Abstract:Claystone, as a typical cohesive-frictional geological material, exhibits complex hardening patterns and dilatancy characteristics due to the dynamic evolution of cohesion and internal friction angle caused by mineral particle interlocking and clay matrix deformation under stress. Based on a cohesion-friction combined hardening pattern and a non-orthogonal plastic flow rule, this study constructed a three-dimensional non-orthogonal elastoplastic constitutive model for claystone. First, the evolution patterns of cohesion and internal friction angle during the hardening/softening processes were inversely derived using the Mohr-Coulomb strength criterion and multiaxial test data of claystone. These evolution patterns were then quantitatively described by introducing two independent hardening/softening functions. Subsequently, a stress-dependent plastic internal variable was proposed to effectively characterize the brittle behavior of claystone under low confining pressure and the ductile behavior under high confining pressure. By employing the fractional derivative, the non-orthogonal direction of the yield function was directly obtained as the plastic flow direction, thereby avoiding the complex process of constructing a plastic potential function in the non-orthogonal flow rule. Finally, the rationality of the model was evaluated through multiple sets of conventional triaxial drained tests. The results showed that the proposed model could reasonably describe the typical nonlinear mechanical behaviors of claystone, including strain hardening/softening characteristics, shear contraction/dilation phenomena, and the brittle-ductile transition under different confining pressures. This study provides a reliable constitutive model for the theoretical analysis and numerical simulation of the engineering mechanical properties of claystone.
