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 (kST) can be used to calibrate the strain sensitivity of the surface bonded FBG. Here we propose an alternative method to determine kST 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 kST 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 kST 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.