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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σ_1-σ_3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m_(ci)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σ_(ci)/σ_F,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。
Crack initiation strength is an important stress threshold in the process of rock failure. Studying the rules of rock initiation is of great significance for revealing the failure mechanism and predicting the engineering properties of surrounding rock. First, the uniaxial and triaxial compression cracking tests of the sandstone samples were carried out. Based on the analysis of the initiation mechanism and the meso-failure characteristics of the samples, it was pointed out that the local tensile stress concentration was the main cause of the initiation of failure and concluded that Open type cracking mode under low confining pressure and sliding type initiation model under high confining pressure. Then, based on the Griffith strength theory, the local maximum tensile stress at the end of rock defects in compressive stress field is analyzed. The size increases with the increase of differential stress σ_1-σ_3, and is greatly affected by surface friction under confining pressure . Aiming at the meso - cracking mechanism of rock, a rule of crack initiation prediction is proposed. The criterion of cracking initiation is introduced into the guideline as the influence coefficient of confining pressure to characterize the friction effect, which is suitable for prediction of initiation and fracture under different confining pressures. The test results of the three groups of cracking tests verify the experimental criteria, and their accuracy and practicability are obviously better than the traditional linear cracking rules. Finally, by analyzing the ratio σ_ (ci) / σ_F of rock initiation strength and peak strength under different confining pressure, it is found that the cracking initiation of the specimen under the confining pressure of 60 MPa is a meso-scale tensile failure mechanism.