Nonlinear Guided-Wave Mixing for Condition Monitoring of Bolted Joints

Bolted joints are fundamental to numerous structural components in engineering practice. Nevertheless, their failure or even their loosening can lead to insufficient performance and reduced structural safety. This study presents a theoretical development and experimental investigation into nonlinear guided-wave mixing for integrity monitoring of bolted joints in plates. Combinational harmonics generated due to nonlinear Lamb wave mixing and contact acoustic nonlinearity at the bolted joints were used to evaluate the applied torque level in the joint. The area of the power spectral density in the region of the sum combinational harmonic bandwidth is found to be highly correlated to the applied torque level at the joint. Moreover, the effect of the number of cycles and thus the time duration of the excitation is investigated. The results show that the combinational harmonics remain robust for different numbers of cycles in detecting bolt loosening. The findings presented in this study also provide physical insight into the phenomena of nonlinear Lamb wave mixing for evaluating applied torque in bolted joints, and the results help further advance the use of nonlinear guided waves for damage detection.

Second Harmonic Generation of Guided Wave at Crack-Induced Debonding in FRP-Strengthened Metallic Plates

The use of fiber-reinforced polymer (FRP) has been widely recognized to be an effective and economical way to strengthen existing structures or repair damaged structures for extending their service life. This study investigates the feasibility of using nonlinear guided wave to monitor crack-induced debonding in FRP-strengthened metallic plates. The study focusses on investigating the nonlinear guided wave interaction with the crack-induced debonding. A three-dimensional (3D) finite element (FE) model is developed to simulate the crack-induced debonding in the FRP-strengthened metallic plates. The performance of using fundamental symmetric ([Formula: see text] and anti-symmetric ([Formula: see text] modes of guided wave as incident wave in the second harmonic generation at the crack-induced debonding is investigated in detail. It is found that the amplitude of the second harmonic and its variation with different damage sizes are very different when using [Formula: see text] and [Formula: see text] guided wave as the incident wave, respectively. The results suggest that it is possible to detect potential damage and distinguish its type based on the features of the generated second harmonic.