While the irrigation and heating method has proven effective for monitoring cracks in underwater concrete structures, its localization accuracy is limited by the spacing of sensing-heating elements. This study aims to introduce an enhanced crack localization strategy that integrates a thin copper tube with a high-density fiber Bragg grating (FBG) array, thereby improving localization accuracy while maintaining monitoring efficiency.
A thin copper tube is embedded between the monitoring and irrigation tubes. A bare optical fiber with a sensing section of multiple fiber Bragg gratings (FBGs)at specific intervals is threaded through it. After coarse crack localization via the irrigation and heating method, the FBG section is moved near the crack. Hot water is then injected into the irrigation tube, forming a localized high-temperature zone around the crack, with efficient heat conduction via the copper tube. The precise crack location is determined based on the characteristic temperature response patterns at each FBG measurement point.
Experimental results under varying crack widths (0.3 mm and 0.8 mm) and water temperatures (45?°C, 55?°C and 65?°C) show that both heating rate and temperature rise amplitude follow a unimodal distribution centered at the crack. The proposed method achieves a localization accuracy of 10 mm (half the FBG spacing), with sensitivity improving as crack width and injected water temperature increase.
This paper presents a novel approach for precise crack localization using optical fibers and active thermal tracing. It innovatively combines a thin copper tube with a movable FBG array to address the final critical step missing in traditional irrigation and heating methods for underwater concrete structures. The study systematically examines temperature rise and heating rate distributions near cracks and their influencing factors. A novel two-stage monitoring strategy is proposed, starting with coarse screening via the irrigation and heating method, followed by precise localization using the proposed technique, significantly improving both accuracy and operational efficiency.
