A. N. MailoUDC 621.921We have assessed the characteristic of local inelasticity distribution, which controls the evolution of microstructural processes in a loaded surface layer of D16T structural aluminum alloy specimen in view of the strain-hardening stages and deformation mechanism changeover. The deformation stages for static tensile conditions are correlated with the characteristic of local inelasticity distribution, in order to determine the relationship between the deformation mechanism changeover and local inelasticity kinetics for the transition point from strain-hardening to strain-softening of the alloy under study.Introduction. Physical processes occurring in a material under action of mechanical loads are described by total deformation diagrams [1]. Tensile diagram can be used for damage characterization of a polycrystal under static deformation conditions. Beyond the elastic region, the diagram contains two basic stages of damage accumulation: strain-hardening and strain-softening [2]. The intrinsic feature of strain-hardening is stress raise with strain increase, which is occurs due to plasticity of a polycrystal. The strain-hardening process intensity diminishes as the material plasticity exhausts. The material exhausts its strain-hardening ability, when a certain level of residual strain is accumulated. Further loading results in the material strain-softening, deformation mechanism changeover and development of a system of microcracks [4]. A number of structurally-sensitive parameters are used for monitoring of damage accumulation in polycrystals. As one of the above parameters, researchers [5] used the elastic module defectiveness, while applicability of the lateral strain coefficient at the strain-softening stage has been proved in [6]. However, the results of [5] indicate that the elastic module defectiveness fails to describe damageability of a polycrystal at the strain-softening stage (descending portion of the tensile diagram). Lebedev et al. [7] used the vibration decrement as a parameter of structurally heterogenic material damageability. The strain-hardening process of a polycrystal is described by monotonic increase in the logarithmic decrement of vibrations [7], which controls the material damageability.The aim of this study is to determine the distribution trends of statistical inelasticity characteristics in surface-layer local zones of the material under study for static deformation conditions. The analysis of a polycrystal damageability by distribution characteristics of the above parameters shows that it has a monotonic trend irrelevant of the scale of structural changes. The monotonic trend of the damageability characteristic is due to the fact that the analyzed relationships provide integral description of variation of the loaded material properties, but fail to control the multistage process of its structural transformation. Studies [8,9] show that techniques using the energy irreversibly dissipated in polycrystals are the most sensible to structural changes. Analysis of the e...

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...

We have developed a method which provides monitoring of the inelasticity kinetics of polycrystal materials by variation of the stress-strain phase-shift angle in the locally loaded surface zone of the material under study. The proposed method allows one to determine current value of damage of investigated aluminum alloy under laboratory conditions of cyclic deformation by variation of statistical characteristics of phase-shift angle distribution. AMg6N aluminum alloy, which is a cyclically hardening material, was used in this study.All structural materials during deformation manifest dissipation of mechanical characteristics due to imperfection of the material structure stipulated by presence of acute angles, lattice defects, asymmetry of structural elements, etc. These factors are stress concentrators on macro-, micro-, and submicroscopic levels. Results of [1] provide a quantitative assessment of a size of a structural element (grain) in a volume of the investigated polycrystal material. Grain dimensions can vary by order of magnitude, while those of grain units -in several times. Imperfection of a polycrystal material structure results in its nonuniform stressed state during loading. Therefore, stress values in local volumes of a material can exceed the nominal ones in two-three times [2][3][4], which results in the inhomogeneous process of plastic deformation in the total volume of a polycrystal material.Under cyclical loading conditions, microplastic strain is one of major factors of variation of mechanical characteristics. Inelastic strain per cycle is used as a quantitative characteristic of microplastic deformation process. In stress-strain coordinates, variation of mechanical characteristics of a material during cyclical loading is described by a closed hysteresis loop. The loop width is proportional to the inelastic strain per cycle, which makes possible quantitative estimation of a polycrystal material damageability [5][6][7]. Under cyclic loading conditions, the stress distribution in a structural material attributed to its imperfection varies cycle by cycle, which results in reduction of the material proportionality and yield stress limits. This phenomenon is known as the Bauschinger effect [8], which implies strain-hardening or strain-softening under cyclic loading conditions. Stressed state nonuniformity can be expressed by the statistical distribution characteristic of microplastic strains [2,3]. The microplastic deformation kinetics must correspond to the pattern of the material damage accumulation as long as varies the fatigue localization volume. There are many methods which provide integral estimation of inelastic strains in a material per cycle: calorimetric method [9], phase method [10], the method of free damped oscillations [11], the Kimball-Lazan method [12], resonance curve method [13], and the dynamic hysteresis loop method [10]. A deficiency of the above methods is that the integral material inelasticity characteristic fails to provide comprehensive description of the material structur...

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An approach has been developed that allows assessing inelastic phenomena in a material based on the param eter o f the phase shift angle distribution between the stress and strain, which is measured in local zones on the surface o f the investigated material. The distribution o f the phase shift angle variance in the service life range investigated allows tracing the kinetics o f the discrete phenomena o f inelasticity in the material studied. It is show n in [5,6] that the process o f plastic deform ation taking place at a m icrolevel under cyclic loading is characterized by regular stage-like nature. The results o f the investigations o f inelasticity variation w ithin the high-cycle fatigue region [7][8][9] show that traditional concepts o f the scattered dam age progressing in stages in m etals and alloys at a m acrolevel can be extended by taking into account the scale level o f the analysis o f inelastic processes o f m aterial deform ation w hich reveal stochastic peculiarities at the m icrostructural level. This approach is of current im portance considering the fact that the characteristics o f inelastic processes in the material that w ere obtained on the basis o f the integral assessm ent do not reflect the discrete character o f the m aterial dam ageability under cyclic deform ation [10].The aim o f this w ork is to investigate the kinetics o f the discrete phenom ena o f inelasticity in the alum inum alloy under cyclic deform ation in the high-cycle fatigue region.