Study of Material Properties under Complex Conditions of Low-Cycle Deformation

Abstract

The operability of structures in complex combined loading modes depends on a significant number of combinations of operational parameters of thermomechanical impacts in terms of loads, temperatures, times, numbers of cycles, and strain rates. The main strain patterns of structural materials under complex conditions are established using combined standard, unified, and special tests under laboratory conditions. With the use of representative substantiations of physicochemical models for strain diagrams in a wide range of loading conditions and with allowance for the scale diversity of models, the material structure, and the responsibility of structures, we propose a step-by-step consideration of the corresponding strain types: elastic, sign-alternating flow, progressive strain accumulation, and their combinations. In this case, structural calculations can be built as a hierarchical system, in which each next level refines the boundaries of permissible impacts toward expansion of the range of acting loads, temperatures, rates, and strain modes, which is associated with an increase in the amount of required initial data and complicates calculations. The proposed methods for schematization of physicomechanical properties and types of state equations for describing strain curves account for the compactness requirements of the initial data and the need to use both standard and unified methods for determining the characteristics of cyclic inelastic deformation and special methods. Both from a theoretical standpoint and from the viewpoint of practical applications, power equations are the most reasonable to describe the kinetics of strain diagrams under the considered conditions. Exponential dependences are suitable to reflect the role of the temperature factor while power dependences are suitable to take into account the time and strain-rate factors and two-frequency loading conditions. Ensuring the maximum possible use of the strain and strength reserves of materials and structures, refined calculations at higher, more complex levels of the considered hierarchy must be based on kinetic dependences describing low-cycle deformation in complex loading modes.