Strength of materials and structures

Troshchenko, V. T.; Kuriat, R. I.

doi:10.1007/s11223-006-0048-z

Cited by 5 publications

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“…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.…”

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“…The mechanism of local changes in plastic properties is activated by nominal cyclic stresses applied to the specimen. As is known from the literature, e.g., [8], cyclic deformation results in the change of material macroproperties, which can be compared with inelasticity kinetics in local volumes. The curve in Fig.…”

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

et al. 2008

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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...

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

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“…The strain-type equations were also constructed for the materials with path-independent behavior [18]. In [19,20], it is shown in which cases it is possible to deduce the equations of the deformation theory of plasticity for the materials considered in [12].…”

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“…2) and viscoelastic (Fig. 3) bodies, and recently deduced constitutive relations for elastoplastic materials [13][14][15][16][17][18][19][20][38][39][40] (Fig. 4).…”

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Classification of the models of materials in continuum mechanics

Lepikhin

2006

Strength Mater

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The classification of the models of materials in continuum mechanics proposed by the author on the basis of the general theory of Noll constitutive relations is developed by using the methods of rational continuum mechanics.Keywords: continuum mechanics, theory of materials simple in Noll's sense, constitutive relations, classification of the models of materials.Classifications play an important role in the development of scientific knowledge [1]. They can be especially useful if they are constructed by using the deductive method according to the most natural basic parameters (which can give the maximum possible number of derived characteristics, i.e, the classification can serve as a source of knowledge about the classified objects) and have the form of inverted trees [1].In [2], it is indicated that "in science, we, in fact, always deal with models and it is not reasonable to speak about the basic concepts of the theory and regularities of nature without relating them to certain specific classes of the models." According to [3], "mechanics does not study natural objects directly; instead of these objects, it considers bodies, i.e., mathematical notions obtained by generalizing some general features common for numerous natural objects." Thus, within the framework of continuum mechanics, we can, in fact, classify not the actually existing materials but only their models (constitutive relations, physical equations, etc.).The development of a classification of the models of materials is closely connected with the methods of their construction. In the present work, we restrict ourselves to pure mechanical models of materials and their classification.At present, there are two basic approaches to the construction of constitutive relations in continuum mechanics: phenomenological and axiomatic. This separation is, to a certain extent, artificial. Both the axiomatic and phenomenological approaches are based on the generalization of experimental data but the former is characterized by a higher level of generalization.In deducing the constitutive relations within the framework of the phenomenological approach, it is customary to use the method of induction. Indeed, the major part of the existing constitutive relations for solid deformable bodies, fluids, and liquid crystals is obtained by using this approach. Thus, the models of elastic, viscoelastic, elastoplastic, elastoviscoplastic, and viscoelastic-viscoplastic materials are developed for solid deformable bodies.In the phenomenological approach, the models of isotropic materials with relatively simple mechanical behavior for low strains are especially well-developed. For the materials with complex response to deformation (first of all, for the elastoplastic and viscoelastic-viscoplastic materials), there are no generally accepted methods used for the construction of constitutive relations, especially in the cases of arbitrary strains and types of symmetry of their properties. The existing numerous procedures of deducing physical equations for the elastoplastic and vis...

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Strain instability in metals at cryogenic temperatures and potentials of practical application

Vorob’ev

Strizhalo

2011

Strength Mater

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Force, deformation, velocity, energy, and other criteria of limiting states at low-temperature serrated yield of metals are examined. The feasibility of their use for setting norms and standards of strength and mechanical tests is demonstrated.Introduction. Low-temperature strength is related directly to optimum account of strain stability losses in metals, i.e., the serrated yield effect, which is observed at temperatures below 30 K [1,2]. This effect is greatly dependent on loading conditions and strain rates, the rigidity of a loading system, on the influence of electromagnetic and other physical fields as well as on design and technology factors, viz prestrains and stress concentrations, object shapes and dimensions [2-9], etc. An abrupt transfer from elastic deformation to high-rate localized strain typical of high-strength structural alloys is considered as a stepwise reduction of loading resistance or instantaneous carrying capacity losses in a structural element due to overstrain or fracture [10]. It should be noted that current standard strength tests for the structures operating at temperatures below 30 K [11][12][13][14] as well as methods for determining the mechanical characteristics of materials [15, 16] do not take account of a low-temperature serrated yield effect. As would be shown below, its account should be based on the criterion estimation of limiting states of metallic materials arising under discontinuous strain. The criteria can be classified, considering anisothermic, adiabatic, and inhomogeneous deformation characteristics as well as the multiple-factor and multiphase nature of this process [17].Limiting States and Criteria. The authors have already presented the results of complex studying the effect of elastic, inertial, kinetic characteristics of a loading system and design-technology factors on the plastic strain, strength, and fracture behavior of alloy specimens in static and quasi-static tension in liquid helium, accounting for the stepwise strain accumulation nature. The major stages are represented by subcritical macrohomogeneous deformation of the material, deformation stability losses, viz strain discontinuity, nonadiabatic deformation just after the discontinuity termination.The investigation offers the answer to several critical challenges: the effect of serrated yield under known loading conditions, overstrain or fracture in the discontinuity or after its termination, changes in serrated yield parameters and standard mechanical characteristics of the material under the influence of loading-or designtechnology-related factors.The solution of those problems is directly relevant to the criterion estimation of the examined effect. It suggests revealing the conditions of the onset of limiting states, including plastic strain stability losses, if serrated yield starts after the stage of monotonous homogeneous plastic strain or the strain discontinuity occurs just after the stage of elastic deformation (Fig. 1). In both cases the basic criterion should be observed, viz thermal in...

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Strength of materials and structures

Cited by 5 publications

References 0 publications

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

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

Classification of the models of materials in continuum mechanics

Strain instability in metals at cryogenic temperatures and potentials of practical application

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