红层软岩压缩破坏的分形特征与级联失效过程

Fractal characteristics of compression failure of red soft rock and cascading failure process

基于不同围压条件下的红砂岩三轴压缩试验结果,采用数字图像相关技术对岩样外表面破坏形态进行全程观测,获得不同阶段岩样外表面破坏图像,分别对岩样破坏后阶段的图像进行分析。通过分形盒维数表征岩样外表面裂纹的变化情况,发现岩样破坏越严重,裂纹分形维数越高,且在最后破坏阶段裂纹分形维数出现突变,揭示岩样的破坏特征。引入级联失效数学模型,结合红层软岩内部的矿物成分与细观结构分布特点,概化出其结构的随机连接模型与破坏方式,分析红层软岩加载作用下的内部节点颗粒'容量-荷载'重分配原则以及节点颗粒失效后其初始荷载传递路径。结果表明,红层软岩在受载时为一个级联失效的分阶段破坏过程,级联因子通过级联路径不断传播、增多,级联失效越发明显,同时结合岩石破坏的变形损伤过程与其轴向应力-应变曲线分段特征,揭示该类岩石破坏的级联失效分级标准,为拓展该领域的研究具备一定的参考价值。

Based on the triaxial compression test under different confining pressures of red sandstone,the failure characteristics at the outside surface of rock samples is monitored by using digital image correlation technique,and the failure images of outside surface of rock samples at different stages are obtained.The images showing the failure of the rock samples are analyzed and the fractal dimension calculated by box-counting method was used to characterize the variation of cracks on the outer surface of rock samples.It is found that the fractal dimension of cracks is proportional to the degree of damage in the rock samples.In addition,at the final stage of the failure in the rock sample,the fractal dimension of cracks appears an abrupt change,demonstrating the failure characteristics of rock samples.Combined with the mineral composition and mesostructure distribution characteristics of the red-bed soft rock,the cascade failure model is introduced to generalize the random connection model and failure mode of the structure.Furthermore,the redistribution principle of'capacity-load'of internal nodular particles under loading,and the initial load transfer path of nodular particles after failure are also analyzed.Results show that the red-bed soft rock under loading presents a multiple stage failure process,i.e.cascade failure.The cascade factor is continuously propagated and increased through the cascade paths.Thus the cascade failure becomes more obvious than before.Combining the deformation and damage process of rock failure and the interval characteristics of its axial stress-strain curve,the cascade failure classification criterion of rock failure is revealed,which has a certain reference value for expanding the research in this field.