Scatter in the fatigue characteristics of steels and its analysis with allowance for cyclic inelastic strains

Abstract

The regularities of the scatter in the characteristics of fatigue and inelasticity of steels 45 and 1Kh13 have been studied under steady-state, step, and block loading. It is shown that the use of the inelastic strain per cycle as a measure of the fatigue damage accumulation rate makes it possible to take into account the scatter in the number of cycles to fracture and in the fatigue limits and to substantiate the cumulative fatigue damage rule.Keywords: fatigue, inelastic strain per cycle, scatter in mechanical properties, number of cycles to fracture, cumulative damage rule. Introduction.It is known that the number of cycles to fracture under high-cycle loading, and under step and block loading in particular, and the magnitudes of the fatigue limit of metals and alloys are subject to appreciable scattering [1-3 and others].It is important to investigate the regularities in this scatter and to justify the characteristics of the fatigue damage accumulation rate that would enable presenting the results of the investigation of the fatigue of metals and alloys without essential scatter.Cyclic inelastic strain is a promising characteristic for assessing the fatigue damage accumulation rate at the stage of nonlocalized fatigue damage, which is a stage where the main fatigue crack leading to final fracture of a specimen or a component is absent [4, 5, and others].Investigations of cyclic inelasticity (cyclic plasticity) are mainly performed in the low-cycle fatigue region at low loading frequencies. In this case, primary consideration is the investigation of interrelation between structural changes in the metal, which lead to the initiation of a fatigue crack, and the characteristics of cyclic inelasticity [6-8 and others], as well as justification of the strain and energy criteria of fatigue with allowance for the mean stress in a cycle, complex stress state, and loading regime [9-13 and others]. At the same time, analysis of many classical regularities in the high-cycle fatigue of metals and alloys such as, for instance, an essential scatter in the number of cycles to fracture and fatigue limits, inconsistency between the test results under step and block loading and the linear cumulative damage rule, etc., does not always involve the use of the results of studying inelasticity.Currently, much attention has been given to the consideration of the process of the fatigue of metals and alloys as a process of the fatigue crack development [14-17 and others]. No matter how attractive these approaches, one should bear in mind that, under high-cycle loading, the initiation stage for a fatigue crack of size 0.05 mm takes up to 90% and more of the total life of smooth specimens [18]. Therefore, the processes occurring at the stage of nonlocalized fatigue damage, for which cyclic inelasticity is an integral characteristic, can be determining for the regularities observed in high-cycle fatigue of metals and alloys.