Oleg Plekhov

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 heat dissipation at the crack tip during fatigue crack propagation

This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE and titanium alloy OT4-0 samples. The investigation of the fatigue crack propagation was carried out on flat samples with different geometries and types of stress concentrators. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at the crack tip.

Heat dissipation energy under fatigue based on infrared data processing

The presented work is devoted to the experimental study of heat dissipation process caused by fatigue crack propagation. To investigate a spatial and time temperature evolution at the crack tip, set of experiments has been carried out using plate titanium specimens with pre-grown centred fatigue crack. An original mathematical algorithm for experimental data treatment has been developed to obtain the power of heat dissipation caused by plastic deformation at the crack tip. The algorithm includes spatial-time filtration and relative motion compensation procedures. The time dependence of the stored energy was calculated as the difference between work caused by plastic deformation near the crack tip and heat dissipation energy obtained from experimental data. As a result, it has been shown that the stored energy has to accumulated during the fatigue test and has to be equal to zero when the crack reaches the critical length corresponding to the failure of sample.

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.

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.