Key Takeaways & Executive Findings
- •• A novel brittleness index based on rock fracture energy is proposed and validated through multi-scale experiments. • Increasing loading rate from 0.1 to 5 mm/min significantly enhances granite brittleness, with the brittleness index Bs rising from 0.17 to 0.28. • Qualitative indices (projectile mass ratio and average lumpiness) also increase with loading rate, confirming the brittleness trend. • The study provides theoretical guidance for predicting rockburst and optimizing hydraulic fracturing in deep underground engineering.
Abstract
In deep underground engineering, rock brittleness is closely associated with rockburst and feasibility of hydraulic fracturing. The loading rate plays a crucial role in determining the severity of rockburst and cuttability. By conducting uniaxial compression tests and single-cycle loading-unloading experiments, the brittle evolution of four types of granite under different loading rates was investigated. During the uniaxial compression process, acoustic emission parameters were used to characterize the crack evolution patterns. Additionally, the macroscopic failure process of the specimens and the post-failure rock fragments were recorded with a high-speed camera, providing multi-scale validation. This study proposes a quantitative brittleness index based on rock fracture energy, and its validity is verified by analyzing the rock failure process and the macroscopic characteristics of rock fragments. This work contributes to advancing research on rock brittleness indices considering the coupling between energy evolution and kinematic mechanisms. The research results indicate that as the loading rate increases from 0.1 mm/min to 5 mm/min, the quantitative evaluation index (Bs) for brittleness increases from 0.17 to 0.28, while the qualitative evaluation indices MF (projectile mass ratio) and l (average lumpiness) increase from 0.3261 to 0.4184 and from 32.96 mm to 38.12 mm, respectively. With increasing loading rates, the brittleness of the rock increases significantly. A series of qualitative and quantitative results, including fractal characteristics and acoustic emission parameters, reveal the crack evolution patterns of granite under different loading rates and confirm the rationality of the brittleness index. This study provides theoretical guidance for practical deep underground engineering applications.
1. Introduction
As a fundamental property of rocks, brittleness is a critical factor in the prevention of geological hazards and rock engineering. Moreover, brittleness aids in predicting rock damage characteristics and behavior, thereby facilitating the implementation of measures to mitigate potential risks. Research on rock brittleness plays a crucial role in predicting rockburst proneness in high-stress rock masses, enhancing the efficiency of mining equipment in hard rock excavation, and lowering the costs associated with unconventional oil and gas resource extraction.
The brittleness of rocks is intrinsically linked to the development of internal fractures, with the stress concentration process leading to brittle damage occurring gradually. The development of internal fractures in rocks is governed by a variety of factors, including stress state, mineralogical composition, temperature fluctuations, and loading rate. Numerous studies have shown that variations exist in the progression of internal rock fractures under low-rate static loading conditions. XIE et al utilized numerical simulation methods to replicate the crack evolution process of porous brittle rock materials, thereby ascertaining that fracture toughness is inversely proportional to porosity and negatively linearly correlated with pore distribution distance. This provides a robust numerical foundation for high-accuracy simulation demonstrations of internal multi-crack interaction phenomena. ZHAO et al performed experimental investigations on granite at different loading rates. Their findings indicated that the intrinsic damage of the specimen escalated with increasing loading rates, and that this damage accelerated significantly at the peak stress. In the study by ZHANG et al, fracture experiments were conducted using a quasi-static test method at a loading rate of 0.002 mm/s. This investigation was compared with modified dynamic fracture tests on marble. The findings indicated that fracture toughness correlated with the loading rate. Subsequent analysis revealed that this phenomenon was due to an intrinsic damage mechanism. GONG et al analyzed the damage process of granite. They found that the movement and distribution of the rock crushing process varied with loading rate. This indicated that crack development within the rock varied with loading rate. Limited research exists on the brittleness of rocks under varying loading rates.
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YIN Tu-bing, LIU Fan, MA Jie-xin, DAI Hao, LU Jian-fei, GUO Wen-xuan, LI Xi-bing (2026). Effect of loading rate on the brittleness index of granite: An experimental investigation. Journal of Central South University. https://doi.org/10.1007/s11771-026-6238-x
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Frequently Asked Questions
What is the main contribution of this study?
The study proposes a new quantitative brittleness index based on rock fracture energy and validates it through experiments on granite under different loading rates, providing insights into rockburst and hydraulic fracturing.
How does loading rate affect granite brittleness?
As the loading rate increases from 0.1 mm/min to 5 mm/min, the brittleness index Bs increases from 0.17 to 0.28, indicating that higher loading rates significantly enhance rock brittleness.
What experimental methods were used?
Uniaxial compression tests and single-cycle loading-unloading experiments were conducted, with acoustic emission monitoring and high-speed camera recording to characterize crack evolution and failure processes.
What are the practical applications of this research?
The findings provide theoretical guidance for predicting rockburst in deep mines and optimizing hydraulic fracturing in unconventional oil and gas extraction.
What are the qualitative indices used to evaluate brittleness?
The qualitative indices include projectile mass ratio (MF) and average lumpiness (l), which both increased with loading rate, confirming the brittleness trend.
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