Investigations in Crack Detection and Monitoring Based on PPP-BOTDA Distributed Sensing Technology

Cracking is a common phenomenon in the structural members experiencing aging and deterioration problems. Structural health monitoring (SHM) based on crack detection and monitoring plays an important role in safety evaluation of the structures. Sensors developed from different technologies have been employed in crack detection and monitoring. Compared to point-style sensors, fiber optic distributed sensors have the advantage of performing ubiquitous sensing, which can locate the cracks directly. Brillouin scattering in optical fibers is sensitive to strain/temperature variations. One key factor affecting the performance of a Brillouin distributed sensing system is the spatial resolution (SR). Pre-pump-pulse Brillouin optical time domain analysis (PPP-BOTDA) is developed and capable of distributed strain/temperature sensing with centimeter-level SR. In this research, the capability of PPP-BOTDA in crack detection and monitoring is studied both theoretically and experimentally. Appearance of a crack in the structural member will create an extra strain distribution in the sensing fiber. The changes in Brillouin gain spectrum (BGS) induced by the extra strain are analyzed by numerical simulation and validated by the single-crack test. The changes in BGS are characterized by Brillouin frequency shift (BFS) and the difference in BGS width (BGSWD). Overall, results from the test match with calculation from numerical simulation. The crack is successfully detected as a peak from BFS and BGSWD. The widening of the crack can be monitored by the amplitude of the peak. Tests on structural members show that the SR employed will affect the performance of the sensing system when multi cracks appear. The capability of PPP-BOTDA system in differentiating neighboring cracks is studied through numerical simulation followed by experimental tests for a series of dual-crack cases considering different crack spacings. It is concluded that PPP-BOTDA can differentiate neighboring cracks when the SR employed is superior to the crack spacing. The conclusion from the dual-crack cases is validated by results from load tests of reinforced concrete (RC) beams. Based on the comprehensive research, better understanding of the results from the tests is achieved, and general guidance is provided for crack detection and monitoring with PPP-BOTDA distributed sensors.