Strain transfer analysis of embedded fiber Bragg grating sensor under nonaxial stress

When a fiber Bragg grating (FBG) sensor is embedded in a structure to sense its strain, a portion of strain is absorbed by the protective interlayer of the fiber optical sensor. If an angle exists between the FBG sensor axis and external principal stress direction of the host material, the strain transfer from the host material to the sensor will be much different than that of the external stress parallel to the sensor axis. A suitable strain transfer model is developed for evaluating the interaction between the surrounding matrix and a length of optical fiber under nonaxial stress. A number of realistic assumptions are introduced to simplify the process of the mathematics rigor. Strain transfer rate is introduced to describe the level of strain loss within the protective interlayer and the amount transferred to the optical fiber core. Theoretical results show that the angle of the optical fiber sensor plays an important role in strain transferring from the surrounding materials to the optical fiber core. The theoretical findings are verified through a series of experiments with FBG sensors. The evaluation error of average strain transfer rate is discussed because of sensor-located angle deviation.