Strain analysis of experimental superposed deformation using calcite twin lamellae

Abstract

Specimens of Indiana limestone have been experimentally deformed twice with the maximum compression direction of the second deformation parallel, perpendicular, or at 45° to the maximum compression direction of the first deformation in order to determine how useful the least-squares strain-gage technique for calcite twin lamellae ( Groshong, 1972, 1974) is in mapping superposed deformations. The superposed deformations were achieved by axially shortening cylindrical specimens (4.76 cm in diameter by 10.00 cm long) to 5.3% strain in dry triaxial compression tests at 100 MPa confining pressure, 10 −4 sec −1 strain rate, and room temperature. Each of these deformed specimens was then cored parallel, perpendicular, or at 45° to the load axis to obtain smaller cylindrical specimens (1.25 cm in diameter by 3.10 cm long) for subsequent deformation under identical experimental conditions with axial shortenings ranging from 2.8 to 8.3%. The straingage technique permits one to compute not only the principal strains associated with the total superposed deformation of the specimen but also to compute an expected shear strain for each measured twin set. Since the sense of shear for twin gliding in calcite is fixed (positive), unfavorably oriented twins for a computed strain tensor yield apparent negative shear strains. Separate analysis of the measured twin sets on the basis of positive and negative expected shear strains permits the recognition of each of the two perpendicular superposed deformations, but for coaxial superposed deformations the technique calculates only the total strain of both deformations. For the single and coaxial deformations the calculated strains are within 10% of the experimental axial compressive strains and the angular difference in direction is only 3°. For perpendicular superposed deformations the error in the magnitude and direction of the calculated strains relative to the experimental strain are twice as large as those for the single or coaxial deformations. When the maximum compression direction of the second deformation is 45° to the maximum compression direction of the first deformation, these errors are four times larger.