Multicycle Fatigue Research Articles

Чисельно-експериментальне дослідження високотемпературної багатоциклової втомної міцності монокристалічних лопаток газових турбін

The subject of the research is anisotropic single-crystal turbine blades of aircraft gas turbine engines (AGTE), as well as their multi-cycle fatigue strength characteristics at high temperatures. The goal of this study was to determine the endurance limit of single-crystal high-pressure turbine blades of an AGTE at high temperatures of 820°C. The objective of the study is to develop a method for calculating and experimentally investigating the characteristics of high-temperature multi-cycle fatigue strength of gas turbine blades made of single-crystal nickel heat-resistant alloys. The research methods used are: method of computer modelling, which was used to create a 3D model of a cooled high-pressure turbine blade; finite element method, which was used to determine the stress-strain state of the blade according to the selected form of resonant vibrations; strain and resonance methods, which were used to determine the fatigue strength characteristics of single-crystal turbine blades under high-temperature loading. The following results were obtained. Literary sources describing theoretical and experimental methods for determining the strength characteristics of blades under combined dynamic and high-temperature loading were analyzed. The developed special installation for testing turbine blades under conditions of temperature and dynamic (resonant) loading was described. A combined method for assessing the multi-cycle fatigue strength at high temperatures is proposed, which includes studies on full-scale turbine blades and finite element calculations. The dynamic and temperature characterization of single-crystal turbine blades selected for the study was carried out and described. Fatigue tests were performed to determine the endurance limit of the blades at a high temperature equal to T = 820 °C. Conclusions. The scientific novelty of the obtained results is as follows. A specialized experimental setup was developed to study the endurance limit of single-crystal AGTE blades under high-temperature loading. A method for combining numerical (ANSYS) and experimental (field tests) study of high-temperature multi-cycle fatigue strength of turbine blades made of single-crystal heat-resistant alloys was developed. The direction of further research is presented.

МЕТОДИКА АНАЛИЗА УСТАЛОСТНОЙ ДОЛГОВЕЧНОСТИ НЕСУЩЕЙ РАМЫ СПЕЦИАЛИЗИРОВАННОГО ВАГОНА ДЛЯ ПЕРЕВОЗКИ ШЛАКА

A technique to assess the strength and fatigue life of the welded load-bearing frame structure of a specialized slag carriage is proposed. This method provides for taking into account the specific operating modes of the carriage, which differ significantly from the operating conditions of mainline freight cars. For this car type, the fatigue life is determined by the modes associated with loading and unloading operations, as well as temperature loads acting on the bearing structure of the frame. The proposed method is based on mathematical and computer modeling of the dynamic loading of a car in operation. Dynamic loads are found on the basis of the developed solid car model. The structural strength is assessed using the finite element method. The fatigue life of the structure is estimated within the framework of a hybrid model that includes elements of low- and multi-cycle fatigue. The approbation of the technique is carried out using the example of a specialized car for transporting 16-VP slag. The results obtained confirm the operability of the studied car structure and correspond well with the results of field tests and analysis of bearing structures in operation.

Comparison of applications results of two speckle methods for study multi-cycle fatigue of structural steel

Comparison of applications results of two speckle methods for study multi-cycle fatigue of structural steel

Scientific literature reviews; Investigating analysis of turbine blade operational failure

As a source of energy in industry and power plants, turbines must have reliability in operation.. One that affects the performance of the turnin is the condition of the blades. The multi-cycle fatigue experienced by the blades of compressors and turbines caused by dynamic stresses caused by the vibration and resonance of the blades in the engine operating time is a common type of failure. Over the past five decades, researchers have studied and investigated turbine blade failure. The main purpose of this article is to provide a brief overview of recent studies on turbine blade failure. The author only focuses on describing some of the failure methodologies used in various studies to investigate turbine blades. This article discusses each of these factors in more detail

A method for studying the frequency stability of materials during tests for multi-cycle fatigue of steel

For trouble-free operation without loss of elastic and inelastic properties of particularly critical elements of electrical-to-mechanical vibration converters during a long period of cyclic operation, it is necessary, in addition to studying the fatigue characteristics of materials used for their manufacture, to study these alloys for frequency stability, since minor deviations in the frequency of natural oscillations lead to unacceptable errors in the operation of such high-precision products. To carry out such studies, we developed and constructed an original installation, in which sinusoidal loading is carried out according to the “soft” scheme of flat samples cantilever bending operating in self-oscillation mode. The frequency of cyclic loading in this installation is generated by current pulses, which are a response to the frequency of the test sample natural oscillations converted using electronics. As a result, frequency equality is achieved in the test process. An algorithm for calculating stresses depending on the loading amplitude of steel samples of different geometric shapes was developed. It is shown that the stress on the sample calculated by the deformation amplitude in all cases is 8 – 10 % higher than the stress calculated by the force, regardless of the shape of the proposed samples. To verify the proposed research method, martensitic-aging steel was tested at loads close to the fatigue limit, since frequency stability in this range is of great interest. We obtained the frequency characteristics in the multi-cycle test area. It was determined that with an operating time of 50 million loading cycles, the frequency change was 0.75 Hz. The dynamics of frequency stability was revealed: the frequency changed most intensively during the first 10 million loading cycles, during this time the frequency changed by 0.54 Hz.

Evolution of the Fracture Surface of Commercially Pure VT1-0 Titanium Subjected to Multicycle Fatigue in a Constant Magnetic Field

Evolution of the Fracture Surface of Commercially Pure VT1-0 Titanium Subjected to Multicycle Fatigue in a Constant Magnetic Field

Щодо розрахунку втомної довговічності призматичних стержнів за умов одновісного асиметричного згину

An alternative approach to solving the fatigue life computing problems for material samples and structural elements under uniaxial asymmetric cyclic loading has been considered. This approach is based on the concept of equivalent stresses, which allows the asymmetric load cycle to be reduced to the equivalent symmetric cycle for the number of fracture cycles. The structure of equivalent stresses is obtained based on the hypothesis of the existence of single isochronous ultimate stress diagrams, which are invariant with respect to the number of cycles to fracture. As basic experiments, to determine the material constants in the original equations, easily reproducible standard tests of smooth samples of the material for short-term strength and fatigue under symmetric and asymmetric bending load cycles are used. Problems of multicycle fatigue analysis of prismatic rods under uniaxial asymmetric bending, considering the influence of mean stress, stress range, and amplitude coefficient, have been solved. Satisfactory agreement of calculation results with experimental data was obtained.

Influence of Constant Magnetic Field upon Fatigue Life of Commercially Pure Titanium.

Cyclic tests of the multicycle fatigue of commercially pure titanium were performed under normal conditions (without a magnetic field) and after exposure to a constant magnetic field of varying density (B = 0.3, 0.4, 0.5 T). It was shown that the application of the constant magnetic field of varying density led to a fold increase in the average number of cycles to destruction of the VT1-0 titanium samples by 64, 123, and 163%, respectively. Scanning electron microscopy revealed that the magnetic field led to a 1.45-fold increase in the critical length of the fracture (the width of the fatigue crack growth zone) and a 1.6-fold decrease in the distance between the fatigue striations in the accelerated crack growth zone of the destroyed titanium samples. It was established that a subgrain (fragmented) structure formed in the area of the fatigue growth of the fracture of the titanium samples. The size of the subgrains corresponded to the spaces between the fatigue striations, which had an inhibitory influence on the microcrack propagation. Collectively, the revealed facts are indicative of a higher material resistance to fatigue fracture propagation and increased operation resources under the fatigue tests in the magnetic field, which correlates with the data on the growth of the average number of cycles to fracture of the VT1-0 titanium samples.

Multi-Cycle Fatigue of Composite Three-Layer Plates with Honeycomb Structure Made by Additive FDM Technologies

The multi-cycle fatigue of three-layer plates with honeycomb structure, which was manufactured using additive FDM technologies from polylactide, is considered. Carbon fiber based on the SIGRAPREG C U200-0/NF-E310/30% pre-preg is chosen as a material for the upper and lower covers. Fatigue analysis of three-layer plates with honeycomb structure is based on their vibration tests. To study the fatigue characteristics of honeycomb structures, special samples were made. Fatigue characteristics of structures are studied on three-layer samples using carbon-plastic sheathings. The experiment was accompanied by finite element simulation of fatigue tests in the ANSYS software complex. The fatigue properties of three-layer plates are studied.

Study of the Stress-Strain State of the Surface Layer During the Strengthening Treatment of Parts

The paper presents experimental studies and obtained statistical models of the influence of processing modes on the quality of the surface layer and cyclic durability of reinforced machine parts. It was established that the main influence on the strengthening effect, the depletion of the plasticity reserve of the metal and the formation of residual stresses are exerted by the effective gap of the cutter, the effective tension and the profile radius of the roller. The results showed that dimensional combined running-in ensures high cyclic durability of strengthened parts under conditions of multi-cycle fatigue load, which reaches 8 million cycles, which is 3.5 times greater than the durability of a non-reinforced part and 1.5 times - the durability of a part strengthened by surface plastic deformation Studies have shown that the greatest cyclic durability of the part is ensured at the minimum values of the effective cutter gap, the maximum values of the profile radius and the value of the effective roller tension of 0.6 mm, which corresponds to the degree of exhaustion of the plasticity reserve by processing with dimensional compatible rolling Ψ ≈ 0.65. On the basis of the results of theoretical and experimental studies, a methodology and algorithm for the design of processing technology by dimensional combined running-in were developed. A computer program has been developed that allows you to calculate the quality of the surface layer and cyclic durability of the part based on the specified modes, as well as to assign rational processing modes that ensure the specified quality of the surface layer and cyclic durability. Based on the results of research, a technological process of strengthening processing of machine parts by combined dimensional running-in has been developed, which allows forming compressive residual stresses in the surface layer of the processed part, as well as increasing productivity up to 2 times while maintaining or improving the specified quality parameters of the surface layer of the processed part.

AN INFLUENCE OF ROLLER STRENGTHENING ON MULTCYCLIC FATIGUE OF SPECIMENS WITH CUTS OF VARIOUS DEPTH AND A PRESSURIZED HUB

The influence of roller strengthening on a residual stresses distribution and multicyclic fatigue of solid cylindrical specimens of 50 mm diameter made of steel 20 with cuts of various depths and a pressurized hub has been examined. As a result, an endurance limit of roller strengthened specimens diminishes. In order to preserve the effect of hardening with a cuts depth increase, its necessary to increase the thickness of a layer with compressive residual stresses. Surface hardening influence on an endurance limit of specimens has been estimated by two criteria: axial residual stresses on a concentrators surface and average integral residual stresses, calculated through the dangerous sections surface layer thickness that equal a critical depth of a non-propagating fatigue crack. Its been stated that its more reasonable to use the average integral residual stresses criterion for an evaluation of roller strengthening influence on an endurance limit of specimens with cuts of various depth and a pressurized hub.

Minimization of dies volume using uniform distribution of hollows in the conditions of multi-cycle fatigue dependence on temperature

The research is devoted to development of a model of dies of optimized shape used in the field of metal forming. Use of solid-cast metal dies at the present stage of production requires large material resources, which makes it actual to develop dies models with minimal volume of material due to effective distribution of hollows in correspondence with the required strength resource. The dies mainly experience compressive non-uniform loading in the conditions of drawing and forming, which can lead to the accumulation of damages and lowering of its strength resource in the conditions of long-term operation of a die at high temperatures; therefore, the optimized area is represented in this work by alternating rod elements and hollows. The paper considers the problem of optimizing the volume of a metal die, presents a model for minimizing the die volume with restrictions on multi-cycle fatigue at various temperature conditions. The dependence between the volume of the optimized area and the number of loading cycles is obtained, taking into account the non-uniform loading of the die for axisymmetric forming. The main approach to reducing the die volume due to distribution of hollows was concluded in increase the stress state of the rod elements experiencing compressive stress to the limit values in accordance with the restriction on multi-cycle fatigue. The number of elements providing the required die stress state is calculated for different temperature conditions and preset geometrical parameters of the rod elements. When calculating the configuration of the optimized die area for axisymmetric forming, experimental curves of dependence between stress state and number of cycles for 12Kh1MF1 steel were considered at the constant temperature. The variants of the material distribution in the internal die area at preset temperatures are presented. Reduction of the optimized die volume was compared with a solid cast die. The constructed mathematical model and the results of calculation of the optimized die volume will significantly reduce the material costs for production of metal forming dies and can be used for further development of the methods for topological optimization of stamping tools, taking into account the features and duration of power and thermal loading. The research was carried out under financial support of the Russian Foundation for Basic Research within the framework of the scientific project No. 19-38-60020\19 “Development of optimization model for dies of stamping accessories via the method of efficient material redistribution”.

Functionally oriented high-temperature composite materials

The authors developed technology for obtaining surface composite materials. This technology includes high-energy mechanical treatment, high-velocity oxygen fuel spraying in a protective atmosphere and subsequent thermomechanical and thermal treatment of ZrCuNiCoTi and cBNNi3AlSiCCoY layers in a protective atmosphere. The processing allowed increasing the adhesive strength of the surface composites, reducing their porosity and improving their functional and operational properties. Staged methods of heat treatment and plastic deformation of surface layers have been developed. These methods stabilize the material structure while reducing residual stresses. On the basis of complex X-ray diffraction and electron microscopic studies, the structural parameters of surface composites were determined. It was shown that the ZrCuNiCoTi alloy is in the austenitic–martensitic state and has a nanocrystalline structure with a grain size of 80–120 nm. Meanwhile, the cBNNi3AlSiCCoY alloy consists of many intermetallic phases and inclusions and has a nanosized structure with a grain size of 100–200 nm. A microhardness study of the surface layers in ZrCuNiCoTi–cBNNi3AlSiCCoY composite showed that thermomechanical treatment increases microhardness. The experimental data were statistically processed. As a result, empirical mathematical dependences of the stress amplitude on cyclic durability were compiled. Mechanical tests included tests of NiCoTiZrHf–cBNCoMo, ZrCuNiCoTi–cBNNi3AlSiCCoY and TiNiZrHfCoCu–cBNCoNiAlY composites for multi-cycle fatigue during bending with rotation.

Improving the efficiency of finishing-hardening treatment of gas turbine engine blades

This paper considers the influence of the technology to finish and strengthen compressor blade tips made of EP718-ID alloy on the characteristics of the surface quality, surface layer, and bearing capacity. Taking into consideration the special role of the finishing-strengthening treatment in the formation of the quality of the surface layer, various options for blade tip processing were investigated. The blade tips were shaped by high-speed line milling. The finishing-strengthening stage of tip machining included manual polishing and ultrasonic hardening operations with steel balls in various combinations. The basic regularities have been established in the formation of the roughness of tip surfaces, the maximum height of micro-irregularities, the surface microhardness, and the propagation depth of the hardened layer, depending on the combination of finishing-strengthening machining techniques. The results of tests are given for multi-cycle fatigue of blade batches treated according to various variants of the technological process. The efficiency of polishing the surface of the tip after strengthening treatment has been established. To restore the quality characteristics of the surface layer after polishing, it is proposed to perform repeated strengthening treatment. It is shown that the use of double deformation hardening technology with intermediate polishing at the finishing-strengthening stage of blade manufacturing makes it possible to increase the endurance limit from 320 MPa to 400 MPa while increasing durability. Technology for the finishing-strengthening stage of machining blades made from nickel alloys, characterized by significant viscosity, has been devised. It is shown that based on the criteria of minimum labor intensity of machining and maximum endurance of blades, it is effective to use double deformation hardening with steel balls in an ultrasonic field with intermediate polishing.

ЧИСЛЕННОЕ МОДЕЛИРОВАНИЕ УСТАЛОСТНОЙ ДОЛГОВЕЧНОСТИ ДЕТАЛЕЙ АВИАЦИОННЫХ ГТД ПРИ ЦИКЛИЧЕСКОМ ДВУХЧАСТОТНОМ НАГРУЖЕНИИ

The problem of assessing the resource characteristics of responsible engineering facilities, taking into account the peculiarities of operational loading modes, is discussed. The processes of fatigue durability of polycrystalline structural alloys under cyclic two-frequency loading (with the combined action of mechanisms of low and multi-cycle fatigue) are considered. A mathematical model describing the processes of cyclic elastoplastic deformation and accumulation of fatigue damage in structural alloys under multiaxial disproportionate modes of combined thermomechanical loading has been developed from the modern positions of damaged medium mechanics and fracture mechanics. The model consists of three interrelated parts: relations determining the cyclic elastic-plastic behavior of the material, taking into account the dependence on the destruction process; evolutionary equations describing the kinetics of fatigue damage accumulation; criteria for the strength of the damaged material. The variant of the defining elastic-plasticity relations is based on the idea of the yield surface and the principle of gradiency of the velocity vector of plastic deformations to the yield surface at the loading point. This version of the equations of state reflects the main effects of cyclic elastic-plastic deformation of the material for arbitrary folded loading trajectories. The variant of kinetic equations of fatigue damage accumulation is based on the introduction of a scalar damage parameter, is based on energy principles and takes into account the main effects of formation, growth and fusion of microdefects under arbitrary complex cyclic loading regimes. A joint form of the evolutionary equation of fatigue damage accumulation in the areas of low- and multi-cycle fatigue is proposed. As a criterion of the strength of the damaged material, the condition of reaching the critical value of the damage value is used. The results of numerical simulation of fatigue durability of a compact sample (model of the disk deflector of the high pressure turbine of an aviation gas turbine engine) are presented with cyclic two-frequency loading. The results of the comparison of the calculated and experimental data showed that the proposed model qualitatively and with the accuracy necessary for practical calculations quantitatively describes the fatigue life of structural elements under cyclic two-frequency loading.

Steel fatigue under non-stationary cyclic loading with low cycle overloads

The regularities and mechanisms of the elastoplastic strain of structural steels of two different types under two-stage cyclic loading regime conditions combining multicycle fatigue loading and hard elastoplastic strain under repeated overloading are considered. Based on experimental results and metallographic studies, repeated elastoplastic overloads on changes in the structural state of the material, mechanisms of fatigue damage accumulation, and lifetime indicators compared to the stationary process of cyclic loading are evaluated. It is shown that the combination of multicycle loading modes and low-cycle overloading leads to a change in fatigue resistance mechanisms of steels and changes in fatigue life depending on the loading history. Predict the effect of pre-elastoplastic loading of different types of metals, a change ratio in lifetime depending on the parameters of the two-step cyclic loading regime is proposed.

Modeling the Multicyclic Fatigue Failure of Rods under Symmetrical Tension/Compression

Modeling the Multicyclic Fatigue Failure of Rods under Symmetrical Tension/Compression

Study of the structure and properties of a high-entropy ceramic composite material

The article describes the technology development of layered surface compositions, including a high-entropy layer of a material with high-temperature memory effect of TiNiZrHfCoCu and a high-temperature ceramic layer based on zirconium dioxide ZrO2 stabilized by Y2O3. The authors also studied the structure, functional and mechanical properties of the abovementioned compositions. The proposed technology was implemented on a patented multifunctional technological complex for the formation of composite layered materials on the surface of steel products in a single technological cycle. On the basis of complex X-ray diffraction and electron microscopic studies, the structural parameters of the constituent layers of the surface composition were determined with subsequent thermal and thermomechanical treatment. The tests for ‘friction-wear’ and mechanical multi-cycle fatigue of the constituent layers of the surface composition showed a decrease in wear intensity and an increase in cyclic durability compared to the steel base. The comparison of wear intensity and cyclic durability of the surface-modified layers of the layered composition TiNiZrHfCoCu-ZrO2(Y2O3) with the composite materials obtained and studied by the authors earlier allowed the recommendation of the surface composition ‘high-entropy alloy TiNiZrHfCoCu-high-temperature ceramics’ for products of various purposes.

Endurance limit and mechanism of fracture of friction welded joints of exploration drill pipes in the conditions of multi-cycle fatigue

The paper presents the results of tests for multi-cycle fatigue of laboratory samples of welded joints of exploration drill pipes in a combination of steel s 32-2-Mn of strength group L (pipe body) with 40-Cr-Ni (tool joint part) and solid samples of pipe body material. Welding of the joint part with the pipe body was carried out by rotary friction welding. Multi-cycle fatigue tests were carried out according to GOST 25502 on a test machine with a two-support rotating specimen under constant torque. Mathematical processing of the results of fatigue tests was carried out, linear regression equations were obtained, fatigue curves were constructed, the endurance limits σ-1 of samples with a welded joint and solid samples were determined, taking into account the relative error of the regression equations. On the basis of metallographic analysis, vulnerabilities in welded joints were identified, in which the initiation and development of fatigue cracks occurred. Fractograms are presented that illustrate the mechanism of fatigue fracture of solid and welded samples. It is shown that the development of a fatigue crack in specimens of welded joints occurs with a greater proportion of microplastic deformation. It has been established that rotary friction welding of a pipe body made of 32-2-Mn steel of strength group L and a joint made of 40-Cr-Ni steel contributes to an insignificant decrease in the fatigue limit of welded joints relative to the pipe body, providing its value at the level of 43-47 % of the ultimate strength of 32-2-Mn steel. The data obtained assume trouble-free operation of the welded joint zone in the drill pipe string made of steel 32-2-Mn, strength group L with welded joints made of steel 40-Cr-Ni.The reported study was funded by RFBR, project number 19-38-90079.

Properties of amorphous Fe-based alloy

The paper presents the results of tests for multi-cycle fatigue of laboratory samples of welded joints of exploration drill pipes in a combination of steel s 32-2-Mn of strength group L (pipe body) with 40-Cr-Ni (tool joint part) and solid samples of pipe body material. Welding of the joint part with the pipe body was carried out by rotary friction welding. Multi-cycle fatigue tests were carried out according to GOST 25502 on a test machine with a two-support rotating specimen under constant torque. Mathematical processing of the results of fatigue tests was carried out, linear regression equations were obtained, fatigue curves were constructed, the endurance limits σ-1 of samples with a welded joint and solid samples were determined, taking into account the relative error of the regression equations. On the basis of metallographic analysis, vulnerabilities in welded joints were identified, in which the initiation and development of fatigue cracks occurred. Fractograms are presented that illustrate the mechanism of fatigue fracture of solid and welded samples. It is shown that the development of a fatigue crack in specimens of welded joints occurs with a greater proportion of microplastic deformation. It has been established that rotary friction welding of a pipe body made of 32-2-Mn steel of strength group L and a joint made of 40-Cr-Ni steel contributes to an insignificant decrease in the fatigue limit of welded joints relative to the pipe body, providing its value at the level of 43-47 % of the ultimate strength of 32-2-Mn steel. The data obtained assume trouble-free operation of the welded joint zone in the drill pipe string made of steel 32-2-Mn, strength group L with welded joints made of steel 40-Cr-Ni.The reported study was funded by RFBR, project number 19-38-90079.