Volume 26, Issue 5 (September 1998)
A Potential Optical Standard for Resistance Strain Gages
The laser-based interferometric strain/displacement gage (ISDG) is an optical technique that measures the change in relative distance between two reflective markers on a specimen surface. When the markers are illuminated with a low-power He-Ne laser, interference patterns are formed in space that can be monitored and related to the relative displacement between them. The ISDG has been used in experiments ranging in duration from 10 μs to 1000 h—a dynamic range of eleven orders of magnitude. This noncontacting technique offers some intriguing possibilities for resistance strain gage calibration. It does not have to be compensated for temperature since there is no thermal expansion of the gage. There are no transverse effects since it measures only the strain along the line between the two markers. It can measure biaxial strains if three markers are placed in an orthogonal pattern. With high-speed detectors, the ISDG can have a very high-frequency response. The fringe motions can be tracked by following the maximums or minimums of the patterns so there is no drift in the electronic instrumentation, which makes the ISDG suitable for calibration of gages used to measure creep strain.
This paper presents a brief overview of the ISDG and discusses application areas where it may be useful in evaluating the performance of resistance strain gages. The two applications where it is most likely to be useful are when elastoplastic strains are measured in regions of high gradients or when the strain is dynamic and elastoplastic.