Non-destructive fiber Bragg grating based sensing system: Early corrosion detection for structural health monitoring

Steel corrosion is known as one of the major structural defects in steel structures such as pipelines. The high resolution of fiber Bragg grating (FBG) sensor and its ability to provide real-time monitoring make it a potential candidate for steel corrosion detection. Nevertheless, fiber optic sensor is now been widely used in civil engineering for structural health monitoring purpose. In this study, a non-destructive corrosion detection approach is developed using FBG sensors. The sensor comprises FBG coated with mixed pH-sensitive hydrogel and PDMS strain-sensitive coating. FBG sensors were embedded on the specimen to monitor the expansion strain caused by rebar corrosion. Specimens were placed in different environment conditions namely air, acidic and alkaline conditions, to experience different corrosion rate. The sensing principle is based on a Bragg wavelength shifts resulted from the induced strain on the FBG due to mechanical expansion and swells, in response to the changes in pH of the coating materials. A significant wavelength shift occurred at about day 20, inducing wavelength shift of 0.27 nm, 0.06 nm and 0.011 nm under acidic, air and alkaline conditions respectively. The relationship between wavelength shift, corrosion rate and strain induced is investigated and validated through this experiment. These sensors can be installed for real-time monitoring and early corrosion detection due to its non-destructive and highly sensitivity performance.