A POTENTIAL OPTICAL STANDARD FOR RESISTANCE-STRAIN GAUGES

The laser-based interferometric strain/displacement gage (ISDG) is an optical technique that measures the change in relative distance betwee n two reflective markers on a specimen surface. When the markers are i lluminated with a low-power He-Ne laser, interference patterns are for med in space that can be monitored and related to the relative displac ement between them. The ISDG has been used in experiments ranging in d uration from 10 mu s to 1000 h-a dynamic range of eleven orders of mag nitude. This noncontacting technique offers some intriguing possibilit ies for resistance strain gage calibration. It does not have to be com pensated for temperature since there is no thermal expansion of the ga ge. 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 frin ge motions can be tracked by following the maximums or minimums of the patterns so there is no drift in the electronic instrumentation, whic h makes the ISDG suitable for calibration of gages used to measure cre ep strain. This paper presents a brief overview of the ISDG and discus ses application areas where it may be useful in evaluating the perform ance of resistance strain gages. The two applications where it is most likely to be useful are when elastoplastic strains are measured in re gions of high gradients or when the strain is dynamic and elastoplasti c.