三轴应力下致密砂岩裂纹展布规律及表征方法

利用三轴试验机和CT扫描设备,开展了多组不同围压作用下的致密砂岩三轴压缩试验及CT扫描试验,得到了不同三轴应力条件下砂岩破坏裂纹的CT图像；利用图像处理技术和统计学原理提取了裂纹展布的几何形态。引入裂纹的宽度、长度、面积和分形维数等几何参数描述了破坏裂纹的空间形貌,分析了不同围压条件下岩石破坏裂纹面积、长度、宽度和分形维数的分布特征,揭示了围压应力对上述裂纹几何特征的影响规律。研究表明：围压应力对破坏裂纹的面积、长度、宽度和分形等几何特征有显著的影响,低围压条件下,裂纹的面积、长度和宽度都较小,随着围压的增加裂纹的面积、长度和宽度也增大,且概率密度分布函数有了明显的差异；破坏裂纹的分形维数随着围压的增大呈指数递减趋势,低围压条件下裂纹的分形维数大,说明裂纹形态复杂,曲线边界粗糙,产生的小裂纹较多,且相互交错分布,形成的裂纹网络结构占据了整个试件二维横截面图。随着围压的增大,破坏裂纹的分形维数减小,说明裂纹形态趋于规则,小裂纹减少,形成了近似直线的光滑主裂纹。

Growth distribution laws and characterization methods of cracks of compact sandstone subjected to triaxial stress

A series of triaxial compressive tests and CT scanning tests on tight sand are carried out using the triaxial testing machine and CT scanning machine. The CT images of fractured cracks subjected to diverse triaxial stresses are obtained. The image processing technique and statistic principles are employed to extract geometric morphology of crack growth. The geometrical parameters are proposed to describe the growth morphology of fractured cracks, i.e., area, length, width and fractal dimension. The distribution characters of geometric parameters are analyzed. The effects of confined pressure on distribution characters of the above geometric parameters are exposed. The results show that the confined pressure has significant effects on the geometrical features of fractured cracks, such as area, length, width and fractal dimension. The area, length and width are all smaller when the confined pressure is smaller. With the increase of the confined pressure, the values of geometrical parameters rise also. The diverse confined pressures lead to different probability density distribution functions of geometrical parameters. The fractal dimensions of fractured cracks accord with the exponential decline law when the confined pressure rises. The fractal dimensions of cracks subjected to low confined pressure are larger than the ones subjected to high confined pressure. When the confined pressure is lower, the cracks with larger fractal dimensions exhibit complex geometric morphology and rough boundary curves. The crack networks with more small cracks occupy the whole cross section of specimens. When the confined pressure is higher, the cracks with less fractal dimensions become more regular and smooth. The main cracks approximating straight line appear and the small cracks disappear.