Thermo-Mechanical Properties of Granite at Elevated Temperatures and Numerical Simulation of Thermal Cracking

A comprehensive data compilation, in respect to thermo-mechanical parameters, of granite exposed to temperatures up to about 1000 degrees C is presented. Most material parameters experience a significant change with increasing temperature connected with thermal-induced cracking. Some of them (tensile strength, Young's modulus, cohesive strength, thermal conductivity) show a continuous change, while the alpha-beta transition of quartz leads to an abrupt jump in some other parameters (Poisson's ratio, thermal expansion coefficient, specific heat). Based on a compilation of these parameters, temperature-dependent relations have been deduced. These relations are combined with constitutive models based on the classical Mohr-Coulomb model with strain softening and tension cutoff. The obtained new constitutive law is validated by uniaxial compression tests on granite samples exposed to high temperatures up to 800 degrees C. The proposed numerical model is able to duplicate the thermal-induced cracking, which results in reduced peak strength, pronounced softening, and transition from brittle to ductile behaviour. Comparison with lab tests in respect to thermal-induced fracture pattern and stress-strain relations shows remarkable agreement. Simulations-supported also by lab tests-show, that up to about 200 degrees C no macroscopic damage occurs in the heated granite before loading; however, significant macroscopic damage occurs beyond 600 degrees C, which leads to reduced strength after cooling.