Strain Transfer Theory Research Of Fiber Bragg Grating Sensors

Optic fiber Bragg grating sensor is a new type of high performance sensor, which is playing an important role in structural healthy monitoring. There is a protective layer exist between structure and fiber to protect fiber Bragg grating because of the limitation of fiber core itself. In the application of fiber Bragg grating strain sensors, a small portion of strain is absorbed by the interlayer in the process of strain transfer from host material to fiber core for the existence of protective layer. As a result, the measurement strain of FBG sensor isn't equal to the real strain of host material. In order to resolve practical engineering problems and perfect the academic researches, strain transfer models between host material and fiber core were developed based on fiber Bragg grating sensor. And the parameters that influence the strain transfer were educed and discussed in detail. Many useful conclusions were obtained for next researches. The main contents of this article include:Firstly, based on the existence strain transferring theory, the important factors that affect the strain transferring are achieved through particular analyses. Every influencing parameter is discussed detailedly. Some useful conclusions are developed, which provides a theoretical basis for next researches and choices of sensor material and size.Secondly, a strain transfer model is developed for evaluating the interaction between the surrounding host material and a length of fiber Bragg grating sensor embedded in it, when there is an angle existence between the sensor axis and the external principal stress direction of the host material. And an average strain transfer rate is used to describe the percentage of strain applied to the host material actually transferred to the sensor. The theoretical results have been confirmed by the experimental data that obtained through fiber Bragg grating strain sensors stuck on steel plate. At last, the average strain transfer rate evaluation error and strain measured error which are induced by sensor location angle deviation is discussed. The conclusions extend the application ranges of strain transferring model and provide the theoretical basements for the engineering applications of fiber Bragg grating sensors.Thirdly, when the Young's modulus of host materials is low, the strain transfer between the host materials and fiber Bragg grating sensor is influenced seriously by matrix modulus. Taking into account the Young's modulus of host material, the strain transfer theory is deduced afresh. The strain transferring theory is extended to low stiffness materials. Correspondingly the strain transferring formulae can be used for fiber Bragg grating sensors, which are embedded in various materials of different modulus. At the same time, the strain transferring influencing parameters are considered roundly. The research results provide a theoretical basis for the application of fiber Bragg grating sensors in various fields.