Study of inelastic cyclic deformations and fatigue in metals

Troshchenko, V. T.; Khamaza, L. A.; Tsybanev, G. V.

doi:10.1007/bf01528118

Cited by 5 publications

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“…6a. The above stages confirm the multistage process of fatigue damage accumulation described in [7], where inelastic strain per cycle was used as a parameter of fatigue damage, insofar as its variation manifested structural transformations in the material under study. Figure 6b depicts the kinetic characteristic of VT14 titanium alloy [23], which has a variation trend similar to that of AMg6N alloy.…”

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“…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.…”

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Inelasticity monitoring of structural materials by the resonance method

Mailo

2009

Strength Mater

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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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Inelasticity monitoring of structural materials by the resonance method

Mailo

2009

Strength Mater

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“…c and endurance limits σ R of alloyed and carbon steels. They showed that for cyclically softening and cyclically stable steels the following relation holds: σ σ A comprehensive study of the possibility of accelerated determination of endurance limits for metals and alloys with the use of cyclic inelastic strains was performed earlier [24][25][26][27][28][29].…”

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“…The possibility of accelerated determination of the endurance limit from analysis of cyclic inelastic deformations was considered in [22][23][24][25][26][27][28][29].…”

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Nonlocalized fatigue damage of metals and alloys. Part 3. Strain and energy criteria

Troshchenko

2006

Strength Mater

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The paper considers methods of accelerated determination of endurance limits of metals and alloys based on the analysis of regularities in their inelastic deformation, strain and energy criteria of fatigue fracture, and methods for life prediction with the use of those criteria.Keywords: endurance limit, accelerated method, strain and energy criteria, life prediction.Introduction. Strain and energy criteria of fatigue fracture of metals and alloys are based on the analysis of either inelastic strains and inelastic strain energy or total strains and strain energy of which inelastic strains and energy are a part. This gives grounds to conclude that the regularities in the nonlocalized fatigue damage form the basis of these criteria.The purpose of the development of the fatigue fracture strain and energy criteria is to establish the relationship between these criteria and the number of cycles to fracture with allowance for the mean stress in a cycle, multiaxial proportional and nonproportional loading, stress concentration, loading conditions, etc. using the characteristics obtained under uniaxial steady-state loading.The criteria, which allow determining the endurance limit without performing a complete cycle of testing in order to plot a fatigue curve, are of special importance when considering the strain and energy criteria of the fatigue fracture. Methods that make it possible to determine the fatigue limit without full-scale tests have come to be known as accelerated methods. Figure 1 shows a fatigue curve for metals and alloys plotted with allowance for the results of recent investigations [1][2][3][4][5] in the coordinates stress-number of cycles to fracture in a wide range of numbers of cycles to fracture, where stage I corresponds to the fatigue fracture in which the main role belongs to the processes on the surface, stage III -to fracture in which the main role belongs to the internal defects (inclusions, pores, etc.), stage II corresponds to the endurance limit when the fatigue fracture is governed by the surface processes, and stage IV to the endurance limit when the fatigue fracture is related to the internal defects.From Fig. 1 it follows that, firstly, the limiting stress amplitude decreases even when the number of the load cycles exceeds 10 7 cycles; secondly, the transition of the fatigue curve from stage I to stage III is accompanied by the appearance of a discontinuity on the fatigue curve, which cannot always be well determined, specifically, when a small number of specimens is tested; thirdly, at stage IV the endurance limit is not always observed, which was noted in [3][4]. Discontinuities on the fatigue curves of aluminum alloys and steels were also observed in the region of stresses corresponding to 10 5 -10 6 cycles to fracture [6][7][8].Results of the experimental investigation into fatigue and inelasticity of metals and alloys known from the literature were obtained for the number of cycles to fracture below 10 8 cycles and, according to the scheme presented in Fig. 1, pertain to fatigue d...

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An energy approach to fatigue tests and crack initiation stage determination

Tsybanev

1994

Strength Mater

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Study of inelastic cyclic deformations and fatigue in metals

Cited by 5 publications

References 1 publication

Inelasticity monitoring of structural materials by the resonance method

Inelasticity monitoring of structural materials by the resonance method

Nonlocalized fatigue damage of metals and alloys. Part 3. Strain and energy criteria

An energy approach to fatigue tests and crack initiation stage determination

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