Evgeniya G. Skripnyak

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

Modeling of Mechanical Behavior of Ceramic Nanocomposites

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

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.

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.

SIMULATION OF MECHANICAL PROPERTIES OF CERAMIC PARTS PRODUCED BY ADDITIVE TECHNOLOGIES IN WIDE RANGE OF LOADING RATES

Inelastic deformation and damage at the mesoscale level of ultrafine grained (UFG) light alloys with distribution of grain size were investigated in wide loading conditions by experimental and computer simulation methods. The computational multiscale models of representative volume element (RVE) with the unimodal and bimodal grain size distributions were developed using the data of structure researches aluminum and magnesium UFG alloys. The critical fracture stress of UFG alloys on mesoscale level depends on relative volumes of coarse grains. Microcracks nucleation at quasi-static and dynamic loading is associated with strain localization in UFG partial volumes with bimodal grain size distribution. Microcracks arise 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.

BRITTLE OR QUASI-BRITTLE FRACTURE OF CERAMIC NANOCOMPOSITES UNDER DYNAMIC LOADING

The multiscale simulation approach was used to study the mechanical behavior of ceramic parts created by the selective laser sintering (SLS) technology. The goal of the research was studding the difference in the elastic moduli and dynamic strength of ceramic parts, manufactured by the high-temperature sintering and the selective laser sintering. The mechanical properties of ceramic parts were determined by statistic averaging of the results of multi scale computer simulations of representative volumes of materials under loadings. It was shown that the probability of the fracture of such parts under dynamic loading depends on the time history evolution of the distribution of local values of specific internal energy and the damage parameter on the mesoscale level. It was found that the stress threshold of the dynamic fracture beginning of ceramic bodies depends on the distribution of the pore sizes and not just on the average porosity.

Mechanical Behaviour of Nanostructured Materials at High Strain Rates. Computer Simulation

Multiscale computer simulation was used for simulation of the damage nucleation and fracture of ZrB2 − ZrO2, ZrB2 – B4C, Al2O3 − ZrO2 − Y2O3, Al2O3 − B4C nanocomposites under dynamic loading. The aim of research was the study transition from brittle to quasibrittle fracture of nanocomposites under dynamic loading. It was shown that isolated microand mesoscale cracks can be nucleate in ceramic nanocomposites near voids under stress pulse amplitude less than the Hugoniot elastic limit. The critical fracture stress on mesoscale level depends not only on relative volumes of voids and particles concentration, but also sizes of corresponding structure elements. Results of simulation have shown the Hugoniot elastic limit and ceramics damage kinetics under dynamic loading depends on a volume concentration of nano-particles and nano-voids clusters.

Computer Simulation of the Propagation of Short Shock Pulses in Ceramic Materials

The new model of mechanical behaviour of nanostructured materials in wide range of strain rates is presented in the paper. The inelastic strains caused by the dislocation mechanisms, twinning, and grain boundary sliding have been taken into consideration. The model was used for computer simulation of the dynamics of shock waves in nanostructured materials. The results testify to distinctions of the mechanical behaviour of nanostructured and course grained materials under shock wave loading. The shear stress of nanostructured materials at high strain rates has been predicted to be less than ones of course‐grained materials due to contributions to inelastic deformation caused by grain boundary sliding.