Strain Transfer Rate Research Articles

表面粘贴式光纤布拉格光栅传感器层状结构对测量应变的影响

When a surface-bonded Fiber Bragg Grating(FBG)sensor is used to measure the strain of a host material,it will effect the strain distribution of the host material.Therefore,this paper explores the relationship between optical fiber strain and host material strain.A theoretical model of strain transfer of the host material to the optical fiber was presented to modify the measured strain,and the interaction between FBG sensor and host material was considered.Finally,the theoretical predictions proposed in this paper were verified by Finite Element Analysis(FEA)and practical experiments.The results show that the error between FEA and theoretical solution is controlled within 5%,and thatbetween theoretical solution and experimental data is controlled within 8%.These mean that the strain transfer mode satisfies the accuracy requirement of surface-bonded FBG sensors.Moreover,the effects of geometrical and material parameters on the average strain transfer rate and the strain transfer rate were analyzed.The obtained results indicate that the average strain transfer rate and the strain transfer rate increase with the Young's modulus of the host material.However,they decrease with increasing the top thickness and bottom thickness of an adhesive layer.

Monitoring of deployment process of shape memory polymers for morphing structures with embedded fibre Bragg grating sensors

This article demonstrates the use of embedded fibre Bragg gratings as vector bending sensor to monitor two-dimensional shape deformation of a shape memory polymer plate. The shape memory polymer plate was made by using thermal-responsive epoxy-based shape memory polymer materials, and the two fibre Bragg grating sensors were orthogonally embedded, one on the top and the other on the bottom layer of the plate, in order to measure the strain distribution in both longitudinal and transverse directions separately and also with temperature reference. When the shape memory polymer plate was bent at different angles, the Bragg wavelengths of the embedded fibre Bragg gratings showed a red-shift of 50 pm/° caused by the bent-induced tensile strain on the plate surface. The finite element method was used to analyse the stress distribution for the whole shape recovery process. The strain transfer rate between the shape memory polymer and optical fibre was also calculated from the finite element method and determined by experimental results, which was around 0.25. During the experiment, the embedded fibre Bragg gratings showed very high temperature sensitivity due to the high thermal expansion coefficient of the shape memory polymer, which was around 108.24 pm/°C below the glass transition temperature ( Tg) and 47.29 pm/°C above Tg. Therefore, the orthogonal arrangement of the two fibre Bragg grating sensors could provide a temperature compensation function, as one of the fibre Bragg gratings only measures the temperature while the other is subjected to the directional deformation.

Strain transfer of surface-bonded fiber Bragg grating sensors for airship envelope structural health monitoring

This paper deals with an improved bonding approach of surface-bonded fiber Bragg grating (FBG) sensors for airship envelope structural health monitoring (SHM) under the strain transfer theory. A theoretical formula is derived from the proposed model to predict the strain transfer relationship between the airship envelope and fiber core. Then theoretical predictions are validated by numerical analysis using the finite element method (FEM). Finally, on the basis of the theoretical approach and numerical validation, parameters that influence the strain transfer rate from the airship envelope to fiber core and the ratio of effective sensing length are analyzed, and some meaningful conclusions are provided.

Analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings

A theoretical analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings (FBGs) subjected to uniform axial stress is presented. The proposed six-layer structure consists of optical fiber, protective coating, adhesive layer, substrate layer, outer adhesive layer, and host material, which is different from the four-layer case of common acknowledgement. A theoretical formula of strain transfer rate from host material to optical fiber is established to provide an accurate theoretical prediction. On the basis of the theoretical analysis, influence parameters of the middle layers that affect the average strain transfer rate of the six-layer surface-bonded FBG are discussed. After the parametric study, a selection scheme of sensor parameters for numerical validation, which makes the average strain transfer rate approach unity, is determined. Good agreement is observed between numerical results and theoretical predictions. In the end, the six-layer model is extended to the general situation of multiple substrate layers, which lays a theoretical groundwork for the research and design of surface-bonded FBGs with substrate layers in the future.

Optical fiber Bragg grating sensor assembly for 3D strain monitoring and its case study in highway pavement

Strain of massive structures, such as highway pavement systems, varies in every direction and it contains all the significant information about the safety of the civil structures. In this paper, optical fiber Bragg grating (OFBG) based sensor assembly packaged in fiber reinforced polymer (FRP), named OFBG based sensor, was proposed for 3D structural strain monitoring. Its dimensions have been firstly optimized according to the host structure characteristic, loading condition and acceptable strain transfer rate; then the stainless annulus and FRP outshoot, which can reduce the stress concentration and strain transfer error, were assembly utilized to develop the proposed OFBG based sensor; a case study of the proposed sensor applied to monitor the 3D strain of various asphalt concrete pavement layers of the highway system under different static loading conditions was presented in the end. In comparison of the in-situ test and simulation results, the proposed OFBG based sensor assembly monitored the 3D strain distribution of the highway pavement system effectively, which proves it is a promising solution for 3D strain monitoring in structural health assessment.

Strain transfer analysis of embedded fiber Bragg grating sensor under nonaxial stress

When a fiber Bragg grating (FBG) sensor is embedded in a structure to sense its strain, a portion of strain is absorbed by the protective interlayer of the fiber optical sensor. If an angle exists between the FBG sensor axis and external principal stress direction of the host material, the strain transfer from the host material to the sensor will be much different than that of the external stress parallel to the sensor axis. A suitable strain transfer model is developed for evaluating the interaction between the surrounding matrix and a length of optical fiber under nonaxial stress. A number of realistic assumptions are introduced to simplify the process of the mathematics rigor. Strain transfer rate is introduced to describe the level of strain loss within the protective interlayer and the amount transferred to the optical fiber core. Theoretical results show that the angle of the optical fiber sensor plays an important role in strain transferring from the surrounding materials to the optical fiber core. The theoretical findings are verified through a series of experiments with FBG sensors. The evaluation error of average strain transfer rate is discussed because of sensor-located angle deviation.

Strain transferring analysis of fiber Bragg grating sensors

We develop an analytical model for the relationship between the strain measured by a fiber Bragg grating sensor and the actual structural strain. The values of the average strain transfer rates calculated from the analytical model agree well with available experiment data. Based on the analytical model, the critical adherence length of an optical fiber sensor can be calculated and is determined by a strain lag parameter, which contains both the effects of the geometry and the relative stiffness of the structural components. The analysis shows that the critical adherence length of a fiber sensing segment is the minimum length with which the fiber must be tightly bonded to a structure for adequate sensing. The strain transfer rate of an optical fiber sensor embedded in a multilayered structure is developed in a similar way, and the factors that influence the efficiency of optical fiber sensor strain transferring are discussed. It is concluded that the strain sensed by a fiber Bragg grating must be magnified by a factor (strain transfer rate) to be equal to the actual structural strain. This is of interest for the application of fiber Bragg grating sensors.

Micromechanical behavior of short-fiber polymer composites

Micro-Raman Spectroscopy (MRS) has been used to measure the load transfer in short-fiber, high-modulus/epoxy composites as a function of angle to the loading direction, and to monitor strain concentrations in short-fiber composites due to fiber/fiber interactions. Toray M40B PAN-based carbon fibers were tested with sized, unsized, elastomer-treated surfaces. The matrix was Shell Epon 828 cured with TETA. It was found that incremental load transfer to the fibers compared well with theory. It was also found that the maximum strain transfer rate did not depend on the angle of orientation. The interface was found to change the strain concentration factors, with a noticeable decrease in the strain concentration factors observed for a weaker, more brittle interface. The strain concentration results were used to modify an existing model of composite strength.