An improved diagnostic method for detection of bolt loosening in thermal protection panels

Research and development efforts are underway to provide structural health monitoring systems to ensure the integrity of thermal protection system (TPS). In this paper, a smart washer was integrated into the TPS with an embedded piezoelectric element used as an actuator to generate the propagating waves as well as a sensor to receive the diagnostic waves. Theoretical and experimental analyses were completed to demonstrate the feasibility of detecting loose fasteners in a TPS. Based on the damping phenomena of ultrasonic waves across the bolted joints, an improved analytical method was proposed to assess the fastener integrity of a bolted structure; the computing process was simplified greatly by using energy and attenuation time as the extracted features. Experimental results show that this method is capable of locating the loosened bolts, as well as quantifying the degree of panel and bracket bolt loosening.