Physical Mesomechanics最新文献

The article discusses the longitudinal tension of two-layered plates of orthorhombic crystals. Expressions for effective Young’s moduli and Poisson’s ratios for different loading directions are obtained. With use of available experimental data on the elasticity constants, variability of effective elastic properties of plates is analyzed for all possible combinations of orthorhombic crystals. Plates with effective Young’s modulus greater than Young’s moduli of the layers and plates with a negative effective Poisson’s ratio are identified.

Mechanisms of fatigue crack nucleation and propagation in metals are considered on the basis of the physical mesomechanics methodology. Evolution in the metal behavior is presented in the direction of increasing scale levels of evolution with growing cyclic equivalent stress or strain energy density. Differences in damage accumulation in the surface layer and internal volumes on the three scales corresponding to very high-cycle, high-cycle and low-cycle fatigue regions are analyzed. The subsurface crack initiation mechanisms leading to origins in the form of a smooth facet or fine-granular area are shown. Both types of origins are created under vortex flow of plastic deformation. Differences in the fatigue crack nucleation on the meso- and macroscales are discussed and illustrated. Fracture features of ductile metals in the ultralow-cycle fatigue region, which is a transition region from cyclic to monotonic static loading conditions, are considered.

A modification of the dislocation-based constitutive model is proposed to describe the deformation of polycrystalline materials taking into account intragranular and grain boundary strengthening. The developed model is based on the introduction of internal variables, physical elastoviscoplasticity theory, and multilevel approach; it is considered at three (macro, meso-1, and meso-2) hierarchically related structural and scale levels. At the upper level (of a representative macrovolume), the response of the material (a measure of the stress state) to thermomechanical effects (given changes in the measure of deformation and temperature) is determined. Elements of the meso-1 and meso-2 levels have the same scales, differing in the way they are described. The meso-1 elements are considered in terms of mechanical variables (stress measures, residual and critical shear stresses, and shear rates in slip systems). At the meso-2 level, the description is carried out in terms of densities and velocities of dislocations. Particular attention is paid to the interaction of dislocations with grain boundaries, which have a significant effect on the behavior of polycrystalline materials. Grain boundaries determine the local stress-strain state and are barriers to dislocation slip, which can lead to dislocation pile-ups in the near-boundary regions. The submodel taking into account the dislocation flow in the vicinity of grain and subgrain boundaries within the dislocation-based model is described in detail. Examples of the model application to the study of loading of bicrystal specimens are given. It is shown that the developed model allows describing dislocation fluxes through the boundary and takes qualitative account of grain-boundary strengthening.

The study numerically investigates deformation and fracture of the eutectic AlSi12 alloy produced by wire-feed electron beam additive manufacturing. A top-down approach is implemented for the numerical and experimental determination of local plastic and strength properties of materials. The alloy structure is experimentally examined on different scales using optical microscopy. Mechanical uniaxial tensile and nanoindentation tests are performed. Based on the experimental data, the finite element models of the layered structure at the macrolevel and dendritic structure at the mesolevel are created. The top-down analysis involves sequential numerical simulations at the macro- and mesolevels to extract the mechanical properties of aluminum in the eutectic material and dendrites. Based on the experimental flow curve and nanoindentation data, hardening functions and ultimate strains are determined for the layer and interlayer materials by numerically simulating the layered structure tension on the macroscale. The obtained layer properties and nanoindentation data are then used to derive the properties of aluminum in the eutectic and dendrites by simulating the dendritic structure tension on the mesoscale. The derived plastic and strength characteristics of aluminum and the eutectic are used to numerically analyze heterogeneous plastic flow and cracking in the dendritic structure with an interlayer containing micron-sized silicon particles.

A Volterra edge dislocation moving in an infinite isotropic elastic medium under shear prestress is considered as a model of a shear rupture in the Earth’s crust formed at seismic depths and propagating at velocities in the range between the elastic shear and longitudinal wave velocities. In the plane strain approximation, the equations of stationary motion of the medium around the dislocation are reduced to an elliptic-hyperbolic system of equations for velocities and stresses, which is integrated using analytic functions of a complex variable and the method of characteristics. The invariant J-integral is used to estimate the deformation energy released during the dislocation motion depending on the velocity, shear stress at infinity, length of a fan adjacent to the dislocation tip, and Burgers vector distribution in the fan.

The paper presents an analysis of crack growth rates and an assessment of fracture mechanisms based on the experimental data on isothermal and nonisothermal fatigue. Test results were obtained in the temperature range of 400–650°C under conditions of harmonic fatigue, creep–fatigue interaction, in-phase and out-of-phase thermomechanical cyclic loading. The object of study was heat-resistant nickel-based alloy EI698. After testing, the specimens underwent detailed fractographic analysis using scanning electron microscopy. It was found that the fatigue fracture diagrams were in the following order in terms of crack growth acceleration: isothermal creep–fatigue interaction, nonisothermal in-phase thermomechanical fatigue, isothermal pure fatigue, and nonisothermal out-of-phase thermomechanical fatigue. The crack growth rate curves were arranged according to the dominant intergranular and transgranular fracture mechanisms, and features of transition from one to another were identified.

The effect of mean stress on the fatigue strength of a polyphthalamide composite with 33 wt % short glass fibers was studied under various loading conditions, including cyclic tension, tension–compression, and compression. The mean stresses were 45, 0, –60, and –120 MPa, and the stress amplitudes ranged from 15 to 80 MPa. The results were compared with the known models by Goodman, Soderberg, and Gerber, highlighting the ambiguity in predicting the effect of mean stress in the region of high positive and negative mean stress values. At negative mean stresses, the limiting stress amplitudes were higher, when the fatigue resistance increased until the material reached its compressive yield stress, after which a decrease was observed again. Differences in the fatigue damage mechanisms were identified in cyclic tension and cyclic compression modes. In the tension mode, fatigue damage developed at the fiber–matrix interface due to adhesive fracture. In contrast, in cyclic compression mode, the main mechanism of fatigue damage accumulation was creep followed by cohesive fracture of the matrix.

The deformation and fracture patterns of Ti–6Al–4V/TiC composites 3D printed by wire-feed electron beam additive manufacturing using Ti–6Al–4V wire pre-electrospark alloyed with carbide-containing electrodes or by simultaneous melting of titanium wire and TiC powder are investigated. It is shown that Ti–6Al–4V/TiC composites obtained by both methods are characterized by the same microstructure and volume fraction of the carbide phase (~2%) but different sizes of TiC inclusions. It is found that the ductility of the Ti–6Al–4V/TiC composite obtained by electrospark alloying of the wire and containing TiC inclusions with a diameter of ~1 μm is significantly higher than that of the specimen 3D printed by simultaneous melting of titanium wire and TiC powder, in which the inclusion sizes vary within 15–30 μm. Different fracture characteristics of the investigated Ti–6Al–4V/TiC composites under uniaxial tensile stress were revealed. Three-dimensional modeling of elastic-plastic deformation and fracture of model Ti/TiC composites under mechanical loading is carried out using the method of movable cellular automata. The influence of the size and hardness of TiC particles, as well as the character of their distribution in the titanium matrix, on the regularities of crack initiation and propagation in the model composite is demonstrated. It is shown that the inhomogeneous distribution of large carbide inclusions in the titanium matrix is a key factor for crack initiation and propagation in the model composites.

The progress of various industries, especially the transportation industry, depends on the materials and structures required by these industries. With the emergence of metamaterials, one of the challenges for researchers is to investigate the effect of these materials on various structures. This research investigates the vibration behavior of a sandwich plate with piezo-electro-magnetic face sheets and the effect of several core models (three types of porous and two types of auxetic cores). The equations of motion for the sandwich plate are determined. These equations are solved using a semi-analytical method, and the vibration of the sandwich plate is obtained. The effects of porosity distribution, electric and magnetic fields, different parameters of the auxetic core, and thickness ratio on the natural frequency are determined. The results show that the effect of the 3rd type of porous distribution on the structure frequency is more than that of other porous distributions. The effect of auxetic cores with negative Poisson’s ratios was also analyzed. The second type of the auxetic core has a more significant effect on the frequency. The findings of this study have practical applications in the aerospace sector and can be used in developing lightweight structures, sensors, and actuators.

The effect of residual stresses on surface and subsurface fatigue fracture under rolling and sliding friction is studied by analyzing internal stresses during rolling and sliding of an elastic cylinder against an elastic half-plane with residual stresses in the subsurface. The macroscopic approach is used to calculate the accumulated contact fatigue damage for different types of residual stress distribution in the subsurface layer and for various contact conditions (friction coefficient, relative slip, etc.). Compressive and tensile residual stresses, which are either constant across the subsurface layer of a given thickness or decrease linearly to zero in it, are considered. It is shown that compressive residual stresses cause a decrease in the equivalent stress amplitude in the subsurface of the half-plane under both sliding and rolling friction. They also decrease the rate of fatigue damage accumulation in the subsurface layers of the friction pair. The obtained results can be used to develop methods for controlling the contact fatigue fracture in the subsurface layers of the friction pair under rolling and sliding friction by inducing the appropriate residual stresses in the interacting materials.