Development of High-Temperature Strain Gauges

Abstract

This paper is restricted to a review of the work carried out in order to develop a strain gauge capable of operating at temperatures up to 1000°C with an inherent accuracy of ±5 per cent. A large number of resistance alloys were tested as unbonded long wires at room temperature and, from the results obtained, a small number were selected for further investigation, in the form of gauges, at high temperatures. The effects of factors such as metallurgical changes, geometric shape and long-term exposure on the behaviour of the gauges were investigated. A number of bonding mediums, some commercially available, were examined with particular reference to creep under load, shear strength and resistance to erosion and thermal shock. Finally some preliminary tests in conjunction with the measurement of steady strains at elevated temperatures were undertaken. It is concluded that the gauge factor is a function of the lattice imperfections of the element wire and, as such, will be temperature-conscious only if the imperfections themselves are affected by temperature variations. In general, any factor affecting the resistivity will affect the sensitivity. The most significant result obtained during the work described is that the gauge factor may be predicted within ±5 per cent at any given temperature, provided certain precautions are observed. Some typical failures under field conditions are discussed as also are the possibilities of operating for protracted periods under steady stress conditions. More stringent requirements for future applications suggest that the wire gauge will be unsuitable, in view of its low resistance in very small sizes, and a possible alternative is briefly outlined.

Resistance Strain Gauges for the Measurement of Steady Strains at High Temperatures

“…The gauge factor appeared to vary at rather less than 0·001 per cent per degC over the temperature range 20-600°C. These results were essentially in agreement with those reported in the earlier investigation (5).…”

“…Characteristics of unbonded wires In view of their high temperature coefficients of resistance at elevated temperatures, the alloys of copper-nickel and chrome-iron tested during the earlier work (5) were not considered for the present application. Alloys of the nickelchromium series appeared suitable and initial tests were carried out on bright annealed Brightray C wires loosely wound on formers and cooled at various rates from IOOO°C.…”

“…In view of this the first system considered during the present work was a semi-empirical one in which the stabilized gauge was cycled over the relevant temperature range in order to determine the temperature-induced apparent strain in the unloaded condition. In operation the apparent indicated strain could be defined as €l = €a+ Ll€ (5) where €l was the apparent strain, €a was the actual strain and Ll€ the empirically determined temperature-induced strain defined in equation (1). The actual strain could therefore be determined by deducting the temperatureinduced strain from the apparent strain.…”

The Measurement of Large Strains Using Electrical Resistance Strain Gages

“…Similarly for moderate plastic strains (up to 0.01 m/m), Kammer and Pardue 9 showed that alloys, such as annealed Constantan, Cupron, and Copel X, have a gage factor K = 2. Likewise Bertoldo 10,11 found that the gage factor of a single wire of various materials tends to 2 in the plastic range.…”

Análise De Viabilidade Técnica De Utilização Da Fibra De Bananeira Com Resina Sintética Em Compósitos

“…This concept came from researches in which the main results were given in terms of experimentally obtained diagrams (ΔR/R) x (ɛ), as conceived by Bertodo (1959), for example. Observing Ohm's law, the resistance is given by Equation 4:…”