UT of Circumferential Cracking
Reinhart & Associates, Inc. has, in the last 40 years, performed boresonic examinations of more than 1,000 steam turbine and generator rotors. The type of damage detected during service on these rotors include mechanical and thermomechanical fatigue cracking in the axial-radial and circumferential-radial planes. These service related defects will propagate perpendicular to the stress induced during service. For example, bore circumferential or hoop stress caused by thermal and mechanical stresses during startup and operation will induce bore surface connected defects in the axial-radial plane. Axial and bending stress applied on the shaft by thermal and mechanical forces will induce surface connected defects in the circumferential-radial plane.
An example of circumferential cracking is depicted on the near figures that shows a HP-IP rotor found to have a circumferential “crack-like” indication detected by liquid penetrant testing (PT) and phased array ultrasonic testing (PAUT) from the outside diameter (OD) surface. The indication was verified by boresonics from the inside diameter (ID) of the bore using axial FWD and AFT 45-degree shear wave scans. The dimensions of the indication were 11 inches long in the circumferential direction and 5 inches deep in the radial direction measured from the rotor outside diameter (OD) surface.