Abstract
This paper describes the design and development of a pneumatic extensometer suitable for the measurement of axial strains of the order 10-5 over a range of ±20 per cent, using a 1-in. gauge length. The pneumatic gauge is capable of measuring strains less than 10˜-5 over a range of approximately 4 times 10-3 thus ensuring adequate measurement of elastic and initial plastic strains. A micrometer screw is used for adjustment to cover the large range and is suitable for measurement of gross plastic strains. The mechanism of the instrument was designed to comply with the following requirements: (1) magnification linear with strain, (2) nozzle and micrometer spindle to be maintained parallel, (3) insensitive to bending strains, and (4) accommodate changes in specimen diameter without adjustment. Consideration of these requirements led to the choice of a parallel-motion mechanism made in two halves to allow for changes in specimen diameter. The measuring units were so placed as to measure the mean strain between the gauge points. The method of calibration is described and also the method for analysing the results for comparison with the theoretical expression derived. This calibration and subsequent use of the instrument have shown that all the requirements listed above have been adequately satisfied.
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