In this paper, we discuss strong ellipticity conditions within nonlinear elasticity. We consider a Cauchy-type simple material model and strain gradient elasticity. The constitutive equations of a simple material are formulated using the strain energy density, which is given as a function of the deformation gradient. In strain gradient elasticity, the strain energy density depends on the first and second gradients of strain. For strain gradient elasticity of third order, it depends on the first, second and third deformation gradients. We formulate strong ellipticity conditions and analyze their relation to infinitesimal stability. These conditions are defined using the strain energy density and its convexity with respect to a particular class of deformations. In gradient elasticity, the strong ellipticity conditions, as defined in the theory of partial differential equations, constrain the form of the constitutive equations and the deformations themselves, depending on the deformation gradient of the highest order. Infinitesimal stability is defined as the positive definiteness of the second variation of potential energy with respect to admissible displacements. We consider the relationship between ellipticity and infinitesimal stability for the first boundary-value problem, which is a boundary-value problem with Dirichlet boundary conditions. We demonstrate an essential difference between the considered models. For example, in a simple material, strong ellipticity implies stability of affine deformations within the first boundary-value problem. However, within strain-gradient elasticity, this statement is generally incorrect. A series of inequalities serves as sufficient conditions. As a particular case of gradient elasticity, we discuss gradient poroelasticity. In this theory, the strong ellipticity conditions are not met; however, there is still an ellipticity property in the sense of Douglis – Nirenberg.
A linear problem of the initial (subcritical) planar stress-strain state under axial compression of a test sample with double-sided fixation sections of finite length is posed and solved. The test sample is made of unidirectional fiber composite material. It is assumed that the axial compression of the test sample is carried out by kinematic loading of the fixed end sections due to the friction forces arising between the rod and the rigid elements of the device and ensuring the implementation of one of the known loading schemes in accordance with existing test standards. In the area of double-sided attachment of the sample, the loading method in question also ensures its compression in the transverse direction. The equations constructed for fixed sections are based on cubic thickness approximation of axial displacements and linear approximation of deflection. Approximations are transformed into another model by subjecting them at the points of the boundary surfaces to the conditions of kinematic coupling with rigid elements of the test device with specified displacements. In the loose section, a cubic approximation is used for axial displacements. A linear approximation is accepted for deflection. Approximations were made based on the thickness of the sample, in addition to which a refined model by S.P. Timoshenko was also used, taking into account transverse compression. For all accepted deformation models, kinematic conditions for the coupling of fixed and non-fixed sections are formulated, equations of their balance are constructed, as well as force conditions for coupling. Taking into account the formulated kinematic conditions of coupling of the marked sections of the sample and the corresponding linear elasticity ratios, analytical solutions of the constructed displacement equations were constructed, using which numerical experiments were conducted to determine the subcritical stress state during kinematic compression of a sample made of unidirectional fibrous composite based on carbon fiber of the ELUR-P brand and of the ХТ-118. binder. A comparison is given of the results obtained in the analytical solution of the problem with the finite element solution when modeling a sample with a set of isoparametric rectangular elements based on the equations of the plane problem of the theory of elasticity
An experimental substantiation of the possibility of determining the parameters of a spherical cavity or inclusion based on the data of uniaxial compression (or tension) tests of samples is presented. For experimental identification of spherical cavity parameters based on mechanical compression (tension) tests using additive 3D printing technologies, a series of cubic shaped samples with spherical cavities made of optically active materials (Anycubic ABS-Like PRO 2 Clear) was produced. In a study using models of transparent disks and cubic samples with spherical cavities printed on a 3D printer, the characteristics of double refraction during compression of photopolymers and compression tests in the frozen state at various temperatures and axial loads were investigated. Interference patterns (maps of isochromatic lines using the digital photoelasticity method) on the sample surface and the distribution of displacement fields (using the digital image correlation method) were obtained. Based on experimentally found stress and displacement fields on the sample surface, the coordinates of the center of a spherical cavity and its radius are determined by calculating the values of the reciprocity functional and invariant interaction integrals found from experimental fields. To implement the method of “freezing” stresses and fixing stressed areas on the printed model, a polaroscopic system combined with a high-temperature loading chamber has been developed. After testing, the spatial sample is cut into plates, which give interference patterns of the difference in the main stresses in the inner sections of the sample: patterns of isochromatic bands in flat plates obtained by cutting a cubic sample with a spherical cavity are found, which makes it possible to estimate the parameters of the stress-strain state inside the sample with a stress concentrator. A comparison of experimental fields with finite element solutions was carried out, and the comparison showed good consistency of the results. A comprehensive technique has been developed for the experimental determination of mechanical fields in loaded bodies, followed by the identification of internal defects based on interference-optical measurements, invariant interaction integrals, and numerical estimates for the identification of hidden inclusions.
Currently, constitutive models of metals and their alloys, created within the framework of a multi-level approach based on crystal plasticity, are widely used. One of the key equations of models of this class is the Hutchinson equation for determining shear rates on slip systems. In this paper, the power parameter influence on the results obtained using a face-centered cubicmetal two-level statistical model was investigated. It was assumed that the scalar factor (prefactor) in the Hutchinson equation is proportional to the intensity of the strain rate. The popular formulation of the hardening law was used, based on the concept of a gradual increase in the density of defects (mainly forest dislocations) that impede the movement of dislocations, until a state is reached in which the processes of annihilation and reproduction of dislocations balance each other. It is shown that with numerical stability of calculations, the power parameter has virtually no effect on the response, so it can be set arbitrarily, only requiring that the calculations stability be ensured. A study of the conditions for ensuring the numerical implementation stability of the model was conducted, and the existence of a certain critical value of the exponent was established, at which the stability of the numerical implementation is violated when using the explicit Euler integration scheme. It was found that this critical value depends on the integration step and a number of model parameters (critical stress, shear moduli). Assuming that only one slip system is active and there are not rotations, a theoretical estimate of the exponent in Hutchinson's law critical value is obtained for the Euler scheme, which is in satisfactory agreement with the computational experiments results.
The actual topic of nondestructive evaluation of strength properties of cast aluminum matrix composites using ultrasonic inspection data is considered. The results of ultrasonic investigations of promising cast isotropic composite material based on aluminum alloy АA1060, reinforced with hollow aluminosilicate microspheres, are presented. Using the echo pulse method, the velocities of longitudinal and shear ultrasonic waves were determined in the specimens containing fractions of microspheres with sizes of 40–80 µm and 100–200 µm, with saturation degree of 5, 10 and 15%. Based on measurements of the propagation times of ultrasonic waves, the values of the Poisson's ratio are calculated. It was found that both the propagation velocities of longitudinal and shear waves and the Poisson's ratio decrease in direct proportion to the degree of saturation of the alloy with hollow aluminosilicate microspheres, while the slope angles of straightlines vary significantly depending on the size of the microspheres. For the specimens of composite containing microspheres 40–80 µm, the corresponding slope angles of the linear dependences are smaller than for the specimens containing microspheres 100–200 µm. It was also found that, with the same degree of saturation, the ultrasonic wave velocities and the Poisson's ratio in the specimens containing smaller microspheres are greater than in the specimens containing larger microspheres. A negative linear correlation of the Poisson's ratio and the tensile strength has been established for the studied sampling. It is shown that the Poisson's ratio, determined using ultrasonic waves, can be used as an informative parameter for nondestructive testing of an aluminum-matrix composite reinforced with hollow aluminosilicate microspheres. Empirical formulas are proposed for evaluation the ultimate strength of composite and the degree of saturation with hollow microspheres from the ultrasonic measurements.
This paper presents a method for determining the elastic moduli of thermoplastics using plate-shaped specimens fabricated by fused deposition modeling (FDM 3D printing). The proposed approach is based on the analysis of guided elastic wave propagation in the sample and the statistical processing of the results of the corresponding inverse coefficient problem solution. Laser Doppler vibrometry was used to register the velocities of vertical vibrations on the surface of the rectangular plate, which were excited by a piezoelectric transducer glued at the surface via cyanoacrylate. The matrix pencil method was applied to the recorded wave signals to determine dispersion characteristics of Lamb waves. The inverse coefficient problem for elastic moduli was solved numerically using the differential evolution method. The objective function was constructed utilizing the property that the Fourier symbol of the Green's matrix for a homogeneous elastic layer tends to infinity at the points corresponding to the measured wavenumbers. To increase the efficiency of the method, the procedure is performed for several lines starting from the center of the source of vibration. Statistical analysis of the elastic moduli recovered using different scan lines was conducted, providing estimates of the elastic properties of the material. A comparison with the results of standardized uniaxial tensile mechanical tests was performed. Therefore, the effectiveness of the developed method for determining and monitoring the elastic characteristics of thin-walled polymer structures manufactured using additive technologies is demonstrated.
A brief description of three computational models used is given. They are aimed at solving problems of large plastic deformations of thin-walled metal structures under internal pressure. The set of models includes a membrane rigid-plastic finite element model and two solid elastic-plastic finite element models, which are intended to solve problems of large plastic deformations in axisymmetric and three-dimensional formulations. The reliability of the resulting numerical solution to the considered problem of limit plastic forming is established by the fact of consistency of the calculation results based on two (alternative) of the three models. The law of strengthening of the highly plastic material of the structure taken into consideration is determined within the framework of the calculation and experimental approach based on the use of standard mechanical characteristics of this material. The results of the calculation and experimental studies on the limiting plastic deformation of the structure containing an element in the form of a toroidal (with two cavities) shell subjected to the action of a large value of internal pressure are presented. Good agreement with the experiment of the obtained calculation forecasts for the limiting values of the applied pressure is noted. This confirms the possibility of assessing the safety factor of such structures within the framework of the approach proposed by V.I. Feodosyev, based on the calculated forecast for the limiting value of the applied load obtained in the described manner.
The patterns of plastic deformation in metals and alloys subjected to complex plastic deformation under both monotonic and cyclic loading are investigated. The theoretical basis of the research is a model developed within the framework of the differential theory of plasticity by professor Yu.G. Korotkikh. A key feature of the model is the concept of a loading (yield) surface and the postulate of the gradientality of the plastic strain rate vector to this surface at the loading point. This form of writing constitutive equations allows for a correct description of fundamental plasticity effects when arbitrary (complex) loading paths are realized. By setting the corresponding material parameters to zero, all main forms of equations for plastic deformation of materials at small strains presented in the literature are obtained as a special case from the general variant of the equations (a system of “nested” models). Problems of mathematical modeling of processes characterized by a change in the orientation of the principal axes of stress tensors, as well as total and plastic strains (so-called non-proportional loading regimes), are investigated. To verify the proposed model, a series of numerical experiments simulating complex plastic deformation of structural steels (using grades 304, 316, and steel 45 as examples) was performed. The analysis was conducted for planar trajectories corresponding to non-proportional deformation regimes under monotonic and cyclic loading. The research results demonstrate that Yu.G. Korotkikh's plasticity model correctly predicts the main effects of complex plastic deformation in structural alloys. The model provides a qualitative description of material behavior for planar loading trajectories of any curvature, and its accuracy is sufficient for practical calculations. A number of characteristic features accompanying the process of non-proportional plastic deformation of alloys have been noted: the presence of a “dip” on the material's deformation diagram when transitioning from a curvilinear deformation section to a rectilinear one; the wavy nature of the material's vector properties on curvilinear trajectories of constant curvature under cyclic loading; the appearance of additional cyclic hardening, and others.
This paper examines the behavior of equilibria in a compressed, elastic, shallow, circular cylindrical shell with a rectangular planform. The shell is subject to internal stresses caused by fields of continuously distributed edge dislocations and wedge disclinations. The compressive load is uniformly distributed over the curved edges of the shell and acts parallel to the cylinder's generatrix. Boundary conditions for free clamping or pinned support of the shell edges are considered. Nonlinear equilibrium equations of the Karman type are derived. A scalar function, called the incompatibility function, appears on the right-hand side of the deformation compatibility equation and depends on the density of dislocations and disclinations. It is established that in the presence of stress sources, the solution to the system of equations is a vector function whose components are the deflection and the stress function. The problem of weak bending of a compressed shallow shell with dislocations and disclinations is also considered and solved using a difference method. In the absence of dislocation and disclination fields, the nonlinear problem has a trivial solution, and after linearization, an eigenvalue problem arises, which determines the critical buckling loads of a compressed shell. To solve the eigenvalue problem, a variational method is used in combination with a difference method. The results of numerical calculations for the first few critical values of the compressive load are presented, and graphs of the components of the corresponding eigenvector functions are plotted. For the linear equilibrium problem of a compressed shell with dislocations and disclinations, the results of numerical calculations and graphs of the components of the vector functions for given values of the compressive load and the incompatibility function are also presented.
The article discusses the problem of topological optimization of mechanical properties of materials based on a numerical-digital approach that involves the integration of numerical modeling and digital prototyping methods. The focus is on the standard implementation of the SIMP (Solid Isotropic Material with Penalization) method, as well as its improved version. The improvement consists in the use of interpolation functions to approximate the density of the material, which allows moving from a piecewise-constant to a continuous distribution, providing a smoothed representation of the structure's topology. As a numerical experiment, the problem of three-point bending of a beam with a rectangular cross-section is investigated. Calculations are performed for three different computational grids, which makes it possible to evaluate the influence of the degree of discretization on the results and compare the effectiveness of the standard and improved approaches. At each iteration, the density distribution is recorded, forming a voxel model of a digital prototype that can be used in additive manufacturing. The efficiency of the algorithm is evaluated based on the internal deformation energy functional. It is shown that the use of interpolation functions reduces the value of the functional by 2-4% and provides more stable convergence compared to the classical SIMP. The results confirm the practical significance of the method, which allows the formation of optimized structures with high rigidity while maintaining the specified volume of material. Additionally, it is noted that the modified approach reduces computational costs and speeds up the optimization process, which is especially important for high-dimensional problems. The proposed method can be effectively applied in mechanical engineering, medical engineering, and additive technologies.
The nature and form of the relationship between the density of mobile dislocations and plastic deformation during the development of plastic flow at different stages of the stress-strain curve of metals are considered. The possibility of constructing such a relationship function based on autowave concepts of the dependences of the autowave length of localized plasticity and the velocity of its propagation on deformation, corresponding to the laws of multi-stage plastic flow in solids, is demonstrated. The autowave mechanism is based on the concepts of the leading role of localization of plastic deformation and generation of self-excited autowave processes during plastic flow, associated with the self-organization of a plastically deformed active medium. The principles of mutual complementarity of the dislocation and autowave approaches to the problem of plasticity of solids and their coordination based on the study of macroscopic autowave patterns of localization of plastic flow are formulated. Both the autowave pattern in the form of mobile and stationary foci of localized deformation on the yield plateau, stages of strain hardening, and dislocation ensembles evolve regularly during plastic flow. A comparison of autowave macroscale quantitative data on the localization of plastic flow, recorded using in situ speckle photography, with microscopic characteristics of dislocations arising in the material under the same deformation conditions, recorded by the acoustic method, is carried out. The difficulty of solving the problem is that the compared micro- and macrostructures have fundamentally different spatial scales, differing by 6-7 orders of magnitude.
The first part of the study, published in the previous issue of the journal, was devoted to estimating fatigue limits using a fatigue curve in tests conducted at several levels of alternating stress amplitudes over a wide range of life spans, as well as tests conducted to estimate the parameters of the fatigue failure similarity equation for subsequent combined statistical modeling with elements of bootstrap estimation and the Monte Carlo method. This article (part two) continues this research. For the same purposes, an up-down fatigue limit assessment is considered. The up-down method is not intended to construct a complete fatigue curve over a wide range of life spans, but it allows for a forced determination of the fatigue limit distribution corresponding to a certain fixed (base) life span. The mathematical model of the “up-down” testing method has a rigorous probabilistic-statistical justification. It is based on the maximum likelihood method and, unlike other accelerated fatigue testing methods such as Pro, Enomoto, and Locatti, does not require a priori information about the characteristics of random variables. It is only necessary to approximately specify the initial stress amplitude level of the cycle and the range between levels during the experiment, which is maintained constant throughout the test. The hypothetical distribution functions considered are the normal law, the log-normal law, and the Weibull law for the fatigue limit. A significant time savings compared to a full-scale fatigue experiment and high accuracy allow this method to be recommended for fatigue limit assessment. Generating a nonparametric fatigue limit distribution function for comparative evaluation of different groups of observations using the binomial criterion is performed using Monte Carlo statistical modeling, as in the first part of the paper.
The problem of reconstructing variable characteristics of functionally gradient bodies under the action of external static loads with restrictions on the region of additional information retrieval is considered. An approach is proposed that allows reducing the inverse problem with data specified on the boundary to the inverse problem in the first statement: to the problem in which it is necessary to find unknown variable coefficients from information on the field specified in the entire region. An analogue of the Galerkin method for constructing an approximation of the field is proposed. An example of an inverse problem for reconstructing the flexural rigidity of a beam cantilevered at one end and bent by various loads, including distributed ones, as well as a force and/or moment applied to the other end is considered. Additional information on the deflection is specified on the part of the beam free of load. Several variants of approximating the deflection function with subsequent construction of the flexural rigidity using regularization according to A.N. Tikhonov are implemented. The developed scheme made it possible to reconstruct the flexural rigidity with high accuracy in the region available for retrieving additional information. In the remaining area, the second derivative of the deflection is completed by continuity in a quadratic manner and the flexural rigidity is found with some error. A series of computational experiments was conducted where the area of additional data gathered 50 and 65 percent of the total. As a second example, an approach is proposed that allows for the reconstruction of compliance (a function inversely proportional to rigidity) in the class of polynomial functions. The problem is reduced to solving an algebraic system with respect to unknown coefficients of the polynomial. The determinant of the system is calculated, the conditionality of the matrix is analyzed depending on the points of information removal about the deflection, and recommendations are given for their selection. A series of computational experiments is presented.
The issue of the origin and formation of dislocated cellular disoriented structures is investigated on the basis of the proposed evolutionary model. The model is a system of material balance equations for two types of dislocations. Accordingly, mobile dislocations and dislocations in a bound state (sedentary). It is assumed that dislocations move under the action of an applied external voltage in opposite directions at a constant speed. We neglect the long-range stress of dislocations, but we take into account their local interaction, namely: reproduction, immobilization, recombination and runoff of dislocations. The model as a whole represents a system of four partial differential equations with complex generation-recombination dynamics. The initial system is transformed to a system for total and excess dislocation density, and possible homogeneous stationary states are found for the new variables. There are two of them: the first is only mobile, the second is mobile and sedentary dislocations for their total density. The excess density in both states is zero. A control parameter of the system is introduced that characterizes the volume fraction of the substructure on which dislocation runoff occurs, decreasing with increasing deformation. Next, a linear and nonlinear analysis of the system is performed. It is established that as the control parameter decreases, the solutions of the system reflected in the bifurcation diagram form three regions. In the first region, the homogeneous state is stable only for mobile dislocations. In the second domain, a uniform solution is stable for both types of dislocations. In the third area, a spatially inhomogeneous periodic solution for total and excess dislocation density becomes stable, as shown by nonlinear analysis, which characterizes the properties of the cellular structure.
Two modifications of the Kolsky method have been implemented for studying the tensile strength properties of brittle materials (rocks) by splitting cylindrical specimens (Brazilian test) as well as cubic specimens (splitting using wedge-shaped attachments on the ends of measuring rods adjacent to the specimen). A series of dynamic splitting tests were conducted on cubic gabbro-diabase igneous rock specimens with an edge size of ~20 mm and cylindrical (Brazilian test) specimens with a diameter of ~22 mm and a thickness of ~11 mm. The splitting strength of rock specimens of both configurations was determined and its dependence on the stress growth rate was plotted. This dependence is consistent with the known tendency for the strength properties of rocks to increase with increasing stress growth rate. The strength properties of granite obtained by splitting cylindrical specimens are, on average, 25–30% higher than those obtained by splitting cubic specimens. The dependence of the material incubation time on the stress growth rate under loading of cylindrical constructed. The constructed dependences of the splitting strength and the time to failure (incubation time of “life”) on the rate of stress growth are supposed to be used in further work to equip some destruction criteria, for example, the Morozov – Petrov structural-time destruction criterion. High-speed video recording of the cracking process of specimens of both geometries allows us to determine the moment of crack initiation and development in the specimen. The video recordings show that the first crack for both types appeared approximately 11 μs after the first frame of video data. The question of precisely synchronizing the electrical process of recording deformation pulses in the pressure bars with the optical process of high-speed video recording of specimen deformation remains open and requires further investigation.
The paper gives a brief overview of material models used in computational practice for predicting the behavior of geological media under conditions of high and low pressure, complex stress conditions under quasi-static and dynamicloading, including the high-speed one. The analysis of the application areas of geomaterial models implemented in the dynamic strength software module of the “Logos” software package is performed. The models are conventionally divided into two categories. A feature of the models of the first category (Grigoryan's and Zamyshlyaev –Evteev's ground environment) is the application of pressure dependence on total volumetric deformation, like the deformation theory of plasticity. The models in this category are easy to implement. The second category of geomaterials (Geologic Cap model, CSCM concrete, FHWA and Mohr – Coulomb models) uses the separation of elastic and plastic deformations. To calculate the increments of plastic deformations, the surface of the plastic potential is used, associated or not associated (Mohr – Coulomb model) with the loading surface. It is also possible to account for plastic volume expansion (dilatation) caused by shear stress at low all-round pressure. The shape of the shear loading surface generally includes a separation surface, a fracture shear surface and/or a lid surface and aoptional dependence on the type of stress state. The dependence of pressure on volumetric deformation in the models of geomaterials of the first group is set directly by the user, the second group is determined by the law of plastic compressibility for the volumetric component of plastic deformation. The models of the second group are complemented by mechanisms of kinematic hardening (the concept of active and reverse stress), viscoplastic behavior (the concept of trial and inviscid stresses), and damage accumulation (the concept of effective and reduced stresses). Examples of test calculations on a single finite element in the dynamic strength software module of the “Logos” software package are given, demonstrating the operability of the algorithms for: plasticity and taking into account the type of stress state on the CSCM geomaterial model under tension, compression and shear; viscoplasticity, damage on the CSCM geomaterial model under compression; cap, kinematic hardening on the Geologic Cap geomaterial model under compression; isotropic hardening – based on the FHWA geomaterial model under compression.
This study presents a comprehensive investigation of the influence of operational defects on the mechanical properties of polymer laminated composite materials. A methodology was developed for introducing defects simulating external operational loads with varying degrees of damage severity. During experimental studies, acoustic emission signals were recorded both during defect formation and subsequent quasi-static tensile tests. The obtained data were used to analyze the correlation between acoustic emission parameters and material failure processes under mechanical loading. The research methodology combines experimental mechanical testing with advanced non-destructive evaluation techniques. A systematic approach to damage assessment was implemented, incorporating quantitative analysis of acoustic emission signals and their correlation with macroscopic material characteristics. The study examined methods for interpreting acoustic emission data to differentiate between various failure mechanisms in the composite structure. Acoustic emission signal analysis employed two principal approaches: interpretation of individual signal parameters and investigation of interparameter relationships. Recognizing that analysis of single acoustic emission parameters provides incomplete damage characterization, the study developed an integrated approach combining temporal and frequency-domain signal analysis, enabling more accurate material condition assessment. The results demonstrate the qualitative influence of operational defects of different magnitudes on the mechanical properties of the composite material. These findings can contribute to the development of diagnostic methods for composite materials used in critical structures. The proposed methodology may be applied for residual life assessment and performance prediction of composite components under operational conditions.
This paper examines complex plastic deformation processes in L63 brass along plane trajectories of disproportionate deformation. The version of Yu.G. Korotkikh's elastic-plasticity equations used in this paper is based on the concept of a yield surface and the principle of the gradient of the plastic strain rate vector to the yield surface at the loading point. This version of the equations of state reflects the fundamental effects of elastic-plastic deformation of the material for arbitrary complex deformation trajectories. By setting the corresponding material parameters from the general version of the equations to zero, as a special case, all of its fundamental forms of plastic equations for small deformations are obtained (a system of “nested” models). Particular attention is given to modeling elastic-plastic deformation processes for disproportionate loading trajectories accompanied by rotation of the principal areas of the stress tensors, total and plastic strains. To assess the reliability and determine the applicability limits of the constitutive plasticity relations, numerical studies were conducted on the complex plastic deformation of L63 brass along planar trajectories of disproportionate deformation: – planar two-link trajectories with varying fracture angles; – planar smooth deformation trajectories of varying curvature; – four semicircles of equal radius with a change in curvature as one circle passes into another; – smooth two-link deformation trajectories: the first section was characterized by proportional torsional deformation, while the second section was characterized by complex deformation along curved trajectories of constant radius with varying fracture angles. The results of the study demonstrate that Yu.G. Korotkikh's model accurately predicts the main effects of complex plastic deformation of L63 brass. The model provides a qualitative description of material behavior for flat loading trajectories of any curvature, and its accuracy is sufficient for practical calculations. A number of characteristic features accompanying the complex elastoplastic deformation of L63 brass are noted (the presence of a “dip” in the stress-strain diagram in flat “fan” experiments, the presence of a similar “dip” when transitioning from a rectilinear to a curved section of deformation, and on curved trajectories of constant curvature where the sign changes-vector properties have a wave-like character, etc.).
Comprehensive studies of the physical and mechanical properties and structure of chromium-nickel austenitic stainless steel 03H17N12M2 (Russian analog of 316L steel) produced by selective laser melting have been carried out. The values of density, tensile strength, microhardness, shear modulus, nanohardness and Young's modulus were obtained, as well as the results of studying the structure and phase composition using scanning electron microscopy, electron backscatter diffraction, and X-ray diffraction. The obtained values of mechanical properties exceed the mechanical properties of steel 316L stainless steel of a similar composition, produced using both traditional technologies and selective laser melting. In order to conduct a more detailed analysis of the features of the material produced by the selective laser melting and to identify the features of the emerging structure depending on the volumetric energy density, additional studies were conducted, such as determining the proportion of low-angle boundaries, determining residual stresses and the proportion of elastic deformation during nanoidentation. It is shown that a change in the volumetric energy density in the considered range 45–130 J/mm³ has a negligible effect on the physical and mechanical properties of the material. However, the cooling rate during selective laser melting, which depends on the volumetric energy density, has a significant impact on the parameters of the emerging structure at the microlevel, such as the type of grain boundaries, the density and mobility of dislocations, etc., which can be an important factor when using the material
A promising mathematical model for the creation of wide-purpose acoustoelectronic devices has been developed for a piezoactive heterostructure consisting of a W-cut lithium tantalate crystal plate deposited through a silicon dioxide buffer layer onto a (0001)-cut ɑ-sapphire substrate, where W defines the normal to the crystal cut surface. The model takes into account the crystal cut orientation, the direction of propagation of a horizontally polarized shear wave, and the geometric dimensions of the heterostructure's constituent elements. This allows for a wide range of parameter variations to achieve maximum values of the electromechanical coupling coefficient. The heterostructure's properties have been studied with and without a buffer layer. For the latter case, the analysis allowed for determining the optimal orientation of the lithium tantalate plate cut and its thickness, which would ensure the optimal level of electromechanical coupling. The maximum value of this coefficient, for a given cut orientation and specific geometric parameters of the heterostructure, is achieved by selecting the wave propagation direction. A study of a heterostructure with a silicon dioxide buffer layer revealed that its introduction significantly increases the electromechanical coupling coefficient. A detailed study of the acoustic wave amplitude and energy flux density distribution across the heterostructure's depth was conducted. Calculations revealed that both the wave amplitude and the degree of elastic energy localization in the piezoactive layer depend on the presence of the buffer layer and the thickness of the piezoactive layer, which directly affects the electromechanical coupling coefficient. Its maximum is observed at a small piezoactive layer thickness. Thus, optimal heterostructure parameter values exist that allow for achieving the maximum electromechanical coupling coefficient. This study is of interest to developers of acoustoelectronic devices and general-purpose devices.achieving maximum EMCC. This study is of interest to developers of acoustoelectronic devices and general-purpose devices.
