Fatigue Strength Characteristics 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.

Variations of VHCF Characteristics by Microstructure of a Spring Steel to UNSM Treatment

<i>The bainitic structure resulting from the austempering of spring steel exhibits high strength and ductility. On the other hand, there appears to be no study on the effects of very high cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of this bainite structure. Therefore, this study compared the fatigue properties of VHCF using spring steel with bainitic and martensitic structures and bearing steel data. This study analyzed the characteristics of microstructure transformation associated with the heat treatment cycles and studied and evaluated the fatigue strength characteristics because of the UNSM in terms of fracture mechanics method and fracture surface analysis through electron backscatter diffraction, scanning electron microscopy fracture analysis, and energy-dispersive spectroscopy analysis. The fatigue limit of UNSM-treated spring steel was improved significantly by approximately 33% to 50% compared to the fatigue test results of the untreated material in the VHCF. In the long life range of bainized spring steel, fish-eye cracks appear in the form of the fine granular area, and fatigue cracks occur in the form of fish-eye cracks that occur in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.</i>

Probabilistic fatigue strength assessment of cross-ply laminates: Exploring effects of manufacturing defects through a two-scale modeling approach

The study presents a two-scale modeling approach allowing for an efficient fatigue strength evaluation on a macro scale considering a micro-mechanical defect characterization of a Carbon Fiber Reinforced Polymer (CFRP) material. The modeling approach integrates a macro model with the effective elastic properties from micro-mechanical simulations considering voids. This enables the analysis of defects’ influence on material fatigue strength using a probabilistic weakest link approach. A CFRP laminate with a cross-ply layup was investigated. Two simulation case studies demonstrate the effect of void content and size on the characteristic fatigue strength. An experimental investigation was conducted testing the laminates in tension–tension fatigue verifying the predicted numerical behavior. The numerical models identify a difference in the characteristic fatigue strength consistent with the fatigue test results. It is numerically concluded that the investigated CFRP material’s fatigue strength is affected by the presence of voids and even with only a slight difference in the global void volume fraction a scatter in fatigue strength is identified.

Characteristic fatigue strength and reliability of dental glass-ceramics: Effect of distinct surface treatments – Hydrofluoric acid etching and silane treatment vs one-step self-etching ceramic primer

The aim of this study was to mechanically characterize through flexural fatigue test two CAD-CAM glass-ceramics according to distinct surface etching protocols. To do so, feldspathic (FELD) and lithium disilicate (LD) glass ceramics were subjected to different surface treatments: (1) control – no treatment (Ctrl); (2) conventional protocol etching with 5% hydrofluoric acid followed by silane coupling agent application (HF + SIL; Monobond N, Ivoclar); or (3) using a self-etching ceramic primer (E&P; Monobond Etch & Prime, Ivoclar). Ceramic discs (N = 120; Ø = 12 mm; thickness = 1.2 mm) were produced from CAD-CAM blocks, with 60 being from FELD (VITABLOCS Mark II, Vita Zahnfabrik) and 60 from LD (IPS e.max CAD, Ivoclar). Next, 20 disks of each ceramic were allocated into three groups: Ctrl, HF + SIL, or E&P. Surface roughness data were collected on all samples before and after surface treatments (except for Ctrl). Cyclic fatigue (n = 15) biaxial flexural strength tests were performed by the piston-on-three-balls geometry (ISO 6872) considering the test parameters established from a monotonic test (n = 5). The monotonic test was carried out at a 1 mm/min loading rate and 500 kgf loading cell until fracture to obtain the failure data. The cyclic fatigue test was executed underwater at a frequency up to 20 Hz, with the first stress being 25% of the monotonic test for 5000 cycles, followed by increments of 5% of the monotonic test at each step of 10,000 cycles until failure (fracture). Complementary fractography, topography and Atomic Force Microscopy (AFM) analyses were performed. Characteristic Fatigue Strength (CFS) and Weibull modulus were analyzed by Weibull analysis using the fatigue test data. Roughness and complementary analysis data were analyzed by one-way ANOVA. The statistical results exhibited similar CFS among Ctrl, HF + SIL and E&P for both glass-ceramics. The survival analysis corroborates the findings, however the Weibull modulus pointed out superior structural reliability of FELD treated with the E&P group compared to HF + SIL. According to the complementary analyses, HF + SIL exhibited a higher surface area than E&P and Ctrl for FELD (p = 0.001). Roughness showed statistically significant differences among conditions for FELD (E&P < Ctrl = HF + SIL; p < 0.05) and no difference for LD (p > 0.05). Therefore, the CFS were not influenced by any condition evaluated for FELD and LD glass-ceramics; however, superior structural reliability (higher Weibull modulus) for the feldspathic ceramic treated with the E&P was observed.

Influence of hot-dip galvanization on the fatigue performance of high-strength bolted connections

In the present work, the influence of hot-dip galvanization (HDG) on the fatigue behaviour of high-strength bolted details is investigated through a set of 15 constant-amplitude tests. Cyclic behaviour of coated specimens is hence compared against results for unprotected samples drawn from literature, i.e., both with reference to neutral and aggressive exposure conditions. Namely, while behaviour of coated samples is moderately inferior as respect to uncoated ones in dry air (–6 % in terms of characteristic fatigue strength, probability of failure PF = 5 %, confidence interval CI = 75 %), their performance still complies with European normative requirements for double covered joints, and galvanization proves to be highly effective when corrosion is likely to occur (with a characteristic strength increase of [+12 %; +52 %]). Finally, fractography analyses on both uncoated and coated specimens were performed, suggesting that hydrogen embrittlement may play a role in reducing fatigue strength of coated joints, that is, especially within the high-cycle fatigue regime.

Study of vibration frequency-fatigue strength action of 6061-T6 aluminum alloy during fillet welding

The research mainly focuses on the fatigue strength characteristics of 6061-T6 aluminum alloy fillet welds under different vibration frequencies. Firstly, by introducing Stress Life Curve (S-N Curve) and Probability Stress Life Curve (P-S-N Curve), the external vibration stress effects of the main load-bearing points in the stress environment of welded joints are analyzed. Subsequently, a vibration test control system is designed to analyze the relationship between vibration frequency and fatigue strength through nominal stress analysis and hot spot stress analysis. The research findings revealed that under the nominal stress analysis method, the P-S-N fitting curve showed a declining trend with an increase in fatigue life for stress variation curves with survival rates of 50 %, 95 %, and 97.7 % at vibration frequencies of 57.5 Hz, 67.5 Hz, and 77.5 Hz. At the same survival rate stress conditions, the fatigue life variation formed by resonance frequency was smaller, and the fatigue life in the resonance state was relatively lower. There is a certain correlation between vibration frequency and fatigue strength, with resonance frequency corresponding to relatively low fatigue life. This research result helps to reveal the fatigue behavior of 6061-T6 aluminum alloy fillet welds under different vibration stresses, providing a reference for the structural safety design of aluminum alloy components.

Evaluation of the contact endurance of machine parts based on the mechanical characteristics of fatigue strength

The condition of contact endurance of machine parts is proposed, in which the maximum equivalent stress in the contact zone of the parts is used as a criterion, and the material endurance limit is used as a critical value. The use of this condition would allow to avoid time-consuming experiments on determination of the contact endurance limit and quickly assess the working ability of parts exposed to periodic contact load. Keywords: contact endurance, strength, fatigue strength, fatigue contact tests. alexmed@bmstu.ru

Determination method of fatigue strength of cement-stabilized Macadam under three-dimensional stress state using equivalent stress as index

The cement-stabilized macadam (CSM) material is in a three-dimensional (3D) complex stress state in the pavement structure. It is difficult for the conventional test method to characterize the 3D fatigue strength characteristics of the CSM. In order to establish a 3D fatigue strength design criterion through simple strength methods, this work conducted the conventional unconfined compressive (UC), direct tensile (DT), and indirect tensile (IT) tests. The corresponding 3D strength model, including three parameters, was determined based on three strength testing methods. In addition, a normalized fatigue model including one parameter was obtained by three fatigue tests. Finally, the fatigue strength model under a 3D stress state was established by combining fatigue and the 3D strength model. The results indicate that the 3D strength model effectively characterized the CSM strength under any stress state, and the difference between the IT strengths predicted by the 3D strength model and obtained by the test is 18.43%. Moreover, the fatigue normalization model reflected the 3D fatigue failure of CSM in a complex stress state. The proposed fatigue strength of CSM based on the equivalent stress has clear physical meaning. This work provided a practical technology to obtain 3D fatigue strength resistance of CSM, which solved the technical problem of inaccurate measurement of pavement structure resistance.

Significant reduction of fatigue crack non-propagation limit caused by damage accumulation mode fatigue crack propagation in a precipitation-hardened punched steel plate

Precipitation-hardened steel exhibits specific characteristics due to the punching process that deteriorates its strength. To investigate these characteristics, fatigue tests were conducted with artificial micro defects on the punching processed surface that imitate the flaws caused by the process. The authors found that changes in material properties and the flaws change the fatigue strength properties. Specifically, the fatigue crack extends in the damage accumulation mode, which does not have the non-propagation mechanism and reduces the crack non-propagation limit. Furthermore, the damage accumulation mode fatigue crack propagation changes the fatigue limit's physical meaning from the fatigue crack non-propagation limit to the initiation limit. Finally, processing conditions that improve the fatigue strength characteristics were proposed.

The methodical peculiarities of the investigation of portal crane rolled steels degradation

Rolled structural steels of port structures, operating under intensive cyclic loading, are particularly susceptible to the degradation of their mechanical properties. Advantages of the investigation of operational degradation of steels based not on fatigue strength characteristics, but on characteristics of resistance to brittle fracture using the example of determining the impact strength of longitudinal and transverse Charpy samples in relation to the rolling direction of sheet metal for 10 local areas at different structural nodes of portal crane are analyzed. This is caused to a great extend by micro-layering along the stretched fibers in the rolling direction of the rolled product. Accordingly, the mechanical properties of the metal become particularly sensitive to the direction of samples cutting in relation to the direction of rolling. Therefore, in order to evaluate the steel operational degradation it is recommended to use transverse samples in which the direction of micro-laayering coincides with the direction of rolling. Possible role of the marine environment in enhancing the degradation of steel due to its flooding properties is also considered in this paper.

TEMPERATURE DEPENDENCE OF ACOUSTIC BIREFRINGENCE OF SHEAR ELASTIC WAVES IN POLYCRYSTALLINE ALUMINUM

The article presents the results of a study of the temperature dependences of the shear elastic waves velocity of polycrystalline aluminum alloy AD0 in the temperature range from 20 °C to 45 °C. For the studied material the temperature dependence of the acoustic birefringence parameter characterizing the a shear ultrasonic wave splitting into two orthogonally polarized waves propagating at different velocities is determined. The calculation of the acoustic birefringence parameter was carried out according to the data of the propagation time precision measurement of shear elastic waves (the measurement error of the propagation time is 1-2 ns), polarized along and across the direction of rolling. Theoretical studies have shown that for materials with weak anisotropy, the temperature dependence of the acoustic birefringence parameter is mainly related to the influence of temperature on the elastic anisotropy of the crystals that make up the polycrystalline material and the characteristic of the crystallographic texture – the coefficient of the orientation distribution function W420. Corrections to the value of the birefringence parameter with temperature change are given. It was experimentally obtained that the acoustic birefringence parameter increases by 9% with increasing temperature, which must be taken into account by using it as a diagnostic parameter to evaluation changes in the characteristics of static and fatigue strength. An algorithm is proposed for estimating the prediction of the temperature dependence of the acoustic birefringence parameter of a single-phase material when the crystallographic texture changes as a result of plastic deformation, for example. A good coincidence of the predicted temperature dependence of the acoustic birefringence parameter with the experimental one was obtained – the deviation error of the calculated curve from the experimental one was 0.2–2% in the temperature range of 20 °С to 45 °С.

Evaluating the strength of a beam-type electric switch mechanism housing

Aim. Choice of method and its validation for the purpose of confirming the feasibility of safe operation of a switch mechanism housing submitted to rolling stock axle loads of 30 tf. Method. As of today, there are neither regulations regarding the strength parameters of load-carrying elements of electric switch mechanisms, nor verification procedures. Given the above, when evaluating the strength of electric switch mechanism housing, the authors used the method used for the purpose of evaluating the strength of load-carrying elements of locomotives per GOST R 55513-2013. That method has a long history of application as part of calculation and testing of motive power and has shown good results. According to the method, strength is evaluated by comparing the safety factor of fatigue strength n with the allowed value for steel structures [n] = 2.0. Result. The activities resulted in the validation of the proposed method of strength estimation by the criterion of fatigue strength. The value of fatigue endurance of electric switch mechanism housing was estimated based on the results of fatigue benchmark tests of three items. Each was stressed by stepping up the amplitude of the applied force after the base number of cycles had been reached. At the first stage, the loading was equivalent to the operational value. After the base number of functions had been reached for each item, the following step was initiated. The tests continued until cracks have been found. In the process of testing, the amplitude of stress was recorded at each level of loading. For the purpose of identifying the fatigue strength characteristics, the reduced fatigue strength of the switch mechanism housing was calculated under the hypothesis of linear addition of fatigue damage subject to the condition of deterministic loading and subsequent processing of the findings using statistical methods. The value of fatigue limit has been obtained for the housing of a tie-type electric switch mechanism equal to 48.4 MPa. The safety factor of fatigue strength was found to be equal to 2.86, which is above the minimal allowed value. Conclusion. It has been shown that the housing of a tie-type electric switch mechanism has a sufficient strength as regards the operational static and dynamic loads. Its safe operation when exposed to axle loads of 30 tf has also been confirmed.

On the origins of fatigue strength in crystalline metallic materials.

Metallic materials experience irreversible deformation with increasing applied stress, manifested in localized slip events that result in fatigue failure upon repeated cycling. We discerned the physical origins of fatigue strength in a large set of face-centered cubic, hexagonal close-packed, and body-centered cubic metallic materials by considering cyclic deformation processes at nanometer resolution over large volumes of individual materials at the earliest stages of cycling. We identified quantitative relations between the yield strength and the ultimate tensile strength, fatigue strength, and physical characteristics of early slip localization events. The fatigue strength of metallic alloys that deform by slip could be predicted by the amplitude of slip localization during the first cycle of loading. Our observations provide a physical basis for well-known empirical fatigue laws and enable a rapid method of predicting fatigue strength as reflected by measurement of slip localization amplitude.

Comparison of compressive fatigue performance of cementitious composites with different types of carbon nanotube

The compressive fatigue performance of cementitious composites largely depends on the generation and connection of fatigue nano/micro-cracks in the composites. These types of cracks are beyond the scope that traditional fibers can restrain, but can be effectively eliminated and inhibited by incorporating nano-materials, especially those with fiber shape, such as carbon nanotube (CNT). Unfortunately, the effect of CNT and its physical and chemical characteristics on the compressive fatigue performance of cementitious composites remains unclear. Therefore, this paper systematically investigated the effect of CNT with different contents and sizes, functional groups and coating layers on the compressive fatigue performance of cementitious composites, including fatigue life, strength, deformation behavior and cracking characteristics. The morphology of the fatigue fracture surface of cementitious composites shows that CNT can refine pore structure, bridge nano/micro-cracks and result in the generation of the multi-directional and network-like fatigue micro-cracks as well as the appearance of the fatigue striation around aggregates. Consequently, by increasing the integrity among each phase of cementitious composites, CNT significantly maximizes the compressive fatigue life (in logarithmic form), strength and failure strain of cementitious composites by 160 %, 43.4 % and 20.6 %, respectively, showing great potential for extending the service life of concrete structures.

Retained Austenite Reduction near Fracture Surface in Repeatedly Quenched SUJ2 Steel

High-carbon high-strength JIS-SUJ2 bearing steel is an alloy having the characteristics of high wear resistance and fatigue strength as a result of quenching. When this kind of high-strength material is exposed to long-lasting low stress application loadings, fisheye fractures can often be seen. In this research, we measured the retained austenite near the fracture surface in JIS-SUJ2. After the measurement, we confirmed the decrease of the retained austenite in the vertical surface under both fisheye fracture and ductile fracture.

Fatigue strength and fracture characteristics of linear friction welded joints of weathering mild steel

AbstractWeathering steel with high phosphorus content usually cannot be joined by fusion welding due to the possible solidification crack. The present study successfully joined the unweldable weathering steel SPA‐H by linear friction welding (LFW) above the A3 temperature. The microstructure, microhardness, and residual stress were measured and analyzed. The stress concentration factor (SCF) of LFW joints was assessed by the finite element method (FEM). The influence of flash on the fatigue strength and fracture characteristics was investigated. The results suggest that the weld center zone (WCZ) consisting of ferrite and bainite shows a necking shape. The ductile fracture of LFW joints occurred at the base metal, resulting in a joint efficiency close to 100%. The possible unbonded flash at the edge of the LFW joints greatly reduced the fatigue life. The proposed S‐N curve illustrates a much higher fatigue strength than the FAT90 design curve.

Experimental Investigation of the Fatigue Damage and Strength Characteristics of Heterogeneous Rock Mass under Cyclic Loading

Rock fractures filled with cementation inevitably lead to rock mass heterogeneity and affect its mechanical properties. The cementation is usually regarded as a defect due to its strength is lower than that of the surrounding rock. To investigate the influence of the defect on fatigue damage and strength of the rock mass under cyclic loading, the defect area ratio was obtained and a series of static and cyclic loading uniaxial tests were carried out. The experimental results reveal that tensile failure with few secondary cracks occurs under static loading. However, microcracks are widely developed in the samples after cyclic loading, especially at the defect interface. The compressive strength decreases with increasing defect area ratio. During cyclic loading, the fatigue damage for samples with small defect area ratios (<10%) is similar to that of intact rock. However, multiple sudden increases in the residual strain and dissipation energy occur when the defect area ratio is greater than 10%. In addition, cumulative residual strain, cumulative acoustic emission (AE) count and cumulative dissipation energy increase with increasing defect area ratio. Cyclic loadings significantly reduce the rock mass strength, and the strength reduction increases with increasing defect area ratio. It is inferred that the incompatible deformation in the heterogeneous rock mass is the main reason for the sample damage and the strength reduction, and the unloading has an essential effect on the initiation, propagation and coalescence of microcracks.

Effect of Prolonged Thermal Exposure on Low-Cycle Bending Fatigue Resistance of Low-Carbon Steel

Using a dynamic mechanical analyzer, the comparative studies of a low-cycle bending fatigue were carried out for AISI 1022 low-carbon steel after extreme thermal exposure, simulating the severe beyond-design-basis accident at nuclear power plants. In the as-delivered state, the steel has a high resistance to low-cycle fatigue (the fatigue strength at N = 3.5 × 104 cycles (σNf) was 360 MPa). Long-term thermal exposure led to a slight decrease in the resistance to low-cycle fatigue of steel: σNf is decreased by 9%. The influence of AISI 1022 steel structure on the characteristics of fatigue strength and fracture mechanisms is analyzed.

МОДЕЛИРОВАНИЕ РАСПРЕДЕЛЕНИЯ ОСТАТОЧНОЙ ПОРИСТОСТИ МЕТАЛЛИЧЕСКОГО ИЗДЕЛИЯ ПРИ АДДИТИВНОМ ПРОИЗВОДСТВЕ С ПОСЛОЙНОЙ ПРОКОВКОЙ

Porosity, which occurs in products made of aluminum-magnesium alloys synthesized by wire-arc surfacing, significantly worsens the characteristics of fatigue strength. The developed technology of hybrid additive manufacturing with layer-by-layer forging of each deposited layer of material is able to minimize the porosity of the product. To select the rational parameters of the technological process, the evolution of the porosity distribution over the cross-section of a linear element after its single forging with a pneumatic hammer is investigated. A numerical model of the process is constructed in the LS-DYNA® package, where the Gurson–Tvergaard–Needleman relations are taken as constitutive equations of plastic deformation of the material and porosity evolution. To determine the parameters of the Johnson–Cook hardening law, tests of the AMg6 alloy were performed in a wide range of strain rates. The impact of the pneumatic hammer in the numerical model was calibrated using an accelerometric and strain-gauged steel target, as well as by distortions of the cross-section of a forged bar made of AMg6 alloy. Calculations according to the model are compared with experimental data, for which two linear segments were made by additive manufacturing with and without layer-by-layer forging, from the cross-sections of which the slots processed for pore visualization were prepared. With this method of pressure treatment, the decrease in porosity in the boundary layer of the workpiece mainly depends on the accumulated plastic deformations and does weakly sensitive the appearance of a stressed state. The model allows you to predict the size of the area under the hammer head depending on the processing mode, within which the porosity is eliminated by forging. The use of such modes will ensure the manufacturing of products without residual porosity in the processes of additive manufacturing by surfacing with layer-by-layer forging.

СТАТИЧЕСКАЯ И ДИНАМИЧЕСКАЯ НАГРУЖЕННОСТЬ ПРУЖИН БУКСОВОГО ПОДВЕШИВАНИЯ ТЕЛЕЖКИ ЭЛЕКТРОПОЕЗДА ЭС2Г

It is possible to determine a service life of an electric-multiple unit and its assemblies by actual time of proper operation or theoretically according to results of comprehensive test of components and parts. According to standards of car calculation on strength, for parts that work in conditions of long and intense influence of dynamic loads it is necessary to determine fatigue strength at multi-cycle loading. Calculation is carried out with the consideration for probabilistic dispersion of fatigue strength characteristics and stochasticity of dynamic loading of a part, on the basis of which a life cycle is determined. Oscillograms of dynamic processes gained at tests on the line in theoretical calculations are substituted by a number of simple cycles. Study results presented in the paper allow determining static and dynamic parameters of springs of the primary spring suspension for electric-multiple unit bogie and a type of a simple cycle and its parameters.