非均质性对砂岩试样宏细观破坏特征影响研究

Study on the effect of heterogeneity on the macro and mesoscopic failure characteristics of sandstone samples

砂岩的细观非均质性是造成应力-应变曲线及破坏特征差异性的本质原因。为探究非均质砂岩的宏细观破坏特征,采用X射线衍射(XRD)精细表征砂岩的矿物成分和含量,通过Fish语言构建矿物、节理力学参数正态分布的UDEC-Tri非均质模型,提出非均质岩样的损伤判别指标,研究了矿物颗粒弹性模量、矿物颗粒空间分布及节理黏聚力的非均质性对岩样宏观破坏及细观损伤的影响规律。结果表明:矿物弹性模量的非均质度(C^(b)_(v))增大,岩样应力-应变曲线的非线性特征增强,切线模量和抗压强度呈线性降低趋势,宏观破坏形式从剪切破坏向劈裂破坏转变。另外,C^(b)_(v)值增大导致更多相邻矿物颗粒屈服不同步,张拉裂纹占比增加,岩样起裂应力与损伤应力降低。C^(b)_(v)值相同时,矿物颗粒空间分布导致相邻颗粒之间出现力学参数“薄弱接触面”,与宏观裂缝扩展路径的一致性较好,是低强度岩样破裂特征离散性的根本原因;相比矿物C^(b)_(v),节理非均质度(C^(j)_(v))对岩样力学性质的弱化作用更显著。随着C^(j)_(v)的增大,岩样张拉裂纹长度逐渐增加直至超过剪切裂纹,小尺度裂纹占比增大,其破裂模式从“单斜面剪切破坏”转变为“X状共轭斜面剪切破坏”。研究结果解释了同一产地、尺寸砂岩试样破坏特征差异性的机制,为建立非均质与试样强度、破坏特征的映射关系提供参考。

The mesoscopic heterogeneity of sandstone works as the fundamental cause of its stress-strain curve and variances in failure characteristics.In this paper,to investigate the macroscopic and mesoscopic failure characteristics of heterogenous sandstone,diffraction of x-rays(XRD)was employed for the precise characterization of mineral components and contents of sandstone,fish language was utilized to establish the UDEC-Tri heterogeneous normal distribution model of minerals and joint mechanical parameters.Indicators to identify damage of heterogenous rock samples were proposed,and the influence rules of heterogeneity in modulus of elasticity,spatial distribution of mineral particles and joint cohesion of minerals on the macroscopic failure and mesoscopic damage of rock samples were studied.The results show that with the increase of heterogeneous level(C^(b)_(v)),the nonlinear characteristics of stress-strain curves of rock samples are enhanced,the tangent modulus and compressive strength decreasing linearly,and macroscopic failure changes from the mode of shear failure to splitting failure.The increased C^(b)_(v) value results in a non-synchronized yield of adjacent mineral particles,a higher proportion of tension crack,and decreased crack initiation stress and damage stress of rock samples;when embodying the identical C^(b)_(v) value,the variance distribution in minerals of rock samples leads to the presence of“weak contact surfaces”,as measured by mechanic parameters,between adjacent particles,which shows good consistency with the extension path of macroscopic crack and serves as the basic cause of the discreteness of crack characteristics in low-strength rock samples.Compared with the mineral C^(b)_(v),joint heterogeneity(C^(j)_(v))embodies a more conspicuous weakening effect on the mechanical properties of sandstone.As C^(j)_(v) increases,the length of tension crack grows to more than that of shear crack,with an enlarged proportion of small-scale crack;and with the increase of joint cohesion C^(j)_(v),the failure mode of rock changes from“shear failure on single slopes”to“X-shaped shear failure on conjugated slopes”.In general,the research findings explain the mechanism of variances in failure characteristics of sandstone samples collected of the identical origin and the same size,as references for building the mapping relation between heterogeneity and sample strength,and between heterogeneity and failure characteristics.