压缩条件下玄武岩柱尺寸效应及破裂机理数值模拟研究

Numerical simulation of size effect and fracture mechanism of columnar jointed basalts under compression

为研究压缩条件下玄武岩柱强度和变形的尺寸效应,及其破裂机理和破坏模式,本研究构建不同尺寸的玄武岩柱图像,然后将细观损伤力学、统计强度理论、连续介质力学相结合,基于RFPA3D-CT的数字图像处理,将玄武岩柱图像转化为有限元网格模型,并分别赋予节理、岩石的材料力学参数,其中,考虑节理及岩石的非均质。开展压缩条件下的玄武岩柱数值试验,分析侧压、垂直柱轴方向(Ⅰ/Ⅱ)对玄武岩柱的力学尺寸效应的影响,以及玄武岩柱破裂过程的应力场演化与损伤破裂特征。研究表明:对于垂直柱轴方向Ⅰ的情况,当侧压为0 MPa,试件抗压强度无明显的尺寸效应;当侧压为2 MPa,试件抗压强度的尺寸效应临界值为4 m;当侧压为4、6、8 MPa,试件抗压强度的尺寸效应临界值为6 m。对于垂直柱轴方向Ⅱ的情况,当侧压为0、2 MPa,试件抗压强度无明显的尺寸效应;当侧压为4 MPa,试件抗压强度的尺寸效应临界值为4 m;当侧压为6、8 MPa,试件抗压强度的尺寸效应临界值为6 m。当侧压为0~8 MPa,随着模型尺寸的增加,试件抗压强度的横观各向异性系数变化范围为0.75~1.15;试件等效变形模量的横观各向异性系数变化范围为0.90~1.15。因此,玄武岩柱可近似为横观各向同性。分析了不同侧压、模型尺寸、垂直柱轴方向(Ⅰ/Ⅱ)条件下玄武岩柱的节理开裂情况、应力集中、裂纹萌生扩展、破碎带形成的全过程,以及声发射特征。

In order to study the size effect of strength and deformation,fracture mechanisms and failure modes of columnar jointed basalts(CJBs)under compression,the images of CJBs with different sizes are constructed,and combined with the meso damage mechanics,statistical strength theory and continuum mechanics,based on the digital image processing of RFPA 3D-CT software,the images of CJBs are transformed into finite element mesh models,and the mechanical parameters of joint and rock are given respectively,in which the heterogeneities of joint and rock are considered.Numerical tests of CJBs under compression are carried out to investigated the influences of lateral pressure and the directions(Ⅰ/Ⅱ)on mechanical size effect of CJBs.The stress field evolution and damaged fracture characteristics of CJBs during fracture are also revealed.The results show that for the directionⅠorthogonal to column axis,when the lateral pressure is 0 MPa,the compressive strength(CS)of specimen is without no obvious size effect;when the lateral pressure is 2 MPa,the critical value of CS size effect of specimen is 4 m;when the lateral pressure is 4,6,8 MPa,this critical value is 6 m.For the directionⅡorthogonal to column axis,when the lateral pressure is 0 MPa and 2 MPa,the CS of specimen has no apparent size effect;when the lateral pressure is 4 MPa,the critical value of CS size effect of specimen is 4 m;when the lateral pressure is 6 MPa and 8 MPa,this critical value is 6 m.When the lateral pressure is 0-8 MPa,with the increase of model size,the variation range of transverse anisotropy coefficient of CS is 0.75-1.15;that of equivalent deformation modulus(EDM)is 0.90-1.15.Therefore,the CJBs can be approximately transversely isotropic.The whole processes of joint cracking condition,stress concentration,crack initiation and propagation,fracture zone formation and acoustic emission(AE)characteristics of CJBs are analyzed under different lateral pressures,model sizes and the directions(Ⅰ/Ⅱ).