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

Localized Temperature Variations in Laser-Irradiated Composites with Embedded Fiber Bragg Grating Sensors

“…While the SmartScan interrogator scanned the spectrum at multi-kHz rates, the time required to detect a temperature gradient is intrinsically limited by the response time of the FBG sensor itself (i.e., when not embedded). Precise measurements of the sensor response time were not taken, though as a rough estimate, the heating response time appeared to be ~50 ms, with a somewhat slower cooling response [ 19 ], similar to results reported elsewhere [ 15 , 16 , 17 ].…”

“…An oven was used to calibrate the sensitivity of both bare and embedded sensors to temperatures as high as 250 °C. In the preliminary characterization [ 19 ], the wavelength shift versus temperature in bare fiber was in good agreement with manufacturer specifications [ 24 ]. The resulting sensitivity was 11.9% ± 5% pm/°C in the measured temperature range, both for sensors in bare fiber and for sensors embedded in the carbon fiber/epoxy composite.…”

“…N 2 gas was applied as necessary to extinguish flames that erupted. For initial tests [ 19 ], the laser power was set to 25% of its maximum power, corresponding to ~25 W of total integrated power across the Gaussian beam profile. To measure the composite response over time, the interrogator scanned the spectrum at 500 Hz, acquiring the peak wavelength of each sensor every 2 ms.…”

“…Though precise measurement of temperature is not essential in this application, reliable estimates of the temperature are still important to optimally design an FBG sensor network that can detect high-energy radiation, so an oven was used initially to calibrate the low temperature sensitivity of bare sensors and embedded sensors. Subsequent tests with a heat gun demonstrated the influence of conduction and convection on the thermal response in all of the composite structures [ 19 ]. In the tests described here, in which energy from a high power laser was absorbed by the composite, the primary mechanism of heat transfer was radiation from the laser strike, but conduction and convective cooling were also evident.…”

“…Before performing experiments using FBGs to detect localized temperature variations in a composite caused by laser irradiation, Equations (2) and (4) were experimentally characterized for the sensors and composites used in this project [ 19 ]. Type I FBG [ 9 ] temperature sensors from ITF Technologies were used in all tests, with nominal reflectivity values near 70% at .…”