Estimation of Displacement Response from FBG Strain Sensors Using Empirical Mode Decomposition Technique

As an important factor in evaluating the safety of civil infrastructures, predictions of displacement become the basis for determining the decrease of structural performance and the degree of aging in general. It is, however, well known that it is not easy to measure the displacement response of civil infrastructures such as suspension bridges due to a lack of appropriate measurement techniques, despite the importance of measurements in the displacement response. Thus, as an alternative for predicting the displacement response indirectly, the conversion of the measured strain signal obtained using Fiber optic Bragg-Grating (FBG) sensors into the displacement response is suggested. In previous studies on the prediction of displacement response using FBG sensors, static displacement was mainly predicted. A known complication in the use of the measured strain signal to predict dynamic displacement is the fact that the measured strain signal includes higher modes, and that the predicted dynamic displacement can be inherently contaminated by broad-band noises. To overcome such a problem, a mode decomposition technique was used. This is a method that estimates the total displacement response combined with each displacement response about the major mode of the structure and the quasi-static displacement responses. In order to verify the suggested algorithm to predict the displacement responses from FBG strain signals, a model experiment and field tests were executed.     

Full-Spectral Interrogation of Fiber Bragg Grating Sensors Exposed to Steady-State Vibration

In this paper we measure for the first time the full-spectral response of a fiber Bragg grating (FBG) sensor subjected to vibration. We consider two cases: with and without an initial spectral distortion due to non-uniform strain along the length of the FBG. Previous work has measured only the dynamic response at a single wavelength which is valid when no spectral distortion is present. FBG sensors are mounted near the notch tip on a double edge notch specimen that is also subjected to harmonic vibration. We measure the full-spectral response of the FBG at 100 kHz applying an interrogator recently developed by the authors. The measurements of the FBG response with an initial spectral distortion clearly show the transient response and are verified through simulation. Finally, we demonstrate that the use of the high-speed, full-spectral interrogator permits the separation of the spectral distortion and the harmonic vibration from the FBG response signal through classical filtering and can therefore be applied to measure non-uniform strain fields in noisy environments.     

A Transducer for Measuring Tensile Strains in Concrete Bridge Girders

Load testing highway bridges is becoming an increasingly common procedure for determining bridge load capacity. Bridge testers frequently measure the strains in critical bridge components due to known loads. Measuring the strains in reinforced concrete bridges can be difficult due to the non-uniform distribution of strain in the tension (cracked) zone. This paper describes the design and testing of a simple yet accurate transducer for measuring flexural tensile strains in reinforced concrete bridges.     

Experimental Measurement and Numerical Validation of Bone Cement Mantle Strains of an In Vitro Hip Replacement Using Optical FBG Sensors

Experimental pre-clinical tests associated with numeric models of cemented implants are important for screening of new implants in the market. The aim of this study was to measure strain profiles and maximum temperature polymerization inside a cement mantle of an in vitro cemented hip reconstruction using optical fiber Bragg grating (FBG) sensors. For this purpose, a hip femoral prosthesis was instrumented with 12 FBG sensors, three in each aspect of the femur, anterior, posterior, medial and lateral. These were positioned at the proximal, middle and distal part of the cement mantle relatively to the stem. Another sensor was placed in the lateral-proximal region of the mantle to measure the maximum temperature of cement polymerization. The strains measured were compared with those obtained with a Finite Element model, both for quaistatic mechanical loading. The results show that the experimental technique used can measure strains inside the cement mantle with good correlation, R²?=?0.970, with the numerical model results. The results present a maximum temperature of polymerization around 110°C inside of cement at proximal region. It was also observed strain concentration in lateral aspect of the femur in polymerization process. The procedure hereby explained can be used to improve experimental pre-clinical tests to measure the strain distribution inside the cement mantle as well as residual strain and temperature variation along with time, as a result of the curing process of cement.     

埋光纤光栅传感器智能土木结构应变监测

采取与土木工程施工特点相适应的操作工艺与保护方法，将光纤Bragg光栅传感器埋入钢筋混凝土结构中，分别在实验室和实际工程施工现场对混凝土内部的应变进行了测量。实验中光纤Bragg光栅传感性能良好，能够监测混凝土内部应变状态，显示出成活率高、绝对值测量、寿命长等优势，为建立基于光纤传感器的结构健康监测系统奠定了基础。     

Torsion measurement using fiber Bragg grating sensors

In this paper, the authors study the potential of using fiber Bragg grating (FBG) strain sensors to measure the torsion deformation theoretically and experimentally. FBG sensors are bonded on the surface of a shaft. When the shaft is under torsion, there is strain induced in the FBG sensor and the Bragg wavelength will shift accordingly. According to the wavelength shift and photoelastic properties of the FBG sensor bonded to the shaft, the torsion deformation of the shaft can be obtained. To minimize the measurement error, the optimal direction of the FBG sensor is obtained. The influences of the orientation deviation of the FBG sensor are discussed. The feasibility of this method is demonstrated by experiment, and the test results agree well with the theoretical analysis.     

Multi-Functional Measurement Using a Single FBG Sensor

This paper describes the measurement of average strain, strain distribution and vibration of a cantilever beam made of Carbon Fiber Reinforced Plastics (CFRP), using a single Fibre Bragg Grating (FBG) sensor mounted on the beam surface. Average strain is determined from the displacement of the peak wavelength of reflected spectrum from the FBG sensor. Two unstrained reference FBG sensors were used to compensate for temperature drift. Measured strains agree with those measured by a resistance foil strain gauge attached to the sample. Stress distributions are measured by monitoring the variation in the full width at half maximum (FWHM) values of the reflected spectrum, using a proposed optical analytical model, described in the paper. FWHM values were measured for both the cantilever test beam and for a reference beam, loaded using a four-point bending rig. The trend of the stress distribution for the test beam matches with our analytical model, however with a relatively large noise present in the experimentally determined data. The vibration of a cantilever beam was measured by temporal analysis of the peak reflection wavelength. This technique is very stable as measurements are not affected by variations in the signal amplitude. Finally an application of FBG sensors for damage detection of CFRP plates, by measuring the natural frequency, is demonstrated. With small defects of different sizes applied to the CFRP plate, the natural frequency decreased with damage size.     

等截面抛物线无铰拱挠度影响线实用解析解

为解决弧长微分表达式复杂导致抛物线拱挠度解析计算繁琐的难题,采取有简单弧长微分表达式的近似曲线拟合抛物线的方法,通过以沿曲线的曲率和为量化指标评价3种曲线(直线)的拟合度,最终得出悬索线拟合度最优的结论。在此基础上,利用基本力学原理,推导出抛物线无铰拱挠度影响线解析计算公式,最后应用于实例,并与有限元分析结果进行对比。算例结果表明,公式计算与有限元分析结果最大相差不超过1%,具有非常高的工程精度,可用于工程实践。     

Fiber Bragg Grating Strain Rosette

0Introduction OpticalFiberBraggGratings(FBGs)sensorshavefounddiverseapplicationsinvariousfields sincetheirdevelopmentinthelate1980s.FBGshaveanumberofsignificantadvantagesoverother fiberopticsensorssuchasmultiplexing,self referencing,optical,mechanicalandt…     

Experimental and Computational Investigation of Low-Impact Velocity and Quasi-Static Failure of PMMA

An investigation of the low velocity impact and quasi-static failure of polymethylmethacrylate (PMMA) based on global and local post-impact strain measurements was conducted. Local strains were obtained from surface mounted Fiber Bragg Grating (FBG) sensors, and they were combined with global measurements from quasi-static indentation and low-velocity impact experiments, and finite-element analyses to obtain detailed maps of how failure spatially initiates and evolves. For both loading regimes, the interactions between the host PMMA specimens and the sensors played a crucial role in the evolution of residual strains. A mapping of the strains clearly shows that strains decrease radially, from high values near the point of impact to far-field values. Sensors located in critical locations, which are near the impact region, had the highest residual strains prior to PMMA fracture. Furthermore, it was determined that strain transfer to the sensor is strongly influenced by the bonding conditions at the specimen’s surface. Due to the debonding of the sensor and the frictional effects associated with the bonding agent, compressive residual strains occurred on the rear-surface. Hence, a detailed understanding of how strain evolves due to sensor-host interactions and catastrophic fracture can be obtained, which can potentially be used to mitigate damage in PMMA for a range of strain-rates.     

<Emphasis Type="Italic">Ex Vivo</Emphasis> Intervertebral Disc Bulging Measurement Using a Fibre Bragg Grating Sensor

Advances using optical fibres as sensors may represent an important contribution for development of minimally invasive techniques in biomedical and biomechanical applications. Concerning spine injuries, intervertebral disc (IVD) degeneration is a major clinical issue since it represents gross structural disruption and it is irreversible. Measuring biomechanical parameters of the IVD should contribute for better understanding on its mechanical response to external applied forces. The purpose of this study was to explore the potential of a Fibre Bragg Grating (FBG) sensor to measure strain caused by bulging of the intervertebral disc under axial compression. Disc bulging is a consequence of IVD compression and a technique to register this behaviour is addressed in this study. Needle-mounted sensors were already used to measure IVD pressure in cadaveric material. In this study we also explored the possibility of using needles only for sensor guiding and positioning leaving sensor directly in contact with the IVD material. An ex vivo porcine dorsal functional spinal unit was instrumented with a FBG sensor and submitted to axial compression. Results suggest the sensor’s ability to measure strain response to load. Bulging of the annulus fibrosus as a consequence of axial compression was confirmed using the FBG sensor. Hysteresis and viscoelastic behaviour were observable suggesting that energy is dissipated by the deformation of the annulus and that unloading time was insufficient for disc recovery. Nevertheless the relatively low strain sensitivity of the sensor as well as signal artefacts caused by transverse loading may constitute a problem in the analysis and interpretation of strain data. The technique may not be suitable for measurement of physiologic bulging being more indicative of the radial force exerted by the annulus.     

斜拉桥合理成桥状态确定的序列二次规划法

本文首先介绍斜拉桥合理成桥状态的概念和现有的斜拉桥索力优化方法。然后基于序列二次规划(SQP)算法，提出了一种用于确定斜拉桥成桥合理状态的实用方法，序列二次规划法。该方法通过建立斜拉桥索力优化的非线性规划模型，以斜拉桥主梁和索塔的弯曲应变能为目标函数，以各斜拉索的索力为设计变量，结构的应力和索力为约束条件，并计人大跨度斜拉桥各种几何非线性因素的影响，采用强次可行序列二次规划法进行优化求解，确定斜拉桥成桥合理状态的索力。运用该方法和空间非线性有限元分析程序分析了某斜拉桥的合理成桥状态，计算结果表明该方法简单、有效。     

Deflection Measurement and Determination of Young’s Modulus of Micro-cantilever Using Phase-shift Shadow Moiré Method

The deflection of micro-structures have been previously measured using optical interferometry methods. In this study, the classical phase-shift shadow moiré method (PSSM) was applied to measure the deflection of a silicon micro-cantilever and to determine the Young’s modulus of the cantilever material. The modulus value was determined from the profile based on deflection equation. A normal white light source and a grating of 40 line pairs per mm were used to generate the moiré fringes. Since the use of white light and high-resolution grating produces low contrast moiré fringes, the fringe visibility was enhanced by applying contrast enhancement and filtering techniques. The Young’s modulus of the silicon cantilever material was estimated to be 165.9 GPa with an uncertainty of ±11.3 GPa (6.8%). The experimental results show that the PSSM method can be successfully applied for characterizing micro-cantilevers. Comparison of the deflection profile from the proposed method and a commercial 3-D optical profiler showed that the measurement range and sensitivity of PSSM are not affected by the poor contrast images.     

基于视觉的桥梁挠度测量方法与研究进展

挠度是评估桥梁承载能力和健康状态最直观的指标。近20年来,基于计算机视觉的桥梁挠度测量方法凭借其非接触式、快速简易安装等优点,被逐步应用于实际测量中。本文从测量原理、测量方式和影响因素3个方面出发,介绍了当前基于视觉的桥梁挠度测量方法与研究进展。在测量原理方面,从相机标定、三维立体视觉、摄影测量、特征检测与匹配4个方面进行了介绍。在测量方式方面,介绍了单相机二维测量、双相机三维测量、基于摄影测量的准静态测量和位移传递串联相机网络多点动态测量。在影响因素方面,介绍了相机自身因素、标定因素、算法因素和环境因素4个方面对测量结果的影响,并总结了目前国内外的研究成果。最后对基于视觉的桥梁挠度测量技术的未来发展趋势做出了展望。     

On the breakup of slender liquid bridges: Experiments and a 1-D numerical analysis

Slender axisymmetric dielectric liquid bridges are made stable by the action of an axial electric field. In this paper, the subsequent dynamics of a slender liquid bridge after turning off the electric field is considered. The evolution in time of the bridge profile is investigated both theoretically and experimentally. A one-dimensional model is used to simulate the dynamic response of the system. Experiments are performed applying an axial electric field to a liquid bridge of 1 mm of diameter, and turning-off the electric field. The evolution of the liquid bridge is recorded using a video camera, and the digitized images are analysed. Good agreement between computations and experiments is found.     

某简支组合梁桥荷载试验研究

以某简支组合梁桥为工程背景,基于现场荷载试验方法,对该桥加固后的实际承载力进行了研究。首先,详细介绍了试验过程中应变和挠度测点的布置方式;然后,利用荷载效应等效原则,结合组合简支梁桥结构的内力分布特点,确定了现场荷载试验中典型截面的最不利荷载工况。最后,将试验结果与理论计算结果进行了对比分析。研究结果表明,静载试验的应变实测值和挠度实测值均小于理论计算值,满足相关规范要求;桥梁结构整体强度和刚度满足设计荷载及正常使用的要求;该桥的跨中横向分布系数分布不均匀,且其横向传递剪力能力较弱。     

Field measured pre-stress concrete losses versus design code estimates

In Florida, pre-stressed concrete bridges with a hybrid structural configuration, consisting of simply supported girders and continuous deck units providing composite action, are prevalent. In this study, an actual bridge with this configuration is instrumented with embedded vibrating wire strain gages during construction. The axial strain data are utilized to determine the time-dependent pre-stress loss variation and distribution in the pre-tensioned concrete girders used for the bridge. In this paper, however, we discuss the bridge instrumentation and monitoring technique used, and the deduced pre-stress losses obtained from field-measured strains up to 150 days, before placement of the composite slab. A comparison of the measured loss with the estimates of the Precast/Prestressed Concrete Institute (PCI) and the American Association of State Highway and Transportation Officials (AASHTO) indicates that the field-measured pre-stress loss is non-uniform across the girder depth, opposed to a uniform distribution implicitly assumed in most codes. The measured pre-stress variation is used in computing the concrete stress distribution in the girder at different depths within the given cross-section. When compared to the concrete stress from using the PCI and AASHTO suggested losses, the stress distribution resulting fron using the field-measured loss is found to be non-linear, and in most cases higher.     

预应力混凝土简支梁桥锚下有效预应力分析

为了确定预应力混凝土简支梁桥在施工张拉与运营养护全寿命过程中结构锚固端锚下有效预应力值,基于梁挠曲变形理论与简支梁受力特性,推导出简支梁桥锚下有效预应力计算公式,并提出了一种预应力混凝土简支梁桥下有效预应力简便检测方法．以某40 m预制简支T梁桥为依托工程,通过对不同工况下简支梁桥跨中挠度值与锚固端有效预应力值进行测量,计算了不同工况下简支梁桥锚下有效预应力值,并与实测值进行了比较．研究结果表明：本文所提简支梁桥锚下有效预应力计算公式能较准确预测锚固端有效预应力值,依托工程简支梁桥锚下有效预应力公式计算值与实测值相对误差最大仅为4.83 %,计算精度满足工程需求;所提出的预应力混凝土简支梁桥锚下有效预应力简便检测方法可为桥梁全寿命预测提供指导与借鉴．     

基于光纤陀螺的桥梁微小形变检测技术

桥梁是现代交通运输业最重要的组成部分,形变又是评价桥梁结构健康状况最重要的指标之一。考虑到传统形变检测技术存在诸多不足,为了实现桥梁微小形变的快速、连续检测,准确直观地定位桥跨最大下挠处,提出了基于光纤陀螺(FOG)的微小形变检测新方法,设计出了基于FOG的桥梁连续线形检测系统(CDMS)。通过对标定路段和大型跨江桥梁结构线形进行测试,并辅以水准仪进行数据比对,结果表明:对于100 m的标定路段,检测精度可以达到5 mm;对于跨径为400 m的跨江大桥,检测精度可以达到2 cm。基于FOG的线形检测系统在桥梁结构测试过程中,无需封路,不影响交通,具有方便、快捷、测量点连续、精度高等特点,尤其是在大跨桥梁的微小形变检测中具有传统技术无可比拟的优势。     

Computation of Full-field Strains Using Principal Component Analysis

The primary output from several full-field deformation measurement techniques, e.g., Digital Image Correlation (DIC), is the displacement vector at a dense grid of points covering the area of interest. Since such displacement data inherently contain noise, they are usually smoothed first and then differentiated to obtain strains. Another common approach is to use finite-element shape functions for the strains and compute them by treating the measured displacements as nodal displacements. In this paper, we propose a novel method for strain calculation from full-field data, based on the multivariate analysis technique of Principal Component Analysis (PCA) using which we first obtain the singular values and singular vectors for each component of the displacement field. By choosing only the dominant singular values and their corresponding singular vectors, we show that the dimensionality of the displacement data is sharply reduced and a significant portion of the noise is eliminated. Moreover, the shapes of the dominant singular vectors offer physical insight into dominant deformation patterns. We demonstrate the accuracy of the proposed technique by applying it to two cases each of homogeneous and inhomogeneous strain fields and show that in all cases the proposed method yields excellent results.