Thermomechanical Fully Coupled Nonorthogonal Elastoplastic Constitutive Model for Concrete

Abstract 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.