Analysis of Mechanical Properties and Failure Modes in Thermally Treated Granite with Structural Defects
Authors: Yang Zheng , Dang Yike , Yang Shangtong , Liu Xiaoyu 
Published: August 2024

ABSTRACT:
Natural rocks usually contain joints, fractures, etc. However, current research on rocks' mechanical properties after thermal treatment focuses on intact rocks. To make up for this deficiency, we investigated the influence of temperature on the strength, deformation, and crack propagation behavior of granite with single fissures at different angles. The results indicate that: (1) Compared to intact specimens, the strength of defective granite is reduced due to the presence of defects. As the crack angle increases from 0° to 90°, peak strength and elastic modulus decrease and then increase. (2) Between 150°C and 300°C, the peak strength gradually increases. Beyond 300°C, the peak strength decreases with rising temperature. The initiation, propagation, and peak stress of cracks are closely correlated with temperature. (3) At lower temperatures, cracks' propagation speed and direction will change when encountering minerals. For specimens treated with high temperatures, the temperature caused many thermally induced cracks to form in the sample. The cracks no longer sought the minimum resistance path during the propagation process, and the resistance encountered by crack propagation was greatly reduced, resulting in more direct and predictable paths of crack propagation.

1. INTRODUCTION
Granite, a crystalline rock commonly found in the Earth's crust, is crucial in understanding crustal movements, geothermal energy development, and deep geotechnical engineering construction. The evolution of its mechanical behavior and fracture mechanisms under high-temperature conditions is particularly significant. As human activities increasingly impact the Earth's environment, the demand for high-temperature geological engineering encompassing nuclear waste disposal and underground energy extraction has surged. Exploring the principles and mechanisms behind temperature's influence on the mechanical properties of granite is thus crucial. Understanding how temperature changes affect thermal expansion, crack development, and strength weakening in rocks holds significant theoretical and practical value. Such knowledge is vital for predicting and preventing geological disasters and optimizing rock engineering designs.
Scholars have conducted many studies to study the effect of temperature on rock mechanical properties and damage, with some widely recognized conclusions. For example, the temperature increases the porosity (Li et al., 2020) and permeability (Jiang et al., 2018). Rock's P-wave velocity and density decrease with the increase of temperature (Guo et al., 2020). Pan et al. (Pan et al., 2023)studied granite's strength and deformation characteristics after high temperature and chemical solution treatment through uniaxial compression tests. This study introduces a new statistical damage constitutive model considering initial thermochemical and loading damage. This model is used to examine the effects of thermal and chemical treatments on the damage evolution of granite during uniaxial compression tests. It was found that, up to 150 °C, chemical treatment results in more significant damage than thermal treatment. However, after thermal treatment at 300 °C or higher, thermal damage progressively increases, and the disparity in chemical damage induced by various solutions diminishes. Zhang et al. (Zhang et al., 2016) investigated the thermal effects on the physical and mechanical properties of rocks by analyzing changes in mineral composition, internal structure, and water content. Based on the temperature range of adhesive water, bound water, and structural water escaping, they divided the temperature into three ranges: from room temperature to 100 °C, 100 °C to 300 °C, and 300 °C to 500 °C. In the second and third stages, the porosity, permeability, and acoustic emission signals of the rock significantly increase, while the P-wave velocity continues to decrease. The rapid increase in sample volume and number of microcracks within the temperature range of 400 °C to 500 °C leads to changes in rock mechanical properties. Yavuz et al. (Yavuz et al., 2010) analyzed the influence of thermal damage on the physical properties of five types of carbonate rocks. Microscopic observation shows that the damage degree of rocks at high temperatures varies depending on particle size, porosity, structure, and texture characteristics. Microscopic analysis and P-wave results indicate that the internal structure of the rock is compacted below 150 °C, and the expansion of calcite does not cause significant damage to the rock structure. The compaction of rock structure leads to an increase in P-wave velocity and a decrease in porosity. Most of the damage occurs within 24 hours of the heating time, and further heating treatment has a relatively small impact on the changes in physical properties.