Electrical Resistance Strain Gauges Research Articles

Contact acoustic nonlinearity and damping properties of completely free vibrating curved and delaminated CFRP plates

This study reports an experimental investigation on the effect of plate curvature and circular inter-ply delamination on the completely free vibration properties of carbon fiber reinforced polymer (CFRP) plates. Impact tests on wire suspended plates were performed and variations in measured natural frequencies and modal damping are reported. Delamination clapping type contact acoustic nonlinearity (CAN) was observed in the presence of a series of higher harmonics in the vibration spectrum. These higher harmonics consisted of five to seven equally distanced peaks grouped in a frequency range of around the double of odd mode M (5,0). Modal damping was highly sensitive to delamination which caused a 33% average increase in damping loss factors. Odd mode M (3,0) was the most affected with an average increase of 51% in damping loss factor, explained by the delamination location relative to the wavelength of that mode shape. Natural frequencies were less sensitive to delamination, on average decreasing 1.0% in flat plates and increasing 3.1% in curved plates. The modal frequencies calculated from embedded fiber optic sensor (FOS) or electrical resistance strain gauge signals agreed with finite element modeling (FEM) to within a 3.1% difference for all modes considered. The damping loss factors deducted from suspended plate tests were consistent with complementary results from dynamic mechanical analysis (DMA). The measured delamination clapping CAN and its related influence on damping can serve to enhance structural health monitoring (SHM) methods.

Accurate measurement of the bond stress between rebar and concrete in reinforced concrete using FBG sensing technology

Recent earthquakes in several developing countries have shown that reinforced concrete (RC) buildings with improper structural detailing experience severe damage under seismic motions. Using low-quality construction materials such as brick aggregates, resulting in low-strength concrete, significantly impacts the bond between rebar and concrete. Accurate evaluation of the bond performance of such low-strength concrete is one of the key issues for seismic safety assessment of RC buildings, especially in Bangladesh; thus, the bond performance is usually evaluated through laboratory tests. However, conventional measurements of bond stress based on rebar strains measured by electrical resistance strain gauges are likely to negatively impact the bond behavior/performance because of the reduced total contact area between rebar and concrete as well as the changing rebar surface boundary conditions. Under the above social and academic backgrounds, in this study, a new measurement technique that applies fiber Bragg grating (FBG) sensors embedded in optical fiber to rebar strain measurements is developed, and its effectiveness is investigated to realize more accurate measurements of the bond stress between rebar and concrete. Two 70% scaled RC beam-column joint specimens in which the beam rebar was anchored in a straight manner were constructed with identical detailing, except for the beam rebar strain measuring methods. The specimens were then subjected to cyclic lateral loading until failure. By comparing the experimental data acquired by the above two different devices (the FBG sensors and conventional strain gauges), it was found that the experimental bond strength on the beam rebar based on the strain data measured by the FBG sensors was much higher than that from the data measured using conventional strain gauges. Which negatively impacted the test data on the beam-column joint’s capacity in the specimen applied the conventional measuring method, indicating the necessity of the presented method not only for accurate evaluation of the bond stress between rebar and concrete but also for seismic safety assessments of RC buildings.

Calibration Experiment and Temperature Compensation Method for the Thermal Output of Electrical Resistance Strain Gauges in Health Monitoring of Structures

Electrical resistance strain gauges are widely used in asymmetric structures for measurement and monitoring, but their thermal output in changing temperature environments has a significant impact on the measurement results. Since thermal output is related to the coefficient of thermal expansion of the strain gauge’s sensitive grating material and the measured object, the temperature self-compensation technique of strain gauges fails to eliminate the additional strain caused by temperature because it cannot match the coefficient of thermal expansion of various measured objects. To address this problem, in this study, the principle of the thermal output of electrical resistance strain gauges was analyzed, a calibration experiment for thermal output in the case of a mismatch between the coefficient of linear expansion of the measured object and the strain gauge grating material was conducted, and the mechanism for temperature influence on thermal output was revealed. A method was proposed to obtain the thermal output curves for different materials by using thermostats with dual temperatures to conduct temperature calibration experiments. A linear regression method was used to obtain a linear formula for the thermal output corresponding to each temperature. The thermal output conversion relationship was derived for materials with different coefficients of linear expansion. An in situ temperature compensation technique for electrical resistance strain gauges that separates the measured strain into thermal and mechanical strains was proposed. The results showed that the thermal output curve for the measured object can be calibrated in advance and then deducted from the measured strain, thus reducing the influence of temperature-induced additional strain on the mechanical strain. In addition, a new method was provided for the calculation of the thermal output among materials with similar coefficients of linear expansion, providing a reference for the health monitoring of asymmetric structures.

Evaluation of thermal-drift effect on strain measurement of energy pile

Fiber Bragg grating (FBG) strain sensors and electrical resistance strain (ERS) gauges are typically utilized to monitor the internal deformation of structures. However, a common problem inherent in these sensors is the unwanted thermal drift in the measurement results. This study first investigated factors influencing the thermal strain measurements of FBG strain sensors and ERS gauges through a series of calibration experiments. The results demonstrate that the relative error caused by the thermal output of the heated connection cables was as high as 90 %. Then a correction method for the thermal drift of the strain measurement was proposed. This correction method reduced the relative difference between the strain readings of bonded ERS gauges and FBG sensors from 82.6 % to around 10–20 %. Finally, FBG strain sensors and ERS gauges were employed to monitor the strain of the energy piles under cooling and heating cycles. The difference between the corrected readings from FBG strain sensors and ERS gauges was less than ± 10 %. The experimental results also reveal that the monitoring reliability of the FBG strain sensors was higher than the ERS gauges.

Interfacial behavior of externally bonded BFRP-to-concrete joints using different epoxy adhesives

The purpose of this study is to investigate the influence of adhesive types on the interfacial behavior of externally bonded basalt fiber reinforced polymer (BFRP) sheet-concrete substrate. Twelve manufactured double-lap shear samples were tested using four bonding adhesives. The strains on the surface of BFRP‐concrete joint were determined using electrical resistance strain gauge together with two‐dimensional digital image correlation technique (2D‐DIC). Experimental results show that properties of different bonding adhesives i.e., stiff (linear elastic, higher strength) and soft (nonlinear elastic, lower modulus, higher ultimate strain) has a significant impact on the failure mode, stiffness, ultimate load, strain and shear stress distribution nonetheless not so much on bond-slip relationship. The failure modes of specimens changed from interfacial debonding or cohesive failure in concrete substrate for stiff adhesive specimens to debonding in adhesive layer when soft adhesives are used. Stiff adhesive interfaces experienced higher ultimate loads from 1.1 to 1.3 times higher than soft adhesive interface. Meanwhile soft adhesives are shown to possess a lower maximum shear stress, higher interfacial fracture energy and longer stress transfer length.

Effectiveness of Selected Strain and Displacement Measurement Techniques in Civil Engineering

The aim of this study was to assess how useful certain selected measurement techniques are in civil engineering. In this work, the focus was placed on the measurement of displacement and strain. Classical methods with an established position in the industry, such as electrical resistance strain gauge measurements and linear variable differential transducers (LVDT), were compared with modern techniques that do not require direct contact with the measured object, such as laser scanning and digital image correlation. A simply supported beam was bent in two types of tests. In the first test, a small load was applied on the beam, causing a slight deflection of the structure of approximately 0.5 mm. This enabled us to assess how effective the tested methods were, given the very precise measurement of the structure. In the second test, a much higher load was introduced, which caused displacement that can realistically be found in actual civil engineering structures. Ultimately, the model went through the plastic phase and was damaged. This enabled the measurement of displacement and strain that were much higher than those of the safe operating range of the structure. Based on conducted examinations, practical conclusions were drawn relative to the analyzed measurement methods.

A computational and experimental assessment of the pipeline stress state under bending load and internal pressure

Main pipelines are subjected to a complex of loads during operation. Monitoring of the stress state of the pipeline wall is necessary for performing strength calculations and evaluating the pipeline reliability.The article is devoted to the method of computational and experimental study of the stress state of a pipe under a bending load and combined action of a bending load and internal pressure.The experiments have been carried out on a laboratory bench. The object of the study is a pipe that has the following characteristics: an outer diameter of 325 mm, a wall thickness of 8.5 mm and steel grade of "14XGS". Electrical resistance strain gages were used to measure the strain of the pipe wall. Formulas for calculating the stress state components of the pipe wall in the elastic-plastic deformation stage are proposed. It is given formulas for calculating the stress state components of the pipe wall in the elastic-plastic deformation stage. Plots of hoop and longitudinal stresses as well as von Mises stress are obtained for the case of bending load on the pipe and the case of combined loading under bending and internal pressure. The areas of maximum values of von Mises stress where the transition to the limiting state is most likely have been determined.When only the bending load is applied, the maximum von Mises stress zone is observed on the lower area of the pipe in its central region. When combined loading under bending and internal pressure, the maximum von Mises stress zone is observed on the lateral area of the pipe in its central region.

Strain analysis of Moso bamboo (Phyllostachys pubescens) subjected to longitudinal tensile force

This study used digital image correlation (DIC) to perform a strain analysis of Moso bamboo (Phyllostachys pubescens) in the tangential section subjected to longitudinal tensile force to compare samples from outer/inner position of the culm wall. Simultaneously, an electrical resistance strain gauge (SG) was used to measure the strain profile for comparison with the strain analysis using DIC. During loading, the rate of the longitudinal strain between different positions was significantly different. As the load time increased, there was no difference in the rate of the transverse strain between the outer and inner samples, while the rate of the longitudinal strain for the inner sample was higher than that for the outer sample. Additionally, DIC showed that the longitudinal direction showed a relatively homogeneous strain contour compared to the transverse direction. Furthermore, the result indicated that the Poisson’s ratio was dependent on the positions of the bamboo in the radial direction of the culm wall. Using SG recording as a reference, the DIC revealed appropriate strain behavior of bamboo in terms of longitudinal and transverse strains within full-field area. Therefore, the DIC recording may help to characterize strain deformation on the bamboo during the application of a tensile force.

Load transfer on instrumented prestressed ground anchors in sandy soil

abstract: This study evaluates load variations in instrumented prestressed ground anchors installed in a bored pile retaining wall system in sandy soil. Data were collected from instrumentation assembled in the bonded length of three anchors, which were monitored during pullout tests and during different construction phases of the retaining wall system. Instrumentation consisted of electrical resistance strain gauges positioned in five different sections along the bonded length. Skin friction distributions were obtained from the field load measurements. Results showed that the skin friction followed a non-uniform distribution along the anchor bonded length. The mobilized skin friction concentrated more intensely on the bonded length half closest to the unbonded length, while the other half of the bonded length developed very small skin friction. The contribution of the unbonded length skin friction to the overall anchor capacity was significant and this should be accounted for in the interpretation of routine anchor testing results. Displacements applied to the anchor head were sufficient to mobilize the ultimate skin friction on the unbonded length, but not on the bonded length. Performance of loading-unloading stages on the ground anchor intensified the transfer of load from the unbonded length to the bonded length. Long-term monitoring of the anchor after lock-off revealed that the load at the anchor bonded length followed a tendency to reduce with time and was not significantly influenced by the retaining wall construction phases.

Determination of deformation characteristics of natural rock using methods of close-range photogrammetry

In the field of civil engineering, traditional deformation sensors are used (measurements of changes of length and widths in the stressed material), such as an electrical resistance strain gauge, deviation meters (mechanical dial gauge, electronic digital indicator, LVDT). These sensors work quite well, but the sample surface preparation and fixing the sensors on the sample surface can be timely and work skill demanding. On the other hand, conventional non-destructive testing methods, such as rebound hammers or ultrasonic pulse propagation have a disadvantage in large data scatterness and low reliability. This paper focuses on a practical, more reliable and cost-effective method for determining deformation characteristics, calculated from digital images using the principle of close-range photogrammetry. To verify the effectiveness of this method, a test was performed on a sample of natural stone from the tunnel Višňové. The data were obtained by these methods and correlated with the used load. After plotting the stress-strain curve, the static modulus of elasticity and the modulus of deformation were determined. These modules were compared with those determined from resistance strain gauge measurements.

High Speed, Localized Multi-Point Strain Measurements on a Containment Vessel at 1.7 MHz Using Swept-Wavelength Laser-Interrogated Fiber Bragg Gratings

Dynamic elastic strain in ~1.8 and 1.0 m diameter containment vessels containing a high explosive detonation was measured using an array of fiber Bragg gratings. The all-optical method, called real-time localized strain measurement, recorded the strain for 10 ms after detonation with additional measurements being sequentially made at a rate of 1.7 MHz. A swept wavelength laser source provided the repetition rate necessary for such high-speed measurements while also providing enough signal strength and bandwidth to simultaneously measure 8 or more unique points on the vessel’s surface. The data presented here arethen compared with additional diagnostics consisting of a fast spectral interferometer and an optical backscatter reflectometer to show a comparison between the local and global changes in the vessel strain, both dynamically and statically to further characterize the performance of the localized strain measurement. The results are also compared with electrical resistive strain gauges and finite element analysis simulations.

Effort and bearing capacity testing of laterally restrained asymmetrical stepped steel I-columns

This paper presents extensive experimental investigations of models of I-shaped steel columns with segmentally prismatically variable stiffness. The models are discretely braced with lateral restraints along their length. With regard to the geometry of the actual columns, the models were made on the semi-industrial scale of 1:2. An external load, in the form of a vertical concentrated force, was applied to the header (in the Ch test series) or to the bracket (in the Cb and Chb test series). Selected static equilibrium paths of the linear displacements and the angles of rotation of column cross sections, determined in each of the test series, are presented and their behaviour is described in detail. Moreover, in the case of the Ch3 and Cb3 models, stress intensity in selected cross sections is examined by means of electrical resistance strain gauges, and the increase in normal stress with regards to the external load is shown. In addition, the experimental investigations included relevant material studies to determine the strength and chemical composition of the steel of which the columns were made.

A novel test method for tendon re-anchorage in bonded post tensioned concrete using ESPI full field measurement

In this paper, a novel test method is proposed to measure the re-anchorage of a ruptured tendon in bonded post-tensioned concrete. Full-field measurements on post-tensioned concrete prisms (500 × 100 × 100 mm) allow a large number of parameters to be considered. The full-field displacement on the concrete surface after the rupture was recorded using 3D Electronic Speckle Pattern Interferometry (ESPI), which is an optical contactless technique. The results in the paper also demonstrate the power of the ESPI measurements, which allow many more measurements to be made (in this study 256 × 51 point measurements of displacement in concrete prisms) in comparison to conventional techniques such as DEMEC and electrical resistance strain gauges. The comparison of the proposed test method results to the 3D non-linear finite element model predictions shows the strength of the proposed test method for studying the re-anchorage phenomenon of ruptured tendons.

Calibration of a Load Measurement System for an Unmanned Aircraft Composite Wing Based on Fibre Bragg Gratings and Electrical Strain Gauges

This paper presents the issues concerning calibration of a measurement system for monitoring the cross-sectional forces and moments of an unmanned aircraft’s wing. A composite cantilever wing with built-in measurement systems based on electrical resistance and Fibre Bragg Grating strain gauges has been made for the purpose of the study. Measurement systems placed along the span of the wing consist of strain gauges arranged in a manner that allows the monitoring of shear force, bending and twisting moments. The calibration process was described in terms of both experimental tests and mathematical formalism. The calibration results were compared for the complete system, consisting of three sensor units, and for various combinations of separated measuring points. For each case, a reading inaccuracy analysis was carried out and conclusions, including recommendations for the design of this kind of measurement system, were formulated.

Bond Stress between Steel‐Reinforced Bars and Fly Ash‐Based Geopolymer Concrete

Geopolymer concrete has been regarded as one of the most important green construction materials, which has been restrained in engineering applications partially due to a lack of bond studies. The structural performance of the reinforced concrete components primarily relies on the sufficient bond between the concrete and the reinforcing bars. Before being utilized in any concrete structure, GPC must demonstrate that it possesses understandable bond behaviour with commercial steel reinforcements. This work presents an experimental investigation on the bond stress of steel bars in reinforced geopolymer concrete (GPC) structures. Standard beam‐end pull‐out tests were conducted on GPC specimens reinforced with 16 mm plain and ribbed bars that were equipped with electrical resistance strain gauges. The longitudinal variation in the bond stress in the GPC beams during the pull‐out tests was calculated and plotted, as well as the stress in steel bars. The cracks on the bond area of the GPC were compared with those of the corresponding ordinary Portland cement concrete (OPC), as well as the steel stress and bond stress. The results showed that the relative slip between plain bar and geopolymer concrete varies from 30–450 microns from the loaded end to the free end when the bond stress decreased by 83%. The relative slip between ribbed bar and geopolymer concrete varies from 280–3,000 microns from the loaded end to the free end when the bond stress decreased by 57%. Generally, GPC is different from OPC in terms of bond stress distribution.

Design and verification of FBG strain gauge

Compared with the traditional electrical resistance strain gauge, the fibre Bragg grating (FBG) is a new material that can be used to produce the strain sensor, which has many advantages such as convenient installation, low temperature drifting, less signal interference and higher transmitted quality. In this study, FBG sensor system configuration, design principle, numerical simulation and loading tests for the FBG strain gauge are expatiated extensively. At the last, one-axial and three-axial strain gauges are produced and some conclusions have been put forward. So the FBG strain gauge can meet the precision of the structure monitoring and replace the traditional sensors.

Yacht performance monitoring in real sailing conditions

In this paper a multi-method approach is used to setup and validate a monitoring system applied to a small sailing boat during real sailing conditions. This monitoring system is able to transform the data coming from some typical devices installed on board into information about the deformed state of the boat. GPS, Wind Data Logger and cameras have been installed on the boat to measure its route and speed, the apparent wind velocity and direction and the positions of the crew members. These data are processed to determine the equilibrium of the boat and estimate the loads applied on it. Then, a CAD/FEM model calculates the effects of these loads on the boat shape. The resulting deformed model is compared with measurements of local strains obtained with Electrical Resistance strain gauges applied on the hull and on reinforcements of the boat. Onboard measuring devices are real-time monitored with a home-made software while the numerical prediction of the global boat deformation is obtained a posteriori once FEM computation is achieved. A test at sea has been performed to check the efficiency of the system: data computed with the proposed procedure have been compared with those coming from the field test.

Strain monitoring of low carbon steel in a corrosive environment using fiber Bragg technology

This paper explores the use of a technique to determine the strain that progressively accumulated in a low carbon steel coupon on exposure to a corrosive medium. The strains developed in the steel coupon were independently measured by an electric resistance strain gauge (ESG) as well as using a fiber Bragg grating (FBG) sensor. The uniaxial strains measured using these two different techniques were compared. The strains accumulated in coupons in both the unrestrained and restrained conditions were measured and the difference in their mechanical response were brought out by the change in volume accompanying the formation of rust, which, in turn, could be sensed by a change in the wavelength by the FBG sensor.

An Investigation of the Temperature-Drift Effect on Strain Measurement of Concrete Beams

This study investigates the temperature-drift effect on strain measurement of concrete beams and proposes a method for determination of the mechanical strain of stressed concrete beams. In the study, wireless electrical resistance strain gauges were used to measure the strain of concrete beams. This study first examined how temperature changes affected the strain gauge attached to concrete beams. Subsequently, a concrete beam experiencing changes in temperature and load was monitored for six consecutive days. The test results showed that the apparent strain response of the concrete beam was significantly affected by temperature changes. After adjusting for the temperature effect, the mechanical strain generated by a load could be obtained. However, temperature-induced drift was still observed in the mechanical strain response. Based on the assumption that temperature changes are slow and gradual, and mechanical strain changes are momentary, an adjacent data subtraction method can be used to eliminate the temperature-induced drift present in the mechanical strain data. The subtraction results show that the mechanical strain generated by a load was accurately obtained. The proposed data-processing method could also be used to find the residual strain of the nonelastic response of a beam subjected to substantial short-term forces.

Spatially continuous strain monitoring using distributed fiber optic sensors embedded in carbon fiber composites

A distributed fiber optic strain sensor based on Rayleigh backscattering, embedded in a fiber-reinforced polymer composite, has been demonstrated. The optical frequency domain reflectometry technique is used to analyze the backscattered signal. The shift in the Rayleigh backscattered spectrum is observed to be linearly related to the change in strain of the composite material. The sensor (standard single-mode fiber) is embedded between the layers of the composite laminate. A series of tensile loads is applied to the laminate using an Instron testing machine, and the corresponding strain distribution of the laminate is measured. The results show a linear response indicating a seamless integration of the optical fiber in the composite material and a good correlation with the electrical-resistance strain gauge results. The sensor is also used to evaluate the strain response of a composite-laminate-based cantilever beam. Distributed strain measurements in a composite laminate are successfully obtained using an embedded fiber optic sensor.