A THERMO-PLASTIC CONSTITUTIVE LAW FOR BRITTLE-PLASTIC BEHAVIOR OF ROCKS AT HIGH-TEMPERATURES

Mechanical properties of rocks change under the influence of temperatu re. Stress at the onset of yielding, ultimate strength, dilatancy, str ain hardening and softening, and the confining pressure at brittle-duc tile transition are all reduced by the increasing temperature. This st udy presents a framework of constitutive modeling of thermo-brittle-pl astic behavior of rocks which encompasses these changes. The constitut ive law is based on a thermo-plasticity theory first proposed for meta ls by PRAGER (1958). Two phenomenological mechanisms have been identif ied as central for the modeling: temperature dependence of the yield l ocus (thermal softening), and temperature dependence of the strain-har dening function (thermally enhanced ductility). Material parameters fo r two rocks, Carrara marble and Westerly granite, were determined on t he basis of additional hypotheses. These parameters are used in numeri cal simulations of triaxial tests at different temperatures. The obtai ned stress-strain curves compare well to the experimental results. The changes with temperature in the stress at the onset of yielding are m ore accurately reproduced than the evolution of hardening or softening . Suggestions for possible improvements and future research directions are indicated.