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No 1 (2019)

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The Effect of Preliminary Plastic Hardening by Tension on the Cylindrical Shells Buckling during Torsion

Abstract

The results of the experimental and numerical analysis of the processes of elastoplastic deformation and loss of stability of tubular specimens from 12Kh18N10T steel under monotonic simple and complex kinematic loading by tensile-torsion are presented taking into account large deformations and non-uniformity of the stress-deformed state. The experimental studies were carried out on a modernized (in the standard version- only tension-compression plus alternating torsion) test complex Z100 ZWICK-ROEL, which allows performing tests for complex loading on tubular samples with simultaneous, synchronized setting of the parameters in the quasistatic loading range: the rate of the longitudinal force change (displacement), the rate of the torque change (angle of twist), the rate of the internal pressure change. Numerical modelling of the samples elastoplastic deformation and loss of stability was carried out in Lagrangian variables. Kinematic relations are formulated in speeds in the metric of the current state, which makes it possible to describe large form changes. The elastoplastic properties of the material are described by the flow theory with nonlinear isotropic hardening. The equation of motion is written in a fixed Cartesian coordinate system and follows from the equation of the balance of virtual powers. The system of equations, supplemented by kinematic boundary and initial conditions, is solved by the finite element method in combination with an explicit integration scheme of a cross type. There is a good agreement of the numerical results with the experimental ones on the residual shape of the samples and the integral characteristics of the deformation process (axial force from axial displacement and torque from the angle of twist). The analysis of the mutual influence of stretching and torsion on the loss of stability of the cylindrical shells has been carried out.

Problems of Endocrinology. 2021;(1):8-17
pages 8-17 views

On Predicting Material and Structural Resources under Cyclic Loading

Bondar V.S., Abashev D.R., Petrov V.K.

Abstract

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.

Problems of Endocrinology. 2021;(1):18-26
pages 18-26 views

Experimental study of postcritical deformation and failure of steels at high temperature

Wildemann V.E., Tretyakov M.P.

Abstract

The work is aimed to studying the strength and survivability of structures on the base of perceptions about the kinetic nature of the destruction of materials. An experimental study of the regularities and effects of the mechanical behavior of structural materials at the stages of elastoplastic and postcritical deformation at elevated temperatures has been carried out. Tests on uniaxial tension of specimens of the heat-resistant high-alloyed corrosion-resistant steel of martensitic class X15CrNi12-2 and the structural alloyed steel of pearlite class 40Cr were implemented in a wide range of temperatures: 22 °C, 200 °C, 400 °C, 500 °C and 600 °C. The methodological issues of testing on the universal servohydraulic test system Instron 8801 with using of the furnace and a high-temperature extensometer are considered. Based on the obtained experimental data, deformation diagrams with developed stages of softening were constructed, and the influence of the strain rate on the postcritical behavior of steel 40Cr was evaluated. Comparison of the main strength and deformation characteristics of the considered steels at different temperatures is carried out. Images of fracture surfaces of samples after fracture at high temperatures, which are characterized by different morphology, were obtained. On the basis of experimental data about the mechanical characteristics of steels, diagrams of the petal type have been constructed, which allow to a realize the complex analysis of properties at various temperatures. It is shown that with increasing the temperature of the test, the decreasing of the strength characteristics, associated with the provision of the static strength of the part, is take place. This is followed the increasing of the deformation resources (residual elongation and relative narrowing after rupture) and the coefficient of the postcritical deformation stage, which has a positive effect on survivability during destruction process.
Problems of Endocrinology. 2021;(1):27-38
pages 27-38 views

Application of the Acoustic Emission Technique to Studying the Damage Accumulation in a Functional Ceramic Coating

Zubova E.M., Lobanov D.S., Strungar E.M., Wildemann V.E., Lyamin Y.B.

Abstract

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.
Problems of Endocrinology. 2021;(1):39-49
pages 39-49 views

Autowave mechanics of metal plasticity

Zuev L.B., Barannikova S.A.

Abstract

The model is proposed for the development of a localized plastic flow in solids. It is based on the representation of an interaction between plasticity carriers and acoustic emission signals in the course of elementary plasticity acts. It is shown that the plastic flow localization is a common feature of all deformation processes and can be observed throughout the process. Specific localization pictures, i.e. patterns of the localized plastic flow, are autowaves that are generated in a deformed medium by ordering its defect structure. Localized plasticity patterns correlate with the work hardening stages revealed in mechanical tests. A two-component model of the development of the localized deformation was proposed and analyzed to take into account the interaction of the elastic and plastic deformation components. This model describes the generation of localized plasticity autowaves in a number of materials. General regularities of the development of the localized plastic flow at different stages of work hardening are found. An elastic-plastic strain invariant is introduced to relate the elastic and plastic properties of materials. It is shown that basic properties of deformed media result from this invariant. Among consequences of the elastic-plastic strain invariant are the dependence of the velocity of the localized deformation of autowaves on the work hardening coefficient, the dispersion equation for autowaves, the scale effect of deformation localization, the dependence of autowave parameters on the grain size, the equation of the localized deformation autowave, the work hardening coefficient, the Taylor-Orowan equation of dislocation dynamics, the Hall-Petch relation, and the dependence of the mobile dislocation density on the strain.
Problems of Endocrinology. 2021;(1):50-64
pages 50-64 views

Investigation of the behavior of cylindrical bodies under conditions of joint tension and torsion under disproportionate loading

Kryukov A.A.

Abstract

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.
Problems of Endocrinology. 2021;(1):65-77
pages 65-77 views

Mechanical behavior of X15CrNi12-2 structural steel under biaxial low-cycle fatigue at normal and elevated temperatures

Lomakin E.V., Tretyakov M.P., Ilinykh A.V., Lykova A.V.

Abstract

The results of the experimental studies of the low-cycle fatigue characteristics of heat-resistant structural X15CrNi12-2 steel for aircraft purposes (chemistry: C - 0.13%; Cr - 12.5%; Si - 0.05%; Ni - 2.05%; Mo - 1.50%, W - 0.70%; Nb - 0.20%; V - 0.20%) under biaxial cyclic loading are presented. For cyclic tests a specialized Instron 8850 two-axes testing system was used which allows the planning of cyclic and static tests with an arbitrary stress sequence under the conditions of tension and torsion. The Epsilon 3550-010M dual-axis dynamic strain sensors for testing at normal temperatures and the Epsilon 3550HT-025M for testing at high temperatures were used to determine the values of axial and shear strains during the experiments. The test methods for biaxial cyclic loading under normal and elevated temperatures are described which allows to analyze the mechanical behavior and structural steel destruction processes under plane stress conditions. The tests results of X15CrNi12-2 heat-resistant alloy under low-cycle fatigue at different temperatures and cyclic strain paths with proportional and non-proportional changes in axial and shear deformations are presented. For different types of tests hysteresis loops are represented in the form of dependences of normal and shear stresses on axial and shear deformations, respectively. It is shown that the durability of X15CrNi12-2 steel in these parameters significantly depends on the cyclic strain path, the shape of the cycle and the test temperature. In the case of non-proportional deformation, the fatigue life of X15CrNi12-2 steel decreases 1.5-2 times as compared with the proportional loading at different test temperatures. Depending on the strain path, a significant decrease in fatigue life at a temperature of 600 ° C by 17-44% in comparison with the room temperature was observed.
Problems of Endocrinology. 2021;(1):78-87
pages 78-87 views

Development of the Digital Image Correlation Method to Study Deformation and Fracture Processes of Structural Materials

Lyubutin P.S., Panin S.V., Titkov V.V., Eremin A.V., Sunder R.

Abstract

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).
Problems of Endocrinology. 2021;(1):88-109
pages 88-109 views

Identification of elastic parameters of composite using experimental data on modal characteristics of samples

Nikhamkin M.S., Solomonov D.G., Silberschmidt V.V.

Abstract

In order to determine the elastic parameters of polymer composites, a mixed numerical-experimental method of identification was developed in the 1990s - 2000s, based on the use of experimental data on natural frequencies and eigenmodes of oscillation of samples. Its practical application involves the choice of a shape and a size of samples, a set used to identify the natural frequencies and eigenmodes of vibration, a methods of their experimental determination, a finite-element model for modal analysis, and an algorithm for solving the identification problem.The object of the research is laminated polymer composite materials reinforced with carbon or glass fibers. The aim of the work is to develop practical aspects of implementation and assessment of the accuracy of the mixed experimental-calculation method for identifying of the elastic parameters of such materials based on experimental data on natural frequencies and eigenmodes of vibration of standard samples. Parameter identification for the material is considered as an optimization problem with an optimization function, which is a weighted sum of squares of differences between the experimental and calculated values of natural frequencies. A procedure was developed to implementing the main steps of the suggested technique: experiments, calculations and control of results. An error analysis of main steps was carried out, and the accuracy of the determined parameters of the ply was estimated. To determine the natural frequencies and eigenmodes of oscillation of the samples, the method of three-component scanning laser vibrometry was used. The experimental technique was established and parameters of the technique were chosen to ensure the necessary accuracy in determining of the natural frequencies. The parameters of the identification procedure and the finite-element model of the sample were selected. To control the obtained values of the elastic parameters, the natural frequencies of the samples were calculated, including those not used in the identification procedure. The error assessment of the in determined elastic parameters was performed on three different series of samples of carbon-fiber-reinforced laminates with the same material of the plies and different ply numbers and stacking orders. The developed technique can be recommended to determine the parameters of material models required for strength and vibrations assessment of products manufactured with layered composites.
Problems of Endocrinology. 2021;(1):110-122
pages 110-122 views

Lifetime of AMg6 Alloy under Consecutive Shock-Wave and Gigacycle Loading

Oborin V.A., Bannikov M.V., Bayandin Y.V., Naimark O.B.

Abstract

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
Problems of Endocrinology. 2021;(1):123-130
pages 123-130 views

A System Development for Monitoring Kinetic Parameters of a Phase Transition in a Fluid-Saturated Soil Based on Fiber Optic Sensors

Prokhorov A.E., Plekhov O.A.

Abstract

The use of fiber-optic sensors (FBG) is a promising direction of developing various science and technology fields. Bragg grating has a number of advantages over standard sensors of deformation, stress, temperature, angles of inclination. In the mining industry, FBG are typically used to monitor temperatures during the construction of an ice barrier, to analyze the occurrence of bends in the sinking of mine shafts or when driving piles. As it is known, the process of thawing and freezing of porous media is accompanied by the effects of mass and heat transfer, the formation of cryogenic flows, changes in the stress-strain state of a medium. Therefore, it is important to develop a laboratory fiber-optic monitoring system to record the temperature and deformation characteristics of the phase transition in a porous medium. In this paper we develop a system analyzing the stress-strain state of porous soils saturated with moisture in the process of occurrence and movement of the phase transition between water and ice. For the formation of a sample with the fiber-optic system placed therein, we used a designed plastic form is used which had been 3D printed. For the analysis of the kinetic parameters of the phase transition and the stress-strain state of the soil, the optical-fiber temperature and strain sensors are used. The paper deals with the compensation methods of temperature deformation on the sensors of mechanical deformation of the rock and the deformation effect on the temperature sensors. Methods of creating a system with a linearly advancing phase transition for the analysis of the kinetic features of the water-ice/ice-water system are considered. The features of the phase transition in a porous water-saturated medium are investigated.
Problems of Endocrinology. 2021;(1):131-139
pages 131-139 views

Mathematical modeling of the stress-strain state in surface hardened thin-walled tubes with regard to the residual shear stresses

Radchenko V.P., Pavlov V.P., Saushkin M.N.

Abstract

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.
Problems of Endocrinology. 2021;(1):140-152
pages 140-152 views

The Influence of Stress Concentrators on the Magnesium Alloy Mechanical Behavior under Deformation at High Strain Rates in the Temperature Range from 295 to 673 K

Skripnyak V.A., Skripnyak V.V., Kozulin A.A., Iohim K.V.

Abstract

The paper presents the experimental results of the mechanical behavior of Mg-3% Al-1% Zn alloy at high strain rates at room and elevated temperatures. The flat samples with smooth working parts and notches with a radius of 10 mm, 5 mm and 2.5 mm were used. The experimental studies were carried out using the high-velocity servo hydraulic test machine Instron VHS 40 / 50-20. The samples were heated with flat ceramic infrared emitters on average from 60 seconds to 160 seconds. The temperature control in the working part of the samples was carried out in real time using a chromel-alumel thermocouple. Data on the stress triaxiality effect on the plastic flow stress and the strain to fracture of the magnesium alloy under tension with strain rates of 100 and 1000 1 / s at temperatures of 295 K, 473 K and 673 K were obtained. The stress triaxiality factor was varied within the range from 0.33 to 0.5. It was found that the value of tensile strain to fracture of the magnesium alloy decreased twice when the stress triaxiality factor increased from 0.33 to 0.5. This effect is realized in a wide range of strain rates and homologous temperatures T/Tm from 0.32 to 0.73 (Tm= 923 K is the melting point of Mg-3% Al - 1% Zn magnesium alloy). The obtained data were used for calibration of plastic deformation and fracture models which are used in computer-aided design of products from MA2-1magnesium alloy.
Problems of Endocrinology. 2021;(1):153-162
pages 153-162 views

Evaluation of the Damage Degree to Carbon-Fiber Composite Materials under Impact

Staroverov O.A., Babushkin A.V., Gorbunov S.M.

Abstract

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.
Problems of Endocrinology. 2021;(1):163-174
pages 163-174 views

Comprehensive analysis of mechanical behavior and fracture processes of specimens of three-dimensional reinforced carbon fiber in tensile tests

Tretyakova T.V., Dushko A.N., Strungar E.M., Zubova E.M., Lobanov D.S.

Abstract

The aim of the work is to develop a procedure of an experimental study related to the inelastic behavior and failure process of a 3D reinforced composite material taking into account the influence of interweaving schemes based on a combined use of optical methods analyzing strain and temperature fields, as well as a method for recording acoustic emission signals. Uniaxial tension tests were conducted for six groups of specimens which preforms were made with the 3D weaving technology in six different ways of weaving. It is noted that the CFRP-samples with the orthogonal and orthogonal conjoint weave scheme are characterized by high values of maximum load compared to the samples with the interlayer reinforcement and layered samples. The choice of the optimal parameters (subset and step) of the correlation processing of digital images in the study of the PEP samples is illustrated taking into account the structural features of the material. We carried out the analysis of changes in the cumulative energy of AE-signals obtained by summing the values of the energy parameter and reflecting the intensity of the accumulation of defects in the material during loading. It was found that the samples with the orthogonal, orthogonal-combined weave scheme, as well as with pairwise interlayer reinforcement, are characterized by a low damage accumulation rate in the material, the process of defect initiation and propagation proceed uniformly. The samples with the pairwise interlayer combined reinforcement with through interlayer reinforcement and layered samples are characterized by the intensity of the crack formation in the material during loading. The results of the analysis and quantitative comparison of parameters (maximum load, ultimate elongation of samples, intensity of local heating of the material at the time of macrodestruction, the maximum value of the cumulative energy achieved at the time of failure, the number of recorded emissions of the AE signal) were obtained for groups of the samples with different weave patterns. It is shown that the multiparameter analysis of the experimental data allows selection of a composite optimal properties during its development in accordance with the required operating conditions.
Problems of Endocrinology. 2021;(1):175-185
pages 175-185 views

The Research of the Mechanical Behavior of Elastically Transformable Composite Structures

Khaliulin V.I., Batrakov V.V., Shabalin L.P., Kiauka M.Y., Bezzametnov O.N.

Abstract

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.
Problems of Endocrinology. 2021;(1):186-197
pages 186-197 views

New Paradigms in Metals Fatigue Description

Shanyavskiy A.A., Soldatenkov A.P.

Abstract

Present processes of damages accumulation in metals under regular cyclic loads were analyzed. Based on the concept of physical mechanics, a sequence of damage accumulation mechanisms was considered in accordance with the stress level increasing. It was shown that the evolution of metals behavior takes place in the direction from micro-, to meso-, and then macroscale levels in accordance with the bifurcation diagram under consideration. It was explained why metals mechanical characteristic called fatigue limit cannot be used for simulation of structures durability and in-service life-time. The problem of the bimodal durability distribution for different kinds of metals was discussed when the bifurcation transition takes place from one scale level to another one. It was shown that in the bifurcation region metals can experience a constant stress level but its reaction appeared in two ways with a different probability because the difference in damage accumulation belonged to the scale above and below the bifurcation region. Mechanisms of the subsurface fatigue cracking in a very-high-cycle-fatigue regime were reviewed. It was demonstrated that the dominant process in damages accumulation under the metal surface and appearance of the subsurface cracking origin is related to sliding deformation and material torsion during material uploading. Test data for fatigue limit determination of aviation structural materials in accordance with the standard were reviewed. The influence of mechanical characteristics of the fatigue limit value was analyzed. It was demonstrated that the major part of the material realized all three scale levels during the stress level increasing from one unit to another. The realization of the low-cycle-fatigue is not the computationally recommended case for the operated complex structures when the mesoscale of metal fatigue does not exist.
Problems of Endocrinology. 2021;(1):198-209
pages 198-209 views

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