Dmitry Bilalov

Nonequilibrium transitions in ensembles of defects attributed to dynamic localization of plastic deformation

Mechanisms of instability and localization of plastic shear during dynamic deformation of metals has been studied theoretically and experimentally. Instability mechanisms are attributed to collective effects in ensembles of microshears in spatially localized areas. Infrared scanning in situ of the instability zone and consequent study of dislocation substructure confirm the crucial role of nonequilibrium transitions in ensembles of defects during development of localized plastic flow. A model of plastic shear instability was developed based on equations that express the correlation between nonequilibrium transitions and mechanisms of structural relaxation and plastic flow.

Structural and mechanical investigation of the estimating reliability of aluminum alloys with consecutive dynamic and gigacycle loading

In this paper we investigate the influence of consecutive dynamic and gigacycle fatigue loads on the lifetime of the aluminum-magnesium alloy AlMg6. Preloading of specimens is achieved during dynamic tensile tests in the split Hopkinson bar device, and quasi-static loadings are carried out on a servo hydraulic tensile machine (Biss Bi-00-100 series). Fatigue tests are conducted on a Shimadzu USF-2000 ultrasonic fatigue testing machine. This machine provides 109–1010 loading cycles with the amplitude from 1 to several dozens of microns and frequency of 20 kHz; this reduces dramatically the testing time in comparison with the classical fatigue testing machines. A New-View 5010 interferometer–profiler of a high structural resolution (0.1 nm) is used for qualitative fracture surface analysis, which provides the data allowing us to find a correlation between mechanical properties and scale-invariant characteristics of damage-induced roughness formed under dynamic and gigacycle fatigue loading conditions.

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.

Scaling invariance of fracture in aluminum alloy under fatigue and dynamic loading

The role of the collective behavior of defect ensembles at the crack tip and the laws of fatigue crack propagation in aluminum-magnesium alloy AMg6 (5056) have been studied under conditions of symmetric tension-compression gigacycle loading at 20 kHz using ultrasonic fatigue testing machine Shimadzu USF2000. 3D New View 5010 interferometer profiler high resolution data of defect induced roughness in the crack process zone under fatigue crack path revealed the existence of two characteristic scales: the scale of the process zone and the correlation length that is the scale when the correlated behavior of defect induced roughness has started.

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.

Numerical simulation and experimental investigation of strain and damage localization in metals under dynamic loading

Simulation of high-strain rate loading of thin-walled cylindrical specimens are represented using the model linking characteristic stages of strain and damage localization and multiscale defects (microshears) kinetics. Shear compression the Kolsky bar test, “In-situ” infrared framing of strain localization areas and the following structural analysis of shear transformation areas are carried out to support the constitutive models and results of numerical simulation.

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.

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.

Numerical and experimental studies of strength characteristics of aluminum alloys under penetration of a barrier

The research is focused on the simulation and experimental investigation of high-strain rate loading to study the mechanisms of plastic flow localization in metals (AlMg6) using a ballistic installation. Unique results of experiments are given in the paper. Numerical simulation using wide-range constitutive equations is done. The paper also shows the results of structural studies.

Structural mechanisms of formation of adiabatic shear bands

The paper focuses on the experimental and theoretical study of plastic deformation instability and localization in materials subjected to dynamic loading and high-velocity perforation. We investigate the behavior of samples dynamically loaded during Hopkinson-Kolsky pressure bar tests in a regime close to simple shear conditions. Experiments were carried out using samples of a special shape and appropriate test rigging, which allowed us to realize a plane strain state. Also, the shear-compression specimens proposed in [1] were investigated. The lateral surface of the samples 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. Use of a transmission electron microscope for studying the surface of samples showed that in the regions of strain localization there are parts taking the shape of bands and honeycomb structure in the deformed layer. The process of target perforation involving plug formation and ejection was investigated using a high-speed infra-red camera. A specially designed ballistic set-up for studying perforation was used to test samples in different impulse loading regimes followed by plastic flow instability and plug ejection. Changes in the velocity of the rear surface at different time of plug ejection were analyzed by Doppler interferometry techniques. The microstructure of tested samples was analyzed using an optical interferometer-profilometer and a scanning electron microscope. The subsequent processing of 3D deformation relief data enabled estimation of the distribution of plastic strain gradients at different time of plug formation and ejection. It has been found that in strain localization areas the subgrains are elongated taking the shape of bands and undergo fragmentation leading to the formation of super-microcrystalline structure, in which the size of grains is ~300nm. Rotational deformation modes give rise to the high angular disorientations of grains. The development of plastic shear instability regions has been simulated numerically. For this purpose, we use a recently developed theory, in which the influence of microshears on the deformation properties of materials has been studied by the methods of statistical physics and thermodynamics of irreversible processes. The results of theoretical and experimental studies suggest that one of the mechanisms of the plastic shear instability and localization of plastic strain at high-velocity perforation is related to structural and kinetic transitions in microshear ensembles. References 1. Rittel D., Landau P., and Venkert A. // Phys.Rev.Lett. 2008. V.101. 165501. P. 1-4.