基于颗粒流的砂岩三轴破裂演化宏-细观机理

Macro-micro evolution mechanism on sandstone failure in triaxial compression test based on PFC2d

  • 摘要: 以重庆地区砂岩的三轴压缩试验应力-应变曲线为基准,完成了二维颗粒流PFC2d数值砂岩试样的细观参数标定。针对0.5~20 MPa围压下数值砂岩在双轴试验中的宏观力学响应,从试样内部位移、3种类型裂纹(总裂纹、拉裂纹和剪裂纹)的数目演化和角度分布等细观层面进行了解释。研究结果表明:PFC2d数值试样能代表物理砂岩的主要宏观力学特性;试验过程中,试样内3种裂纹数目均呈“S”型累积,其突变点与宏观应力-应变曲线的弹、塑和破坏特征点对应;随着围压增大,由试验过程中剪裂纹始终占主导逐渐变为拉裂纹始终占主导,拉裂纹的占比逐渐增大;对于整个试样和破裂带,破裂带内拉裂纹的占比始终高于整个试样;总裂纹和剪裂纹角度分布的主方向明显,与加载方向相同;拉裂纹的角度分布较为均匀,在加载方向、破裂面及共轭破裂面方向的集中度相对明显。

     

    Abstract: Laboratory and PFC2d (particle flow code in 2-dimension) numerical tests were both employed to investigate into the macro-micro evolution mechanism of sandstone in Chongqing area, China.Triaxial compression tests of sandstone were carried out by RMT-150C rock mechanics test system in laboratory, with the obtained stress-strain curves, appropriate micro parameters of PFC2d sandstone samples were calibrated by letting numerical sandstone samples show similar macro characteristics to the corresponding laboratory samples.Based on a series of biaxial compression tests of PFC2d sandstone samples under the confining pressure of 0.5-20 MPa, the macro mechanical behaviors of sandstone were explained from micro level, i.e., the deformation distribution of samples, the cumulative number and angular distribution laws of three kinds of crack (total crack, shear crack and tension crack) during the failure process.Numerical tests results show that the cumulative numbers of three kinds of cracks all increase as S-shaped curves, and the changing points are consistent to the elastic, plastic and failure points of corresponding macro stress-strain curves, which indicates that the occurrence and growth of cracks determine the macro behaviors of rock.With the confining pressure increasing, the dominant type of crack is no longer shear crack but tension crack, and the number proportion of tension crack in the whole sandstone sample and fracture band are both increased.In the whole samples and fracture bands under all confining pressure levels, the proportion of tension crack in fracture band are slightly bigger than that of whole sample, and the principal directions of angle distribution for total cracks and shear cracks are always along the axial loading direction.However, the angle distribution of tension cracks are uniform, cracks close to the angle of fracture surface, axial loading direction and conjugate fracture surface are relatively obvious.

     

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