Ultrasonic Flaw Detection Using a Time Shifted Moving Average

The Retirement For Cause (RFC) inspection system uses both eddy current and ultrasonics for the real-time inspection of gas turbine engine components. The ultrasonic inspection module uses a squirter technique to couple the ultrasound to the engine part. The current flaw detection requirement for the ultrasonic system is 0.020 inch diameter, mal-oriented, penny-shaped, internal voids and extensive testing has shown that the squirter technique is comparable to immersion for the detection of these type defects [1,2]. The signal-to-noise ratio is nearly the same for both techniques with only occasional water noise signals occuring in the squirter technique. Additionally, on complex geometries, low amplitude reflections from the nozzle are sometimes present. A typical scan of a bore of an engine part (see Figure 1) has approximately 40,000 A-scans with each A-scan having 300–1000 digitized points; thus even an occasional noise signal can add up to many false indications over the course of an entire scan. For example, using the above numbers a noise signal 0.01% of the time would result in 2000 “flaw indications” in one scan. To help reduce the number of noise-induced flaw indications, an algorithm has been developed to allow the system to distinguish between a signal from an actual defect and one induced by stray reflections, electrical, or water noise