基于Griffith强度理论的岩石裂纹起裂经验预测方法研究

 裂纹起裂强度是岩石破坏过程中的重要应力阈值，研究岩石起裂准则对于揭示其破坏机制及预测围岩工程性质有着重要意义。首先进行青砂岩试样的单轴及三轴压缩起裂试验，并基于多种应变响应分析其中的起裂机制及细观破坏特征，指出局部张拉应力集中是起裂破坏的主因，总结提出低围压条件下的张开型起裂模型及高围压条件下的滑动型起裂模型。然后基于Griffith强度理论分析压应力场中岩石缺陷端部的局部最大张拉应力，其大小随差应力 的升高而增大，同时在围压条件下受表面摩擦作用的影响较大。针对岩石细观起裂机制提出起裂预测经验准则，准则中引入起裂参数 作为围压影响系数以表征摩擦作用，从而适用于不同围压条件下的起裂破坏预测。利用3组起裂试验结果对经验准则进行验证，其准确性及实用性明显优于传统线性起裂准则。最后通过分析不同围压下岩石起裂强度与峰值强度之比 ，发现试样在围压60 MPa以下时其起裂破坏属于细观张拉破坏机制。

A revised empirical method for predicting crack initiation based on Griffith strength criterion

Crack initiation stress is an important stress threshold in rock failure process. A better understanding of the rock failure mechanism can be obtained by studying its initiation criterion and a more reliable prediction of the rock properties can be acquired. The crack initiation tests on green sandstone from Chongqing under uniaxial and triaxial compression were conducted. The crack initiation mechanism and micromechanical feature were analyzed based on the multiple strain response，and it was found out that the induced tensile stress concentration caused by rock defects was the main reason for crack initiation. The initiation model of opening crack under the low pressure of confinements and the initiation model of sliding crack under the high pressure of confinements were obtained based on the above analysis. The maximum tensile stress around the rock defects was studied by employing the Griffith theory. The tensile stress increases as the deviatoric stress rises，which is strongly influenced by the friction effects under the high confining pressure. An empirical criterion of crack initiation prediction based on the different crack initiation patterns was proposed by introducing the parameter as the influence coefficient of the confining pressure. The parameter ，closely related to the internal frictional angle of rock，characterizes the friction effects in crack initiation，so the proposed empirical criterion can be applied to conditions of different confining pressures. The revised empirical criterion of crack initiation was verified with three sets of triaxial test on sandstone，granodiorite and granite，and its validity and applicability are much better than the conventional linear crack initiation criterion. It was also found that the crack initiation mechanism based on the analysis of is mainly tensile under the confining pressure of 60 MPa.