Method for In-Situ Strain Transfer Calibration of surface bonded Fiber Bragg Gratings

This paper reports on a method for strain calibration of fiber optic sensors which have been adhesively bonded to a structural member. Generally, the coefficient of strain transfer ( $k_{\mathrm {ST}}$ ) can be used to calibrate the strain sensitivity of the surface bonded FBG. Here we propose an alternative method to determine $k_{\mathrm {ST}}$ which is based solely on observing the temperature-dependent behaviour of the adhesively mounted FBG sensor. In this paper we prove the validity of our method by comparing $k_{\mathrm {ST}}$ determined via a conventional strain (mechanical) calibration and via our alternative temperature (thermal) calibration. A set of 18 surface bonded FBGs with coefficients ranging from 0.5 to 1 are investigated. The average deviation of $k_{\mathrm {ST}}$ determined via a temperature calibration versus a mechanical strain calibration was ~5%, with a maximum deviation of 13% obtained from the set of bonded sensors measured in this work. This led to an average relative accuracy of < 6%, when the strain sensors were calibrated using our temperature-based calibration method.