Optical And Mechanical Interaction Of Structurally Integrated Optical Fiber Sensors

Abstract

The mechanical and optical interaction of structurally integrated optical fiber strain sensors are explored with fiber and moire’ interferometry. A single mode optical fiber is embedded in a four point bend specimen to investigate both the phase shift induced in the optical fiber by the specimen strain field, and the stress concentrations induced in the specimen by the fiber acting as a stiff inclusion. The phase shift measured with a Mach-Zehnder fiber interferometer is under predicted by the combination of simple beam theory and the three-dimension optical phase-strain relation. This result is expected since, unlike surface mounted optical fiber sensors, embedded sensors significantly perturb the strain state in the immediate vicinity of the fiber. A hybrid analysis using moire* interferometry and finite element methods is used to better estimate the true average strain state in the optical fiber, which is in turn used to calculate the optical phase shift. However, the phase shifts calculated by this method over predicts the measured phase shifts. Possible reasons for the over prediction include the material property choice for the optical fibers, and the use of a plane stress finite element analysis. Both problems are discussed below. Finally, the hybrid stress analysis technique is used to investigate the micromechanical stress state in a 500fxm square region local to the embedded optical fiber. The most significant stress concentration attains a value of 1.72, and the fiber induced perturbation in the stress state is restricted to within three fiber diameters from the fiber center.