Structural health monitoring (SHM) is the set of non-intrusive techniques used for early damage detection and recognition in structures. Using advances from communication and sensing technology in the past two decades, various SHM systems have been developed to ensure infrastructure safety.;Bolted joints in most aerospace structures are useful components that enable easy assembly and disassembly for maintenance purposes. However, human workmanship engaged in the process of assembly makes an integrity check of the bolted joints a necessary process. Robust monitoring of such bolted joints is a critical task for increasing reliability in aerospace structures.;This dissertation investigates methods for SHM of L-shape bolted joints. These methods include using ultrasonic signals and thermal contact resistance metrics correlated to contact pressure to assess the joint's integrity. Moreover, contact pressure distribution at the joint surface was examined using pressure sensitive films. Experimental measurements showed that there were areas on the joint interface with very low to no contact pressure. Such low pressure can result in local shear slip. Furthermore, it is experimentally and numerically demonstrated that thermal contact resistance across bolted joints, although difficult to measure, has the ability to monitor joint integrity. It is also demonstrated that ultrasonic signals are a sensitive, yet easy to use, technology for monitoring the integrity of bolted joints. The proposed ultrasonic based feature proved capable of describing the integrity of bolted joints at the time of assembly. Time-dependent effects of ultrasonic transmitted signals and the SHM feature were studied on the L-shape bolted joints. Experiments showed that time has a significant effect on the ultrasonic signals transmitted through the joint interface.
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