Vladimir V. Skripnyak

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.

The Effect of a Severe Plastic Deformation by Groove Pressing on the Grain Structure of the Al-Mg Alloy

In this article, the effect of a severe plastic deformation (SPD) achieved by groove pressing (GP) on the grain structure and mechanical properties of a rolled sheet Al-Mg alloy was investigated. The study of the microstructure of the samples before and after processing was carried out by means of electron backscattered diffraction (EBSD). The mechanical properties of the samples were experimentally studied under uniaxial tension in quasi-static conditions, and microhardness testing was implemented. It was found that the conventional yield strength and ultimate tensile strength increase by the factor of 1.4 and 1.5, respectively; and the microhardness increases by approximately 2.7 times after four machining sequences of the rolled sheet alloy. A bimodal grain structure, consisting of two grain types with particular features, is formed in the samples after four machining sequences of GP.

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

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.

Modeling of Mechanical Behavior of Ceramic Nanocomposites

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.

Numerical simulation of deformation behavior of aluminum alloy sheets under processing by groove pressing method

The results of theoretical estimation of capabilities of the material structure modification of 1560 aluminum alloy sheets under processing by severe plastic deformation are presented in this paper. Severe plastic deformation of flat specimens is effected by the constrained groove pressing method in original dies with trapezoidal teeth. The numerical simulation results of the sheet specimen treatment process by severe plastic deformation were used for dies designing. The stress-strain state of flat aluminum alloy specimens and the steel dies at high processing temperature, support reaction force during pressing and the degrees of plastic strain accumulation at the optimum mode of pressing were estimated. The main numerical result is the value of accumulated plastic strain in the specimen per one pressing cycle which is about 1.14. Large degrees of strain are the reasons of grain structure and material texture changes, which leads to inevitable change of its physical-mechanical properties. Increasing the number of pressing cycles leads to proportional increase of the degree of accumulated plastic strain.

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

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.

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

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

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.

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

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.