Investigation of strain transmission of surface-bonded FBGs used as strain sensors

Bonding an FBG on a substrate as a sensor using an adhesive, the strain transferred from the substrate through the bonding layer to FBG is smaller than that on the substrate. The strain transmission loss becomes large when the substrate is thin and/or made by a low-modulus material, e.g. the polyimide film used in lithium battery and chip on film manufacturing industry. Moreover, the FBG and the bonding layer affect the original strain distribution on the thin and low-modulus substrate. As a result, the substrate strain sensed by the FBG is underestimated and thus required to be corrected. Based on elasticity, an analytical model is proposed to characterize the strain transmission of an FBG used as a strain sensor when it is surface-bonded on a structure using an adhesive. The proposed strain transmission formula takes the influences caused by the stiffness of substrate and FBG as well as the bonding layer characteristics, i.e. the length, thickness and shear lag parameter into consideration. Validated respectively by numerical simulations using finite element method and experiments, this formula provides a simple but accurate correction for the bonded FBG to reflect the true structural strain.