MEASUREMENT OF THERMAL-EXPANSION COEFFICIENTS USING AN IN-FIBER BRAGG-GRATING SENSOR

A simple method to measure the thermal expansion coefficient using a s urface-mounted Bragg-grating sensor is presented. This method uses a s ingle, uniform-pitch Bragg-grating fibre that is only partially glued onto the specimen. To prevent cross talk between the two Bragg wavelen gths reflecting from glued and free sections, a preloading is applied to the fibre before it is partially glued onto the specimen. After rel ease, two Bragg wavelengths are observed. The Bragg wavelength reflect ed from the sensor section not glued onto the specimen is used to meas ure temperature variations; the Bragg wavelength reflected from the se nsor section glued onto the specimen is affected by variations in ther mal strain. Therefore, the thermal expansion coefficient can be determ ined by calculating the spectral separation of the two Bragg wavelengt hs. The measured thermal expansion coefficient of an aluminium plate i s (24.9 +/- 2.1) x 10(-6)degrees C-1 for every 5 degrees C in the rang e 35-75 degrees C. It is in good agreement with the value of 25 x 10(- 8)degrees C-1 for aluminium. However, the scheme will lead to incorrec t results if there is a temperature gradient across the Bragg grating or if the material to be measured is inhomogeneous.