三轴应力下致密砂岩的裂纹发育特征与能量机制

CRACKS DEVELOPMENT FEATURES AND ENERGY MECHANISM OF DENSE SANDSTONE SUBJECTED TO TRIAXIAL STRESS

开展致密砂岩三轴压缩试验及CT扫描试验,获得不同围压作用下岩石破坏裂纹的几何形态CT图像;利用图像处理、统计学等方法构建破坏裂纹形态的三维空间模型,分析不同围压对破坏裂纹几何形态和分布特征的影响规律,基于能量理论揭示不同三轴应力下岩石破坏时裂纹扩展的能量机制。研究表明:围压对岩石破坏裂纹的形态、数量和空间分布特征有很大的影响。当围压较低时,破坏裂纹数量众多、形态复杂,最终形成主裂纹和次生裂纹交叉分布的裂纹网络结构;围压较高时,最终形成的破坏主裂纹数量减少,次生裂纹消失,形态复杂的裂纹网络被近似直线的破坏裂纹所取代;围压对岩石破坏裂纹扩展的能量耗散和能量释放特征有显著的影响。随着围压的增加,单位体积内的可释放弹性应变能线性增加,而耗散能则呈线性递减趋势。低围压时破坏裂纹的耗散能较大,从而产生几何形态复杂、数量众多的微裂纹。而高围压时的耗散能较少,产生的破坏裂纹数量减少,几何形态趋于简单化、规则化。

A series of triaxial compressive tests and CT scanning tests on dense sandstones were carried out. The CT images of failure cracks in rocks under different confining pressures were obtained. Using the image processing technique and the statistic principle, three-dimensional geometric models of failure cracks are established. Based on these three-dimensional models, the influences of confining pressure on geometric form and distribution features of failure cracks were analyzed. The energy theory was employed to reveal the intrinsic energy mechanism of crack growth characteristics in rocks subjected to triaxial stress. The results show that the confining pressure had great effects on the forms, the quantity and the special distribution features of failure cracks in rocks. When the confining pressure value is lower, the great number of the failure cracks occurs with complex formation. When rocks are completely frac^xred, the main cracks and secondary cracks cross each otherforming a crack network. When the confining pressure is higher, the number of main cracks decreases and secondary cracks disappear. The cracks network with complicated geometrical form is replaced by linear failure cracks in similar. The confining pressure has significant influences on the characteristics of energy dissipation and energy release of the failure cracks. With the increase of the confining pressure, the releasable elastic strain energy increases linearly and the dissipation energy gradually decreases. Under the lower confining pressures, rocks have more dissipated energy leading to a large number of failure microcracks with complex geometric formations. While under the higher confining pressures, rocks have less dissipation energy leading to less failure cracks with the regular geometric formations.