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Mechanical properties and energy of sandstone under cyclic loading in evolutionary pattern experimental studies

Xu Daqiang, Zhang Peisen, Yan Wei, Zhang Xiaole, Dong Yuhang, Niu Hui

Archives of Mining Sciences

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2023

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Vol. 68, no. 2

351--370

EN

In order to explore the mining failure law of deep coal seam floor and clarify the mechanical behavior and energy change in the floor strata during mining, the mechanical properties and energy evolution law of sandstone under cyclic loading with different confining pressures (20, 30, 40 MPa) were studied using the Rock Top multi-field coupling tester. The results are as follows: (1) the hysteresis phenomenon of a rock stress-strain curve under cyclic loading is evident. Moreover, the hysteresis loop migrates to the direction of strain increase, and the fatigue damage caused by cyclic loading has a certain weakening effect on the peak strength of rock; (2) both the number of cycles and the axial strain show a nonlinear change characteristic that satisfies the quadratic function relationship. Among them, the stress level of the rock is the main factor affecting the fitting effect; (3) under the same confining pressure, with an increase in cycle level, the macroscopic deformation of the rock increases, the accumulation of fatigue damage in the sample increases, and the irreversible deformation of the rock increases, which leads to an increase in energy input and dissipation; (4) in terms of elastic energy and dissipation energy, elastic energy plays a dominant role. In the initial cycle, the rock is destroyed, and the rock energy loss is great. After the second cycle, the input energy is mainly stored in the rock in the form of elastic energy, and only a small part of the input energy is released in the form of dissipation energy; (5) the confining pressure can improve the efficiency of rock absorption and energy storage, enhance the energy storage limit of rock, and limit the dissipation and release of partial energy of rock. The greater the confining pressure, the more evident the limiting effect, and the more significant the dominant position of elastic energy; and (6) the change in the energy dissipation ratio can be divided into three stages: rapid decline stage, stable development stage and rapid rise stage. The greater the increase in dissipation energy, the greater the degree of rock damage. The evolution process of the energy dissipation ratio can reflect the internal damage accumulation process of rock well, which can be used as the criterion of rock instability.

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The influence of loading rate on the mechanical behavior and energy evolution characteristics of hard and soft rock under triaxial compression

Deng Jihui, Chen Chao, Wu Xiaoning

Journal of Theoretical and Applied Mechanics

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2022

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Vol. 60 nr 3

495--508

EN

To investigate the influence of loading rate and confining pressure on the mechanical behavior and energy evolution characteristics of hard and soft rock, high strength sandstone and low strength granite were subjected to triaxial compression tests with different loading rates. The results show that significant differences exist in the stress-strain curves for sandstone and granite. The confining pressure has a significant effect on the stress-strain curve, while the loading rate has a smaller effect on the stress-strain curve. As the confining pressure increases, the peak axial strain, peak axial stress, total energy, elastic energy and dissipated energy of sandstone and granite increase, the proportion of dissipated energy to total energy of sandstone and the proportion of elastic energy to total energy of granite are reduced. As the loading rate goes up, the peak axial stress, total energy and elastic energy increase in both sandstone and granite. The ultimate failure pattern of sandstone is a typical single inclined plane shear failure, while the ultimate failure pattern of granite consists of a single inclined plane shear failure and a vertical split failure. The loading rate has no significant effect on the macroscopic failure pattern, the elastic and dissipated energies are proportional to the total energy of sandstone and granite.

3

Prefabricated fractured rock under stepwise loading and unloading

Zhang Wei, Zhao Tongbin, Yin Yanchun

Journal of Theoretical and Applied Mechanics

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2022

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Vol. 60 nr 1

167--179

EN

After the engineering rock mass has been affected by comprehensive effects of mining and disturbance, it experiences an input of external energy as well as dissipation and release of internal energy. From the viewpoint of energy, characteristics of rock failure are studied, and the law of rock unstable deformation and energy evolution is analyzed. The damage mechanism of rock is revealed easily. A gradual loading and unloading test of fractured rock is carried out to analyze deformation characteristics of the fractured rock during the load- -bearing process, and to study the law of energy dissipation and release under different load- ing and unloading stress levels. The results show that: (1) the load-bearing time, loading and unloading stress level, stress at crack initiation and peak stress of fractured rock gradually decrease with an increase in the number of cracks, and the descending speed decreases grad- ually with the increase in the number of cracks; (2) the strain at crack initiation and peak strain of the fractured rock increase gradually with an increase in the number of cracks, and the gathering speed decreases with the increase of the number of cracks; (3) the released strain energy and dissipation energy of fractured rock increase with an increase of loading and unloading stress levels, and the increasing rate gradually slows down; (4) the number of acoustic emission events is positively related to the degree of rock damage, and the increase in the number of cracks will prolong duration of the number of acoustic emission events. The results have a theoretical value for energy evolution and deformation damage of fractured rock masses, and also provide experimental experience to study the instability precursor information of rock materials from the viewpoint of energy.

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Energy characteristics and micro cracking behaviors of deep slate rock under triaxial loadings

Zhang Anlin, Feng Tao, Jiang Liangwen, Wang Dong, Wang Zhewei, Zhang Ru, Feng Gan, Zhang Zhilong, Deng Jianhui, Ren Li

Acta Geophysica

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2022

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Vol. 70, no 4

1457--1472

EN

Sichuan-Tibet railway tunnels are generally long and deep, and some of them pass through layered slate, which is prone to failure under high in situ stress. Insight into the mechanical behaviors of deep slate under different stress environments is critical to the safety of engineering practices in this area. To this end, we studied the energy characteristics and microcracking behaviors of slate samples under different triaxial loadings. The results showed that the confining pressure increased the strength, elastic modulus, Poisson’s ratio and crack characteristic stresses of the studied slate. As the confining pressure increased, the storage of elastic strain energy during the prepeak stress stage increased, and its release during the postpeak stress stage was inhibited. Additionally, we obtained the slate failure precursors by analyzing the acoustic emission (AE) evolution characteristics, i.e., “a sudden increase in AE metrics, followed by a quiet period, and then a sudden increase again” in terms of the AE count rate and AE energy rate after obvious expansion of the sample. Then, we further observed that with an increase in confining pressure, a gradual transition of the failure pattern from compressive shear to tensile shear occurred. Finally, we discussed the influence mechanisms of confining pressure on the slate failure behaviors and concluded that the change in the effective shear stress along the foliation planes played an important role in the failure mode transition.