Sensitivity Enhancement of Long-gage FBG Sensors for Macro-strain Measurements

In our recent study, a structural health monitoring strategy based on distributed fiber optic sensing techniques has been proposed to utilize the strain responses throughout the full or some partial areas of structures to detect the arbitrary and unforeseen damage. However, to perform this strategy more effectively, there is an increasing demand for improving the ability of such sensors to measure small structural responses, especially for the cases such as damage identification based on ambient vibration tests, fatigue crack monitoring of steel structures and crack detection of reinforced concrete structures. This work puts forward a novel packaging design for long-gage fiber Bragg grating (FBG) sensors to enhance the measurement sensitivity of strain responses. The basic idea is to utilize two materials of different stiffness to package the in-tube optical fiber and impose the deformation within the gage length largely on the short-gage sensing part of FBG. Proof-tests indicate that the measurement sensitivity of the innovative FBG sensor can be artificially enhanced by adjusting the two recoating materials and their respective lengths. A series of tests are carried out to verify the ability of the improved sensors to detect small crack and measure slight vibration.