3D numerical prediction of thermal weakening effects on granite

This paper presents a numerical method to predict the temperature weakening effects on tensile and compressive strength and stiffness of granitic rock. Thermally induced cracking, leading to degradation of the material stiffness and strength, is modelled in the continuum sense by using a damage-viscoplasticity model based on the Drucker-Prager criterion with a rounded tensile cut-off surface. The governing thermo-mechanical initial/boundary value problem is solved with an explicit (in time) staggered method while using extreme mass scaling to increase the critical time step. Rock heterogeneity is described as random clusters of finite elements assigned with the constituent mineral, here Quartz, Feldspar, and Biotite, material properties further randomized by Weibull distribution. In the present approach, only Quartz thermal expansion coefficient is assumed temperature dependent due to its strong and anomalous temperature dependence upon approaching the α-β transition. In the numerical testing, the sample is first volumetrically heated to a target temperature. Then, the uniaxial tension and compression tests are performed on the cooled down numerical samples. The simulations demonstrate the validity of the proposed approach as the experimental weakening effects on the rock strength and stiffness as well as the macroscopic failure modes, both in tension and compression, are realistically predicted in a non-circular way, that is, not using the temperature dependence of any material parameter, save Quartz thermal expansion, as an input data.