Technology development for multiple internal strain measurement in composite structures using embedded fiber optic sensors

This dissertation describes a single micro-optical fiber sensor capable of measuring three strains simultaneously in a composite structure subjected to ballistic impact loading. This single transducer is based on cascading four micro Fabry-Pérot cavities to measure three normal strains and one shear strain in the plane of the optical fiber cross-section.; The focus of this work is directed towards the sensor fabrication and signal processing techniques. The fabrication includes designing and fabricating new optical side-hole fibers with specific mechanical properties, manufacturing new partially reflective mirrors to be used in Fabry-Pérot cavities. Fabry-Pérot cavities made from side-hole fibers have an anisotropic phase response to transverse loading, which is important to determine the strain state perpendicular to the fiber axis.; The demodulation uses passive quadrature demodulation combined with path matched differential interferometry (PMDI) to accommodate the strain rates and ranges generated by ballistic impact testing. The signal processing includes an investigation of cross-talk between multiplexed Fabry-Pérot cavities and the phase recovering from the measured intensities.; Finally the sensor has been embedded into the composite material while maintaining a desired orientation and its response to transverse loading has been verified.; This dissertation also includes results from 2D and 3D finite element analysis and 2D closed form analysis to establish the transformation between fiber core and composite host strain states, which is important for expressing the desired strains in the host composite in terms of the measured optical phase shift in the fiber sensor.