High-temperature high-elongation resistance strain gauges

Abstract

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.