Strain transfer in distributed fiber optic sensor with optical frequency domain reflectometry technology
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Abstract:
Present research on strain transfer in optical fiber sensors focuses on high-precision, point-bonded fiber Bragg grating sensors. The spatial resolution of a traditional distributed fiber optic sensor (DOFS) reaches submeter or meter, and strain transfer in these sensors is rarely noticed. The accuracy of the DOFS based on optical frequency domain reflectometry used in our paper is improved significantly compared to previous sensors. It has an accuracy of 0.1 and a spatial resolution of 1 mm. Therefore, the strain transfer performance of such a high-precision DOFS should be investigated further. We fixed a DOFS on a host structure using an adhesive, and the factors influencing strain transfer were studied, including the “endpoint effect,” bond length, shear modulus of the coating and adhesive layers, and thickness of the adhesive layer. A theoretical strain transfer model for a DOFS was designed and the theoretical strain transfer coefficient was calculated using numerical simulations. Furthermore, an evaluation of the theoretical model was implemented using a uniform strength beam. The experimental results are consistent with the simplified two-layer strain transfer model presented in our paper. The effect of the bond length on strain transfer is summarized from experimental and numerical simulation results, i.e., a larger adhesive length leads to a higher strain transfer coefficient and smaller “endpoint effect.” If the shear modulus of the fiber coating layer and the glue is high, we obtain a high train transfer coefficient and small “endpoint effect.” These conclusions provide a useful reference for the application of DOFS in practice.Keywords:
Reflectometry
Fiber Bragg Grating
Structural Health Monitoring
Fiber bragg grating has become one of the widely used optical measurement technologies in the field of aviation structure monitoring because of its light weight, high sensitivity and immunity to electromagnetic interference. A strain transfer model is established for the surface attached fiber grating, the function between real strain and sensing strain is obtained. Basing on finite element simulation, strain field distribution of the sensing structure is analyzed and the distribution curve of strain transform rate is drawn. The strain calibration system of fiber grating is built to verify the accuracy of the model. The average error of this model is superior to 5.5% in the measurement range of 0-3000με. The study improves the measurement precision of surface bonded fiber grating sensor, which also provides academic bases and references for the aircraft structural health monitoring.
Fiber Bragg Grating
Strain (injury)
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Fiber optic sensors represent one of the most promising technologies for the monitoring of various engineering structures. A major challenge in the field is to analyze and predict the strain transfer to the fiber core reliably. Many authors developed analytical models of a coated optical fiber, assuming null strain at the ends of the bonding length. However, this configuration only partially reflects real experimental setups in which the cable structure can be more complex and the strains do not drastically reduce to zero. In this study, a novel strain transfer model for surface-bonded sensing cables with multilayered structure was developed. The analytical model was validated both experimentally and numerically, considering two surface-mounted cable prototypes with three different bonding lengths and five load cases. The results demonstrated the capability of the model to predict the strain profile and, differently from the available strain transfer models, that the strain values at the extremities of the bonded fiber length are not null.
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Waveguide
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Optical fiber strain sensors, in particular, the fiber Bragg grating (FBG) type, are widely applied in different applications. The most common installation method is surface-attached. In principle, the optical fiber strain sensor with adequate sampling and signal processing techniques is usually more accurate than electrical resistive strain gauge. However, the strain of the surface of structure may not transfer to the sensing element perfectly. The ratio between the measured and actual strain can be correlated by a strain transfer factor (STF). However, it depends on the material and geometrical properties of the optical fiber and adhesive. It is noneconomical and impractical to measure the STF for every installed sensor. It is desirable to identify the most of the sensitive parameters on the variation of the STF so that the quality control and assurance procedure can be performed more efficiently. In this paper, a quantitative global sensitivity analysis, called extended Fourier amplitude sensitivity test will be performed to compute the first-order and total sensitivity indexes based on a well-established semi-analytical/empirical mechanical model of three material and five geometrical parameters of both integral and optical FBG type optical fiber strain sensor with two different kinds of polymeric coating under three types of strain field in 16 different configurations. From the detail analysis, the most of the sensitive parameters on the STF are bond length, the thickness of adhesive beneath the optical fiber and the deviation of grating position, which are related to workmanship instead of the material properties of the optical fiber and adhesive.
Fiber Bragg Grating
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Comparing with general optical fiber sensors performing localized measurement, distributed optical fiber sensors can measure along an optical fiber, and they have large measuring range. The surface-mounting method with epoxy adhesive is general in attaching optical fiber sensors to structures, This is also appliable to the structural integrity monitoring with Brillouin-scattering distributed optical fiber sensors. In this paper, Brillouin-scattering distributed optical fiber sensors, which are attached to the surface of a structure with epoxy adhesive, was verified with the finite element method. From the analysis results of strain transfer through the structure, optical fiber coating, cladding and core, the strain transfer rates were calculated. And the influence of the epoxy free-end was also studied.
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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.
Fiber Bragg Grating
Strain (injury)
PHOSFOS
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We propose and demonstrate a 0.5 mm resolution distributed fiber temperature and strain sensor with position-deviation compensation based on the Optical Frequency Domain Reflectometry. The position-deviation compensation helps maintain the cross-correlation of the Rayleigh spectra, which degenerates at the higher resolution. Experimental results reveal a 0.5 mm strained fiber segment recognized at the end of a 25 m fiber, with 50,000 equivalent measuring points. The temperature repeatability of ± 0.9 °C (12.5 pm) is obtained from 50 °C to 500 °C with an 18 m gold-coated fiber, and the strain accuracy of ± 15 µɛ is also achieved within ± 2500 µɛ using a polyimide coated fiber. The small-scale, high spatial-resolution, and electromagnetic-immune distributed optical fiber sensors can be applied to address the test challenges in astronautics, advanced materials, and nuclear facilities, where high temperature, large strain change, space radiation, and complex electromagnetics presents.
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As a study of a special structure that FBG sensor is bonded between the composite shell surface and the protective coating in this paper, a strain transfer function is established and a finite element method (FEM) was conducted to the validate function, and the calculation error is controlled within 5%.
Strain (injury)
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As the package structures of a sensor seriously interfere the strain responses measured by a Fiber-optic Bragg Grating(FBG),this paper focuses on the relationship of measured strain and true strain in the actual measurement.It establishes a strain transfer function for embedded FBG sensors and verifies the validity of the transfer function and the influence of different parameters on the measured strain.Firstly,based on mechanical characteristics of embedded FBG sensors,the shear stress distribution with a form of polynomial is presented,then the strain transfer function is established and verified by taking a numerical method and an experiment.Finally the influence of sensor length,cementation layer modulus and cementation layer thickness on the measured strain is analyzed.Experimental results indicate that the strain transfer function is valid.Moreover,the thinner the cementation thickness and the higner the cementation modulus are,the more convenient the strain transfer is.The strain transfer function satisfies the accuracy requirement of embedded FBG sensors because the calculation error is controlled within 5%,which is considered as a guidance for its practical application.
Fiber Bragg Grating
Strain (injury)
Cementation (geology)
Strain gauge
Stress–strain curve
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