Inorganic Materials最新文献

An analytical approach that provides obtaining conservative estimates for the probability of fatigue brittle failure in the structural components of technical systems taking into account the scatter in the initial size of cracklike defects described by an exponential probabilistic distribution is presented. The operational loading is considered both as a deterministic process (with the loading cycles of constant amplitude and frequency) and as a random one (a steady-state narrowband Gaussian random loading). The crack growth kinetics is described on the basis of the modified Paris equation that takes into account the effects of the stress ratio (the loading cycle asymmetry). The parameters of the Paris law are considered as deterministic quantities. An example of the assessment of fatigue failure probability for an element of a linear pipeline section containing an axial crack on the inner surface and loaded by an internal pressure is presented. A comparative analysis of the results obtained with and without taking into account the random nature of the operational loading is performed. It is shown that neglecting the random nature of the operational loading leads to nonconservative estimates obtained for the fatigue failure probability, which can differ by an order of magnitude from calculation data taking into account the stochastic nature of the loading process. The developed method can be used in the implementation of probabilistic and risk-based approaches to providing strength, service life, and safety for technical systems under real operation conditions and in adjusting standard operating programs in terms of choosing the frequency and scope of nondestructive testing.

Coatings made of materials that can efficiently absorb radiation, e.g., ferrite materials, are used to reduce the level of electromagnetic radiation in rooms containing household or industrial equipment. It is known that significant dissipation of the radiation energy can be provided by the thickness of the shielding coating which should be comparable to the electromagnetic wave length in the material, which, in turn, significantly decreases at high magnetic permeability and dielectric constant inherent in a radiofrequency-absorbing material. Radiofrequency-absorbing ferrite coatings are characterized by a high level of heat resistance, a low level of flammability, and a small (10–20 mm) thickness. However, at frequencies less than 40 MHz, the plate thickness should be higher than 30 mm to provide an efficient absorption level. In this case, the weight and cost of such coatings exhibit a significant increase. This paper presents the results of studying the effect of sintering temperature and titanium, calcium, and bismuth oxide dopants exerted on the dielectric constant of Ni- and Mn-Zn radiofrequency-absorbing ferrites. Reagent grade starting oxide components with a content of basic substance higher than 99.6 wt % have been used to synthesize samples on the basis of traditional oxide technology. It is shown that alloying with bismuth and titanium oxides is rather efficient for obtaining radiofrequency-absorbing ferrites that combine a high level of magnetic permeability and high dielectric constant. The obtained results can be used in the manufacture of radiofrequency-absorbing ferrite materials operating in the megahertz range.

A mathematical model for analysis of temperature–time conditions of arc surfacing upon fabrication of steel-aluminum compositions has been developed and verified. In the course of simulation, the database of SVARKA software has been supplemented with thermophysical properties (thermal conductivity and thermal capacity at constant pressure and volume) of the considered materials as a function of heating temperature. The geometric model of the object during simulation of arc surfacing has been preset as a single body, which can consist of various materials, for instance, in the case of formation of functional coatings based on nonferrous metals on steel substates. The parameters of the heat loads of the heating source are as follows: motion speed of motion, power, distribution along and across seam, as well as existence and grade of surfacing material. The heat propagation for argon arc surfacing using a non-consumable electrode has been calculated according to the design with a normal circular source located on the surface of a flat layer and exposed to limiting action of the sheet bottom plane. The selected calculation design reflects all the main features of argon arc surfacing, including the welding arc heat input to a massive body from its surface, low pressure of welding arc, and insignificant penetration of active spot into liquid metal. It has been demonstrated that, owing to accounting for thermophysical properties of the Fe–Al intermetallic layer located in diffusion zone, the mathematical model with uncertainty not exceeding 8% makes it possible to determine the heating temperature not only at steel–aluminum interface but also at any point of the specimens both upon joining of transitional bimetallic steel-aluminum elements with aluminum or steel structures and upon formation of functional aluminum coatings by surfacing, including composite materials.

The capabilities of the numerical simulation of technological processes are limited by the accuracy and efficiency of determining the properties of materials continuously changing under repeated heating and cooling. The parameters of structural transformations are the principal factors affecting the properties of alloyed steels. In this paper, we present a method for determining the parameters of relationships describing C-shaped curves in the experimental diagrams of isothermal decomposition of austenite. The proposed approach makes it possible to reconstruct the entire C-shaped curve using a relatively small fragment near the “nose” (based on three points). The joint processing of a series of curves provides determining the parameters of ferritic, pearlitic and bainitic transformation kinetics. However, one should take into account the distinctive features of the diffusion decomposition of austenite. For example, ferrite and pearlite are formed in overlapping temperature ranges and have similar mechanical properties, but their combining into a single ferrite-pearlitic structure complicates the construction of a mathematical model for the transformation. The bainitic transformation is a transient one between diffusion and diffusionless transformations. In a part of the transformation temperature range the limit of conversion level is a function of temperature (just as in the case of martensitic transformation). It has been shown that, for the case of ferrite-pearlitic transformation, the best results can be obtained with the use of Kolmogorov–Avrami equation, whereas for the case of bainitic transformation, the best results can be obtained with the use of Austin–Rickett equation modified to take into account an incomplete conversion level of the transformation.

The flow of a gas or liquid can be visualized by recording and analyzing successive images of the particle distribution on the surface of the object determining the parameters of motion. However, this approach allows visualizing the distribution of solid particles without evaluating particle mass and density. The paper outlines results on determining the mass and density of suspended particles through exposing the particle flow to acoustic radiation. We used the method of visual image processing in flow of particles entrained by an acoustic field having known frequency and amplitude during at least two periods of acoustic oscillations. We also took into account the relaxation of particles in the measuring plane confined using a light sheet. The basic mathematical expressions are derived from estimating the mass, density, velocity field, and shape of particles using digital image processing and temperature measurement in the flow region. A schematic diagram is constructed and the design of a device to implement the diagnostic method is outlined. The approach proposed can be applied to determine parameters of suspended particles in medicine, biology, ecology, powder metallurgy, and other areas of science and technology.

The operability of structures in complex combined loading modes depends on a significant number of combinations of operational parameters of thermomechanical impacts in terms of loads, temperatures, times, numbers of cycles, and strain rates. The main strain patterns of structural materials under complex conditions are established using combined standard, unified, and special tests under laboratory conditions. With the use of representative substantiations of physicochemical models for strain diagrams in a wide range of loading conditions and with allowance for the scale diversity of models, the material structure, and the responsibility of structures, we propose a step-by-step consideration of the corresponding strain types: elastic, sign-alternating flow, progressive strain accumulation, and their combinations. In this case, structural calculations can be built as a hierarchical system, in which each next level refines the boundaries of permissible impacts toward expansion of the range of acting loads, temperatures, rates, and strain modes, which is associated with an increase in the amount of required initial data and complicates calculations. The proposed methods for schematization of physicomechanical properties and types of state equations for describing strain curves account for the compactness requirements of the initial data and the need to use both standard and unified methods for determining the characteristics of cyclic inelastic deformation and special methods. Both from a theoretical standpoint and from the viewpoint of practical applications, power equations are the most reasonable to describe the kinetics of strain diagrams under the considered conditions. Exponential dependences are suitable to reflect the role of the temperature factor while power dependences are suitable to take into account the time and strain-rate factors and two-frequency loading conditions. Ensuring the maximum possible use of the strain and strength reserves of materials and structures, refined calculations at higher, more complex levels of the considered hierarchy must be based on kinetic dependences describing low-cycle deformation in complex loading modes.

A comparative analysis of typical stress-strain diagrams obtained for in-plain shear of 25 unidirectional and cross-ply reinforced polymer matrix composites under quasistatic loading was carried out. Three of them were tested within the framework of this study, and the experimental data on other materials were taken from the literature. The analysis of the generalized shear-strength curves showed that most of the tested materials exhibit a similar deformation pattern depending on their initial shear modulus: a linear section is observed at the beginning of loading, whereas further increase in the load decreases the slope of the curve, reaching the minimum at the failure point. For three parameters (end point of the linear part, maximum reduced deviation of the diagram, tangent shear modulus at the failure point) characterizing the individual features of the presented stress-strain diagrams, approximating dependences on the values of the reduced initial shear modulus are obtained. At the characteristic points of the deformation diagrams, boundary conditions are determined that can be used to find the parameters of the approximating functions. A condition is proposed for determination of the end point of the linear section on the experimental stress-strain curve, according to which the maximum deviation between the experimental and calculated (according to Hooke’s law) values of the shear stress in this section is no more than 1%, thus ensuring rather high accuracy of approximation in the linear section of the diagram. It is recommended to use the results of this study when developing universal and approximating functions relatively simple in structure that take into account the characteristic properties of the experimental curves of deformation of polymer composite materials under in-plane shear of the sheet. The minimum set of experimental data is required to determine the parameters of these functions.

One of the most critical manufacturing defects of cast metal-matrix composites is a nonuniform porosity distribution over the composite volume. This nonuniformity not only leads to local softening, but also plays a key role in the evolution of the damage process under external loads. In this work, we present the results of the study of a local porosity in disperse-strengthened reactive cast aluminum matrix composites. The studies were performed using a laser-ultrasonic method based on statistical analysis of the amplitude distribution of backscattered broadband pulses of longitudinal ultrasonic waves in composites. Laser excitation and piezoelectric detection of ultrasound were performed using a laser-ultrasonic transducer. Two series of reactive cast aluminum matrix composites were analyzed: reinforced by in situ synthesized Al₃Ti intermetallic particles in different volume concentrations and by Al₃Ti particles with the addition of synthetic diamond nanoparticles. It has been found that, for both series of composites, the amplitude distribution of backscattered ultrasonic pulses is approximated by a Gaussian probability distribution applicable for statistics of a large number of independent random variables. The empirical dependence of the half-width of this distribution on the local porosity in composites is approximated by the same nearly linear function regardless of the size and concentration of reinforcing particles. This function was used to derive the calculating formula for determining the local porosity in the studied materials. The obtained results can be used to reveal potentially dangerous domains with a higher porosity in reactive cast metal matrix composites.

Unidirectional composites exhibit the highest strength when stretched along the fibers. However, the proper determination of the strength faces great methodological difficulties. The main problems of tensile testing of polymer composites consisted in development of the specimen shape and the method of specimen fixation which ensure the minimum impact of the stress concentration near the grips on the strength measurements. A conventional shape of the specimen with fillets is unsuitable for unidirectional polymers owing to the splitting occurring in the fillet zones upon loading. Therefore, the specimens are usually standardized in the form of rectangular strips fixed using tabs or special grips which provide constant transverse forces. However, with such a specimen shape, a significant stress concentration inevitably occurs at the edge of grips, and the lower the ratio of the interlayer shear modulus to the longitudinal Young’s modulus, the greater the stress concentration impact. For the purpose of the most correct determination of the strength, we propose to use specimens with smoothly varying dimensions at the same cross-sectional area which ensures keeping the total number of unbroken fibers in each section. The specimen thickness decreases when moving from the working part of the specimen to the gripping part, whereas the width (while maintaining the section area) grows to prevent the collapse of the specimen resulting from transverse forces in standard self-tightening grips. Analytical and FEM modeling is performed to select a rational contour shape. Technological equipment has been developed, and a procedure of manufacturing testing specimens has been worked out. The tensile test of specially manufactured curvilinear reinforced specimens showed higher strength values compared to standard rectangular strips or specimens with semicircular fillets.

In this paper, we present the results of computational and experimental studies of changes in patterns of resistance to deformation and damage accumulation under conditions of irregular low-cycle loading, which are compared with similar data of a regular cyclic elastoplastic deformation upon equal loading. An irregular mode of low-cycle loading implemented in research is adopted as a uniform distribution of the change in the stress amplitude between specified maximum and minimum levels at the symmetric shape of the cycle. This mode was reproduced on test equipment by introducing into the control program of the corresponding functional dependence of changes in the stress amplitude in the cycles. Obtained in the irregular loading mode, the data on a cycle-by-cycle kinetics of both cyclic and unilaterally accumulated strains were recorded in a database and then compared with the data for a regular loading. This made possible their analytical description by the corresponding state equations with the correction of the diagram parameters of cyclic elastoplastic deformation, accounting for the irregularity conditions of loading modes. The experimental results are presented as test mode diagrams, curves of a low-cycle fatigue of the studied material in the soft and hard loading modes, diagrams of a cycle-by-cycle kinetics of the cyclic and accumulated strains in the regular and irregular modes, and kinetic diagrams of damage accumulation under these conditions. The summation criteria that include the deformation characteristics of accumulated damage are used. It is shown that, with allowance for change in the nature of the strain development during irregular low-cycle loading, the criterion dependences when accepting the condition of reaching the limiting state can be used to estimate the durability and compare it with the similar data in regular modes.