Highly-localized thermal response measurements in composites using embedded fiber Bragg grating temperature sensors

In this research, fiber Bragg grating (FBG) temperature sensors are embedded in composites in order to detect highly localized temperature gradients in the composite structures. The primary goal is to perform structural health monitoring on a composite structure. A secondary goal is to use the sensors as a diagnostic tool to determine the optimal composite materials, architectures, or structures that are the least susceptible to thermal damage. Initial results will be discussed for two composite materials using a single sensor to measure temperature variations. The tests include measurements of the temporal and spatial thermal response of the composite resulting either from an applied heat source or to high energy radiation incident on the surface. Additional tests demonstrate the response using a 3x2 array of sensors to simultaneously measure the temperature at three varying depths in the composite, using three FBGs aligned with the heat source, and three FBGs located a short lateral distance (3cm) away from the heat source. In addition, since FBGs respond to strain as well as to temperature, any strain in the composite is coupled into the embedded fiber and is also detected by the FBG sensors. Initial measurements demonstrate the simultaneous response of FBG sensors to both temperature and strain. The various components of strain that are observed in the composite will be discussed, and possible methods to isolate these components and mitigate their response will be considered.