Characteristics of exponential strength criterion of rock in principal stress space

The exponential strength criterion with direct form denotes influences of the minor principal stress and intermediate principal stress on strength. The parameters in the strength criterion have definite mechanical background. Based on the conventional compression strength, the shear strength tends to be constant as normal stress increases; and the crack angle predicted by Mohr circle is closed to 45. However, the practical crack angle may not agree with the theoretical value because of small difference between envelope and Mohr circle near the tangent point and the heterogeneity of rock. The parameters in strength criteria are not sensitive to fitting errors because of dispersion of test data in the true triaxial compression; the best fitting solution determined by the minimum fitting error may not be able to describe the strength property of rock correctly. The characteristics of exponential strength criterion in principal stress space are studied. It is suggested that the parameters in strength criterion should be determined based on the fact that the influential index of the minor principal stress is equal to that when the intermediate principal stress is equal to the minor principals stress under the condition that the stress tends to be infinite. And the misfit of test data does not significantly increase.