Structural health monitoring of bolted joints using linear and nonlinear acoustic/ultrasound methods

The structural health monitoring (SHM) of structures is acquiring a key role in the present time. An in situ system able to assess the health state of bolted joints would save money and maintenance time, by allowing quick assessment of residual life and degradation state of structures. In the last decades, SHM systems based on linear acoustic/ultrasound methods have been investigated extensively. The scope of this study was to develop a reliable index able to assess the loosening/tightening health state of a bolted structure based on linear and nonlinear acoustic/ultrasound parameters. In particular, for the linear acoustic/ultrasound method, a tightening/loosening state index based on the first-order acoustic moment was developed. This method is based on the assumption that a change of signal energy would be recorded for different loosening/tightening states. As for the nonlinear methods, under single- and multi-frequency excitation, high-harmonics generation and sidebands modulation indices were developed. The developed tightening/loosening state index trend was very well reproduced by an analytical expression where they are expressed as function of the torque applied. In particular, the fully loosened (kissing bond) and tight state of bolted structures can be clearly identified by the measured plateau region. The proposed analytical trend approximates the experimental results with excellent correlation. By knowing this trend and measuring the proposed indices, it would be possible to know the torque applied on the bolt and therefore assess the health state of a bolted structure.