Mesoscopic fracture evolution of granite under different thermal disturbances

Abstract

Hot-stage polarizing microscopy technique was employed to investigate the mesoscopic fracture evolution characteristics of granite throughout the entire process from room temperature to real-time high temperature and then to cooling. The study analyzed the influence of mineral types, temperature, cooling medium, and the heating and cooling progress on the microcrack development in granite. Additionally, the contributions of heating and cooling to the damage of granite were discussed. The research indicates that crack evolution follows a characteristic trend: the number of small cracks increases, and larger cracks form through the coalescence and propagation of smaller ones during heating. The thermal fracture threshold for granite was identified at 300 °C. The three main minerals in granite exhibit distinct area change behaviors with temperature. After natural cooling, mineral areas show a slight increase compared to the pre-treatment state. Following thermal shock in water, these areas decrease marginally relative to their extent at 600 ℃ yet remain significantly larger values than initial ones. Thermal shock cooling induces more extensive fracturing in granite compared to natural air cooling. Furthermore, the heating process contributes more significantly to the overall damage than the subsequent cooling stage. This study enhances the understanding of mesoscopic evolution in thermal disturbances treated rocks and provides a theoretical basis for assessing rock stability in high-temperature engineering environments.

1. Introduction

Rocks, being brittle materials with low thermal conductivity [1], comprise minerals with distinct thermal expansion coefficients. Temperature fluctuations induce thermal stress, resulting in rock damage [2]. We define such temperature-change-induced effects as thermal disturbances. The heating rate is a critical factor controlling rock damage extent, which is paramount in applications such as geological nuclear waste disposal and thermomechanical rock breaking [3]. Moreover, during the stimulation of hot dry rock reservoirs, different cooling media (water or liquid CO2) cause markedly different levels of thermal shock damage [4, 5]. Consequently, investigating rock thermal damage evolution under various thermal disturbances is crucial for advancing the understanding of rock thermodynamic behavior and for guiding related engineering practices.

Researchers globally have investigated rock failure through diverse testing methodologies [6−8]. Thermal disturbances can induce significant alterations in multiple rock parameters [9−11], and the heating process initiates rock cracking [12−14]. The increase in heating temperature promotes the development of internal cracks within rocks. These cracks undergo progressive propagation, interconnection, and coalescence, ultimately forming extensive fracture networks. While such networks enhance rock permeability [15], they also significantly reduce rock mass strength [16]. Additionally, the presence of air, which has a much lower thermal conductivity than the rock matrix, further reduces the overall thermal conductivity of the material [17]. During the thermal shock cooling process, sharp temperature gradients induce further fracturing, which weakens the mechanical and physical properties of the rock [18, 19].

Understanding failure mechanisms requires tracking the entire process of crack evolution. However, experimental studies under real-time high-temperature conditions remain limited due to technical challenges. Some researchers have turned to numerical simulations to investigate rock fracturing under thermal disturbances [20, 21], yet these models often lack intuitive correspondence with actual fracture processes. XU et al [22] conducted real-time high-temperature observations and identified 300 ℃ as the threshold for widespread crack coalescence. However, their study did not capture continuous thermal fracture evolution or incorporate cooling effects into the analysis. To address these gaps, we utilize high-temperature hot-stage microscopy to examine the full heating-cooling process in granite.