Mechanics of deformation of structurally nonuniform media

It is shown theoretically and experimentally that the complete deformation of structurally nonuniform media consists of two components: macrodeformation, which is determined by the relative displacement of the elements in the structure (grains, subgrains, phases, and so on) as a whole, and microdeformations, which represent the deformations of these elements themselves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 82–86, November, 1981.     

Contribution of surface tension energy during plastic deformation of nanomaterials

The surface tension energy of crystallites in polycrystalline materials having different microstructures and in nanomaterials has been estimated. A hypothesis is proposed, according to which the yield stress of materials is determined by the balance of elastic energy and the surface tension energy of crystallites. The independently measured value of surface energy can be used to estimate the yield stress of polycrystalline materials.     

Nucleation of deformation twins on gliding grain-boundary dislocations in nanomaterials

A new micromechanism of nucleating deformation twins in nanocrystalline and ultrafine-grained materials under action of severe mechanical stresses has been proposed and theoretically described. The mechanism is a subsequent splitting of grain-boundary dislocations into lattice partial and sessile grain-boundary dislocations. Ensembles of gliding partial dislocation forms deformation twins. The energy characteristics of this process are calculated. The nucleation of the twins is shown to be energetically profitable and can be athermic (without an energy barrier) under conditions of severe mechanical stress. The dependence of a critical stress at which the barrier-less nucleation of twins took place on the widths of these twins is calculated.     

Micromechanics of the transition from intergrain to intragrain deformation in nanomaterials

A micromechanism of the transition from intergrain sliding to intragrain glide by nucleation and emission of lattice partial dislocations at grain-boundary dislocations is proposed and described theoretically. The energy characteristics of this process are calculated. It is shown that the nucleation of lattice partial dislocations is energetically efficient and can occur athermally (without the energy barrier) under conditions of the action of ultrahigh mechanical stresses. The critical stresses required for the athermal nucleation and emission of dislocations are calculated.     

Emission processes accompanying deformation and fracture of metals

Present-day physical methods of investigation reveal that the fracture and plastic deformation of metals is accompanied by emission processes, in particular, by luminescence and emission of electrons. All the metals studied thus far exhibit a capability of luminescence. The intensity, duration, and spectrum of mechanoluminescence are different for different metals. The intensity is determined by the mechanical and thermal characteristics. For a given metal, the intensity depends on dislocation density in the structure and the sample loading rate. The spectrum of noble metals is governed by the electronic structure of surface states. The dynamics of mechanoluminescence and electron emission (exoemission) depends on the rate of stress variation in the sample under study. This permits one to consider the mechanoluminescence and exoemission not only as physical characteristics but also as a potential tool for probing surface states in metals and the kinetics of emergence of mobile dislocations on the surface with a high time resolution. Fiz. Tverd. Tela (St. Petersburg) 41, 841–843 (May 1999)     

Computer simulation of the deformation and fracture of crystals

Methods for computer simulation of strength testing of crystals are proposed. The methods employed are similar to usual static methods, and they are used to investigate deformation and fracture of perfect fcc crystals having different orientations with respect to the tensile force. A strain-induced phase transition from the fcc to the hcp structure is detected, and the formation and displacement of crystal twins are observed. Plastoelastic deformation and fracture of crystals are investigated.     

Mechanisms of deformation and fracture of the Al-Zn-Mg alloy

Laws of dislocation substructure evolution in the Al-Zn-Mg alloy subjected to compression and tension in different structural states are compared with laws of forming a deformation relief. It is established that long aging of the alloy changes the deformation localization mechanisms compared to its evolution in the alloy subjected to short-term aging. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 62–67, November, 2007.     

The gradient deformation criterion for brittle fracture

A new fracture criterion based on the assumption that brittle fracture occurs when the strain gradient reaches its limiting value is formulated. The presence of a strain gradient at the boundary of a body’s temperature drop is shown analytically. The results of an experiment with specimens under an abrupt change in temperature are presented.     

Influence of nanotwin generation near crack twins on the fracture toughness of nanomaterials

A theoretical model of microscopic mechanisms of the nucleation and development of deformation twins in nanocrystalline materials has been developed. Within the model, we have studied the generation of deformation twins near crack tips, which occurs through multiple nanoscopic shears that represent nanoscopic regions of an ideal plastic shear. It has been shown that the nucleation of such nanotwins near crack tips reduces the high local stresses that arise near these tips. Thus, the generation and development of nanotwins near crack tips increases the fracture toughness of brittle nanocrystalline materials and serves as an efficient mechanism of improving the crack resistance of deformed nanocrystalline materials.     

Mesomechanics of plastic deformation and fracture of partially crystalline polyethylene

From the standpoint of physical mesomechanics, we have investigated plastic deformation mechanisms and the mechanical properties of partially crystalline polyethylene. We show that from the very beginning, plastic deformation occurs at the mesoscopic level. Fracture is preceded by fragmentation of the material. The observed stages of the process of plastic deformation of polyethylene are qualitatively similar to the stages of this process for metallic materials. The effect of electron bombardment on the mechanical properties of polyethylene is explained by the size reduction in the mesoscopic substructure formed on plastic deformation. Tomsk Polytechnical University. Zhilin University, People’s Republic of China. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 48–53, January, 1997.     

Structural conditions of deformation and fracture of oriented crystalline polymers

Deformation of supermolecular structure elements of oriented crystalline polymers and nucleation of initial submicroscopic cracks induced by stress have been studied by the small-angle x-ray scattering technique. It is shown that the intrafibrillar amorphous interlayers have low strength and high deformability. The rupture of the weakest amorphous interlayers leads to nucleation of initial submicrocracks. The influence of submicrocracks on deformation around such cracks is revealed. The micromechanics of deformation and fracture of polymers is discussed.     

Electromagnetic phenomena entailed by deformation and fracture of dielectric solids

Mechanoelectric effects caused by elastic deformation of glasses and marbles are studied in a neutral environment and with weak electric polarization of samples. It is found that the electric potentials that are produced by bending a sample are opposite in sign in compressed and stretched regions. The mechanoelectric effects increase or decrease depending on the direction of the electric field applied to the sample. It is concluded that the electric polarization and the polarization induced by mechanical deformation are of a common nature. Electromagnetic precursors of earthquakes are discussed.     

Dynamics of deformation bands and fracture of the aluminum-magnesium alloy 5556

The dynamics of deformation bands at the prefracture stage of the aluminum-magnesium alloy 5556 has been examined using the high-speed video recording. It has been revealed that, in the artificially aged and recrystallized alloys, the correlations between the propagating deformation bands and the development of the main crack are of different character.     

Kinetics and mechanism of copper fracture during deformation in surface-active baths

Relations have been derived here between the macroscopic characteristics of liquid metal embrittlement (durability _c under creep and strain _c prior to rupture under tension) and the parameters which characterize the micromechanism of fracture (surface energy at the crystal bath interface, energy of grain boundaries, temperature, structure of the crystal-bath interface, etc.), on the basis of test data indicating that the subcritical stage of microcrack development governs the fracture process, and on the assumption that transition to supercritical fracture occurs when the crack angle at the tip opens to its critical width _c. It is also shown here that, as the rate of subcritical crack development changes by three orders of magnitude, the magnitude of the critical angle _c changes only by a factor of 3.0 and may, to the first approximation, be regarded as independent of the bath composition. The values of _c and _c calculated according to this approximation agree closely enough with values based on tests.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 22–29, July, 1976.