Three‐dimensional hardening‐softening elastoplastic modeling of nondilatant rock‐like materials with nonorthogonal flow rule

Existing constitutive models have rarely addressed the anisotropic hardening‐softening deformation of nondilatant rock‐like materials under true‐triaxial stress conditions. A three‐dimensional elastoplastic constitutive model incorporating a novel nonorthogonal flow rule is developed to describe this behavior. The three‐dimensional yield function combines a power function in the meridian plane with the Matsuoka‐Nakai criterion in the deviatoric plane, while its parameters are formulated as piecewise functions governed by a plastic internal variable. The proposed nonorthogonal flow rule employs the dilation and plastic deflection angles to independently control the plastic flow directions in the meridian and deviatoric planes. The dilation angle is determined from the symmetric component of plastic volumetric strain with respect to plastic shear strain in the deviatoric stress direction, whereas the plastic deflection angle is determined from the plastic shear strain in the Lode angle direction or its symmetric component, depending on its sign. Fractional orders regulate the evolution of the corresponding plastic shear strains, enabling a flexible description of anisotropic plastic flow. The model is validated against true‐triaxial experimental data for sandstone and cement stone. It accurately reproduces the suppression of intermediate principal strain dilatancy in sandstone and the transition of intermediate principal strain from dilation to contraction in cement stone with increasing intermediate principal stress, demonstrating its accuracy and applicability. The proposed model provides a theoretical basis for stability assessment of deep underground engineering involving nondilatant rock‐like materials.