Calculation of Life Characteristics for Structural Alloys under the Mutual Influence of Fatigue and Long-Term Strength

The processes of degradation of the initial strength properties of polycrystalline structural alloys are considered under mechanisms that combine low-cycle fatigue and long-term strength of the material. From the point of view of mechanics of damaged medium (MDM) and fracture mechanics (FM), a mathematical model that describes the processes of cyclic viscoplastic deformation and damage accumulation in structural alloys under multiaxial disproportionate modes of combined thermomechanical loading has been developed. The model consists of three interrelated components: relations that determine the cyclic viscoplastic behavior of the material by taking into account the dependence on the fracture process; evolutionary equations describing the kinetics of damage accumulation; criterion of the strength of the damaged material. The viscoplasticity model is based on the idea of the existence of plasticity and creep surfaces in stress space and the principle of gradient vectors of plastic and creep strain rates to the corresponding surface at the loading point. This form of the equations of state reflects the main effects of cyclic viscoplastic deformation of the material for arbitrary complex loading trajectories. The form of kinetic equations for damage accumulation is based on the introduction of a scalar damage parameter and on energy principles. It also takes into account the main effects of the formation, growth and merging of microdefects under arbitrary complex modes of combined thermomechanical loading. A joint form of the evolutionary equation for damage accumulation in the areas of low-cycle fatigue and long-term strength of the material is proposed. As a criterion for the strength of a damaged material, the condition for reaching a critical value is used. The material parameters and scalar functions included in the constitutive relations of the MDM mathematical model are obtained. The results of numerical simulation of the processes of deformation and damage accumulation in structural alloys under the mutual influence of low-cycle fatigue and long-term strength of the material are presented. The results of comparison of calculated and experimental data show that the proposed MDM model qualitatively and with the accuracy necessary for practical calculations quantitatively describes the durability of materials under the mutual influence of low-cycle fatigue and long-term strength of the material.