Effect of surface compensation for imaging through doubly-curved surfaces using a 2D phased array

In order to characterise and size defects located beneath complex surface geometries in components when using ultrasonic non-destructive testing methods, correct surface compensation is of critical importance when generating images. The effect of the accuracy of surface compensation when generating Total Focusing Method (TFM) images through a doubly-curved surface with a 128-element sparse 2D phased array has been investigated. An aluminium test specimen was created with an axi-symmetric surface with a Gaussian profile in order to create a surface that curves in multiple directions. By determining the resolution of a defect for different assumed surface profiles, it has been shown that an extracted surface works best when compared to a planar surface parallel to the array and a 2D fitted planar surface. The results show that, even for surfaces that are curved very slightly, incorrect surface compensation can lead to significant image de-focusing.