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| Brittleness characteristics of quasi-static triaxial coal rock based on characteristic stress |
| LIU Xiaohui1，2，ZHENG Yu2，HAO Qijun2，GUI Xin2，XUE Yang2 |
| (1. Key Laboratory of Fluid and Power Machinery，Ministry of Education，Xihua University，Chengdu，Sichuan 610039，China；2. School of Energy and Power Engineering，Xihua University，Chengdu，Sichuan 610039，China) |
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Abstract The triaxial compression tests of coal rock under quasi-static strain rate were carried out by using MTS815 test system，and the failure characteristics and energy evolution law of coal rock were analyzed. By using energy dissipation theory to determine the damage stress，a brittleness index reflecting the pre/post-peak mechanical behavior was established based on characteristic stress to analyze the brittleness traits of coal rock under different strain rates and confining pressures. The results show that stress-strain curves of quasi-static triaxial coal rock have obvious stage characteristics. As the confining pressure increases and the strain rate decreases，the post-peak stress drop declines，failure mode transitions from tensile failure to shear failure，and the brittle failure characteristics of coal rock are gradually weakened. According to the irreversible damage in coal rock due to energy dissipation，the dissipation energy rate is defined as the ratio of the dissipation energy to the total strain energy，and the minimum of the curve of the dissipation energy rate is determined as the damage stress point. Comprehensively considering the degree of crack propagation in pre-peak and stress drop in post-peak，a brittleness index Bstress of quasi-static triaxial coal rock is established with the combination of the damage stress，the peak stress and the residual stress. Bstress decreases as the confining pressure increases and the strain rate decreases，indicating that the brittleness of coal rock is weakened. Comparing with other brittleness indexes，it is found that Bstress can better depict that the brittleness of coal rock is promoted by the strain rate and restrained by the confining pressure. Further analysis of the dispersion degree of each brittle index under the same group of parallel tests is carried out by using the coefficient of variation，finding that the dispersion of Bstress is minimal and it is more reasonable to characterize the brittleness of triaxial coal rock under quasi-static strain rate.
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2021, Vol. 40 
