Multi-parameter measurement in composite structures using embedded fiber optic sensors

This dissertation attempts to extend the state-of-the-art of multi-parameter fiber optic sensors for structurally embedded applications. Three main purposes have been addressed in this dissertation for the applications of fiber optic sensors embedded in composite structures: (1) simultaneously measuring both strain and temperature, (2) simultaneously measuring two strain components, and (3) build the foundation for simultaneously measuring four components of the complete strain state.; First of all, a dual-parameter hybrid sensor made by cascading an in-line fiber etalon and an in-fiber Bragg grating to simultaneously measure axial strain and temperature is presented. This sensor system is analyzed in great details including the optical approach, the micromechanics of the sensor embedded in the composite, and the numerical stability. The experimental results demonstrating the functionality of the dual-parameter sensor by bonding it to the surface of an aluminum cantilever beam and embedding it in a graphite/epoxy composite cantilever beam are given.; Secondly, the extension of the sensor system described above is made to simultaneously measure axial strain and transverse strain in the composite cantilever beam. The requisite micromechanics is developed for the two-strain sensor using concentric cylinder models. The internal axial and transverse strains are measured with the fiber optic two-strain sensor and two resistance strain gages mounted on the surface of the beam. They show very consistent results. The two-strain sensor results are promising, and suggest that it might be possible to develop a multi-axis strain sensor.; Finally, a single micro-optical fiber sensor capable of simultaneously measuring four key elements of the complete strain state at one point in composite material is proposed. The objective of this part is to obtain four strain components in a composite host material (far field strains) using the measured strains from the optical fiber sensor that is embedded in the composite material. The measured strains are then translated into strains in the composite material using approximate transformation matrices and a micromechanical finite element analysis for the sensor and the host material. The fiber transducer is based on cascading four micro Fabry-Perot cavities fabricated from three very different types of optical fibers. These types of fiber are selected because each has a completely different optical response to applied strain, therefore enabling four independent optical measurements to be used to solve for the three normal strains and one in-plane shear strain. The development of the sensor configuration design and micromechanics analysis has been discussed. The sensor design includes designing new type of optical fiber, fabricating in-line micro-attenuators, and embedding the side-hole fibers in the composite at desired orientation. Finite element model was used to design side-hole fiber for maximized sensitivity to transverse strains. Two-dimensional closed form analysis has been used to establish the transformation between fiber sensors and composite host strain state. Then, four linear equations are used to express the desired strains in the host composite in terms of the measured optical phase changes in the multi-strain sensor.