An optical device for measuring bending strain to 5000 microstrain and compatible with optical fiber installations

An optical sensor is described which can be attached to a structure and used as a gage for measuring bending strain. This device can be adjusted to maximize the gage factor for predetermined strain ranges. The sensor consists of glass capillaries coated on the outer surfaces with an optical absorbing layer followed by a reflecting layer. A mechanical strengthening layer can be included to extend the range of strain response. A source laser beam from an optical fiber is injected into one end of the gage. The light remaining in the beam after traveling through the gage is collected via another optical fiber. The optically active layer is adjusted during manufacture to provide a predetermined gage factor. For a given thickness of the absorber layer, the detected light is proportional to the amount of bending. Thus, by rigidly affixing the sensor to a structural member, the strain experienced by the member can be monitored.     

Optical grating diffraction method: from strain microscope to strain gauge

The grating diffraction method for direct strain measurement is reviewed. Two systems which use this method are presented. The first system is a compact strain microscope. A Leitz optical transmitting microscope with white light source is reconstructed by developing a loading and recording system. Gratings with median density of between 40 and 200 lines/mm are used. With the help of a Bertrand lens, the Fourier spectrum of the grating is formed with high image quality on the CCD sensor plane. Software is developed to precisely, quickly and automatically determine the diffraction spot centroids. The second system is a new strain sensor based on a high-frequency grating and two Position Sensor Detectors (PSDs). The grating, attached to the surface of the specimen, is illuminated by a focused laser beam, generally with a frequency of 1,200 lines/mm. The centroids of diffracted beam spots from the grating are automatically determined using two PSD sensors connected to a personal computer. The shift of diffracted beam spots due to specimen deformation is detected. Strain sensitivity of one micro-strain can be obtained, as can a 0.4 mm spatial resolution for strain measurement. The system can be used for both static and dynamic tests.     

Modeling, design, fabrication, and testing of a fiber Bragg grating strain sensor array

The modeling, design, simulation, fabrication, calibration, and testing of a three-element, 15.3 cm fiber Bragg grating strain sensor array with the coherent optical frequency domain reflectometry (C-OFDR) interrogation technique are demonstrated. The fiber Bragg grating array (FBGA) is initially simulated using in-house software that incorporates transfer matrices. Compared to the previous techniques used, the transfer matrix method allows a systemwide approach to modeling the FBGA-C-OFDR system. Once designed and simulated, the FBGA system design is then imprinted into the core of a boron-germanium codoped photosensitive fiber using the phase mask technique. A fiber optic Fabry-Perot interferometric (FPI) strain gauge calibrator is then used to determine the strain gauge factor of a single fiber Bragg grating (FBG), and the results are used on the FBGA. The FPI strain gauge calibrator offers nondestructive testing of the FBG. To test the system, the FBGA is then attached to a 75 cm cantilever beam and interrogated using an incremental tunable laser. Electric strain gauges (ESGs) are then used to independently verify the strain measurements with the FBGA at various displacements of the cantilever beam. The results show that the peak strain error is 18% with respect to ESG results. In addition, good agreement is shown between the simulation and the experimental results.     

Fabrication and Sensing Performance of Smart Composite Structures Using Optical Fibre Sensors

This paper determines the performance of Fibre Bragg Grating (FBG) sensors for strain sensing applications in carbon fibre composite materials. Carbon fibre laminates in either cross-plied or quasiisotropic stacking sequences were fabricated using T300/Hexcel 914 prepregs. The FBG optical sensors were either surface attached, or embedded within laminates. The sensor orientation was aligned either parallel or transverse to the adjacent carbon fibre layers. The composite structures with integrated FBG sensors were subjected to static tensile loading. A scanning fibre Fabry-Perot filter was used to monitor the reflected Bragg wavelengths. The optical sensor embedded between two 90° carbon fibre plies shows a high sensitivity to multi-site cracking formed in the transverse plies. The embedding in 90° plies seems to change the local stress distributions and to become a source of crack initiation. Efficient stress transfer from the host materials to the sensors is dependent upon incorporation methods, the thickness of the adhesive layers, and the location of the sensors.     

A Novel Strain Sensor for Reinforced Concrete Structures

Abstract:  Reinforced concrete (RC) is the most commonly used structural material in civil engineering applications. RC structures have long-term service lives under normal loading conditions; however, overload caused by misuse or statistically remote events such as earthquakes may create damages that, if not detected in time, may eventually cause failure. Hence, it is important to monitor RC structures to take necessary precautions and save human lives. A long-gauge strain (LGS) sensor has been developed to monitor these structures. While it has been developed mainly with concrete applications in mind, the new sensor can also be used in a variety of applications, including measuring strains in pipelines, steel structures, and the like. The proposed sensor system has a very low cost compared with the commercially available competing systems. Prototypes of the proposed strain sensors have been built and calibrated. Test results prove the accuracy, repeatability and reliability of the proposed strain sensor. When the LGS sensor was incorporated into a concrete beam there was very good agreement between the experimental measurement of strain using the LGS sensor when compared with two strain-gauged parallel steel rebars in the same concrete beam.     

Theoretical Analysis on Strain Transfer Error of FBG Sensors Attached on Steel Structures Subjected to Fatigue Load

Fibre Bragg grating (FBG) sensors have been increasingly adopted to detect the dynamic strain of structures. When the sensor is attached on the surface, adhesive material is employed to assist the installation, which leads to indirect contact of sensing fibre and the monitored structure. To correct the strain transfer error induced by the shear lag effect and improve the measurement accuracy of FBG sensors under dynamic response, strain transfer mechanism of a three‐layered testing model constituted of sensing fibre, adhesive layer and host material has been studied in this paper. Laboratory test on steel beam attached with FBG sensor under fatigue load has been projected to investigate the feasibility of the derived strain transfer formula, and numerical simulation by MATLAB has been used as a supporting tool to offer the reference dynamic strain. Based on the analysis, sensitive parameters that affect the strain transfer coefficient have been discussed to instruct the application design of FBG sensors. Results indicate that strain transfer coefficient under dynamic response is much lower than that in static state, and error modification is particularly significant; in the dynamic testing model, bonded length, shear modulus and thickness of adhesive layer are more sensitive, which should be precisely selected in practical engineering to guarantee the effective strain measurement.     

Comparison studies between two wavelet based crack detection methods of a beam subjected to a moving load

In this paper, the performances of two wavelet based damage detection approaches to find the location and the size of a crack in a beam subjected to a moving load are compared. In the first approach, designated as ‘Fixed Sensor Approach’, a sensor is assumed to be located at the mid span of the beam. Thus in this approach, Continuous Wavelet Transform (CWT) coefficient of the time varying deflection attributed to the beam at mid span is used. In the second approach, the sensor is attached to the moving load and CWT coefficient of moving sensor is analyzed. The crack is modeled as a rotational spring which its stiffness is obtained from fracture mechanics, and modal expansion theory is used to find the deflection of cracked beam. The two approaches are compared for several cases of moving load speeds, crack sizes and crack locations, and also several scales of CWT. In the both approaches, wavelet Gaussian 4 is employed. It is shown that, the highest magnitude of CWT coefficient occurs at the exact location of crack and its value depends on damage size. As a result, it can be considered as a damage index. It is found that the moving sensor approach is more effective than the fixed sensor. Thereafter, the effect of beam parameters such as length, width, modulus of elasticity and density and also, crack size and location on the damage index were investigated based on the moving sensor approach. The obtained damage index has two significant features: (a) it has an explicit expression and shows the effects of all parameters clearly, (b) it is not dependent upon the response of the undamaged beam. From the obtained explicit relation for damage index, it is illustrated that the damage index does not depend on material properties of a homogeneous beam. Moreover in a rectangular cross section beam, the damage index is independent of width and has a linear relationship with the height to length ratio. It is shown that using this damage index, small cracks with a depth more than 10% of the beam height can be detected.     

Finite dynamic deformations of smart structures

We study transient finite deformations of a neoHookean beam or plate with piezoelectric (PZT) patches bonded to its upper and lower surfaces. The constitutive relations for the PZTs are taken to be linear in the Green-Lagrange strain tensor but quadratic in the driving voltage. A finite element code using 8-noded brick elements has been developed and validated by comparing computed results with either analytical solutions or experimental observations. For flexural waves propagating through a cantilever beam, the sensor output is influenced a little by the presence of a defect placed symmetrically about the centroidal axis. A simple feedback control algorithm is shown to control the motion of a neoHookean plate subjected to an impulsive load.     

压电梁振动的多输入多输出主动控制

对表面上贴有多个用作驱动器和传感器的压电陶瓷片的“压电梁”结构，导出了从驱动器到传感器的频响函数公式，作为压电结构设计和振动控制的数学模型。提出了压电梁对缓变周期扰动振动环境的多输入多输出振动抑制方法。     

用光纤干涉仪检测I形复合材料梁腹板/翼缘连接处的分层

介绍了一种在超声回弹波频谱分析基础上用光纤干涉仪来检测I形复合材料梁腹板/翼缘连接处分层的方法, 利用超声发射器在I形梁中产生应力波, 用表面粘贴的光纤干涉仪来接收应力波产生的输出信号, 对此信号进行频谱分析可找到I形梁的分层位置。理论分析和实验都表明了此方法探测复合材料梁腹板/翼缘连接处分层的可行性。      

In Situ Cross-Calibration of In-Fiber Bragg Grating and Electrical Resistance Strain Gauges for Structural Monitoring Using an Extensometer

This study addresses the direct calibration of optical fiber strain sensors used for structural monitoring and is carried out in situ. The behavior of fiber-Bragg-grating-based sensor systems when attached to metal bars, in a manner representative of their use as reinforcement bars in structures, was examined and their response calibrated. To ensure the validity of the measurements, this was done using an extensometer with a further calibration against the response of electrical resistance strain gauges, often conventionally used, for comparison. The results show a repeatable calibration generating a suitable geometric factor of extension to strain for these sensors, to enable accurate strain data to be obtained when the fiber-optic sensor system is in use in structural monitoring applications.     

用光纤干涉仪检测I形复合材料梁腹板/翼缘连接处的分层

介绍了一种在超声回弹波频谱分析基础上用光纤干涉仪来检测I形复合材料梁腹板/翼缘连接处分层的方法,利用超声发射器在I形梁中产生应力波,用表面粘贴的光纤干涉仪来接收应力波产生的输出信号,对此信号进行频谱分析可找到I形梁的分层位置。理论分析和实验都表明了此方法探测复合材料梁腹板/翼缘连接处分层的可行性。     

Transverse Shear and Normal Deformation Theory for Bending Analysis of Laminated and Sandwich Elastic Beams

The influence of transverse normal strain on bending analysis of cross-ply laminated and sandwich beams is presented. A higher-order shear deformation beam theory is developed. Euler-Bernoulli classical, Timoshenko first-order and simple higher-order theories have been also used in the analysis. The governing equations for a beam composed of orthotropic layers and subjected to any given mechanical load distribution are derived. Making use of Navier-like approach, exact solutions are obtained for cross-ply laminated and sandwich beams subjected to arbitrary loadings. Numerical results for beams with the simply-supported boundary conditions are presented. The effects due to transverse normal strain, transverse shear deformation and number of layers on the static response of the beams are investigated.     

带有力反馈控制的三明治式微机械干涉加速度计

设计了一种静电力反馈控制的三明治式微机械干涉加速度计,加速度计由敏感芯片、半导体激光器、光电二极管以及相应的驱动电路和反馈控制电路组成.敏感芯片为玻璃-硅-玻璃3层结构,通过硅-玻璃键合体硅工艺制成.硅质量块由铝梁支撑,底部玻璃基片上有金属光栅和电极,通过在质量块和底部玻璃基片上的电极之间施加电压可以调节质量块与玻璃基片间的间隙.入射激光照射到敏感芯片上的光栅上,产生衍射光束,其光强随质量块与下玻璃的间距而变化.反馈控制电路通过测量衍射光强的变化来改变质量块与底电极之间的电压,使得质量块与底部玻璃基片的距离保持为入射光波长1/8的奇数倍,从而提高输出线性度,改善灵敏度,增大量程.     

Pointwise fiber Bragg grating displacement sensor system for dynamic measurements

A method for setting up a fiber Bragg grating (FBG) sensor which can measure the pointwise, out-of-plane or in-plane dynamic displacement is proposed. The proposed FBG sensor is reusable. A multiplexing demodulation system based on a single long-period fiber grating is used in this study. The experimental results of the steady-state motion for a multilayer piezoelectric actuator and the dynamic response of a cantilever beam subjected to impact loadings are presented. These results indicate that the proposed displacement sensor has the ability to measure the out-of-plane dynamic displacement with high sensitivity. Measurements for a piezoceramic plate excited by high frequency show that the proposed displacement sensor also has the ability to provide the in-plane dynamic displacement up to 20 kHz.     

1 x N star coupler as a distributed fiber-optic strain sensor in a white-light interferometer

A novel technique of using a 1 x N star fiber optic coupler as a distributed strain sensor in a white-light interferometer to measure the distribution of strain is presented. The measuring principle and 1 x 4 star coupler with four fiber optic strain sensors are demonstrated. The experiment is performed with four sensors attached to a combination plastic specimen.     

White-light interferometric fiber-optic strain sensor from three-peak-wavelength broadband LED source

A white-light fiber-optic interferometer working in the spatial domain, using a special three-peak-wavelength LED as a light source and one of the two arms of the Michelson interferometer as a strain sensor, is presented. Based on the Gaussian spectrum distribution function, a simple spectrum-decomposing method is used to describe the special three-peak-wavelength LED source. Experimental and theoretical analyses show that this special three-peak-wavelength LED source can be used in white-light interferometry to simplify the problem of central-fringe identification. A white-light Michelson fiber-optic strain-sensing system that uses a tapered cantilever beam is described. Experimental results show that the strain measured by the fiber-optic sensor is linear and always less than the surrounding matrix.     

Simultaneous sensing of the strain and points of failure in composite beams with an embedded fiber optic Michelson sensor

A fiber optic Michelson sensor was embedded in composite beams to sense the internal strain and points of failure of the composite structures. The bending deformation and matrix cracking were investigated by four-point bending tests of cross-ply composite beams with the embedded fiber optic sensor. The failure points of composite beams were detected by using both a PZT sensor and a fiber optic sensor in order to investigate the fiber optic failure signals. The failure due to matrix cracks in a composite beam was confirmed by the edge replica method. The digital processing of the fiber optic signal was carried out to determine the strains and failure points of composite beams. The failure points were observed from the processed failure signal by high-pass filtering. The initial failure strain of the composite beam was measured and processed from the fiber optic strain signal after low-pass filtering.     

FBG应变挠度法标定及过程仿真

介绍了一种基于挠度分析的等强度悬臂梁FBG传感器应变标定测试方法和装置,研究了FBG传感器应变传递过程中的相关参量的仿真分析方法,精确测量了应变传递过程中的相关参量,并且针对不同载荷下的等强度悬臂梁的表面应变和挠度进行了理论分析和仿真。同时解析了挠度与梁表面应变、FBG传感器应变的关系,给出了标定后FBG传感器的应变灵敏度,并对挠度法测量FBG传感器的标定结果进行了不确定度评定。当包含因子k=2时,FBG传感器应变灵敏度的相对扩展不确定度＜0.5％,能够满足高精度的应变测试要求,为高精度FBG传感器的应变标定提供了技术支持。     

Brillouin Sensing Cable: Design and Experimental Validation

We report on the design and experimental validation of a distributed Brillouin-based optical fiber sensor embedded into concrete structures for temperature and strain measurement. A composite-made wave-like coating designed by finite-element analysis ensures the sensor is transferring optimally temperature and strain fields from the concrete to the optical fiber, where Brillouin scattering takes place. During all experiments, sensors have been interrogated with a commercially available Brillouin optical time-domain reflectometer unit. First, temperature sensitivity of the Brillouin frequency shift were evaluated in PANDA and SMF28 optical fibers, before wrapping them into the specific sheath for embedment into a 3 m-long reinforced concrete beam. Temperature measurements during concrete beam casting agreed with reference measurements, and showed the significant sensor coating influence. A month later, strain measurements performed during a four-point bending experiment showed promising results: linearity and reliability of measurements were demonstrated, under tensile as well as compressive loadings.