A new thermodynamically based model for creep and cyclic plasticity

Creep and cyclic plasticity are two common mechanical behaviors in engineering, and their interaction significantly affects the performance and degradation of corresponding components and structures, especially at high temperatures. How to build a physically based model for describing materials' mechanical response under complex loading is still a challenge. This study proposes a thermodynamically based model for describing complex creep plasticity behaviors, such as cyclic plasticity and creep. Unlike existing models, the proposed model can describe time-dependent cyclic plasticity and clarify the contribution of various creep mechanisms. Starting from the fundamental thermodynamic laws, Helmholtz free energy and dissipation analysis considering different creep mechanisms are introduced. Then, constitutive equations are derived according to the physical hypothesis of normality of dissipation space. In the proposed model, plastic deformation is thought to depend on static plasticity and creep, which is further decomposed into three parts based on diffusion and dislocation activities, and the contribution of each creep mechanism is explicitly described. Finally, a series experimental data of Ti-6-4 is used to verify the utility of the proposed model. Satisfactory modeling results are obtained, including primary creep, steady creep, power-law breakdown, and creep cyclic plasticity interactions, confirming the validity of the proposed model.