Extensive teaching of physics for students of technical specialties with the help of experimental units and computer simulation software

The paper studies the issues of the extensive teaching of physics for the stu-dents of technical specialties with the help of experimental units and com-puter simulation software. The authors proposed and experimentally tested a pedagogical method for increasing the students’ interest in mastering the study material in physics. The article presents the experimental units devel-oped by the authors and patented as utility models for the study of dielectric hysteresis and the tunnel effect, developed by the authors and patented as useful models. Computer models for calculating the current in an electrical circuit and for calculating the interaction potential of particles created by the authors and patented as computer programs are shown. The methods of per-forming work by students on experimental units and with the help of com-puter simulation software are described. In conclusion, an increase in the in-terest and involvement of students in project and research work after the in-troduction of experimental units and computer models into the educational process, and as a result, an increase in motivation for learning in general and in-depth study of physics in particular, was noted

Temperature and strain rate dependence of microstructural evolution and dynamic mechanical behavior in nanocrystalline Ti

The mechanical behavior of commercial purity titanium with a nanocrystalline (NC) grain size was investigated using split Hopkinson pressure bar tests at high strain rates and over a range of temperatures. The study was accompanied by detailed microstructural investigations before and after compression testing. The results show that rotary dynamic recrystallization operates during compressive deformation at strain rates of ~3000 and ~4500 s?1 at temperatures from 298 to 573 K but cells form at 673 K. The dynamic mechanical behavior of NC Ti shows a strong dependence on temperature and strain rate such that the flow stress and the strain hardening rate both increase with increasing strain and decreasing temperature. A constitutive equation is derived to relate the flow stress to the temperature, strain rate and true strain and to predict the yield strength and the peak stress of NC Ti subjected to dynamic deformation at elevated temperatures

СВЕРХПЛАСТИЧЕСКОЕ ПОВЕДЕНИЕ АЛЮМИНИЕВОГО СПЛАВА 1420 С МЕЛКОЗЕРНИСТОЙ СТРУКТУРОЙ

The superplastic behavior of fine-grained 1420 Al-Mg-Li alloy was investigated using a modern electron microscopy technique based on automatic analysis of electron backscattered diffraction patterns (EBSD analysis). The generally accepted idea that grain boundary sliding is dominant during superplastic flow suggests the preservation of an equiaxed fine-grained structure with predominantly high-angle grain boundary misorientation in the material. The present study revealed that heating prior to the onset of deformation leads to some grain growth due to static recrystallization, and superplastic deformation is accompanied by dynamic grain growth and continuous dynamic recrystallization. Continuous recrystallization has a more significant effect on microstructural changes. This mechanism involves the transverse division of pre-elongated grains into subgrains that ultimately transform into chains of nearly equiaxed small grains, resulting in a bimodal grain structure. The data obtained, including significant strain hardening, noticeable grain elongation, the formation of a well-defined dislocation structure and subboundaries within grains, as well as the development of a pronounced crystallographic texture, provide convincing evidence of the occurrence of intragranular slip during superplastic flow throughout the entire volume of the material..

Chain Model for Carbon Nanotube Bundle under Plane Strain Conditions

Carbon nanotubes (CNTs) have record high tensile strength and Young’s modulus, which makes them ideal for making super strong yarns, ropes, fillers for composites, solid lubricants, etc. The mechanical properties of CNT bundles have been addressed in a number of experimental and theoretical studies. The development of efficient computational methods for solving this problem is an important step in the design of new CNT-based materials. In the present study, an atomistic chain model is proposed to analyze the mechanical response of CNT bundles under plane strain conditions. The model takes into account the tensile and bending rigidity of the CNT wall, as well as the van der Waals interactions between walls. Due to the discrete character of the model, it is able to describe large curvature of the CNT wall and the fracture of the walls at very high pressures, where both of these problems are difficult to address in frame of continuum mechanics models. As an example, equilibrium structures of CNT crystal under biaxial, strain controlled loading are obtained and their thermal stability is analyzed. The obtained results agree well with previously reported data. In addition, a new equilibrium structure with four SNTs in a translational cell is reported. The model offered here can be applied with great efficiency to the analysis of the mechanical properties of CNT bundles composed of single-walled or multi-walled CNTs under plane strain conditions due to considerable reduction in the number of degrees of freedom

EVOLUTION OF THE CARBON NANOTUBE BUNDLE STRUCTURE UNDER BIAXIAL AND SHEAR STRAINS

Close packed carbon nanotube bundles are materials with highly deformable elements, for which unusual deformation mechanisms are expected. Structural evolution of the zigzag carbon nanotube bundle subjected to biaxial lateral compression with the subsequent shear straining is studied under plane strain conditions using the chain model with a reduced number of degrees of freedom. Biaxial compression results in bending of carbon nanotubes walls and formation of the characteristic pattern, when nanotube cross-sections are inclined in the opposite directions alternatively in the parallel close-packed rows. Subsequent shearing up to a certain shear strain leads to an appearance of shear bands and vortex-like displacements. Stress components and potential energy as the functions of shear strain for different values of the biaxial volumetric strain are analyzed in detail. A new mechanism of carbon nanotube bundle shear deformation through cooperative, vortex-like displacements of nanotube cross sections is reported

Delocalized nonlinear vibrational modes in graphene: second harmonic generation and negative pressure

With the help of molecular dynamics simulations, delocalized nonlinear vibrational modes (DNVM) in graphene are analyzed. Such modes are dictated by the lattice symmetry, they are exact solutions to the atomic equations of motion, regardless the employed interatomic potential and for any mode amplitude (though for large amplitudes they are typically unstable). In this study, only one‐ and two‐component DNVM are analyzed, they are reducible to the dynamical systems with one and two degrees of freedom, respectively. There exist 4 one‐component and 12 two‐component DNVM with in‐plane atomic displacements. Any two‐component mode includes one of the one‐component modes. If the amplitudes of the modes constituting a two‐component mode are properly chosen, periodic in time vibrations are observed for the two degrees of freedom at frequencies ω and 2ω, that is, second harmonic generation takes place. For particular DNVM, the higher harmonic can have frequency nearly two times larger than the maximal frequency of the phonon spectrum of graphene. Excitation of some of DNVM results in the appearance of negative in‐plane pressure in graphene. This counterintuitive result is explained by the rotational motion of carbon hexagons. Our results contribute to the understanding of nonlinear dynamics of the graphene lattice