Kaiser效应方向独立性的控制参数理论分析

基于Kaiser效应由裂纹扩展释放弹性波的认识,当远场应力为压应力时,从断裂力学角度对Kaiser 效应随加载方向变化规律进行了分析,建立了临界应力${{\sigma }_{\text{c}}}$与裂纹面方向夹角$\beta $、裂纹面摩擦系数$f$的理论联系,揭示了Kaiser 效应方向独立性机理及其控制参数。结果表明：Kaiser效应方向独立性受临界应力相对值影响,其控制参数为初始加载方向与微裂纹面方向的夹角${{\beta }_{1}}$、裂纹面的摩擦系数$f$。夹角${{\beta }_{1}}$越大,FR值大于1.1所需加载偏转相对角度$\gamma $越小,加载方向偏转后所得应力与初始加载应力相差越大,Kaiser效应方向独立性越明显,即Kaiser效应测量初始应力精度与岩体初始微裂纹分布有关。摩擦系数$f$越小,FR值大于1.1所需加载偏转相对角度$\gamma $越大,加载方向偏转后所得应力和初始加载应力相差越小,Kaiser效应方向独立性越不明显,即岩样如越湿润,Kaiser效应测试初始应力结果离散性越大。以上结论与既有试验在变化规律上具有一致性,并通过特征曲线对比确定了试验控制参数为微裂纹面摩擦系数$f$小于0.5、初始加载方向与微裂纹面方向夹角${{\beta }_{1}}$在30°~40°之间,这均与试验条件较为吻合。以上结论可为进一步研究Kaiser效应机理提供参考。

Theoretical analysis of controlling parameters of direction independence of Kaiser effect

Based on the acknowledgement that the Kaiser effect is induced by the released elastic wave of crack propagation when the far-field stress is a compressive one, the variation of the Kaiser effect with loading direction is analyzed from the perspective of fracture mechanics. The theoretical relationship among the critical stress ${{\sigma }_{\text{c}}}$, the direction of the crack surface $\beta $ and the friction coefficient $f$ of the crack surface is established. The mechanism of the direction independence of the Kaiser effect and its control parameters are revealed. The results show that the direction independence of the Kaiser effect is affected by the relative value of the critical stress, and the control parameters include the angle between the initial loading direction and micro-crack surface ${{\beta }_{1}}$ and the crack surface friction coefficient $f$. When the angle ${{\beta }_{1}}$ is larger, the relative angle of loading deflection $\gamma $ is smaller to keep FR value greater than 1.1. There is greater difference between the stress obtained after the deflection of the loading direction and the initial loading stress, and the direction independence of Kaiser effect is more obvious. The accuracy of the initial stress measured by the Kaiser effect is related to the distribution of the initial microcrack of the rock mass. The smaller the friction coefficient $f$, the larger the relative angle $\gamma $ required for the FR value to be greater than 1.1, and the smaller the difference between the meseared stress and the initial loading stress after the deflection, the less obvious the independence of the Kaiser effect. The watter the rock sample, the greater the dispersion when measuring the initial stress with Kaiser effect. The above conclusions are consistent with the rules of the previous test results. By comparing the characteristic curves, the control parameters are established: friction coefficient on the microcrack surface is less than 0.5, the angle between the initial loading direction and the microcrack surface is about 30°~40°, which agrees well with the test conditions. They can provide references for further studies on the mechanism of the Kaiser effect.