Evaluation of bolt loosening using a hybrid approach based on contact acoustic nonlinearity

Bolted connections are widely used in various industrial sectors. Different loading conditions to which the connections are subjected can result in looseness of bolts, entailing timely detection to avoid system failure due to this. Linear ultrasonic inspection-based methods are currently the prevailing approaches in non-destructive evaluation (NDE) to serve this purpose. However these methods may lose their effectiveness when used to detect slight looseness of bolts. In this study, two nonlinear NDE approaches for detection of bolt loosening, using second harmonic nonlinearity of Lamb waves and vibro-acoustic modulation (VM), respectively, were developed. An analytical model based on Hertzian contact theory was proposed to establish a correlation between applied torques on a bolted joint and its contact acoustic nonlinear behaviours. These two approaches were validated experimentally by detecting bolt loosening in both metallic and composite connections. In the first approach, Lamb waves were introduced by a low-profile piezoceramic wafer, and an acoustic nonlinear parameter was calculated from the magnitude of extracted second harmonic of the Lamb waves; in the second approach, two harmonic excitations (i.e., a low-frequency vibration and a high-frequency probing wave) were produced by a shaker and a piezoceramic stack actuator, respectively, and the response sidebands were identified in spectra, to ascertain a modulation index. The variation of the nonlinear parameter and the modulation index, subjected to the torque applied on the bolt, has been found in a good agreement with theoretical prediction using the model. Results have demonstrated that the developed approaches are accurate, reliable and effective to detect bolt loosening at an early stage in a wide range of operational frequencies.