Use of focused pulse-echo ultrasonics for non-destructive inspection of thick carbon-carbon structures

The reinforced carbon-carbon (RCC) heat shield components on the Space Shuttle's wings must withstand harsh atmospheric re-entry environments where the wing leading edge can reach temperatures of 3000 degrees F (about 1650 degrees C). 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 develop quickly an acceptable health monitoring programme. 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 analysis 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 less 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.