Method for processing of the results of low-cycle fatigue tests

Abstract

We propose a method for processing the data of low-cycle fatigue testing metals. This method enables us to determine the characteristics of the materials in various periods of the fatigue life. The results ob- tained by using the proposed method make it possible to take into account all visible changes in the cy- clic properties of the material in the course of fatigue-life calculations. This is of especial importance for materials characterized by the absence of stabilization period. In the method of fatigue-life calculations based on the local strain and stress analyses (1), it is customary to use the data on materials determined in the course of fatigue tests in the area of low-cycle fatigue. The proce- dures of realization of the tests and processing of their results correspond to the ASTM E 606-04: Standard Prac- tice for Strain-Controlled Fatigue Testing. According to the directions specified in this document, the low-cycle properties of metals are determined on the basis of constant-amplitude tests for several (minimum five) levels of controlled stresses or strains (2). For each level of strains or stresses, at least three fatigue tests are performed. In each fatigue test, instantaneous values of the loading force and strains formed in the specimen are recorded for the selected loading cycles. The recorded values of stresses and strains enable to carry out the analysis of the basic parameters of the hysteresis loop and deduce relations between these parameters. One of the two characteristics evaluated on the basis of the performed fatigue tests is the diagram of cyclic strains describing the relationship between the plastic strain amplitude eap and the stress amplitude σ a . It is assumed that these parameters obey an exponential relationship from the stabilization period. In the bilogarith- mic coordinate system, this relationship is described by the Morrow equation (3):