Abstract
This paper is a personal account of the origin and development of the twinned-calcite strain gauge, its experimental verification, and its relationship to stress analysis. The method allows the calculation of the three-dimensional deviatoric strain tensor based on five or more twin sets. A minimum of about 25 twin sets should provide a reasonably accurate result for the magnitude and orientation of the strain tensor. The opposite-signed strain axis orientation is the most accurately located. Where one strain axis is appreciably different from the other two, that axis is generally within about 10° of the correct value. Experiments confirm a magnitude accuracy of 1% strain over the range of 1–12% axial shortening and that samples with more than 40% negative expected values imply multiple or rotational deformations. If two deformations are at a high angle to one another, the strain calculated from the positive and negative expected values separately provides a good estimate of both deformations. Most stress analysis techniques do not provide useful magnitudes, although most provide a good estimate of the principal strain axis directions. Stress analysis based on the number of twin sets per grain provides a better than order-of-magnitude approximation to the differential stress magnitude in a constant strain rate experiment.
Highlights
This paper is a personal account of the origin and development of the calcite strain gauge calculation technique along with a discussion of subsequent experimental studies that tested and added to the concept
The experimental results show that the strain gauge calculation is accurate to 20 or 30% of the magnitude value; the order-of-magnitude discrepancy is real
The experimental results show that the odd-signed principal axis orientation is the most accurately located
Summary
This paper is a personal account of the origin and development of the calcite strain gauge calculation technique along with a discussion of subsequent experimental studies that tested and added to the concept. I developed the twinned-calcite strain gauge calculation technique while studying for my PhD with Bill Chapple at Brown University. I decided to measure strain in natural buckle folds to see how it fit with the mathematical models. This was to be conducted using a measurement technique probably known nowhere else, the method of Conel [1]. A bit of serendipity is that John Spang was already at Brown studying for his PhD with Chapple. We learned how to measure calcite twins (later the two of us were occasionally known as the calcite twins) Another bit of serendipity was that John Imbrie was in the department teaching a class on advanced statistical methods
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