Multicycle Fatigue Research Articles

Vibrational Stresses of Damaged Steam Turbine Blades After Renovation Repair

The last-stage blades of K-1000-60/3000 steam turbines operate in a humid steam environment, which causes erosion damage in the blades and reduction in their residual life. The relevance of this work is related to the need to continue the safe operation of such turbine blades. A number of variants of the finite-element models of individual blades and last-stage blades in the disk-blade systems of the above turbines are considered. Results of the numerical study of the influence of blade part removals in erosion damage zones after renovation repair on the vibration characteristics of individual blades and blades in the disk-blade system are presented. An analysis of the stress-strain state under the conditional load from the steam flow during the forced oscillations of individual blades and blades in the disk-blade system is carried out. The loads are given as evenly distributed and linearly variable on blade surfaces. The dependence of the maximum equivalent vibration stresses on excitation frequency is determined. It is assumed that the physical and mechanical properties of the blade material are preserved (as for the original version) after the renovation repair of blades and processing of their surfaces. There is a significantly greater reduction in the vibration stresses of blades in the disk-blade system than in the stresses of individual blades. Graphs of the dependence of the maximum stresses on excitation frequency both for undamaged individual blades and blades in the disk-blade system after their renovation repair are given. Various variants of blade part removals in areas of blade leading and trailing edges are considered. It is shown that with decreasing chords of blades after renovation repair, frequency regions of increased vibration may appear in lower blade parts. In the lower parts of individual blades and blades in the disk-blade system, the maximum stresses increase in comparison with their values in undamaged blades. With the change in the stress-strain state of rotor blades in comparison with the original version of undamaged blades, the possibility of extending their safe lifetime in case of multi-cycle fatigue is assessed. The safe lifetime of the considered blades with a chord of at least 150 mm after their renovation repair can be extended according to their stresses, if the cyclic symmetry of the disk-blade system is not violated, and the physical and mechanical properties of the material are preserved after the processing of damage removal zones on blade trailing edges.

Fatigue Strength of Structural Steel DAN400 Plate Produced by Thermomechanical Treatment Technology with Accelerated Cooling

Fatigue properties of contemporary low-carbon micro-alloyed structural steel grades have not been studied sufficiently in spite of a large amount of available reference information. This slows down and hinders introduction of the latest generation of structural steel grades into production chains for manufacturers of heavy machinery and structures. According to multicycle fatigue test results for plate steel grade DAN400 36 mm thick with σy = 450 MPa, Cequ = 0.27%, and Pwi = 0.12% produced by thermomechanical treatment technology with accelerated cooling (TMTT + AC) the fatigue strength (σ–1, MPa) on a base of two million cycles, the index of the left-hand branch of the fatigue curve m = 1/α, and coefficients C and α in the equation for the left-hand branch of the fatigue curve σ–1 = С·N–α are determined. An experimental fatigue curve is constructed on double logarithmic coordinates for the test steel.

ПРИМЕНЕНИЕ КОМПЛЕКСА ИОННО-ПЛАЗМЕННЫХ ТЕХНОЛОГИЙ ДЛЯ ЗАЩИТЫ ЛОПАТОК КОМПРЕССОРА ВЕРТОЛЕТНОГО ГАЗОТУРБИННОГО ДВИГАТЕЛЯ ОТ ЭРОЗИОННОГО ИЗНОСА И ФРЕТТИНГА

Describes the application of the technology of applying erosion-resistant and fretting-resistant ion-plasma coatings to protect compressor blades of VT8M-1 alloy from erosion wear and fretting, presents the results of tests of compressor blades for erosion resistance and vibration fatigue, samples of VT8M-1 alloy for fretting wear, multi-cycle fatigue and long-term strength, metallographic and metallophysical studies. It has been established that multilayer coatings TiN/CrN and Ti+TiN increase, respectively, the erosion resistance of the feather and the resistance to fretting wear of the locking part of titanium GTE compressor blades while maintaining their fatigue strength.

Estimation of Damage Rotor Shaft of Turbogenerators with a Diode Brushless Excitation System

The damage to the rotor shafts of high-power turbo generators is estimated as a result of the resonant in-teraction between the generator and the exciter caused by the operation of the automatic excitation regulator (AER) by mathematical modeling. The analysis covers turbo generators with a capacity of 300 to 1000 MW with a diode brushless excitation system (DBES). The simulated circuit includes a turbo generator with a block transformer, a power transmission line, an exciter in the form of a reversed synchronous generator with a rotat-ing rectifier diode converter unit, a static thyristor converter, and a sub-exciter in the form of a synchronous machine with permanent magnets. When modeling the electrical part, an approach is used from the positions of its own coordinates, which ensures maximum methodological consistency of the models of the listed devices and allows directly reproducing resonant phenomena at torsional vibration frequencies with the determination of instantaneous values of currents, voltages and electromagnetic moments of the turbo generator and exciter. The mechanical system is presented taking into account the exciter and sub-exciter as a seven-mass system. AER is introduced into the mathematical model by means of transfer functions with corresponding coefficients and time constants. The control system of the thyristor converter is represented in the model by a generator of line-arly increasing signals, a body for comparing these signals with the signal from the AER and a control pulse generator. To assess the damage rate, the deformation criterion for soft and hard loads in the zone of low-cycle fatigue and the force criterion in the zone of multi-cycle fatigue were used. A comparative quantitative assess-ment of the damage in the neck of the G-E shaft line in the resonant interaction between the exciter and the generator with different automatic excitation control systems is given. The influence of attenuation of electro-magnetic transients and damping of torsional vibrations on the damage values is analyzed. The results ob-tained can be used to analyze the functioning and determine the settings of the AER.

Damage accumulation in cyclically stable steel under low-cycle loading and elevated temperature

Based on the strain-kinetic fatigue fracture criterion, the kinetics of accumulated damages in structural carbon steel under low-cycle loading at an elevated temperature (150°C), at which the material begins to develop stress aging processes, is estimated. It is shown that stress aging, strengthening the material, narrows the durability area in which there is quasistatic destruction, the limiting case of which is the buckling failure of plastic strain, like single static destruction. This increases the fatigue fracture range, the limiting case of which is the formation of the primary fracture. The recoverable plastic strain causes the main damaging effect. Stress aging contributes to an increase in damage from elastic strain, although in the area of low-cycle failure, the share of damage from elastic strain remains small, growing with increased durability, and in the area of multi-cycle fatigue, this share becomes prevalent. Hardening from stress aging inhibits the development of damage from the one-way accumulated strain. The author confirms the validity of applying the strain-kinetic criterion of fatigue fracture to describe the damage kinetics and limit states during the cyclic elastic-plastic strain of carbon steel in the presence of weak stress aging.

Совершенствование конструкции подкладки контррельса стрелочного перевода

The element of the upper structure of the railway track - the switch counter-rail lining is studied. A variant of the serial design of the PKZhDL-65-4 counter-rail lining is considered. The analysis of the failure rate of counter-rail linings is carried out, and the results of observations of the condition of the linings in different regions are presented. The destruction zone is localized and its character as a multi-cycle fatigue destruction is determined. The information about the conducted complex of experimental studies for investigation of the stress-strain state in the dangerous zone of the lining is presented. To carry out the design studies, the design schemes of the counter-rail lining were developed. Calculated studies of the influence of various factors on the level of stresses acting in the hazardous area of the structure are performed. The influence of the type of the design scheme, the stiffness of the rubber shock-absorbing lining and the properties of the sleeper material is studied. Variants of counter-rail lining designs for reducing the level of stresses acting in the dangerous zone are proposed. Computational studies were conducted to optimize the proposed lining options in order to reduce material consumption. The results of testing of new lining structures on the switches of the South Ural Railway are presented. The information about the conducted complex of experimental studies for investigation of the stress-strain state in the dangerous zone of the lining is presented. To carry out the design studies, the design schemes of the counter-rail lining were developed. Calculated studies of the influence of various factors on the level of stresses acting in the hazardous area of the structure are performed. The influence of the type of the design scheme, the stiffness of the rubber shock-absorbing lining and the properties of the sleeper material is studied. Variants of counter-rail lining designs for reducing the level of stresses acting in the dangerous zone are proposed. Computational studies were conducted to optimize the proposed lining options in order to reduce material consumption. The results of testing of new lining structures on the switches of the South Ural Railway are presented.

The Role of Laser Shock Processing Treatment in the Growth Dynamics of Fatigue Cracks in Specimens of Ti-6Al−4V Titanium Alloys Damaged by Foreign Objects

Foreign object damage (FOD) is one of the main factors reducing the service life of wings, engine blades, aircraft compressors, and highly loaded machine components. This article presents a detailed study of the peculiarities of the growth rates of fatigue cracks upon FOD in the vicinity of the leading edge of a Ti-6Al−4V titanium alloy blade exposed to preliminary laser shock processing treatment. The FOD and the laser shock processing treatment were modeled using 3D numerical analysis by the finite-element method. The resulting range of changes in the effective stress intensity ratios takes into account both the fields of residual stresses caused by FOD and the crack closure effect due to residual compression stresses. The derived effective stress intensity ratios are used to analyze the growth rates of fatigue cracks in complex residual stress fields. Such actual operational loading modes are considered as low-cycle and multicycle fatigue, separately and in combination.

ИССЛЕДОВАНИЕ МНОГОЦИКЛОВОЙ УСТАЛОСТИ ЭЛЕМЕНТОВ ОБОРУДОВАНИЯ И МАШИН ПРИ СЛОЖНОМ НАПРЯЖЕННОМ СОСТОЯНИИ И НЕСТАЦИОНАРНОМ НАГРУЖЕНИИ

A computational and experimental method for estimating the fatigue life of structural elements of machines operating under complex cyclic stress conditions and non-stationary loading is considered. Comparison of the results of the calculations and experimental data indicated the effectiveness of the kinetic equation of multi-cycle fatigue damage based on the energy concentration of fatigue failure.

DAMAGING AND DESTRUCTION OF REDUCTION TRAINS OF MI-8 HELICPTER AND ITS MODIFICATIONS

The results of quantitative factographic study of the Mi-8 helicopter (its modifications) reduction trains with different types of gear coupling, the destruction of which occurred while in flight due to the presence of preliminary material damage at the stage of production of the gear wheels and during their operational employment are presented in a generalized form. Contact teeth destruction as the main cause of failures of high-speed gear wheels in well-lubricated enclosed gears are considered. The causes of the wearing and sticking of the teeth along their lateral surfaces as well as scoring and destruction depending on the origin of the cracks are revealed.
 The main causes of damaging and destruction of helicopter reduction trains, locations and origination of fatigue cracks spots are identified. Different types of contact and fatigue destructions due to various reasons are considered.
 Dynamic loading of gear wheels disks as a result of their rotation damages the material of the wheels in the area of multi-cycle fatigue due to the repeatability of flight load cycles.
 Lamination or deep contact destruction of the material, which sometimes take place in gears with surface hardened (cemented, tempered, nitrided, etc.) teeth, are much more dangerous than surface destruction of the material since sufficiently large fractions of the hardened material layer are split away from the teeth that can quickly break down an entire gear train.
 The main causes of contact teeth destruction are load concentration along the length and profile of the teeth, the presence of residual tensile stresses on the working surfaces of the teeth, adverse operational damage to the tooth profiles and negative effect of lubricating fluid on the teeth.
 For a repaired gear train the cause of the uneven load distribution along the length of the teeth may be the poor assembly of the gear wheels and other transmission parts which leads to unsatisfactory contact of the gear teeth in the assembled unit.
 The revealed peculiarities of the destruction process indicate regular crack proliferation from flight to flight. According to the operation conditions of the gear weel, its regular extensive loading refers only to the full cycle of starting and stopping the reducer train or unloading the helicopter in flight, which in this case corresponds to the cycle of helicopter operation in one flight. Therefore, the number of detected fatigue lines corresponds to the number of helicopter flights.
 To increase the smoothness of gear train running which affects positioning accuracy, reduce wear during running-in, decrease noise, eliminate early jamming, and ultimately, to increase the resource, it is necessary to eliminate the edge interaction of the teeth, which causes their excessive wear.

Structural behavior and mechanical properties of high-entropy coatings synthesized by HVOF

This paper discusses the technology, structure and functional–mechanical properties of surface layers made of high-entropy materials with a high-temperature shape memory effect, nickel (Ni)–cobalt (Co)–titanium (Ti)–zirconium (Zr)–hafnium (Hf), nickel–copper (Cu)–titanium–zirconium–hafnium and titanium–nickel–zirconium–hafnium–cobalt–copper. The developed technology and equipment for the formation of high-entropy coatings on the surface of steel products included mechanical activation of powders, high-velocity oxygen fuel (HVOF) spraying in a protective atmosphere and subsequent thermal and thermomechanical treatment. This allowed carrying out this process in a single technological cycle. The authors have developed statistical models of the formation process of highly entropic HVOF coatings in a protective atmosphere according to the criteria of adhesive strength and porosity with optimization of parameters. Based on complex X-ray diffraction and electron microscopy studies, the authors determined the structural parameters of high-entropy alloy coatings obtained by HVOF, followed by thermal and thermomechanical treatment. It was shown that all three alloys, nickel–cobalt–titanium–zirconium–hafnium, nickel–copper–titanium–zirconium–hafnium and titanium–nickel–zirconium–hafnium–cobalt–copper, which make up the surface compositions, are in an austenitic–martensitic state with a grain size of 32–120 nm. The authors performed calorimetric studies of the functional properties of the coatings. The studies showed the manifestation of austenitic–martensitic transformations. The authors also carried out tests for ‘friction wear’ and mechanical multicycle fatigue of the obtained high-entropy compositions on a steel surface.

DAMAGING AND DESTRUCTION OF REDUCTION TRAINS OF MI-8 HELICPTER AND ITS MODIFICATIONS

The results of quantitative factographic study of the Mi-8 helicopter (its modifications) reduction trains with different types of gear coupling, the destruction of which occurred while in flight due to the presence of preliminary material damage at the stage of production of the gear wheels and during their operational employment are presented in a generalized form. Contact teeth destruction as the main cause of failures of high-speed gear wheels in well-lubricated enclosed gears are considered. The causes of the wearing and sticking of the teeth along their lateral surfaces as well as scoring and destruction depending on the origin of the cracks are revealed.
 The main causes of damaging and destruction of helicopter reduction trains, locations and origination of fatigue cracks spots are identified. Different types of contact and fatigue destructions due to various reasons are considered.
 Dynamic loading of gear wheels disks as a result of their rotation damages the material of the wheels in the area of multi-cycle fatigue due to the repeatability of flight load cycles.
 Lamination or deep contact destruction of the material, which sometimes take place in gears with surface hardened (cemented, tempered, nitrided, etc.) teeth, are much more dangerous than surface destruction of the material since sufficiently large fractions of the hardened material layer are split away from the teeth that can quickly break down an entire gear train.
 The main causes of contact teeth destruction are load concentration along the length and profile of the teeth, the presence of residual tensile stresses on the working surfaces of the teeth, adverse operational damage to the tooth profiles and negative effect of lubricating fluid on the teeth.
 For a repaired gear train the cause of the uneven load distribution along the length of the teeth may be the poor assembly of the gear wheels and other transmission parts which leads to unsatisfactory contact of the gear teeth in the assembled unit.
 The revealed peculiarities of the destruction process indicate regular crack proliferation from flight to flight. According to the operation conditions of the gear wheel, its regular extensive loading refers only to the full cycle of starting and stopping the reducer train or unloading the helicopter in flight, which in this case corresponds to the cycle of helicopter operation in one flight. Therefore, the number of detected fatigue lines corresponds to the number of helicopter flights.
 To increase the smoothness of gear train running which affects positioning accuracy, reduce wear during running-in, decrease noise, eliminate early jamming, and ultimately, to increase the resource, it is necessary to eliminate the edge interaction of the teeth, which causes their excessive wear.

The cross-disciplinary approach to analysis and forecast of operational damage tolerance of the oil pipeline system – part 2

The paper presents the cross-disciplinary approach to the analysis of the oil pipeline system based on the methodologies of tribo-fatigue and mechano-thermodynamics. The pipeline section is analyzed as a complex system pipe-soil- flow of liquid subject to the set of mechanical, thermal and friction loads. It is shown that theses loads are mainly repeatedlyalternated, and the pipe metal works in the multi-cycle fatigue conditions. The procedure of resonance accelerated fatigue tests is proposed, and their results are presented. Also, the unorthodox method of integrated wear-fatigue tests of the pipeline steel was proposed with the model of simultaneous pressure and wall friction actions. The presented field test results of pipes subject to the long-term operation showed that their fracture may occur not only in the near-weld zone, but also in the vicinity of internal corrosion damages. New models of three-dimensional stress-strain state and volumetric damage tolerance for the system pipe-soil-liquid flow were developed. These models were applied with regard to the pipe internal corrosion damages, defined using the inline inspection technique. A new efficient method to describe static and cyclic elastic-plastic fracture of the pipe steel with crack using the transverse strain is proposed and tested. Results of the computer-simulated propagation of the crack-like damage are based upon the model of deformed solid with dangerous volume. The new model is proposed for risk and safety assessment with regard to the ultrasonic inspection data. The algorithm of the ‘oil line pipe’ problem solution is presented for drafting a short-term plan of particular R&D actions.

Restoring the Fatigue Life of Surface-Hardened Airplane Parts by Repeated Shot Hardening

The restoration of the fatigue life of high-strength airplane parts by repeated shot peening in conditions of multicyclic fatigue is explained.

Torsional Oscillations and Damageability of the Rotor Shaft of the Turbogenerators Fitted with Automatic Regulated Excitation

The assessment of damage to the rotor shaft of the turbine-generator due to resonant interaction between generator and exciter, due to automatic excitation regulator (ARV) by mathematical modeling. The thyristor system of independent excitation is used in the work. The simulated circuit includes a turbogenerator with a block transformer, a power line, a synchronous generator and a gate-controlled exciter Converter. When modeling the electrical part, an approach is used from the position of its own coordinates, which provides maximum methodological consistency of the models of the listed devices and allows directly reproducing resonant phenomena at frequencies of torsional vibrations with the determination of instantaneous values of currents, voltages and electromagnetic moments of the turbo generator and exciter. The mechanical system is presented taking into account the exciter as a six-mass system. ARV is introduced into the mathematical model by means of transfer functions with corresponding coefficients and time constants. The control system of the gate Converter of the exciter is represented in the model by a generator of linearly increasing signals, a body for comparing these signals with the signal from the ARV and a control pulse generator. The deformation criterion for soft and hard loads in the zone of low-cycle fatigue and the force criterion in the zone of multi-cycle fatigue were used to assess the damage. A comparative quantitative assessment of the damage to the generator rotor shaft under resonant interaction at all subsynchronous frequencies of torsional vibrations and in near-resonant modes is given. The influence of attenuation of electromagnetic transients and damping of torsional vibrations on the damage values is analyzed.

Torsional Vibrations of the Steam Turbine Shaft Line and Estimation of the Residual Life of Its Elements

A comparative quantitative assessment of the damage and residual life of the shaft line elements for differ-ent types of high-power steam turbines at the end of their design life is made by mathematical modeling. The analysis covers all elements of the shaft line: from the steam turbine Central pump to the turbine generator ex-citer. The simulated circuit includes turbo generators, transformers, gate converters, AC and DC power lines. When modeling, an approach is used from the position of proper coordinates, which provides maximum meth-odological consistency of the models of the listed devices and allows you to directly reproduce electromagnetic and mechanical transients with the determination of instantaneous values of currents, voltages, electromagnet-ic and torsional moments. To estimate damage, we used the deformation criterion for soft and hard loads in the zone of low-cycle and force criterion in the zone of multi-cycle fatigue. The influence of the number of starts and running time of a steam turbine on the damage and residual life of its shaft elements is studied. When de-termining the remaining life, in addition to starts, other abnormal operating modes of the turbo generator were taken into account during the turbine operating time: short circuits and their disconnections, unsuccessful high-speed automatic re-activation, subsynchronous resonance due to both the operation of the control system of the PPT and the automatic generator excitation regulator. The influence of attenuation of electromagnetic transients in the generator and damping of torsional vibrations on the degree of reduction of the residual life of the shaft elements is analyzed. The results obtained can be used for a comprehensive solution of the issue of further operation of steam turbines that have spent their design life.

Very High Cycle Fatigue Properties of Ferritic-Pearlitic Steel after Micro-Shot Peening

An alloyed ferritic-pearlitic steel is studied after micro-shot peening with balls from a high-speed steel. The morphology and the roughness of the surface of the specimens are determined. Specimens with smooth and notched surfaces are subjected to multi-cycle fatigue tests. The fatigue resistance of the specimens with a smooth surface and with a notched surface increases by 22 and 27%, respectively, after the shot-peening. Observation of crack growth in specimens with small holes shows that micro-shot peening hinders crack propagation.

The cross-disciplinary approach to analysis and forecast of operational damage tolerance of the oil pipeline system – part 1

The paper presents the cross-disciplinary approach to the analysis of the oil pipeline system based on the methodologies of tribo-fatigue and mechanothermodynamics. The pipeline section is analyzed as a complex system pipe-soil- flow of liquid subject to the set of mechanical, thermal and friction loads. It is shown that theses loads are mainly repeatedly-alternated, and the pipe metal works in the multi-cycle fatigue conditions. The procedure of resonance accelerated fatigue tests is proposed, and their results are presented. Also, the unorthodox method of integrated wear-fatigue tests of the pipeline steel was proposed with the model of simultaneous pressure and wall friction actions. The presented field test results of pipes subject to the long-term operation showed that their fracture may occur not only in the near-weld zone, but also in the vicinity of internal corrosion damages. New models of three-dimensional stressstrain state and volumetric damage tolerance for the system pipe-soil-liquid flow were developed. These models were applied with regard to the pipe internal corrosion damages, defined using the inline inspection technique. A new efficient method to describe static and cyclic elastic-plastic fracture of the pipe steel with crack using the transverse strain is proposed and tested. Results of the computer-simulated propagation of the crack-like damage are based upon the model of deformed solid with dangerous volume. The new model is proposed for risk and safety assessment with regard to the ultrasonic inspection data. The algorithm of the ‘oil line pipe’ problem solution is presented for drafting a short-term plan of particular R&D actions.

ВІБРАЦІЙНА НАВАНТАЖЕНІСТЬ ТА РУЙНУВАННЯ ЗУБЧАСТИХ ПЕРЕДАЧ ВЕРТОЛІТНИХ РЕДУКТОРІВ

Роглянута вібраційна навантаженість високошвидкісних зубчастих передач вертолітних редукторів на злітному і крейсерському режимах експлуатації, розподіл навантаження між зубцями в зоні багатопарного зачеплення, динамічні навантаження в зачепленні зубчастих коліс та їх пошкоджуваність і руйнування.

Simulation of Microcontact Interactions during Gas-Abrasive Wearing of Steels with Allowance for Strain Hardening

In this paper, we present a model of the effect of the contact interaction velocity and the mechanical properties of contacting materials on contact stresses and deformations in carbon structural steels caused by a solid particle flow. The existence of three critical states of the material exposed to impact deformation is considered. The first state is characterized by the formation of a plastic flow zone in the surface layer (transition from perfectly elastic to constrained elastic–plastic deformation). The second state is characterized by the exit of this zone to the part surface (transition from constrained elastic–plastic deformation to a free plastic flow, i.e., dynamic indentation). The third state is characterized by the destruction of surface layers due to tensile stresses on the contour of the contact spot values equal to the true tensile strength of the deformable material under tension with allowance for strain hardening. The characteristic values of the dynamic microcontact interaction velocities that correspond to the ranges of the existence of each of the critical state forms were determined. The dominant mechanisms of erosive wear of parts for each of the states (multi-cycle fatigue wear for the first state, low-cycle fatigue wear with an incubation period for the second state, and intensive formation of erosion damage with a minimal incubation period for the third state) were indicated.

Improving the Carrying Capacity of Structural Elements of Vehicles by the Corrosive Strength Criterion

The problems of improving the accuracy of the definition of resistance to corrosive fatigue of the shafts of transmission gears, which operate under the effect of atmospheric precipitation and under variable complex loading (joint cyclical bend and torsion), were considered. According to the results of fatigue tests, the equations of the families of fatigue lines for the probability interval of no-failure P(N) = 50–99.9% were derived. The dependences to determining the coefficients of strength and durability in a wide interval of multicycle fatigue are obtained. The nomogram of these coefficients for a probabilistic estimate of the carrying capacity of the shafts at N = 105–108 cycles is given.