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。     

基于双光纤光栅的冰压力传感器研究

冰压力是高纬度地区结构的重要荷载,然而传统的基于电阻应变计开发的冰压力传感装置在稳定性与耐久性上遇到难以跨越的难题。光纤光栅是目前在智能材料系统与结构健康监测研究与应用最为广泛的敏感材料之一,具有分布式绝对测量、抗腐蚀能力强等优点。本文基于双光纤光栅应变测量原理,考虑冰压力测试装置的环境条件,设计开发出冰压力传感装置,详细推导和试验验证了该装置的传感特性,并将试验与理论结果进行了对比。研究结果表明该装置具有温度自补偿、测量值与荷载作用点无关、线性度和重复性好、精度较高等优点,具有良好的应用前景。     

Three-dimensional strain rosettes: Pattern selection and performance evaluation

The configuration of three-dimensional strain rosettes is shown to be directly related to precision and reliability. Equations are given for computing these parameters, and several rosette configurations are compared accordingly. Experimental techniques developed at Istituto di Metrologia “G. Colonnetti” are described. A case history is given, in which the analysis of deviations between expected and actual embedded rosette results throws some light on complex interactions between model and deadweight loading machine.     

Flat vs. Stacked rosettes

Special four-element flat rosettes were used to measure residual stresses and stress concentrations. The resulting data indicated that flat strain-gage rosettes are sometimes quite inadequate for measuring principal stresses. Since all equations used to determine principal stresses from strain-gage rosettes assume a single-point measurement for all elements, it was decided to evaluate, experimentally, both flat and stacked rosettes when used in different types of stress fields. The two rosette types were found to perform equally well in constant stress fields. In nonconstant stress fields where the stress gradient was linear and of moderate level, both rosette types again performed satisfactorily. When these rosettes were subjected to nonlinear stress gradients, the data from the stacked rosette were much superior. Even a small nonlinearity in the stress gradient across a flat rosette caused significant errors in the results. A mathematical model is used to evaluate the performance of flat rosettes subjected to theoretical linear and nonlinear stress gradients. The theoretical sets of strain readings were programmed into a computer in the same manner as would be done with sets of actual test data. The theoretical data agreed quite well with the experimental data obtained and also provided information on flat-rosette performance under loading conditions difficult to simulate in the laboratory.     

Measuring small internal pressures along a tube during steady flow

A tube that can accurately measure small strains and pressure profiles during flow of non-Newtonian paste explosives has been designed and strain gaged. Equiangular rosette strain gages were installed along the length of a 6-mm-diam, 0.46-m-long thin-walled aluminum tube. The rosettes were oriented in the classical stress-gage configuration to measure circumferential stress and, hence, internal pressure independent of other stresses. The tube was static calibrated on a floating-piston pressure calibrator. Steady flow calibration was accomplished by extruding a viscous Newtonian silicone oil. Inlet pressure ranged from 0.52 to 2.1 MPa (75 to 300 psi). For the low-pressure 0.52-MPa silicone-oil extrusion, the full-scale strain levels varied from 6 to 53 μm/m. For all eight strain-gage stations, the maximum deviation from a linear pressure profile was equivalent to 0.5 μm/m. A pulsed-current-excitation signal-conditioning and digital data-acquisition system provided the necessary stability and precision to measure these unusually low-strain levels accurately.     

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.     

An experimental method of determining thermal strains

If a stress-freezing photoelastic plastic is allowed to cool through the freezing band while supporting a temperature profile, the resulting strain system consists of two parts, a frozen part and an unfrozen part. In the experiments reported in this paper, approximately 95 percent of the unfrozen part bears a known theoretical relationship to the room-temperature modulus, coefficient of expansion and Poisson's ratio of the material. It is argued that if, after freezing, electrical-resistance strain-gage rosettes are attached to the model, and the main body of the model removed from the rosette and the surface to which it is attached, the surface thermal strains corresponding to the initial temperature profile are obtained.     

Fiber Bragg grating sensor package for submicron level displacement measurements

Experimental Techniques - In this article, the design and development of a Fiber Bragg Grating (FBG) based displacement sensor package for submicron level displacement measurements are presented. A...     

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.     

The interferometric strain rosette technique

An interferometric strain rosette can be used to measure three in-plane strains. The strain rosette consists of three microindentations produced on an object surface. Upon illuminating the indentations with laser light, each pair of indentations acts as a two-point source generating a pair of Young's interference fringe patterns. When the object is deformed, the distance between the indentations is altered. By measuring the change of spacing of the Young's fringes, the strain in the direction of the separation of the indentations is measured. Three indentations are at the vertexes of an equilateral triangle to constitute a strain rosette. Each pair of the microindentations enables measurement of an axial strain in the indentation separation. The rosette measures simultaneously three in-plane strains in the directions of the triangular sides. As three in-plane strains are measured, the in-plane shear and two normal strains can be found. Compared with a resistance strain rosette, the interferometric strain rosette has great features such as non-contacting and a short gage length. In addition, the interferometric strain rosette can measure large elastoplastic strains and is applicable to measurements at elevated temperatures. The theory with experimental evaluation is presented. Measurement sensitivity of the technique is discussed. Potential applications and limitations of the technique are to be described as well.     

Use of Spectral Conditions to Separate Strain and Temperature Effects in Fibre Bragg Grating Sensors Embedded in Load-Carrying Anisotropic Laminates

This work presents a further development of the methods of simultaneous determination of temperature and axial strain using a single fibre Bragg grating (FBG). The reflected spectrum of composite-embedded FBG sensors has been analyzed in order to separate temperature and load effects. We found out that during the curing process of the laminates, a superstructure has been introduced on the FBG. Analyzing this effect, a temperature and strain separation was implemented by simply calculating and comparing the integrated intensity of the spectral response signal. A mathematical four-parametric model has been developed to calculate the reflected spectrum of a superstructured fibre Bragg grating. The mathematically achieved data has been evaluated with experimentally determined data from fibre Bragg gratings embedded in glass-fibre reinforced composite materials.     

Fiber Bragg Grating Based Stress Measurement for Films Deposited onto Cylindrical Surfaces

Shape of a substrate directly influences the residual stress in thin film coatings. In this study, a method involving Fiber Bragg Grating (FBG) was used to measure residual stress in a film deposited on a cylindrical surface. An FBG has a cylindrical surface and its Bragg wavelength shifts continuously when a film is being deposited on the sensor’s surface. Herein, we calculated the residual strain in the film from the wavelength shift of the Bragg grating by studying the transfer of the residual strain of the cylindrical film to the core of the optical fiber substrate during deposition. By employing the energy method, we derived expressions that related the strain in the core of fiber to the residual strain in single layer films, bilayer films, and multilayer cylindrical films. As an example, we demonstrated a detailed process for testing the stress and the strain distribution across a nickel (Ni) film electrodeposited on the surface of a nickel-phosphorus (Ni-P) alloy-coated optical fiber. The results indicated that the measured strain repeatability was less than 500 μ? and the strain sensitivity was more than ?2 × 10^?3 pm/μ?, when the thickness of the film was less than 5 μm. The negative sign on the strain sensitivity indicated that the tensile strain in the film produced compressive strain in the core of the optical fiber. The FBG sensor system has high test speed, and integrates measurement and signal transmission. This method provides an effective and convenient approach to measure stress in a film deposited on a cylindrical surface.     

Single-strand strainwire technique: Extensions and refinements

This paper is the first of three papers on a refined internal strainwire technique. Availability of sensitive data-acquisition equipment has made possible a detailed investigation that resulted in reductions in both gage length and excitation voltage with little reduction in the quality of the information. The method of measuring internal strains by embedded lengths of strain-gage wire has been extended to the construction and evaluation of strain rosettes. Experimentally measured principal strains on an interior plane of a compressed square prism are comared for statistical and theoretical consistency.     

A new rosette design for more reliable hole-drilling residual stress measurements

A new six-element strain gage rosette is presented that can greatly improve residual stress measurement accuracy when using the hole-drilling method. The proposed rosette consists of three pairs of sector-shaped radial and circumferential gages connected as half-bridges. This rosette design increases effective strain sensitivity by a factor of 2.3 compared with a standard ASTM rectangular rosette, and can identify stresses at one-third greater depths from the measurement surface. Experimental measurements confirm theoretical strain response calculations within 3–4 percent. Apart from a small increase in time to complete the electrical connections, the practical use of the proposed rosette is identical to that of a conventional three-element rosette.     

Analysis of a high intensity shear zone between overlapping fiber ends in a polymer matrix composite

The formation of high intensity shear zones in a glass fiber reinforced thermoplast is studied numerically. The thermoplast is characterized by a finite strain elastic-viscoplastic constitutive relation and the calculations are carried out using a dynamic finite element program where plane strain conditions are assumed to prevail in the direction of the thickness. Different ratios of the elongation strain and the transverse strain are studied to consider the effect of different levels of stress triaxiality and the effect of these states on the shear zone development and emerging strain and stress concentrations. Comparing a case of embedded infinitely stiff fibers to a case with glass fiber reinforcement shows little difference thus illustrating that the glass fibers act approximately as infinitely stiff. Fiber spacing and fiber width are shown to influence the shear zones and the stress fields that develop as the highly deformed region approaches the limit resulting from network stiffening in the polymer. A simple analysis assuming periodicity is included in order to study the mechanical behaviour of the polymer matrix between fiber ends with long overlap.     

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.     

Wave propagation in layer with two preferred directions

This article is concerned with overall or macroscopic properties of a composite material with no distinction made between the fibres and the matrix which they are embedded in. All the properties with dimensions larger than the fibre diameter and spacing are regarded as averaged over a volume of material. The systems of particular interest here are in the fibre reinforced composites with the fibres being very much stiffer and stronger than the matrix.Laminated plates of fibre-reinforced material are often fabricated from prepreg tapes, laid up according to some specific arrangement of fibre orientation and then bonded together. An angle-ply laminate is formed by alternating plies so that the families in adjacent laminas are inclined by angle ϕ and −ϕ to given direction alternately. The process of fabricating a multilayered plate of this material gives rise to a laminate in which the plies are separated by resin rich layer, and when this layer is thin enough that its thickness is negligible it may be regarded as plate reinforced by two families of fibres. Problems shall be considered in three dimensions, but attention shall be restricted to linear elasticity theory. The plate under consideration is reinforced by two mechanically equivalent families of fibres, but with no other preferred directions, so that it is locally orthotropic with respect to the plane of the fibres and to the two planes that orthogonally bisect the fibres.In this article linear elastic stress–strain relation is employed to derive dispersion curves for plane harmonic waves propagating in a plate of finite thickness but of infinite lateral extent. Attention is restricted to waves propagating in the plane parallel to stress free plate faces where waves travelling at any angle to one of the families of very strong fibres are examined. The dispersion equations, relating the phase velocity to the wavelength, are obtained. The fundamental modes are examined for symmetric as well as for anti-symmetric deformations. This leads to full understanding of displacement field as well as stress field.     

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.     

Dynamic strain measurements of a circular cylinder in a cross flow using a fibre Bragg grating sensor

 A fibre Bragg grating (FBG) sensor was proposed as an alternative to strain gauges to measure the strain ɛ of a vibrating cylinder in a uniform cross flow. In order to validate the measurements of the FBG sensor, the transverse fluctuating bending displacement Y of the cylinder was also measured using a laser vibrometer. The two measurements were found to be consistent in terms of the natural frequency of the fluid–structure system and the vortex shedding frequency. The spectral coherence between ɛ and Y at the same point of the cylinder attains 1 at these frequencies, thus indicating a near perfect correlation between the two quantities. When the transverse bending displacement is small, the measured ɛ and Y are linearly related. Therefore, the results indicate that the FBG sensor can be used with confidence to measure the fluctuating strain arising from the vortex-induced forces on a structure in a uniform cross flow. As such, it can be used in conjunction with a laser Doppler anemometer to study fluid–structure interactions in flow-induced vibration problems. Furthermore, it is expected that the FBG sensor, because of its physical uniqueness, will have an important role to play in the study of fluid–structure interaction problems with multiple structures arranged in an array. Received: 17 August 1998 / Accepted 27 January 1999