Crack Damage Detection in Tunnel Structures Based on Non-contact Laser Vibration Measurement

Tunnel engineering is widely used in China and is one of the most important infrastructure in the field of transportation. In order to ensure their safe operation, it is necessary to conduct inspections and damage detection of tunnel structures. The traditional manual inspection has low efficiency and huge consumption. The research on automatic tunnel inspection technology using the concept of structural health monitoring has great engineering significance. However, the embedded piezoelectric accelerometers used in tunnel structural health monitoring (SHM) face significant limitations due to their contact-based measurement approach and narrow bandwidth. These constraints often lead to reduced accuracy and flexibility during the monitoring process. Additionally, the vibrations experienced by tunnels during their service life are challenging to capture precisely, further complicating effective SHM. To address these issues, laser vibrometer, as a non-contact vibration measurement technology, offers several advantages. It provides high precision, a wide bandwidth measurement range, high sensitivity, and long-distance measurement capabilities, which facilitate more convenient and accurate collection of structural vibration responses. Based on these advantages, a novel detection method based on the principles of laser doppler vibrometer (LDV) using homodyne interferometry and transmissibility function theory is proposed and studied in this paper. In this method, the structural response data was directly exploited to extract damage characteristics, providing a more reliable and adaptable solution for SHM. The paper begins by analyzing the principles of laser homodyne interferometry, a key component of the LDV system. Following this, the study explores operational modal analysis (OMA) based on transmissibility function theory, identifying and proposing indicators for structural damage detection and localization. A laser homodyne interferometry measurement system is designed and implemented, followed by both numerical simulations and scaled model experiments to validate the proposed methodology. During the simulations and experiments, response signals are collected from various sensor points on the structure. These signals are then processed using the Random Decrement Technique (RDT) and Hilbert-Huang Transform (HHT) to extract the natural frequencies of the structure. Subsequently, the transmissibility function is employed to extract damage characteristics, with curvature mode and curvature mode change rate serving as the primary indicators for assessing the extent and location of the damage. The experimental results demonstrate that by using the proposed method, tunnel damages can be effectively identified and localized under ambient excitation conditions. This capability is particularly advantageous in real-world applications where precise vibration measurement is challenging. The non-contact nature of the LDV system, combined with advanced signal processing techniques, offers a substantial improvement over traditional SHM methods, making it a valuable tool for the non-destructive testing of tunnel structures.