当井壁岩石所受的应力超过其强度时就会发生井壁失稳。
If the stress that ACTS on shale around wellbore exceeds its intensity, borehole instability will happen.
结合压缩应力应变关系可进一步确定弹塑性失稳点的位置，从而导出压缩强度的理论公式。
Further determination of the elastoplastic instability point is facilitated by the compressional stress strain relation, which then gives rise of the final formula of the compressive strength.
本文根据砂岩储层的物理力学特征，运用断裂力学理论，给出了储层裂缝的应力强度因子的计算方法，定量地分析了开发过程中储层裂缝失稳扩展的判据、裂纹扩展方向。
Using the LFEM theory, the author provided the computing method of stress strength factor of crack and the criteria of crack instability propagation and propagation angle.
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