UC Berkeley

Brillouin scattering–based distributed fiber optic sensing (Brillouin-DFOS) technology is widely used in the long-term health monitoring of large-scale structures to provide early warnings of structural degradation for timely maintenance and renewal. Material cracking is one of the key mechanisms contributing to structural failure. However, the conventional strain measurement using a Brillouin-DFOS system (Brillouin optical time-domain analysis/reflectometry (BOTDA/R)) has a decimeter-order spatial resolution, making it difficult to measure the highly localized strain generated by a sub-millimeter crack.

This research develops and implements two crack analysis methods based on Brillouin scattering spectrum (BSS) data to overcome the spatial resolution–induced crack measurement limitation of the BOTDA/R system. The first method extracts the maximum strain within the spatial resolution around the measurement points by taking the derivative of the BSS data and tracking their local minimums. The crack can be located by comparing the variation of the extracted maximum strain within the spatial resolution around different measurement points along the fiber. The performance of the method is demonstrated and verified by monitoring the opening of a synthetic crack between two wood boards in a laboratory test. In the test, the crack width estimation error is $\pm$0.15 mm for a crack as narrow as 0.23 mm. The method is also applied to a thin bonded concrete overlay of asphalt pavement experiment, in which the growth of a transverse joint penetrating the concrete–asphalt interface is monitored. The method successfully distinguishes the position, traces the strain variation, and estimates the width of the sub-millimeter crack using a Brillouin-DFOS system with a 75-cm spatial resolution.

The second method back-calculates the strain profile by decomposing the BSS data. The method uses the least squares–based algorithm, non-negative least squares, to back-calculate the strain profile within the spatial resolution around each measurement point. The performance of this method is verified using the same wood board separation test as the first method. With the strain profile being calculated accurately, the crack width estimation error is improved to $\pm$0.005 mm for a crack as narrow as 0.23 mm compared to $\pm$0.15 mm error of method I. The method is also applied to monitor crack openings within a slag cement-cement-bentonite (SCCB) beam under four-point bending. The method can identify not only the crack generation but also the occurrence of slippage from the estimated strain profile. It successfully measures a sub-millimeter crack in the SCCB beam with an error of $\pm$0.005 mm.

One of the advantages of the DFOS technology is that it is designed for multi-purpose applications; that is, a fiber instrumented in the structure and connected to different types of interrogators can be used to take many types of measurements, such as strain, temperature and vibration. In this research, a fiber embedded in pavement is connected to an optical frequency domain reflectometry (OFDR) system, and DFOS technology is applied to detect and monitor the movement of vehicles by measuring their induced dynamic deformation of the pavement. From the footprint formed by the distributed dynamic strain profiles measured by the OFDR system, the moving direction and speed of the vehicles are accurately evaluated. Different vehicles and pedestrians are distinguished from each other based on their induced strain values and footprint patterns.

This research therefore develops two BSS-based methods to improve the performance of a BOTDA/R system in measuring the heavily concentrated deformation induced by cracks and investigates the ability of DFOS technology to detect and track vehicles.