Verification of a built-in health monitoring system for bolted thermal protection panels

The integrity of thermal protection systems (TPS) is crucial to ensure a successful mission of space exploration vehicles. In this paper, an attenuation-based built-in diagnostic technique is demonstrated through a carbon-carbon (C-C) panel for the detection of bolt loosening under extreme environments. The proposed technique is based on the attenuation properties of propagating waves, which depend on the torque level and contact material at the bolted-joint interface. A smart washer was developed with an embedded piezoelectric element used as an actuator to generate the propagating waves as well as a sensor to receive the diagnostic waves. The washers were installed in each bolt on the TPS panel. During the course of the investigation, a complete diagnostic system including smart washers, diagnostic algorithms, and electronic hardware was developed to verify the proposed attenuation technique. Experiments which simulate the acoustic environments during the re-entry process were conducted using a shaker in the AFRL to verify the technique. The test results revealed that the proposed system successfully identify the bolt loosening and failure. More tests are being considered to include temperature effect.