Intrinsic single-mode polymer optical fiber sensors for large strain applications

This project develops an intrinsic, single-mode, polymer optical fiber (POF) used as a high-strain sensor for applications such as health monitoring of a civil infrastructure subjected to earthquake loading. Optical fibers in general are insensitive to electromagnetic fields, lightweight and relatively non-intrusive as compared to conventional strain gauges. Polymer optical fibers are more advantageous than glass optical fibers due to their high fracture toughness relative flexibility in bending durability in harsh chemical and environmental conditions and high elastic strain limit. Moreover, conventional electrical resistance strain gages are reliable for steel structures only up to about 3% strain for cyclic loading conditions and are less reliable for concrete structures, particularly for strains above 1%. POFs have the potential strain range of 6-12% which is viable to measure extreme loads in civil structures where material strains can exceed 2% in reinforced concrete and 5% in steel.Single-mode POFs are still fairly new and experimental. It is well known that they posses a higher attenuation compared to their glass counterpart. However, current improvements in manufacturing have allowed interferometric sensing capabilities which in turn offer high accuracy strain measurements. In order to evaluate the sensing response of the POF, the optical properties as well as the mechanical properties must be assessed. The opto-mechanical response is formulated for the POF based on a second-order photoelastic effect as well as a second-order solution for the deformation which occurs during loading. It is shown that four independent constants are required for small deformations, including two mechanical and two photoelastic properties. For large deformations six additional constants are required, including two mechanical and four photoelastic properties.Mechanical testing is performed under tension at strain rates ranging from 0.01 min-1 to 305 min-1. Repeatable results at each strain rate shows a failure strain between 22% and 36%. The elastic modulus tends to increase slightly and the yield point tends to increase dramatically with an increase in testing rate. With an increase in strain rate the yield strain increases until it levels off at &sim4.6%. The normal modulus, E 0, is calculated through a second-order equation in order to compensate for finite deformation of the fiber and the best fit value of E0 will be dependent upon the strain range used. For each strain rate E0 tends to increase with a larger strain range but eventually decreases at each correlating yield strain. A separate Mach-Zender interferometric system is created to measure the change in phase shift of the light propagating through the fiber as well as a correlating mechanical system which applies tension to the POF in a controlled fashion. Fringe counting is utilized to measure the phase shift for a known displacement. The measured values of d&phiv/dL at the initial loading condition from the two final specimens of the current study, namely 137x105 rad/m and 136x105 rad/m, match the calculated value from bulk properties of PMMA and the values previously reported in the literature. The magnitude of the optomechanical nonlinear term is shown to be of the same order of magnitude as the mechanical nonlinear term. Therefore, we cannot exclude these unknown constants from the nonlinear term of the phase shift equation.