Experimental and theoretical study on the shale crack initiation stress and crack damage stress

In order to study the crack initiation mechanism and damage evolution of shale during the rock progressive failure process,some conventional triaxial compression experiments under different confining pressures were conducted by using shale specimens with different bedding plane dip angles. Based on the inflection points of crack volume strain and specimens’ volume strain,the crack initiation stress and crack damage stress were determined respectively. The experiments results indicated that with the constant confining pressure,the inclination of bedding plane had little influ- ence on the crack initiation stress while the dip angle of bedding plane would affect the crack damage stress signifi- cantly,which induced that the failure patterns and strength of shale specimens were depended on the inclination of bedding plane. When the dip angle of bedding plane was same,the ratio of crack initiation stress and rock strength ele- vated with the increase of the confining pressure,but the ratio of crack damage stress and rock strength varied very little. This phenomenon demonstrated that the confining pressure had a significant effect on the crack initiation while had less influence on the crack unstable propagation. The crack initiation mechanism of shale was analyzed based on the compression-shear mixed mode crack in fracture mechanics. The model was verified as reasonable by experimental da- ta. The length of the compression-shear crack was predicted as 0. 985 mm by the presented model,which had a good a- greement with the existing literatures. The numerical results of the model showed that as the length of the compression- shear crack was over 3 mm,the value of crack initiation stress could be seen independent on the crack length.