受拉加载方向变化对Kaiser效应点准确度的影响

基于Kaiser效应由裂纹扩展释放弹性波产生的认识,依据断裂力学理论就加载方向变化对Kaiser效应的影响进行分析。研究了远场应力为拉应力时,二维Ⅰ,Ⅱ型混合裂纹扩展的临界应力相对值、FR比值与加载方向偏转角之间的关系。结果表明：加载方向对Kaiser效应点反应岩石记忆先期荷载值的准确度有较大影响,其变化大小与临界应力相对值正相关。若第一次加载方向与裂纹面垂直,偏转角度从0°~90°变化,裂纹扩展临界应力相对值和FR比值均不断增大。偏转角为0°~20°时,FR比值变化范围为1~1.1,Kaiser效应点记忆较为准确,但第二方向所记忆的荷载并非该方向的正应力值。偏转角为90°时,临界应力相对值和FR比值均趋近于无穷大,裂纹在该裂纹面方向上不能发展,说明Kaiser效应消失。这与部分学者的试验结果一致,说明此类劈裂试验中Kaiser效应的产生,裂纹扩展是主要原因,也在某种程度上解释了该类试验加载方向偏转角度越大,Kaiser效应越来越不明显的问题。

Influence of tensile loading direction on accuracy of determining Kaiser effect point

The Kaiser effect is caused by the elastic waves generated by the crack propagation. Based on this common understanding and the theory of fracture mechanism, the influence of the loading direction on the Kaiser effect is discussed when the far field stress is the tensile one. The relationship between the critical stress of the crack propagation for I + II mixed mode in plane, the value of Felicity ratio (FR) and the rotational angle of loading are studied. The results show that the loading direction has important influence on whether the Kaiser effect can correctly reproduce the information on the peak stresses of the rock experienced in the past. If the loading in the first cycle is perpendicular to the crack and the rotational angle changes from 0° to 90°, the critical stress of the crack propagation and the FR value will increase with the rotational angle. When the rotational angle varies between 0° to 20°, the FR will value the changes between 1 to 1.1, and the “Memory” of the Kaiser effect is more accurate, while the stress memorized by rock in the second direction is not the normal one according to that direction. When the rotational angle is 90°, both the critical stress of the crack propagation and the FR value increase to infinite, and the crack isn’t able to propagate in its crack surface. Hence it results in complete disappearance of the Kaiser effect. They are consistent with those of some other researchers. Therefore it can be concluded that the crack propagation is the main cause of the Kaiser effect in this kind of split tests. It can also explain why the Kaiser effect becomes more and more inapparent with the increasing rotation angle.