The search for an adequate reference strain measuring technique for use in the materials testing laboratory has spanned several decades. Early methods relied on properly calibrated extensometers. Two examples are Tuckerman's Optical Strain Gage and Bergqvist's highly refined inductive and strain gage based extensometers. Length standards were used to calibrate these devices Initially, mechanical displacement mechanisms were used as extensometer checking devices, but with the advent of lasers, the Michelson interferometer has become a dominant displacement measuring tool Advances in stabilizing hardware and electronic detectors have led to the availability of Michelson systems with a resolution better than 5 nm. Microelectronics manufacturing techniques have been used to replicate entire Michelson systems on a silicon chip Advances in detector technology have also led to interferometric linear encoders with resolutions commensurate to classical interferometers. New techniques are being developed that can report strain and do not require initial calibration by displacement measuring instruments Moire interferometry, using a reference grating frequency of 2400 lines mm, has many valuable traits for providing laboratory strain measurements traceable to the National Institute for Standards and Technology (NIST). Experience with strain calibration by more indicates a measurement uncertainty of less than 0.1% corresponding to an accuracy of better than 1 μm m. Bonded metallic electrical resistance strain gages possess essentially limitless resolution, but accuracy must be established. When properly calibrated, bonded strain gages operating within their elastic range could serve as Class A extensometers. This paper reviews some of the more interesting approaches from the past, analyzes some current methods being investigated, and comments on possibilities for the future concerning the difficult task of finding a suitable calibration technique for use with strain measuring devices.
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