Annotation of the results obtained in 2020
As a result of the work carried out in the frame of the project, it became clear that plastic deformation and the accumulation of fatigue damage during low-cycle and high-cycle fatigue have one common feature which can be conditionally characterized as a surface effect during the accumulation of metal damage. The surface effect includes all types of damage: microcracks, pores of nano-, micro- and meso-scales, structural defects, as well as the accumulation of hydrogen, and is concentrated in a surface layer of 1-2 grain sizes (about 100 μm) thick. For plastic deformation, this effect has been known since the beginning of the 20th century; moreover, in single crystals during plastic deformation, damage is also observed in a layer 40-50 μm thick. For fatigue damage and hydrogen accumulation associated with fatigue and plastic deformation, this effect was discovered and systematically investigated as a result of the project work. Our comprehensive studies allowed us to substantiate that in many types of alloys and in a number of composite materials a thin damaged surface layer is formed during plastic deformation and fatigue. In the case of metals it is saturated with hydrogen according to the mechanism of "saturation of metals with hydrogen during fracture", first studied by A. Yu. Khrustalev. The method of acoustic damage, the foundations of which were laid in the works by V.I. Erofeev and E.A. Nikitina, establishes a connection between the sound speed and the metal damage. At the same time, it turned out to be difficult to practically apply the results of measuring the absolute values of the sound speed since it is affected by many factors with a similar magnitude including mechanical stress, internal metal structure, temperature, alloy chemical composition. As a result of the project, it was experimentally established and theoretically substantiated that the acoustic anisotropy is a stable parameter, which depends mainly on the initial anisotropy, internal mechanical stresses and metal damage. It is important that the value of acoustic anisotropy associated with damage at small and medium plastic deformations, associated saturation of metals with hydrogen and developed fatigue of metals is about an order of magnitude larger than the value associated with mechanical stresses. Thus, we were able to reveal and substantiate the new content of the acoustic damage method. It is shown as a result of the project, that with the acoustic anisotropy value of the order of a few percent, nearly all this value can be associated with the initial acoustic anisotropy and damage. On the other hand, due to the surface effect it is possible to isolate the acoustic anisotropy associated with internal stresses and use the standard techniques for acoustic tensometry by means of mechanical removal at the measurement site of a metal layer with a thickness of about 50-100 μm and standard accounting for the initial anisotropy. The important results of the project are: (i) theoretical substantiation of the strong influence of damage to a thin surface layer on the acoustic anisotropy of flat metal samples; (ii) a formula connecting the main components of the damage tensor and the magnitude of acoustic anisotropy; and (iii) obtaining the distribution of the magnitude of the damage tensor components over the depth of a flat sample on the basis of experimental measurements of the acoustic anisotropy of metal samples. A fundamentally new result is also the possibility of performing measurements by the acoustic damage method with a large initial anisotropy of the material (about 30-45%) that was theoretically proven and experimentally confirmed on samples made of a single-crystal heat-resistant alloy. This type of anisotropy associated with the anisotropy of the elastic moduli is characteristic of the single-crystal heat-resistant alloys and many other composite and nanocomposite materials. Thus, as a result of the project, the possibility of acoustic diagnostics and acoustic quality control at the micro and nanoscale of metallic and non-metallic composites and nanocomposites has been substantiated, which is extremely important for modern technology in which the composite materials are used more and more widely. Separate elements of the complex study can find independent application: new methods for measuring acoustic anisotropy (methods for measuring angular diagrams of acoustic anisotropy); results of studies by the method of angular diagrams of geological cores; methods for measuring the spatial and energy distribution of hydrogen in metals; methods of finite element modeling of acoustic wave propagation in medium, evolutionary type damage accumulation models, taking into account the anisotropy of the properties of a single crystal material; methods of modeling piezoelectric generators of sound waves and finite element modeling of their interaction with a viscoelastic continuous medium; models of dynamic redistribution of hydrogen in the surface layer of a loaded metal, built on the basis of a general model of a bi-continuous material; models of wave propagation in media of a random structure based on stochastic analysis; new models of the effect of localization of plastic deformation. The results of the project can be used directly as a fundamental basis of methods for technical diagnostics of parts and assemblies of machines and structures to control strength and reliability. 1. The possibility of using measurements of acoustic anisotropy to determine the degree of damage to metals, including damage associated with fatigue, large plastic deformations and hydrogen degradation, has been substantiated. Thus, the most common types of degradation of mechanical properties and destruction of metal parts, including those prone to corrosion, can be diagnosed. 2. The possibility of using measurements of acoustic anisotropy for determining the orientation of the main axes of anisotropy of mechanical elastic moduli, as well as determining mechanical stresses and damage in metals and non-metals with a large initial anisotropy, has been substantiated. On this foundation are possible: * Fast non-destructive testing of the correct orientation of the main axes in the part which can be used in the production of gas turbine blades and various parts from composite materials (including polymer); * Control of mechanical stresses under working load in machine parts and structures; * Technical diagnostics of damage accumulation at the level of rafting of nanostructural elements in gas turbine blades, the appearance of significant concentrations of microdefects in parts, and changes in the degree of adhesion and porosity growth in composite materials. In all these cases, it is possible to develop the methods for both non-destructive diagnostics during input and output production control, and diagnostics during the operation of machines and structures. After additional development, it is possible to apply the project results in petromechanics and geology, especially in terms of determining the microporosity and anisotropy of underground formations, which is especially important in field development and calculations of hydraulic fracturing parameters. Project results also have more indirect consequences. First of all, their implementation will ensure the safe functioning of all the most important industrial, energy and transport facilities. They will also increase the economic efficiency of the most modern production with the use of nanostructured and composite materials by reducing the cost of defective products and the possibility of setting up effective technological modes of production taking into account the results of non-destructive testing.

 

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Annotation of the results obtained in 2018
The main objective of the project is to develop the theoretical and experimental foundations of the method of acoustic damage to assess the strength and durability of structural elements in the process of their manufacture and operation. The studies were carried out in two main areas, which are acoustic and hydrogen diagnostics, both by means of experimental methods and theoretical approaches. An analysis of correlation of the results was carried out. The diagnostics of the elements of critical structures in thermonuclear power engineering was carried out by using the methods of acoustoelasticity and acoustic damage. The diagnostics was carried out on real elements made of nanostructured silicon carbide, specifically on two types of samples obtained by the industrial sintering method: on the model mirror of the light collection system of the projected ITER fusion reactor, and also on a fragment of the carrying structure of the reactor destroyed by vibration. The analysis of the obtained results confirmed the possibility of applying the method of acoustic damage to the detection of structural defects and the assessment of damage to composite structures made of nanostructured silicon carbide. Acoustic anisotropy and the spatial-energy distribution of dissolved hydrogen concentrations of samples of single-crystal materials of various crystallographic orientations subjected to cyclic effects resulting in low-cycle fatigue were studied. The performed theoretical studies covered a wide class of problems, including the consideration of fatigue cracks, and were confirmed by a finite element calculation. The study of changes in the spatial-energy distribution of dissolved hydrogen concentrations under low-cycle fatigue loading of steel samples with periodic surface modification due to mechanical processing was performed. The purpose of this part of the project was to clarify the nature of non-uniformity in the spatio-temporal distribution of hydrogen, both in depth and in length of the research object. The studies were conducted in parallel theoretically and in tests. Qualitative features were identified in order to identify the accumulation of plastic deformations and microdefects in the elements of critical structures based on the angular diagrams of acoustic anisotropy. The main goal of this part of the work was to clarify the role of factors affecting acoustic anisotropy, including anisotropy of the elastic properties of the material, active or residual stresses, plastic deformations and internal or surface defects. Analysis of the evolution of the angular diagrams of acoustic anisotropy provided additional information in determining the nature and separation of the contributions of various factors. Finite element modeling of the effect of surface microcracks on the speed of passage of ultrasonic waves in materials was carried out. This study is necessary to build a numerical counterpart of an industrial device that implements the idea of acoustoelasticity. To this end, a two-dimensional finite-element model of the passage of elastic waves initiated by a piezo-actuator was constructed in samples containing a network of microcracks in a thin surface layer, based on a two-layer heterogeneous approximation. An analytical approach was developed to describe the propagation of high-frequency waves in elastic media with a random distribution of elastic and mass characteristics in order to take into account spatial heterogeneity, microstructure, damage and microcracks within a single approach. This part of the project is dictated by the need to clarify the significance of non-ideality of the material in the results of theoretical and simulation parts of the project, made under the assumption of ideal materials. As a result of the research, the following main scientific results were obtained. The results of an experimental study of the acoustic anisotropy fields of a composite mirror of a system for collecting light from polycrystalline silicon carbide made it possible to obtain and analyze the distribution field of the main values of the damage tensor in the mirror design. A correlation was established between acoustic anisotropy, principal values of the damage tensor and dissolved hydrogen concentrations for various loading modes of standard compact single-crystal samples from nickel-based super alloys. The parameters of hydrogen transport models were obtained, which are necessary to identify the spatial parameters of damage to the samples under cyclic loading and the parameters of the bicomponent model of the material containing hydrogen, for further use in simulating the effects of acoustic damage. Mathematical models of hydrogen-containing material were developed for further use in modeling effects of acoustic damage, as well as mathematical models for calculating the main values of the damage tensor in the initially anisotropic material based on data on the propagation speeds of waves of different polarization. The dependences of acoustic anisotropy on crystallographic orientation of single-crystal samples were obtained before and after fatigue tests. Angular diagrams of acoustic anisotropy were obtained in the case of complex multi-axial stress-strain state of samples that underwent fatigue tests and in the case of step uniaxial loading of samples up to large plastic deformations and failure. The dependence of the wave propagation time on the distribution density of microcracks in a thin surface layer was established. Expressions were also obtained for the speed of passage of elastic waves in the sample material for the case of direct modeling of a network of surface cracks and for modeling their presence with a homogeneous layer with a modified shear modulus corresponding to the density of surface microcracks under consideration. As a result of solving the problem of wave propagation in an elastic medium with a random distribution of elastic and mass characteristics, expressions for the mean field and dispersion of the propagating harmonic wave for characteristic cases of inhomogeneities in materials were obtained in closed form.

 

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Annotation of the results obtained in 2019
The main goal of the second year of the project was to develop the experimental and theoretical foundations of the acoustic damage method for monitoring the destruction of porous materials and structural elements subjected to extreme thermal-mechanical loading. The studies were carried out in two main directions and included the analysis of the anisotropy of the elastic properties, porosity and damage of cores from oil-bearing rocks and modeling the process of fracture of single-crystal heat-resistant alloys under conditions of high-temperature creep. Also, the development of the acoustic damage method to assessment of the stress field, residual plastic deformations and damage to structural materials which are widespread in industry, including steel and aluminum rolled products, was continued. The physical-mechanical properties and porosity of cores from sandstone extracted from an 4 km deep oilwell were diagnosed. The results obtained indicate the possibility of obtaining a quantitative, rather than only a qualitative, estimate of anisotropy of the mechanical properties of oil-containing geological materials using the acoustic damage method. This is of great importance for the exploration of oil fields, since it allows us to predict the result of drilling and hydraulic fracturing. Experimental studies of the spatial and energy distribution of the hydrogen concentration and the acoustic anisotropy in single-crystal samples of heat-resistant nickel-based alloys destroyed during high-temperature creep tests were carried out. The analysis of the results made it possible to separate the influence of crystallographic orientation from the effect of creep on the value of acoustic anisotropy, which can be used to estimate the resource, durability, and reliability of blades and guide vanes of the gas turbines. The nature of evolution of the angular diagrams of acoustic anisotropy was studied under uniaxial elastic-plastic tension of structural elements of rolled metal in the deformation range up to the limit of temporary resistance. As a result of the research, a technology was proposed for assessing the anisotropic nature of the propagation of transverse ultrasonic waves in different directions of polarization, which can improve the accuracy of the standard methodology for assessing the stress-strain state of metal structures using acoustic anisotropy. A study was made of the spatial and energy distribution of the concentrations of dissolved hydrogen in steel samples after monotonous tensile tests up to the limit of temporary resistance. The recommendations for modifying the surface of the samples proposed as part of the work made it possible to improve their mechanical characteristics and increase their bearing capacity. A finite element simulation of the processes of direct and inverse piezoelectric effect was realized by excitation and recording of transverse waves by a piezoelectric sensor in a material with defects filled with gaseous hydrogen. The results of electromechanical analysis underlie the future model of the digital twin of the acoustic anisotropy analyzer used to study the hydrogen embrittlement of metals. An analytical approach was developed for the propagation of high-frequency waves in essentially heterogeneous media with a random distribution of elastic and mass characteristics. It allowed one to overcome the shortcomings of previously developed statistical approaches by introducing new independent and dependent dynamic variables. The studies carried out yielded the following main scientific results: • The ultrasonic measurements in different directions of the orientation of a system of orthogonally polarized shear waves made it possible to analyze the anisotropic nature of the overall measure of damage in various core sections from oil-saturated sandstone and to construct angular diagrams of acoustic anisotropy and the velocity field of ultrasonic waves within the core. • A mathematical model was developed for calculating acoustic anisotropy in a single-crystal material with cubic syngony after damage accumulation due to creep under uniaxial and multiaxial stress state, verified experimentally for crystallographic directions [100] and [101]. • A new effect has been experimentally discovered that consists in the crucial influence of crystallographic orientation on the acoustic anisotropy of single-crystal samples, which is more than an order of magnitude greater than the contribution of creep to acoustic anisotropy, which is consistent with theoretical estimates obtained in the first year of the project. • Three basic energy states of hydrogen and its critical concentrations in single-crystal samples were determined after thermomechanical tests by analyzing the experimental dependences of the hydrogen flux on the extraction temperature and the results of numerical simulation of the process of multi-channel hydrogen diffusion. • Correlation dependences between acoustic anisotropy and damage tensor measures were obtained for various symmetrization schemes of the effective stress tensor, and relations were proposed for predicting the value of acoustic anisotropy based on the results of calculation of the main damage based on experimental values of elastic wave velocities. • The dependences of propagation time and propagation velocity of transverse waves in a medium with a mesh of microcracks filled with hydrogen were obtained by means of direct numerical simulation and the approach based on the homogenization of the damaged layer in a material with a system of surface cracks. • Analytical expressions are obtained for the spatial decay coefficient of a high-frequency wave propagating in a medium with random elastic and mass characteristics for the case of an essentially heterogeneous elastic medium.

 

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