On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part 1: influence of hydrogen trapped by inclusions

The fracture surfaces of specimens of a heat-treated hard steel, namely Cr–Mo steel SCM435, which failed in the regime of N = 10⁵ to 5 × 10⁸ cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat-treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.     

S‐N curves in the very‐high‐cycle fatigue regime: statistical modeling based on the hydrogen embrittlement consideration

The continuous increment of durability and reliability requirements for many machinery components is significantly enhancing the research activity in the Very‐High‐Cycle Fatigue (VHCF) characterization of metallic materials, in particular of high‐strength steels for critical structural applications. According to the model, the VHCF strength of high‐strength steels can be estimated from the projected area of the ‘Optically Dark Area’ (ODA), which plays a key role in the VHCF response of high‐strength steels: more than 95% of the total VHCF life is consumed in the ODA formation, with crack growing even though the Stress Intensity Factor (SIF) is below the threshold for crack growth. Following the hydrogen embrittlement theory proposed by Murakami, hydrogen is supposed to assist crack growth within the ODA. The present paper proposes a general SIF formulation for the analytical model of the hydrogen assisted crack growth within the ODA. Starting from the general SIF formulation, a general expression for the material fatigue limit is obtained in the paper. The statistical method for the estimation of the parameters involved in the proposed model is finally illustrated in the paper and numerically applied to an experimental dataset.     

Interior failure mechanism and life prediction of surface‐treated 17Cr‐Ni steel under high and very high cycle fatigue

The understanding of very high cycle fatigue (VHCF) mechanisms is critical to the development of life prediction approach. For this purpose, high cycle fatigue (HCF) and VHCF properties of a surface‐treated 17Cr‐Ni steel were investigated under axial loading with stress ratio of 0. This steel exhibits the constantly decreasing S‐N characteristics associated with the inclusion‐fisheye induced failure under the HCF and the inclusion‐FGA (fine granular area)‐fisheye induced failure under the VHCF. The cyclic pressing plays an important role in the FGA formation process, but the FGA still can be observed for the stress ratio of zero due to the slight crack closure effect. Two life modelling approaches associated with related failure mechanisms in the HCF and VHCF regimes are proposed based on the agreement between experimental and predicted results.     

A rapid scatter prediction method for very high cycle fatigue

Very high cycle fatigue (VHCF) tests are often performed by a high‐frequency fatigue test system, such as ultrasonic fatigue test machine. In the paper, simple VHCF tests and cumulative fatigue (low cycle fatigue plus VHCF) tests are performed to investigate the fatigue behaviour, respectively, for a low carbon manganese steel. The test results in Wöhler diagram show a large scatter in VHCF regime. Continuum damage mechanics model is extended to VHCF region to estimate the remaining fatigue life. A rapid fatigue failure probability prediction method is applied and extended to VHCF regime in order to evaluate the fatigue dispersion based on multi‐scales model and fatigue dissipation analysis.     

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

It is well‐known that the high cycle fatigue (HCF) strength of steel components is influenced by a lot of factors depending on both material, loading (including environment), specimen or component geometry (design), and manufacturing process. Based on a literature review of a lot of experimental data, a synthesis is proposed in this paper to discuss the effect of the structural and operational factors on the very high cycle fatigue (VHCF) characteristics of steels. HCF and VHCF regimes are distinguished in terms of failure mechanisms and S‐N curve shapes for high and low strength steels. Then, the effect of the microstructural and mechanical features on the VHCF resistance is debated as different parameters (microstructure, inclusion size type and depth, hydrogen, environment, maximum tensile strength, and residual stresses). Next, the influence of the loading conditions is addressed by taking into account both the frequency effect, the highly stressed volume, the loading type, and loading ratio. Finally, the influence of the testing techniques used in VHCF experiments is discussed.     

Very-high-cycle fatigue crack initiation and propagation behaviours of magnesium alloy ZK60

The aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.     

Fisheye failure analysis and life design approach for case-carburized gear steel based on statistical evaluation of defect size

The high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of a case-carburized gear steel were investigated under tension and compression, and the distribution characteristics of defect size was analyzed using four statistical functions including Statistics of Extreme Values (SEV), Generalized Pareto distribution (GP), Generalized Extreme Values (GEV) and Exponential Generalized Pareto distribution (EXPGP). Results show that the interior inclusion-induced failure with the existence of fisheye becomes the predominant failure mode in HCF and VHCF regimes. With the increasing of applied stress amplitude, the effect of crack size within the fisheye on fatigue life is gradually reduced. Based on this, a fatigue strength prediction model associated with the variation of crack size was developed. At a given probability, the predicted fatigue strength is the highest by the estimated maximum inclusion size using the GEV distribution, then followed by using Gumbel distribution and by using GP distribution, and the lowest by using EXPGP distribution, which is inversely proportional to the evaluated maximum inclusion sizes. Based on the good agreement between the predicted and experimental results, the proposed approach based on statistical evaluation of defect size can be well used to predict the HCF and VHCF fatigue lives with interior fisheye failure.     

Investigation of high cycle and Very-High-Cycle Fatigue behaviors for a structural steel with smooth and notched specimens

The high cycle and Very-High-Cycle Fatigue (VHCF) properties of a structural steel with smooth and notched specimens were studied by employing a rotary bending machine with frequency of 52.5 Hz. For smooth specimens, VHCF failure did occur at fatigue cycles of 7.1 × 10⁸ with the related S–N curve of stepwise tendency. Scanning Electron Microscopy (SEM) was used for the observations of the fracture surfaces. It shows that for smooth specimens the crack origination is surface mode in the failure regime of less than 10⁷ cycles. While at VHCF regime, the material failed from the nonmetallic inclusion lies in the interior of material, leading to the formation of fisheye pattern. The dimensions of crack initiation region were measured and discussed with respect to the number of cycles to failure. The mechanism analysis by means of low temperature fracture technique shows that the nonmetallic inclusion in the interior of specimen tends to debond from surrounding matrix and form a crack. The crack propagates and results to the final failure. The stress intensity factor and fatigue strength were calculated to investigate the crack initiation properties. VHCF study on the notched specimens shows that the obtained S–N curve decreases continuously. SEM analysis reveals that multiple crack origins are dominant on specimen surface and that fatigue crack tends to initiate from the surface of the specimen. Based on the fatigue tests and observations, a model of crack initiation was used to describe the transition of fatigue initiation site from subsurface to surface for smooth and notched specimens. The model reveals the influences of load, grain size, inclusion size and surface notch on the crack initiation transition.     

贝/马复相钢超高周疲劳行为及非夹杂起裂

贝/马复相钢具有较低的夹杂物敏感性,组织因素对其超高周疲劳性能具有显著影响。组织因素引起的"非夹杂起裂"成为贝/马复相钢重要的裂纹萌生方式,贝/马复相组织的类型、形态、均匀性、细化程度等都对钢的超高周疲劳性能具有显著影响。讨论了组织纯净化、组织细化和残余奥氏体对贝/马复相钢超高周疲劳性能及其裂纹萌生机制的影响,在合理控制夹杂物水平的基础上,调控复相组织,可以在1 600MPa级别的贝/马复相钢中,获得超高周(循环周次大于108)疲劳强度达到900MPa的优异性能。同时对非夹杂起裂机理进行了初步探讨。     

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.     

Short fatigue crack propagation during low-cycle,high cycle and very-high-cycle fatigue of duplex steel – An unified approach

The present paper reviews experimental results on the fatigue damage of austenitic–ferritic duplex steel under various load levels ranging from LCF to VHCF, placing the focus towards the relationship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. A combination between fatigue testing of electropolished specimens and analytical electron microscopy (SEM/EBSD, TEM) revealed that under LCF loading conditions almost all the ferrite and the austenite grains showed plasticity, while under HCF and VHCF loading conditions, slip band formation was limited to the softer austenite grains and a low plastic activity is observed in the ferrite. Once being formed, the bands generate high stress concentrations, where they impinge the α–γ phase boundaries, eventually, leading to the crack initiation. This is discussed by applying a numerical simulation approach based on the finite-element (FEM) and the boundary-element (BEM) method.     

Evaluation of crack growth behavior and probabilistic S–N characteristics of carburized Cr–Mn–Si steel with multiple failure modes

The unexpected failures of case-hardened steels in long life regime have been a critical issue in modern engineering design. In this study, the failure behavior of a carburized Cr–Mn–Si steel under very high cycle fatigue (VHCF) was investigated, and a model for evaluating the probabilistic S–N curve associated with multiple failure modes was developed. Results show that the carburized Cr–Mn–Si steel exhibits three failure modes including the surface flaw-induced failure, the interior inclusion-induced failure without the fine granular area (FGA) and the interior inclusion-induced failure with the FGA. As the predominant failure mode in the VHCF regime, the interior failure process can be divided into four stages: (i) the small crack growth around the inclusion, (ii) the stable macroscopic crack growth outside the FGA, (iii) the unstable crack growth outside the fish-eye and (iv) the momentary fracture outside the final crack growth zone. The threshold values are successively evaluated to be 2.33 MPa m^1/2, 4.13 MPa m^1/2, 18.51 MPa m^1/2 and 29.26 MPa m^1/2. The distribution characteristics of the test data in transition failure region can be well characterized by the mixed two-parameter Weibull distribution function. The developed probabilistic S–N curve model is in good agreement with the test data with multiple failure modes. Although the result is somewhat conservative in the VHCF regime, it is acceptable for safety considerations.     

The observation of compliance change in the transition from hydrogen to air environment during the fatigue test

A compliance change was observed during fatigue testing of ASTM A710 HSLA steel using constant “K” CDCB specimen. The compliance decreased from 1.296 × 10⁻⁵ mm/N to 1.235× 10⁻⁵ mm/N when the environment was changed from hydrogen to air under the fatigue test conditions of f = 0.2 Hz, R = 0.1 and Δ K = 10 MPa✓m. The compliance change was observed in all fatigue testing while changing the environment from hydrogen to air. This compliance change can be explained numerically using the differential method for the design factors of the CDCB specimen. It was found from the calculation that the compliance change corresponded to a 6.3% change in Young’s modulus. It is proposed that the increased compliance resulted from the decreased Young’s modulus, the reduced Young’s modulus resulted from the increased lattice dilation which in turn resulted from a significantly increased hydrogen concentration at the crack tip region. The increased hydrogen concentration at the crack tip resulted from stress-induced hydrogen diffusion at the crack tip region.This work was conducted at Illinois institute of Technology (IIT).     

VHCF properties of nitrided 18Ni maraging steel thin sheets with different Co and Ti content

High cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of two 18Ni maraging steels with different cobalt and titanium content and similar static strength are investigated. Ultrasonic fatigue tests are performed with thin sheets with nitrided surfaces at load ratio R = 0.1. The specimens are mounted on a carrier and are forced to joint vibrations at approximately 20 kHz. The increase of Co content and the elimination of Ti improved the HCF and VHCF strength of 18Ni maraging steel. TiN inclusions if Ti is present and Al₂O₃ inclusions in the Ti free material with sizes (area_INC)^1/2 smaller than 10 µm were preferential crack initiation locations. Considering inclusions as initial cracks, the minimum stress intensity range for VHCF failure is 1.2 MPam^1/2 for TiN inclusions and 1.8 MPam^1/2 for Al₂O₃ inclusions. Data scatter may be slightly reduced if lifetimes are presented versus stress amplitudes multiplied by (area_INC)^1/12 rather than in an S–N diagram.     

High‐cycle and very‐high‐cycle fatigue behaviour of a titanium alloy with equiaxed microstructure under different mean stresses

The fatigue behaviour of a titanium alloy Ti‐6Al‐4V with equiaxed microstructure (EM) under different values of tensile mean stress or stress ratio (R) was investigated from high‐cycle fatigue (HCF) to very‐high‐cycle fatigue (VHCF) regimes via ultrasonic axial cycling. The effect of mean stress or R on the fatigue strength of HCF and VHCF was addressed by Goodman, Gerber, and Authors' formula. Three types of crack initiation, namely, surface‐with‐RA (rough area), surface‐without‐RA, and interior‐with‐RA, were classified. The maximum value of stress intensity factor (SIF) at RA boundary for R < 0 keeps constant regardless of R in HCF and VHCF regimes. The SIF range at RA boundary for R > 0 also keeps constant regardless of R in VHCF regime, but this value decreases linearly with the increase of R for surface RA cases. The microstructure observation at RA regions gives a new result of nanograin formation only in the cases of negative stress ratios for the titanium alloy with EM, which is explained by the mechanism of numerous cyclic pressing.     

Ultrasonic fatigue testing of cast steel G42CrMo4 at elevated temperatures

The fatigue life of cast steel G42CrMo4 in two different heat treatment conditions was investigated at room temperature (RT), 473 K and 773 K up to the range of very high cycle fatigue (VHCF), that is, 10⁹ cycles. The fatigue life is determined by casting defects, the hardness of the steel matrix and by temperature. Fatigue life data were discussed in correlation with crack‐initiating defects analysed on fracture surfaces. The SN curves obtained at RT and at 473 K show a large scatter. However, the SN curve at 773 K exhibits a larger slope parameter and a significantly reduced scatter. It is shown that the fatigue behaviour of the cast steel G42CrMo4 changes from 473 to 773 K in the range of VHCF. The fatigue lives of the specimens tested at 773 K were described with a crack growth model.     

Untersuchungen zur Wechselwirkung von Versetzungen und Ausscheidungen in einer Nickelbasis‐Superlegierung im VHCF‐Bereich

Over the past years the fatigue behaviour of structural materials at very high number of cycles (VHCF) has become a widely acclaimed topic of scientific and technical interest. One of the most important application fields for nickel‐base superalloys are turbines, which demand fatigue lives exceeding 10⁷ cycles. Hence, in this study the interrelationship between dislocation slip behaviour, precipitation condition (peak‐aged and overaged) and fatigue behaviour of the nickel‐base superalloy Nimonic 80A was investigated in the VHCF range focusing on the influence of the predominant microstructural feature. Surprisingly, the overaged condition of Nimonic 80A shows a slightly higher fatigue strength in the VHCF regime as compared to the peak‐aged condition. Accordingly, the VHCF‐behaviour does neither correspond to the static strength nor to the fatigue behaviour in the conventional range. Microstructural features evoked by cyclic deformation (e. g. development of slip markings at surface grains and dislocation/precipitate interaction) were characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) providing preliminary explanations for the unexpected fatigue behaviour. A comparison of cyclic deformation curves both for the conventional and the VHCF range illustrates the pronounced difference in global and local strain range and its influence on the damage evolution during fatigue testing.     

Very high cycle fatigue behaviors of Mn–Si–Cr series Bainite/Martensite dual phase steels

The present work studied the very high cycle fatigue (VHCF) behaviors of Mn–Si–Cr series Bainite/Martensite (B/M) dual phase high-strength low-alloy (HSLA) steels to ensure safe applications in railway and oil field. Four kinds of chemical compositions and seven types of B/M steels were designed and studied by ultrasonic fatigue test. The conception of apparent and intrinsic VHCF strength is addressed. Results show the effect of heat generation cannot be ignored in B/M steels in ultrasonic fatigue test, and higher is the intrinsic VHCF strength of materials, more remarkable is the negative effect of self-heating. More importantly, the VHCF property of B/M steels is less sensitive to the inclusion than that of tempered martensite steels, and the amount of retained austenite puts little effect on the VHCF property.     

Fatigue strength of spring steel with small scratches

In compression coil springs subjected to cyclic load, fatigue cracks can sometimes initiate and propagate from scratch‐like small defects produced during the manufacturing process and degrade the fatigue strength. In this study, torsional and rotating bending fatigue tests were conducted to examine the fatigue behavior of a high‐strength spring steel (JIS G 3561, SWOSC‐V) in the presence of small scratches. The sensitivity of the HCF and VHCF strength to small scratches was evaluated based on the parameter model.     

Thermal fatigue and failure of electronic power device substrates

Electronic power devices used for transportation applications (automotive and avionics) experience severe temperature variations, which promote their thermal fatigue and failure. For example, for power modules mounted on the engine of an aircraft, temperature variations range from −55 °C (in the worst case of storage before takeoff) to +200 °C (flight). Direct bonded copper (DBC) substrates are used to isolate chips (silicon dies) from their base plates. For large thermal amplitudes, the failure occurs in DBC substrates, which are copper/ceramic/copper sandwiches. The Weibull approach was used to model the brittle fracture of the ceramic layer from a natural defect. Furthermore, geometric singularities in the upper ceramic/copper interface are at the origin of cracks that grow by fatigue along the interface and finally bifurcate and break the ceramic layer. It is discussed how the framework of linear elastic fracture mechanics (LEFM) can be used to characterize the stress field around singularities and the associated risk of failure. These two criteria and the finite element method, allow analysing how a thermal loading history may modify the risk of failure of DBC substrates. It was shown, in particular, that three overcooling cycles should produce an “overload retardation effect”. Experimentally, applying three “overload cycles” (−70 °C, +180 °C), prior to thermal fatigue cycles (−30 °C, +180 °C), increased very significantly the fatigue life of DBC substrates. This result shows that the fatigue life and the reliability of power electronic devices could be optimized using a thermo-mechanical approach of the problem and suitable failure criteria.