| The thermal damage of rocks is a key and frontier scientific issue in rock mechanics and rock engineering.In this thesis,the mechanisms and a mechanical model of rock thermal damage are explored by a developed thermal damage assessment system and numerical modeling.The main contributions and innovations of this thesis consist of five aspects: a method of evaluating thermal damage after temperature treatment by using ultrasonic waves,a method of evaluating thermal damage under high-temperature by using acoustic emission,a thermal damage evolution model for both heating and cooling processes,two mechanisms for heating damage and cooling damage,and a coupled numerical model considering thermal damage for the long-term stability of deep geothermal engineering.First,a system integrating ultrasonic testing and acoustic emission monitoring is developed for rock damage evaluation after high-temperature treatment and under realtime temperature.Following this,for damage evaluation,a multifrequency ultrasonic approach is proposed for extracting the static initial modulus of rock.Both the physical experiments and numerical simulations demonstrate the rationality of the proposed approach.A damage assessment of rocks is conducted after temperature treatment by the proposed approach,and reasonable results are obtained.Moreover,a real-time thermal damage model for rock is established based on acoustic emissions.The proposed model can describe the damage evolution of rock during both the heating and cooling stages.Using a thermomechanical coupled distinct lattice spring model,the variation in the damage variable in the experiment is reproduced,thus proposing the damage mechanisms for rock: the nonuniform deformation of materials for heating damage and the crack propagation in defective grains for cooling damage.Based on the rock damage model,a multiphysics coupled model is established wherein thermal damage,creep deformation,and confining pressure can be considered.The long-term mechanical behavior of reservoir rock encountered in deep geothermal exploitation is numerically analyzed.Our numerical results indicate the reasonability and applicability of the coupled model in actual geothermal engineering practices.The main achievements of the whole Ph D.work and future research works are summarized and prospected in the conclusion part of the thesis. |
