Retirement for Cause (RFC) Eddy Current Inspection System Calibration Improvement

摘要

Examination of the reliability test data from the eddy current Retirement For Cause (RFC) systems installed at the-San Antonio and Oklahoma City ALCs has revealed occasional systematic shifts in the probability of detection (POD) curves created from inspecting specimens containing fatigue cracks. Analysis showed that the shift was not related to the fatigue cracks. The most likely cause was thought to be the gain calibration process uses electro-discharge machined (EDM) notches as the artifact that causes the eddy current response, and it was thought that some of the shift in the POD curves was due to variability of the notches. EDM notches have been used in eddy current systems because of their similarity to fatigue cracks. However, the RFC eddy current systems were designed such that the calibration process did not require crack-like artifacts in order to successfully perform gain calibration. The research identified two candidate technologies fix the new calibration artifacts. A photolithography technique was investigated to produce both conductive lines and marked-off areas that caused reproducible eddy current responses. Precisely placed wire segments were also investigated aa a technique to produce uniform and repeatable eddy current responses. Both types of calibration artifacts were fabricated and extensively tested for repeatability, uniformity, and ease of manufacture. Ultimately, the wire-type artifact was selected for more extensive testing due to the ease in which they were produced- Many wire-type artifacts were constructed having the same size, shape, and & fit conventional EDM notch calibration blocks used on the RFC systems.