Grain-scale failure mechanism of porous sandstone: An experimental and numerical FDEM study of the Brazilian Tensile Strength test using CT-Scan microstructure
Grain-scale failure mechanism of porous sandstone: An experimental and numerical FDEM study of the Brazilian Tensile Strength test using CT-Scan microstructure
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DOI：
10.1016/j.ijrmms.2020.104348
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发表时间：
2020-08
影响因子：
7.2
通讯作者：
Bin Chen;J. Xiang;J. Latham;R. Bakker
中科院分区：
文献类型：
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作者：
Bin Chen;J. Xiang;J. Latham;R. Bakker
许多广泛使用的岩石断裂数值模型基于有效“完整”强度参数的细观实验室测试表征，忽略了微观结构的影响。因此，它们无法解释晶界和孔隙对裂纹扩展的影响，因此不足以用于利用点应力和压痕应力的岩石破坏模型。因此，了解涉及钻头扣和/或喷水的深部钻探过程（其中岩石载荷集中在矿物颗粒较少的区域）将需要具有微观结构的模型。为了研究颗粒岩石在不同情况下的微观破坏机制，我们以多孔砂岩为目标，并引入了一种新颖的工作流程，该工作流程由基于计算机断层扫描 (CT) 的微观结构构建方法和补充的机械数值方法组成。该构造方法提取真实的岩石微观结构，并将大量体素 CT 扫描数据转换为明显更少的三角形元素。采用有限离散元法（FDEM）和基于晶粒的模型（GBM）来求解力学问题。利用数值结果和破坏后 CT 扫描试验数据，对巴西试验期间砂岩的微观破坏机制进行了彻底分析。说明了压应力和拉应力链的建立、微裂纹形核、局部松弛、链切换和利用孔隙的最终裂纹路径发展，揭示了时间和空间上的微观到宏观失效机制。巴西拉伸试验期间样品中的断裂路径主要由孔隙和晶间边界决定。据估计，晶间接头的拉伸强度至少是中尺度样品完整拉伸强度的3.67倍，而孔隙占断裂路径的72.76%。通过数值案例研究了胶结分布和微观不连续性的影响。
Many widely used numerical models of rock fracture based on mesoscale laboratory test characterisation of effective ‘intact’ strength parameters neglect microstructure effects. They therefore cannot explain grain boundary and pore effects on crack propagation and consequently are inadequate for models of rock destruction that exploit point and indentation stresses. Understanding deep drilling processes involving drill-bit buttons and/or water-jetting where rock loading is concentrated in domains with fewer mineral grains will therefore require models with microstructure. To investigate microscale failure mechanisms of granular rocks in diverse scenarios, we target a porous sandstone and introduce a novel workflow consisting of a computerized tomography (CT) based microstructure construction approach and a complementary mechanical numerical approach. The construction approach extracts the realistic rock microstructure and transforms the large voxel number CT-scan data into significantly fewer triangular elements. The finite-discrete element method (FDEM) with grain-based model (GBM) is adopted to solve the mechanics. The microscale failure mechanism of sandstone during the Brazilian test was thoroughly analysed using the numerical results together with the post failure CT-scan test data. The build-up of compressive and tensile stress chains, micro-crack nucleation, local relaxation, chain switching and final crack-path development exploiting pores was illustrated, revealing the micro-to-macro failure mechanism in time and space. Fracture paths in the specimens during Brazilian tensile test were dominated by the pores and the inter-grain boundaries. The tensile strength of the inter-grain joints was estimated to be at least 3.67 times the mesoscale specimen's intact tensile strength, while the pores account for 72.76% of the fracture path. The influence of the cementation distribution and microscale discontinuities was investigated with numerical cases.