Strain Transferring Mechanism Analysis Of Surface-bonded FBG Sensors

Surface-bonded fiber Bragg grating (FBG) sensor has broad prospectsin structural health monitoring (SHM) of aircraft, ocean platform, etc. dueto the advantages of lightweight, high sensitivity and immunity toelectromagnetic interference, resistance to corrosion, high reusability,distributed or quasi-distributed measurement, and so on. There existprotective coating and adhesive layer between the fiber and the hoststructure, and their Young’s modulus usually differ greatly. Therefore, astrain transfer coefficient is caused between the real strain of the hoststructure and the strain FBG measured after the fiber is bonded onto thehost structure. The analysis of the strain transfer mechanism ofsurface-bonded FBG sensors has a great theoretical and practicalsignificance.In this paper, the strain transfer mechanism of surface-bonded FBGsensors had been investigated. Firstly, a theoretical equation was derivedfrom the proposed4-layer surface-bonded FBG model to predict the straintransfer relationship between the host structure and the fiber. A numericalanalysis was carried out to validate the theoretical equation, and therelationship between the average strain transfer rate and the parameters ofthe layers was analyzed. Then, the strain transfer mechanism of the6-layermodel adding the substrate layer was studied and then extended to thegeneral situation of multi-substrate layers. On the basis of the theoreticalapproach, the Young’s modulus of the host structure was brought into thederivation process of the theoretical equation, and the strain transfermechanisms of4-layer,6-layer and multi-substrate-layer models wereestablished respectively. The relevant numerical validation and parameter study were performed. Finally, the case that the axial of the sensor had acertain angle with the primary stress of the host structure was analyzed.The strain transfer equation was derived, and the relevant numericalanalysis and parameter study were carried out respectively.Conclusions from this paper would provide a theoretical reference forthe design and application of surface-bonded FBG sensors in structuralhealth monitoring.