Particle flow simulation of the crack propagation characteristics of granite under cyclic load
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摘要: 從細觀角度、采用顆粒離散元法開展了預制裂隙花崗巖循環加卸載的數值模擬試驗。首先,使用圖像處理技術識別花崗巖中的不同細觀組分、結合室內單軸壓縮試驗結果對細觀力學參數進行了標定。然后,通過編制顆粒流代碼追蹤裂隙的類型和擴展過程,分析巖石破壞過程中裂隙發展的階段性特征。結果表明:不同傾角裂隙巖石的新生裂隙走向與預制裂隙貫通方向基本一致;根據新生裂隙的優勢傾向分組得到裂隙起裂角與預制裂隙傾角的關系:傾角β≤45°時剪切和張拉裂隙的起裂角單調遞減,傾角β≥60°時剪切和張拉裂隙的起裂角單調遞增;循環擾動荷載增加了裂隙巖體的軸向變形,軸向累積殘余應變曲線呈反S形、提高擾動荷載應力上限促使曲線進入加速階段;試件峰值強度隨裂隙傾角增大表現出先減小后增大的趨勢,峰值強度為實驗室完整巖石單軸抗壓強度的63% ~ 89%,反映了較為明顯的劣化現象;在循環荷載作用下,剪切裂隙和張拉裂隙增長曲線表現出明顯的變化特點,在裂隙不穩定擴展階段中張拉裂隙數目增長速率顯著大于剪切裂隙,對分析巖石變形破壞過程具有一定的參考意義。Abstract: The microcracks in natural rock masses considerably impact the stability of the underground engineering structures. The mechanical properties of the cracked rock masses contribute considerably to the strength of the rock masses and their compression failure mechanism. The instability and failure of the surrounding rocks are often induced by the propagation and penetration of these internal cracks. In practical engineering, rock mass excavation is a process involving dynamic disturbance. The mechanical properties of the rocks under cyclic load are considerably different from those of the rocks under static load. The characteristics and development of microcracks are the main factors influencing rock fatigue failure. From the microscopic viewpoint, the particle-based discrete element method is used to conduct the cyclic loading and unloading tests of the preexisting cracked granite. First, the microcompositions of granite are determined using image processing techniques, and the micromechanical parameters are calibrated based on the indoor uniaxial compression test results. The stage of crack development during rock failure is analyzed by compiling particle flow code to track the type and propagation process of cracks. Results indicate that the orientations of new cracks in fractured rocks with different dip angles are similar to those of the preexisting cracks. Further, the relation between the crack initiation angle and the inclination angle of the preexisting cracks is obtained according to the tendency of new cracks. The crack initiation angle of shear and tension cracks decreases and increases monotonically, respectively, when the inclination angle β ≤ 45° and β ≥ 60°. The cyclic disturbance load increases the axial deformation of the fractured rock mass, and the axial cumulative residual strain curve exhibits an inverse S-shape when entering the acceleration stage faster with the increasing upper stress limit. The peak strength of the model specimen shows a decreasing trend followed by an increasing trend with the increasing fracture inclination. The peak strengths of the laboratory-intact rock are 63% to 89%, indicating an obvious deterioration phenomenon in the rock materials. The growth of shear and tension cracks show different characteristics under cyclic load; the growth rate of tension cracks is considerably higher than that of shear cracks during the unstable crack development stage. The results presented in this study may be used as reference to investigate the deformation and failure mechanisms of rock materials.
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Key words:
- microstructure /
- cyclic load /
- crack orientation /
- crack propagation /
- particle flow
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表 1 花崗巖細觀力學性質參數
Table 1. Microscale mechanical parameters of granite
Mineral component Particles forming grains Linear parallel bond model Minimum particle radius forming grain, Rmin/mm Maximum to minimum radius ratio, Rmax/Rmin Young’s modulus, Ec/GPa Ratio of normal to shear stiffness of the particle, kn/ks Young’s modulus, $ {\bar E_{\rm{c}}} $/GPa Ratio of normal to shear stiffness of the parallel bond, ${\bar k_{\rm{n}}} $/$ {\bar k_{\rm{s}}} $ Shear bond strength, $ {\tau _{\rm{c}}} $/MPa Friction ratio, $ \bar \varphi $ Tensile–shear bond strength ratio, ${\bar \sigma _{\rm{c}}} $/$ {\bar \tau _{\rm{c}}} $ Feldspar 1.2 1.66 45.5 1.15 28.0 1.6 51.0 0.5 1.0 Quartz 1.2 1.66 33.0 1.15 22.6 1.6 81.6 0.8 1.0 Mica 1.2 1.66 11.2 1.15 5.9 1.6 15.3 0.15 1.0 表 2 新生裂隙傾向和傾角分布統計
Table 2. Statistics of the distribution of tendencies and inclinations for newly generated cracks
Tendencies and inclinations for preexisting cracks Shear cracks Tension crack Tendency grouping Average inclination/(°) Percentage/% Tendency grouping Average inclination/(°) Percentage/% 90°∠0° 151°–160° 65 6.3 61°–70° 77 8.0 211°–220° 60 6.3 251°–260° 62 8.0 90°∠30° 261°–270° 62 11.9 71°–80° 61 10.5 241°–250° 48 9.5 141°–150° 72 10.5 90°∠45° 261°–270° 54 17.4 71°–80° 52 18.5 251°–260° 45 15.2 91°–100° 64 11.1 90°∠60° 261°–270° 51 9.5 81°–90° 64 14.3 251°–260° 51 7.9 61°–70° 69 10.7 90°∠90° 261°–270° 45 8.5 131°–140° 88 11.1 181°–190° 43 6.4 21°–30° 48 5.6 表 3 峰值強度統計
Table 3. Statistics of peak strengths
Inclination angle of rock specimen, β/(°) Peak strength under cyclic load/MPa Peak strength under cyclic load to the uniaxial strength of the intact rock ratio Cycles 0 84.4 0.67 21 30 79.3 0.63 24 45 91.3 0.73 24 60 96.7 0.77 24 90 112 0.89 24 259luxu-164 -
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