Studies on Identification of Mechanical Loading Applied to Composite Plates Using Distributed Fiber Optic Sensors

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The purpose of the present study is to develop a fundamental theory of mode couplings in modal-domain polarization-maintaining optical fibers subjected to an arbitrary three-dimensional stress state, and to demonstrate its validity through comparison with some experiments for possible applications to smart materials/structures.Mode couplings in polarization-maintaining optical fibers embedded in materials under different loading conditions were theoretically derived to demonstrate the effects of embedded material properties as well as types of loadings such as axial tension, pure bending, torsion and transverse compression. Practically useful formulas were derived for mode coupling equations by expanding the stresses into Taylor series around the center of the optical fiber.Considering a sensitive response of mode couplings under transverse compressive loadings, load sensor modules were developed. For stable and quantitative measurements, the following design criteria were establishied : (1)to protect the optical fiber without coatings, (2)to attach a sensor head, (3)to load the optical fiber normal to the fiber axis, (4)to fix the angle between fiber principal and loading directions to 45 degree, and(5)to fix the loading length to 1 mm. Different types of load sensor modules were designed and fabricated for performance tests, and practical miniature load sensor modules were successfully developed.In addition, some attempts were made to detect stress concentrations due to cracks or notches. Different sensitivities between sensor outputs and crack lengths were noticed under Mode I and Mode II loadings. The sensitivity were found to be extremely high when the beat length of fundamental modes of the optical fiber was equivalent to the crack length. However, the sensitivity became low when the crack length is less than 5% of the beat length of fundamental modes of the optical fiber.