Elasto-Plastic Bonding of Embedded Optical Fiber Sensors in Concrete

Fiberoptic sensors are increasingly employed for sensing and measurement of strains in structural materials. The glass core of the optical fiber senses the strain through intensity fluctuations, interference, or frequency modulation. Brittleness of the glass core limits practical usage, and therefore, the glass core of optical fibers is coated with low modulus softer protective coatings. The protective coating alters the strain transduction capabilities of the sensor. It absorbs a portion of the strain, and hence only a segment of structural strain is sensed. The study reported here corrects for this error through development of a theoretical model to account for the loss of strain in the protective coating of the optical fiber. The model considers the coating as an elasto-plastic material and formulates strain transfer coefficients for elastic, elasto-plastic, and plastic phases of coating deformation. The theoretical findings were verified through laboratory experimentation. The experimental program involved fabrication of interferometric optical fiber sensors, embedment within mortar samples, and tensile tests in a closed-loop servo-hydraulic testing machine. The elasto-plastic strain transfer coefficients developed in this study were employed for correction of optical fiber sensor data and results were compared with conventional extensometers.     

Processing and microstructures of fiber bragg grating sensors embedded in stainless steel

The benefit of layered manufacturing is the ability to create physical parts with little or no restriction from shape complexity. Using a laser-assisted shape deposition manufacturing (LASDM) methodology, fiber Bragg grating (FBG) sensors were embedded into stainless steel structures. The embedding sequence included sputtering a thin metallic film as a conductive layer on the silica fiber, electroplating the fibers with a Ni protective layer, and then laser cladding with stainless steel. Microstructural studies of each layer and interface were conducted. The grain size of electroplated nickel was approximately 150 nm before laser cladding. The electroplated nickel recrystallized during and after the laser cladding. The nickel grain size gradually increased from very fine (∼1 - to 10-μm diameter) in the immediate layer surrounding the optical fibers to approximately 200 μm near the interface with the laser-deposited stainless steel. The deposited stainless steel appeared as a coarse dendritic structure with a grain size larger than 100 μm. The Ni/fiber interface shows full contact between the silica fibers and the Ni matrix of adequate thickness before and after laser cladding. Microstructural studies were used to conclude that the degradation of the embedded sensors is due to thermally induced stress from laser deposition if a thinner Ni layer is used. Strain and temperature measurements were successfully performed using the embedded FBG sensor, thus demonstrating the viability of this manufacturing technique.     

Debonding and Calibration Shift of Optical Fiber Sensors in Concrete

Fiber optic sensors have recently be considered for strain monitoring in concrete structures. The calibration factor of the sensor depends on the strain distribution along the fiber. When an embedded fiber is under strain, debonding may occur, causing the strain distribution and hence the calibration to change. Since interfacial properties that govern debonding are sensitive to environmental conditions, the calibration factor can also change when exposed to various environments. In this paper, a theoretical framework is developed to quantify the effect of environmental conditions on calibration shift. To illustrate the application of the theoretical approach, pullout test results on specimens subjected to various environmental conditions are first analyzed to obtain interfacial parameters. With these parameters, the effects of applied strain, environmental conditions, and fiber length on the calibration factor of two kinds of fiber optic sensors are quantified with the use of a strain transfer model. Based on the results, design guidelines to minimize calibration shift can be identified.     

Field Pullout Testing and Performance Evaluation of GFRP Soil Nails

Glass fiber–reinforced polymer (GFRP) materials provide practical solutions to corrosion and site-maneuvering problems for civil infrastructures using conventional steel bars as reinforcements. In this study, the feasibility of using GFRP soil nails for slope stabilization is evaluated. The GFRP soil nail system consists of a GFRP pipe installed by the double-grouting technique. Two field-scale pullout tests were performed at a slope site. Fiber Bragg grating (FBG) sensors, strain gauges, linear variable displacement transformers (LVDTs), and a load cell were used to measure axial strain distributions and pullout force-displacement relationships during testing. The pullout test results of steel soil nails at another slope site are also presented for comparison. It is proven that the load transfer mechanisms of GFRP and steel soil nails have certain difference. Based on these test results, a simplified model using a hyperbolic shear stress-strain relationship was developed to describe the pullout performance of the GFRP soil nail. A parametric study was conducted using this model to study some factors affecting the pullout behavior of GFRP soil nails, including nail diameter, shear resistance of soil-grout interface, and ratio of interface shear coefficient to the Young’s modulus of the nail. The results indicate that the GFRP soil nail may exhibit excessive pullout displacement and thus a lower allowable pullout resistance than with the steel soil nail.     

Distributed Strain Measurement in Steel Bridge with Fiber Optic Sensors: Validation through Diagnostic Load Test

Fiber optic sensing technologies are emerging as valid alternatives for the health monitoring of civil structures. Distributed sensors based on Brillouin scattering add the unique capability of measuring strain and temperature profiles along optical fibers. Measurement is performed by establishing the correlation between fiber strain and temperature, and the frequency shift of the Brillouin backscattered light induced by a monochromatic light pulse. The technology holds potential for use on large structures and integrated transportation infrastructure. Its effectiveness has been assessed through scaled laboratory experiments, whereas field validation is limited to very few demonstration projects conducted to date. This paper presents a pilot application of Brillouin optical time domain reflectometry to measure strain profiles along the steel girders of a continuous slab-on-girder bridge subjected to diagnostic load testing. One of the exterior continuous girders required heat-straightening after falling during construction due to wind. The significance of applying a distributed measurement technique lies in the potential to assess the global girder response, which would be impractical and uneconomical using discrete measurement techniques. A 1.16?km long sensing circuit was installed onto the web of four girders. The circuit comprises bare optical fiber sensors, and a novel adhesively bonded fiberglass tape with embedded sensing fibers for strain measurement and thermal compensation. The strain profiles were first converted into deflection profiles and validated against discrete deflection measurements performed with a high-precision total station system. Structural assessment based on comparison of the strain profiles with the results of three-dimensional finite-element analysis of the bridge superstructure, and with specification mandated criteria, indicated that the response of the girder under investigation was within the design limits, and did not pose serviceability concerns. Factors that may affect measurement accuracy are finally discussed on the basis of the experimental and numerical results.     

Comparison of biomechanical properties of the incisor periodontal ligament among different species

BACKGROUND: The aim of this study was to obtain a more precise understanding of the mechanical properties of the periodontal ligament in continuously erupting incisors by comparing the shear stress-strain relations among teeth from four closely related species. METHODS: Four species of experimental animals (mice, hamsters, rats, and rabbits) were used. Transverse sections of the left mandibular incisors were cut from the incisal, middle, and basal regions of each incisor. The tooth was pushed out of the alveolar bone in an extrusive direction at 5 mm/min using a materials testing machine. The maximum shear stress, maximum shear strain, tangent modulus, and failure strain energy density were estimated from the resulting stress-strain curve. Polarized light microscopic observations of collagen fibers were also made. RESULTS: All the biomechanical measures tended to decrease from the incisal toward the basal regions in all species. There were large species differences, especially in the incisal region, with the greatest maximum shear stress and failure strain energy density in hamsters. The greatest tangent modulus and the smallest maximum shear strain were observed in mice. The birefringent fiber architectures of the periodontal ligaments in the four species appeared to be similarly organized; the incisal periodontal ligament appeared to have more organized and thicker collagen fibres than did the middle and basal ligaments in the four species. CONCLUSIONS: These results suggest that the regional differences in the biomechanical properties of the periodontal ligament depend upon the developmental stages of the periodontal collagen fibers that may be related to the general arrangement, diameters, and densities of the collagen fiber bundles and the fiber insertions into the alveolar bone and cementum. The species differences in the biomechanical properties may be due to differences in the width of the periodontal ligament and the waviness as well as the strength and stiffness of the periodontal collagen fibers.     

On creep of unidirectional fiber composites with fiber damage

The roles of broken fibers in the creep of continuous fiber reinforced composites are studied theoretically. The unidirectional fiber composite is modeled using a cylindrical cell consisting of a single broken fiber and a shell of the fiber material embedded in an elastic-power law creeping matrix. The time-dependent creep behavior of the composite is calculated using finite elements; both longitudinal and transverse loads are considered. It is shown that when fibers are broken, the increase in the overall creep strain of the composite and the axial stress in the intact fibers can be significant. It is also demonstrated that applied transverse tension can reduce the composite creep strain and the normal stress in the fibers; the opposite is true with applied transverse compression. Matrix plasticity is found to have very limited effect on the creep behavior of the composite. The accuracy of the McLean formula for undamaged composite is also examined.     

Theoretical Modeling of Signal Loss versus Crack Opening for a Novel Crack Sensor

The degradation of concrete structures is always accompanied by the formation of cracks. Crack monitoring is hence useful in assessing the “health condition” of the structure. In our previous investigation on an optical-fiber crack sensor, we have successfully demonstrated the possibility of crack detection and monitoring without requiring prior knowledge of crack locations. Also, a single fiber can be employed to monitor a number of cracks. In practical applications, the signal loss versus crack opening relationship of the sensor needs to be properly “designed” to satisfy performance requirements on sensitivity to small cracks and/or the total number of detectable cracks (which is limited by the maximum loss at each crack). In this paper, a theoretical model for the signal loss versus crack opening relationship is developed through a combination of mechanical and optical analyses. Using concrete beams with embedded sensors, crack monitoring experiments are conducted. Test results are found to be in reasonably good agreement with predictions from the model. With the verified model, simulations are carried out to study the effect of various parameters on sensor performance. The potential application of the theoretical model to generate sensor design guidelines is hence demonstrated.     

The effect of hip stem material modulus on surface strain in human femora

Human femora were used to compare the changes in bone surface strain resulting from decreasing the material modulus of a collarless hip stem to determine whether a highly elastic stem increased bone loading. Three substrate materials were tested: titanium (modulus of elasticity 110 GPa), carbon fiber composite (modulus of elasticity 52 GPa), and polymethylmethacrylate (PMMA, modulus of elasticity of 1.9 GPa). Two separate analyses were performed in which femora were implanted randomly with one of the three stem types. Results showed that assembly strains did not differ significantly among different materials. There was a large strain reduction in the proximal region of the femora for all stem substrates relative to the intact femur. Although there was statistically greater surface shear strain as the material modulus decreased, the PMMA stem did not substantially increase bone loading.     

Flexural Strengthening of RC Beams with Cement-Based Composites

In this paper, the effectiveness of fiber-reinforced cementitious matrix (FRCM) materials for the strengthening of reinforced concrete (RC) beams is experimentally investigated. Bending tests on RC beams strengthened with different FRCM materials, made out of (1)?carbon fiber nets; and (2)?poliparafenilenbenzobisoxazole (PBO) fiber nets embedded in cement-based matrix, are performed. For case (2), different net shapes, cementitious matrices, and a number of net layers were considered. Depending on the type of fibers and matrix, different flexural debonding failure modes are identified. The fiber strain at debonding is evaluated by comparing the experimental results with those obtained with two different theoretical models. The results obtained in this study confirm the effectiveness of FRCM materials for the strengthening of RC structures and encourage further experimental and theoretical work on the topic. A better understanding of the debonding phenomenon is crucial for an optimal design of the strengthening material. The way in which the nature of fibers and matrices and the number of layers control the performance of the strengthened members is also investigated in the present paper.     

Lifetime enhancement of ultrasmall fluorescent liquid polymeric film based optodes by diffusion-induced self-recovery after photobleaching

The major concern with optodes, especially miniaturized ones, has been their photobleaching limited lifetime. Liquid polymer [highly plasticized poly(vinyl chloride)] films are commonly used to prepare fluorescent optical fiber sensors. A major advantage is the ease of their fabrication. It is demonstrated here that, with proper choice of excitation power and illumination time, the sensor will completely recover itself from photobleaching after each measurement. This self-recovery is demonstrated on single-mode optical fibers with 80 microns diameter (3.1 microns active region) and on near-field scanning optical microscope pulled fiber tips with submicrometer diameter (250 nm active region). The single-mode optode can be used for 30,000 measurements with only a 5% signal loss at a signal/noise of > 66. This opens the way for prolonged ratiometric application of such optodes.     

Construction Effect on Load Transfer along Bored Piles

The load transfer behavior along bored piles is affected by details of pile construction particularly those imposing stress and moisture changes to the surrounding soils. An investigation involving moisture migration tests, in situ horizontal stress measurements, and borehole shear and pressuremeter tests shows clear effects of construction that lead to subsequent changes in soil properties. The construction of bored piles in Singapore and the region often involves casting of concrete either in unsupported “dry” boreholes or in “wet” boreholes filled with water. It is necessary to differentiate these two extreme construction conditions in bored pile design. Based on triaxial compression and pressuremeter tests on the residual soil of the Jurong Formation in Singapore, the variation of soil modulus with shear strain can be described by a hyperbolic function. A procedure is recommended for assessing the combined effect of stress relief and soaking on soil modulus by introducing a modulus reduction factor. Modulus degradation curves from pressuremeter tests with the borehole conditions properly simulated are found capable of producing load transfer curves that are comparable to those deduced in the field.     

光纤传感器的原理及应用

传感器在人们日常生活中已经得到了广泛的应用,随着新一代传输媒介光纤的问世,传感器得到进一步的发展,出现了光纤传感器。本文主要介绍了光纤传感器的相关知识包括光纤的基本知识以及光纤传感器的分类结构等,并重点介绍了光纤传感器的原理及其在各方面的广泛应用。     

Fiber Optic Sensors for Bridge Monitoring

Advances in the production of optical fibers made possible the recent development of innovative sensing systems for the health monitoring of civil structures. The main reasons for this development are the reduced weight and dimensions of fiber optic sensors, the strong immunity to electromagnetic interference, the improved environmental resistance and the scale flexibility for small-gauge and long-gauge measurements. These systems can provide high-resolution and measurement capabilities that are not feasible with conventional technologies. In addition, they can be manufactured at a low cost and they offer a number of key advantages, including the ability to multiplex an appreciable number of sensors along a single fiber and interrogate such systems over large distances. For these reasons, it is evident that fiber optic sensors will change the instrumentation industry in the same way fiber optics has revolutionized communications. This paper provides an overview of the intensity modulated and spectrometric fiber optic sensors and techniques to assess the condition of existing structures in order to enhance the durability of the new bridges, increasing lifetime and reliability and decreasing maintenance activities. Application of these sensors to monitoring strain, temperature, inclination, acceleration, load measurements, ice detection, vehicles speeds and weights, and corrosion and cracking of reinforced and prestressed concrete structures will be described.     

Characterization of Damage in Triaxial Braided Composites under Tensile Loading

Carbon fiber composites that utilize flattened, large tow yarns in woven or braided forms are being used in many aerospace applications. The complex fiber architecture and large unit cell size in these materials present challenges for both understanding the deformation process and measuring reliable material properties. In this paper composites made using flattened 12k and 24k (referring to the number of fibers in the fiber tow) standard modulus carbon fiber yarns in a 0°/+60°/?60° triaxial braided architecture are examined. Standard straight-sided tensile coupons were tested with the 0° axial braid fibers either parallel to (axial tensile test) or perpendicular to (transverse tensile test) the applied tensile load. The nonuniform surface strain resulting from the triaxial braided architecture was examined using photogrammetry. Local regions of high strain concentration were examined to identify where failure initiates and to determine the local strain at the time of failure initiation. Splitting within fiber bundles was the first failure mode observed at low to intermediate strains. For axial tensile tests the splitting was primarily in the ±60° bias fibers, which were oriented 60° to the applied load. At higher strains in the axial tensile test, out-of-plane deformation associated with localized delamination between fiber bundles or damage within fiber bundles was observed. For transverse tensile tests, the splitting was primarily in the 0° axial fibers, which were oriented transverse to the applied load. The initiation and accumulation of local damage caused the global transverse stress-strain curves to become nonlinear and caused failure to occur at a reduced ultimate strain for both the axial and transverse tensile tests. Extensive delamination at the specimen edges was also observed. Modifications to the standard straight-sided coupon geometry are needed to minimize these edge effects when testing the large unit cell type of material examined in this work.     

Carbon fibers coated with chromium carbide using the liquid metal transfer agent technique

Protective coatings of chromium carbide were applied to PAN type carbon fibers by a liquid metal transfer agent (LMTA) technique using copper and tin as transfer agents, and the effect of the coating on the strength of the fibers was evaluated by performing single fiber tensile tests. The coatings were examined metallographically, by X-ray diffractometry, and by scanning electron microscopy. It was found that a tin transfer medium produced a smoother and more uniform coating than a copper transfer medium. The smallest carbide coating obtained using a copper transfer medium was approximately 1 μm, while coatings as thin as approximately 0.14 μm were obtained using a tin transfer medium. It was found that wetting of the fibers by the tin and copper alloys is associated with the spontaneous formation of a carbide layer of significant thickness, after which the carbide layer was found to grow parabolically with time and with an apparent activation energy of 139 kJ/mol. The strength of the carbon fibers decreased with increasing coating thickness.     

Development and cellular applications of fiber optic nitric oxide sensors based on a gold-adsorbed fluorophore

A new design for optochemical sensors has been applied to the development of a nitric oxide selective fiber optic sensor. This sensor is composed of a fluorescein derivative dye attached to colloidal gold. The fluorescein dye rearranges as nitric oxide adsorbs onto the gold, inducing a decrease in the fluorescence intensity of the dye. This mechanism has allowed preparation of fiber optic dye-based nitric oxide sensors, which have been made ratio-metric by addition of reference dye microspheres. Previously developed fast, selective optical sensors for detection of aqueous nitric oxide involved a protein, such as cytochrome c'. The new fluorescein derivative chemical sensors have characteristics similar to those of the protein-based biosensors, including fast response times, excellent selectivity, and complete reversibility. In addition, the chemical sensors have advantages such as greater stability and commercially available components. These sensors were utilized to measure nitric oxide production by BALB/c mouse macrophages.     

Study of Plastic Shear Localization via the Flow Theory of Mechanism-Based Strain Gradient Plasticity

The plastic shear localization is studied via the flow theory of mechanism-based strain gradient plasticity. The shear strain rate displays a rather sharp decrease from the center to the boundary of shear band, and gradually approaches the uniform remote shear strain rate outside the shear band. The shear band thickness obtained analytically is linearly proportional to the (length of) Burgers vector, and also depends on the softening modulus and mesoscale cell size. The maximum shear strain rate in the shear band, however, is sensitive to other material properties such as the ultimate tensile strength and intrinsic material length in strain gradient plasticity.     

Analysis of the breakage process during the measurement of the mechanical strength of guinea pig tympanic membranes

We analyzed the breakage process of the tympanic membrane in relation to the material mechanics, using the microtesting system which we have reported previously. 111 fresh tympanic membranes from 56 guinea pigs were used. We calculated the stress-strain curve from the load-displacement curve which was obtained in this experiment. In addition, we observed the morphological changes during the breakage process of the tympanic membrane using optical microscope and scanning electron microscope (SEM). We observed multiple notches (small peaks) on the stress-strain curve. The first notch was considered to correspond to the elastic limit of the tympanic membrane. This first notch appeared at the point when the stress was 4.86 MPa and the strain was 0.10. Comparing the stress-strain curve to morphological changes, this notch in the stress-strain curve appeared when the stress suddenly decreased due to a slit on the tympanic membrane. The elastic modulus of the fresh tympanic membrane of guinea pigs was calculated as 5.71 x 10(-2)mN/microns 2. The maximal stress was 18.5 MPa and this value represented the maximal strength of the radial fiber bundles. From these results, we can speculate that from a mechanical point of view, the tympanic membrane of the guinea pig is a combined material consisting of the radial fiber bundles and circular fibers and that at the site of mechanical injury both fibers strengthened their interactions with each other.     

Design and Evaluation of Fiber-Reinforced Polymer Bond-Type Anchorages and Ground Anchors

This paper presents laboratory tests on a bond-type anchorage system and a full-scale ground anchor with fiber-reinforced polymer 9-bar tendons. A cement-based mortar filled steel tube was used to simulate a rock mass environment. Resistance strain gauges, fiber optic sensors, embedded strain gauges, and linear variable displacement transducers were used to monitor the behavior of the anchor and to study load transfer mechanism of the tendon to surrounding rock mass. The test results show that the developed bond-type anchorage performs well for post-tensing applications with the tendons. The tested anchor presents an acceptable tensile behavior for each loading stage investigated (up to a load level of 0.6ffu, where ffu=guaranteed capacity of the tendon) in accordance with existing codes. The anchor is similar in strain distribution profile to conventional steel anchors, except giving a shorter load transfer length. It is expected that the anchor require a minimum anchor bonded length of 2,000?mm with plain cement grouts.