Oleg Naimark

Elastic-plastic transition in iron: Structural and thermodynamic features

The structural and thermodynamic features of the elastic-plastic transition in armco iron and its plastic deformation are studied. Energy storage in iron is shown to have a nonlinear character and be accompanied by wavelike heat dissipation. To describe the energy balance in the plastically deformed metal, a theoretical model is proposed based on a statistical description of the evolution of an ensemble of typical mesodefects (microshears). Moreover, a procedure is developed to experimentally determine the dependence of the potential of the medium on the mesodefect density using infrared scanning data.

Nonlinear and structural aspects of transitions from damage to fracture in composites and structures

Abstract Statistical theory of defect evolution allows us to obtain nonlinear kinetic equations for tensor parameter of microcrack density. The laws of spatial damage localization on various structural levels are defined by the nonlinearity of the microcrack accumulation in the condition of intensive interaction of the defects. This leads to the multiscale generation of failure centers. The relation between typical nonlinearity of damage kinetics and spatial failure localization is the theoretical background for the explanation of experimental results and numerical simulation of fracture in heterogeneous materials. Topological features of fracture development were investigated numerically using percolation model of failure cluster growth.

Crack propagation: Dynamic stochasticity and scaling

The results of theoretical and experimental investigations provide explanation of the mechanism of stochastic branching during crack propagation in a solid, relating this phenomenon to changes in the fundamental symmetry relationships in a nonlinear system representing the solid with defects. The character of the stochastic behavior of quasibrittle materials is determined by nonlinear dynamics of the ensemble of microscopic defects under the conditions of kinetic transitions. The transition from dispersed to macroscopic fracture is accompanied by the appearance of multiple fracture zones in the regime of explosion-like instability development over a discrete spectrum of spatial scales.

Nonlinear dynamics of the blow-up structures in the ensembles of defects as a mechanism of formation of earthquake sources

A statistical thermodynamic description for the evolution of the ensemble of defects in a geological medium is suggested for modeling the formation of the potential earthquake source. The evolution of the medium with defects is described in terms of two order parameters, the damage parameter and the structural scaling parameter, which are the unknowns in the evolution equations. Based on the obtained constitutive relationships, a mathematical model is constructed and the final stage of the earthquake source formation and propagation of the slow waves is numerically modeled. Our results can be interpreted as the model visualizations that render, in terms of a seismic process, the physical hypothesis implying the common formation mechanism of the defects (discontinuities) over a wide range of spatial scales.

Experimental investigations of the laws of energy dissipation during dynamic deformation of nanocrystalline titanium

The mechanical and thermodynamic characteristics of nanocrystalline (NC) titanium have been experimentally studied under the conditions of dynamic compression at a strain rate of (2–5) × 103 s−1. The samples of NC titanium were cut from a rod processed by equal-channel angular pressing and had a characteristic grain size of 300 nm. It is established that a 25% increase in the dynamic limit of elasticity in the NC titanium (compared to an initial coarse-grained sample) is accompanied by qualitative changes in the process of energy absorption and dissipation in the material. The amount of dissipated energy remains approximately constant and independent of the rate and amplitude of loading in the entire range of strain rates studied.

Experimental study of energy accumulation and dissipation in iron in an elastic-plastic transition

Modern experimental studies of the temperature evolution at the surface of plastically deformed metals show that processes of energy accumulation and dissipation in the material are nonlinear even in the quasi-static case and obviously depend on the loading conditions as well as past history of deformation. Considerable intensification of investigations in this field, which has been observed during the last decade, is due to the fact that this class of problems has wide application apart from its fundamental importance. High-sensitivity IR detectors used in such experiments make it possible to develop new effective methods of nondestructive control, which are based on analysis of thermal precursors of deformation localization and failure. In this study, the features of propagation of thermal waves on the surface of pure iron under an elastic-plastic transition are investigated and time dependences of energy accumulation rate are obtained during quasi-static deformation.

Peculiarities of the elastic-plastic transition and failure in polycrystalline vanadium under shock-wave loading conditions

Specific features in the deformation of polycrystalline vanadium under shock-wave loading conditions have been studied by experimental and theoretical methods. Analysis of the full wave profiles was used to determine the dynamic elastic limit and spall fracture stress in a range of stain rates of 105–107 s−1. The laws of elastic–plastic transition at the shock-wave front, mechanisms of elastic precursor relaxation, and spall fracture development are discussed.

Experimental study of thermodynamic and fatigue properties of submicrocrystalline titanium under high cyclic and gigacyclic fatigue regimes

This paper presents an experimental study of the mechanical and thermal behavior of titanium samples (Grade 2 and Grade 4) with different grain sizes under cyclic loading. The self-heating test demonstrates that the structure of the material has a strong effect on the dissipation ability of titanium. The threshold of energy dissipation corresponding to the transition through the fatigue limit is shown for coarse-grained titanium. On contrary, submicrocrystalline samples exhibit the dependence of continuous energy dissipation on the applied stress amplitude. Analysis of the fatigue properties of titanium in a gigacyclic regime provides evidence that grain grinding improves substantially the fatigue properties of the material.

Kinetics of defect accumulation and duality of the weller curve in gigacycle fatigue of metals

We propose a model describing the kinetics of accumulation of defects under cyclic loading in metals. Analysis of experimental data on the initial defect distribution and of the role of the free surface of the sample in straining makes it possible to explain the features of generation of fatigue cracks in the bulk, which is typical of the gigacycle fatigue regime at a low stress level. The duality of the Weller curve in the gigacycle fatigue regime is attributed to the emergence of a fine-grain region in the form of a dissipative peaking structure in the defect ensemble.

Scale invariance in dynamic fragmentation of quartz

In the work, we studied statistical regularities in fragmentation of cylindrical quartz specimens under dynamic loading. The original equipment used in the study ensured integrity of the specimens after impact for the determination of fragment size distribution (spatial scaling) and allowed recording fractoluminescence pulses at newly formed fracture surfaces for the determination of pulse spacing distribution (temporal scaling). The results of experimental data processing show that the size distributions for both the spatial parameter (fragment size) and the temporal parameter (fractoluminescence pulse spacing) are described by a power function. This enables us to refer dynamic fragmentation of quartz to phenomena exhibiting self-organized criticality.