Electrical Strain Gauges Research Articles

Reliability and accuracy of embedded fiber Bragg grating sensors for strain monitoring in advanced composite structures

This work investigated issues for an efficient and reliable embedding and use of Fiber Bragg Grating (FBG) sensors for strain monitoring of composite structures with particular regard to the manufacturing process of components in the nautical field by means of the vacuum bag technique in autoclave. CFRP material laminates with embedded FBGs were produced and the effect of the curing process parameters on the light transmission characteristics of the optical fibers was initially investigated. Two different types of coating, namely polyimide and acrylate, were tested by measuring the light attenuation by an Optical Time Domain Reflectometer. Tensile specimens were subsequently extracted from the laminas and instrumented also with a surface-mounted conventional electrical strain gage (SG). Comparison between the FBG and SG measurements during static tensile tests allowed the evaluation of the strain monitoring capability of the FBGs, in particular of their sensitivity (i.e., gage factor) when embedded.

Evaluation of bending strain measurements in a composite sailboat bowsprit with embedded fibre Bragg gratings

Acrylate-coated Fibre Bragg Gratings (FBGs) were embedded in a sailing boat bowsprit tubular section manufactured using the vacuum bagging process in autoclave with the aim of measuring strains under realistic loading conditions. In order to establish an effective procedure for sensor integration, flat tensile specimens with embedded sensors were firstly produced under different processing conditions, using advanced composite material employed in the marine industry. Information obtained in this way was used to manufacture a sailboat bowsprit with an array of gratings embedded between the last inner plies. In order to assess the sensitivity and accuracy of the embedded sensors, the bowsprit was subjected to bending tests and the results were compared to analytical values and to data obtained from electrical strain gages. Results are very encouraging but reveal that great attention must be paid both to the integration process and to the in situ calibration of the embedded sensors.

Displacement and Strain Field Measurement in Steel and RC Beams Using Particle Image Velocimetry

AbstractConventional deformation measurement techniques, such as utilizing a LVDT, a demountable mechanical strain gauge (demec gauge), and an electrical strain gauge, despite their vast usage in experimental studies, suffer several disadvantages, including high cost, time-consuming specimen preparation, and the inability to obtain the whole displacement or strain fields. The aforementioned drawbacks on one hand and the importance of deformation measurements in all civil engineering laboratory tests on the other hand have led researchers to develop other measurement techniques, especially image-based techniques, capable of evaluating whole deformation fields. Among all of these recently developed methods, particle image velocimetry (PIV) has attracted much interest in all branches of civil engineering experimental investigations; PIV is a powerful image-based technique for evaluating both displacement and strain fields by analyzing successive digital images of a field undergoing deformation. Despite its g...

Comparative analysis of tooth-root strength using stress–strength interference (SSI) theory with FEM-based verification

Gear geometry plays a significant role in deciding the tooth deflection, magnitude of stress induced and stress distribution. Accurate evaluation of the stress state and distribution of stresses at the tooth root requires application of experimental methods to determine, such as electric resistance wire strain gauges, or photoelastic gauges, etc. These experimental methods are especially complicated, expensive, and laborious. Therefore, they are applied mainly in special cases. In the recent years, the methods of probabilistic design have been commonly accepted in the design of mechanical components and systems to assist the designer in making decisions on the best balanced design with respect to several design criteria. In probabilistic design, it is common to use parametric statistical models to calculate the probability of failure obtained from the Stress–Strength Interference (SSI) theory, which more nearly models the true situation. This paper discusses the application of the SSI theory that has been used to support the detailed gear design as part of the “Design for Reliability” approach to evaluate the tooth-root strength with finite element method-based verification. The SSI theory is formulated to predict the effect of the root fillet generated by rack or hob tool with and without protuberance on the gear system reliability. The results indicate the high fillet root stresses and gear tooth deflection in gears with the lowest tip radius of the generating tool.

Experimental Study on Measuring the Steel Stress via FBG Sensors

Studying the bond stress-slip relationship between concrete and corroded steel bar by cutting the steel bars into two separate parts and attaching electric strain gauges in the slots is no longer suitable. To overcome the disadvantages of electric strain gauges in the measuring the stress of corroded steel bars, this paper introduced a new kind of FBG sensor measuring steel stress. By calibration tests, the proportion coefficient between variation of wavelength and steel strain was confirmed as 0.0012. The bond behavior between concrete and steel bar was also investigated by performing pullout tests on beam end specimens.

State of Stress Determination in a Water Drinker by Numerical and Experimental Methods

This paper presents the evaluation of residual stresses and stresses induced by external loads in a water drinker by numerical and experimental methods. The part, used in animal husbandry, is made of grey cast iron (EN GJL 200). Numerical simulations were based on the finite element method, using ANSYS Workbench software to determine the stresses produced by external loads and finite volume method, using NovaFlow software, to determine residual stresses. The results obtained by numerical methods, were validated by experimental methods. Applied experimental methods consist of electrical resistive strain gauge technique to determine the state of stress in various loading conditions and the blind hole method to determine residual stresses, respectively.

Ultimate Tendon Stress in CFRP Strengthened Unbounded HSC Post-Tensioned Continuous I-Beams

The use of unbounded tendons is common in prestressed concrete structures and evaluation of the stress increase in unbonded tendons at ultimate flexural strength of such structure has posed a great challenge over the years. Based on the bending experiment for two-span continuous post-tension beams with unbounded tendons and externally applied CFRP sheets, the monitoring of the stress increment of unbounded tendons is made in the loading process. For these aims, in this paper there are presented results of two continuous un-bonded post-tensioned I-beams were cast with high strength concrete (HSC) and monitored by electrical strain gauges. The beams are made of which are compared with the theory proposed by different codes. The results indicate that the ACI 318-2011 provides better estimates than AASHTO-2010 model whereas this model provides better estimates than BS 8110-97. Comparison of experimental ultimate tendon stress increase of strengthened and non-strengthened beams casted with HSC indicates that increase in tendon stress at an ultimate state in strengthened unbounded post-tensioned beam is lower than non-strengthened unbounded post-tension beam casted with HSC.

An Experimental and Numerical Investigation of a Grid Composite Cylindrical Shell Subjected to Transverse Loading

This paper presents an experimental and numerical study of a stiffened composite cylindrical shell with clamped-free boundary condition under transverse end loading. Electrical strain gauges were employed to measure the strains. Likewise a finite element analysis has been used to validate the experimental results. The finite element method (FEM) software used was ANSYS11. Fairly good agreements were observed between the numerical and the measured results. In addition, a comparison between the stiffened and unstiffened composite shell was carried out. It was found that the intersection of two stiffeners has an important effect in reducing Von Mises stress in the shell. Failure analysis based on Tsai–Wu failure theory was also performed. It was concluded that stiffening of the shell by helical stiffeners increases the load capacity considerably.

Application of sensor technologies for local and distributed structural health monitoring

SUMMARY The work presented in this paper deals with the application of different sensor technologies for fatigue crack damage monitoring of metallic helicopter fuselages. A test programme has been conducted, consisting of seven fatigue crack propagation tests on aluminium panels (skin with riveted stringers) representative of the rear fuselage of a helicopter. Electrical crack gauges and comparative vacuum monitoring sensors have been used locally to monitor propagating cracks. A network of optical fibre Bragg gratings is presented as a valid possibility for distributed monitoring (based on strain field dependence on damage), alternative to consolidated electrical resistance-based strain gauges. A Smart Layer based on piezoelectric transducers that emit and receive Lamb wave signals has also been analysed in this paper for distributed monitoring. Two damages have been considered: a skin crack artificially initiated on a panel bay and a skin crack propagating from a rivet hole after stringer failure. Damage sensitivity has been evaluated and compared among the considered technologies, thus providing useful recommendations for each considered system, together with advantages and drawbacks concerning their suitability for on-board installation and monitoring. The damage index sensitivity to operative condition, load in particular, is verified and compared with damage effect for distributed networks. A finite element model able to describe any selected feature sensitivity to the monitored damage is also presented as a useful tool for the optimization of the structural health monitoring system design process. Copyright © 2013 John Wiley & Sons, Ltd.

Deflection estimation of a wind turbine blade using FBG sensors embedded in the blade bonding line

Estimating the deflection of flexible composite wind turbine blades is very important to prevent the blades from hitting the tower. Several researchers have used fiber Bragg grating (FBG) sensors—a type of optical fiber sensor (OFS)—to monitor the structural behavior of the blades. They can be installed on the surface and/or embedded in the interior of composites. However, the typical installation positions of OFSs present several problems, including delamination of sensing probes and a higher risk of fiber breakage during installation. In this study, we proposed using the bonding line between the shear web and spar cap as a new installation position of embedded OFSs for estimating the deflection of the blades. Laboratory coupon tests were undertaken preliminarily to confirm the strain measuring capability of embedded FBG sensors in adhesive layers, and the obtained values were verified by comparison with results obtained by electrical strain gauges and finite element analysis. We performed static loading tests on a 100 kW composite wind turbine blade to evaluate its deflections using embedded FBG sensors positioned in the bonding line. The deflections were estimated by classical beam theory considering a rigid body rotation near the tip of the blade. The evaluated tip deflections closely matched those measured by a linear variable differential transformer. Therefore, we verified the capability of embedded FBG sensors for evaluating the deflections of wind turbine blades. In addition, we confirmed that the bonding line between the shear web and spar cap is a practical location to embed the FBG sensors.

Fiber Bragg Grating Sensors for Fatigue Monitoring of Composite

Fiber Bragg grating (FBG) sensor can be used to monitor the mechanical behavior of composite. The internal strain of composite during a constant stress amplitude fatigue testing process was monitored with FBG sensors in this paper. FBG sensor as a fatigue indicator can reveal the decrease of Young's modulus of the specimen, which is a direct index of damage. FBG sensor can not only be embedded in composite laminates to detect fatigue damage, but also has excellent durability compared with other sensors such as electric strain gauge. After 1 million cycles, the FBG sensor can still keep good sensibility. FBG sensor as a fatigue indicator is a novel sensor to monitor, evaluate and give crash alert for the health state of composite during its whole service life. And also, a new and simple model of stiffness degradation was developed. This model can be used to predict the fatigue life of composite material.

Measurement Model for the Maximum Strain in Beam Structures Using Multiplexed Fiber Bragg Grating Sensors

This study develops a strain measurement model for beam structures subjected to multiloading conditions by defining the strain-shape function and participation factors to overcome the limitations of strain measurements using fiber Bragg grating (FBG) strain sensors. Using the proposed model, the maximum strain in a beam is obtained by the sum of the strains caused by the different loadings acting separately. In this paper, the strain-shape functions for various loading and support conditions are provided, and a system of equations is defined to calculate the participation factors. Furthermore, the influence ratio is defined to identify the influence of each loading on the value of the total strain. The measurement model is applied to the monitoring of the maximum strain in a 4 m long steel beam subjected to two concentrated loads. For measurements during the test, seven FBG sensors and nine electric strain gauges (ESGs) were attached on the surface of the bottom flange. The experimental results indicate a good agreement between the estimated strains based on the model and the measured strains from ESGs. Furthermore, the dependency of the locations for the FBG sensors installed at the beam structure on the selection can be avoided using the measurement model.

Analytical Model for Estimation of Maximum Normal Stress in Steel Beam‐Columns Based on Wireless Measurement of Average Strains from Vibrating Wire Strain Gages

AbstractIn this article, an analytical model is developed for estimation of the maximum normal stress in a beam‐column member subjected simultaneously to an axial load and bending moments. The analytical model is derived by defining the relationship between the maximum strain and average strains measured wirelessly from vibrating wire strain gages. On the basis of the experimental tests on a two‐dimensional steel frame structure with beam‐column members, the performance of the estimation model is evaluated by comparing the maximum strains directly obtained from electrical strain gages and the estimated maximum strains from the model. Using the model, a practical monitoring of the maximum stress in steel beam‐columns is made possible by measuring three average strains at arbitrary locations.

A Strain-Based Load Identification Model for Beams in Building Structures

A strain-based load identification model for beam structures subjected to multiple loads is presented. The number of sensors for the load identification model is the same as the number of load conditions acting on a beam structure. In the model, the contribution of each load to the strains measured by strain sensors is defined. In this paper, the longitudinal strains measured from multiplexed fiber Bragg grating (FBG) strain sensors are used in the load identification. To avoid the dependency on the selection of locations for FBG sensors installed on a beam structure, the measured strain is expressed by a general form of a strain sensing model defined by superimposing the distribution shapes for strains from multiple loads. Numerical simulation is conducted to verify the model. Then, the load identification model is applied to monitoring of applied loads on a 4 m-long steel beam subjected to two concentrated loads. In the experiment, seven FBG sensors and nine electrical strain gages (ESGs) were installed on the surface of the bottom flange. The experimental results indicate a good agreement between estimated loadings from the model and the loads applied by a hydraulic jack.

Loading Behavior Investigation of Composite Single Lap Adhesive Joints

Adhesive bonding as a promising join technique for composites is not widely used for the absence of absolutely understanding of loading behavior. In this paper both electric-strain gages and FBG sensors were employed to get the internal and surface strain of adhesive joint. In the experiment, nonlinear behavior is observed. Then the test data were compared with FEM results. It is showed that they meet each other well, FEM could get the stress distribution in adhesive bond composite joint, and both biggest peel stress and shear stress are appear at the end of adhesive.

Experimental investigation of channel-section composite profiles’ behavior with various sequences of plies subjected to static compression

This paper deals with the buckling of thin-walled channel-section composite columns subjected to static compression. It was assumed that the columns were supported with articulated joints at both ends. For experimental testing, three series of specimens were manufactured with autoclaving technique. The specimens had identical dimensions but differed about ply sequence. The Hexcel′s HexPly M12 carbon-epoxy prepreg was used in order to fabricate the channel-section profiles. During the stand tests minimal critical forces of the real structure and the corresponding buckling modes were determined with an application of electrical strain gauges. In addition, post-critical equilibrium paths for small overloads—150% of the critical load for the ideal structure—were determined. The experimental results were compared to the ones obtained numerically with the finite element method (FEM).

Consumed Fatigue Life Assessment of Composite Material Structures by Optical Surface Roughness Inspection

A strong knowledge of the fatigue state of highly advanced carbon fiber reinforced polymer composite (CFRP) structures is essential to predict the residual life and optimize intervals of structural inspection, repairs, and/or replacements. Current techniques are based mostly in measurement of structural loads throughout the service life by electric strain gauge sensors. These sensors are affected by extreme environmental conditions and by fatigue loads in such a way that the sensors and their systems require exhaustive maintenance throughout system life.This work is focused on providing a new technique to evaluate the fatigue state of CFRP structures by means of evaluating the surface roughness variation due to fatigue damage. The surface roughness is a property that can be measured in the field by optical techniques such as speckle and could be a useful tool for structural health monitoring. The relation between surface roughness and fatigue life has been assessed on CFRP test specimens. A tensile fatigue load with an R=0.1 (T-T) and a maximum load of 60% of the material ultimate strength has been applied. The surface roughness of the specimens has been determined from the surface topography measured by a high precision confocal microscope. Results show that the surface roughness of the specimens increases with the accumulation of fatigue cycles in such a way that the roughness could be taken into account as a fatigue damage metrics for CFRP.

Benchmarking of deformation and vibration measurement techniques for nuclear fuel pins

Understanding the mechanical interactions between the coolant and the core structure in nuclear reactors helps to determine the lifetime, health or optimal design of the reactor core. The flow of the coolant produces vibrations in the reactor core containing the fuel assemblies that consists of a matrix of fuel pins. We report on an evaluation of the performance of different vibration measurement techniques considered for measuring the flow induced vibrations on a fuel pin mock-up. These techniques include a laser Doppler vibrometer (LDV), a grid method (GRID), fiber Bragg grating sensors (FBGs), electrical strain gages and two types of accelerometers. In this paper we first show the practical aspects of the validation experiments before proceeding with the influence of the techniques on the pin dynamics. Finally we compare the signal-to-noise ratio (SNR) and the level of determination of the response signal of the sensors for low amplitudes and low frequencies. We conclude that for our setup the optical techniques and MEMS-type accelerometer prove to offer superior performance. Considering the space constraints, we believe that the fiber Bragg gratings are the best candidate for vibration monitoring in nuclear reactor core mock-ups.

Experimental and numerical stress analysis of glass fiber-reinforced polymer (GFRP) -stiffened shells with cutout under axial loading

In the present study, the stress analysis of thin-walled glass fiber-reinforced polymer (GFRP) cylindrical shells with and without cutout subjected to axial loading was carried out by using experimental and finite-element method. The effect of cutout type on the stress distribution of the shell was described. In the experimental procedure electrical strain gauges were used to measure the experimental strain at specific point of the structure. Finite-element analysis was included in a static analysis that predicts stress distribution and a buckling analysis that predicts the local buckling response of the models. The results illustrated that the stress concentration factor was completely related to cutout position on grid-stiffened shells. In particular, a local buckling response occurred in the shell near the cutout edge where the maximum stress gradient was reported. In addition, it has been concluded that stiffeners have good influence on reducing the stress concentration and changing the direction of the destructive loads on the shell. Key words: Stress analysis, cutout, stiffened shells, composite shells, stress concentration factor (SCF).

Use of fiber Bragg grating sensors for monitoring concrete structures with prestressed near-surface mounted carbon fiber–reinforced polymer strips

Prestressed fiber–reinforced polymer composites have gained popularity as a structural rehabilitation technique; however, the prestress loss remains a critical issue in terms of material efficiency and structure safety. This article presents a fiber Bragg grating–based technique for prestress loss monitoring in reinforced concrete beams strengthened by prestressed near-surface mounted carbon fiber–reinforced polymer strips. Bare fiber Bragg grating sensors are used to monitor the time history of the prestress loss in near-surface mounted strips with different prestress levels during both the construction stage and the ultimate loading phase. The prestress losses measured by the fiber Bragg grating sensors are further compared with the data reported by the conventional electrical strain gauges. The results show that the fiber Bragg grating sensors can effectively monitor the prestress loss when properly integrated into the near-surface mounted strips. They also provide a reliable method for crack damage detection in concrete.