AbstractPolycarbonate (PC) has diverse applications in different industries such as transportation, electronics, biomedical and solar energy sectors. Polycarbonate is used as a material for structural components that are usually complex in shape and subjected to severe mechanical loading. The presence of notches such as holes, grooves, or cuts reduces the load-carrying capacity of structural components because of the stress concentration. Therefore, it is essential to understand the mechanical behavior of polycarbonate in the presence of different notch geometries. Machining of inclined notches at different angles to the applied load is simple, but this can produce complex mixed-mode I/II states that exist in real-life applications. The present study is performed on PC specimens with U-notches of different geometry. They differed in depths, radii, and angles. These specimens were tested under quasi-static loading, and selected specimens were analyzed using digital image correlation. Two linear elastic methods were used to analyze the fracture of U-notched PC specimens: the theory of critical distance with the point method (TCD-PM) and the strain energy density with the equivalent material concept (SED-EMC). Satisfactory estimates with the error between –4% and 2.5% were achieved using the TCD-PM method. Estimates derived by the SED-EMC method were mostly within the error of about ±13%.

AbstractThe strain and temperature characteristics of the shape memory effect are studied in bending tests of a composite material with the nylon 66 matrix reinforced with nickel-titanium wire containing 55.7 wt % Ni. It is shown that deformation mechanisms in the matrix and the reinforcing filler influence the shape memory behavior of the composite. The В2 → В19′ martensitic transformation and dislocation slip processes occurring in nickel titanium and resulting in unrecoverable deformation determine thermomechanical properties of the composite. The plastic deformation and stress relaxation mechanisms in the polyamide matrix should also be taken into account, as they can either promote or prevent the shape memory effect in the reinforcing filler at different stages of the composite deformation. It is proposed that the main performance characteristics of shape memory composites are the critical strain εcr0.2, at which 0.2% of unrecoverable strain is accumulated, and the shape recovery start and finish temperatures determined after prestraining to εcr0.2 in a cooled state. The strain and temperature characteristics of the shape memory effect are compared between the composite material and the reinforcing filler. The viscoelastic behavior of the composite matrix is shown to decrease the critical strain from 9% in the reinforcing NiTi filler to 5% in the composite. The composite material exhibits a slight decrease (by approximately 5°C) in the shape recovery temperatures compared to the reinforcing NiTi filler.

AbstractThe present work is aimed at analyzing free longitudinal vibrations of nanorods composed of a functionally graded (FG) material with deformable boundaries within a hardening nonlocal elasticity approach. For this purpose, a FG nanorod composed of the ceramic and metal constituents is considered to be elastically supported by means of axial springs at both ends. Then the analytical method based on the association of the Fourier sine series and the Stokes transformation is developed to solve the free axial vibration problem of a FG nanorod with both deformable and nondeformable boundaries. Free axial vibration of a restrained FG nanorod is first studied within hardening nonlocal elasticity. To show the validity and profitability of the proposed analytical method, the presented Fourier series method with the Stokes transformation is used for the analysis of axial vibration of a rigidly supported homogeneous nanorod by setting the appropriate spring stiffness values. The main superiority of this new approach is in its power of dealing with numerous boundary conditions to determine longitudinal vibration frequencies of FG nanorods. Using the present solution method, various numerical applications are given for different small-scale parameters, gradient index, and nanorod length.

AbstractThis work investigates the effect of a longitudinal magnetic field on the mechanical buckling of a single-walled carbon nanotube (SWCNT) integrated in an elastic Kerr medium. The structure is assumed to be homogeneous and therefore modeled using a nonlocal Euler–Bernoulli theory (NL-EBT). The present model targets thin structures and takes into account the small-scale effect. The elastic matrix is described by the Kerr model, which takes into account the normal pressure and the transverse shear strain. Using the nonlocal elasticity theory and considering the Lorentz magnetic force obtained from Maxwell relations, the stability equation for buckling analysis of a simply supported SWCNT under a longitudinal magnetic field is derived. The governing equations of the system are determined via the virtual work model and resolved by Navier’s method. The obtained results are compared with those found in the literature. It can be observed that the effects of the magnetic field, nonlocal parameter, lower spring parameter Kw, upper spring parameter Kc, and intermediate shear layer parameter KG are significant and must be taken into account for this kind of analysis.

AbstractIn this work, the propagation of planar waves in a homogeneous micropolar thermoelastic medium is studied while the entire body rotates with a uniform angular speed. The coordinate system of the rotating medium is assumed to be stationary, and therefore the kinematic equations have two additional terms, namely, the gravitational and the Coriolis accelerations. The problem is addressed based on the two-temperature thermoelastic model with higher-order time derivatives and dual-phase lag, which can explain the effect of microscopic features in nonsimple materials. With certain boundary conditions and the normal mode approach, the variations in temperature, displacement, microrotation, and thermal stresses induced by heating are derived. In the absence of rotation and two-temperature factor, comparison is made with the results of classical thermoelastic models.

AbstractThis research is devoted to the study of the flexural response and buckling analysis (thermal and mechanical) of functionally graded (FG) nanoscale plates integrated in an elastic medium. The structure is modeled on the basis of a refined integral plate theory with four unknowns incorporated into Eringen’s nonlocal elasticity theory. The material properties of the plate are considered to be graded continuously over the entire thickness of the nanoplate. The elastic medium is simulated like Pasternak’s two-parameter elastic foundations. The equilibrium equations are determined from the principle of virtual displacements. The results for simply supported FG nanoscale plates are deduced and compared with those available in the literature. Parametric studies are carried out to demonstrate the impacts of the inhomogeneity parameter, nonlocal parameter, elastic medium stiffness, and plate geometric ratios on the behavior of FG nanoscale plates.

AbstractSingle-walled carbon nanotubes (SWCNTs) in an elastic medium under a longitudinal magnetic field have piqued the interest of researchers as elements utilized in nanoelectro-magneto-mechanical systems (NEMMS). This work presents the vibration analysis of embedded SWCNTs using the nonlocal second-order strain gradient elasticity theory. Considering the surface effect, the characteristic equation of motion for a SWCNT embedded in a polymer matrix under a longitudinal magnetic field is formulated and derived. The dependence of the distinct natural frequency of SWCNTs on the nanotube chiral angle and diameter is clarified. The effects of various parameters on the vibration characteristics of SWCNTs are examined and discussed, including the longitudinal magnetic field, surface effect, chiral index, chiral angle, chirality of SWCNTs, vibrational mode number, aspect ratio (length-to-diameter ratio), nonlocal and material length scale parameters. The numerical findings of this work might be helpful in the study and implementation of embedded SWCNTs as NEMMS devices.

AbstractElectron beam additive manufacturing with simultaneous feeding of two dissimilar metal wires was used to obtain Ti-6Al-4V specimens successively alloyed with 0.6, 1.6, 6.0 and 9.7 wt % Cu. The specimens were characterized for microstructure, phases, and mechanical properties. Increasing the copper content in the alloy from 0.6 to 9.7 wt % resulted in the refinement of primary β-Ti grains and the columnar-to-equiaxed grain transformation owing to the effect of constitutional undercooling on grain nucleation and growth. The grain growth restriction factor was calculated to substantiate the microstructural evolution from columnar to equiaxed grains. Admixing with up to 6.0 wt % Cu resulted in the formation of ultrathin α-Ti platelets, while increasing the copper content to 9.7 wt % Cu led not only to further thinning of α-Ti platelets but also to the formation of refined α′-Ti and α″-Ti phases. Intermetallic Ti2Cu particles were precipitated due to the β → Ti2Cu + α eutectoid decomposition of primary β-Ti grains and then plausibly induced heterogeneous nucleation of α-Ti platelets. A combined effect of solid solution hardening, precipitation hardening, and grain boundary hardening was achieved and allowed increasing the microhardness, ultimate tensile stress, tensile yield stress, and compression yield stress of Ti-6Al-4V/Сu specimens.

AbstractThe paper proposes a microstructural model of tetragonal single-crystalline barium titanate for analyzing how the state of its domain structure can influence the simulation accuracy of dielectric hysteresis curves with regard for domain interactions and for stress and electric field inhomogeneities in the single crystal. Hysteresis curves based on finite element homogenization are presented for all eight types of second-rank laminate domain patterns satisfying the compatibility conditions. It is shown that the properties of domain structures are substantially anisotropic under loading in different directions and that the dielectric hysteresis for different domain patterns differs greatly. The proposed model allows one to describe the effects of domain hardening and unloading nonlinearity. The results of calculations using the model agree well with experimental data at different cyclic load amplitudes.

AbstractUnderstanding the mechanical response of polymer components fabricated by fused deposition modeling (FDM) is an important issue. Therefore, the present study deals with the effects of raster angle and layer orientation on the tensile properties and fracture toughness of acrylonitrile butadiene styrene (ABS) specimens produced by the FDM method. Two groups of specimens are considered. The first group includes specimens with the same layer orientation and the four different raster angles 0°/90°, 15°/–75°, 30°/–60°, and 45°/–45°. Specimens in the second group have the fixed raster angle 45°/–45° and three different layer orientations. Tensile tests are performed using dumbbell specimens, and semicircular bending (SCB) specimens were used for fracture mechanics tests. The critical value of J-integral obtained from finite element simulations is used as a parameter to characterize fracture properties. In the first group of specimens, the critical value of J-integral for the 45°/–45° specimen is 4389 J/m2 while it is about 1880 J/m2 for the 0°/90° specimen. In the second group, the vertically printed specimens have the least fracture resistance 1004 J/m2, while this value reaches 5934 J/m2 for the specimens in which the precrack is perpendicular to the printed layers. In addition, the fracture surface of tensile specimens is analyzed using scanning electron microscopy for the mesomechanical study of failure in the printed specimens. Lastly, the crack path in SCB specimens is explored experimentally to understand how the raster angle and layer orientation affect the fracture trajectory and to justify different values of fracture loads.

AbstractThe paper presents a brief analysis of the accumulated data on fast dynamics of earthquake ruptures and their qualitative comparison with the numerical results on supershear rupture propagation along homogeneous and heterogeneous fault surfaces. Calculation methods and parameters are described. The numerical results indicate that the rupture velocity in strong earthquakes can vary in a wide range, exceeding significantly the Rayleigh wave velocity treated as the maximum possible crack velocity in conventional fracture mechanics. It is shown that a necessary condition for the transition to supershear rupture propagation along the heterogeneous contact surface is the presence of a sufficient number of asperity contact spots that experience rapid frictional weakening during shear. The rupture velocity can periodically decrease or increase in the case of a heterogeneous fracture surface. Systematic variation in fault properties along strike increases the probability of supershear rupture in old contact segments.

AbstractWave propagation through the interface of elastic bodies was studied under perfect contact and slip boundary conditions occurring during friction and surface wave propagation along interfaces in multiphase and heterogeneous materials. The dependences of the interfacial Fresnel coefficients and strain amplitudes on the wave incidence angle were calculated for perfect contact and slip conditions. The results obtained can be used to analyze the effect of boundary conditions, properties of contacting bodies, and loading conditions on wave processes and strains at the interface.

AbstractThe formation of ultrafine-grained structures is a well-known method of decreasing the temperature and/or increasing the strain rate to provide superplasticity in titanium alloys. Under certain conditions, dissolved hydrogen can produce a plasticizing effect on titanium alloys, showing up as a decrease in their stress and/or an increase in their ultimate strain. Here we study the effect of 0.3 wt % of dissolved hydrogen on the structure, phase state, and superplastic properties of an ultrafine-grained (α + β) Ti-Al-V-Mo system (VT16 alloy) at a temperature of 823–923 K. The ultrafine-grained structure of Ti-Al-V-Mo (VT16 alloy) and Ti-Al-V-Mo-0.3 wt % H (VT16-H alloy) results from severe plastic deformation via uniaxial compression with a change in the strain axis and in the temperature from 1023 to 823 K. In the temperature range used, the presence of hydrogen in the solid solution of VT16 alloy decreases its superplastic properties. During deformation, hydrogen is redistributed in the bulk of the material by elastic stress fields and is accumulated in the most stressed regions, leading to plastic strain localization and to a decrease in the strain to fracture. The release of hydrogen from VT16-H alloy during deformation activates its β → α transformation and associated diffusion redistribution of its alloying elements, contributing to the accommodation of grain boundary sliding and to the increase in the strain to fracture.

AbstractFatigue failure is the most common type of failure in aircrafts, motor vehicles, medical devices, and other engineering systems, and its study is crucial for predicting the service life of such systems. Of particular interest is to investigate the fatigue failure of new bulk ultrafine-grained nanostructured metal materials produced by severe plastic deformation. The major fatigue failure parameters such as the maximum cycle stress σmax, which characterizes the applied failure load, and the cycle asymmetry coefficient R = σmin/σmax, which characterizes the loading conditions, can be determined with resort to fracture mechanics, in particular, to the relation between the sizes of plastic zones at the tip of a growing crack and the fatigue fracture parameters. Here we demonstrate the possibility of determining σmax and R from the depth of plastic zones beneath the surface of fatigue fractures on the example of coarse- and ultrafine-grained materials with different lattice types: carbon steels (steels 20 and 45), austenitic steel (07Cr13Ni4NMn20), and aluminum (D16) and magnesium (Mg6Al) alloys exposed to three-point bending. The depth of plastic zones beneath the surface of fatigue fractures was measured by the X-ray method. The maximum cycle stress σmax was estimated from the relation between the depth of a monotonic plastic zone hy at a given crack length l and the factor Kmax.

AbstractAdditive manufacturing of metal materials is one of the most promising technologies in modern industry. A wide variety of current additive manufacturing techniques allow rapid prototyping and industrial production of different-sized products from various structural and functional materials. The structure and physical-mechanical properties of the metal products fabricated by electron-beam additive manufacturing (EBAM) within nonstationary metallurgy in a local molten pool often differ from those of the products fabricated by conventional metallurgy due to different crystallization mechanisms, sequence and completeness of phase transformations, and heterogeneous/homogeneous chemical composition of the resulting material. The possibility to control local metallurgical processes in the molten pool is the key advantage of the EBAM technology. It allows one to control the structure, composition, and properties of mono- and polymetallic, graded, composite and heat-resistant materials in order to obtain products with the desired chemical composition, macroscopic architecture, and microscopic structural parameters. As any new industrial technology, the EBAM method requires the development of scientifically based approaches to the choice of materials and production conditions. Here we provide an overview of the scientific approaches developed for electron-beam additive manufacturing of products from metals and alloys using wire or rods as a feedstock. The range of the studied materials includes additive materials based on copper, bronze, aluminum, nickel, titanium alloys, and different steels, as well as aluminum-based functionally graded materials and copper-based graded materials. The most important research findings are summarized.

AbstractA three-level constitutive model is proposed for describing the deformation of polycrystalline materials, which is based on crystal elasto-viscoplasticity and the introduction of internal variables. The structure, mathematical formulation, and implementation algorithm of the model are discussed. The element of the upper structural-scale level is the representative macrovolume. The elements of mesolevels 1 and 2, which are identical in scale, are crystallites (grains, subgrains, fragments, depending on the required element size). The description at mesolevel 1 is performed in terms of thermomechanical variables (stresses, strains, strain rates). The behavior of meso-2 elements is described in terms of dislocation densities and velocities. Particular attention is paid to the formation of barriers on split dislocations. As an example, the model is applied to study proportional and nonproportional cyclic loading of samples with substantially different stacking fault energies. It is shown that barriers are more readily formed in materials with low stacking fault energy, leading to their additional cyclic hardening under nonproportional loading.

AbstractDue to its high deterioration rate in the physiological environment, the clinical application of magnesium (Mg) in bone repair has been restricted. Graphene oxide (GO)-plasma electrolytic oxidation (PEO) coatings were effectively applied to Mg alloy to enhance corrosion resistance and biocompatibility. The structure, biocompatibility, electrochemical characteristics, and long-term corrosion performance of composite coatings were studied in the present paper. The incorporation of GO to the PEO layer decreased wettability of all samples, resulting in hydrophobic behavior. The amount of GO incorporated in the PEO layer had a minor effect on the film thickness, but the pore size of the PEO coating decreased as the amount of GO increased. PEO/GO coatings have better corrosion resistance than counterpart PEO coatings and bare samples, according to electrochemical tests. The results also demonstrated that corrosion resistance increases significantly as GO concentration increases, owing to the fact that GO nanosheets in the coating operate as a barrier to the electrolyte diffusion route, preventing aggressive electrolytes from accessing the substrate. Because of dramatically decreased Mg ion release and changes in pH value in the culture medium, all of the PEO and PEO/GO coatings could improve MG63 cell attachment and differentiation compared to the bare Mg alloy sample. The as-prepared PEO/GO coating on Mg alloy is attractive for medical applications due to its satisfactory corrosion resistance and biocompatibility.

AbstractThis paper considers dynamic rock fracture from the viewpoint of the force and energy limiting criteria formulated from the concepts of the structural-temporal approach. For each limiting criterion set at a given fracture time, the incubation time is calculated as a key constant material characteristic within the proposed approach which depends on the scale and is the main measure of the material response. Experimental literature data on three-point bending are used to discuss the strain rate dependences of rock fracture toughness with varying specimen notch length and fracture work. Using the dynamic fracture of coal and granite as an example, it is shown that the incubation time determined by the force criterion is independent of the specimen notch length. Comparison is made of the marble incubation times obtained from the strain rate dependences of fracture toughness and fracture work.

AbstractStrength measurement results are reported for hot-rolled cast cold-resistant structural alloy steel 09CrNi2MoCu subjected to shock compression up to 15.5 GPa within the strain rate range of 105–106 s–1. Specimens fabricated by direct laser deposition were used to study the effect of the deposition direction and shock compression amplitude on the Hugoniot elastic limit and critical stresses during spall fracture. The strength characteristics were determined by analyzing the full waveform data recorded during loading by a VISAR laser Doppler velocity interferometer. It was found that the spall strength of the cast steel specimens is almost independent of the shock compression pressure, but strongly depends on the strain rate before spalling. The spall strength of the additively manufactured specimens is slightly lower than that of the hot-rolled cast steel specimens and does not depend on the deposition direction. The α ↔ ε phase transformation expected at 13 GPa was not observed in experiments on cast steel 09CrNi2MoCu with the maximum shock compression pressure.

AbstractThe description of cyclic plasticity requires experimental determination of the constants entered into respective resulting equations. In this paper, a method is proposed for determining the parameters and constants of a Lemaitre-type damage accumulation model on the example of P2M rotary steel. The model is based on the Voce isotropic and the Armstrong–Frederick kinematic hardening law. The method of experimental determination involves standard uniaxial tension tests as applied to the parameters of isotropic hardening, and low-cycle fatigue tests, to the constants of damage accumulation and parameters of kinematic hardening. The method is applicable to any alloy that fits the model representations. Using the constants and parameters found, the behavior of cylindrical P2M steel specimens under cyclic loading is modeled by finite element simulation and their fatigue curve is plotted. The predicted fatigue life of P2M steel correlates well with experimental data.