Highly Sensitive Low-Energy Laser Sensing Based on Sweep Pulse Excitation for Bolt Loosening Diagnosis

The hammering method used by inspectors is generally applied to inspection of looseness of bolted parts. This method entails active sensing through the input of an excitation signal produced by the inspectors, and has the advantage of practical experience acquired over many years. The sound (vibration) and sensation produced upon excitation by a hammer can be used as indices, but also leads to the disadvantage of unavoidable qualitative testing and human error. Laser remote sensing (LRS) uses the same diagnostic principle as the hammering test for remote, high-speed, and quantitative inspection. However, the accuracy of LRS must be improved and the required pulse energy must be reduced. Hence, we propose the sweep pulse excitation method, which realizes LRS with low pulse energy through the resonance oscillation caused by sweeping the laser irradiation frequency. A performance test on bolted joints was conducted with regard to bolt loosening diagnosis using two methods, namely, the conventional single-pulse excitation method and the sweep pulse excitation method. With the sweep pulse excitation method, the laser-induced vibration is produced only by absorbing the laser pulse energy which is caused by thermo-elastic waves and without using the elastic wave induced by the laser ablation. The experimental and calculation results reveal that, with the proposed method, the measurement accuracy improved by 104 times, while the required pulse energy decreased by 102 times. The adaptability of the low-pulse energy laser contributes toward the development of inspection systems because it achieves size and cost reduction, and inspection convenience.