基于SENDB试样的砂岩复合脆性断裂行为研究

基于岩石裂缝多为三维的事实,对不同裂缝长度和支座间距组合下的单边切槽深梁(SENDB)试样3个形状因子(即量纲一化的Ⅰ型和Ⅱ型应力强度因子、量纲一化的T应力)进行三维标定,并与已有的二维数值标定结果进行细致对比,相关结果为科学使用SENDB试样开展岩石复合断裂韧度测试,讨论试件厚度对岩石复合断裂韧度计算的影响等奠定了基础。结果表明,由三维模型获得的几何因子相比二维模型所得结果更大,故使用二维计算结果确定的断裂韧度将比实际值小,对水力压裂这类"造缝"岩石工程设计可能带来不利影响。最后,基于SENDB试样开展从纯Ⅰ型到纯Ⅱ型的砂岩复合断裂韧度测试,并利用广义最大周向应力(GMTS)准则从理论上研究SENDB砂岩试样裂纹尖端的三维微破裂区与载荷复合度的关系。研究表明,随着裂缝倾角b的增加,由最大周向应力确定的微破裂区范围从中间大两头小变为中间小两头大;其次,裂纹在扩展方向上的临界微破裂区并非GMTS准则所采用的不变量r0=(2π)-1(KIc/st)2;更为关键的是,纯Ⅰ型裂纹和纯Ⅱ型裂纹对应的临界微破裂区半径并不相等,进而从根本上否定了GMTS准则的理论假定,相关结果为合理使用GMTS准则提供了参考并指明了GMTS准则的改进方向。

Investigation on mixed-mode fracture behavior of sand stone using a SENDB specimen

Based on the fact that rock fractures are usually three-dimensional，the dimensionless stress intensity factors(i.e. YI and YII) of mode I and mode II and the dimensionless T-stress of the single edge notched deep beam(SENDB) specimen were calibrated using 3D numerical models，and compared with the results obtained with 2D numerical models from a reference. The 2D models usually provide the low values for SIFs and T* than the 3D models. Accordingly，the fracture toughness calculated with a 2D model was lower than the real fracture toughness value of rock，which may have an unfavorable effect in the design of fracturing-related rock engineering. The SENDB specimens made of sandstone were tested in a full range of mixed modes from pure mode I to pure mode II. The micro-crack zones(MFZ) were theoretically estimated based on the criterion of generalized maximum tangential stress (GMTS). Under the mode-I dominated loadings，the maximum MFZ was found in the middle of the specimen and the minimum MFZ on the sample surface. While for the mode-II dominated fractures，the size of the MFZ on sample surface is much larger than that in the middle of the sample. The critical radius of the MFZ for mode I fracture was much smaller than that for mode II fracture， indicating that the critical radius r0 of the MFZ in the crack propagation direction was not an invariant. It is obvious that the hypothesis of a constant r0 adopted by the GMTS criteria is questionable.