Modeling of multi-axial Bragg grating fiber optic sensors.

摘要

In this research, the Fiber Bragg Grating sensors are systematically studied. A unique superposition method is created to predict wavelength profile based on the strain distribution inside the sensor. Different load cases are studied by both experiments and simulations. FEM models are created to calculate strain distributions in structure and sensors for all cases studied. A unique characterization method of thermo-optical coefficient is presented based on the oil bath experiment. The step-shape load case is studied for non-embedded sensors. Partially loaded sensor generated two independent peaks. A unique transverse load transfer mechanism for embedded sensor is revealed. The transverse-direction strain in the fiber is the combination of the direct transverse load transferring and Poisson strain from the axial direction. Parametric study of the host material's modulus is conducted for the embedded sensors under both thermal and mechanical loads. The results show that the host material's modulus has great impact on the strain level in the sensor, which will further add complexity of signal interpretation. Strain gradient's effect on the sensor signal is studied by two experimental setups. 3-point bending and 4-point bending beam provide symmetric and asymmetric strain gradient respectively. The strain gradient will cause significant widening of the sensor's wavelength profile. The strain sensing capability of multi-axial multi-wavelength FBG sensor and single-axial sensor is evaluated. The multi-axial sensors have problems of K matrix inversion, transverse-direction load transferring and host material property dependency that prevent its real-life application. The single-axial sensor also has system error due to the transverse-direction load transferring. New single-axial sensor design is proposed to avoid this problem.