Development of constitutive parameters from true triaxial tests performed on Castlegate Sandstone

A series of true triaxial constant mean stress tests was performed on Castlegate Sandstone with the intent of determining the effect of the intermediate principal stress on localization. Tests were performed by first hydrostatically loading the specimens to the desired mean stress (30, 60, 90, 120, 150 MPa) then switching the test mode to maintain constant mean stress. This was done by releasing the minimum principal stress, increasing the maximum principal stress and varying the intermediate principal stress in proportion to achieve the desired stress state. The stress states were defined by maintaining a constant Lode angle throughout the test. Five Lode angles were tested ranging from axisymmetric compression to axisymmetric extension (30, 14.5, 0, -14.5, -30°). Results showed that as the Lode angle decreased the loads required to cause localization decreased and the band angle (defined as the angle between the band normal and the direction of maximum compression) decreased with increasing mean stress.

Comparing the experimental results with localization predictions required development of constitutive parameters from the experimental data. This resulted in development of a strain separation process to determine elastic and plastic constitutive parameters from the experimental data. The strain was separated into four components: A) strain due to change in stress at constant modulus, B) strain due to stress dependence of the modulus, C) strain due to plastic strain dependence of the modulus, and D) plastic strain. Information derived from the strain separation process was used to develop constitutive parameters; which were applied to the Rudnicki and Rice (1975) localization criterion and the results were compared to experimental deformation bands. Results indicated that away from the axisymmetric stress states, at low mean stress, the strain separation process provides results which correlate well with experimental results. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.