Electrical Resistance Strain Gauges Research Articles

Correcting for the effects of temperature on strain gauges —‐use of computer–logger on Milford Haven Bridge project

The effects of temperature changes on strain gauge readings depend partly on the characteristics of the gauge and measuring equipment, and partly on the thermal properties of the structure being tested. The effect on the gauge can be numerically corrected, or compensated for by experimental techniques, and if required, the effect of temperature changes on the structure can be reduced by careful selection of the time of reading. Correction of Demec, vibrating wire and electrical resistance strain gauge outputs is discussed.Experience with a computer controlled data logger used to correct for thermal effects on a 1/4‐bridge electrical resistance strain gauge installation is described. The use of this system on the Milford Haven Bridge is estimated to have reduced potential temperature errors from between ±100 to 200 μstrain to about ±20 to 30 μstrain.

Large-Scale Model Test of Drilled Pier in Sand

A technique is presented for assessing the magnitude of skin friction for pier foundations in sand. The technique employs the results of interface direct shear tests between sand and the construction material; the results are presented in hyperbolic equation form. The test pier was 16 in. (407 mm) in diameter and 15 ft (4.6 m) long. The test pier was heavily instrumented with Carlson stress meters, electrical resistance strain gages, and calibrated hydraulic rams on the top and bottom of the pier. Comparisons of predicted and measured values of skin friction showed agreement within 5%. Bonded resistance strain gages on the vertical steel reinforcement are the most reliable instrumentation method. The magnitude of skin friction for point bearing piers in sand in prototype installations may approach 20% - 30% of the total load capacity. /ASCE/

Strain-gage instrumentation for ammunition testing

This paper deals with a feasibility study on the use of electrical-resistance strain gages for quality-assurance testing of ammunition. A special combination of strain gages was selected to measure the quantity ( $$ \in _\theta + v \in _z $$ ) on the outer surface of a test barrel. This combination of strain gages was essentially a stress gage which has several advantages over commercially available types. The combined strain signal was proportional to $$\sigma _\theta $$ which was related to the internal ballistic pressure. The results of ballistic firings and hydraulic tests are reported which demonstrate the feasibility of this method for acceptance testing of ammunition.

Behaviour of brazed pipe flanges with separate clamping rings

Three pairs of aluminium-brass 100-mm-bore flange models with different surface finishes of the joint faces have been subjected to known bolting forces and internal pressures up to seven times the rated pressure at room temperature. Hard and soft flat gaskets were used and it was found that the joint efficiency was independent of gasket material, surface finish, and initial bolt tension. Surface strains on the outer surfaces of the tubes and flange rings were measured with electrical-resistance strain gauges where these could be fitted. These results are also presented. The flanges did not fail; tests ended when the loose steel rings touched owing to excessive deformation, so that the gaskets were prevented from sealing.

Strain distribution within compressed circular cylinders

A theoretical solution has been obtained for the stress distribution throughout a cylindrical specimen loaded in compression, which takes into account different degrees of friction at the end surfaces. To check the solution experimentally, a cylindrical model was constructed from epoxy sheets, with electrical-resistance strain gages embedded between the sheets and also bonded to the cylindrical surface. With the specimen under axial compression, strain measurements were recorded within the elastic range of the epoxy. Reasonable agreement with the theoretical solution was observed for the two types of end conditions. In one case the cylinder was in direct contact with the platens of the testing machine, in the other case a Teflon sheet was placed between the two surfaces.

Strain gauge measurement on plastics

A description is given of tests carried out to investigate the problem of signal instability encountered in the use of electrical resistance strain gauges on plastics. Time response from switch‐on is indicated for 120 ohm wire and foil gauges of various types and sizes, bonded to epoxy resin and energised at 25 V. Wire gauges show a reduction in signal and foil gauges an increase. Results of subsequent tests demonstrate the effects of initial strain, energising voltage, thermal diffusivity of the specimen material and gauge grid area. An indication of gauge surface temperature obtained by thermocouple, is related to degree of drift and energising voltage. Signal stability is improved by coating the gauge with a wax/aluminium‐powder mixture. A data logger with pulse excitation is recommended as the best system and limiting energising voltages are stated.

Plastic behaviour of oblique flush cylindrical nozzles in cylindrical pressure vessels: An experimental investigation

Results of seven tests on cylindrical pressure vessels with flush cylindrical nozzles are reported. Two of the test specimens had radial nozzles and the rest had nozzles oblique to a radial line but normal to the longitudinal axis of the vessel. The specimens were machined out of solid 8 in diameter aluminium alloy bar. Electrical resistance strain gauges were used to measure the distribution of strain. Deflections were measured using clock-gauges and displacement transducers.

Reinforced discontinuous bend subjected to uniform internal pressure: Effect of reinforcing ring stiffness

This paper deals with the results of an experimental study undertaken to assess the stress-reducing effect of internally bracing a traditional constant-depth elliptical reinforcing ring. Stress profiles, calculated from strain readings recorded by electrical-resistance foil strain gauges bonded to inner and outer surfaces of the cylindrical limbs of model Araldite bends, are presented for four reinforcement conditions: constant-depth (traditional) elliptical ring; constant-depth ring with internal cruciform bracing; constant-depth ring with a single brace across the minor diameter; perfectly rigid reinforcing plate. When a traditional reinforcing ring was used the greatest stress-concentration factor (referring to circumferential membrane stress remote from the discontinuity as unity) was found to be 4.15. This was reduced to 2.90 by the use of a ring with a single minor-diameter brace and was further reduced to 2.70 when a ring having a cruciform brace set across major and minor diameters was used. The effect of an infinitely rigid reinforcement was to reduce the maximum stress-concentration factor to 2.60.

The heating effect of strain gauges on Araldite models

The rise in temperature was measured in the locality of an excited electrical resistance strain gauge fixed to the edge of unloaded Araldite plates, during a period of 30 min. The tests were carried out in air, in transformer oil and also when the gauge was encapsulated in the Araldite.

The diffractographic strain gage

A method for measuring strain using diffraction of light from a single aperture is described, and results of a comparison tensile test with an electrical-resistance strain gage are presented. The “diffractographic strain gage” is shown to have high sensitivity, linearity, accuracy and temperature compensation and the ability to operate in a variety of hostile environments. It is furthermore simple, inexpensive, and the data can be collected by eye without assistance from further instrumentation.

An Experimental Study on the Deformation of Materials under High Hydrostatic Pressures : 2nd Report, On Torsional Deformation

Many papers on the behaviour of materials under hydrostatic pressure have been published; but it is still not sufficiently clear whether hydrostatic pressure has any effect on the elasto-plastic properties of metals. This may possibly be due to the difficulties in the measurement of the stress and strain under hydrostatic pressure, where various measuring errors are likely to be involved. In the present investigation on torsion, in order to eliminate the largest conceivable of these measuring errors, namely those due to friction and distortion of torsion grips, a new apparatus is introduced. In this apparatus, the specimen, with a torque bar connected in series, is twisted in a high pressure vessel and both the strains of the specimen and torque bar are measured by electric-resistance strain gages. The effects of hydrostatic pressures up to 4000 kg/cm2 on the deformation of aluminum and a zinc alloy and on the lower yield point of low carbon steel were investigated, and the results are as follows: (1) With aluminum, in both annealed and extruded states, there is no effect on the shape of stress-strain curve. (2) With low carbon steel, there is no effect on the lower yield point. (3) With a zinc alloy, there is evidence of a little rise in the stress-strain curve.

Measurement of residual stresses in rolling contact

This paper describes a technique for the experimental determination of residual stress. Starting with a stressed part for which the stress distribution is required, the first step is to cut two mutually orthogonal strip specimens from the part. Each specimen is analysed by repeatedly carrying out two basic operations: removal of residually stressed material in thin layers of known depth and measurement of the resulting strain in the specimen. With these data, the stress distribution in each specimen can be inferred. If the strain imparted to each specimen by its removal from the original part has been measured, then the stress distribution in the original part is also known. Layer removal by electropolishing, combined with strain measurement using electrical resistance strain gauges enables automated data recording with an attendant reduction in time and labour compared with previous techniques. This method leads to improvements in productivity of residual stress measurements compared with previous techniques and may be applied to a variety of different geometries. Three different experimental configurations are described and some experimental results obtained with them are presented.

A strain-gauge force platform for studying human movement.

For a study of the postural aspects of human movement a strain-gauge force plarform was designed to meet the following requirements: (1) that it measure horizontal and vertical forces independently; (2) that it have a range of 800 lb. in each axis; (3) that it be as rigid as the average floor; (4) that it be compact and movable, and compatible wirh standard instrumentation and recording systems. In carrying out the design, electrical resistance strain gauges were used because of their versatility in application and because their resolution is limited only by the instrumentation driving them. The sensing element is an aluminum tube with .25-in. wall thickness and 2.50-in. outer diameter. Wall thickness is reduced somewhat in the areas where gauges are mounted in order to increase sensitivity. The tube is supported on the ends by aluminum plates M in. thick. The loading platform is attached in the center. The strain gauge systems, which have outputs of 1.3 mv. at full load, are so arranged and wired that they measure and add the rwo forces which are transmitted to the ends of the tube in each plane. In this way the output of the unit is independent of the location at which the load is applied.

THE EFFECT OF CROSS‐SENSITIVITY ON THE DETERMINATION OF STRESS BY STRAIN GAUGES

Some free‐filament wire strain gauges used at high temperatures have a higher than normal cross‐sensitivity so that errors in the principal stress calculated by standard formulae may be significant. The magnitude of the error is shown and the formulae are modified to obtain true principal stresses. As the modifications are simple, all results obtained from electrical resistance strain gauges can be corrected similarly.

Finite-element stress analysis of a non-linear problem: A connecting-road eye loaded by means of a pin

The stress analysis of a connecting rod under a tensile load represents a non-linear problem, since the distribution of the forces, acting between the eye of the rod and the pin, changes as the loading proceeds. This is due to the tendency of the eye to deform, thus wrapping itself around the pin as the clearance between the parts is reduced. In consequence, the stress pattern in the eye also changes. Such a problem has been solved with a linear elastic finite-element computer programme. A connecting-rod eye was idealized as a plane-stress problem and special elements were introduced to connect the pin and rod together. These elements were assigned stiffnesses which were changed in a prescribed manner after each iteration until compatibility of forces and displacements, with due allowance for initial clearances, was obtained at each connecting point. Although eight iterations were required to achieve the correct displacements, it was found that the circumferential stresses did not change appreciably after the first few iterations. Good agreement was found between the calculated stresses and those measured in a laboratory test with electrical-resistance strain gauges.

High-temperature high-elongation resistance strain gauges

The paper outlines an investigation undertaken in an attempt to evolve an electrical-resistance strain gauge for the measurement of slowly varying strains in the range ±5 per cent over the temperature interval 20°C-400°C in oxidizing environments. A critical review of published work suggested that few pertinent data existed for plastically strained conductors and that only alloys having a high degree of electrical stability and a strain sensitivity close to 2·0 would be suitable. Two alloys appeared to possess such properties, namely one containing nearly equal molecular proportions of nickel and copper and one approximating to 4(Ni3Cr)Ni3Al. Subsequent tests on long free spcimens revealed that the electrical stability was dictated by metallurgical factors and in some way related to the fatigue ductility. Only the nickel-chromium-aluminium alloy was adequately stable, electrically and metallurgically, over the temperature range of interest, provided it was in the disordered annealed condition. Measurement errors became too high at fatigue strains above ±1·5 per cent at 400°C. Tests were carried out on flat-grid gauges with bonding media having shear strengths in excess of about 1000 lb/in2, this being the apparent limiting value for faithful transfer of strains of up to ±1·5 per cent from the specimen to the strin-sensing grid. These largely corroborated earlier tests on free specimens and suggested that strains of up to ±11/2 per cent could be detected with an error of about 10 per cent at 400°C. The zero drift amounted to an equivalent strin of 400 μin/in and this required separate correcton. The problems of temperature compensation for slow cycling rates were similar to those met in conventional steady-strain measurements with electrical-resistance strain gauges and were in no way alleviated by the high strain output available.

Magnetostriction and Anomalous Thermal Expansion of Chromium

Crystal distortions in Cr associated with antiferromagnetic ordering have been established using electrical-resistance strain gauges. Between 123 and 311\ifmmode^\circ\else\textdegree\fi{}K the observations confirm the existence of an orthorhombic structure associated with the presence of a transversely polarized spin-density wave.

高張力鋼(80kg/mm<SUP>2</SUP>級)溶接部の応力―ひずみ特性

For purposes in evaluating the elastic and plastic behaviors of weld zones, tensile tests and alternating plane bending tests beyond the elastic range were carried out on the transverse-weld specimens of a 80 kg/mm2 class heat-treated high-strength steel, and the individual stress-strain curves for the weld metal, heat-affected zone and base metal were observed in detail. The strain was measured with electric-resistance strain gauge of 1 mm gauge-length sticked on each zone. Also, these curves were compared with those examined on the quenched or and tempered specimens of the base metal. The tests revealed the following:The over-heated zone, which exhibited high hardness and low ductility, began to yield gradually at considerably small stress as against the abrupt yielding of the base metal, the 0.02% offset elastic limit of the zone being about half of the yield point of the base metal. This low elastic limit again resulted in oilquenched specimens, whose microstructures were similar to that of the over-heated zone in the weld.In the cyclic bending each stress-strain hysteresis loop for the weld metal, softened zone and base metal widened with decrease in elastic limit as cycling continued, while the loop for the over-heated zone became rather narrow due to strain hardening in similar way for the quench-hardened base metal.

AN ELECTRICALLY WEIGHED LYSIMETER FOR MEASURING EVAPORATION RATES

Abstract An electrically-weighed lysimeter using electrical resistance strain gauges has been developed to measure the daily and diurnal rates of evaporation of water from crop and/or soil surfaces. The lysimeter, with a plan area of 0.4 m 2 , records changes of weight of the order of 20 g (equivalent to 0.05 mm depth of water) over a range of 20 kg in a total weight of 110 kg. Continuity of vegetation between the lysimeter and the surrounding area is facilitated by confining the weighing mechanism to the space directly beneath the container. The weighing device consists of a mechanical system that supports the soil container and offsets a pre-determined proportion of its weight, and an electrical system that registers changes in load as changes in electrical output. The changes in load are transmitted to the sensing elements of a load transducer consisting of two machined aluminum weighbars that become loaded as beams after filling the lysimeter with soil. Each weighbar has a simple cantilever at either end to which electrical resistance strain gauges are affixed. The strain gauges are wired into a Wheatstone bridge circuit and the output is registered on a digital voltmeter and/or potentiometric chart recorder. The output voltage changes by an amount that is proportional to the change in load. Fifteen of these load cell lysimeters have been installed in a vinery-type glasshouse.

An electrically-weighed lysimeter for measuring evaporation rates

An electrically-weighed lysimeter using electrical resistance strain gauges has been developed to measure the daily and diurnal rates of evaporation of water from crop and/or soil surfaces. The lysimeter, with a plan area of 0.4 m 2, records changes of weight of the order of 20 g (equivalent to 0.05 mm depth of water) over a range of 20 kg in a total weight of 110 kg. Continuity of vegetation between the lysimeter and the surrounding area is facilitated by confining the weighing mechanism to the space directly beneath the container. The weighing device consists of a mechanical system that supports the soil container and offsets a pre-determined proportion of its weight, and an electrical system that registers changes in load as changes in electrical output. The changes in load are transmitted to the sensing elements of a load transducer consisting of two machined aluminum weighbars that become loaded as beams after filling the lysimeter with soil. Each weighbar has a simple cantilever at either end to which electrical resistance strain gauges are affixed. The strain gauges are wired into a Wheatstone bridge circuit and the output is registered on a digital voltmeter and/or potentiometric chart recorder. The output voltage changes by an amount that is proportional to the change in load. Fifteen of these load cell lysimeters have been installed in a vinery-type glasshouse.