Simple, Convenient Isotropic Failure Surface

A simple, convenient isotropic failure surface is developed that includes the classical failure criteria of maximum‐octahedral‐shear stress and Mohr‐Coulomb as special cases. In addition, the failure surface is general enough to generate a smooth failure curve in the octahedral plane that passes through experimental data for triaxial compression (TXC), a deviatoric state of torsion (TOR), and triaxial extension (TXE) as functions of pressure. To demonstrate the versatility of the failure surface, examples using data for limestone and Monterey sand are considered. Detailed analysis of the normal‐to‐the‐failure surface in stress‐space indicates that this normal is uniquely defined everywhere on the surface but that it may be discontinuous at TXC and TXE. The relationship of this result with the common notion that this normal is undefined at the “corners” of the Tresca failure surface is explained.