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

The fatigue properties of FV520B-I up to 10⁹ cycles when the surface roughness R_a ≈ 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.     

Influence of non-metallic inclusions on fatigue life in the very high cycle fatigue regime

The present paper deals with the influence of non-metallic inclusions on fatigue life in the high cycle fatigue and the very high cycle fatigue regime. For that purpose, several castings of steel 42CrMo4 (AISI 4140, DIN EN 1.7225) were produced by using recently developed novel metal-melt filters. The specimens were tested in hot-isostatically pressed and heat treated condition. After fatigue failure every fracture surface was intensively investigated by scanning electron microscopy in order to define the type, the size, the chemical composition, the morphology and the location of the crack initiating discontinuity. Subsequently, Murakami’s √area model was used for the evaluation of the influence of non-metallic inclusions on the fatigue life. In the present investigation four common types of chemical compositions of crack initiating discontinuities were identified. Furthermore, four different internal failure types and their influence on the fatigue life in cast steel were investigated and described. Thus, the present contribution proposes a basic correlation determined from fatigue lives in case of various internal crack initiation types. The key parameters for fatigue life prediction in case of internal fatigue failure in the very high cycle fatigue regime are (i) the size of the crack initiating discontinuity, (ii) the inclusion depth and (iii) the crack initiating failure type.     

Crack path in torsion loading in very high cycle fatigue regime

Torsion fatigue tests have been conducted at 20 kHz ultrasonic fatigue testing systems, and compared to the torsion fatigue data generated on 35 Hz conventional fatigue test machine to determine if there are any frequency effects, for steels including D38MSV5S steel and 100C6 steel. Results indicated that the S-N curves exhibit decrease in fatigue strength beyond 10⁷ cycles. The initiation in the Gigacycle regime is related to defects sometimes located beneath the surface which shows a competition between the maximum shear at the surface and the stress concentration under the surface, even in torsion.     

Approaches to fatigue life assessment applied in the very high cycle regime

Designers and calculation engineers are becoming increasingly interested in the latest results on very high cycle fatigue (VHCF). Often, the influence of loading with a very high number of cycles on component behaviour is estimated conservatively, but the exact safety margin is unknown. This paper gives an overview of failure mechanisms in the HCF‐ and VHCF‐regions and of material and component related influences, which have to be considered in the fatigue life assessment. The state of the art of design codes, recommendations from the literature and initial investigations on variable amplitude loading in the VHCF‐regime are presented. This review indicates that further research activities are necessary to improve fatigue life assessment in order to allow a reduction of safety margins.     

Factors influencing the GBF size of high strength steels in the very high cycle fatigue regime

The influences of major factors including applied stress amplitude, inclusion size and hydrogen content on granular-bright-facet (GBF) size of high strength steels in the very high cycle fatigue regime were studied in this article. It was found that the GBF size is determined by the applied stress amplitude and material hardness. If the applied stress amplitude is lower, the GBF size is larger. When a specimen containing bigger inclusions, the applied stress amplitude to form GBF can be reduced which results in the increase of GBF size. Hydrogen has different effects on the GBF size. The related reasons were discussed.     

Very high cycle fatigue of a high strength steel under sea water corrosion: A strong corrosion and mechanical damage coupling

This paper is focused on the effect of sea water corrosion on the gigacycle fatigue strength of a martensitic–bainitic hot rolled steel R5 used for manufacturing off-shore mooring chains for petroleum platforms in the North Sea. Crack initiation fatigue tests in the regime of 10⁶ to 10¹⁰ cycles were carried out on smooth specimens under three different environment conditions: (i) without any corrosion (virgin state) in air, (ii) in air after pre-corrosion, and (iii) in-situ corrosion-fatigue under artificial sea water flow. A drastic effect of sea water corrosion was found: the median fatigue strength beyond 10⁸ cycles is divided by 5 compared to virgin state specimens. The crack initiation sites were corrosion pits caused by pre-corrosion or created during corrosion-fatigue under sea water flow. Furthermore some sub-surface and internal crack initiations were observed on specimens without any corrosion (virgin state). Crack propagation curves were obtained in mode I in air and under sea water flow. Calculation of the stress intensity factor at the tip of cracks emanating from hemispherical surface pits combined with the Paris–Hertzberg–Mc Clintock crack growth rate model showed that fatigue crack initiation period represents most of the fatigue life in the VHCF regime. Additional original experiments have shown physical evidences that the fatigue strength in the gigacycle regime under sea water flow is mainly governed by the corrosion process with a strong coupling between cyclic loading and corrosion.     

Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

A review on fatigue fracture modes of structural metallic materials in very high cycle regime

In S–N diagrams for high strength steels, experimental data in the usual surface fracture mode appears at higher stress levels with fewer loading cycles, whereas the data in the interior fracture mode tends to appear at lower stress levels with the very long fatigue life. Thus, the duplex S–N property was usually confirmed for those high strength steels in such a very high cycle regime. In the case of interior fracture mode, there can be several different types of the crack initiation with/without nonmetallic inclusion at the crack initiation site, and different crack initiation types can be found even for the surface fracture modes in the conventional fatigue life region. In the present work, the authors have attempted to review the overall feature of these fatigue fracture modes appearing at the usual life regime and the very high cycle regime.     

On the determination of the bending fatigue strength in and above the very high cycle fatigue regime of shot-peened gears

Demands on modern gearboxes are constantly increasing, for example to comply with lightweight design goals or new CO₂ thresholds. Normally, to increase performance requires making gearboxes and powertrains more robust. However, this increases the weight of a standard gearbox. The two trends therefore seem contradictory. To satisfy both of these goals, gears in gearboxes can be shot-peened to introduce high compressive residual stresses and improve their bending fatigue strength. To determine a gear’s tooth root bending fatigue strength, experiments are conducted up to a defined number of load cycles in the high cycle fatigue range. However, investigations of shot-peened gears have revealed tooth root fracture damage initiated at non-metallic inclusions in and above the very high cycle fatigue range. This means that a further reduction in bending load carrying capacity has to be expected at higher load cycles, something which is not covered under current standard testing conditions. The question is whether there is a significant decrease in the bending load carrying capacity and, also, if pulsating tests conducted at higher load cycles—or even tests on the FZG back-to-back test rig—are necessary to determine a proper endurance fatigue limit for shot-peened gears. This paper examines these questions.

     

Fatigue crack initiation and growth on a steel in the very high cycle regime with sea water corrosion

This paper is devoted to the effect of corrosion on the gigacycle fatigue strength of a martensitic-bainitic hot rolled steel used for manufacturing offshore mooring chains for petroleum platforms. Smooth specimens were tested under fully reversed tension between 10⁶ and 10¹⁰ cycles in three testing conditions and environments: (i) in air, (ii) in air after pre-corrosion and (iii) in air under real time artificial sea water flow. The fatigue strength at greater than 10⁸ cycles is reduced by a factor more than five compared with non-corroded specimens. Fatigue cracks initiate at corrosion pits due to pre-corrosion, if any, or pits resulting from corrosion in real time during the cyclic loading. It is shown that under sea water flow, the fatigue life in the gigacycle regime is mainly governed by the corrosion process. Furthermore, the calculation of the mode I stress intensity factor at hemispherical surface defects (pits) combined with the Paris-Hertzberg-Mc Clintock crack growth rate model shows that fatigue crack initiation regime represents most of the fatigue life.     

Notable size effects on very high cycle fatigue properties of high-strength steel

Very high cycle fatigue (VHCF) properties were compared between two types of specimens: enlarged specimens and our standard specimens. Fatigue tests were conducted by ultrasonic fatigue testing; the material used was commercial spring steel. All tests ended in internal fracture, with large-size effects observed, i.e., the enlarged specimens showed lower VHCF strength than the standard specimens. Most of the internal fracture origins were oxide-type inclusions that were larger in the enlarged specimens than in the standard specimens, indicating the size effect to be caused by the difference in oxide-type inclusion sizes at the origins of internal fractures. The large-size effect strongly urges the use of large specimens when conducting VHCF tests on high-strength steel. Moreover, the large-size effect implies that fatigue strength cannot in this case be determined using the conventional S-N curve approach, since the S-N curve depends on the specimen size. The evaluation of the VHCF strength thus needs two steps: an estimation of the maximal inclusion size, followed by an estimation of the VHCF strength based on the maximal inclusion size.     

Effects of surface treatments on high cycle corrosion fatigue of metallic implant materials

Long term corrosion fatigue properties of two materials which are candidates for skeletal implants — cold worked c.p. Niobium and c.p. Tantalum — have been investigated. Fatigue properties have been compared to two implant materials in clinical use — c.p. Titanium and Ti–6Al–7Nb alloy. Constant amplitude fatigue experiments (S–N curves) were performed at ultrasonic frequency (20 kHz) with two different surface structures (ground surface and blasted and shot peened surface) in ambient air and in a corrosive fluid similar to the body fluid in the oral cavity. The endurance limit at 2×10⁸ cycles of all materials decreased by 5–20% if they were cycled in corrosive fluid instead of ambient air. The loss of fatigue strength is more pronounced for ground Ti–6Al–7Nb alloy and c.p. Ti than for ground c.p. Nb cw and c.p. Ta cw. Fracture surfaces show a more pronounced embrittlement of ground Ti–6Al–7Nb alloy and c.p. Titanium after cycling in corrosive fluid than ground c.p. Tantalum and c.p. Niobium. A beneficial influence of surface structuring by blasting and shot peening on the fatigue properties was found for all materials in both environments. Fatigue loading using ultrasonic frequency allows one to select appropriate implant materials and to determine their very-high cycle corrosion-fatigue behaviour within reasonable testing times. Though the obtained high-frequency values may not be fully representative of actual in vivo behaviour, they are regarded as useful material characterizing values.     

The characteristics of granular-bright facet in hydrogen pre-charged and uncharged high strength steels in the very high cycle fatigue regime

In this study, the effect of hydrogen on fatigue strength of high strength steels in the very high cycle fatigue regime was further discussed. It is found that the calculated results of fatigue strength by modified Murakami’s expression are in good accordance with the experimental results in ±15% error band. The relationship between fatigue life (N _f) and the ratio of granular-bright facet (GBF) to inclusion size (\frac?{A_\textGBF } ?{A_\textinc } ) \left({\frac{{\sqrt {A_{\text{GBF}} } }}{{\sqrt {A_{\text{inc}} } }}}\right) for quenching and tempering (QT) specimens and pre-charged specimens by soaking (SK) and cathodic (CD) charging can be approximately expressed by \frac?{A_\textGBF } ?{A_\textinc } = \fracR_\textGBF R_\textinc = 0. 2 5N_\textf ^{0. 1 2 5} {\frac{{\sqrt {A_{\text{GBF}} } }}{{\sqrt {A_{\text{inc}} } }}} = {\frac{{R_{\text{GBF}} }}{{R_{\text{inc}} }}} = 0. 2 5N_{\text{f}}^{ 0. 1 2 5} ; however, the value of \frac?{A_\textGBF } ?{A_\textinc } {\frac{{\sqrt {A_{\text{GBF}} } }}{{\sqrt {A_{\text{inc}} } }}} for specimens pre-charged by high-pressure thermal hydrogen charging is obviously greater than that for QT specimens and pre-charged specimens by SK and CD charging at an identical N _f. The stress intensity factor range at the periphery of the GBF, ΔK _GBF, was calculated in this work. It is found that the value of ΔK _GBF is not a constant but approximately proportional to (?{A_\textGBF } ) ^{1/ 3} (\sqrt {A_{\text{GBF}} } )^{ 1/ 3} . Besides it is also found that ΔK _GBF decreases with the increase of hydrogen content.     

Crack growth process in GBF area of SUJ2 bearing steel in very high cycle fatigue regime

Abstract

The crack growth process in GBF of high strength bearing steel JIS SUJ2 is qualitatively investigated by low–high two-step variable amplitude loading (TSAL) tests. It is demonstrated that the GBF size reduces with the increase in applied stress amplitude, and that the GBF size at a given stress amplitude is a constant which is independent of inclusion size at the crack origin. The fatigue life at a given stress amplitude, which is proportional to ?8 power of the inclusion size, reduces with increasing inclusion size. It was shown by TSAL tests that GBF was formed at the earlier stage (approximate 10⁵ cycles corresponding to less than 1% total fatigue life in the regime N_f≧10⁷ cycles) and propagated hardly until near to the final life (more than 90% total fatigue life). After that the GBF crack will propagate rapidly to the ultimate size in a short time.     

Analysis of endurance limits under very high cycle fatigue using a unified damage approach

A critical evaluation of endurance limits under very high cycle (giga-cycle range) fatigue is presented. The available experimental results are analyzed using the unified damage approach developed earlier by the authors. It is shown that the experimental evidence supports the theory that endurance limits at very high cycle fatigue are related to thresholds for crack propagation of incipient cracks from stress-concentration sites. The crack propagation behavior of the incipient cracks is related to the accumulation of internal stresses that is needed to augment the preexisting stresses. These in turn cause the incipient cracks to initiate at stress concentrations which either grow or arrest depending on the magnitude and gradient of the internal stresses. Micro-deformations at localized stress concentrations, such as inclusions, can further augment the local internal stresses needed for the incipient crack to nucleate and propagate. The presence of trapped hydrogen at inclusions can reduce the resistance to deformation, and hence reduce crack initiation and growth process. Understanding the role of preexisting stress concentrations (such as notches, inclusions, and hard second phases) provides a bridge for understanding the fatigue damage evolution in a smooth specimen compared to that in a cracked specimen. Kitagawa diagram helps provide this necessary bridge. Several critical issues related to fatigue testing method; fracture surface analysis, load history and environment are raised in this paper and briefly discussed in a self-consistent manner based on our unified damage approach.     

Influence of mechanical surface treatments on the high cycle fatigue performance of TIMETAL 54M

TIMETAL 54M (in the following Ti-54M) is a newly developed (α + β) titanium alloy with nominal composition Ti-5Al-4V-0.6Mo-0.4Fe. The alloy can provide a cost benefit over Ti-6Al-4V due to improved machinability and formability. These attractive properties might be a driving force for replacing Ti-6Al-4V in many aircraft as well as biomedical applications. Since HCF performance is one of the most important requirements for these applications, it is essential to improve this property by microstructural optimization and by mechanical surface treatments such as shot peening or ball burnishing. The latter improvement is mainly the result of induced near-surface severe plastic deformation which results in work-hardening and the generation of compressive residual stresses that retard fatigue crack propagation. The main aim of the present study was to investigate the potential fatigue life improvements in Ti-54M due to shot peening and ball-burnishing. The process-induced residual stresses and stress-depth profiles were determined by energy-dispersive X-ray diffraction (ED) of synchrotron radiation with the beam energy of 10-80 keV. Results on Ti-54M and Ti-6Al-4V will be compared and correlated with the mean stress and environmental sensitivities of the fatigue strengths in the microstructures.     

Fatigue behaviour of SiCp-reinforced aluminium composites in the very high cycle regime using ultrasonic fatigue

The fatigue behaviour of a 2009/SiC/15p‐T4 DRA composite has been examined in the very high cycle fatigue (VHCF) regime where 10⁷≤N_f≤ 10⁹ cycles. Ultrasonic fatigue was used to achieve the very high cycle counts. Careful processing yielded a composite with a very homogeneous particle distribution with minimal clustering. Fatigue crack initiation was observed almost exclusively at AlCuFe inclusions with no crack initiation observed at SiC particle clusters. Fatigue lives at a given stress level exhibited minimal scatter and subsurface crack initiation was observed in all cases. This behaviour is consistent with the presence of a low number density of critical inclusions that are responsible for crack initiation very early in fatigue life.     

Through thickness property variations in friction stir welded AA6061 joint fatigued in very high cycle fatigue regime

Ultrasonic fatigue tests were performed on friction stir welded AA6061 joint to investigate very high cycle fatigue (VHCF) behaviors. As a result, almost all the fatigue cracks are initiated from local plastic slip markings around the boundary between thermo-mechanically affected zone and heat affected zone. The fatigue strength decreases from the top to root of the welded joint, owing to the variation of plastic deformation history and temperature distribution through the thickness. In fractography, the fatigue crack initiation site is surrounded by a semicircular flat zone, of which the formation in VHCF regime accounts for more than 98% of the total fatigue life.     

Numerical simulation of the coupling between thermal dissipation and fish‐eye crack growth in very high cycle fatigue regime

In this paper, we study the temperature field associated with the propagation of a fatigue crack in a very high cycle fatigue regime during ultrasonic fatigue testing. We use a Paris–Hertzberg crack growth law to compute the evolution of the crack and a perfectly elastic–plastic constitutive law to compute the plastic dissipation per cycle at the tip of the crack. A thermomechanical finite element model is proposed to estimate the evolution of the temperature field during the crack propagation. Numerical results obtained agree fairly well with experimental results.