Single-Mode Polymer Optical Fiber Sensors for Large Strain Applications
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0969-W05-05
Single-Mode Polymer Optical Fiber Sensors for Large Strain Applications Sharon M. Kiesel1, Kara Peters1, Tasnim Hassan2, and Mervyn Kowalsky2 1 Mechanical and Aerospace Engineering, North Carolina State University, Campus Box 7910, 3211 Broughton Hall, Raleigh, NC, 27695 2 Department of Civil, Construction and Environmental Engineering, North Carolina State University, 208 Mann Hall, NCSU Campus Box 7908, Raleigh, NC, 27695
ABSTRACT This paper characterizes an intrinsic, single-mode, polymer optical fiber (POF) sensor for use in large-strain applications such as civil infrastructures subjected to earthquake loading or systems with large shape changes such as morphing aircraft. The opto-mechanical response was formulated for the POF including a second-order (in strain) photoelastic effect as well as a second-order (in strain) solution for the deformation of the POF during loading. It is shown that four independent mechanical and opto-mechanical constants are required for the small deformation regime and six additional independent mechanical and opto-mechanical constants are required for the large deformation regime. The mechanical nonlinearity of a typical polymer optical fiber was experimentally measured in tension at various loading rates. The secant modulus of elasticity measured at small strains, roughly up to 2% strain, was measured to be ~4GPa whereas at larger strains, roughly up to 4.5% strain, the secant modulus was measured to be ~4.8GPa. As the loading rate was increased the yield strain increased, ranging from ~3.2% at 1mm/min to ~5% at 305 mm/min. INTRODUCTION A sensor that can measure large strains would have significant immediate implications for structural testing and monitoring of localized failure mechanisms due to inelastic deformation and damage accumulation. In the case of steel structures, conventional strain gages for cyclic load applications can be reliable up to about 3% strain [1]. These limitations are particularly important for civil structures subjected to extreme loads where material strains can exceed 2% in the concrete, and 5% in the reinforcing steel. Strain gage devices on previous concrete structure experiments have failed to monitor strain levels beyond 1%. As a result, researchers are forced to use approximate methods and observations to attempt to quantify behavior. While polymer optical fiber (POF) systems have been used considerably for chemical and environmental monitoring, only recently has their use in the monitoring of mechanical properties been explored. Xiong et al. [2] demonstrated 6% strain before failure of POF and cites a potential 13% increase with the improvement of manufacturing techniques. POFs are also less brittle than silica based optical fibers which have already been used extensively for monitoring civil structures. Therefore, POF sensor systems could offer a larger strain range measurement capability along with more long-term survivability. Recent advances in the fabrication of singlemode POFs have made it possible to extend POFs to interferome
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