Yu. I. Meshcheryakov

Nonequilibrium processes in condensed media: Part 1. Experimental studies in light of nonlocal transport theory

Based on experimental research in shock loading of solid-state materials it is shown that among the important dynamic characteristics of the process, like spatial-temporal mass velocity profiles of shock waves, are the mass velocity variation, velocity defect, and structural instability threshold recorded in real time. Analysis of these characteristics depending on the strain rate, target thickness, and structural state of material demonstrates that conventional approaches of continuum mechanics fail to provide their adequate interpretation and simulation of shock wave processes. A new concept of shock wave processes in condensed media is proposed. The concept, being based on nonlocal nonequilibrium transport theory, allows describing the transition from elastic to hydrodynamic response of a medium depending on the loading rate and time. A nonstationary elastoplastic wave model is proposed for describing the relaxation of an elastic precursor and formation of a retarded plastic front during the wave propagation in a medium with regard to structural evolution. Analysis of the experimental data shows that the division of stresses and strains into elastic and plastic components is incorrect for shock loading.

Structural Viscosity of Solids

Simple estimates of the coefficient of dynamic viscosity of some metals are considered. Experimental results on shock loading of aluminum, copper, and steel plane specimens are given. It is shown that the coefficient of dynamic viscosity depends on the characteristic size of the structural level of plastic strain at which loading-energy dissipation is considered. It was found that the main level that determines the viscosity of materials under high-velocity loading is the mesoscopic level with a characteristic size of ≈ 10 μm. Key words: viscosity, high-velocity loading, mesoscopic level, interferogram.

Effect of processes at the compression pulse front on spalling strength of a material and resistance to high-velocity penetration

Tests on backward spalling of 38KhN3MFA structural steel, D-16 aluminum alloy, M-2 copper, 02Kh18K9M5-VI maraging steel, KhN75VMYu alloy, beryllium, and other materials show that spalling strength correlates with the threshold of structural instability of a material to compression at the leading edge of the compression pulse. It is shown that the threshold of structural instability to compression obtained in experiments on uniaxial deformation of flat targets determines the strength of resistance to high-velocity penetration in the Alekseevskii–Tate model.

Noncrystallographic structural deformation levels in strongly excited systems

ConclusionTherefore, the analysis performed in the previous sections on the experimental data and results of modelling by the molecular dynamics method permits making a deduction on the possibility of the formation of strongly excited systems of noncrystallographic structural deformation levels during loading. As an experimental investigation showed, for all the kinds of static loading utilized their origination is associated with the pre-fracture stages. Crack propagation over the noncrystallographic interfacial boundaries of the fragments indicates this. Under shockwave loading the macroflux motion with the grains is also a new, larger-scale dynamic deformation level (compared with the dislocation level).

Effect of velocity nonuniformity on the dynamic recrystallization of metals in shock waves

AbstractA series of mechanical tests on D16 aluminum alloy samples under uniaxial strain conditions in single and double impact loading regimes showed that dynamic recrystallization in localized shear bands takes place only in the latter case, with the second (additional loading) pulse delayed by 0.5–0.7 μs relative to the first shock-wave front. It is established that, in addition to well-known conditions (γ ≥ 3,

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

\dot \gamma

Turbulence and dissipative structures in shock-loaded copper

≥ 104 s−1, T ≥ 0.4Tm), a determining role in the dynamic recrystallization process is played by the nonuniformuty (variation) of mass velocity at the leading front of the compression pulse.

Oscillations of the plastic wave front under high‐rate loading

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.