L-1 Dovetail UT
In order to examine 100% of the external surfaces of the finger type dovetail on low pressure (LP) rotors, it is required that the dovetail pins be removed along with the blades. The nondestructive evaluation (NDE) performed after removal of the pins and blades include visual testing (VT) and wet fluorescent magnetic particle testing (WFMT). The usual periphery NDE done without dovetail pin and blade removal only examines the inlet and exhaust faces of the side finger dovetails but no examination of the rest of the disk rim is possible. The Sherco U3 LP rotor L-1 disk rim failure root cause analysis (November 2011) performed by Thielsch Engineering Inc. shows that stress corrosion cracking in the finger type dovetail develops at the first cross section change as shown in Figure 1 above.
A rotor that has similar Finger type dovetails was examined using a special ultrasonic testing (UT) technique for rotors that have a central bore. The UT technique examined the finger dovetail stress concentration areas where cracking normally develops between pin holes in the circumferential-axial plane as shown in Figure 2 below. The UT technique examined the dovetails from the bore surface up to approximately 24 inches of metal path using 2.25 MHz longitudinal wave transducers with wavelengths of 0.102 inches. Additional scans using 2.25 MHz shear wave transducers were also performed to try to detect the presence of partial to through-wall cracking. Figure 3 below shows a representative scan of L-1 dovetails with no crack-like indications.