Strain Gauge Technique Research Articles

Accurate measurement of elastic modulus of specimen with initial bending using two-dimensional DIC and dual-reflector imaging technique

Common optical methods are not suitable for accurate measurement of elastic modulus of specimens with initial bending because it is rather difficult to separate the axial strain caused by the axial load from the surface strain which consists of the axial strain and the bending strain caused by additional bending moment. Inspired by the strain-gauge technique, averaging the two strains on the opposite surfaces can eliminate the effect of bending strain of the specimen with initial bending, which is easily implemented by dual-reflector imaging and two-dimensional digital image correlation (2D-DIC). With dual-reflector imaging, the front and rear surfaces of the specimen are recorded by using a single digital camera. Consequently, the strains of two optical extensometers constructed on these two surfaces are obtained with common 2D-DIC, and averaging these strains can eliminate the effect of bending moment and out-of-plane motion of the specimen. Therefore, the elastic modulus can be determined with high measurement accuracy. The effectiveness of the proposed method was first verified using self-manufactured tensile equipment which leads to large out-of-plane motion during the test. Furthermore, uniaxial tensile tests of stainless-steel specimens, including static and continuous tensile tests, were conducted to evaluate the accuracy of the proposed method. The experimental results show good agreement between elastic modulus obtained using the proposed method and a strain gauge and the relative error between them is less than 0.5%, which shows excellent performance of the proposed method on accurate measurement of elastic modulus of specimens with initial bending.

A strain gage technique for mode I notch stress intensity factor of sharp V-notched configurations

A simple and robust single strain gage technique for accurate measurement of mode I notch stress intensity factor (NSIF) for sharp V-notched configurations has been proposed. The technique is supported by strong theoretical foundation, and it allows the strain gage to be placed reasonably away from the notch tip thus avoiding various problems associated with singularities. Importantly, in contrast to the available techniques, the present strain gage technique also prescribes proper radial locations (optimal locations) for the strain gage which are extremely useful in accurate measurement of NSIFs. A general theory and procedure has been developed for the estimation of these gage locations a priori for any given sharp V-notched configuration. These locations are shown to be dependent on the notch angle and notch length to width ratio. Experiments have been conducted using linear strain gages of 1 mm gage length and the results show that accurate NSIFs can be measured using the proposed single strain gage technique. The numerical results of the present investigation clearly show that very accurate (of the order of less than 1% error) mode I NSIF can be measured using the proposed single strain gage technique if the gages are placed at the optimal locations. The present results also demonstrate that highly inaccurate (of the order of 45% error) and unacceptable NSIFs would result if the gages are placed at non-optimal gage locations.

Effect of low-shrinkage monomers on the physicochemical properties of experimental composite resin

This study was conducted to determine whether novel experimental low-shrinkage dimethacrylate co-monomers could provide low polymerization shrinkage composites without sacrifice to degree of conversion, and mechanical properties of the composites. Experimental composites were prepared by mixing 28.6 wt% of bisphenol-A-glycidyl dimethacrylate based resin matrix (bis-GMA) with various weight-fractions of co-monomers; tricyclo decanedimethanol dacrylate (SR833s) and isobornyl acrylate (IBOA) to 71.4 wt% of particulate-fillers. A composite based on bis-GMA/TEGDMA (triethylene glycol dimethacrylate) was used as a control. Fracture toughness and flexural strength were determined for each experimental material following international standards. Degree of monomer-conversion (DC%) was determined by FTIR spectrometry. The volumetric shrinkage in percent was calculated as a buoyancy change in distilled water by means of the Archimedes’ principle. Polymerization shrinkage-strain and -stress of the specimens were measured using the strain-gage technique and tensilometer, respectively with respect to time. Statistical analysis revealed that control group had the highest double-bond conversion (p < .05) among the experimental resins tested. All of the experimental composite resins had comparable flexural strength, modulus, and fracture toughness (p > .05). Volumetric shrinkage and shrinkage stress decreased with increasing IBOA concentration. Replacing TEGDMA with SR833s and IBOA can decrease the volumetric shrinkage, shrinkage strain, and shrinkage stress of composite resins without affecting the mechanical properties. However, the degree of conversion was also decreased.

Structural, Magnetic, and Magentostrictive Properties of Dy1−x (Tb0.2Pr0.8) x Fe1.93 (0≤ x ≤ 0.5) Compounds

The structural, magnetic, and magnetostrictive properties of light rare earth Pr-contained Dy1−x (Tb0.2Pr0.8) x Fe1.93 (0≤ x ≤ 0.5) compounds were investigated by means of X-ray diffraction, vibrating sample magnetometer and strain gauge technique. A single Laves phase with MgCu2-type structure was obtained in the composition range of 0.0≤ x ≤ 0.25. The lattice parameter of the Lave phase increases with increasing x due to the large radius of Pr3+ and Tb3+. The Curie temperature of the Laves phase decreases with increasing x, indicating weakening strength of 3d-4f coupling. The saturation magnetization decreases with increasing x, which could be ascribed to the opposite magnetic moment alignments of Pr and (Td,Dy) with Fe. The magnetostriction of Dy1−x (Tb0.2Pr0.8) x Fe1.93 increases with increasing x, peaks at x = 0.25 and then decreases with further increasing x.

Measurement of friction and noise from piston assembly of a single-cylinder motorbike engine at realistic speeds

This paper presents a thorough experimental study of piston assembly friction and noise in a single-cylinder motorbike engine operating at low speeds. The friction of the piston ring pack is evaluated using a foil strain gauge with minimal cylinder modification on the thrust side. The technique involves transmitting deformations through the cylinder bore and recording reflections from the lubricated interface as the piston assembly passes. Under these conditions, the piston side forces and the thermal deformations on the output side of the strain gauge sensor are critical. Therefore, the proposed methodology is designed under controlled operating conditions. The overall deformation of the piston assembly is analysed to measure the primary reflection due to friction between the piston assembly and the cylinder wall. Simultaneously, the piston assembly noise is recorded on the thrust side of the engine block using a microphone. Taking measured noise data into account, possible piston slap events resulting from varied engine speeds are taken into account using continuous wavelet signal analysis. The calibration procedure for both tests is also illustrated. The measured friction results show that the strain gauge technique is a challenging work in providing realistic results to enhance current technology. For low engine speeds, a higher contribution is noted by boundary friction at the top dead centre reversal, extending to the position of maximum combustion pressure in the power stroke. Furthermore, the main contribution of the piston slap is estimated at the thrust side when the piston assembly passes at the beginning of the combustion stroke. These results can also be attributed as data to validate piston ring models in terms of friction and piston slap.

The Effects of Cavity Preparation and Composite Resin on Bond Strength and Stress Distribution Using the Microtensile Bond Test.

To evaluate the effect of flowable bulk-fill or conventional composite resin on bond strength and stress distribution in flat or mesio-occlusal-distal (MOD) cavity preparations using the microtensile bond strength (μTBS) test. Forty human molars were divided into two groups and received either standardized MOD or flat cavity preparations. Restorations were made using the conventional composite resin Z350 (Filtek Z350XT, 3M-ESPE, St Paul, MN, USA) or flowable bulk-fill (FBF) composite resin (Filtek Bulk Fill Flowable, 3M-ESPE). Postgel shrinkage was measured using the strain gauge technique (n=10). The Z350 buildup was made in two increments of 2.0 mm, and the FBF was made in a single increment of 4.0 mm. Six rectangular sticks were obtained for each tooth, and each section was used for μTBS testing at 1.0 mm/min. Polymerization shrinkage was modeled using postgel shrinkage data. The μTBS data were analyzed statistically using a two-way analysis of variance (ANOVA), and the postgel shrinkage data were analyzed using a one-way ANOVA with Tukey post hoc test. The failure modes were analyzed using a chi-square test (α=0.05). Our results show that both the type of cavity preparation and the composite resin used affect the bond strength and stress distribution. The Z350 composite resin had a higher postgel shrinkage than the FBF composite resin. The μTBS of the MOD preparation was influenced by the type of composite resin used. Irrespective of composite resin, flat cavity preparations resulted in higher μTBS than MOD preparations ( p<0.001). Specifically, in flat-prepared cavities, FBF composite resin had a similar μTBS relative to Z350 composite resin. However, in MOD-prepared cavities, those with FBF composite resin had higher μTBS values than those with Z350 composite resin. Adhesive failure was prevalent for all tested groups. The MOD preparation resulted in higher shrinkage stress than the flat preparation, irrespective of composite resin. For MOD-prepared cavities, FBF composite resin resulted in lower stress than Z350 composite resin. However, no differences were found for flat-prepared cavities. FBF composite resin had lower shrinkage stress than Z350 conventional composite resin. The μTBS of the MOD preparation was influenced by the composite resin type. Flat cavity preparations had no influence on stress and μTBS. However, for MOD preparation, composite resin with higher shrinkage stress resulted in lower μTBS values.

Metal explosion chambers: designing, manufacturing, application

Designing of explosion chambers is based on research investigations of the chamber body stress-strain state, which is determined by numerical computation and experimentally by the strain gage technique. Studies show that chamber bottoms are the most loaded elements, and maximal stresses arise in chamber poles. Increasing the shell thickness around poles by welding-in an insert is a simple and saving way to solve this problem. There are structural solutions, enabling reliable hermetic closure and preventing leakage of detonation products from the chamber. Explosion chambers are employed in scientific research and in different industrial applications: explosive welding and hardening, synthesis of new materials, disposal of expired ammunition, and etc.

Experimental Investigations on the Stress State in the Bearing Structure of a Special Wagon to Transport Liquid Iron

Abstract In order to increase productivity, to reduce energy consumption and for environmental protection, a special wagon intended for the transport of liquid pig iron, in steel mills, it was designed and manufactured. This wagon consists of an especial chamotted torpedo, with a capacity of 300 tons, rested, at each end, by means of a box beam, on two buggies, with four axles each one. To evaluate the mechanical behavior of this wagon during service, its prototype was investigated experimentally in terms of the stress and strain state. Due to difficulties encountered related to the long duration of loading and unloading time of the torpedo pot with liquid cast iron and due to very large dimensions of the carriage gauge, experimental investigations were performed on the torpedo pot and box beam models, manufactured at scale. The buggy structure was investigated on a special stand, set up in the laboratory of Strength of Materials Department of the Polytechnic University of Bucharest. The torpedo pot and box beam were investigated using two combined experimental techniques (brittle coatings technique and resistive strain gauge technique). Brittle coatings technique was used to identify the principal stresses directions, which then were applied electro-resistive transducers. Using strain gauge amplifiers the principal strains were measured, and principal stresses in measurement points were calculated. The results of these studies have led to changes of constructive solutions in the case of some substructures, contributing to increase the operational safety of the wagon.

Material Characterization of Hybrid Composite: Experimental Using Strain Gauge/DIC with Finite Element Modelling Macro/Micro Scale

The study presents the multi methods of determining mechanical properties and mechanical characterization under tensile loading of hybrid composite in the form of experimental technique involving measurement of strain using strain gauge and digital image correlation (DIC) technique utilizing open source platform Ncorr to compute the strain on surface of hybrid composite. The method of micro mechanical modelling using Finite Element Modelling (FEM) in the mode of representation volume element (RVE) method and macro scale FEM using commercial software Ansys have been performed to compute the modulus of elasticity in direction of uniaxial tensile loading. The four methods then compared which yields very consistent results with each other. It is observed that all four methods are reliable in determining mechanical properties of unidirectional single composite as well as hybrid composite. The experimental involved the use of ASTM D3039 standard tensile test for hybrid composite and strain are measured using strain gauges and DIC.

Accurate Measurement of Mixed Mode (I/II) Stress Intensity Factors Using Strain Gages

Abstract Determination of accurate mixed mode stress intensity factors forms the basis for the understanding and analysis of mixed mode fracture. In this context, experimental methods for the measurement of stress intensity factors (SIFs) play a vital role. However, very few strain gage based techniques are available for measurement of mixed mode SIFs KI and KII in isotropic materials. In addition, no useful recommendations on appropriate radial locations of strain gages ensuring accurate SIFs are available in literature. Recently, authors have theoretically proposed a mixed mode strain gage technique and attempted to address the long standing problem of assessment of appropriate strain gage radial locations for the accurate measurement of the SIFs. They have shown that there exist certain gage radial locations called as optimal locations for the accurate measurement of SIFs, which can be easily assessed with their proposed methodology for any mixed mode (I/II) configurations. In the present investigation, experiments were conducted to study the efficacy of the authors' proposed mixed mode strain gage technique and existence and usefulness of the optimal strain gage locations in accurate measurement of SIFs KI and KII. The present experiments were also aimed to show the consequences of placing the gages at invalid or non-optimal gage locations. The experimental results clearly show that highly accurate mixed mode SIFs can be measured using the proposed mixed mode strain gage technique if the gages are pasted at the optimal locations. On the other hand, the results also exposed that highly erroneous and unacceptable measurement of SIFs can result if the gages are located at non-optimal locations. Furthermore, the present results clearly demonstrated the need of knowing the optimal strain gage locations a priori, which can be assessed using the methodology proposed recently by the authors.

Experimental and numerical investigation of stress in a large-scale steel tank with a floating roof

Research on steel tanks, particularly large-scale oil storage tanks, has increased significantly with the rapid development of the petrochemical industry. The focus on these thin-walled structures is related not only to the cost of the infrastructure, but also because failures due to accidents may generate huge economic costs, environmental losses, and casualties. The aim of the present study is an extensive experimental and numerical investigation of stress in a large-scale oil tank with a floating roof. Stresses in the shell and bottom of a large-scale oil storage tank are measured by a resistance strain gauge technique. Meanwhile, an improved 3-D finite element model is developed to analyze stresses in the tank. By comparing calculated results with experimental data, the validity of the developed finite element model is verified. Both experimental and numerical results suggest that the stress on the fillet joint is relatively high in the tank. The finite element model is used to study the effects of three key steel tank parameters on the stress of the fillet joint in the oil storage tank: the thickness of the annular bottom plate at the base, the radial width of the projection of the annular bottom plate outside the shell, and the radial width of the concrete ringwall. In addition, a new method related to the design of the annular bottom plate and concrete ringwall is proposed. This method will be helpful in improving the design of large-scale oil tanks and assessing their structural integrity.

Imperfection sensitivity of post-buckling of FML channel section column

In the paper the post-buckling behaviour of thin-walled channel cross-section columns made of Fiber Metal Laminate subjected to an axial compressions is considered. The main interest is focused on the initial imperfection influence on the post-buckling response and thus the sensitivity analysis with application of Monte Carlo simulations is performed. A different shapes and amplitudes of geometrical imperfections – all corresponding to a particular buckling mode or to a few superimposed, have been applied in numerical models. The numerical simulations have been performed using developed FEM software based on Koiter's asymptotic theory for hundreds of buckling mode cases. The obtained results of numerical calculation have been compared with results of experimental investigations. Both - the numerical computations and laboratory experiments were performed on short FML channel section columns with four different layer arrangements. The strain-gauge technique, laser measurement and digital image correlation system were employed to determine deformation modes in laboratory tests. A comparison of the results shows that geometrical imperfections have substantial impact on the post-buckling response of considered columns.

External spur gear root bending stress: A comparison of ISO 6336:2006, AGMA 2101-D04, ANSYS finite element analysis and strain gauge techniques

The International Organisation for Standardisation ISO 6336:2006 and the American Gear Manufacturers Association AGMA 2101-D04 are the two most predominant empirically based analytical gear stress analysis methods for establishing contact and root bending stress in involute spur and helical gears. Although based on the same fundamental principles, they have evolved to such an extent that, for various reasons, they are not necessarily in agreement. The use of commercial universal numerical finite element software has become an increasingly popular alternative for gear stress analysis and this research compares the root bending stresses in external spur gears established using ISO 6336:2006, AGMA 2101-D04 and numerical finite element analysis (ANSYS) with experimental strain gauge validation.

Calculus and experimental investigations on the caissons models used in construction of mooring keys

The special quay infrastructure for mooring heavy boats is made of reinforced concrete caissons, in the form of thin-walled structures. These caissons are placed on the bottom of the sea, by means of radiators and filled with stone quarry. On these structures are built access roads for heavy trucks, rolling paths for rotary cranes and railways. The behaviour of these structures under service tasks has been investigated both by MEF and experimentally, using two combined techniques (reflexion-photoelelasticity and resistive strain gauge technique). The results of the researches carried out on two models of caissons showed that the maximum stresses recorded in their walls are below the allowable limit, and there is a good agreement between the calculation and experimental data.

A strain gage technique for the determination of mixed mode stress intensity factors of orthotropic materials

A theoretical frame work is developed for strain gage based determination of mixed mode (KI/KII) stress intensity factors (SIFs) in slant edge cracked plate (SECP) made of orthotropic materials. Using three parameter strain series around the crack tip and appropriate stress functions, the present formulation shows that mixed mode SIFs in orthotropic materials could be determined using only four strain gages. A finite element based methodology is developed to determine the upper bound on the radial location (rmax) of strain gages ensuring accurate determination of SIFs. Proposed technique is applied to numerical simulation of [02/90]2S and [0/±45/90]S glass-epoxy SECP laminates to demonstrate accurate determination of mixed mode SIFs by placing the gages within rmax. Results from the present work provide clear guidelines in terms of number of strain gages and their suggested locations for accurate determination of mixed mode SIFs in orthotropic materials.

Reduced Venous Compliance in Young Women with Type 1 Diabetes - Further Aggravated by Prolonged Elevated Levels of HbA1c.

Young patients with diabetes present with reduced compensatory responses to hypovolemic stress. Less compliant veins could be a contributing factor, since roughly two-thirds of the blood volume resides in the venous system as a blood reservoir, adjusting proper venous inflow to the heart. The aim of this study was to measure venous compliance and lower limb blood pooling during hypovolemic stress, and to correlate them to indices of diabetes severity and glucose control. Fifteen young women with type 1 diabetes (DW) and 18 healthy age-matched women (C) were subjected to lower body negative pressure (LBNP) (11-44 mmHg), creating hypovolemic stress. Lower limb blood pooling was measured with strain gage technique and venous compliance calculated as the relationship between ∆V/∆P. DW presented with reduced blood pooling (e.g., blood pooling during LBNP of 44 mmHg, DW, 1.69 ± 0.10; C, 2.10 ± 0.08 (ml/100 ml), and P = 0.003). Calculated venous compliance was also reduced in DW (e.g., compliance at 20 mmHg, DW, 0.046 ± 0.003; C, 0.059 ± 0.002 (ml/100 ml/mmHg), and P = 0.002). A progressive reduction in both venous compliance (P < 0.007) and blood pooling (P < 0.005) was seen with increasing level of HbA1c, and furthermore, less strongly associated with presence of microvascular disease (signs of retinopathy). Women with type 1 diabetes present with both reduced venous compliance and blood pooling. The reductions were particularly present in patients with long-standing poor glycemic control.

Environmental Thermal Cycle Analysis and Testing of Elastomeric Mounted Objective Lens Cell for Head-Up Display System

This paper presents the determination of principal stresses, strains, and shear stresses due to thermal cycle load experimentally as well as using Finite Element Analysis (FEA) in the objective lens cell of Head-Up Display for aircraft application. The principal stresses and strains in the objective cell are obtained by adopting two methods namely a highly correlated FEA and strain gauge technique. The stress and strains results obtained from aforesaid both the methods were analyzed and found satisfactory. The results obtained were validated with allowable stress tolerance limits of the glass laid down by some researchers and rule of thumb in optics.

Theoretical and experimental research on turbo-generator shaft alignment using strain gauge method

Shaft alignment is a key factor for the safe and reliable operation of a turbine unit. This paper presents an approach to estimate the turbo-generator shaft alignment using strain gauge technique with two measuring sections nearby each coupling. The change of shaft bending moment is linearly proportional to the change of bearing elevation and reaction force. So the bearing load distribution can be identified from the strain values and bending moments which are measured using strain gauge. The transfer matrix method is used to calculate the detailed shaft alignment data including deflection, slopes, bearing reaction forces, influence coefficients, sag/gap values. Experiments were carried out on a test rig with 3 rotors, 2 couplings and 6 bearings. It was shown that the strain gauge identification results agree well with the results using the jack-up method. The maximum relative error of each bearing loads is almost 8.7%. The sag/gap values using the strain gauge method are close to those using the laser alignment method. The maximum relative error is less than 7.5%. Results also show that the strain gauge method is sensitive to possible disturbances in the bearing elevation. In the end, strain gauge alignment technique with two measuring sections nearby each coupling was applied on the shaft alignment estimation of a 600 MW turbine unit successfully. The bearing loads and shaft deflection were obtained accurately.

Self Sensing Concrete using Carbon Fibre for Health Monitoring of Structures under Static loading

Background/Objective: To find an easy method to health monitor structures. Method: Self sensing or smart behavior has been observed in mortar or concrete with the addition of small amounts (0.2 to 0.5% by volume of cement) of short (5mm length) carbon fibres. It is seen that there is an increase in electrical resistance on loading upto crack propagation or fracture. On reaching the inelastic stage, the resistance change is not reversible. Findings: A method is developed which can be used in place of, often used strain gage technique or fibre optic technique for health monitoring of structures. There is an increase in resistance during fibre pull out in the elastic range. The change in elastic resistance was measured by a four probe method and was seen to be reversible for elastic deformation. Also, the crack propagation and fibre breakage of the specimen can be identified by irreversible resistance change. The stress vs strain and resistance vs strain graphs when plotted show similarity. Application: This phenomenon can be made use of to find the real time weight of vehicles in traffic and finding stress values of a loaded structure etc.

Design Development and Performance Analysis of Leaf Spring Testing Set up in Elastic Domain

Leaf spring is the simplest type of suspension spring commonly used in wheeled vehicles and its main cause of failure is large bending under load. Leaf spring is generally designed by modeling it either as curved cantilever beam with tip concentrated load or curved beam under three point bending. In spite of the widespread theoretical static analysis of leaf spring, there is lack of experimental fidelity in the ensuing models. Moreover, there is insufficiency in modeling the roller supports at eye ends and in addition, asymmetry in master leaf geometry and presence of drill hole in master leaf have not been considered properly. This paper presents design development and performance analysis of two different leaf spring testing rigs. The design considerations, detail descriptions of the components along with their manufacturing details are furnished here. The first set-up is a general purpose set-up for clamping structures, which models leaf spring as curved cantilever beam under tip concentrated load by considering only its one half. Experiment is carried out in this set-up to obtain deflection profiles of master leaf spring using image processing technique. Comparisons with theoretical results identify several insufficiencies in the present experimental model such as unknown clamping force, non-consideration of asymmetry in master leaf geometry and inappropriate modeling of support arrangement at eye ends. To overcome such insufficiencies, the second set-up is designed which simulates three point bending test. This set-up introduces roller supports at eye ends and considers asymmetry in the master leaf geometry. In both the cases, deflection profile is obtained using image processing technique and in the second set-up, strain gauge technique is adopted for strain measurement.