Electrical Resistance Strain Gauges Research Articles

Force measurement on fracture site with external fixation.

A force measurement device has been designed to monitor the mechanical properties of fracture site with external fixation. Forces are measured through electric resistance strain gauges mounted on fixation framework and the measurement results are displayed on an LCD screen. The device features a force range of 0-10 kg with linearity and repeatability less than 1% and accuracy less than 0.1 kg.

Relationship of Bond Stress, Steel Stress, and Slip in Reinforced Concrete

This paper presents an experimental analysis to establish the fundamental relationship between bond stress, steel stress, and slip in reinforced-concrete structures. Tests were conducted on double pullout specimens reinforced with 25-mm plain round mild steel, cold-worked and hot-rolled ribbed bars that had been fully instrumented internally with electrical resistance strain gauges. The results provided examples of the longitudinal variation of the steel strain (analogous to that between cracks). The method has proved to be capable of providing sufficient data for plotting the distribution of steel stress, bond stress, and slip between flexural cracks. The relationship between bond stress, steel stress, and slip was derived from the steel strain function, and has been represented by empirical formulas.

Verification of the duplex gauge using variable strains and temperatures

AbstractFour steel specimens were equipped with “duplex nances”, a recently proposed transducer based on two different electrical resistance strain gauges, capable of correcting the effects induced by temperature. The examined innovative transducer, in comparison with traditional strain measuring systems, has the advantage of utilising two strain gauge channels of the same type, which significantly simplifies automatic measuring systems. The specimens were subjected to temperature and strain field variations in the ranges of=30–150°Cand=100–700μm/m respectively The temperature and strain values obtained from the duplex gauges were compared with those measured by means of traditional methodology, i.e. a strain gauge and a thermocouple.

A TECHNIQUE FOR BIAXIAL DAMPING MEASUREMENT

A description is given of a novel technique by means of which experimental studies may be made of the energy dissipation behaviour of materials subjected to biaxial (i.e., combined stress) loading. A specimen in the form a thin cylinder is subjected to simultaneous but separately controlled fluctuating fluid pressures internally, externally and axially. Pressure control is achieved by electro-hydraulic servo-systems and strain response is measured by miniature electrical resistance strain gauges. The apparatus is believed to be unique in its ability to apply biaxial with uniform stress distribution and uniform ratio of principal stresses over the full range −1 ≤ σ 2/σ 1≤ 1 with adequate control and accuracy and absence of means tress. Hysteresis loops in the two principal directions are recorded. The principles of the apparatus and the special features of its design are discussed. Experimental results are presented. The errors associated with operation of the apparatus correspond to loss factors of the order of 0·001–0·002 and hence the apparatus may be considered satisfactory for studies of materials of loss factor 0·01 and higher.

Experimental results and finite element modelling of an embedded polarimetric sensor

‘Smart structures’ or ‘smart skins’ will require structurally integrated sensing systems that can operate in practical situations. Optical sensing techniques are receiving considerable attention for the monitoring of such systems. In this work an optical-fibre sensor has been embedded, without removing its acrylate protective coating, in a glass fibre-reinforced polymer. A single-ended polarimetric sensor was utilized with a large dynamic range for strain measurements up to 2000 με, at which value the sensor failed. The optical measurements show good agreement with conventional electrical resistance strain gauge measurements. Finite element modelling was used to investigate the stress/strain distributions within the composite material and the embedded optical fibre. The modelling results show excellent agreement with the experimental results and suggest that the soft acrylate coating is debonding, thus reducing the sensor's dynamic range.

Characterizing the temperature dependence of electronic packaging-material properties

A computer-controlled, temperature-dependent material characterization system has been developed for thermal deformation analysis in electronic packaging applications, especially for printed wiring assembly warpage study. For fiberglass-reinforced epoxy (FR-4 type) material, the Young's moduli decrease to as low as 20–30% of the room-temperature values, while the shear moduli decrease to as low as 60–70% of the room-temperature values. The electrical resistance strain gage technique was used in this research. The test results produced overestimated values in property measurements, and this was shown in a case study. A noncontact strau]n measurement technique (laser extensometer) is now being used to measure these properties. Discrepancies of finite-element warpage predictions using different property values increase as the temperature increases from the stress-free temperature.

Mechanical Hysteresis in Force Transducers Manufactured from Precipitation-Hardened Stainless Steel

A non-linear, anelastic stress-strain relationship, hereafter called mechanical hysteresis, which leads to a significant error in the output signal of electrical resistance strain gauge force transducers, has been confirmed to be a general phenomena in precipitation-hardened stainless steels. The mechanism of mechanical hysteresis has been found to be due to the material behaviour; nevertheless, the design of the force transducer and details of the electrical resistance strain gauges could impose further minor effects on the size of the error. Some of the secondary influencing factors (for example loading and mounting details) have been eliminated in the course of the investigation. Using a specially developed heat treatment process, it has been possible to substantially reduce the hysteresis error, which in turn improves the force transducer accuracy.

Propagation of microcracks in concrete under uniaxial compression

This Paper reports the findings of a series of short-term sustained loading tests on concrete cylinders in which the extent of microcracking in concrete was continuously monitored using a novel non-destructive method. The approximate crack areas within the cross-sections of these concrete cylinders were estimated from the measured transverse and axial strain readings obtained with electrical resistance strain gauges attached to the surface of the cylinders. The experimental results showed that microcracks were initiated at a stress-strength ratio ranging between 0·15 and 0·44 (rather than at a specific value), but they did not propagate under sustained loads unless the stress–strength ratio exceeded a threshold value of about 0·5. Beyond this threshold level, the rate of microcrack propagation generally increased with increasing stress but, up to a certain stress–strength ratio (ranging between 0·8 and 0·9), decreased with time during sustained loading. It was also observed that this particular stress–strength ratio had the same range of values as the stress at which the volume of concrete became a minimum. Beyond this critical stress–strength ratio, the rate of microcracking increased rapidly with stress and increased with time under sustained loading, indicating that failure of concrete was imminent.

Investigation of Bioabsorbable Screw Usage for Long Bone Fracture Repair in the Horse: Interfragmentary Compression and Axial Load Response in Equine Cadaver Longbone Fractures

SummaryThe purpose of the study was to compare biomechanical features of bioabsorbable, self-reinforced poly-L-lactic acid (SR-PLLA) and stainless steel (ASIF) 4.5 mm cortical screws. Tests were conducted in equine cadaver third metacarpal bones in which a simulated complete and displaced condylar fracture had been created by osteotomy.An evaluation of the amount of interfragmental compression achieved with each screw type was performed using electrical resistance strain gauges placed in proximity to the osteotomy. This revealed a substantial increase in interfragmental compression when ASIF screws were used compared to the SR-PLLA screw. A hybrid of the two screw types did however achieve adequate interfragmental compression. Comparison of the strength of fracture fixation through response to axial loading revealed significant difference between use of three ASIF screws and a hybrid of one SR-PLLA and two ASIF screws (p = 0.04) or two ASIF (p = 0.01) screws alone. It appeared that the hybrid configuration did have a mechanical advantage over the two ASIF screw arrangement.The results indicate that the SR-PLLA screw is inferior to the stainless steel screw as a means of primary fracture reduction in the horse. A less mechanically demanding role may yet enable usage of the bioabsorbable screws in equine fracture repair, although they proved inadequate in this testing environment.This study examined the inter-fragmental compression and response to axial loading of bioabsorbable and stainless steel 4.5 mm cortical screws in simulated condylar fractures in equine cadaver longbones. The stainless steel screw was superior in both compression and response to axial loading. It was concluded that the bioabsorbable screw did not possess characteristics that would allow it to bring about adequate primary fracture repair.

The Design and Analysis of a Quick-Release Pressure Vessel Door Closure

A quick-release door closure design (Bandlock 2) is described. The paper presents a case study of the design and analysis of an existing closure geometry. Four techniques for stress analysis of the closure are described: two numerical (finite element and boundary element) an experimental (electrical resistance strain gauge) and an approximate (strength of materials) approach to the calculation of stresses. The stress results are presented in an unnormalized form for an ANSI class 600 test pressure.

A precision six-load-component transducer: A design incorporating finite-length measurement paths

The design of an instrument is described that measures three resultant force components and three resultant moment components acting on a surface. Within the framework of linear elastostatics of an isotropic homogeneous material the device separates to a given precision the six resultant load components. Sensor paths of finite length are employed. Moreover if fiber-optic differential displacement sensors are used rather than traditional electrical resistance strain gages, the range and sensitivity of the instrument can in principle be improved without sacrificing the device stiffness. The primary reason for these improvements is that a complete solution to the equations of elasticity allows certain displacements to be measured over large distances and be combined to yield all of the resultant load components. These displacement measurements over a long distance accommodates the use of fiber-optic interferometric sensors. The use of optical sensors in contrast with electrical-resistance gages, has the potential to allow the measurement precision and range to scale with the geometry of the device rather than the maximum strain in the instrument. It becomes possible by virtue of these features to produce a better instrument.

A temperature calibration of a temperature/strain transducer based on two strain gauges

AbstractThe operation of a temperature/strain transducer, called “duplex gauge”, based on two electrical resistance strain gauges, that are temperature compensated for use on materials with different coefficients of linear expansion, was briefly recalled. Three steel specimens were equipped with “duplex gauges” and were subjected to temperature variations and no external loads were applied. The chosen experimental setup (i) to measure the outputs of the gauge temperature compensated for steel and titanium silicate and (ii) to impose known temperature, assured good accuracy and precision in evaluation of the temperature calibration curve. The obtained results showed that the strain differences could be considered, at least for the experimental arrangement examined, a linear function of temperature over the range of 20 to over 140°C. The prepared calibration chart, that allows the correction of temperature induced apparent strain, will be used in future research when generic temperature and strain fields are applied to specimens.

Calibration Techniques for Extensometry: Possible Standards of Strain Measurement

Abstract The search for an adequate reference strain measuring technique for use in the materials testing laboratory has spanned several decades. Early methods relied on properly calibrated extensometers. Two examples are Tuckerman's Optical Strain Gage and Bergqvist's highly refined inductive and strain gage based extensometers. Length standards were used to calibrate these devices Initially, mechanical displacement mechanisms were used as extensometer checking devices, but with the advent of lasers, the Michelson interferometer has become a dominant displacement measuring tool Advances in stabilizing hardware and electronic detectors have led to the availability of Michelson systems with a resolution better than 5 nm. Microelectronics manufacturing techniques have been used to replicate entire Michelson systems on a silicon chip Advances in detector technology have also led to interferometric linear encoders with resolutions commensurate to classical interferometers. New techniques are being developed that can report strain and do not require initial calibration by displacement measuring instruments Moire interferometry, using a reference grating frequency of 2400 lines mm, has many valuable traits for providing laboratory strain measurements traceable to the National Institute for Standards and Technology (NIST). Experience with strain calibration by more indicates a measurement uncertainty of less than 0.1% corresponding to an accuracy of better than 1 μm m. Bonded metallic electrical resistance strain gages possess essentially limitless resolution, but accuracy must be established. When properly calibrated, bonded strain gages operating within their elastic range could serve as Class A extensometers. This paper reviews some of the more interesting approaches from the past, analyzes some current methods being investigated, and comments on possibilities for the future concerning the difficult task of finding a suitable calibration technique for use with strain measuring devices.

Numerical analysis of high strain rate splitting-tensile tests

Experimental splitting-tension tests were conducted on 2-in. diameter concrete specimens in a Split Hopkinson Pressure Bar at strain rates of 4.4, 10.6, and 14.7/sec. The specimens were instrumented with electrical resistance strain gages and break circuits to detect crack initiation and growth. Experimental results indicate that there is a shift of crack initiation time relative to the peak stress. Also, experimental strength vs strain rate data reveal that the dynamic tensile strength of concrete is significantly higher than the static tensile strength. A comprehensive numerical analysis was conducted on the splitting-tensile experiments to investigate the effects of varying the uniaxial tensile strength of the concrete on the crack initiation time, stress state, crack growth characteristics, and failure mode in the concrete specimens. The results of the numerical analyses are used to enhance the understanding of concrete tensile strength strain rate sensitivity.

Shear measurement using strain gages under large deformation and rotation

The measurement of shear strain under finite deformation and large rotation by using electrical-resistance metallic foil strain gages is studied both analytically and experimentally. Equations for calculating shear strain and axial and circumferential stretches are derived based on the kinematics of general tension-torsion deformation mode. These equations are applied to analyzing pure torsion experimental data. Comparison is made between results obtained with strain gages and a rotary transducer. It is shown that, in case of large rotation, one simple equation can be used to calculate the shear strain up to 30 percent with adequate accuracy.

Pressure vessel manufacturing: Mechanical analysis of gas bottles with convex end-plates

The three-centre geometry design for the bottom end-plates of gas bottle cylinders is analysed using an approximate method involving Finite Element techniques. Shell elements are adopted to obtain the equivalent stress distribution along a generatrix of the bottle. The maximum stress profile in the bottom zone is plotted versus normalized geometrical parameters. The area which bounds acceptable solutions according to basic Standards Codes is outlined in the diagrams. Experimental measurements by means of electrical resistance strain gauges are compared with the numerical forecasts. The good correlation which was obtained confirms the validity of the proposed approach in dealing with industrial applications.

The heating effect of electrical resistance strain gauges applied to low thermal conductivity materials

Gauge heating effects were examined for the case where an electrical resistance foil gauge is bonded to a low thermal diffusivity material. These include i) the effectiveness of a temperature compensating gauge, ii) the influence on localized creep and relaxation behaviour in the material and iii) the relative order in which the bridge is energised and the testpiece loaded. Calculated temperature distributions are confirmed from experiment. This has enabled recommendations to be made for the allowable specific power corresponding to a given temperature rise in a gauge grid of known configuration.

Free bending fatique of axially pre-loaded spiral strands

The response of a large diameter and a multi-layered spiral strand to an applied moment is considered in some detail for a given mean axial load. Carefully conducted large-scale experiments have cast some light on an interesting phenomenon observed in previously reported bending fatigue experiments The first wire to fail was invariably the wire which entered the socket on the bending neutral axis rather than the wires in the ‘extreme fibre’ positions, as might usually be expected. Using a series of electrical resistance strain gauges placed on the individual wires at the mouth of the socket, the previously reported theoretical predictions that interwire slippage is greatest at the neutral axis (in terms of simple beam bending theory), and least at the extreme fibre positions, have been confirmed. In line with the theoretical predictions reported elsewhere, the test results show that, in general, the onset of deviations from no-slip interlayer shear interaction occurs at rather small levels of bending movements near the socket. Based on the theoretical and experimental findings, which identify interlayer fretting as the primary cause of wire fractures, a newly developed parameter capable of predicting spiral strand free bending fatigue life has been proposed. Unlike the traditional extreme fibre maximum direct stress approaches, the proposed parameter takes the interwire/interlayer fretting phenomenon into account.

Pullout tests of epoxy-coated reinforcement in concrete

The bond strength and slip of epoxy-coated reinforcing bars in concrete have been evaluated by carrying out single pullout and double pullout tests. In extended single pullout tests, slip measurements were made while tensile force was applied to reinforcing bars embedded in concrete. In double pullout tests, 20 cycles of load were applied at levels of steel stress between zero and 0·5 times characteristic steel strength. Strains were measured by electrical resistance strain gauges glued inside the bars. Both epoxy-coated and uncoated bars were used in the investigation, to obtain comparative results. The strain gradient along the bar was found to be less for the coated reinforcement. In general, the epoxy coating was found to increase slip in bond and thereby reduce the bond performance of coated bars.

Strain gauge method for measuring polymerization contraction of composite restoratives

Post-gel polymerization contraction of composite restoratives produces a volumetric change in phase with the development of a modulus of elasticity and distributes contractile stresses through the resin hard tissue interface into the tooth. A new method for monitoring the polymerization contraction of composite restoratives utilizes electrical resistance strain gauges. The strain gauge system was calibrated with dial gauge measurements of the bulk expansion of gypsum products. Three composite types (microfilled, hybrid and posterior) were evaluated for polymerization exotherm, contraction during curing, and contraction for various shades. A 60-s curing time was used. The posterior composite (P-50) demonstrated the lowest exotherm and polymerization contraction. The contraction for Silux Plus dark grey was significantly lower than all other shades of all materials. The strain gauge method appears to be well suited for real-time measurement of the curing process and provides a means for studying the kinetics of polymerization.