Development of a Rotating Ferromagnetic Resonance Eddy Current Probe for Inspecting Small Radius Curved Surfaces on Gas Turbine Engine Components

The Retirement-for-Cause (RFC) program has a target flaw size of 0.25 mm long × 0.13 mm wide that must be reliably detected in complex geometries such as the key slot in the jet engine interstage seal shown in Figure 1. A rotating eddy current probe has been developed in an ongoing exploratory development program for advanced NDE methods, which is based on a YIG (Yttrium Iron Garnet) ferromagnetic resonance (FMR) probe. This rotating probe is designed to inspect the small radius (1.8 mm) corners of the key slot. The FMR probe is utilized in an active mode in which it operates as the resonant element in an oscillator circuit (active FMR probe) at frequencies in the order of 800 MHz. A laboratory breadboard signal processor system has been fabricated which converts the active FMR probe output into voltage levels corresponding to it’s frequency and amplitude. It has been demonstrated in previous work2 that flaw and liftoff information can be generated with phase separation from the frequency and amplitude of an active FMR probe. The data presented in this paper, however, is processed only from the amplitude signal. Due to the physical restrictions put on the fabrication of the small radius active FMR probe, we were unable to achieve flaw/liftoff separation by mixing the frequency and amplitude signals as in our previous work. However, we feel that flaw/liftoff separation from the physically small probes is possible with some redesign in the oscillator circuit