The reinforced carbon-carbon (RCC) heat shield components on the Space Shuttle's wings must withstand harsh atmospheric reentry environments where the wing leading edge can reach temperatures of 3,000°F. Potential damage includes impact damage, micro cracks, oxidation in the silicon carbide-to-carbon-carbon layers, and interlaminar disbonds. The thick, carbon-carbon and silicon-carbide layers in the heat shield panels are difficult to inspect and existing techniques were deemed to be inadequate. Since accumulated damage in these structures can lead to catastrophic failure of the Shuttle's heat protection system, it became imperative for NASA to quickly develop an acceptable health monitoring program. NASA selected a multi-method approach for inspecting the wing leading edge which includes eddy current, thermography, and ultrasonics. Sandia Labs produced the in-situ ultrasonic pitch-catch inspection method. Optimum combinations of custom ultrasonic probes and data anlaysis were merged into the overall inspection method needed to properly survey the heat shield panels. Comprehensive validation tests revealed that the ultrasonic pitch-catch inspection system is capable of reliably finding flaws of 0.25" diameter or greater in the heat shields. The inspection system is now in use at NASA to complete "Return-to-Flight" certification inspections prior to each Shuttle launch.
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