Effect of cyclic loading on local structural changes in a heat-resistant alloy

Abstract

The methodological approach to local inelasticity evaluation for cyclically loaded metallic alloys by its random distribution is proposed.Introduction. Data on dislocation structure evolution in cyclically loaded structural materials point to the local nature of structural changes [1][2][3]. Thus, microplastic strains, evolving at the initial damage stage, are prone to local distribution over microvolumes. Microvolumes with maximum structural changes exhibit a higher sensitivity of their elements to external loads. It stems from local stress concentrations, nonuniformity of impurity and alloy additive concentrations, closeness to a free surface. The polycrystalline structure formed in such a way displays distinct characteristics typical of a certain macrovolume [4][5][6].Physicomechanical properties of a polycrystalline body (strength, plasticity, inelasticity, etc.) are integral characteristics of the mechanical behavior of the material in deformation, which represent general response of discrete microstructural elements with random distribution of local properties of a structural component or lab specimen to the applied load. The initial distribution of nonuniform structure microproperties varies in line with mechanical damage and is somewhat dependent on the above factors. Each damage state is accompanied by a distribution of microvolume properties whose kinetics displays the steady-state regular trend typical of given loading conditions. Thus, damage may be represented by a certain random distribution of local properties, showing kinetics of structural changes. The damage of structural materials leads to changes in macroproperties, e.g., inelastic strains [7,8] or hardness [9], which points to possible correlation between the random distribution of microproperties and their macroscopic analogs.The object of the present study is to substantiate the methodology of quantitative damage estimation in fatigue based on the kinetics of local cyclic strain nonlinearity data scattering.Investigation Procedure. The nonlinearity of cyclic strains is represented by the phase shift between the applied stress and the material response, viz strains due to resonance oscillations of the material volume bounded by the contact loading zone on a surface layer 0.1 mm thick. Phase shifts were measured on standard specimens, prepared from an ÉP202 (KhN67VMTYu) heat-resistant nickel-base alloy, after their fracture in fatigue tests at 100 Hz and 10 kHz frequencies in symmetrical tension-compression. The procedure of measurements is described elsewhere [10][11][12]. The tests were performed in the circular zone of the cylindrical surface of the test portion of fractured specimens near the main crack.Investigation results are presented in Figs. 1-4. As is seen in Figs. 1 and 2, the frequency of cyclic loading exerts considerable influence on the fatigue resistance of this alloy. However, it is established in [13] that an increase in the frequency of symmetrical loading up to 10 kHz does not lead to any changes in structural and phase t...