Determination of the subcritical crack growth parameters in rocks using the constant stress-rate test

Subcritical crack growth plays an important role in evaluating the long-term stability of structures in rocks. The characteristics of subcritical crack growth can be described by a relationship between the stress intensity factor and the crack velocity. This paper presents the results of studies conducted to validate the constant stress-rate test for determining subcritical crack growth parameters in rocks, compared with the conventional testing method, the double torsion test. Various types of specimens including the Brazilian disk, three-point bending, grooved disk, single edge notched bend, and compact tension specimens have been tested on Coconino sandstone under the constant stress-rate scheme and the subcritical crack growth parameters have been determined. The results of the constant stress-rate test are in good agreement with the results of double torsion test. More importantly, the stress-rate tests can determine the parameter A with a much smaller standard deviation than the DT test. Thus the constant stress-rate test seems to be a valid test method for rocks. The results show that the parameter n is almost constant regardless of the test method and specimen geometry, but the parameter A varies with the test method and specimen geometry.