Mechanical behavior of optical fiber strain sensors

As optical fiber sensors take an increasingly important role in structural health evaluation, it is vital that a thorough understanding of the mechanical behavior of these sensors be examined. Previous studies have usually looked at the behavior using uncoated fiber. However this study examines the behavior of both coated and uncoated fibers. A theoretical assessment using a three-dimensional finite element model for both coated and uncoated optical fibers is presented. Results show that the coating stiffness can significantly affect the strain transfer from the member under load to the optical fiber. With a stiff coating (or no coating), the fiber will exhibit maximum sensitivity, but the calibration factor (i.e., the fraction of strain transmitted to the fiber) can be affected by the thickness of the glue. For a soft coating, the calibration factor is not as strongly affected by glue thickness, but the strain sensitivity can be quite low. The three-dimensional finite element model can provide guidelines for the optimized design of strain sensors. In addition, a straightforward analytical solution shows good equivalence with the theoretical solution under certain conditions. Experiments using an interferometer have been conducted to verify the results of the theoretical study and have shown good correspondence.