Effect of pressure on electrical resistance strain gages

The pressure effect is defined as the indicated strain response minus the expected response of a strain gage under hydrostatic pressure. The problem is that the expected strain response only considers the strains at the test surface and neglects the grid compression perpendicular to this surface. In this paper, this effect will be included in the analysis, and a simple equation is derived relating the pressure effect to the grid compressibility. The relation is tested by comparing predictions with literature data. It turns out that for most constantan gages the pressure effect is about 0.55/MPa and is independent of the grid design. Furthermore, it is shown that the alternative way of calculating the pressure effect from gage factor and transverse sensitivity (using Bridgman factors) does not compare well with measurements.This research was undertaken while the author was at the University of Salerno, Italy.     

Transverse sensitivity of bonded strain gages

Transverse sensitivity of strain gages usually causes unavoidable errors in two-dimensional stress analyses. Formulas have been derived for correcting these errors but involve transverse-sensitivity values which, in most cases, were not available to the user. This paper describes a method for determination of transverse-sensitivity values using a testing apparatus specially built for this purpose. Results from testing a wide variety of wire and foil strain gages are tabulated and mechanisms of the foil-gage transverse sensitivity discussed.     

Effects of pressure on small foil strain gages

This paper describes the behavior of small foil strain gages under high pressure. Effects of pressure were determined and calibration curves were established in prelininary experiments. The calibrations were then used for correcting measured strains in pressure vessels. Preliminary experiments at room temperature were conducted on small foil strain gages for pressures up to 35,000 psi. The effects of pressure on the gages bonded with a cynoacrylate contact cement, a room-temperature epoxy cement, a high-temperature epoxy cement and a filled epoxy resin were evaluated. Because the contact cement was least affected by pressure and was easiest to apply, it was chosen for use in successive experiments with different gage installations. Calibration curves were determined for strain gages of 0.031-, 0.062- and 0.125-in. gage lengths. The compensating gages were under atmospheric pressure. The calibrations included the pressure effects of gages bonded on both concave and convex surfaces, and the effect of tensile prestrains. Data could be duplicated for successive pressure tests and for several gage installations. The calibration curves proved to be an effective way for obtaining accurate readings from the foil strain gages bonded internally to a pressure vessel.     

Transient response of bonded strain gages

Transient-response characteristics of bonded strain gages have been studied by measuring the elastic step wave produced in a steel bar. A quenched-steel bar with a circumferential notch is stretched statically along its axis until it fractures at the notch. Thus a new cross section is suddenly created in a bar under static tension. At this moment, a sharp step wave of zero stress is produced from the new section, and it travels in the bar at the velocity of sound. By measuring this step wave with strain gages, it is shown that the rise time of the gage itself is less than 0.5 μs+0.8L/c, whereL is the gage length andc is the velocity of longitudinal elastic wave in the bar.     

Vulcanized-rubber protection for strain gages in a seawater environment

New materials and techniques are described for using vulcanized rubber to protect strain gages installed on the components of a controllable-pitch propeller (CPP) system of a Navy destroyer. Strain gages protected by vulcanized rubber have survived many months of full-scale sea trials of the CPP system.     

Resistance strain gages as physiological transducers on trees

A tree responds to its environment in several ways. One important way is through changes in the rate of transfer of water through the stems of the tree as it transpires. These changes induce extremely small variations in internal water stress and, therefore, in the size of the capillaries through which the water moves. The overall effect is minute changes in the diameter of the stems. By monitoring these changes, it is possible to observe an immediate response of the tree to its environment. This paper describes how electrical-resistance strain gages, bonded directly to the bark of a living-tree trunk, were used to measure these changes. It discusses bonding techniques, circuit design, instrumentation and the response of a red-maple tree to diurnal variations in its environment. Substantial changes in the circumference of the trunk, as large as 1900 microstrains, were observed on a 4-cm-diam tree during a daily cycle when the tree was under physiological stress. The overall results indicated good correlation with classical botanical theory and prior experimental data on the subject. In addition to its primary purpose, the paper illustrates the benefits possible from interdisciplinary cocperation and interaction between the fields of experimental mechanics and physical botany as related to forestry. The method described has potential importance in many aspects of tree husbandry. These include capabilities for remote sensing of forest-tree growth, maximizing seedling growth in large nursery operations, and efficient water management in orchards for maximum productivity with minimum irrigation. It may also provide a helpful new technique for use in basic studies in plant and tree physiology.     

Development of temperature-compensated resistance strain gages for use to 700°C

This paper describes the development and evaluation of temperature-compensated resistance strain gages for use to 700°C (1292°F). These gages included single-element gage and double-element gages. The filament of single-element gages was fabricated from specially developed Fe?Cr?Al?V?Ti?Y alloy wire. When bonded to high-temperature Ni-based alloy GH30, the apparent strain from room temperature to 700°C was less than 1800 μm/m. Active grids of double-element gages were fabricated from specially developed Pt?W?Re?Ni alloy wire and Pt-W9.5 wire. The compensating elements were fabricated from Pt?Ir20 alloy wire. When bonded to different high-temperature alloys, by proper adjustment of the ballast resistor in series with the low-resistance Pt?Ir element, apparent strain in the related temperature range was less than 1000 μm/m. Equations to compute the allowable deviation of resistance of compensating element and of ballast resistor for the purpose of attainment of certain measurement accuracy were derived. This paper should be of particular interest to those involved in strain measurement at temperatures to 700°C.     

The effects of high pressure on foil strain gages

The purpose of this investigation was to determine whether a linear pressure-strain response was possible for gages subjected to hydrostatic pressures to 140 ksi. This was motivated by the desire to use this information to measure the elastic-plastic behavior of material at the inside surface of thick-walled cylinders subjected to high internal pressure. This paper shows the effects of fluid pressure to 140 ksi on four different types of foil strain gage. Linear pressure-strain curves were obtained for these gages mounted on flat surfaces of tungsten, steel and aluminum specimens. The linear strains of several materials due to pressure are compared with the compressibility constant (1–2ν)/E as calculated from experimentally determined values ofE and ν, whereE is defined as the modulus of elasticity and ν is Poisson's ratio. Experimental results show the percent deviation between the constants to be a function of the material, being greatest for tungsten and least for aluminum. The fact that a linear pressure-strain response was obtained makes it possible to correct the readings for strain gages mounted on flat surfaces of materials subjected to direct hydrostatic pressure. Temperature effects as a function of pressurization rate were investigated. Various gage failures encountered along with photomicrographs showing probable causes are presented.     

Pressure effects on the response of foil strain gages

The effect of pressure on the gage factor (the strain coefficient of resistance) for a bonded constantan gage was investigated by measuring the compliance of single-crystal quartz in the direction of itsc-crystallographic axis as a function of pressure and by comparing these results with those derived from ocoustic-velocity data. The gage factor is indicated as increasing with pressure at a rate of approximately 0.4 percent per kilobar. This increase may be due to one or both of two factors: an increase in the resistivity-strain tensor or a thinning and stiffening of the gage cement.     

The response of strain gages to longitudinally sweeping strain pulses

The current state-of-the-art for estimating the maximum rise time of a strain gage which is subjected to an axially sweeping strain pulse ist _rg≤0.8L/c+0.5μsec wheret _rg is the 10/90 rise time of the strain gage,^* L is the gage length andc is the strain-pulse velocity. This paper shows that the effect of the 0.8L/c term can be significantly reduced by utilizing an analytical compensation technique. In addition, it is shown that the 0.5 μsec additive constant can be reduced to approximately 0.1 μsec by reinterpreting data published by a previous author.     

Attainable accuracy of strain measurements with thum-sevenson-weiss mechanical-inductive strain gages

This paper determines the accuracy that can be attained with the Thum-Svenson-Weiss mechanical-inductive strain-measuring method. After stating advantages and disadvantages of this measuring method, the strain gage, the electrical parts and the calibration equipment are described. The determination of accuracy is based on the laws of statistics considering all the errors as distributed completely at random. The behavior of the electrical parts, of the mechanical parts, of the strain gage and of the calibration equipment are separately investigated. It is evident from these investigations that the strain measurement with the Thum-Svenson-Weiss mechanical-inductive strain gages is one of the most accurate strain-measuring methods to date.     

Use of FBG strain gages on a pipeline specimen repaired with a CFRE composite

Experimental Techniques - Laboratory tests were conducted to understand and describe how the reinforcement layers of a carbon fiber epoxy composite material can enable a steel line pipe specimen...     

Response of foil strain gages to hydraulic pressure

A series of tests to determine the effect that hydraulic pressure has on the output of electrical-resistance foil strain gages is described. Tests were conducted on Micro-Measurements gages, type EA-13-125BT-120 for pressures up to 20.7 MPa and frequencies up to 150 Hz, both for purely dilatational strain as well as various combinations of dilatational and distortional strains. The results showed that gage performance due to the combined effects of hydrostatic pressure and distortion of the structure differed from the condition predicted when pressure and distortion were considered separately. Rate of pressurization (and, hence, strain rate) had no measurable effect within practical limits.     

Calibration of high-temperature strain gages with the aid of a clamping device

Strain measurements at high temperatures are increasingly required to record the loads on components under operating conditions. This paper describes experience gained with using a clamping device to calibrate high-temperature strain gages which allows the precision of such measurements to be increased far beyond the data given by the strain-gage manufacturer.     

Effects of ultrahigh vacuum on the response characteristics of strain gages

Strain gages are extensively used in spacesimulation research; yet, little if any information has been reported about the environmental effects on the gage installations themselves. Since ultrahigh vacuum affects the physical and chemical properties of materials, one may expect that the response characteristics of strain-gage installations will also be affected. A study was initiated to determine the behavior of a number of strain-gage installations subjected to ultrahigh-vacuum environments. This paper presents the results of the first phase of the program. The gages and adhesives were selected to provide optimum chance for failure in order to establish a time parameter for following tests and, more importantly, to verify the extent and nature of failure possible under the environmental test conditions. Data were obtained on a number of strain-gage-per-formance characteristics. The performances of the gage installations varied widely being dependent in part upon gage and adhesive composition, whether a gage was used as an active or inactive device, and the level of strain to which a gage was subjected. Detailed pre- and post-test examinations showed that there was little permanent damage to any of the installations.     

Elevated-temperature effects on strain gages on the YF-12A wing

A general study is made of the effects of structural heating on calibrated-strain-gage load measurements on the wing of a supersonic airplane. The primary emphasis is on temperature-induced effects as they relate to slope changes and thermal shifts of the applied load/strain relationships. These effects are studied by using the YF-12A airplane, a structural computer model, and subsequent analyses. Such topics as the thermal environment of the structure, the variation of load paths at elevated temperature, the thermal response characteristics of load equations, elevated-temperature load-measurement approaches, the thermal calibration of wings, and the correlation of strains are discussed. Ways are suggested to measure loads with calibrated strain gages in the supersonic environment.Paper was presented at 1978 SESA Spring Meeting held in Wichita, KS on May 14–18.     

Determination of thermal-expansion characteristics of metals using strain gages

A method has been developed to measure thermal-expansion characteristics of metals using bonded resistance strain gages. The method allows rapid and accurate determination of expansion properties at similar or lower cost (depending on the particular application) than conventional dilatometric techniques. Other advantages include elimination of a need for perfectly flat sample material, elimination of specimen machining, and applicability to structures and components. To utilize this technique, the ‘apparent strain’ of the gage is determined by attaching it to a ‘standard’ material for which the thermal-expansion characteristics are accurately known and subtracting the known thermal response of the material from the total gage output. ‘Apparent strain’ is therefore the temperature-induced output of the gage when bonded to a material having a thermal-expansion coefficient of zero. When the gage is then attached to a test material and cycled through the same temperature range, this ‘apparent strain’ is subtracted from the total gage output to obtain the actual unit-length change of the test material. Using this technique, mean-expansion coefficients of experimental alloys were determined over the temperature range ?320°F (?196°C) to room temperature.     

Investigation of strain gages for use at cryogenic temperatures

Strain sensitivity and resistance changes in Advance, Karma, Budd Alloy, Nichrome V, and stabilized Armour D foil gages, and Nichrome V-platinum temperature-compensated gages were evaluated at cryogenic temperatures. The more promising gage types, as determined from these studies, were tested to strain levels up to 11,000 μin./in. with tensile specimens. The other performance characteristics that were investigated included zero drift, creep, hysteresis, linearity and gage element size effects. A mounting method developed for foil gages to be used at cryogenic temperatures is described.