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The great many advantages to using composite materials in structural applications are often overshadowed by the fact that composites possess relatively low impact resistance and the fact that the damage from impacts is often at a location that makes detection difficult and costly. An inexpensive and reliable method of detecting location and severity of damage in composites would be of great benefit. Embedded optical fibers have previously been used to detect damage using an optical fiber fracture concept, where the optical fiber is intended to fracture at the load that causes damage to the composite host, which in turn prevents light from being transmitted through the fiber. Untreated optical fibers have been embedded in composites subject to loading for this purpose, and the strengths of the optical fibers were found to be such that only severe damage could be detected. To tailor the sensitivity of the fiber, an etching technique was developed to predispose optical fibers to break at a known load. Ductile metallic coated fibers have been suggested as a potential alternative to fiber breakage-type damage sensors, where permanent deformation in the coating is used as a metric of thermal or mechanical overloads. One possible advantage offered by this configuration over etched fibers is that the information includes more than just binary (go or no go) data. The actual magnitude of the event may be recoverable from the residual strain data. If the metal coated optical fiber sensor concept proves feasible for damage detection, then the mechanical and physical attributes that are desirable in a coating must be determined. Sirkis and Dasgupta used non- linear analytical techniques to investigate behavior of a metal coated fiber (not embedded) subjected to axial tension and uniform thermal loading, and found that the concept was feasible. Chang and Sirkis then tested this type of sensor in uniaxial tension and by embedding them in graphite/epoxy plates subjected to impact loading. However, the analysis provided by Sirkis and Dasgupta is of little use in designing the embedded sensors because it did not account for the host material system. This paper remedies this limitation by using non-linear analytical techniques to determine the strains and stresses in a metallic coated optical fiber embedded parallel to the reinforcing fibers in a unidirectional fiber reinforced composite material. While this analysis can be used to investigate the relationship between coating material properties and sensor performance, this paper focuses on the effect that structurally embedded, ductile metal coated optical fiber sensors have on the strength characteristics of the host composite system.
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