Thermomechanical Fully Coupled Nonorthogonal Elastoplastic Constitutive Model for Concrete

This study establishes a thermomechanical fully coupled nonorthogonal elastoplastic model, grounded in thermodynamic principles, to capture concrete's strength degradation, plastic strain accumulation, and temperature-enhanced dilatancy under thermomechanical coupling. The temperature-dependent degradation mechanism of concrete cohesion-friction characteristics is analyzed, and a temperature-dependent strength criterion is formulated. Based on the evolution of the yield surface on the meridional and deviatoric planes, a closed-form temperature-dependent yield function is proposed to characterize the influence of temperature on cohesion, frictional characteristics, and hardening-softening behavior. Under the small-deformation assumption, the stress-strain-temperature constitutive relationship is established by treating stress, hardening parameters, and temperature as constitutive variables, and a nonorthogonal flow rule is employed to realistically capture the temperature-dependent dilatancy behavior of concrete. Model analysis demonstrated that, under nonsteady stress-temperature loading paths, the predicted responses strictly satisfy the thermomechanical consistency condition throughout the loading process. Comparisons with multiple sets of experimental data obtained under constant-temperature and temperature-varying loading paths confirmed that the proposed model can accurately reproduce the strength degradation, plastic deformation response, and dilatancy evolution of concrete at different temperatures. The developed model provides a reliable theoretical tool for evaluating the performance and durability of concrete structures under thermomechanical coupling.