combined to produce a single image of the specimen's entire surface. The surface profile is then extracted and discretised for interior volumetric imaging. The results show that the root mean square (RMS) error between the ultrasonically extracted surface and the true surface is 0.04 mm and 95% of absolute errors are less than 0.07 mm. Finally, the positions of visible defects are measured, using (i) the depth above the back wall and (ii) the lateral distance from a notch on the specimen's surface, and compared to their true values. The study shows that the standard deviation of depth and lateral position measurements is 0.68 mm and 0.89 mm respectively. Defects that are located beneath regions of sufficiently steep surface curvature were unable to be imaged.