Study of the Regularities of Resistance to Deformation and Damage Accumulation under Irregular Low Cycle Loading

In this paper, we present the results of computational and experimental studies of changes in patterns of resistance to deformation and damage accumulation under conditions of irregular low-cycle loading, which are compared with similar data of a regular cyclic elastoplastic deformation upon equal loading. An irregular mode of low-cycle loading implemented in research is adopted as a uniform distribution of the change in the stress amplitude between specified maximum and minimum levels at the symmetric shape of the cycle. This mode was reproduced on test equipment by introducing into the control program of the corresponding functional dependence of changes in the stress amplitude in the cycles. Obtained in the irregular loading mode, the data on a cycle-by-cycle kinetics of both cyclic and unilaterally accumulated strains were recorded in a database and then compared with the data for a regular loading. This made possible their analytical description by the corresponding state equations with the correction of the diagram parameters of cyclic elastoplastic deformation, accounting for the irregularity conditions of loading modes. The experimental results are presented as test mode diagrams, curves of a low-cycle fatigue of the studied material in the soft and hard loading modes, diagrams of a cycle-by-cycle kinetics of the cyclic and accumulated strains in the regular and irregular modes, and kinetic diagrams of damage accumulation under these conditions. The summation criteria that include the deformation characteristics of accumulated damage are used. It is shown that, with allowance for change in the nature of the strain development during irregular low-cycle loading, the criterion dependences when accepting the condition of reaching the limiting state can be used to estimate the durability and compare it with the similar data in regular modes.