Conventional Fatigue Limit Research Articles

Wire arc additive manufactured AWS ER100S-G steel: Very high cycle fatigue characterization

This paper presents a comprehensive study on the very high cycle fatigue (VHCF) characterization of a wire arc additive manufactured (WAAM) AWS ER100S-G steel. The demand for high-performance materials with superior fatigue properties has grown exponentially. However, the VHCF behavior of large-format WAAM’d structures remains relatively unexplored. In this study, a series of VHCF tests were conducted under fully reversed cyclic loading conditions to investigate the extended fatigue life (performance) of the WAAM ER100S-G steel. The VHCF properties, relative to conventional fatigue employing a servohydraulic testing system, of the WAAM ER100S-G steel were evaluated by analyzing the stress-life (S-N) curves, fatigue crack initiation, and propagation behavior, in a statistical framework, and the fracture surfaces. The results revealed the controlling mechanisms of VHCF failure of the WAAM ER100S-G steel and the fatigue response of the material beyond the conventional fatigue limit of 107 cycles. The findings provide valuable insights into the influence of WAAM-induced defects/microstructure on the extended fatigue performance of WAAM ER100S-G steel, which can aid in optimizing fatigue and durability design guidelines for additive manufacturing applications in high-cycle and very high-cycle fatigue domains.

High/very-high fatigue properties and microstructure evolutions of 9Cr3W3Co turbine rotor steel at room temperature and 650 °C

The study aimed to investigate the high and very-high fatigue properties of 9Cr3W3Co turbine rotor steel at room temperature (RT) and 650 °C. The resulting S–N curves demonstrated a continuous downward trend, without exhibiting a conventional fatigue limit. The fatigue strength corresponding to 5 × 107 cycles is approximately 53.9% of yield strength at RT, increasing to around 55.4% at 650 °C. Meanwhile, compared to the fatigue strength at 5 × 107 cycles at RT, it at 650 °C decreased by 53.3%. Surface crack initiation was found to be the main fatigue fracture failure model, observed at both RT and 650 °C. Microscopic analysis revealed that severe dislocation motion and grain deformation occurred within the material matrix during the cyclic loading process at both RT and 650 °C, resulting in the formation of abundant low-angle grain boundaries, dislocation lines and sub-grains. At RT, the presence of the M23C6 precipitated phase imposed a strong pinning effect, while the MX phase within the martensite laths effectively impeded the movement of dislocation lines, thereby enhancing fatigue properties. However, at 650 °C, the emergence of newly precipitated Cu-rich phases was observed, which can reduce the free path of dislocations, hinder slip motion of dislocations, and thus compensate for the negative effect caused by high-temperature microstructural degeneration.

Historical review and future prospect for researches on very high cycle fatigue of metallic materials

AbstractSince the first paper on the fatigue of metallic materials by J. Albert in 1837, tremendous numbers of papers have been published in various journals by many researchers all over the world. Based on such a long history, several papers on the very high cycle fatigue (VHCF) in the life‐time longer than 107 cycles had appeared in some journals during the period of 1980's. One characteristic finding in these works is the fact that the metallic material can fail even at the stress level lower than the conventional fatigue limit. This fact means that the conventional fatigue design of mechanical structures cannot give the safety of the practical products in the very high cycle regime. Due to this fact, fatigue properties of structural materials in very long life regime has become an important subject; and a lot of studies have been carried out, and many important results have been accumulated until now. Typical aspects on VHCF property are summarized as follows: (1) the fatigue crack tends to occur around the interior inclusion, (2) fine granular area (FGA) is formed around such an inclusion, and (3) duplex S–N characteristics appear in the VHCF regime. In this paper, a brief historical review together with the future prospect on the very high cycle fatigue of metallic materials is attempted for the sake of reference to facilitate the research in this area.

Comparison between Fractal and Statistical Approaches to Model Size Effects in VHCF

Size effects concern the anomalous scaling of relevant mechanical properties of materials and structures over a sufficiently wide dimensional range. In the last few years, thanks to technological advances, such effects have been experimentally detected also in the very high cycle fatigue (VHCF) tests. Research groups at Politecnico di Torino are very active in this field, observing size effects on fatigue strength, fatigue life and fatigue limit up to the VHCF regime for different metal alloys. In addition, different theoretical models have been put forward to explain these effects. In the present paper, two of them are introduced, respectively based on fractal geometry and statistical concepts. Furthermore, a comparison between the models and experimental results is provided. Both models are able to predict the decrement in the fatigue life and in the conventional fatigue limit.

Defect-Driven Topology Optimisation: TopFat algorithm validation via 3D components re-design for real industrial applications

In recent years, Additive Manufacturing (AM) emerged as a disruptive technology, able to manufacture complex geometries and features, which are not producible through traditional manufacturing processes. The diffusion of AM has therefore permitted to exploit the advantages of the design based on Topology Optimization (TO) algorithms, which provide the material distribution allowing for maximizing the component stiffness with the minimum mass. In order to guarantee the structural integrity of the designed part, constraints on the maximum allowable stress can be set. If the component is subjected to quasi-static loads, the yield stress is generally considered as the limit stress. In case of fatigue loads, the conventional fatigue limit can be, on the other hand, considered. However, it is well known that the fatigue response of parts produced through AM process is controlled by the manufacturing defects (e.g., porosity, incomplete fusion defects). The influence of defects is generally not considered when components are designed through TO algorithms, even if it drives the structural response. Only recently, a defect-driven TO algorithm, named TopFat, has been developed by the Authors. In the present paper, a methodology based on TO algorithms commercially available for the design against fatigue failures from defects is proposed. The influence of defects and the dependency between the defect size and the component volume is assessed through models available in the literature. The methodology has been applied to design components currently used for aerospace applications. To conclude, this study aims at providing a guideline for safely designing real AM parts considering the presence of defects by means of commercial TO software.

Improvement in Fatigue Performance of 2024-T3 Al Alloy Via Single Toroidal Roller Burnishing

Mechanical surface treatment, based on static severe plastic deformation of the surface layer, is a cost-effective approach to improving surface integrity and thus the operating properties of metal components. The present work examines the possibility of using single toroidal roller burnishing (STRB) to implement the deep rolling concept. The effect of the STRB process parameters on the fatigue behavior of 2024-T3 Al alloy specimens is investigated. The specimens were manufactured using a newly developed STRB device. The conventional fatigue limit of 2024-T3 Al alloy is established using the S–N curve approach and tangent method. The STRB implementation as the deep rolling process for 2024-T3 Al alloy specimens provides a maximum conventional fatigue limit of 256 MPa. Compared to the reference condition, the fatigue limit has increased by 38.4%. Calculated at the number of cycles corresponding to the conventional fatigue limit, the fatigue life is enhanced more than 2000 times over. The increase in the number of passes enhances the fatigue strength: as a result the conventional fatigue limit of the treated 2024-T3 Al alloy increased. Increasing the number of passes has practical meaning for up to six passes, after which the effect is negligible.

Single toroidal roller burnishing of 2024-T3 Al alloy implemented as mixed burnishing process

Based on a comprehensive experimental study, single toroidal roller burnishing (STRB) of the 2024-T3 Al alloy can be successfully implemented as a mixed burnishing process. Optimum values of various governing factors provided minimum roughness and significant enhancement of the fatigue life of the treated specimens. With a planned experiment, regression analysis, and optimization procedure based on a genetic algorithm, the optimum factor values were established under a minimum roughness criterion. The derived model predicted a minimum roughness Ra = 0.074 μm. The experiment with optimal process parameters provided an average roughness (Ra) of 0.01 μm. STRB under these optimal conditions yields a relatively homogeneous surface in terms of microhardness with a surface microhardness increase coefficient of 37.6%. The parametric study of the residual surface hoop and axial stresses conducted via X-ray stress analysis shows that the STRB with near-optimal process parameters introduces significant residual stresses. STRB of the 2024-T3 Al alloy, implemented as a mixed burnishing process, produces a mirror-finish surface, improves the fatigue life by more than 2000 times, and increases the conventional fatigue limit by 35.1% compared to the reference condition.

Fatigue Study of the Pre-Corroded 6082-T6 Aluminum Alloy in Saline Atmosphere

This work studies the influence of the saline atmospheric corrosion on the fatigue strength of 6061-T6 aluminum alloy. For this purpose, this alloy was subjected to tests in a salt spray corrosion chamber at different exposure times (1, 2, and 3 months) according to ASTM B117 standard. The morphological study of the pits was carried out by confocal microscopy. Subsequently, fatigue tests were performed at variable stresses whose maximum stress (Smax) was between 30% and 95% of the yield strength (S0) in order to keep them within the zone of elastic behavior of the material. Data were analyzed using the Basquin equation and the maximum likelihood function method. The results show a similar decrease in the conventional fatigue limit (2 × 106 cycles) after one month (98 MPa) and two months (91 MPa) of corrosion. After three months of corrosion, the material showed a very important reduction in the fatigue limit (68 MPa) with respect to the uncorroded material (131 MPa). The data of Se/S0 (fatigue limit/yield strength) versus the ratio Pm/Dm (pit average depth/pit diameter at zero depth) can be fitted to a logarithmic curve.

EA4T railway axle steel fatigue behavior under very high-frequency fatigue loading

One of the most loaded components of the train, with very long operation time, is a railway axle. During its operation, the conventional fatigue limit of 107 cycles is reached easily and information about fatigue behavior for a higher number of cycles is missing. Therefore, the fatigue behavior of railway axle EA4T heat-treated steel with a bainitic microstructure was examined under very high-frequency fatigue loading. S-N curve shift to the higher number of cycles at the same stress amplitude due to the testing frequency increase was observed. However, the same fatigue endurance limit of 375 MPa was determined for 107 cycles at a frequency of 120 Hz and for 1010 cycles at a frequency of 20 kHz. Due to homogenous and fine bainitic structure in a whole tested volume of material, the same fatigue crack initiation and propagation mechanism was observed to be characteristic for all the tested specimens, regardless of the used testing frequency and stress amplitude. Surface and subsurface non-inclusion fatigue crack initiation was responsible for the failure of the specimens. Obtain results confirm, that fatigue endurance limit determined by traditional high fatigue test can be used also for several orders higher number of cycles.

Fatigue behavior of 316 L stainless steel weldment up to very-high–cycle fatigue regime

In nuclear power plant, 316 L weldments are widely used in fabricating the primary coolant pipe and in-core components, which usually suffer from very-high-cycle fatigue. Therefore, in this study, the high-cycle fatigue behaviors of 316 L stainless steel with an argon-arc welding joint were studied up to very high-cycle regime by using an ultrasonic fatigue testing system. Microscopic examination indicated that both the base steel and welding seam showed austenitic equiaxed grains. The grain size and micro-hardness values of the welding seam were comparable to those of the base 316 L steel. And heat affect zone was not evidently observed at the weld joint. Fatigue tests showed that fatigue failure still occurred in 107–109 cycles, indicating that the conventional fatigue limit does not exist for 316 L weldment. The fatigue strengths of 316 L weldments were close to those of the base 316 L steel. Scanning electron microscopic examinations revealed that multiple fatigue cracks initiated at specimen surface in the high cycle and very-high-cycle fatigue regime. A modified Murakami’s model was suggested to take into account the effect of surface roughness and Vickers hardness on fatigue life/strength of 316 L weldment. The fatigue strength predicted using the proposed model showed good agreement with the experimental results.

Effect of Basketweave Microstructure on Very High Cycle Fatigue Behavior of TC21 Titanium Alloy

This paper discusses the effect of basketweave microstructure on the very high cycle fatigue behavior of TC21 titanium alloy. Ultrasonic fatigue tests at 20 kHz are done on a very high cycle fatigue (VHCF) property of the alloys with 60 μm and 40 μm basketweave size, respectively. Results show that the alloys illustrate step-wise S-N characteristics over the 105–109 cycle regimes and that fatigue fracture in both of the alloys occur beyond the conventional fatigue limit of 107 cycles. Subsurface crack initiation occurs at low stress amplitude. A fine granular area (FGA) is observed along the α lamella at the subsurface crack initiation site. The mechanism for the subsurface crack initiation is revealed using layer-by-layer-polishing, due to the micro-voids that are introduced at the granular α phase. The colony of α lamella is due to the local stress concentrated between them under the cyclic load. The stress intensity factor range at the FGA front is regarded as the threshold value controlling the internal crack propagation. Furthermore, the effect of the baseketweave size on the very high cycle fatigue limits of the TC21 titanium alloy is evaluated based on the Murakami model, which is consistent with the experimental results. The fatigue life of TC21 titanium alloy is well predicted using the energy-based crack nucleation life model.

Design of Shaft Respecting the Fatigue Limit for Ultra-High Number of Cycles

Advances in engineering solutions in recent few decades caused that conventional fatigue limit (for steels and cast irons given for number of 107 cycles) is no more sufficient. Construction parts of newly introduced transport vehicles, operating at high velocities and at long distances, reach during their lifetime very high numbers of loading cycles, in order of 109. For this reason, values of fatigue limits for 109 cycles must be considered in design and construction of transport vehicles. In this study, authors present how dramatically will change the design of shaft, when fatigue limit for 109 cycles is considered.

Fatigue life assessment regarding different influences on the HCF/VHCF behavior of a martensitic steel

Modern applications require a special treatment when the conventional specimen size is much larger than the component size. Additional to that, high sophisticated materials are used for highly loaded components. Often the conventional fatigue limit is exceeded and loads are applied in the VHCF regime. Focus was put on the lifetime calculation and the implementation of investigated fatigue data of a X5CrNiCuNb-16-4 type steel. Two specimen geometries with diameters D7.5=7.5 mm and D2.5=2.5 mm were tested at R=-1, at room temperature and up to 109 cycles to failure. The application of different software tools (FEMFAT, fe-safe) showed several issues based on the current results. Results showed a change of crack initiation mechanism to subsurface crack initiation at approx. 2x106 cycles to failure. The gradient based correction of the reference fatigue data showed a good applicability up to 2x106 cylces. The application of fe-safe allows the flexible modification of S/N parameters over the whole cycle range. The usage of the actual material configuration introduced several important questions regarding the fatigue data and the implementation into lifetime calculation tools.

Fatigue life assessment regarding different influences on the HCF/VHCF behavior of a martensitic steel

Modern applications require a special treatment when the conventional specimen size is much larger than the component size. Additional to that, high sophisticated materials are used for highly loaded components. Often the conventional fatigue limit is exceeded and loads are applied in the VHCF regime. Focus was put on the lifetime calculation and the implementation of investigated fatigue data of a X5CrNiCuNb-16-4 type steel. Two specimen geometries with diameters D7.5=7.5 mm and D2.5=2.5 mm were tested at R=-1, at room temperature and up to 109 cycles to failure. The application of different software tools (FEMFAT, fe-safe) showed several issues based on the current results. Results showed a change of crack initiation mechanism to subsurface crack initiation at approx. 2x106 cycles to failure. The gradient based correction of the reference fatigue data showed a good applicability up to 2x106 cylces. The application of fe-safe allows the flexible modification of S/N parameters over the whole cycle range. The usage of the actual material configuration introduced several important questions regarding the fatigue data and the implementation into lifetime calculation tools.

Experimental, analytical and numerical analyses of constant and variable amplitude loadings in the very high cycle fatigue regime

Mechanical components often exceed a very high number of cycles in their fatigue life. Within the very high cycle fatigue (VHCF) regime, different damage mechanisms occur and the conventional fatigue limit is not valid any more, which has to be considered in a safety–relevant design. Especially in reference to operation loads or variable amplitude loading insufficient knowledge exists. Therefore, within the scope of this paper, the cumulative frequency distributions Felix/28 and WISPER are applied by blocks on a high strength steel by means of an ultrasonic fatigue testing system. The results are compared to those of constant amplitude loading at different stress ratios. Due to the variable amplitude loadings arrest marks are produced within the fish–eye surrounding the inclusion. The sizes and the area, where arrest marks are observable, as well as the spacings between the arrest marks are influenced by the different load sequences. By counting and measuring the arrest marks an average crack growth rate for the crack propagation within the fish eye can be calculated. For further studies a three–dimensional finite element model is created to work out the stress distribution surrounding internal inclusions or cavities and to compare the obtained stress intensity factors of a circumferential crack initiating at the imperfection with analytical solutions. Because VHCF failure is highly dependent on the type of the imperfection and also on the interaction to the surrounding matrix, the stiffness of the inclusion as well as the contact formulation between the matrix and the inclusion in the finite element model are modified. Finally, three–dimensional crack propagation simulations are performed to investigate the influence of crack closure behaviour and the mean stresses.

Study on Very-High-Cycle-Fatigue Property of Aero-engine Blades Based on Subcomponent Specimen

The cyclic load number of aero-engine blade during its service life is very likely beyond 10 7 , which is regarded as the conventional fatigue limit. Moreover, surface strengthening is very often used in the manufacturing process of blade. The conventional testing method in the VHCF regime cannot exactly reflect the stress state of the blade, including the mechanism of crack initiation and propagation. To study the fatigue behavior and effects of laser shock peening, a kind of dissymmetrical bending fatigue subcomponent specimen was designed and the laser shock peening model was established. Experiment about TC17 was accomplished by the Ultra-High Cycle bend fatigue system. It is found that the fatigue damage occurs beneath the surface and the S-N curve is continuously rather than multi-step declining in the VHCF regime. Process of surface strengthening has a significant effect on fatigue performance of TC17 titanium alloy.

Interior Fracture Mechanism Analysis and Fatigue Life Prediction of Surface-Hardened Gear Steel under Axial Loading.

The interior defect-induced fracture of surface-hardened metallic materials in the long life region has become a key issue on engineering design. In the present study, the axial loading test with fully reversed condition was performed to examine the fatigue property of a surface-carburized low alloy gear steel in the long life region. Results show that this steel represents the duplex S-N (stress-number of cycles) characteristics without conventional fatigue limit related to 107 cycles. Fatigue cracks are all originated from the interior inclusions in the matrix region due to the inhabitation effect of carburized layer. The inclusion induced fracture with fisheye occurs in the short life region below 5 × 105 cycles, whereas the inclusion induced fracture with fine granular area (FGA) and fisheye occurs in the long life region beyond 106 cycles. The stress intensity factor range at the front of FGA can be regarded as the threshold value controlling stable growth of interior long crack. The evaluated maximum inclusion size in the effective damage volume of specimen is about 27.29 μm. Considering the size relationships between fisheye and FGA, and inclusion, the developed life prediction method involving crack growth can be acceptable on the basis of the good agreement between the predicted and experimental results.

Effect of fretting wear on very high cycle bending fatigue behaviors of FV520B steel

In order to investigate the effect of fretting wear on very high cycle bending fatigue behaviors of FV520B steel, fatigue tests with and without fretting wear were carried out by using ultrasonic fatigue testing system. The specimens for two fatigue testing methods were designed and the stress and displacement distributions were obtained by finite element method. The S–N curves were compared and analyzed based on fatigue data. The micromorphology characteristics of specimen surface and fracture surface were observed. The experimental results indicate that there exists no “conventional fatigue limit” and fatigue life decreases with the increase of applied stress for two fatigue testing methods. For fatigue test without fretting wear, the initiation and propagation of fatigue crack are mainly ascribed to surface machining defects and surface stress concentration. For fatigue test with fretting wear, surface damage including adhesive wear, abrasive wear and surface material removed in sliding contact promotes the initiation and propagation of numerous cracks on surface, which brings about the higher deviation of fatigue data. So the fatigue life is significantly reduced compared to that of the smooth specimen.

Very high cycle bending fatigue behaviors of FV520B steel under fretting wear

Abstract

The fatigue behavior and mechanism of FV520B-I with large surface roughness in a very high cycle regime

Abstract The fatigue properties of FV520B-I up to 109 cycles when the surface roughness Ra ≈ 0.6 were tested and compared with two groups of previously obtained test results. The test results showed that the S-N curve continuously moved downward and the transition stress at which the crack origin changed from the surface to the subsurface decreased with an increase of surface roughness, and the conventional fatigue limit finally appeared. The initiation mechanism of subsurface cracks in a very high cycle fatigue regime was independent of surface roughness. The surface fatigue limit and the high cycle fatigue life were predicted by relevant models. The competition mechanism between surface cracking and subsurface cracking was further discussed.