Linear Variable Displacement Transformer Research Articles

Influence of salt (NaCl) on hydric and hygric dilatation of clay-rich rocks

• Hydric (liquid water) dilatation decreases with increasing salinity of pore water. • Salt contamination reverses hygric (water vapour) dilatation phases of clay-rich rocks. • Crystallisation of NaCl develops higher strain than hydration of clay interlayers. • Changes in volume occur more frequently once clay-rich rocks are exposed to salts. • Salt weathering of clay-rich rocks produces granular disintegration and scaling. The mechanisms through which crystallisation of salts and hydration of swelling clays cause deterioration of rocks are generally understood. However, how these two decay mechanisms operate synergistically is less clear. In this study, we investigated the impact of a salt (NaCl) on the deterioration of clay-rich basaltic scoria from which the churches forming the UNESCO World Heritage Site of Lalibela in Ethiopia are hewn. The basaltic scoria is rich in smectite (swelling clay) and zeolitic minerals which have micropores and an affinity for efficient water absorption. Hydric and hygric swelling experiments were carried out on 50 mm cube quarry samples and continuous measurements were taken with Linear Variable Displacement Transformers (LVDT). Hydric (liquid water) tests involved partially submerging the samples in deionised and saline solutions. Hygric (water vapour) tests were done in an environmental cabinet where the RH fluctuated between 60% to 90% RH with salt-free and salt-contaminated samples. The results showed that contamination with salts alters both hydric and hygric behaviour. Hydric swelling was reduced in most samples when contaminated with salt, while the expansion and contraction phases in hygric tests were reversed after contamination with NaCl. Continuous measurement of dilatation allowed us to quantify the strain developed during the phase transition of NaCl. Due to the crystallisation pressure, hygric dilatation increased significantly in salt-contaminated samples. Salt-contaminated samples also showed frequent episodic changes in volume that corresponded both with the hydration/dehydration of clay interlayers and crystallisation/dissolution of NaCl. These volume changes correspond with the damage profile on the surface of the samples, i.e., contour scaling and granular disintegration. The results indicate that the mechanism through which salt contamination accelerates the decay of clay-rich rocks is via an increase in the frequency and degree of hygric dilatation.

Effect of bonded length on the load response and failure mode of pull-out tests of GFRP bars embedded in concrete

• The stress transfer mechanism occurs within the superficial layer of resin. • The free end of the bar slips prior to attaining the peak load. • Load when the free end starts to slip and bond strength depend on bonded length. • Oscillations in the load are associated with the presence of a carbon fiber yarn. • Peak load versus bonded length is compared with the ACI 440.1R-15 formula. In this paper, the experimental results of pull-out tests of glass fiber-reinforced polymer (GFRP) bars embedded in concrete are presented and discussed. The experimental work is designed to investigate the effect of the bonded length on the bond behavior of the bars. Five bonded lengths are considered. The first bonded length is equal to 5 times the bar diameter ( d b ), which is the bonded length recommended in ASTM D7913. The other four bonded lengths are equal to 10 d b , 20 d b , 30 d b , and 40 d b . Loaded-end displacement is obtained from the measurements of three linear variable displacement transformers (LVDTs). Pull-out tests are performed in displacement control. For some specimens, the free-end displacement is measured by two additional LVDTs. The load responses in terms of applied load versus machine stroke, loaded-end slip, and free-end slip are plotted and compared for the different bonded lengths. The average shear stress is plotted versus the bonded length. Finally, the maximum axial stress in pull-out tests is plotted versus the bonded length and compared with the prediction of ACI 440.1R-15.

Mechanical response of full-scale geosynthetic-reinforced asphalt overlays subjected to repeated loads

This study aims at evaluating the influence of geosynthetic reinforcements on the structural improvement of asphalt overlays placed on distressed pavement layers using repeated load tests. Full-scale instrumented pavement models were constructed in an indoor steel tank measuring 1000 mm in length, 1000 mm in width and 1000 mm in depth. Full-scale instrumented pavement models consisted of a 650-mm-thick weak subgrade, 250-mm-thick base, 90-mm-thick distressed asphalt layer, binder tack coat, geosynthetic reinforcement (except in control sections), and 50-mm-thick hot mix asphalt overlay. Sensors used in the instrumentation program included earth pressure cells and linear variable displacement transformers installed on the subgrade and surface layers, respectively. Four different geosynthetic types, including woven geo-jute mat (GJ), polypropylene geogrid (PP), polyester geogrid (PET), and fiberglass geogrid composite (FGC) were adopted as asphalt reinforcements. A servo-hydraulic actuator was used to replicate a live traffic wheel load by applying an equivalent single axle contact pressure of 550 kPa at a frequency of 1 Hz. Repeated load tests were terminated after 100,000 load cycles and the behaviour of geosynthetic-reinforced full-scale models was compared with that of unreinforced model. Performance indicators, including Traffic Benefit Ratio (TBR) and Rut Depth Reductions (RDR), were estimated and repeated load test results indicated an increase in the structural performance of geosynthetic-reinforced full-scale models in relation to that of unreinforced model. Among the geosynthetic-reinforced models considered in this study, the FGC-reinforced model showed a comparatively better performance with a maximum TBR of 20 at a permanent deflection of 5 mm and the highest RDR of 56% after 100,000 load cycles, respectively. Maximum reductions of 56% in surface deflection and of 30% in vertical pressure on the subgrade were also observed after 100,000 load cycles in the FGC-reinforced model.

Use of Image Correlation to Measure Macroscopic Strains by Hygric Swelling in Sandstone Rocks

Some materials undergo hygric expansion when soaked. In porous rocks, this effect is enhanced by the pore space, because it allows water to reach every part of its volume and to hydrate most swelling parts. In the vicinity, this enlargement has negative structural consequences as adjacent elements support some compressions or displacements. In this work, we propose a normalized cross-correlation between rock surface texture images to determine the hygric expansion of such materials. We used small porous sandstone samples (11 × 11 × 30 mm3) to measure hygric swelling. The experimental setup comprised an industrial digital camera and a telecentric objective. We took one image every 5 min for 3 h to characterize the whole swelling process. An error analysis of both the mathematical and experimental methods was performed. The results showed that the proposed methodology provided, despite some limitations, reliable hygric swelling information by a non-contact methodology with an accuracy of 1 micron and permitted the deformation in both the vertical and horizontal directions to be explored, which is an advantage over traditional linear variable displacement transformers.

Investigation of the compressive behavior and failure modes of unconfined and FRP-confined concrete using digital image correlation

This paper presents a detailed study on the development of axial, lateral and Von Mises strains measured by contact and non-contact methods under axial compression fiber reinforced polymer (FRP) confined and unconfined concrete specimens. The focus of the study was on the correlation between macro responses and localized deformations and the effects of FRP confinement on the localization of deformations. A total of 10 specimens were experimentally tested with a range of instrumentation to measure different types of strain. Digital image correlation (DIC) was used to obtain full-field strain evolution during compressive loading of concrete specimens confined with various fiber reinforced polymers. The effect of confinement material on the compressive behavior and strain distribution of FRP-confined concrete was investigated. The evolution of axial, lateral and Von Mises strains were recorded and examined during axial loading, including post-peak strain softening behavior. Data recorded by the typical contact method of linear variable displacement transformers (LVDTs) and strain gauges were used to validate the non-contact DIC method. The results obtained from DIC data in this study showed that expansion of shear zone for unconfined concrete is more localized than FRP-confined specimens. The results also show that specimens confined with CFRP induced homogenous compressive behavior, whereas GFRP and BFRP specimens with lower lateral stiffness displayed local strain concentration and less homogenous response. It was also observed that DIC provides a more accurate estimation of the ultimate condition compared to the contact methods due to the ability of DIC to capture the evolution of full-field strains.

Biaxial Creep Behavior of Nb-Modified Zircaloys

Zirconium-niobium (Zr-Nb) alloys are used as cladding materials to encapsulate radioactive fuel in nuclear reactors. They possess excellent corrosion resistance at high temperatures making it possible to achieve high fuel burnup, directly increasing the thermal efficiency of the reactor. While they are commonly used in recrystallized (Rx) form in boiling water reactors, there is a need to understand the effect of cold work and stress relief (CWSR) on the biaxial creep characteristics of these materials due to their use in pressurized water reactors. In this study, the biaxial creep behaviors of as-received Zr-Nb alloys, HANA and Zirlo®, have been investigated at 500°C and 400°C, respectively, using internally pressurized tubing superimposed with axial load under varied hoop σθ to axial σz stress ratios of 0 to 2 while monitoring both the axial and hoop strains using a linear variable displacement transformer and a laser telemetric extensometer, respectively. The crystallographic textures and creep loci of these as-received Zr-Nb alloys have been evaluated to correlate with the previous studies on recrystallized HANA4 and CWSR Zircaloy-4. The creep locus of HANA4 was found to be unaffected by initial state (CWSR or Rx) and showed close correspondence to planar isotropy while the creep locus of CWSR Zirlo exhibited more resistance to axial deformation than diametrical as per CWSR Zircaloy-4 reported earlier. These differences are shown to arise from grain-shape anisotropy of the CWSR Zirlo and Zircaloy-4. The simulated creep loci using crystallite-orientation distribution functions in conjunction with prism slip models showed excellent agreement with experimental results.

Influence of the width of the specimen on the fracture response of concrete notched beams

This work presents an experimental study performed to investigate the influence of the width and size of the specimen on the fracture response of concrete notched beams. Thirty-eight beams with six cross-sections has been tested using a three-point bending (TPB) test setup, which is designed according to the draft of a report developed by Joint ACI/ASCE 446 Technical Committee. Two depths (70 mm and 150 mm) and three widths (35 mm, 70 mm and 150 mm) of the beam are considered. 3D digital image correlation (DIC) is employed for 18 specimens. The load-deflection response obtained from DIC is compared with the load-deflection response obtained from the readings of two linear variable displacement transformers (LVDT). Load responses, peak loads, strain profiles, and failure modes of TPB tests are presented and discussed. The authors observe that a potential width effect is combined with the well-known size effect. In addition, the evaluation of the fracture energy from the load response might be influenced by the width of the beam. Finally, the strain profiles along the notched cross-section obtained from DIC suggest that the evaluation of the fracture energy from the load response might be misleading.

Assessment of digital image correlation method in determining large scale cemented rockfill strains

A conventional contact method (using linear transducers) and a non-contact method are deployed to measure the axial and lateral deformations of large scale cylindrical cemented rockfill specimens. Experimental works incorporating two pinhole cameras to create one stereovision by digital image correlation shows that the non-contact method is as reliable for testing large cylindrical specimens as measurements done by using linear variable displacement transformer and string potentiometer. Considering this particular large specimen, the experiment resulted in the acceptable mean difference between lateral strain using both methods is 5.1 percent, and 14.5 percent for the axial strain. This occurrence is inevitable due to the heterogeneity of the concrete system and the placement of the monitoring point in digital image correlation method, although the comparison of stress-strain relationship in both methods still indicates a conformity. Based on the results of the present experiments, the authors recommend the non-contact method for a detailed investigation of the material behavior during the uniaxial compressive strength tests. Full field strain measurement enables this digital method to examine local strains near cracks at any point, a very useful tool for studying material deformation behavior.

WITHDRAWN: Influence of the width of the specimen on the fracture response of concrete notched beams

Abstract This work presents an experimental study performed to investigate the influence of the width and size of the specimen on the fracture response of concrete notched beams. Thirty-eight beams with six cross-sections has been tested using a three-point bending (TPB) test setup, which is designed according to the draft of a report developed by Joint ACI/ASCE 446 Technical Committee. Two depths (70 mm and 150 mm) and three widths (35 mm, 70 mm and 150 mm) of the beam are considered. 3D digital image correlation (DIC) is employed for 18 specimens. The load-deflection response obtained from DIC is compared with the load-deflection response obtained from the readings of two linear variable displacement transformers (LVDT). Load responses, peak loads, strain profiles, and failure modes of TPB tests are presented and discussed. The authors observe that a potential width effect is combined with the well-known size effect. In addition, the evaluation of the fracture energy from the load response might be influenced by the width of the beam. Finally, the strain profiles along the notched cross-section obtained from DIC suggest that the evaluation of the fracture energy from the load response might be misleading.

High-cycle fatigue life prediction of reinforced concrete deep beams

Concrete elements deteriorate as a result of continuous application of compressive fatigue loads. Irreversible deformation accumulates; hence, the effect on embedded steel reinforcing bars capacity and concrete resistance should be accounted for in the fatigue analysis of concrete structures. Experimental investigations were conducted to study the fatigue behaviour of eight small-scale reinforced concrete deep beams with a shear span to effective depth ratio of 1.25. Percentages of the diagonal cracking load from monotonic tests were used as fatigue loads. The deformation evolution within the shear spans of the deep beams were obtained by estimating the average principal and shear strain evolutions from the strain transformation analysis of LVDT (Linear Variable Displacement Transformer) data. Mid-span deflections and reinforcement strain evolutions with proximity to a major concrete crack location were obtained. In all beams, failure occurred with fracture of the longitudinal reinforcement at the intersection with the major concrete crack. Maximum strain evolutions for shear reinforcement measured at regions around the bends were observed to be lower than the strain evolutions observed in the longitudinal reinforcement. This was attributed to the governing arch mechanism common with deep beams.The strut and tie method was modified to predict the fatigue life of the deep beams tested by modifying the constitutive models and effectiveness factor of concrete with fatigue damage models. To achieve this, the irreversible compressive fatigue strain in concrete is considered as a pseudo-load. The crack initiation life and the progressive crack growth of steel reinforcement are accounted for using strain-life models and linear elastic fracture mechanics, respectively. Within the developed algorithm, failure will occur when one of the evolving forces in either the concrete strut or steel reinforcement approaches the corresponding residual resistance capacity.

Influence of FRP-to-Concrete Gap Effect on Axial Strains of FRP-Confined Concrete Columns

This paper reports on an experimental investigation on the influence of FRP-to-concrete interface gap, caused by concrete shrinkage, on axial compressive behavior of concrete-filled FRP tube (CFFT) columns. A total of 12 aramid FRP (AFRP)-confined concrete specimens with circular cross-sections were manufactured. 3 of these specimens were instrumented to monitor long term shrinkage strain development and the remaining 9 were tested under monotonic axial compression. The influence of concrete shrinkage was examined by applying a gap of up to 0.06 mm thickness at the FRP-to-concrete interface, simulating 800 microstrain of shrinkage in the radial direction. Axial strain recordings were compared on specimens instrumented with two different measurement methods: full-and mid-height linear variable displacement transformers (LVDTs). Results of the experimental study indicate that the influence of interface gap on stress-strain behavior is significant, with an increase in interface gap resulting in a decrease and increase in the compressive strength and ultimate axial strain, respectively. It was also observed that an increase in interface gap leads to a slight loss in axial stress at the transition region of the stress-strain curve. Finally, it is found that an increase in the interface gap results in a significant decrease in the ratio of the ultimate axial strains obtained from mid-section and full-height LVDTs.

Axial Strains in FRP-Confined Normal- and High-Strength Concrete: An Examination of Strain Measurement Methods

This paper presents an experimental investigation into the axial strain measurement method of fiber reinforced polymer (FRP)-confined normal-and high-strength concrete (NSC and HSC). A total of 15 FRP-confined concrete specimens with circular cross-sections were tested under monotonic axial compression. Axial strain recordings were compared on specimens instrumented with three different measurement methods: axial strain gauges and full-and mid-height linear variable displacement transformers (LVDTs). The ultimate conditions are tabulated for each measurement method and key experimental outcomes discussed. The results indicate that axial strains of FRP-confined HSC are highly sensitive to the instrumentation arrangement and significant differences occur in the results obtained from different measurement methods.

Response of pear-jujube to different soil water potentials during budding and flowering stages

本试验选取4年生梨枣(<i>Ziziphus jujuba</i> Mill.)为试验材料, 设置-31~-51 kPa、-41~-84 kPa、-59~-132 kPa和-161 kPa(不灌溉)4个土壤水势水平, 结合茎直径微变化、营养生长与生殖生长量, 分析了梨枣在萌芽开花期对土壤水势的响应。结果表明: 1)在梨枣萌芽期, 土壤水势在-41~-84 kPa范围波动时, 茎直径日最大收缩量最小, 营养生长与生殖生长最旺盛, 是该时期适宜的土壤水势范围; 花芽分化适宜的土壤水势范围为-41~-132 kPa。梨枣萌芽期适当的水分亏缺不影响花芽分化, 但减弱了营养生长量, 优化了营养生长与生殖生长的比例。2)在梨枣开花期, 土壤水势范围为-54~-78 kPa时, 茎直径日最大收缩量最小, 坐果率较高, 为开花期适宜的土壤水势范围; 土壤水势为-79~-114 kPa时坐果率最高。开花期轻度的水分亏缺可显著提高坐果率。3)土壤水势过高或过低都抑制梨枣的营养生长、生殖生长和坐果, 但水涝能促进茎直径生长, 水分缺亏则抑制茎直径生长。4)在平水年, 梨枣的萌芽期无需灌水, 自然降雨即能满足其萌芽展叶、花芽分化的需要, 但开花期, 降雨不能满足梨枣高坐果率的需要。

A review of nanometer resolution position sensors: Operation and performance

Position sensors with nanometer resolution are a key component of many precision imaging and fabrication machines. Since the sensor characteristics can define the linearity, resolution and speed of the machine, the sensor performance is a foremost consideration. The first goal of this article is to define concise performance metrics and to provide exact and approximate expressions for error sources including non-linearity, drift and noise. The second goal is to review current position sensor technologies and to compare their performance. The sensors considered include: resistive, piezoelectric and piezoresistive strain sensors; capacitive sensors; electrothermal sensors; eddy current sensors; linear variable displacement transformers; interferometers; and linear encoders.

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.

Physical modelling of sliding failure of concrete gravity dam under overloading condition

Sliding within the dam foundation is one of the key failure modes of a gravity dam. A two-dimensional (2-D) physical model test has been conducted to study the sliding failure of a concrete gravity dam under overloading conditions. This model dam was instrumented with strain rosettes, linear variable displacement transformers (LVDTs), and embedded fiber Bragg grating (FBG) sensing bars. The surface and internal displacements of the dam structure and the strain distributions on the dam body were measured with high accuracy. The setup of the model with instrumentation is described and the monitoring data are presented and analyzed in this paper. The deformation process and failure mechanism of dam sliding within the rock foundation are investigated based on the test results. It is found that the horizontal displacements at the toe and heel indicate the dam stability condition. During overloading, the cracking zone in the foundation can be simplified as a triangle with gradually increased height and vertex angle.

Effect of Varying Water-to-Powder Ratios on the Setting Expansion of White and Gray Mineral Trioxide Aggregate

Introduction Clinicians commonly mix mineral trioxide aggregate (MTA) to a desired consistency rather than use the recommended amounts of powder and water. The purpose of this in vitro study was to evaluate how varying the water-to-powder (WP) ratio affects the setting expansion of white MTA (WMTA) and gray MTA (GMTA). Methods Eight combinations (n = 5) of WMTA and GMTA were mixed using varying WP ratios. Randomized samples were placed in a linear variable displacement transformer and submerged under Hank's balanced salt solution for 25 hours. Expansion was compared using a 2-way analysis of variance (α = 0.05). Results Mean percent expansions ranged from 0.058%–0.093% for WMTA and 2.15%–2.56% for GMTA. GMTA expanded significantly more than WMTA at p = .001. No differences in expansion were found between WP ratios ( p = .218). No significant interaction was found between the WP ratio and material type ( p = .228). Conclusions GMTA expanded significantly more than WMTA; however, varying the WP ratio did not affect the setting expansion.

Monitoring Internal Displacements of a Model Dam Using FBG Sensing Bars

Conventional instrumentation has difficulty to measure displacements inside laboratory-scale physical models. In this paper, an innovative fiber Bragg grating (FBG) sensing bar has been developed to perform high-accuracy and real-time monitoring of internal displacements. According to Euler-Bernoulli beam theory, the strain distribution measured by the quasi-distributed FBG sensors adhered on the bar surface can be used to compute the displacement profile along the sensing bar. The effectiveness of the FBG sensing bar has been verified by calibration tests. In the physical model of the Wudu gravity dam, two FBG sensing bars developed by the authors were installed, together with linear variable displacement transformers (LVDTs) and strain rosettes. The FBG sensing bars successfully captured the variation of internal displacement profiles in the model dam. The monitoring results during the overloading test were presented, based on which the deformation mechanism of the dam-foundation system is explained in details. The displacements measured by the FBG sensing bars are validated by their good agreements with those from LVDTs.

Setting Expansion of Gray and White Mineral Trioxide Aggregate and Portland Cement

One possible reason for the sealing ability of mineral trioxide aggregate (MTA) is its slight expansion upon setting. Both gray mineral trioxide aggregate (GMTA) and white mineral trioxide aggregate (WMTA) are composed of approximately 75% Portland cement (PC). WMTA differs from GMTA in its lower content of tetracalcium aluminoferrite. This difference in composition may affect setting expansion. The purpose of this study was to compare the hydroscopic linear setting expansions of GMTA, WMTA, and PC with a new device. Materials were mixed with water, placed into a cylindrical mold, and covered with sterile water or Hank’s balanced salt solution (HBSS). Expansion changes were measured by using a linear variable displacement transformer dilatometer. One-way analysis of variance and post hoc tests (α = 0.05) showed the mean expansion at 24 hours was 1.02% for GMTA, 0.29% for PC, and 0.08% for WMTA in water immersion and 0.68% for GMTA and 0.11% for WMTA in HBSS immersion. GMTA expanded significantly more than WMTA in either water or HBSS immersion.

Irradiation creep and precipitation in a ferritic ODS steel under helium implantation

Ferritic oxide dispersion strengthened (ODS) steel, PM2000, has been homogeneously implanted with helium under uniaxial tensile stresses from 20 to 250 MPa to maximum doses of about 0.75 dpa (3000 ppm He) with displacement damage rates of 5.5 × 10 −6 dpa/s at temperatures of 573, 673 and 773 K. Straining of a miniaturized dog-bone specimen under helium implantation was monitored by linear variable displacement transformer (LVDT) and meanwhile by their resistance also measured by four-pole technique. Creep compliance was almost constant at 5.7 × 10 −6 dpa −1 MPa −1 for temperatures below 673 K and increased to 18 × 10 −6 dpa −1 MPa −1 at 773 K. The resistivity of PM2000 samples decreased with dose and showed a tendency to saturation. Subsequent transmission electron microscopy observations indicated the formation of ordered Fe 3− x Cr x Al precipitates during implantation. Correlations between the microstructure and resistivity are discussed.