Strain monitoring mechanism of geogrids based on optical fiber sensing technology

The fiber Bragg grating (FBG) sensor is an important tool to measure the surface strain of geogrid. There is a strain transfer coefficient between the real strain of geogrid and the strain measured by FBG sensor due to the existence of adhesive layer. In this study, by taking surface-bonded bare fiber Bragg grating as the research object, the strain transfer formula between fiber layer and matrix layer is derived by establishing the theoretical model of fiber layer, adhesive layer and matrix layer. In addition, the effects of material parameters on the strain transfer coefficient and the average strain transfer coefficient are analyzed. Then, through laboratory tensile test and verification test, the correctness of this newly proposed theory for strain transfer is verified. The results show that the maximum error between the measured value and the theoretical value is about 15%. According to the parameter analysis, the influence of different material parameters on the average strain transfer coefficient can be arranged in a descending order as follows: the FBG bond length, the thickness of the adhesive layer, the shear modulus of the matrix layer, the width of adhesive layer and the shear modulus of adhesive layer, accounting for 33%, 23%, 20%, 13% and 11% respectively.