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

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Abstract
The paper reviews the principal aspects of an investigation carried out between 1958 and 1962 in order to develop a wire resistance strain gauge for the measurement of steady stresses in stationary and rotating turbojet components operating at temperatures up to 600°C in oxidizing atmospheres. The characteristics of a number of alloys are reviewed and the behaviour of long unbonded wires drawn from nickel-chromium, iron-chromium-aluminium and platinum-tungsten alloys are discussed in detail. The effect of such factors as annealing, ordering and oxidation of the alloy's lattice on the resistance-temperature characteristics of the wire are considered. On the basis of such findings three alloys are selected for test in the form of flat grid strain gauges. The results obtained from a large number of such gauges over periods of time in excess of 200 hours are presented. It is concluded that only wires having low temperature coefficients of resistance and oxidation rates, as well as a high degree of stability, are suitable for the manufacture of strain gauges for the measurement of steady strains. Provided they were suitably heat-treated, some alloys of the nickel-chromium-aluminium, iron-chromium-aluminium and platinum-tungsten systems appeared suitable and the results obtained under typical laboratory and field conditions are presented. In general it appeared possible to prepare strain gauges having a temperature coefficient of resistance corresponding to an apparent strain of less than 30 μin/in degC (900 lb/in2 in steel) and a time-induced drift of less than 3 μin/in h (90 lb/in2 in steel). The accuracy of the measurements is therefore largely dependent upon the accuracy to which temperature compensation may be effected. A number of temperature compensating circuits are examined as well as the effect of such factors as current heating, stray magnetic fields and joints on the final accuracy. The shear strength and insulation resistance are given for a number of adhesives and cements chosen because of their compatibility with the strain gauge filaments.

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