Defect detection of spherical heads by time-of-flight diffraction

Abstract

Inspecting spherical heads with ultrasonic time-of-flight diffraction (TOFD) is restricted in quantitative accuracy, since the travel path of lateral wave is not along with the structured surface. Meanwhile, a layered dead zone related to lateral wave generates at the near-surface zone and is greater than that in flat plate. In this paper, the vertical inner-wall defects outside and inside the dead zone in the girth weld of spherical head are detected and located by establishing the quantitative relationship between concerned signals (diffracted longitudinal wave or mode-converted wave) and defect depths. Focusing on the aluminum alloy spherical head with 60.0 mm curvature radius and 20.0 mm thickness, the positioning errors of defects with depths of 15.0 mm and 5.0 mm in simulation and experiments are no more than 9.6 % at 5 MHz inspection frequency. Finally, we verify the theoretical relationship by combining simulated and experimental images, analyze the variation of the dead zone in spherical head with the scanning position under different curvature radii and discuss the influence of curvature radius on location accuracy.