CALIBRATION TECHNIQUES FOR EXTENSOMETRY - POSSIBLE STANDARDS OF STRAIN-MEASUREMENT

The search for an adequate reference strain measuring technique for us e in the materials testing laboratory has spanned several decades. Ear ly methods relied on properly calibrated extensometers. Two examples a re Tuckerman's Optical Strain Gage and Bergqvist's highly refined indu ctive and strain gage based extensometers. Length standards were used to calibrate these devices. Initially, mechanical displacement mechani sms were used as extensometer checking devices, but with the advent of lasers, the Michelson interferometer has become a dominant displaceme nt measuring tool. Advances in stabilizing hardware and electronic det ectors have led to the availability of Michelson systems with a resolu tion better than 5 nm. Microelectronics manufacturing techniques have been used to replicate entire Michelson systems on a silicon chip. Adv ances in detector technology have also led to interferometric linear e ncoders with resolutions commensurate to classical interferometers. Ne w techniques are being developed that can report strain and do not req uire 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 moire indicates a measurement uncertainty of less than 0.1%, corresponding to an accuracy of better than 1 mu m/m. Bonded metallic electrical resistance strain gages pos sess essentially limitless resolution, but accuracy must be establishe d. When properly calibrated, bonded strain gages operating within thei r elastic range could serve as Class A extensometers. This paper revie ws some of the more interesting approaches from the past, analyzes som e current methods being investigated, and comments on possibilities fo r the future concerning the difficult task of finding a suitable calib ration technique for use with strain measuring devices.