Physical Mesomechanics最新文献

A modification of the Lemaitre damage model is proposed based on the introduction of a function sensitive to the Lode parameter. The modified model is imported into the ANSYS software as a dynamically linked custom tag library. The model takes into account isotropic hardening based on the exponential Voice model and kinematic hardening based on the Armstrong–Frederick model. Limit state curves are obtained by numerical finite element analysis for three types of experimental cylindrical specimens: a compression specimen under additional external pressure, a circular-notch specimen under uniaxial tension, and a hollow cylindrical specimen under combined tension, torsion, and internal pressure. The advantages and disadvantages of the proposed model are considered. Recommendations are given for choosing model parameters to predict limit states under multiaxial loading.

The Lennard-Jones potential approach is used to investigate the effect of pressure on the ultrasonic and elastic properties of the rare-earth ternary TbNiAl intermetallic compound. The second- and third-order elastic constants of TbNiAl are considered using the potential model. The pressure-dependent higher-order elastic constants are studied, and it is observed that the elastic constants of the TbNiAl compound increased monotonously with pressure. The hexagonal TbNiAl compound is mechanically stable up to the pressure 20 GPa according to the Born elastic stability criteria. The Voigt–Reuss–Hill approach is used to compute such elastic parameters as Young’s modulus, bulk modulus, Poisson’s ratio, and shear modulus in the pressure range 0–45 GPa. Hardness, melting temperature, and anisotropy are also determined for the intermetallic TbNiAl compound. The pressure-dependent velocities and attenuation of ultrasonic waves in this ternary compound are evaluated. The computation results are also satisfactory in estimating the Debye temperature and thermal conductivity K_min under different pressure. It is observed that TbNiAl has a significant anisotropy at zero pressure, which becomes stronger as the pressure increased. This ternary compound behaves as its purest form at higher pressure and is more ductile, which is demonstrated by the minimum attenuation.

The origins and formation mechanisms of neotectonic structures in a part of the Mongolian-Siberian region were identified by geodynamic zoning based on multivariate statistical analysis of numerical data that describe geological-geophysical and geological-geomorphological processes. These processes in the regional lithosphere were described by a set of 11 geological and geophysical parameters, divided into three main groups using the hierarchical method of cluster analysis. The first group includes the seismic moment, the density of active faults, the recent horizontal strain rates, and the magnitude of the deep heat flow. The second group involves the thicknesses of the earth’s crust and exogenously active layer, the recent horizontal crustal velocities, and the amplitudes of vertical neotectonic movements. The third group includes gravity anomalies and the lithospheric thickness. The spatial grouping of the parameters by cluster analysis (K-means method) yields seven clusters, whose spatial position and composition are determined by the geological history, geological structure, geodynamic evolution of the region, and the recent strain rates. Some clusters characterize large rigid lithospheric blocks, while other clusters describe large active fault systems in the studied region. The search for latent factors that make the greatest contribution to the dispersion of the geological and geophysical parameter values was carried out using the principal component method, which allows minimizing the number of factors. Four main factors were identified for areas that differ in the morphology and origin of neotectonic structures: (i) higher horizontal compressive and tensile strains, (ii) dynamic effect of mantle anomalies, manifested in uplifts and doming, (iii) activation of thinned lithosphere within the boundaries of lithospheric plates or large blocks, and (iv) active shear deformation of the earth’s crust. The results of clustering and factor analysis of numerical data describing geological-geophysical and geological-geomorphological processes within the Mongolian-Siberian region are interpreted in the framework of physical mesomechanics.

This paper discusses the formation of ultrafine-grained (UFG) structure and nanosized second-phase precipitates in commercially pure Grade 4 titanium subjected to severe plastic deformation by high pressure torsion at room temperature with subsequent heat treatment. It was found that the combined processing of Grade 4 titanium provides very high tensile strength (σ_B ≈ 1500 MPa), which significantly exceeds the previous results for this material. Analysis of the strengthening mechanisms showed that the superstrength of commercially pure titanium is due to several factors: UFG structure formation, dispersion strengthening from second-phase nanoparticles, high dislocation density, and grain boundary segregation. The contribution of these strengthening mechanisms is evaluated and compared with experimental data.

This paper investigates the fatigue behavior of smooth specimens made of VT3-1 titanium alloy under fully reversed loading conditions. Mathematical modeling results are presented for the fatigue fracture of smooth specimens under high cycle torsional fatigue loading. The fatigue quasi-crack initiation and growth are calculated using a multimode two-parameter model of fatigue damage accumulation. The surface and subsurface quasi-crack initiation under torsional loading is studied. The numerical results are in good agreement with the experimental data and can be used to predict the change in crack growth mechanisms under complex multiaxial loadings such as torsion.

Carbon nanoadditives are widely used as modifiers for various materials. An important issue is the effect of modification on the material properties, including the sensitivity to temperature changes. In this study, we performed fixed-temperature indentation of polyurethane samples produced by mortar technology with fullerenes and carbon nanotubes. It was found that the addition of modifiers not only changes the mechanical and rheological properties of the material, but also makes these properties more temperature dependent. Based on solving an axisymmetric contact problem of constant loading rate indentation of a viscoelastic half-space, a method was developed for determining material properties from experimental indentation curves obtained at different rates. The properties of the original and modified polyurethanes were determined at three fixed temperatures. The modifiers produced different effects: nanotubes increased stiffness, while fullerenes reduced it. The effect of ion plasma surface treatment, leading to the formation of a hard carbonized nanolayer, on the indentation results at different temperatures was also investigated.

The paper proposes a new method for adhesion strength assessment of polymer coatings which is based on Rockwell indentation with experimental data processing via finite element simulation in terms of fracture mechanics (cohesive zone model, CZM). With the example of a titanium-alkoxide epoxy composition deposited on low-carbon steel, it is shown that when the Rockwell indenter penetrates perpendicular to the coating surface, circular buckling delamination around its indent occurs due to adhesive bond rupture by radial shear with extrusion of the coating material from beneath the indenter. The parameter controlled in the simulation is the width of coating delamination zones formed in indentation experiments at a constant indentation depth. The conditions of adhesive contact are specified using the CZM bilinear law, which describes the relation between the tangential adhesive stress and the adhesive bond elongation under shear in the contact plane of interacting surfaces. The criterion of quantitative adhesion strength assessment is the ultimate specific surface energy of adhesive failure. The simulation gives an optimum value of the ultimate specific surface energy of adhesive failure of the coating at CZM parameters that provide the best convergence of the numerical and experimental data.

High residual porosity in superplastically deformed brass carries the risk of reducing the mechanical properties. Multicomponent brasses demonstrate lower residual porosity, associated with a lower grain size and more effective accommodation of grain boundary sliding. In this paper, the microstructural evolution of the surface and bulk structure of the binary brass and aluminum-bearing brass during steady-state superplastic deformation is compared. After superplastic deformation, dislocation pile-ups and dislocation walls are revealed in the α grains of both alloys, indicating the activation of the dislocation slip/creep mechanism. It is shown that aluminum reduces the contribution of grain boundary sliding along the phase boundaries from ~75 to ~30% and causes strain localization in the β-phase region with the formation of ultrafine grains with the size below ~300 nm as a result of dynamic recrystallization. Alloying with 0.4% Al reduces the flow stress by 20%, increases the relative elongation by a factor of 1.5, and decreases the fraction of residual porosity by a factor of 3. This leads to a much lower loss of room temperature strength in superplastically deformed alloys.

We present a refined engineering theory of cracks based on a two-parameter strength criterion. Unlike the basic theory, the refined approach utilizes an improved algorithm for the regular stress component computation. This improvement allows extending the engineering theory to longer cracks. The two-parameter Leonov–Panasyuk–Dugdale fracture criterion serves as a basis. A coupled fracture criterion includes a strain-based criterion, which is formulated at the tip of the true crack, as well as a stress-based criterion, formulated at the tip of the fictitious crack. Based on the refined criterion, quasi-brittle fracture curves are constructed for a compact specimen, a strip with an edge crack, and a four-point bending beam. To validate the new refined fracture criterion, we present simulation results of quasi-brittle fracture for structures made from various virtual materials. The corresponding virtual materials are modeled using a nonlocal damage theory accounting for the average size of the aggregate state of the material. Additionally, various classes of damage accumulation hypotheses are considered. Analysis of various types of virtual materials provides insights into the impact of hypotheses behind the engineering theory. For each type of material, the influence of the microstructural length scale on the overall structural strength is investigated. The analysis shows that the refined engineering theory has a wider range of applicability as compared to the basic theory based on two-parameter strength criteria.