Monitoring of early looseness of multi-bolt connection: a new entropy-based active sensing method without saturation

Abstract

Monitoring the looseness of bolted connection, especially at the inception or early stage, is important to ensure its integrity and the wellbeing of structures. In the past decades, significant progress, including several state-of-the-art structural health monitoring approaches, has been made to characterize health status of bolted connections. One of these approaches is the active sensing method with merits such as low cost and easy-to-operate; however, its current damage index (DI) may saturate at high levels of bolt preload, which will impede its application in detecting early looseness of bolted connections. As the main contribution of this paper, we apply the multiscale permutation entropy algorithm to develop a new entropy-based DI that eliminates the saturation phenomenon of current DI (i.e. the signal energy), enhancing the performance of the piezoceramic-enabled active sensing method in monitoring bolt early looseness. Multiple experiments were conducted on a multi-bolt connection with two surface-bonded piezoceramic transducers to verify the effectiveness of the proposed method. Finally, after processing the received signals through the new DI, we demonstrated that the proposed method effectively eliminated the saturation problem, which is significant in detecting early looseness of bolt connections.