Behavior of rock salt, limestone, and anhydrite during indentation

The indentation hardness test is being investigated as a means of studying the behavior of rock under stress sufficiently great to be of geologic interest. In this study, indentation and compression of a limestone, marble, anhydrite, and artificial rock salt are compared. Indentation of these rocks with a 136° pyramidal indenter produces an approximately hemispherical deformed zone beneath the surface indentation. Within this zone the average microscopic character of the rock is similar to that deformed in compression tests at low to moderate confining pressure and room temperature. In detail, the deformation of the rock salt is predominantly due to gliding; both single grains and rock work-hardened just under the indenter. The deformation of the other rocks was due to a combination of gliding and micro-fracturing. Inasmuch as translation gliding in the calcite, as shown by coloration during X-irradiation, occurred in spite of abundant fractures formed at the same time, large frictional forces must have developed at surfaces of fractures and along grain boundaries. Approximate stress and strain fields were found for the deformed zone, from which average values of stress difference, confining pressure, and strain could be determined. These three quantities agreed fairly well with a point on a stress-strain curve obtained from a confined compression test of a cylinder of the same material. Vickers hardness is 3±0.3 times the compressive stress difference which is taken at a confining pressure of one-sixth of the hardness for strain of 7 (±5) per cent. At present, the greatest use of the indentation hardness in geology is in comparative studies of the strength of nonporous, crystalline, relatively isotropic rocks.