Bolt early looseness monitoring using modified vibro-acoustic modulation by time-reversal

Structural health monitoring (SHM) of bolted joints has played a vital role in estimation of bolt looseness and prediction of residual service life of bolted connections, thus saving money and significantly improving the efficiency of maintenance routines across industries. In the past decades, several SHM methods, particularly acoustic/ultrasonic methods, have been used to identify the health status of bolted connections. Compared to the linear ultrasound techniques such as active sensing, the vibro-acoustic modulation (VAM) method that is based on nonlinear ultrasonic features has proven its efficiency in bolt early looseness monitoring; however, some drawbacks impede its practical use. The main contribution of this paper is to develop a modified VAM (MVAM) that can circumvent existing problems with practical implementation and provide higher sensitivity. First, the shaker used in the traditional VAM was replaced by a piezoceramic transducer to improve its practicality. Moreover, instead of sine waves, linear swept sine signals were used for both low-frequency (LF) pump vibration and high-frequency (HF) probe wave. In other words, no a priori knowledge of the structural condition is needed, which further broadens the scope of application. Subsequently, the time reversal (TR) method was applied to overcome problems including signal energy dissipation and low signal-to-noise ratio (SNR) in traditional VAM. Moreover, the noise-assisted multivariate empirical mode decomposition (NA-MEMD) and multiscale multivariate sample entropy (MMSE) were used to develop a new damage index (DI) for bolt early looseness monitoring. Finally, multiple repeated experiments were conducted to verify the accuracy of the proposed method and its ability to simplify bolt early looseness monitoring in terms of practical operation, by comparing the proposed MMSE-based DI with nonlinear DI of traditional VAM method. Published by Elsevier Ltd.