Mathematical modelling of elastoplastic deformation and accumulation of material damage under proportional and non-proportional, isothermal and non-isothermal cyclic loading is considered. On the basis of the version of the theory of plasticity (which is a particular version of the theory of inelasticity and class one-dimensional flow theories) with combined hardening. The range of applicability of the version of the theory of plasticity is limited to small deformations of initially isotropic metals at temperatures when there are no phase transitions and strain rates, when dynamic and rheological effects can be neglected. A set of material functions closing the version of the theory of plasticity is given, and a basic experiment, based on the results of which material functions are determined. On the basis of the integration of the evolution equation for microstresses (deviator of the displacement of the loading surface center) with a rigid symmetrical cyclic loading with a constant swing of plastic deformation under conditions of the uniaxial stress state, the equation of the curve of low-cycle fatigue (from up to cycles) is obtained. To determine the parameters entering the equation of the curve of low-cycle fatigue, the results of the basic experiment are sufficient. For SS304 stainless steel, a low-cycle fatigue curve is constructed, which is compared with the experimental data in the range from up to cycles to failure. Also for SS304 stainless steel, processes of non-linear summation of damages are analyzed for two-block rigid cyclic loads. It is shown that the transition from a larger swing of deformations to a smaller one significantly reduces the overall durability. The results of the calculations are compared with the results of the experiments. Further, the fatigue of SS304 stainless steel is considered under proportional and disproportional hard cyclic loads ranging from up to cycles to failure. It is shown that the greatest damaging effect belongs to loading along the trajectory of deformations in the form of a circle. The calculated fatigue curves for different deformation trajectories are compared with the results of the experiments. Predicting the structural material resource under nonisothermal cyclic loading is carried out on the basis of the durability analysis of the edge of the combustion chamber of the diesel piston and the uncooled conical nozzle of the LPRE nozzle under heat changes. In the first case, a uniaxial stress state (simple non-isothermal loading) is realized at the edge of the combustion chamber, and in the second case a biaxial stress state (complex non-isothermal loading) is realized too. We predicted the resource on the basis of the kinetic equation of damage accumulation, included in the equation of the theory of plasticity, and also on the basis of the deformation-kinetic criterion of low-cycle fatigue. Estimation of the resource on the basis of the deformation-kinetic criterion gives overestimated results (by a factor of 5-6) in comparison with the results obtained on the basis of the kinetic equation of plasticity theory and the corresponding experimental results with a good agreement of the latter.

The article is devoted to an experimental study of the mechanisms of damage accumulation in functional ceramic coatings by using the acoustic emission technique. The study and description of the damage evolution in a functional coating makes it possible to determine the destruction moment of this material under operating conditions. Mechanical tensile tests were conducted on steel samples and steel samples coated with a ceramic coating on the universal electromechanical system Instron 5989 at room temperature. A continuous recording of acoustic emission signals was conducted using the AMSY-6 system and AE144A broadband sensors with a frequency range of 100-500 kHz. As an additional method of fixing the moment of destruction of a ceramic coating a system was used to register non-uniform fields of displacements and deformations based on the method of correlation of digital images. During the tests the acoustic emission system, the optical system, and the testing machine were synchronized. The recorded acoustic emission signals were filtered by a threshold value and were filtered using a band-pass digital filter. The frequencies of the spectral maximum are extracted using the fast Fourier transform. Diagrams of dependences of acoustic emission parameters (peak amplitudes, frequencies of the spectral maximum, energy parameter) versus time for all the samples were plotted. A comparison of the obtained graphs for the samples with and without coating is made. The ranges of amplitudes and frequencies that characterize the damage accumulation and destruction of the ceramic coating were determined. Thus, during these works, a technique was implemented to evaluate the behavior of the functional coating applied to the material. The effectiveness of applying the method of recording and analyzing acoustic emission signals to solve this problem was shown.

The work is devoted to studying the behavior of cylindrical bodies of structural steels in the conditions of joint tension and torsion under complex loading. The study is aimed at studying and subsequent modernization of the method of increasing the fatigue life of cylindrical products. It consists in creating the product favorable axial compressive residual stresses in the near-surface area due to the successive elastoplastic deformation, first by tension, and then, during fixation of the longitudinal deformation obtained by tension, by torsion. A mathematical model of elastoplastic deformation by joint tension and torsion of a homogeneous cylindrical body, which allows to calculate the distribution of residual stresses created in the body, is constructed. To check the adequacy of the obtained solution and determine the required material parameters of the model, tests were performed on cylindrical samples of steel 15Cr2MnMoV. The necessary studies were carried out at the Center for Experimental Mechanics of Perm National Research Polytechnic University using the Instron 8850 universal two-axis servo-hydraulic test system, which allows for loading by joint tension and torsion. According to the results of the experiments, graphs of the longitudinal force and torque versus the twist angle were obtained with the deformation sequences studied. By comparing the experimental and calculated dependencies, the adequacy of the developed model was confirmed and the range of deformation modes was established, in which it reflects the behavior of the material with an accuracy acceptable for practice. Instead of the existing method of deformation, which includes a single torsion of a product in a state of tension, a new method is considered, consisting in reversional (alternating) torsion of a cylindrical body in a state of tension. Deformation by sequential tension and reversional torsion allows to provide a favorable (from the standpoint of increasing fatigue life) distribution of residual axial stresses over the cross section of the body with minimum values of residual shear stresses.

A number of problems related to the digital image correlation method from the standpoint of both the hardware and software development and testing, as well as solving problems in the field of fatigue fracture mechanics are described. Testing of the developed computer stereo vision system was carried out using a series of stereo pairs reflecting the change in the location of the object in space as well as plane and out-of-plane deformations. It is shown that the error in determining spatial coordinates does not exceed 0.75 units, while the error in computing the strain tensor components in case of a system with a single camera is two orders of magnitude larger than that at using the stereo machine vision system. An algorithm for automatic crack detection on optical images and calculating its tip coordinates was proposed and tested. It is shown that when the frame size is 2000×1000 pixels the coordinates of the crack tip might be determined with an average error of about 56 pixels, while the average error of the crack area determination does not exceed 1.93%. A modified incremental algorithm for calculating displacements on a series of stereo pairs is proposed, which allows one to estimate large magnitude displacements during serial processing of images. An algorithm for measuring the J-integral using the digital image correlation method has been developed. It is shown that the deviation of the calculated J-integral values from the model ones is on average 1.75 %. A quantitative characterization of the fatigue crack growth process in metal alloys was carried out using the technique based on the digital image correlation (in terms of fracture mechanics) including the calculation of the fatigue crack growth rate da / dN , maximum strain (εmax) and effective cycle asymmetry ( R eff).

The paper presents the experimentally implemented test program for very-high-cycle loading (number of cycles 107-109) samples produced from massive planar targets (aluminum alloy AMg6) and subjected to the plane wave loading method (explosive generator). Shock-wave loading modes provided a controlled damage to simulate structural changes in fan blade materials under conditions of a high-speed collision with solid particles. Very-high-cycle loading was performed using the ultrasonic testing machine Shimadzu USF-2000 that allows testing samples on the basis of 108-1010 cycles with an amplitude of up to several tens of micrometers and a frequency test of 20 kHz. A significant reduction by 34% of the fatigue strength on the basis of 109 cycles for the AMg6 alloy pre-loaded with a shock wave is shown. The technique of the "in situ" determination of fatigue damage is based on the analysis of the amplitude-frequency characteristics corresponding to the change in effective elastic properties. It allowed us to explore the damage development stages taking into account nonlinear kinetics of the defects accumulation in the process of cyclic loading in multi- and gigacycle fatigue modes. The anomalous change of elastic properties of the material at critical levels of damage is established. A quantitative correlation between mechanical properties and scale-invariant characteristics of the topography of the fracture surface are formed in the processes of dynamic loading and gigacycle according to profilometry (interferometer-Profiler New View 5010 with a resolution of 0.1 nm). For the samples subjected to the preliminary shock-wave deformation, a decrease in the Hurst index in comparison with the undeformed samples was established. The latter is associated with an intensive fragmentation in the formation of dislocation ensembles during shock wave loading, which complicates the formation of an ordered system of defects under the subsequent fatigue loading

We suggest the phenomenological mathematical model of the stress-strain state reconstruction in the surface hardened thin-walled tube with an inner diameter 45 mm and outer diameter 51.5 mm made of steel EI961 and treated by diamond smoothing of the outer surface. It is shown that if all stress components depend on radius only, then the components are in the cylindrical coordinate system. The experimental research is made for the samples which were softened under two load modes (radial force) of the diamond ball attachment of 200 and 300 N value. The experimental values of residual stresses , , and in the surface layer are obtained by the ring and strip method using the layer-by-layer electrochemical pickling of the hardened layer. The experimentally measured values of the strip beam deflection, split ring angular opening and axial displacement of cut edges relative to each other are used for this purpose. The hardening anisotropy parameter which relates the axial and circumferential components of plastic strain is included in the mathematical model. To solve the formulated problems we use the hypotheses of plastic incompressibility of the material, the absence of secondary plastic deformations of the material in the surface layer compression area and the hypotheses of flat sections and straight radii. We present the method for solving the stress-strain state reconstruction boundary value problems, which allows obtaining the missing component and all residual plastic strain components. The validation of the computational data obtained by mathematical modelling for adequacy to the experimental data for the two modes of hardening is made. There is a close agreement between the computational and experimental data. The numerical values for the hardening anisotropy parameter are given. By using this parameter we are able to theoretically describe the observable experimental layering of axial and circumferential stresses in depth of the hardened layer. It is theoretically and experimentally established that the absolute values of maximum shear stresses is an order of magnitude smaller than the absolute values of maximum normal stresses. We also discuss the questions of the effect of shear stresses on high-cycle fatigue and creep of the hardened thin-walled tubes. The main results of the research are illustrated by the tabular data and corresponding diagrams of the residual stresses distribution in depth of the hardened layer.

The work presents the results of an experimental research of changes in the residual mechanical properties of the samples of carbon-fiber composites of various structures under combined low-velocity impact and static loads. The main objectives of this study were to obtain new experimental data on the processes of deformation and fracture of carbon-plastic composite materials under shock impact and static loads, as well as the creation of methods analyzing the degree of damage and tolerance to damage after shock loads using different non-destructive testing. Eight types of reinforcement structures were selected for the research. The experimental part was carried out at PNRPU Experimental Mechanics Center using the CEAST 9350 drop-weight test systems and the electromechanical Instron 5982. The test method was based on the standards ASTM D7136 and D7137. After the impact, the depth of the dents and, using non-destructive testing by the sherography method, the size of the discontinuity of the samples were measured. Damage tolerance was assessed using the magnitude of damage after impact. An assessment of the prospects of using the entered values for the design and manufacture of products from carbon-fiber composite materials of various reinforcement structures is given. The calculation of the destruction as a result of the impact was made. The process of the low-velocity impact is described in the diagrams of the dependence of force on displacements at various energy levels. The test results are presented in the form of diagrams of the maximum breaking load under compression describing the change in residual mechanical properties after the impact. The features of using direct and indirect methods of assessing the performance characteristics of the damaged samples are noted. The effect of the impact on the nature of fracture and deformation properties of the samples of carbon-fiber composite materials is analyzed.

The paper presents the computational and experimental studies of the stress-strain state of the composite flexible joint during transformation of the rod structure. A flexible joint consists of two co-acting tape springs and acts as an actuator for the self-deployment of large-scale space structures. Flexible joints that have different reinforcement patterns and shapes of cut-outs have been studied. Full-scale tests of the joint were carried out using a tailored facility that ensures its complete folding and unfolding and records joint moments for each folding angle. The strain pattern for each angle was recorded using a system of photo and video recording based on DIC (Digital Image Correlation) - VIC 3D. To identify elastic properties of the flexible joint, the materials mechanical testing was performed and tensile and compressive Young modulus and ultimate strength were determined. To reduce the volume of full-scale tests, a micromechanical model of the material taking in account the properties of reinforcement fibers, resin and weaving pattern was simulated in Digimat. Verification of micromechanical model was performed based on tensile and compressive properties. Other mechanical properties of the material were determined via virtual testing in Digimat system. Finite element modelling of joint folding and deployment processes was carried out in Ansys Workbench and LS-Dyna. Calculations were carried out for various structures of the joint based on the explicit and implicit methods. Dynamic behavior, geometric nonlinearity, progressive failure and self-contact of joints surface were taken into account in the computational model. As a result, strain pattern and maximal joint moment were determined. The computational strain and joint moment have a good agreement with the experimental data. Based on the results of the study, a comprehensive computational and experimental method to determine rational properties of flexible joints for transformable composite structures is suggested.