Strain monitoring in composite-strengthened concrete structures using optical fibre sensors

In this paper, the mechanical behaviour of the composite-strengthened concrete structures is addressed. Optical fibre sensor presents a great deal of potential in monitoring the structural health condition of civil infrastructure elements after strengthening by externally bonded composite materials. The use of embedded optical fibre sensor for strain and temperature monitoring enables to reveal the status of the composite-strengthened structure in real-time remotely. In this paper, an experimental investigation on the composite-strengthened concrete structures with the embedment of fibre-optic Bragg grating (FBG) sensors is presented. Single- and multiplexed-point strain measuring techniques were used to measure strains of the structures. Frequency modulated continuous wave (FMCW) method was used to measure strains in different points of the structure with using only one single optical fibre. All strains measured from the sensors were compared to conventional surface mounted strain gauges. Experimental results show that the use of the embedded FBG sensor can measure strain accurately and provide information to the operator that the structure is subjected to debond or micro-crack failure. Multiplexed FBG strain sensors enable to measure strain in different locations by occupying only one tiny optical fibre. Reduction of strength in composite laminate is resulted if the embedded optical fibre is aligned perpendicular to the load-bearing direction of the structure.     

Measuring changing strain fields in composites with Distributed Fiber-Optic Sensing using the optical backscatter reflectometer

Background/purposeMeasurements of strains in critical components are often required in addition to finite element calculations when evaluating a structure.MethodsThis paper describes how standard optical fibers, bonded to the surface or embedded in a laminate, can measure strain fields along the entire length of the fiber, using the optical backscatter reflectometer.ResultsA strain field measurement can be much better compared to simulations than the more common single point measurements with strain gauges or Bragg Gratings. Changes of the strain field can be related to damage development and can be used for structural health monitoring. Practical aspects of using the fibers are also discussed.ConclusionDistributed Fiber-Optic Sensing was successfully embedded and bonded to a composite joint. Adhesive damage was identified and the strain field agreed well with FE-Analysis.     

Composite cure simulation scheme fully integrating internal strain measurement

This study proposes a new approach to determine key material parameters for stress/strain calculation of curing composite laminates and validate the simulation. Specifically, fiber Bragg grating (FBG) strain sensors are embedded in a composite laminate and the two key parameters for simulation, composite shrinkage strain and stiffness change during curing, are simultaneously determined from in-situ measurements by the embedded sensors. Furthermore, the simulation is validated using internal strain change during curing. This paper begins by presenting an overview of the proposed simulation scheme and by comparing it with previous approaches to highlight its advantages. Material parameter determination using a shear-lag effect at the edge of the embedded sensors is then described and the practical procedure to obtain the key parameters is demonstrated using a carbon/epoxy laminate. Finally, cure simulation is conducted for validation. Further extension to more general cases including thermoplastic composites is also discussed.     

MEASUREMENT OF INTERNAL STRAIN IN CONCRETE

Methods of strain measurement in concrete have been investigated experimentally. This paper reports work on embedded resistance strain gauges mounted on concrete and other forms of gauge carrier, techniques for strain gauging embedded steelwork, results of gauge factor tests on briquetted and unbriquetted vibrating wire gauges, and gauge responses to temperature variations. Proposals are outlined for future work particularly on stress meters as distinct from strain gauges.     

The Effects of Debonding on the Low-Velocity Impact Response of Steel-CFRP Fibre Metal Laminates

The effect of metal-composite debonding on low-velocity impact response, i.e. on contact force–central deflection response, deformation profiles and strains on the free surfaces was studied. We focused on type 2/1 fibre metal laminate specimens made of stainless steel and carbon fibre epoxy layers, and tested them with drop-weight impact and quasi-static indentation loadings. Local strains were measured with strain gauges and full-field strains with a 3-D digital image correlation method. In addition, finite element simulations were performed and the effects of debonding were studied by exploiting cohesive elements. Our results showed that debonding, either the initial debonding or that formed during the loading, lowers the slope of the contact force–central deflection curve during the force increase. The debonding formation during the rebound phase was shown to amplify the rebound of the impact side, i.e. to lower the ultimate post-impact deflection. The free surface strains were studied on the laminate’s lower surface at the area outside the debond damage. In terms of in-plane strains, debonding formation during impact and indentation, as well as the initial debonding, lowered the peripheral strain and resulted in a positive change in the radial strain.     

基于内埋光纤Bragg光栅传感器的复合材料固化过程监测

为了全面了解复合材料的固化特性,在对碳纤维增强树脂基复合材料固化变形进行数值仿真分析的基础上,将自行设计的光纤Bragg光栅（FBG）传感器埋入复合材料中,实时在线监测复合材料固化过程中温度和应变的演变。预浸料铺层方式为[0₁₁/90₁₁],分别在层合板0°和45°方向的典型位置埋入FBG温度和应变传感器,采用热模压方式固化成型复合材料层合板,并对成型后的层合板进行连续2次降温处理,实时记录固化过程中FBG传感器中心波长的变化。结果表明:在相同的温度条件下,复合材料在第1次降温初始阶段的压应变绝对值明显小于在第2次降温初始阶段的压应变绝对值,表明复合材料在第1次降温过程中仍在进行FBG传感器可检的“后固化”反应;此外,层合板变形的FBG传感器监测数据与有限元模拟结果吻合良好。因此,采用内埋FBG传感器的方法能够实时监测复合材料固化过程,为更全面地分析复合材料固化特性提供了一种可靠有效的方法。      

The Reinforcement Effect of Strain Gauges Embedded in Low Modulus Materials

The reinforcement effect of electrical resistance strain gauges is well‐described in the literature, especially for strain gauges installed on surface. This paper considers the local reinforcement effect of strain gauges embedded within low Young modulus materials. In particular, by using a simple theoretical model, already used for strain gauges installed on the surface, it proposes a simple formula that allows the user to evaluate the local reinforcement effect of a generic strain gauge embedded on plastics, polymer composites, etc. The theoretical analysis has been integrated by numerical and experimental analyses, which confirmed the reliability of the proposed model.     

含冲击损伤复合材料层压板压缩破坏机制试验研究

为了研究含冲击损伤复合材料层压板的压缩破坏机制, 通过前后表面超声C扫描获得分层损伤在层压板内的分布情况, 使用应变片获取层压板两面的应变场, 采用声发射观察层压板压缩破坏过程。研究发现: 冲击分层损伤在厚度方向上的非对称分布引起层压板内部出现严重的应力集中, 致使纤维断裂并最终导致层压板整体破坏; 导致含冲击损伤层压板压缩破坏的主要原因为纤维断裂而非分层扩展。     

The effect of a sliding load on strain gauges embedded under the contact surface

Strain gauges embedded in plastic models can be used in three dimensional stress analysis. When gauges are located immediately below a surface subjected to the tangential traction of a sliding load, unexpected values are recorded. This phenomenon was investigated by suitably loading a semi–infinite solid with normal and shear stresses and comparing theoretical strain values with the experimental results. The comparison showed that strain gauges embedded in epoxy can be successfully used under sliding loads provided that the gauge depth below the contact surface is greater than a critical value.     

A new structural health monitoring system for composite laminates

The increasing use of composite laminates in safety critical structures has prompted the development of a robust structural health monitoring system for laminates, which uses metastable ferrous alloy inserts embedded within the laminate during component construction to provide an indication of the peak tensile strain encountered by the laminate. The metastable ferrous alloy insert has an austenitic crystal structure at room temperature, but upon application of strain, this transforms to a thermodynamically stable martensite, resulting in a change in magnetic susceptibility, which can be correlated with the peak strain experienced by the material (strain memory effect). This paper presents the test results that show that it is possible to manufacture a smart laminate in this fashion, and that sufficient strain is experienced by the insert to provide a significant change in magnetic susceptibility, thereby warning of a high strain level in the laminate. Various insert geometries and laminate thicknesses are also tested for their effect on the susceptibility measurements.     

A Probabilistic Evaluation of the Least Squares Strain Tensor Derived from a Three-Dimensional Modular Strain Rosette Using the Monte-Carlo Technique

Abstract:  Strain gradients give rise to a number of problems in the field of embedded three-dimensional strain measurement. In order to avoid these problems a modular type three-dimensional strain rosette was embedded into known strain fields and the data from the individual gauges compared with theoretical predictions. Finally, the least squares strain tensor was predicted from experimental data analysed using the Monte-Carlo technique and the theoretical results forecast from finite element data taking into account the mechanical properties of the carrier, plug and prismatic bar. Some of the experimental results were found to correlate well with the theoretical values but some values in the least squares strain tensor, in particular under compression and torsional loading, departed considerably from the theoretical values. It was found that the effect of the measurement errors in the individual gauges combined with the matrix operations in the least squares strain tensor were responsible for biasing the resultant tensor data. However, the modular technique provided a solution to the problem of strain gradients.     

Fibre-optic phase-strain response in unidirectional composites

Fibre-optic Fabry-Perot strain gauges are embedded in unidirectional laminates subject to longitudinal stress for the purpose of determining their phase-strain response. The laminated coupons used were Kevlar/epoxy and graphite/polyetheretherketone. Cantilever-beam bending was used to load the coupons while monitoring the phase of the fibre-optic strain gauges. The phase-strain response was found to be repeatable, linear and symmetric with respect to tension and compression. The phase-strain sensitivity was found to be the same as that predicted by a free-fibre uniaxial stress model.     

Local strain in a 5-harness satin weave composite under static tension: Part I - Experimental analysis

This paper presents an experimental method for determining the local strain distribution in the plies of a thermoplastic 5-harness satin weave composite under uni-axial static tensile load. In contrast to uni-directional composites, the yarn interlacing pattern in textile composites causes heterogeneous strain fields with large strain gradients around the yarn crimp regions. In addition, depending on the local constraints that are imposed by the surrounding plies, the deformation behavior of the laminate inner layers may vary from that of the surface layers, which are relatively more free to deform, compared to the inner layers. In order to validate the above hypothesis, the local strains on the composite surface were measured using digital image correlation technique (LIMESS). Internal strains in the composite laminate were measured using embedded fibre optic sensors (FOS).Based on the DIC results, the strain profiles at various locations on the composite surface were estimated. Using the FOS results, the maximum and minimum strain values in the laminate inner layers were evaluated. Comparison of the local strain values at different laminate positions provides an estimate of the influence of the adjacent layers on the local longitudinal strain behavior of a satin weave composite. Part II of this paper elucidates the local strain variation computed using the meso-FE simulations. In addition to the comparison of numerical and experimental strain profiles, Part II presents the maximum and minimum strain envelopes for the carbon-PolyPhenelyne Sulphide (PPS) thermoplastic 5-harness satin weave composite.     

The Performance of Three-Dimensional Strain Rosettes Evaluated when Embedded into a Sphere

Abstract:  Two spheres were manufactured, each containing planes of embedded strain gauges that were arranged so that individual gauge readings could be combined in a variety of ways to calculate the least squares strain tensor. Each sphere was tested by loading along its axis of symmetry. It was found that the particular pattern of rosette chosen produced satisfactory predictions of the orthogonal strains but minor off-axis gauge misalignments combined with the matrix operations in the least squares strain tensor led to misleading forecasts of the shearing strains. The method of manufacture used to embed the gauges was largely responsible for these errors. The investigation confirmed that three-dimensional strain rosettes should be made from square plugs and the direction cosines measured on the plug prior to embedment in the model.     

Höchste Präzision in kurzer Taktzeit

Highest precision in short cycle time – highly sensitive thin fiim strain gauges produced in an industrial high‐rate‐sputtering system Commonly, sputtered strain gauges are already used for highly precise pressure sensors. By using special highly sensitive piezoresistive nanocomposites consisting of metal nanoparticles embedded in an insulating matrix made from diamond‐like carbon (DLC) the sensitivity to strain can be significantly increased. Essential parameters for the characterization of the thin films used as sensor layer are the strain sensitivity (described by the gauge factor) and the temperature coefficient of resistance (TCR). Conventionally used NiCr alloys have a gauge factor of approximately 2. By using metal‐DLC nanocomposite films a factor 5 to 10 higher strain sensitivity in combination with a TCR close to zero was reached on laboratory scale. At first, the highly sensitive films were produced by static deposition using a box coater which led to quite long process times. By using a dynamic deposition process in the same machine the throughput of samples was slightly increased. But for using these highly sensitive films on industrial scale much higher cost and process efficiency is necessary. Hence, the process was transferred to a highrate sputtering system. A 20 times higher throughput of samples was reached in combination with a higher strain sensitivity compared to the dynamic process in the box coater. The used high‐rate sputtering system is also commercially available with enlarged process chambers which enables for a further up‐scaling for efficiently industrial production.     

An Analysis of the Factors Influencing the Data Derived from a Plug Type Three-Dimensional Strain Rosette under Compression and Torsion

Abstract:  The embedding of three-dimensional strain rosettes embedded into epoxy models provides an experimental technique for analysing complex structures; however, this technique has been known to produce data that were difficult to explain in terms of their physical significance. To gain a greater insight into the behaviour of a three-dimensional strain rosette used in this way, a three-dimensional strain rosette was embedded into each of two separate prismatic bars of square cross-section and subjected to fundamental tests of compression and torsion in standard commercial testing machines. In initial tests on a bar containing a three-dimensional strain rosette (Bar A) the data derived from the individual gauges sometimes departed from the theoretical values by more that 30  μ e. After critical evaluation of the procedures used for making and testing Bar A, further tests were carried out on Bar B, which led to a reduction in the difference between theoretical and experimental data to 14  μ e, acceptable for most practical purposes. The use of square plugs containing three-dimensional strain rosettes which are embedded into square cavities in the model, and the measurement of the actual direction cosines of the gauges on the square plug prior to embedment is a distinct advantage over the use of cylindrical plugs. In addition, the use of testing machines with a fixed base as opposed to a floating lower platen is recommended.     

FeGa/BTO/FeGa层状复合结构中的磁电效应

采用区域熔炼法制备了FeGa磁致伸缩材料并测量了其磁致伸缩性能.利用该材料制备了FeGa/BTO/FeGa层状复合结构,并对该样品的磁电性能进行了系统研究.结果表明,该材料在共振频率95kHz下,磁电性能高于低频下性能7～10倍.磁电电压随直流偏磁场的变化发生明显变化,出现5.97×104A/m的优化偏置场,这主要是由FeGa层的压磁系数q随偏磁场的变化所致.3层复合材料的磁电系数与交流驱动场变化呈线性关系.另外,较薄的BTO压电层可以提高压应力,从而获得较高的磁电性能.     

光纤布拉格光栅监测复合材料固化

复合材料层合板在固化成型过程中形成的残余应变和应力是影响材料质量的重要因素,它与预浸料铺层在固化工艺过程中产生的应变密切相关。在研究和测试了光纤布拉格光栅应变和温度传感器传感特性的基础上,将二者埋入复合材料预浸料铺层,在热压釜成型工艺过程中监测了材料内部的温度和应变发展历程,由此获得对称正交层合板的宏观残余应变。监测结果表明,单向和对称正交层合板在固化结束后都将产生收缩,对称正交层合板铺层内的残余应变平行于纤维方向为压应变,垂直于纤维方向为拉应变。光纤光栅传感器为复合材料固化监测及层合板残余应力分析提供了一种新的工具,为实现复合材料从制造到服役的全寿命、一体化监测提供了可能。      

CALL混杂复合材料的弯曲试验研究

本文用高灵敏度云纹干涉法对CALL混杂复合材料在纤维方向和垂直于纤维方向的弯曲及破坏特性进行了实验研究,得到了弯曲试件横截面上的剪应变分布规律及破坏形式。实验结果表明:碳纤维/环氧树脂层的剪应变明显大于铝层的剪应变,但各自沿截面呈抛物线分布。纤维方向弯曲试件的破坏形式是分层或碳纤维/环氧树脂层剪切破坏;垂直于纤维方向弯曲试件的破坏由受拉面碳纤维/环氧树脂层的拉伸破坏所致。本文工作为进一步深入研究CALL材料的力学性能提供了重要的实验依据。     

Investigation on strengthening and strain sensing techniques for concrete structures using FRP composites and FBG sensors

This paper reports the results of an experimental investigation on the mechanical properties of the laboratory size rectangular concrete beams with the notch formation on the tension surface strengthened by using fibre reinforced plastic composites, Fibre-optic Bragg grating (FBG) sensors have been embedded at the interface between the concrete surface and external bonded composites to measure the strain variations when the strengthened structures were subjected to three-point bending load. Multiplexing strainsensing technique has been used for measuring the strains in different locations of the beam by using one single optical fibre. External bonded strain gauges were also attached on the surfaces of composites to measure the surface strain and the results were compared with that obtained from the embedded FBG sensors. The observation in the experiments showed that the flexural load capacity of the strengthened concrete beams was increased significantly after strengthening by laying-up the compsite laminate for both plain and notched concrete structures. Debond and concrete failure occurred at the plate end region when the thick composite laminate was used. The embedded FBG sensors could provide the indication to the operator when the structure serviced in strange conditions. The strains measured at the interfaces from the sensors were higher than that from the surface bonded strain gauges in general.

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Résumé Les investigations expérimentales présentées dans cet article portent sur les propriétés mécaniques d'éprouvettes de béton rectangulaires entaillées en surface de la zone de tension puis renforcées au moyen de polymères renforcés de fibres (FRP). Des capteurs à réseau de Bragg à fibre optique (FBG) ont été placés à l'interface entre le béton et le composite afin de mesurer les variations des contraintes quand la structure est soumise à une charge de flexion en trois points. Un multiplexage des capteurs est utilisé pour mesurer les contraintes en différents points de l'éprouvette en n'utilisant qu'une seule fibre optique. Des jauges d'extensométrie ont aussi été placées en surface du composite, leurs résultats sont alors comparés à ceux obtenus à partir des capteurs FBG. Ces expériences ont montré que la résistance au fléchissement des poutres est nettement améliorée grace au renforcement par le composite, ce tant pour les structures intates, que pour les éprouvettes entaillées. Lorsque la poutre est renforcée par une plaque épaisse du composite laminé, la rupture de la liaison avec le béton, ainsi que la rupture du béton, ont lieu au bord de cette plaque. Les capteurs à réseau de Bragg à fibre optique permettent de fournir des indications à l'opérateur lorsque la structure est en service dans des conditions peu usuelles. Les contraintes mesurées par les capteurs. FBG aux interfaces sont en général plus importantes que celles mesurées par les jauges situées en surface du composite.