Sergey Uvarov

THEORETICAL AND EXPERIMENTAL INVESTIGATION OF THE DISSIPATED AND STORED ENERGY RATIO IN IRON UNDER QUASI-STATIC AND CYCLIC LOADING

This work is devoted to an experimental investigation of energy dissipation in metals under plastic deformation and to the development of a thermodynamic model to study the cold work accumulation under plastic deformation and failure. The proposed model is based on a statistical description of collective properties of mesoscopic defects and on dividing the plastic deformation into two parts (dissipative and structural). The structural plastic strain was considered as an independent thermodynamic variable that allowed us to determine the thermodynamic potential of the system. The derived constitutive relations were applied for numerical simulation of tensile and cyclic tests. The numerical results demonstrate a good agreement with experimental data.

Analysis of fragmentation statistics of alumina tubular specimens

Fragmentation statistics of shocked Al2O3 tubular specimens are studied. The parameter of the tube k = d/r2 is less than unity, where d is thickness of the tube, r2 is inner radius of the tube. Samples were fractured under electro-explosion shock wave loads initiated in the liquid. The mass of collected fragments in the liquid was about 98%. The mass of fragments was estimated by the “weighting” and “photography” techniques. Fragment distributions N(m) with mass greater than a specified value m were analyzed depending on specific energy of pulse load. It has been established that the inflection point in the distribution is shifted toward smaller scale with an increasing specific energy w. The analysis of fragmentation statistics revealed power and exponential laws depending on the value of k and fragment dimension (2D and 3D fragments). The power distributions are characteristic for small fragments (3D-statistics, mfrag < mmean and d* < d, where d* is characteristic fragment size) and exponential 2D-stati...

Statistical Laws of Dynamic Fragmentation of ZrO2 Ceramics

Dynamic fragmentation of ceramic samples with different porosity were carried out using modified Hopkinson bar setup, which allow us to keep samples safe (in order to define fragment size distribution) and to measure fractoluminescence impulses occurred on the fracture surfaces (in order to establish the distribution of intervals between impulses). The analysis of experimental data reveals that the fragment size distribution and distribution of interval between fractoluminescence impulses obeys a power law, which exponent depends on ceramics porosity.

Numerical simulation of spall failure in metals under shock compression

The developed statistical model of solid with mesoscopic defects allowed the formulation of phenomenological model in terms of two independent variables—the defect density tensor and structural scaling parameter and the simulation of shock wave propagation in the linkage with structural relaxation phenomena. It was established the link of the Hugoniot elastic limit with kinetics of structural transition (mathematically related to the defect density tensor) in the structural metastability area, that has generally thermally‐activated character. The development of plastic front is described as the consequence of self‐consistent structural (orientation) transition in microshear ensemble that is realized due to the kinetics of structural scaling parameter. The present investigation was directed to describing of the failure phenomena. Two cases are discussed (quasi‐brittle and elastic‐plastic behavior).

Study of plastic strain localization mechanisms caused by nonequilibrium transitions in mesodefect ensembles under high-speed loading

The behavior of specimens dynamically loaded during split Hopkinson (Kolsky) bar tests in a regime close to simple shear conditions was studied. The lateral surface of the specimens was investigated in-situ using a high-speed infrared camera CEDIP Silver 450M. The temperature field distribution obtained at different time allowed one to trace the evolution of plastic strain localization. The process of target perforation involving plug formation and ejection was examined using a high-speed infrared camera and a VISAR velocity measurement system. The microstructure of tested specimens was analyzed using an optical interferometer-profiler and a scanning electron microscope. The development of plastic shear instability regions has been simulated numerically.

Experimental and numerical study of plastic shear instability under high-speed loading conditions

The behavior of specimens dynamically loaded during the split Hopkinson (Kolsky) bar tests in a regime close to simple shear conditions was studied. The lateral surface of the specimens was investigated in a real-time mode with the aid of a high-speed infra-red camera CEDIP Silver 450M. The temperature field distribution obtained at different time made it possible to trace the evolution of plastic strain localization. The process of target perforation involving plug formation and ejection was examined using a high-speed infra-red camera and a VISAR velocity measurement system. The microstructure of tested specimens was analyzed using an optical interferometer-profilometer and a scanning electron microscope. The development of plastic shear instability regions has been simulated numerically.

Localized instability of plastic deformation at dynamic loading caused by nonequilibrium transitions in defect ensembles

Mechanisms of plastic shear localization in a material under dynamic loading are studied theoretically and experimentally. These mechanisms are associated with collective effects occurring in the ensemble of microdefects in spatially localized regions. In-situ infra red scanning of the instability zone and a subsequent analysis of the dislocation substructure lend support to the supposition that nonequilibrium transitions in defect ensembles play a decisive role in the development of localized plastic flow. The equations relating nonequilibrium transitions to the mechanisms of structural relaxation and plastic flow are used to simulate localizations of plastic deformation.

Regularities of fracture pattern formation in alumina ceramics subjected to dynamic indentation

In this paper the process of dynamic indentation, causing deformation and fracture of alumina ceramics, is investigated. The dynamic indentation experiments were carried out on the original setup based on the split Hopkinson bar technique. The regularities of structure evolution caused by indenter penetration are studied using the computer tomography data of the samples subjected to different loads. The investigation revealed the existence of comminuted area in the vicinity of the indenter and the formation of multiple cracks in the zone lying below. It was found that the higher is the applied indentation load, the denser is the crack pattern and larger are the cracks. A similarity of such a mechanical behavior between the examined material and dentin taken as a biocomposite is discussed.

Structural and mechanical aspects of the formation of adiabatic shear bands under dynamic loading and during target perforation

We have studied, both experimentally and theoretically, the mechanisms of plastic shear localization during the dynamic deformation (adiabatic shear band formation) of metals. Instability mechanisms are frequently associated with collective effects in microdefect ensembles in spatially localized regions. Infrared in-situ scanning of instability regions and further investigation of a dislocation substructure confirm that non-equilibrium transitions play a key role in defect ensembles during the evolution of a localized plastic flow. The use of equations reflecting a relationship between non-equilibrium transitions and plastic flow has made it possible to perform simulations for plastic shear localization.

The effect of initial porosity of a sample under electric explosion loading

Tubular Al2O3 ceramic samples were destroyed under shock wave loading using electrical explosion wire (EEW) in a liquid. The fragments were classified into two types: quasi-two-dimensional (2D) samples, the characteristic size of which, d*, was greater than (or equal to) the tube wall thickness; and three-dimensional (3D) samples of the size d* < d. The study of the influence of the initial sample porosity indicates a direct effect on the formation of 3D fragments, and the distribution of the porosity is described by the power law function with an exponent differing from that of the 3D fragments size distribution. According to the scenario, the initial defects (pores) provide conditions for multi-site fracture of ceramics under high-rate loading. The instability of fast crack propagation leads to micro-branching, which creates conditions for the formation of the 3D objects. The 3D fragments size distribution is described by the power law, which corresponds to the micro-branch distribution.