With embedded sensors, the structures are capable of monitoring parameters at such critical locations not accessible to ordinary sensors. Recently, fiber optic sensor emerges as one promising technology to be integrated with structures. Embedding of fiber optic sensors into composites and some metals, especially those with low melting points, have been reported. However, all reported embedding techniques so far are either complicated or difficult to achieve coherent bonding with low residue stresses. Thus, it is of strong interest to pursue some economical ways to embed fiber optic sensors into metallic structures with low residue stresses. In this work, a new technique is proposed for embedding fiber optic sensor into metallic structures, such as nickel, with minimized residue stress. Fiber Bragg Grating (FBG) sensors have been embedded into nickel structures. Thermal performance of such embedded FBG sensor is studied. Higher temperature sensitivity is demonstrated for the embedded FBG sensors. For temperature measurements, the embedded FBG sensor yields an accuracy of about 2 °C. Under rapid temperature changes, it is found that thermal stresses due to the temperature gradient in the metallic structures would be the main cause for errors.

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Fiber Bragg grating sensor systems have wide application in the area of civil structures. The advantages of fiber grating strain sensors over electrical strain gauges such as greatly reduced size, EMI resistance, and higher temperature capability make them ideal choices for smart structure applications. Some of these fiber grating sensor systems can measure or detect multiaxis strain, transverse strain, temperature, bridge scouring, ice, and traffic flow. SPIE Vol. 3325 • 0277-786X198/$1o.oo Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/01/2015 Terms of Use: http://spiedl.org/terms

Fiber optic Bragg gratings packaged in long gage configurations are being used to measure static and dynamic strain in structures and structural models to monitor structural health and predict damage incurred from a seismic event. These long gage sensors are being used to experimentally verify analytical models of post-earthquake evaluation based on system identification analysis. This fiber optic deformation measurement system could play a significant role in monitoring/recording with a higher level of completeness the actual seismic response of structures and in non-destructive seismic damage assessment techniques based on dynamic signature analysis. This new sensor technology will enable field measurements of the response of real structures to real earthquakes with the same or higher level of detail/resolution as currently in structural testing under controlled laboratory conditions.

A methodology is described for determining a relation (K-matrix) between wavelength shifts and (a) axial strain, (b) two transverse strains and (c) temperature change experienced by a multi-parameter Bragg grating sensor. The sensor is formed by writing gratings at two wavelengths in polarization maintaining fiber. The methodology is based on separate experimental calibrations of sensor response to transverse loading (diametral compression), axial loading and temperature changes. Strains produced in the core by the loadings or temperature changes used in the calibrations are determined by finite element analyses.