Structural Levels Of Deformation Research Articles

ФИЗИЧЕСКОЕ ОБОСНОВАНИЕ СТРУКТУРНЫХ УРОВНЕЙ ДЕФОРМАЦИИ ТВЕРДЫХ ТЕЛ

With the help of harmonic analysis of different-scale finite element models of cubic cells, a physical substantiation of the structural levels of deformation of solids is presented, which consists in the fact that with a decrease in the cells from the macroscopic to the mesostatic level of deformation, the number of resonance peaks of the deformation parameters in comparable scale frequency ranges decreases from seven or eight to two or three and further, during the transition to the microscale level, such a change is repeated again.

Structural Scale Levels of Plastic Deformation and Fracture of High-Strength Titanium Alloy Welds

Structural scale levels of plastic deformation and fracture of welded joints have been studied for two high-strength titanium alloys with a low (VT18U alloy) and a high (VT23 alloy) content of the bcc s phase. Ultrasonic forging and its combination with high-current pulsed electron beam treatment were used to activate nanoscale structural levels of deformation and fracture in welds in order to increase the fatigue life of welded structures. Ultrasonic forging provides an effective dispersion and nanostructuring of surface layers in the VT18U welded joints with a 4.6-fold increase in their fatigue life. The dispersion and nanostructuring of the VT23 laser welded joints is achieved only by ultrasonic forging combined with high-current electric pulse treatment, in which longitudinal dispersion of s bands occurs with the formation of orthorhombic a " nanolaths. In so doing, the fatigue life of the VT23 welds increases twice, but the effect depends on the power of the high-current generator and electrical pulse parameters. The fracture micrographs of the treated VT23 welded joints reveal nanofibrous bands responsible for ductile fracture and for the reduction of the fatigue crack growth rate. The structural changes and the increase in the fatigue life of the studied titanium alloy welds are associated with the activation of nanoscale structural levels of deformation and fracture induced by ultrasonic forging or by its combination with high-current pulsed electron beam treatment.

Hydrodynamic thrust generation and power consumption investigations for piezoelectric fins with different aspect ratios

Bio-inspired hydrodynamic thrust generation using piezoelectric transduction has recently been explored using Macro-Fiber Composite (MFC) actuators. The MFC technology strikes a balance between the actuation force and structural deformation levels for effective swimming performance, and additionally offers geometric scalability, silent operation, and ease of fabrication. Recently we have shown that mean thrust levels comparable to biological fish of similar size can be achieved using MFC fins. The present work investigates the effect of length-to-width (L/b) aspect ratio on the hydrodynamic thrust generation performance of MFC cantilever fins by accounting for the power consumption level. It is known that the hydrodynamic inertia and drag coefficients are controlled by the aspect ratio especially for L/b< 5. The three MFC bimorph fins explored in this work have the aspect ratios of 2.1, 3.9, and 5.4. A nonlinear electrohydroelastic model is employed to extract the inertia and drag coefficients from the vibration response to harmonic actuation for the first bending mode. Experiments are then conducted for various actuation voltage levels to quantify the mean thrust resultant and power consumption levels for different aspect ratios. Variation of the thrust coefficient of the MFC bimorph fins with changing aspect ratio is also semi-empirically modeled and presented.

Numerical modeling of viscoelastic behavior of filled rubber vulcanizates

The paper focuses on the numerical implementation of constitutive relations of a continuum viscoelastic model, which describes the behavior of filled rubber vulcanizates under finite deformations. In the model, the additive decomposition of the strain rate tensor and the concept of macro and structural levels of deformation are used. The equilibrium hyperelastic behavior of a polymer binder is represented by relations that are based on the generalized tube model. Rheological properties are represented as differential relations. Various types of a term in the expression for free energy potential responsible for relaxation properties are tested. The viscous flow coefficient is analyzed as a function of continuum parameters and internal tensor variables. Simulation results are compared with the experimental data for filled rubber vulcanizates under uniaxial cyclic loading.

Scale levels of deformation and fracture and mechanical properties of 25CrlMolV steel before and after nonisothermal loading

In the work, the peculiarities of tensile deformation and fracture of 25CrlMolV steel subjected and not subjected to thermal cycling were studied on the basis of measured mechanical characteristics and data of optical and electron microscopy. It is shown that the deformation of the steel specimens in the initial state features very low strain hardening and considerable elongation at the stage of necking. After thermal cycling, the yield stress decreases, the plasticity grows markedly, and the strain hardening becomes more pronounced. The results of study are discussed with resort to metallographic data. Fractographic analysis suggests that the fracture pattern is governed by regularities of the nucleation of micropores, their coalescence, and formation of a central opening mode macrocrack. According to the experimental results, the specimen is fractured by a mixed “local shear - cleavage” mechanism and the character of fracture is strongly affected by secondary microcracking and severe intragranular plastic deformation. On the basis of the obtained results, a model of structural levels of deformation and fracture of the specimens under static tension is proposed.

Nanoscale deformation irregularities of γ-irradiated poly(methyl methacrylate)

Development of deformation jumps in the creep of poly(methyl methacrylate) (PMMA) has been studied. The structural levels of deformation have been determined from the creep rate oscillation periods (deformation jumps) measured by the interferometric method. Special attention is given to a new method of data processing, which enables one to reveal previously undetectable nanoscale deformation jumps. By the example of PMMA specimens preliminarily exposed to γ radiation with doses D=55–330 kGy and unexposed specimens, the presence of nanoscale deformation jumps with the values dependent on the dose D and time of creep has been shown. The obtained results confirm the existence of 10–20-nm domains in amorphous polymers and make it possible to study the multilevel organization of the deformation process, starting from the nanoscale.

Calculation of mesoscopic strain characteristics for the study of the behavior of porous ceramics under uniaxial compression

The paper studies the deformation behavior of zirconia ceramic specimens with different porous structure under uniaxial static compression. In specimens with large pores the latter refer to an active intermediate structural level of deformation. Its involvement into the resistance to plastic deformation governs a considerable strain increase (by 3 times) at fracture when ultimate strengths in both specimen types are comparable. The stage of deformation processes at the mesolevel is determined by the construction and analysis of the dependence of external stress on shear strain intensity logarithm. The obtained data make it possible to offer a scheme of structural deformation levels for studied materials, which is based on the analysis of the stage character of the σ-γ dependence.

Fatigue failure mechanism of polycrystalline materials at the mesoscopic level

Systematic studies of the mesoscopic mechanisms of deformation of polycrystalline materials of lead and its alloys have been carried out under conditions of sign-alternating bending at room temperature. It has been shown that fatigue failure is due to the evolution of vortices of mesoscopic substructures. Multiple slip separated in adjacent grains is the basis for this kind of deformation. This causes extremely strong localization of the displacement in individual favorably oriented grains and self-organization of these grains in agreement with regular structural levels of deformation. In polycrystalline lead, the mesoscopic substructure has a block character, with each block containing several grains. The elements of such substructures are nucleated in stress mesoconcentrator zones which arise at the grain boundaries under conditions of intense grain boundary slippage. In the course of cycling they gradually propagate through the whole transverse cross section of the sample, which completes its failure. Alloying substantially changes the character of the mesoscopic substructures which are formed. We have considered the different types of vortex mesoscopic substructures and studied their connection with cyclical endurance of the alloy. Recommendations for increasing the fatigue endurance of plastic polycrystalline materials are given.

Role of solid-solution strengthening and interactions in the dislocation ensemble in the formation of flow stress in austenitic nitrogen steel

The paper reports on an experimental investigation of the role of structural levels of deformation in the formation of the flow stress of austenitic nitrided steel. The contributions to the yield stress and the flow stress from the structural levels of various scales have been determined and it has been found that small-scale levels make the main contribution to the strain resistance. A detailed study of the dislocation ensemble has shown that undergoes strong relaxation at different stages of deformation. The laws of solid-solution strengthening and its effect on substructural transformations are examined.

Evolution of mechanisms of plastic deformation in porous metals

An experimental study was made of the structure of deformed porous iron with a porosity P ranging from 4 to 40% within the range of strains e up to fracture. Measurements are made of grain-boundary slip and rotations of structural elements and their individual contributions to the total strain as a function of P and e. It is established that there is a change in the dominant mechanisms of plastic deformation with an increase in porosity, this change being connected with a change in the topological characteristics of the system and corresponding to the transition from intergranular slip to a group of mechanisms that ensures movement of the grains as a whole. The established laws are analyzed on the basis of the concept of structural levels of deformation.

Relationship of a gauge model of an elasto-plastic medium to the Mindlin theory

We examine the relationship between the simplest gauge model of a material with the Mindlin theory that takes into account microstructure in linear elasticity. We establish a connection between the dynamical equations for the two models. The connection allows us to relate an unknown material constant in the gauge Lagrangian to the inertial properties of the structural elements. We obtain an estimate of the unknown constant and the corresponding characteristic frequency for the dimensions of elements with different structural levels of deformation.

Evolution of substructures under microlevel and mesolevel plastic deformation

The existence of a hierarchy of structural levels of plastic deformation can be considered to be an experimentally and theoretically proven fact [1–3]. Mescheryakov [1] showed that a noncrytallographic level of deformation arises in elastoplastic waves, manifesting itself as macrofluxes of particles of the medium; the velocity of the particles relative to each other at velocity has dispersion and the particles move in the direction of the wave propagation. Displacement of macrofragments of the crystal, which is also a manifestation of noncrystallographic structural levels of deformation, has been detected in highly excited systems [2]. The relaxation approach used increasingly to describe plastic deformation assumes that defects are created, move, and are restructured during deformation in a way so that the level of stresses inside the material drops. The nonuniformity of the stress field gives rise to nonuniform plastic deformation and local shears and rotations at points of stress concentration. These concepts make it possible to use the principles of synergetics to build specific theoretical models and to consider loaded material as a nonequilibrum dissipative structure [3]. To date, however, the construction of the theory describing multilevel plastic deformation processes has not been completed. In particular, it is not yet known what levels are added, depending on the rate and duration of the loading and on how the levels are linked.

Structural deformation and fracture levels of polycrystals under different kinds of loading

A survey is given in this paper of the researches [3-12] devoted to the systematic investigation of the turning deformation modes and their associated structural deformation levels of polycrystals. Results of investigations executed on identical materials (lead and alloys with it as base) but for different kinds of loading, active deformation of one sign, creep, and sign-variable loading, are considered.

Noncrystallographic structural deformation levels in strongly excited systems

Therefore, the analysis performed in the previous sections on the experimental data and results of modelling by the molecular dynamics method permits making a deduction on the possibility of the formation of strongly excited systems of noncrystallographic structural deformation levels during loading. As an experimental investigation showed, for all the kinds of static loading utilized their origination is associated with the pre-fracture stages. Crack propagation over the noncrystallographic interfacial boundaries of the fragments indicates this. Under shockwave loading the macroflux motion with the grains is also a new, larger-scale dynamic deformation level (compared with the dislocation level).

Physicomechanical model of the elastoplastic properties of materials taking account of the structural levels of deformation and the kinetic properties of real crystals

On the basis of an earlier-proposed translational-rotational model of a continuous medium, reflecting a hierarchy of structural levels of deformation and destruction, the influence of diverse temperature-velocity conditions of loading on the properties of materials is investigated. The applicability of the theory for the description of unified equations of the mechanical properties of crystals in nonsteady conditions of deformation is established. Analysis and comparison of the calculations results with experiment is undertaken for uniaxial loading.

Translation-rotation continuum model allowing for structural levels of deformation and fracture

A method is proposed for the theoretical description of deformation and fracture occurring in crystals simultaneously at different structural scale levels. The theory developed in the article takes into consideration the deformation and fracture contributions from translational and rotational channels of plasticity and the mutual influence of mass-transfer processes from one level to another.

Structural levels of deformation in solids

Structural levels of deformation in solids