The work is devoted to the determination of local stresses arising in the vicinity of a micropore containing methane under high pressure. Equations are proposed for describing the physical processes that have a decisive influence on the failure of a hollow steel cylinder loaded with internal hydrogen pressure. It is important to note that the high-temperature hydrogen attack (HTHA) itself (cracking of the hollow steel cylinder under the influence of local stresses) is not considered. The local failure criterion is the sum of the hoop stress arising under the influence of hydrogen pressure within the steel cylinder cavity and the hoop local stress arising under the influence of methane pressure within the micropore. To describe the accumulation of methane concentration in the hollow steel cylinder, a diffusion equation with one initial and two boundary conditions is used, where the mass transfer condition is assumed at the outer boundary radius. Porosity, defined as the relative increment in the area of an elementary ring, is considered as a measure of material discontinuity. A new displacement relationship was established from the expression for porosity, where a hyperbolic dependence of displacement on radius was previously accepted. A relationship between pressure, concentration, porosity, and temperature was proposed as the equation of state for methane within a micropore, satisfying the corresponding initial conditions. It should be emphasized that this is the first time that the equation of thermal state for a gas has been considered in practice. It is hypothesized that for localized failure to occur, the sum of the hoop stress and the hoop local stress must reach the break creep strength. Calculations showed that at the micropore boundary, the hoop local stress significantly exceeds the hoop stress. This circumstance led to the conclusion that hoop local stress must play a key role in the cracking of a hollow steel cylinder.
The paper suggests a method integrating an applied version of the theory of inelasticity into a finite element complex for computations of the stress-strain state, strength and service life of structural elements operating under long-term loading. The paper presents the experimental results of smooth specimens made of bronze alloy under creep conditions at different loading levels to obtain material characteristics used to identify the material parameters of BrH08-Sh bronze alloy at temperatures of 400, 500 and 600 °C. The possibility of applying the theory variation is considered by conducting computational and experimental studies on cylindrical specimens with a stress concentrator in the form of a V-shaped annular groove with a radius of 1 mm. Tests of specimens under creep conditions were carried out at temperatures of 400, 500 and 600 °C until failure. The experiment was simulated and the distribution of stress intensity and accumulated inelastic deformation in the finite element model of a specimen with a groove was obtained. The integration of the model into the finite element complex was carried out using the algorithm, which allows calculating stresses and strain at the end of the computation step using the components of the strain increment tensor. The algorithm makes it possible to separate the areas of monotonic and cyclic deformation in the space of the inelastic strain tensor using the memory surface. Comparison of the computed and experimental data shows that this version of the applied theory of inelasticity allows for a sufficiently accurate modeling of creep and assessment of the long-term strength of structural elements with stress concentrators.
This study evaluates fracture toughness of polymer fiber-reinforced composites in terms of the critical stress intensity factor (SIF) at the onset of the splitting crack initiation at the tip of a transverse notch under four-point bending of unidirectional carbon fiber-reinforced plastic specimens. In addition to the nonlinearity analysis of loading diagrams, as adopted in linear fracture mechanics, advanced techniques including acoustic emission (AE), digital image correlation (DIC), and numerical processing of experimental loading diagrams were developed to more accurately determine the moment of splitting the crack initiation. The loading diagrams were processed to identify the local load reduction, corresponding to the formation of a longitudinal splitting crack. A sliding window method based on the standard deviation of the load from an approximating function was used for processing. This moment was further determined using AE techniques with algorithms for analyzing disruptions in time series data. The most informative AE parameters for this task were identified as the average duration and specific energy of the recorded pulses. By dividing the critical plane into three distinct segments, each associated with different sources of AE, the moment of time series disruption was identified, correlating with the onset of the splitting crack formation in the composite specimen and the emergence of a new segment. For verification, the DIC method was employed to analyze strain fields at the notch tip. Through the integrated processing of AE impulse streams, deformation diagrams, and strain fields, the average critical load was determined for a series of specimens corresponding to the formation of a splitting crack in the composite. This critical load was used to calculate the splitting fracture toughness of the carbon fiber-reinforced composite in terms of the critical SIF at the notch tip.
Controlled synthesis of composites attracts attention due to the possibility obtaining a homogeneous product in terms of composition and structure. This also applies to intermetallic composites. In this case, the heat release in reactions is summarized with the heat due to external heating, which can cause self-sustaining regimes such as thermal explosion or combustion. Non-equilibrium thermokinetic processes are accompanied by mechanical stresses and strains, which can affect the process dynamics and synthesis result. In this paper, a coupled model is analysed for the bulk synthesis of a composite from Ni-Al blend, which takes into account the interrelation of thermal, chemical and mechanical phenomena. The model includes a heat conduction equation with an additional heat source of chemical and non-chemical nature. The kinetic equation is added to the model to deter the mass fraction of the synthesis product. To estimate thermal stresses and strains, the known solution is used for the mechanical equilibrium problem of a multiplex cylinder corresponding to the model reactor. A cylinder is uniformly heated from the side surface. The analytical solution of the equilibrium problem after substitution into the thermokinetic model makes it possible to identify equivalent reaction parameters and equivalent thermophysical properties depending on the mechanical moduli. For the numerical implementation of the obtained nonlinear model, a numerical algorithm was developed; a program has been compiled and debugged that allows for detailed parametric research. In the calculations, the temperature fields, the final fraction of the reaction product, as well as the values of stresses and strains were determined in the whole investigated area at different instants of time. As a result, it is shown that taking into account the coupling of thermal, chemical and mechanical processes changes all process characteristics including stress values of different physical natures. The stress values depend on the reaction initiation conditions and geometrical parameters of the reactor.
The paper considers the methodology and principle of refining the load-carrying capacity or ultimate strength of composites using an acoustic emission recording system for ambiguous diagrams. The aim of this work was to increase the objectivity of the results of experimental studies of the mechanical characteristics of composite fiber-reinforced materials with an ambiguous loading diagram based on additional measurements during standard testing of specimen. Obtaining a set of new, more objective data on the behavior of promising composite materials during deformation and fracture. As a result of studying the mechanical behavior of knitted 3D carbon fiber specimen using standard methods, it was possible to record and formulate the concept of an ambiguous deformation (loading) diagram. An algorithm has been developed for refining the ultimate strength (load-bearing capacity) of composite materials in the case of ambiguity in deformation or loading diagrams. The ultimate strength or load-carrying capacity are understood solely in the interpretation of the standard in question. The algorithm is based on the concept and algorithm for determining the critical value of the cumulative energy of acoustic emission during the deformation of a composite material specimen - an acoustic analogue of the ultimate strength (load-carrying capacity). The algorithm is an analysis of the extrema of the approximation of the time-domain sweep of the energy parameter of acoustic emission, a polynomial function with the allocation of a primary localized array of this parameter. For the analysis, the hypothesis is considered that the basic parameter of the ultimate strength (load-carrying capacity) - Pcr is located in the region of half the distance between the maximum and minimum of the primary localized array of the time-based scan of the energy parameter of acoustic emission. A comparative analysis of the strength values (load-carrying capacity) obtained without and with the use of a hypothesis for promising 3D carbon fiber reinforced plastics was carried out. The significant efficiency of the described method is demonstrated, often with multiple changes in the values of the ultimate strength or load-carrying capacity of composite materials.
The paper analyzes the structure, mechanical and physical properties of orthopedic screws made of 316L medical steel manufactured by laser powder bed fusion (L-PBF). The microstructure and internal defects of the L-PBF samples were examined using a Tescan MIRA LMS scanning electron microscope. The relative density of the printed samples was measured by hydrostatic weighing using an OHAUS electronic analytical balance. The Vickers microhardness of the SLM samples was determined using a Metolab 502 microhardness tester. A WYKO NT1100 non-contact profilometer-profilograph was used to measure the roughness parameters. One of the indicators of 3D printer printing quality is the use of powder with certain characteristics. Before printing, a certification analysis of the powder was performed, which showed that the powder particles were rounded, there were no inclusions, and the size was about 50 μm. It was found that the 3D printing parameters, in particular the laser scanning speed, affect the surface quality (roughness) and the internal structure of the product with areas of different thickness. The results showed that at a low scanning speed, defects (pores) in the massive parts of the L-PBF screw were minimal, and in thin places (thread), the number of defects was higher compared to the massive part. An increase in the laser beam speed leads to an increase in the size and number of defects in the massive part of the product. As for the thin part of the screw (thread), the effect of the laser beam scanning speed on defects was found to be nonlinear: with an increase in the scanning speed, defects first decreased and then sharply increased.
A three-dimensional numerical analysis was carried out to study the stress-strain state of a single-mode optical fiber with a copper protective coating during cooling from the drawing temperature to room temperature. The problem was solved within a quasi-stationary thermoelastic formulation without axial symmetry, where the initial temperature of each structural layer was defined by its softening or melting point. In the calculations, the softening temperature of each layer was taken according to the impurity content in the quartz glass. The simulations were performed using COMSOL Multiphysics v.6.2. Residual stress and strain fields were obtained, as well as the longitudinal and transverse strain components in the fiber core. Additional simulations for fibers with silica cladding diameters of 125 μm and 200 μm without the metal coating to analyze the cooling process of the silica part of the fiber from its softening temperature to ambient conditions. This approach makes it possible to isolate the contribution of the metallic coating to the overall deformation, since variations in the cladding diameter can affect the stress distribution in the core even before the copper layer is applied. It was found that increasing the cladding diameter from 125 μm to 200 μm, while keeping the copper thickness constant, decreases the transverse strain in the core by about 36 %. The proposed thermoelastic model is relatively simple but physically consistent and requires further experimental verification. With refinement, it may serve as a basis for the design and analysis of metal-coated optical fibers intended for operation in harsh environments.
Based on the analysis and comparison of atomic physical and engineering models of solid matter, the existing classification of volumetric forces used in the resistance of materials and the theory of elasticity is expanded (magnetic, electromagnetic, inertial, gravitational). This fundamentally new addition is justified by replacing the force of interatomic interaction with a statistically averaged analogous equivalent in the form of a quasi-elastic volumetric load acting between infinitesimal material particles of a solid body which are displaced during its deformation. For continuous homogeneous isotropic materials obeying Hooke's law, using the fundamental law of conservation of mechanical energy, a general theory has been developed for determining the quasi-elastic body force function, the three projections of which Xk , Yk , Zk on the coordinate axes x, y, z linearly depend on the corresponding displacements u, v, w in x, y, z direction. The proposed calculation method is illustrated by the simplest example from the classical course of resistance of materials to uniaxial tension by external static forces P of a linearly elastic weightless beam of constant cross-section. In the course of a mathematically accurate solution to this innovative problem: 1) it was proven that the new quasi-elastic mass force depends on the location (coordinates) of an arbitrary point of the body, the external load acting on the structure, its volume and Poisson’s ratio μ, and at μ = 0,5 this volumetric load becomes zero; 2) the relevance and great importance of taking into account the quasi-elastic volumetric force in the equilibrium equations in the process of mathematical modeling of the stress-strain state and design of load-bearing structures has been confirmed.
Spatially reinforced composite materials with mutually orthogonal fiber arrangements are relatively rarely used in load-bearing structures. This is due to their low stiffness and shear strength in the orthotropic planes. This is especially significant in composites with a pyrocarbon matrix, whose strength characteristics are significantly lower than those of structural polymer binders. However, in certain critical structures or their components, due to geometrical features and other factors, the use of orthogonal reinforcement is preferable. One such structure is the acetobular component of a hip joint endoprosthesis, which is half a hollow sphere loaded on its inner and outer surfaces by contact pressure. For biomedical reasons, carbon-carbon composites (CCCMs) are used to manufacture this component, and spherical symmetry dictates orthogonal reinforcement in a spherical coordinate system. The weak point is the polycrystalline pyrocarbon matrix, damage to which most often initiates failure of carbon composites. This paper examines damage initiation in an acetabular component made of orthogonally reinforced CCCM. Based on simple engineering macroscopic strength criteria, the matrix should not fail under loads of up to 3000 N on the endoprosthesis structure. However, at the scale of individual grains, highly fluctuating mesostresses are realized, the amplitude of which can significantly exceed the magnitude of macrostresses. This paper analyzes stochastic mesostresses in matrix grains in the most heavily loaded zones of the carbon-carbon composite. It is shown that damage in the grains begins with a load of 2208 Newtons. The analysis was conducted using the method of integral equations for mesodeformations. Probability densities of the mesostress distribution were constructed, from which the probabilities of various possible damage types were calculated as functions of the overall load on the endoprosthesis. It has been demonstrated that the acetabular component design under consideration is more reliable in terms of damage initiation than the unidirectional carbon fiber composite (CCM) endoprosthesis stem (liner) traditionally used in clinical practice.
This study investigates the stability of joint nonlinear stationary vibrations of a hysteresis-type elastic dissipative characteristics plate and dynamic absorber under harmonic excitation. The research focuses on a mechanical system consisting of an elastic plate and a dynamic absorber designed to protect the structure from vibrations. The elastic-dissipative relationships of both the plate and the absorber are formulated based on the Pisarenko-Boginich hypothesis, which makes it possible to account for the internal resistance forces typical of hysteresis-type materials. Using the harmonic linearization method, analytical solutions to the system’s nonlinear differential equations were obtained. The derived expressions describe the dependence of the vibration amplitude and frequency on the interaction between the plate and the dynamic absorber. The stability of the stationary vibration modes of the system was examined by means of the vertical tangents method. The analytical results demonstrate that elastic plates with hysteresis-type dissipation exhibit effective vibration damping within specific frequency ranges of excitation. This implies that such materials can significantly reduce the amplitude of steady-state vibrations when appropriately tuned with a dynamic absorber. The findings of this research provide valuable insights for the design and optimization of vibration protection systems. The obtained stability conditions were numerically analyzed depending on the plate material and the dissipative properties of the dynamic absorber damping element. Relevant conclusions were drawn and recommendations were given. The change in the stability field was analyzed and the active parameters in this were determined.
Methods of improving the quality of surface layers can make major contributions in mechanical engineering by increasing reliability of products. Formation of a favorable stress-strain state in the surface layers allows us to increase wear resistance, fatigue strength and performance characteristics of machine parts. Today, there are various methods of strengthening parts by surface plastic deformation, e.g. rolling, roller running, ultrasonic treatment, etc. The choice of a specific method depends on the purpose of each part, its geometry and requirements for performance characteristics. Special technological problems arise when processing cylindrical parts of small diameters such as axles, power studs, rollers, which are easily bent. For non-rigid parts characterized by a length-to-diameter ratio of more than 15, a strengthening method using flat wedge plates has been developed. This method is presented and studied in this paper. It aims at evaluating the stress-strain state in the deformation zone and residual stresses in hardened cylindrical parts depending on the geometric shape of the working tool and the main parameters of the hardening process with flat wedge plates. The research method suggests modeling in the ANSYS Workbench software environment taking into account the bilinear diagram of the sample material deformation. Numerical experiments were carried out at different angles of the wedge section β (from 30° to 90°) and degrees of relative compression Q (from 0.5 to 4%), which made it possible to determine the patterns of distribution of temporary and residual stresses, as well as the intensity of plastic deformation both along the length and radius of the workpiece. It was found that the angle β of the wedge section of flat plates equal to 45° is the most rational, since it ensures a uniform distribution of residual stresses, prevents local overloads and promotes hardening without damaging the surface layer. The recommended value of relative compression during strengthening with flat wedge plates is to be in the range Q = (1.3-1.5) %.
A research study was carried out on the spectral problem of maximizing the first natural frequency of a heterogeneous elastic and viscoelastic circular plate with variable stiffness under various edge fixation conditions. The mechanical elastic or viscoelastic properties of the material are used as control functions. Viscoelastic properties are taken into account within the framework of the complex modulus principle. Several approaches to solving the spectral optimization problem are considered: analytical, semi-analytical and numerical schemes. In the model problem for a circular plate hinged at the edge, an analytical solution is obtained - the optimal distribution of viscoelastic modules-long-term and instantaneous modules, taking into account the condition for the average distribution of the plate stiffness over the volume. The Rayleigh ratio was constructed, as well as a generalized functional based on the Lagrange multiplier method, taking into account the additional condition. The optimality condition was found, which consists in the constancy of the energy characteristic. In the case of a circular plate stiffly fixed at the edge, a numerical scheme for optimal control of material characteristics in the class of polynomials is proposed to find the optimal distribution of complex modules, taking into account their positivity and evenness properties. The proposed scheme is based on the Galerkin method and the least squares method for determining the coefficients of the polynomial expansion of control functions. A numerical scheme for solving the spectral problem of finding the coefficients of the polynomial expansion of control functions is also proposed, which launches a heuristic search for the corresponding parameters. Calculations were performed for each of the proposed schemes for solving optimization problems. The limits of applicability, disadvantages and advantages of each approach were identified. A comparative analysis of the results was performed using model problems as examples.
Nowadays, highly elastic materials showing pronounced rheological properties that contribute to the reduction of peak amplitudes of vibrations in resonance modes are widely used in the designs of vibration-protective devices. The design of such structures in modern conditions requires the construction of a mathematical model, the most important part of which are the defining relations for the description of viscoelastic and hyperelastic behavior of the material and an adequate methodology for the identification of material constants. The purpose of this study is to construct a methodology for determining the visco-hyperelastic properties of a material based on quasi-static test data. In describing the hyperelastic behavior of the material, the Mooney - Rivlin strain energy potential with two constants and Neo-Hookean potential with consideration of compressibility were used, and when viscoelasticity was taken into account, the relaxation function was expanded into Prony series. Semi-analytical expressions of stress dependence on principal strains and time with account of visco-hyperelastic behavior of the material in cases of uniaxial stressed and uniaxial deformed states are obtained. To select the optimum distribution of approximation parameters of relative shear moduli and range of relaxation times, the effect of changing viscoelastic parameters on the magnitude of hysteresis and maximum stresses in the cycle at different strain rates was evaluated. The results of full-scale cyclic tests in free tension and compression and constrained compression were used to calculate material constants. A procedure for determining the visco-hyperelastic properties of the material has been developed, which consists in minimizing the difference between the results of mechanical tests and the response values calculated using the constructed semi-analytical expressions using the Nelder - Mead method. As a result, the hyperelastic and viscoelastic material properties were determined, which agree with the experimental curves with reasonable accuracy. It is shown that the Neo-Hookean potential with its simpler form allows describing the data of mechanical tests on a par with the Mooney - Rivlin potential with two constants. The calculated material properties were verified by comparing the shock absorber test data and the results of numerical simulation of the shock absorber deformation.
The results of determining the opening values of an artificial notch, which simulates a crack in a real composite material by using the electronic speckle interferometry (ESPI) method are presented. The main feature of the presented approach is that precision optical interference measurements of the opening values are carried out directly on the borders of the notch. High-quality interference fringe patterns caused by local removal of material between two exposures are visualized. The data obtained provide a quantitative description of the fields of purely deformational displacements, both in the "close field" and in the far vicinity of the notch top. On this basis, the distribution of the notch opening along its length is constructed when a rectangular composite plate with a hole is stretched. The notch consists of two branches that are directed in opposite directions from the intersection points of the contour of the hole and the short symmetry axis of the rectangular plate. The tensile load acts perpendicular to the notch line. It is shown that the sequence of applying a two-sided notch has practically no effect on the measurement results of the displacement components. Experimental data were obtained for notches of different lengths at different levels of external load. The dependences of the notch opening average values at notch initial points on the notch average length and the values of external tensile stresses are constructed. The obtained dependences demonstrate the obvious nonlinearity of the deformation process in the irregular zone of the coupons, both with a change in the length of the notch and the external load. These data are of considerable interest from the point of view of verifying the calculation methods used to determine the parameters of fracture mechanics for cracks in composite structures.
