Vladimir A. Skripnyak

Effect of structural factors on mechanical properties of the magnesium alloy Ma2-1 under quasi-static and high strain rate deformation conditions

The elastic limit and tensile strength of deformed magnesium alloys Ma2-1 with different structures and textures were measured with the aim of finding a correlation between the spectrum of defects in the material and the resistance to deformation and fracture under quasi-static and dynamic loading conditions. The studies were performed using specimens in the as-received state after high-temperature annealing and specimens subjected to equal-channel angular pressing at a temperature of 250°C. The anisotropy of strength characteristics of the material after shock compression with respect to the direction of rolling of the original alloy was investigated. It was shown that, in contrast to the quasi-static loading conditions, under the shock wave loading conditions, the elastic limit and tensile strength of the magnesium alloy Ma2-1 after equal-channel angular pressing decrease as compared to the specimens in the as-received state.

Mechanical Behavior of Light Alloys with Bimodal Grain Size Distribution

Deformation and damage at the meso-scale level in representative volumes (RVE) of light ultrafine grained (UFG) alloys with distribution of grain size were simulated in wide loading conditions. The computational models of RVE were developed using the data of structure researches aluminum and magnesium UFG alloys on meso-, micro -, and nanoscale levels. The critical fracture stress on meso-scale level depends not only probabilistic of grain size distribution in RVE but relative volumes of coarse grains. Microcracks nucleation is associated with strain localization in UFG partial volumes in alloys with bimodal grain size distribution. Microcracks branch in the vicinity of coarse and ultrafine grains boundaries. It is revealed that the occurrence of bimodal grain size distributions causes the increasing of UFG alloys ductility, but decreasing of the tensile strength. The distribution the shear stress and the local particle velocity takes place at mesoscale level under dynamic loading of UFG alloys with bimodal grain size. The increasing of fine precipitations concentration not only causes the hardening but increasing of ductility of UFG alloys with bimodal grain size distribution.

Multiscale Simulation of Porous Quasi-Brittle Ceramics Fracture

Multiscale computer simulation approach has been applied to research mechanisms of failure in ceramic nanostructured ceramics under dynamic loading. The obtained experimental and theoretical data indicate quasi-brittle fracture of nanostructured ZrB2 ceramics under dynamic compression and tension. Damage nucleation and accumulation in quasi brittle nanostructured ceramics were simulated under impact loadings. Fracture of nanostructured ultra-high temperature ceramics under pulse and shock-wave loadings is provided by fast processes of intercrystalline brittle fracture and relatively slow processes of quasi-brittle failure via growth and coalescence of opened microcracks. For nanostructures ZrB2 ceramics with porosity of 7 %, the compressive strength at strain rate of 1800 s-1 is equal to 2440±50 MPa, the tensile strength at strain rate of 300 s-1 is equal to 155±20 MPa.

Dependence of the Longitudinal Velocity of Sound in Constructional Ceramic Materials on Pressure and Damage Rate

The effect of porosity and concentration of planar microcracks on the velocity of elastic waves in polycrystalline ceramic materials on the basis of SiC, Al2

Strength and plasticity of Fe-Cr alloys

O3, B4C, and ZrO2 is numerically studied. The mechanical behavior of ceramics is described using the model of a damaged medium. Various dependences that describe the relationship between the effective moduli of elasticity of the medium material and the relative volume of damages are analyzed as applied to predicting wave dynamics. For porosities up to 20%, a satisfactory prediction of the velocity of longitudinal waves in ceramics is ensured by the use of exponential and linear dependences. Within this range of porosities, the velocity of elastic waves decreases linearly with increasing relative volume of damages. The influence of the pulse amplitude on the velocity of elastic waves is analyzed. It is shown that the velocity of elastic waves in constructional ceramics increases in proportion to pressure up to 5% within the range of pulse amplitudes that do not exceed the Hugoniot limit of elasticity. Numerical values of coefficients in the relation between the velocity of the longitudinal elastic wave and the velocity of material particles are determined for ceramic materials considered. As the Hugoniot limit of elasticity is exceeded, the values of the coefficients decrease by 10–30% for different ceramic materials. The resultant values of the coefficients are in good agreement with experimental data found in the literature.

FRACTURE OF THIN METAL SHEETS WITH DISTRIBUTION OF GRAIN SIZES IN THE LAYERS

High-chromium steels are attractive as promising structural materials for applications in nuclear facilities. Using the multilevel modeling, yield stress values of precipitation-hardened Fe-Cr steels are predicted in the temperature range up to 1115 K and pressures up to 10 GPa. The adiabatic curve obtained demonstrates a good correlation with the experimental data for a Fe-Cr-Ni alloy in the pressure range up to 10 GPa.

Modeling of Mechanical Behavior of Ceramic Nanocomposites

The influence of ultra-fine grained surface layers on the fracture of light alloys sheets was studied by the method of multiscale simulation. The deformation and damaging of 3D structured elementary volumes of thin metal sheets with structured surface layers under tension and compression were calculated. The modified smooth particle hydrodynamics method was used for numerical simulation. It was found that inelastic deformation and the damage are localized at the boundary between ultrafine-grained and coarse grained layers. The deflection of cracks caused by the residual stresses lead to ductility increasing of metal sheets with layered ultrafine-grained and coarse-grained structure. Fracture of thin sheets of aluminium, magnesium and titanium alloys with nanostructured and fine grained surface layers under loading has probabilistic character and depends on parameters of layered structure.

Characteristic Features of Physical and Mechanical Properties of Ultrafine-Grained Al–Mg Alloy 1560

Deformation and damage occurring at the meso-scale level in structured representative volumes (RVE) of modern nanocomposites in wide loading conditions were simulated. The computational models of a structured RVE of ceramic nanocomposites were developed using the data of structure researches on meso-, micro -, and nanoscale levels. The critical fracture stress on meso-scale level depends not only on relative volumes of voids and inclusions, but also on the parameters of inclusion clusters. The critical fracture stress at the meso-scale level depends not only on relative volumes of voids and strengthened phases, but also on sizes of corresponding structure elements. In the studied ceramic composites the critical failure stress is changed non-monotonically with growth of the volume concentration of strengthening phase particles. At identical porosity, concentration of nanovoids in the vicinity of grain boundaries causes the decrease in the shear strength of nanostructured and ultrafine-grained ceramics. It is revealed that the occurrence of bimodal distributions of the local particle velocity at the meso-scale level precedes the nucleation of microcracks. At mesoscale level of ceramic nanocomposites the pressure and particle velocity distribution don’t display a resonance behavior under submicrosecond single shock pulse loading or repeated pulse loadings.

Fracture mechanisms of zirconium diboride ultra-high temperature ceramics under pulse loading

Specimens of Al–Mg alloy 1560 of ultrafine-grained structure were obtained by the method of severe plastic deformation based on multiple equal-channel angular pressing. Impact on physical and mechanical properties of the processed material and fracture pattern of specimens was studied. Tensile tests showed an increase of the offset yield strength and resistance to rupture with decrease in the ultimate deformation. The obtained specimens have increased microhardness values compared to the initial ones. It was established that the last cycle of pressing determines the structural orientation of macroscopic shear bands occurring at an angle to the specimen longitudinal axis while passing connection of channels. It affects the physical and mechanical properties of the material and fracture pattern. The quality control of the obtained specimens by the method of ultrasonic defectoscopy and X-ray tomography confirmed the absence of macroand microdefects when following the matched optimal regime of processing.

Influence of grain size distribution on the mechanical behavior of light alloys in wide range of strain rates

Mechanisms of failure in ultra-high temperature ceramics (UHTC) based on zirconium diboride under pulse loading were studied experimentally by the method of SHPB and theoretically. The obtained experimental and numerical data are evidence of the quasi-brittle fracture character of nanostructured zirconium diboride ceramics under compression and tension at high strain rates and the room temperature. Damage of nanostructured porous zirconium diboride-based UHTC can be formed under stress pulse amplitude below the Hugoniot elastic limit. Fracture of nanostructured ultra-high temperature ceramics under pulse and shock-wave loadings is provided by fast processes of intercrystalline brittle fracture and relatively slow processes of quasi-brittle failure via growth and coalescence of microcracks.