摘要
为了研究边界对花岗岩常规三轴离散元模拟试验的影响,使用颗粒流模拟软件(PFC3D),基于三维等效晶质模型(Grain‑Based‑Model)方法生成花岗岩模型,同时利用有限元-离散元(FDM‑DEM)耦合建模技术,分别建立刚性、柔性膜以及有限元单元(“Shell”)3种边界下的离散元三轴试验,结果表明:相较于“Shell”边界,刚性边界会提高岩样发生应力集中现象的概率,影响岩样的破坏形态;柔性膜边界会导致岩样在三轴加载过程中所受的实际围压大于目标围压,并显著提高岩样的残余强度;而“Shell”边界下的岩样所受围压稳定,发生应力集中的概率最小。综合分析可得:在花岗岩离散元三轴模拟中“Shell”边界是合适的选择。
花岗岩是由不同种类较大晶体颗粒与非晶体颗粒等构成的非连续、非均质且力学行为较为复杂的岩石,且在诸多掘进工程中极为常见,杨达
许多学
对此,基于最新的有限元-离散元(FDM‑EDM)耦合技术,使用PFC分别生成“刚性”、“柔性膜”、“Shell”单元三种边界下的花岗岩三轴试验模型,进而分析“刚性”、“柔性膜”、“Shell”单元3种边界对花岗岩常规三轴模拟试验的影响。
花岗岩中的矿物颗粒是影响花岗岩力学性质的重要因素之一,也是岩石非均质性的重要体现,花岗岩中矿物成分主要为长石、石英、黑云母以及其它矿物等,根据张涛
颗粒流程序中不能直接生成三维等效晶质模型(Grain‑Based‑Model),须借助其软件中自带的“块体(Rblock)”单元或者“几何体(geometry)”单元来间接生成三维等效晶质模型。张涛
上述过程最终实现了花岗岩三维等效晶质模型的建立,但其生成步骤十分繁琐,且“块体”单元与“几何体”单元之间需要多次转化,降低了计算效率,为此本文在其基础上通过直接使用“Rblock”对初始颗粒模型进行划分,从而简化了生成步骤,提高了计算效率。花岗岩模型生成过程中的3个阶段如

图1 花岗岩模型生成示意
Fig.1 Schematic diagram of granite model generation
阶段一:确定试样尺寸,构建初始颗粒模型,如
阶段二:“块体”单元分组。在初始颗粒模型外部构建一个外接“几何体单元”,将外接“几何体”单元再次转化为“块体”单元,对“块体”单元进行分组,如
阶段三:生成三维等效晶质模型。通过判断“块体”单元与初始模型颗粒位置的关系对初始模型颗粒进行分组,从而生成三维等效晶质模型,如
颗粒流程序中自带多种接触模型,其中平行粘结模型(Parallel Bond Model)适合用来模拟岩石颗粒之间的接
微观参数 | 密度/ (kg· | 体积含量/% | 颗粒最小半径/cm | 颗粒最大半径/cm | 线性部分 | 粘结部分 | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
线性弹性模量/GPa | 线性刚度比 | 摩擦因数 | 粘结弹性模量/GPa | 粘结刚度比 | 粘聚强度/MPa | 拉伸强度/MPa | 摩擦角/(°) | |||||
石英 | 2650 | 23 | 0.028 | 0.056 | 70 | 1.2 | 0.48 | 70 | 1.2 | 240 | 120 | 19.5 |
黑云母 | 3150 | 26 | 55 | 2.8 | 0.32 | 55 | 2.8 | 120 | 60 | 17.2 | ||
长石 | 2600 | 49 | 55 | 1.2 | 0.67 | 55 | 1.2 | 170 | 85 | 22.3 | ||
其他矿物 | 1600 | 2 | 30 | 3.7 | 0.4 | 30 | 3.7 | 80 | 40 | 23.7 | ||
晶间接触 | 45 | 1.3 | 0.8 | 45 | 1.3 | 50 | 27 | 34 |
边界的建立和伺服是三轴模拟试验的关键步骤,前面已介绍过如何建立花岗岩模型的方法,因此这里主要介绍本文中花岗岩三轴压缩试验的3种边界。
PFC可以直接生成常规三轴模拟试验中的刚性边界,刚性边界属于刚体,受力后不会产生任何变形,但通过刚性伺服函数可以获得径向方向上的一个平移自由度,细观上表现为刚性边界顶点的径向位移,宏观上表现为刚性边界的整体放大与缩

图 2 3种不同边界条件示意图
Fig. 2 Schematic diagram of three different boundary conditions
蒋成龙
室内试验所用的橡皮膜在力学本质上属于弹塑性物体,而在模拟弹塑性物体时,有限元软件比离散元有巨大优

图 3 Shell单元示意图
Fig. 3 Example of Shell element
(1) |
(2) |
为在离散元中研究边界对花岗岩三轴压缩试验的影响,设计9种岩样在3种不同边界下的模拟实验,共计27组模拟试验,结果见
围压/MPa | 岩样编号 | 加载速率/(mm∙ |
---|---|---|
6 | X61 | 0.5 |
X62 | 1.0 | |
X63 | 1.5 | |
12 | X121 | 0.5 |
X122 | 1.0 | |
X123 | 1.5 | |
24 | X241 | 0.5 |
X242 | 1.0 | |
X243 | 1.5 |
为对比不同情况下的岩样变形破坏情况,同时鉴于文章篇幅,本文仅列出了X61岩样在3种边界下的变形破坏情况,如

图 4 试样变形形态对比
Fig. 4 Comparison of deformation of different samples
(3) |
为对上述猜想进行验证,列出模拟岩样的体积应变曲线,鉴于文章篇幅,本文仅列出X61岩样在3种边界情况下的体积应变曲线,如

图 5 3种边界下X61试样的应力‑体积应变曲线
Fig. 5 Stress vs volume strain curves of X61 specimens under three boundaries
三轴试验中岩样的应力-应变曲线是反映不同边界对花岗岩三轴试验影响的主要形式,

图 6 3种边界下不同围压及加载速率下试样应力-应变曲线
Fig. 6 Axial and radial stress vs strain curves under three boundaries at different confining pressure and loading velocity
从
根据

图 7 三种边界下岩样峰值应力与加载速率关系
Fig. 7 Relationship between peak stress and loading rate of rock samples under three boundaries

图 8 三种边界下岩样弹性模量与加载速率关系
Fig. 8 Relationship between elastic modulus and loading rate of rock samples under three boundaries
从
从
荷载在试样通过颗粒间的接触力链传递,分析接触力链可从细观角度上对比分析不同边界对岩样三轴压缩实验的影响,因此绘制出3种边界下岩样在轴应变(7%)的接触力链图,如

图 9 3种边界下岩样接触力链图(轴应变为7%)
Fig. 9 Contact force chain of rock samples under three boundaries()
从
本文使用颗粒流程序研究了刚性、柔性膜、“Shell”3种边界对模拟花岗岩常规三轴试验的影响。针对3种边界下花岗岩的宏观破坏形态和应力应变关系、力链分布等模拟结果进行了分析。通过本文研究,得出结论如下:
(1)边界影响岩样的加载效应和围压效应。岩样在不同边界下,其受到加载速率和围压的影响程度不同,从上面的三轴模拟实验中可以看出,相同围压下,刚性边界下岩样的峰值应力易受加载速率的影响,相同加载速率下,柔性膜边界下岩样的峰值应力受围压影响最大。
(2)边界对岩样的破坏变形有显著影响。从
(3)刚性边界和柔性膜边界在三轴加载过程中无法对岩样施加精准且稳定的围压。其中刚性边界由于刚性伺服函数的连续运行之间存在时间间隔以及刚性伺服函数确定实际围压值时所存在的细小误差,导致岩样在刚性边界下无法获得稳定的围压。而柔性膜边界在三轴加载过程中根据柔性膜伺服理论计算出的柔性膜表面积略大于实际的柔性膜边界表面积,导致岩样在柔性膜边界下所受的实际围压大于目标围压,所以岩样在柔性膜边界下的峰值应力易受目标围压影响且残余强度普遍大于其他2种边界下岩样的残余强度。
(4)刚性边界对岩样在三轴加载过程中应力集中现象的产生具有影响。从
综合对比刚性边界、柔性膜边界和Shell边界,发现在Shell边界下,岩样围压施加简单精准且在加载过程中其所受围压恒定不变,应力集中发生概率较小,因此在花岗岩离散元常规三轴模拟时,Shell边界是合适的选择。
参考文献(References)
杨达,陈宝义,曹宏宇,等.基于冲击载荷的硬质合金球齿碎岩机理研究[J].钻探工程,2022,49(1):142-152. [百度学术]
YANG Da, CHEN Baoyi, CAO Hongyu, et al. Study on rock fragmentation mechanism of carbide spherical teeth based on impact load[J]. Drilling Engineering, 2022,49(1):142-152. [百度学术]
汤凤林, Нескоромных В. В.,宁伏龙,等.金刚石钻进岩石破碎过程及其与规程参数关系的研究[J].钻探工程,2021,48(10):43-55. [百度学术]
TANG Fenglin, NESKOROMNYH V. V., NING Fulong, et al. Research on the rock fragmentation process and its relationship with drilling parameters in diamond drilling[J]. Drilling Engineering, 2021,48(10):43-55. [百度学术]
张程,赵大军,张书磊,等.基于岩石表面位移场的超声波振动下花岗岩损伤特性试验研究[J].钻探工程,2021,48(3):39-45. [百度学术]
ZHANG Cheng, ZHAO Dajun, ZHANG Shulei, et al. Experimental study on damage characteristics of granite under ultrasonic vibration based on the displacement field of the rock surface[J]. Drilling Engineering, 2021,48(3):39-45. [百度学术]
Li X F, Li H B, Zhao J. 3D polycrystalline discrete element method (3PDEM) for simulation of crack initiation and propagation in granular rock[J]. Computers and Geotechnics, 2017,90:96-112. [百度学术]
Chen Q, Zhang C, Yang C, et al. Effect of fine‑grained dipping interlayers on mechanical behavior of tailings using discrete element method[J]. Engineering Analysis with Boundary Elements, 2019,104:288-299. [百度学术]
Tang C. Numerical simulation of progressive rock failure and associated seismicity[J]. International Journal of Rock Mechanics and Mining Sciences, 1997,34(2):249-261. [百度学术]
Li H, Yang C, Ding X, et al. Weibull linear parallel bond model (WLPBM) for simulating micro‑mechanical characteristics of heterogeneous rocks[J]. Engineering Analysis with Boundary Elements, 2019,108:82-94. [百度学术]
李博,朱强,张丰收,等.基于矿物晶体模型的非均质性岩石双裂纹扩展规律研究[J].岩石力学与工程学报,2021,40(6): 1119-1131. [百度学术]
LI Bo, ZHU Qiang, ZHANG Fengshou. et al. Influence of meso‑structure heterogeneity on granite strength and deformation with particle flow code[J]. Chinese Journal of Rock Mechanics and Engineering, 2021, 40(6): 1119-1131. [百度学术]
胡训健,卞康,谢正勇,等.细观结构的非均质性对花岗岩强度及变形影响的颗粒流模拟[J].岩土工程学报,2020,42(8): 1540-1548. [百度学术]
HU Xunjian, BIAN Kang, XIE Zhengyong, et al. Numerical simulation of large‑scale triaxial tests on soil‑rock mixture using DEM with three‑dimensional flexible membrane boundary [J]. Chinese Journal of Geotechnical Engineering, 2020,42(8):1540-1548. [百度学术]
金磊,曾亚武.基于三维柔性薄膜边界的土石混合体大型三轴试验颗粒离散元模拟[J].岩土工程学报,2018,40(12):2296–2304. [百度学术]
JIN Lei, ZENG Yawu. Numerical simulation of large‑scale triaxial tests on soil‑rock mixture using DEM with three‑dimensional flexible membrane boundary[J]. Chinese Journal of Geotechnical Engineering, 2018,40(12):2296-2304. [百度学术]
Xu Wenjie, Hu Liming, Gao W. Random generation of the meso-structure of a soil‑rock mixture and its application in the study of the mechanical behavior in a landslide dam[J]. International Journal of Rock Mechanics and Mining Sciences, 2016,86:166-178. [百度学术]
Cheung G, O’Sullivan C. Effective simulation of flexible lateral boundaries in two‑ and three‑dimensional DEM simulations[J]. Particuology, 2008,6(6):483-500. [百度学术]
Thomas P A, Bray J D. Capturing nonspherical shape of granular media with disk clusters[J]. Journal of Geotechnical and Geoenvironmental Engineering, 1999,125(3):169-178. [百度学术]
Qu T, Feng Y T, Wang Y, et al. Discrete element modelling of flexible membrane boundaries for triaxial tests[J]. Computers and Geotechnics, 2019,115:103-154. [百度学术]
张涛,蔚立元,鞠明和,等.基于PFC3D-GBM的晶体-单元体尺寸比对花岗岩动态拉伸特性影响分析[J].岩石力学与工程学报,2022,41(3):28-38. [百度学术]
ZHANG Tao, WEI Liyuan, JU Minghe, et al. Study on the effect of grain size‑particle size ratio on the dynamic tensile properties of granite based on PFC3D-GBM[J]. Journal of Engineering Geology, 2022,41(3):28-38. [百度学术]
刘静,李江腾.基于颗粒流的大理岩三轴循环加卸载细观损伤特性分析[J].中南大学学报(自然科学版),2018,49(11): 2797-2803. [百度学术]
LIU Jing, LI Jiangteng. Analysis on meso-damage characteristics of marble under triaxial cyclic loading and unloading based on particle flow simulation[J]. Journal of Central South University (Science and Technology), 2018,49(11):2797-2803. [百度学术]
HE Pengfei, KULATILAKE P H S W, YANG Xuxu, et al. Detailed comparison of nine intact rock failure criteria using polyaxial intact coal strength data obtained through PFC3D simulations[J]. Acta Geotechnica, 2018,13(2):419-445. [百度学术]
PARK B, MIN K B. Bonded‑particle discrete element modeling of mechanical behavior of transversely isotropic rock[J]. International Journal of Rock Mechanics & Mining Sciences, 2015,76:243-255. [百度学术]
Lu Z, Yao A, Su A, et al. Re‑recognizing the impact of particle shape on physical and mechanical properties of sandy soils: A numerical study[J]. Engineering Geology, 2019,253:36-46. [百度学术]
Ding X, Zhang L, Zhu H, et al. Effect of model scale and particle size distribution on PFC3D simulation results[J]. Rock Mechanics and Rock Engineering, 2014,47(6):2139-2156. [百度学术]
毛海涛,黄海均,严新军,等.非饱和紫色土三轴试验颗粒流宏细观参数关系研究[J].工程地质学报,2021,29(3):711–723. [百度学术]
MAO Haitao, HUANG Haijun, YAN Xinjun, et al. Numerical study on macroscopic and microscopic parameters of particle flow in unsaturated purple soil trixial test[J]. Journal of Engineering Geology, 2021,29(3):711-723. [百度学术]
蒋成龙,许成顺,张小玲,等.三维柔性边界构建方法及其对砾质土变形发展影响的离散元数值研究[J].土木工程学报, 2021,54(5):77-86. [百度学术]
JIANG Chenglong, XU Chengshun, ZHANG Xiaoling, et al. Three‑dimensional flexible boundary construction method and its influence on the deformation development of gravel soil by discrete element simulation[J]. China Civil Engineering Journal, 2021,54(5):77-86. [百度学术]
Shivakumar P N, Sivakumar K C. A review of infinite matrices and their applications[J]. Linear Algebra and its Applications, 2009,430(4):976-998. [百度学术]
甘霖.循环应力-温度作用下花岗岩常规三轴力学行为研究[D].长春:吉林大学, 2021. [百度学术]
GAN Lin. Study on conventional triaxial mechanical behavior of granite tender [D]. Changchun: Jilin University, 2021. [百度学术]
MARTIN C D. The effect of cohesion loss and stress path on brittle rock strength[J]. Canadian Geotechnical Journal, 1997,34(5):698-725. [百度学术]