Evolution of microcracks and energy of granite during shear test with PFC3D
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摘要:
为了探究法向应力对岩石剪切过程中细观损伤演化的影响，开展了不同法向应力作用下的花岗岩直剪试验与PFC3D数值模拟试验，结合声发射信号监测及声发射信息特征值RA（上升时间与振幅的比值）与AF（声发射振铃计数与持续时间的比值）的分析，对花岗岩在不同法向应力作用下剪切变形过程中细观微裂纹及能量演化特征进行了研究，研究结果表明：基于平行黏结接触模型建立的PFC3D模型，不仅在宏观力学参数与破坏模式上与岩石物理试验相近，而且在岩石细观裂纹与能量演化规律上也与岩石物理试验基本一致；花岗岩剪切破坏过程中主要产生张拉裂纹，峰值应力点前产生的微裂纹只占裂纹总数的10%～30%，且法向应力越大，峰值应力点前岩石内部产生的微裂纹数量越少；花岗岩剪切变形过程中，声发射信号可以分为平静期、稳定期和加速期，法向应力越大，平静期越明显，即法向应力对岩石内部微裂纹的生成起抑制作用；随着法向应力的增大，花岗岩剪切破坏过程中，声发射累计振铃计数与岩石内部微裂纹总数均逐渐增大；随着法向应力的增大，花岗岩剪切破坏过程中产生的剪切裂纹数量及其占微裂纹总数的比例均逐渐增加；花岗岩剪切变形过程中外力做功转化为弹性能与耗散能，随着法向应力增大，岩石剪切破坏所需总能量逐渐增大，弹性能与耗散能近似线性增大，且其中耗散能所占的比例逐渐增大。
Abstract:In order to investigate the influence of normal stress on the internal meso-scale damage process during rock shearing, direct shearing experiments and PFC3D numerical simulation experiments of granite under different normal stresses were carried out, combined with acoustic emission signal monitoring and analysis of acoustic emission information characteristic values of RA (ratio of rise time to amplitude) and AF (ratio of acoustic emission ring count to duration). The results show that the PFC3D model based on the parallel adhesive contact model is not only similar to rock physics experiments in macroscopic mechanical parameters and failure modes, but also basically consistent with rock physics experiments in the evolution of meso-cracks and energy in rock. Tensile micro-cracks are mainly produced in the process of shear failure of granite, and the microcracks produced before the peak stress point only account for 10%−30% of the total number of cracks, and the larger the normal stress, the smaller the number of microcracks produced inside the rock before the peak stress point. In the process of granite shear deformation, the acoustic emission signal can be divided into calm period, stable period and accelerated period, the greater the normal stress, the more obvious the calm period, i.e. the normal stress inhibits the generation of microcracks inside the rock.With the increase of the normal stress, the cumulative acoustic emission ring count and the total number of microcracks inside the rock gradually increase in the process of granite shear damage. With the increase of normal stress, the number of shear cracks produced during the shear damage of granite and its proportion to the total number of microcracks gradually increased. The work done by the external force during the shear deformation of granite is converted into elastic energy and dissipation energy, with the increase of normal stress, the total energy required for rock shear failure gradually increases, and the elastic energy and dissipation energy increase approximately linearly, in which the proportion of dissipation energy gradually increases.
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Keywords:
- rock mechanics /
- meso-cracks /
- granite /
- direct shear test /
- PFC3D simulation
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图 2 高温高压岩石真三轴剪切试验系统
Figure 2. True triaxial rock shear experimental system of high temperature and high pressure
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图 4 物理试验与PFC模拟结果对比
Figure 4. Comparison of physical experiment and PFC simulation results
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图 5 物理试验与PFC模拟花岗岩剪切应力–位移曲线对比
Figure 5. Shear stress-displacement curves of granite of physical experiment and PFC simulation
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图 6 物理试验与PFC模拟花岗岩剪切破坏示意
Figure 6. Physical experiment and PFC simulation of granite shear failure diagram
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图 7 不同法向应力作用下花岗岩剪切应力、声发射能量、累计振铃计数与时间曲线
Figure 7. Shear stress, acoustic emission energy, cumulative ringing counts and time curve of granite under different normal stress
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图 8 典型声发射波形参数及裂纹分类示意
Figure 8. Schematic of typical acoustic emission waveform parameters and crack classification
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图 9 花岗岩剪切变形过程AF-RA值分布散点图与密度
Figure 9. Scatter plot and density plot of AF-RA value distribution during shear deformation of granite
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图 10 不同法向应力作用下花岗岩剪切变形过程中AF-RA值分布密度
Figure 10. Distribution density of AF-RA values during shear deformation of granite under different normal stresses
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图 11 PFC数值模型剪切变形过程内部微裂纹统计
Figure 11. Statistics of internal microcracks in PFC numerical model during shear deformation process
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图 12 花岗岩剪切变形过程中细观裂纹演化过程
Figure 12. Evolution of micro cracks in granite during shear deformation
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图 13 花岗岩剪切过程中剪切应力与裂纹数量关系曲线
Figure 13. Curves of shear stress and crack numbers in granite during shear process
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图 14 PFC数值模型与物理试验花岗岩剪切过程总能量随法向应力变化曲线
Figure 14. Variation curve of total energy with normal stress during shearing of granite by PFC numerical model and physical experiment
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图 15 法向应力10 MPa下花岗岩内部能量演化规律
Figure 15. Internal energy evolution law of granite under normal stress of 10 MPa
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图 16 花岗岩内部能量随法向应力变化规律曲线
Figure 16. Curves of energy variation with normal stresses of granite
表 1 校正后的PFC3D模型细观参数
Table 1 Mesoscopic parameters of PFC3D model
细观参数 释义 标定值 $ {R}_{{\mathrm{min}}} $ 最小颗粒半径/mm 0.1 $ {{R}}_{\mathrm{m}\mathrm{a}\mathrm{x}} $/$ {{R}}_{\mathrm{m}\mathrm{i}\mathrm{n}} $ 最大最小颗粒半径比 1.66 $ {E}_{{\mathrm{c}}} $ 颗粒接触模量/GPa 3.78 $ {{k}}^{\mathrm{*}} $ 颗粒刚度比 0.5 $ \mathrm{\mu } $ 颗粒摩擦因数 0.5 $ {\bar{E}}_{{\mathrm{c}}} $ 平行黏结模量/GPa 3.78 $ {\bar{{k}}}^{\mathrm{*}} $ 平行黏结刚度比 0.5 $ {\bar{\sigma }}_{{\mathrm{c}}} $ 平行黏结抗拉强度/MPa 72 $ \bar{c} $ 平行黏结黏聚力强度/MPa 108 $ \bar{{\varnothing }} $ 内摩擦角/（°） 40 
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表 2 PFC模拟与物理试验所得花岗岩剪切强度参数
Table 2 Shear strength parameters of granite obtained by PFC simulation and physical experiment
法向应力/MPa $ {\tau } $/MPa ${G} $/GPa 物理试验 PFC模拟 物理试验 PFC模拟 10 55.24 52.92 2.31 2.39 20 67.03 66.75 2.61 2.67 30 82.68 81.35 2.8 2.78 40 93.59 92.66 3.14 3.08 50 107.32 105.04 3.47 3.43
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