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Ignatovich, I. M. Zakiev, UDC 620.178.14:191.4(045) and D. I. BorisovThe structural-deformation inhomogeneity of the surface layer is assessed by the method of scratching with recording the variations of the tangential force of resistance to the motion of an indenter. For the depths of penetration of the Vickers indenter comparable with the parameters of surface roughness, we determine a function used to describe the inhomogeneity of structuraldeformation properties of the material along the scanning path without taking into account the influence of the surface topography.
Ignatovich, I. M. Zakiev, UDC 620.178.14:191.4(045) and D. I. BorisovThe structural-deformation inhomogeneity of the surface layer is assessed by the method of scratching with recording the variations of the tangential force of resistance to the motion of an indenter. For the depths of penetration of the Vickers indenter comparable with the parameters of surface roughness, we determine a function used to describe the inhomogeneity of structuraldeformation properties of the material along the scanning path without taking into account the influence of the surface topography.
A model o f the formation and evolution o f a local plastic deformation zone at the crack tip is proposed based on the analysis o f the main physical processes taking place in a metallic material under the action o f cyclic loads. An equation o f fatigue crack growth rate curves, which explicitly accounts fo r the loading frequency, was derived. The equation applies to the whole range o f crack lengths from short cracks to macroscopic ones. K e y w o r d s : local plastic deform ation, surface energy, fatigue strength, fatiguecrack grow th resistance, loading frequency.In tro d u c tio n . Fracture o f a m aterial and structural elem ent under external therm om echanical loading is a tw o-stage process. The first stage involves damage accum ulation in the m aterial and com es to an end w hen the param eters o f the local plastic deform ation zone reach their critical values, w hich corresponds to the beginning o f the form ation o f one or several cracks. The second stage is characterized by the crack propagation up to a com plete body failure. N owadays, there are attem pts to describe the entire process o f fatigue fracture from a single perspective, w ith the leading role given to the process zones w hich are form ed both during the first (incubation) period and at the crack grow th stage [1][2][3][4][5][6]. The grow ing fatigue crack is regarded as a sharp notch and its grow th is m odeled as repeated crack initiation events w hich follow the sam e law s as those governing the initiation o f the prim ary crack.The m ajor characteristics o f the loading conditions include the frequency of the acting load. The few m odels considering the frequency either contain it in an im plicit form [7], or cover only one or several m aterials [8][9][10], or are difficult to apply in practice [11].B ased on the above, w hat seems topical to us is the creation o f unified m odels covering the w idest possible range o f factors that influence the fatigue fracture process, relying on the analysis o f physical processes that take place in a m etallic m aterial, and having a sufficiently simple, easy-to-use m athem atical form.P h y sic a l F o u n d a tio n s o f th e F r a c tu r e M odel. E arlier w e analyzed experim ental data, both our ow n and those from literature, on the processes o f fatigue dam age accum ulation and fatigue crack propagation in m etallic m aterials using such techniques as optical, transm ission electron, and scanning electron m icroscopies com bined w ith a quantitative data processing and the determ ination o f the residual electrical resistivity and internal friction. This m ade it possible to establish basic general laws for the evolution o f the m aterial structure and variation o f the fractographic characteristics under cyclic loading. D etailed results o f these investigations are given in [12]. H ere w e outline only the key issues.
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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