基于NDB试样的脆性岩石拉剪复合断裂行为研究

Experimental Investigation on the Tensile-shear Fracture Behavior of Brittle Rocks Using NDB Specimens

岩石复合断裂机理一直是岩石力学领域的研究热点。首先,利用新提出的单边切槽深梁(NDB)试样开展从纯Ⅰ型到纯Ⅱ型全复合度加载下的砂岩复合断裂试验,获得了系列载荷复合度下的砂岩复合断裂强度及砂岩断面。其次,研究砂岩复合断裂强度与随载荷复合度的变化行为,讨论了岩石复合断裂强度特征,并使用最常用的最大切应力(MTS)准则、最小应变能密度因子(MSED)准则、最大能量释放率(MERR)准则和改进的R(IR)准则预测了砂岩复合断裂强度。最后,使用激光扫描仪获得砂岩复合断面并进行断面重构,利用分形理论定量刻画岩石复合断裂面的粗糙度,研究岩石在复合载荷作用下的内在破裂机制。结果表明:砂岩等效断裂韧度随载荷复合度的增加逐步减小,IR准则与MERR准则可很好地预测砂岩复合断裂强度,MTS和MSED准则严重高估了Ⅱ型为主的砂岩复合断裂强度;砂岩断面不同复合载荷作用下的砂岩断面分统计特征具有统计意义上的相似性,一定程度上预示着不同复合载荷作用下的砂岩试件具有相似的内在破裂机制;考虑到脆性材料的强度特点,结合复合载荷作用下裂缝扩展行为,可确定拉剪复合载荷加载下岩石内在断裂机制的确为拉破坏。

Mixed-mode fracture is of concern in rock mechanics. To better understand the failure behavior of rocks under mixed-mode loading,several groups of mixed-mode fracture tests were conducted on sandstone using notched deep beam（NDB） specimens. In this study, the fracture loads and corresponding fracture surfaces under different mixed-mode loadings were obtained, and the relation between mixed-mode fracture strength of the sandstone specimens and loading mixity was discussed. Using three widely accepted fracture criteria（i.e., the MTS, MSED and MERR criterions）, the fracture loads were theoretically estimated, and our results show that the IR-criterion and the MERR criterion agreed well with the experimental data, whereas the MTS and MSED criterions significantly over-predicted the fracture strength of the sandstone specimens for mode-II-dominated loadings. Finally, a surface topography measurement for each rough fracture surface was conducted using a laser profilometer, and the fracture surfaces were experimentally obtained and numerically reconstructed. Using the fractal method, the mixed-mode fracture surface topography properties of these surfaces were then theoretically investigated. In our study, no obvious relationship between the fractal dimension and the initial loading mixity was observed. Therefore, the investigated surfaces were almost statistically identical. Such results emphasized that the failure mode of rocks induced by mixed-mode loading is essentially the tensile mode.