Focusing aberration corrections for ultrasonic inspections of disk forgings when using a surface compensating mirror and segmented annular phased array

Abstract

Phased array transducers are playing an increasing role in ultrasonic nondestructive evaluation inspection applications, and one area of their use is in the inspection of critical jet engine components such as titanium alloy turbine disk forgings. Inspection of these forging disks is performed during stages of their manufacturing, particularly at an intermediate stage when the forging disk has a deliberate “sonic shape.” A forging’s sonic shape, from which the final disk shape will be machined, is conducive to ultrasonic testing inspections due to its simple entry surfaces. These entry surfaces are primarily planar or conical surfaces.

In prior work, forgings from General Electric Aircraft Engines, Pratt & Whitney, and Honeywell Engines and Systems were ultrasonically inspected through their planar interfaces, accomplished with a 10 MHz, segmented annular, compound spherical, phased array transducer designed to perform inspections through planar interfaces. Proof-of-concept research used this array along with surface compensating ultrasonic mirrors to inspect through the conical entry surfaces in these forgings. While successful, it was believed that the results of these inspections fell below what would be possible due to non-ideal focusing conditions and other focusing aberrations.

To correct for focusing aberrations when inspecting through forging material planar and curved interfaces, three progressively more sophisticated ray-tracing algorithms were developed to generate delay time sets for phasing transducer array elements, including an initial 2D method from prior work, a refined 2D method designed to more accurately account for refraction at interfaces, and a 3D method designed for circumferentially phasing the segmented annular array.

Ultrasonic inspections using these methods were performed on two sets of forging material specimens, with either planar or curved interfaces, thicknesses ranging from 0.2 inches to 2.7 inches, and each containing a 1/128-inch-diameter flat bottom hole (#1/2 FBH) reflector. FBH responses from planar interface specimens allowed comparison of the initial and refined 2D inspection methods. The refined 2D inspection method used on the planar and curved interface specimens evaluated the performance of the surface compensating mirror. The refined 2D and 3D inspection methods were compared using the curved interface specimens.