Enhanced elastoplasticity-based frictional-collisional model for solid–fluid phase transition of granular media

Accurately describing the solid-like and fluid-like behaviors of granular media is crucial in geotechnical engineering. While the unified frictional-collisional model, integrating rate-independent frictional and rate-dependent collisional stresses, is widely used for solid–fluid phase transitions, an effective model is still under investigation, and comprehensive analyses are lacking. This study addresses these gaps by developing an enhanced elastoplasticity-based frictional-collisional model. The frictional stress is modeled using a critical-state-based elastoplasticity approach, and the collisional stress is formulated through an enhanced kinetic theory incorporating particle stiffness. Subsequently, comprehensive element simulations are conducted to explore the effects of concentration, particle stiffness, and strain rate paths on the model. The proposed model’s effectiveness is also validated against experimental data. Finally, a detailed comparison with the typical μ (I) rheology model and a state-equation-based phase transition model is conducted. Our analyses show that the developed model effectively captures strain rate path and particle stiffness through the collisional stress component, while concentration-dependent characteristics are captured through both frictional and collisional stress components. Through comparative analyses, we also found that both the state-equation-based and elastoplasticity-based models depict solid-like behavior and replicate the rheology of granular media in a fluid-like state, similar to the μ (I) model. However, they differ in implementing critical state theory: the state-equation-based model acts as a partial-range phase transition model, describing stress evolution from the critical state to the fluid-like state, while the proposed elastoplasticity-based model serves as a full-range phase transition model, covering stress evolution from the initial to the fluid-like state.