B. K. Barakhtin

Dynamic and fractal properties of SP-28 steel under high-speed loading conditions

Using an interferometric method to record the velocity of the free surface of a target subjected to two-dimensional shock loading, it is shown experimentally that the decrease in the compression pulse amplitude is due to the nonstationary nature of mesoscale processes — the amplitude decrease is progressively larger for higher rates of change of the variance of the mesoparticle velocity. It is shown theoretically that the loading rate influences the spallation strength of a material in a planar collision only if the variance of the particle velocity is nonzero. A fractal analysis of the spallation surfaces of steel samples is performed by quantitative fractography methods. An expression relating the fractal dimension of the spallation fracture surface and the variance of the mesoparticle velocity is derived. For typical values of the load pulse parameters for which back-side spallation occurs the fractal dimension agrees satisfactorily with the fractal dimensions for triadic Koch islands.

Multifractal analysis of structural modifications in a cumulative jet

Modifications in the structural-mechanical state of a copper sheath deformed under the conditions of formation and extension of an axisymmetric cumulative jet are studied using optical metallography and electron metallography with the assistance of multifractal analysis.

Dynamic cracking resistance of metallic materials during rapid propagation of a self-similar crack

A model is proposed to determine the dynamic cracking resistance KID of metals and alloys for the case of a rapidly moving fractal or self-affine crack. The values of this characteristic correlate with the fractal dimension Df of the future contour of a crack surface profile. KID is lower or higher than KIC depending on the fractal dimension.

Statistical characteristics of the multiscale spallation of metal targets during dynamic loading and their relation to the mechanical properties of the materials

The damages that form in plane targets made of three steel grades during their shock loading at a rate of 100–650 m/s are statistically processed, and the size distributions of defects are determined. In the general case, the damage accumulation in this strain rate range is shown to be not self-similar. The dynamic fracture toughnesses of the steels are determined, and this characteristic is found to be related to a change in the mechanical properties of the material and the particle velocity range in it on the structural level of the meso II scale.

Influence of velocity nonuniformity in a medium on the penetration of cumulative jets and extended flyer plates

Obstacle materials into which plane cumulative jets with an initial impact velocity of 2.5–3.5 km/s penetrate are studied metallographically. The results suggest that penetration takes place in the nonuniform velocity field of the medium. A refined hydrodynamic model for the fast penetration of jets and flyer plates is put forward and verified experimentally.

Turbulence and dissipative structures in shock-loaded copper

Shock-loading tests of polycrystalline copper M3 under conditions of uniaxial deformation at impact velocities of 100 to 700 m/s were performed. It was established that a threshold deformation rate exists above which dissipative structures in the dynamically deformed material arise in the form of local regions of cellular type, with a size of 15–25 μm, separated by shear plastic bands. The basic size of cellular structure domains is on the nanometer scale. The microhardness of the material within the cellular structures is somewhat higher than in the bands of plastic deformation that separate these structures. At threshold deformation rates and above it, the defect of the mass velocity, the difference between the impactor velocity for symmetrical collision and the free surface velocity at the plateau of the compression pulse, increases sharply as does the spall strength of the material.

On the Shock-Induced Structures in Copper

Shock loading of M3 copper within strain rate range of 5·10 6 -5,7·10 6 s -1 reveals a nucleation of structural objects of 5-30 µm in diameter, which present the three dimensional frameworks composed from shear bands of 50-200 nm spacing. The structures are shown to be nucleated by means of interference of longitudinal and periphery release waves. Transition of the material into structure unstable state responsible for the shear banding happens when rate of change of the velocity variance at the mesoscale becomes higher than the rate of change of the mean particle velocity. The sites of nucleation of 3D-structures are speculated to be the staking faults generated under action of chaotic velocity pulsations relevant to dynamic deformation. The physical model for formation of 3D-structures takes into account the intersection of the partial dislocations and Lomer - Cottrell barriers.

Shock-wave behavior of structural nitrogen-bearing steel after heat treatment under various conditions

Abstract04Kh20G11N6M2AFB steel is subjected to shock tests in the following two states: after high-temperature mechanical treatment (HTMT) and after HTMT followed by quenching. The dynamic yield strength, the spall strength, and the structural transition threshold induced by shock loading are determined. It is shown that these parameters weakly depend on the shock loading rate in the steel after HTMT and increase slightly in the steel quenched from a temperature of 1100°C. In both cases, the mass velocity defect at a compression pulse plateau increases sharply beginning from a certain threshold strain rate, which indicates a high energy absorption ability of the steel.

Deformation and fracture mechanisms and structural changes in coarse-grained copper under shock-wave loading

Results of experiments on shock-wave deformation of M2 copper under uniaxial loading are presented. Light, scanning, and transmission electron microscopy methods are used to reveal specific features of mechanisms of deformation and fracture of copper during the formation of a main spall crack. The parameters of spall strength, damage, and self-similarity of the spall crack contour are determined.

STUDIES ON LOCALIZED STRAIN IN SHEET METALS UNDER IMPACT TENSION AND SHEAR

Results of studies on the microstructure of plane sheet specimens after their impact loading at different rates are discussed. Near the notch tips, in the area of localized strain, several layers of different microstructure were revealed. The formation of fine grains is assumed to be determined by the process of dynamic recrystallization at increased local temperatures due to intensive plastic deformation. The boundaries between the layers of different microstructure as well as increased pore concentrations in the area of localized strain near the outer surface point to the realization of the three-dimensional stress-strain state in the material.