Very high cycle fatigue properties of bainitic high carbon–chromium steel

Fatigue properties of bainitic 100Cr6 (SAE 52100, JIS SUJ2) steel are investigated in the high cycle and very high cycle fatigue (VHCF) regime. Fully reversed tension–compression fatigue tests are performed with ultrasonic fatigue testing equipment. Specimens are grinded which leads to surface compression stresses and increased surface roughness. About 1/3 of the specimens failed after crack initiation at interior Al₂O₃? or TiN-inclusions and 2/3 failed after surface crack initiation at scratches or cavities. When inclusions are considered as cracks, failures can occur at minimum stress intensity range of 2.8 MPa m^1/2, and maximum stress intensity range without failure is 3.3 MPa m^1/2. Facets are visible close to the inclusion in some specimens, and the stress intensity range at the border of the facet is approximately 4.5 MPa m^1/2. Murakami’s model can well predict the endurance limit at 10⁹ cycles for internal failures considering the area of the inclusion in the evaluation. Surface fatigue crack initiation can lead to failure above 10⁸ cycles. When scratches are considered as cracks, minimum stress intensity range of 2.5 MPa m^1/2 can propagate surface cracks to failure. Fracture mechanics approach showed several similarities to literature results of the same material tested in tempered martensite condition.     

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.     

Cyclic torsion very high cycle fatigue of VDSiCr spring steel at different load ratios

Cyclic torsion fatigue tests with superimposed static torsion loads are performed with VDSiCr spring steel with shot-peened surface in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Fatigue properties are investigated at load ratios R = 0.1, R = 0.35 and R = 0.5 up to limiting lifetimes of 5 × 10⁹ cycles with a newly developed ultrasonic torsion testing method. Increasing the load ratio reduces the shear stress amplitude that the material can withstand without failure. Fatigue cracks are initiated at the surface in the HCF regime. In the VHCF regime, cracks are preferentially initiated internally in the matrix, below the surface layer with compression residual stresses, and less frequently at the surface. Cyclic and mean shear stresses with 50% survival probability in the VHCF regime are presented in a Haigh diagram. Linear line approximation delivers a mean stress sensitivity of M = 0.33 for load ratios between R = −1 and R = 0.5.     

Fatigue behavior and plane-strain fracture toughness of sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy

In this study, the tensile properties, high cycle fatigue behavior and plane-strain fracture toughness of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy were investigated, comparison to that of sand-cast plus T6 heat treated magnesium alloy which named after sand-cast-T6. The results showed that the tensile properties of the sand-cast alloy are greatly improved after T6 heat treatment, and the fatigue strength (at 10⁷ cycles) of the sand-cast Mg–10Gd–3Y–0.5Zr magnesium alloy increases from 95 to 120 MPa after T6 heat treatment, i.e. the improvement of 26% in fatigue strength has been achieved. The plane-strain fracture toughnesses K_IC of the sand-cast and sand-cast-T6 alloys are about 12.1 and 16.3 MPa m^1/2, respectively. In addition, crack initiation, crack propagation and fracture behavior of the studied alloys after tensile test, high cycle fatigue test and plane-strain fracture toughness test were also investigated systematically.     

Pit-to-crack transition under cyclic loading in 12% Cr steam turbine blade steel

Fatigue measurements were performed up to the very high cycle fatigue regime in order to investigate pit-to-crack transition in 12% Cr steam turbine blade steel. Pre-pitted and smooth specimens were tested in air and aerated 6 ppm Cl⁻ solution. S–N curves for different stress ratios were determined and a stress ratio dependent critical pit size for pit-to-crack transition with subsequent failure was found. Early crack initiation and small crack growth were observed in the process of development and by fractography using field emission scanning electron microscopy. Fatigue crack growth rates (FCGR) for cracks emanating from pits were determined. Good similarity of FCGR curves for short and long cracks was obtained by normalising the stress intensity factor ranges with the threshold values.     

Short cracks initiated in Al 6013-T6 with the focused ion beam (FIB)-technology

The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner cracks at a stress ratio of R = 0.1, R = 0.7 and R = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T) – specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The precracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with short corner cracks were compared with the long fatigue crack growth test data. The short cracks grew at ΔK-values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK-values and the same stress ratios. A model was created on the basis of constant K_max-tests with long cracks that gives a good and conservative estimation of the short crack growth rates.     

Influence of hydrogen content and microstructure on the fatigue behaviour of steel SAE 52100 in the VHCF regime

The fatigue life of the bearing steel 52100 (100Cr6) in bainite and martensite conditions was investigated up to 2 × 10⁹ cycles. The tests were performed under cycling tension (R = 0.1) and tension–compression (R = −1) on a piezo-electric ultrasonic testing equipment. The specimens are designed with a cylindrical part in the highly stressed centre. Due to grinding, compressive residual stresses are found at the surface, hence crack initiation solely occurs subsurface. Prior testing half of the specimens was charged with hydrogen. The hydrogen content varies from 0.6 as initial condition to 3 ppm after charging. The increased hydrogen content decreased the endurance limit to nearly half of the value of uncharged conditions and crack initiation changed: Conditions with low hydrogen content failed from chromium carbides, titanium nitrides or slag agglomerations. Conditions with 3 ppm hydrogen failed from slag agglomerations, often in combination with aluminium magnesium oxides, and manganese sulphides. Next to the inclusions a fine granular area (FGA) could be observed in some cases, and nearly all fractured surfaces showed a fisheye surrounded by an Optically Bright Zone (OBZ) with the crack initiating inclusion in its centre. Furthermore, selected specimens where analysed using secondary ion mass spectroscopy in a time of flight setup (ToF-SIMS) to ascertain the local hydrogen content. From the results it is assumed that hydrogen accumulates in the cavity at inclusions or bonds to the inclusion if it contains silicon.     

Fatigue propagation of short and long cracks in gaseous hydrogen environment in 3.5NiCrMoV steel

Turbo generators for nuclear plants are mostly equipped with hydrogen cooling systems. Current practice of characterizing the growth of fatigue cracks on the basis of fracture mechanics primarily relies on fatigue tests for long cracks which are typically of several millimeters in length. However, in view of extended life for the plants, the damage tolerance evaluation of such fatigue-critical engineering components requires understanding of the propagation of cracks of significantly smaller dimensions. Then the near threshold of short cracks is investigated and compared to the behavior of long crack by experiments under 4 bar hydrogen atmosphere. The short crack fatigue propagation in hydrogen atmosphere is shown similar to that in air, growing faster than the long crack and at ΔK ranging below the long crack threshold; this effect is related to a reduced crack closure shielding. The propagation behavior of long crack under hydrogen atmosphere is shown similar to that obtained in air in the low rate range, i.e. when the maximum of the stress intensity factor K_max is lower than a critical level of about 16 MPa m^1/2 with higher crack growth rate than in high vacuum. This environment effect is related to the presence of residual water vapor in both gases. For higher K_max, much faster growth rates under hydrogen atmosphere in comparison to air and vacuum are observed and related to hydrogen assisted intergranular propagation combining fatigue and sustained loading damage. The results are discussed on the basis of micrographic observations supporting the involved mechanisms.     

Fatigue of 2024-T351 aluminium alloy at different load ratios up to 1010 cycles

Fatigue properties of 2024-T351 aluminium alloy are investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Endurance tests are performed with ultrasonic equipment at 20 kHz cycling frequency at load ratios of R = −1, R = 0.1 and R = 0.5 up to 10¹⁰ cycles. Additional servo-hydraulic tests between 8 and 10 Hz at R = 0.1 show no frequency influence on fatigue lifetimes. Linear lines in double logarithmic S–N plots are used to approximate data. Slope exponents of approximation lines increase with increasing numbers of cycles for all load ratios. Failures above 5 × 10⁹ cycles (R = −1 and R = 0.1) or 10¹⁰ cycles (R = 0.5) occur, and no fatigue limit is found. Fatigue cracks leading to failures above 10⁹ cycles are initiated at the surface or slightly below at broken constituent particles or at agglomerations of fractured particles, which are probably Al₇Cu₂(Fe, Mn). Specimens stressed with more than 10¹⁰ cycles at R = −1 without failure show several cracks starting at constituent particles. Maximum crack lengths are 30 μm, which is considerably below grain size.     

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.     

Crack growth behavior of 7075-T6 under biaxial tension–tension fatigue

Crack growth behavior of aluminum alloy 7075-T6 was investigated under in-plane biaxial tension–tension fatigue with stress ratio of 0.5. Two biaxiality ratios, λ (=1 and 1.5) were used. Cruciform specimens with a center hole, having a notch at 45° to the specimen’s arms, were tested in a biaxial fatigue test machine. Crack initiated and propagated coplanar with the notch for λ = 1 in L–T orientation, while it was non-coplanar for λ = 1.5 between L–T and T–L orientations. Uniaxial fatigue crack growth tests in L–T and T–L orientations were also conducted. Crack growth rate in region II was practically the same for biaxial fatigue with λ = 1 in L–T orientation and for the uniaxial fatigue in L–T or T–L orientations, while it was faster for biaxial fatigue with λ = 1.5 at a given crack driving force. However, fatigue damage mechanisms were quite different in each case. In region I, crack driving force at a given crack growth rate was smallest for biaxial fatigue with λ = 1.5 and for uniaxial fatigue in T–L orientation, followed by biaxial fatigue with λ = 1 and uniaxial fatigue in L–T orientation in ascending order at a given crack growth rate.     

Structure property relation of hybrid biocomposites based on jute,viscose and polypropylene: The effect of the fibre content and the length on the fracture toughness and the fatigue properties

In the present study, the extent of jute and viscose fibre breakage during the extrusion process on the fracture toughness and the fatigue properties was investigated. The composite materials were manufactured using direct long fibre thermoplastic (D-LFT) extrusion, followed by compression moulding. The fracture toughness (K_IC) and the fracture energy (G_IC) of the PP–J30 composites were significantly improved (133% and 514%, respectively) with the addition of 10 wt% viscose fibres, indicating hindered crack propagation. The addition of viscose fibres resulted in three times higher fatigue life compared with that of the unmodified jute composites. Further, with the addition of (2 wt%) MAPP, the PP–J30–V10 resulted in a higher average viscose fibre length of 8.1 mm, and the fracture toughness and fracture energy increased from 9.1 to 10.0 MPa m^1/2 and 28.9 to 31.2 kJ/m², respectively. Similarly, the fatigue life increased 51% compared with the PP–J30–V10, thus demonstrating the increased work energy due to hindrance of the propagation of cracks.     

Equivalent crack size model for pre-corrosion fatigue life prediction of aluminum alloy 7075-T6

Corrosion damage can significantly reduce the service life of aluminum alloy structures and endanger the structural integrity of aircraft. Here, aiming at center-hole sheet specimens of aluminum alloy 7075-T6, uniaxial fatigue tests and post-fracture analysis are performed to investigate the effect of corrosion pits on the pre-corrosion fatigue behavior. Then the best correlated parameters between corrosion pits and equivalent cracks are identified through Pearson correlation analysis. It is found that for single-crack initiations ar_ECS (equivalent crack depth 1 aspect ratio) vs. ar_cri (critical pit depth 1 aspect ratio) are best correlated with correlation coefficient of 0.9, while the best correlated parameters for multi-crack initiations are ar_ECS (equivalent crack depth 1 aspect ratio) vs. r_cri (aspect ratio) with correlation coefficient of 0.69. Equivalent crack size (ECS) models are correspondingly developed with these best correlated parameters for single- and multi-crack initiations, respectively. The pre-corrosion fatigue lives predicted with our models agree well with the experimental results and the maximum error factor is about 1.6.     

Experimental study on the fatigue failure mechanism of QP-16 type ball-eye under asymmetric load

The ball eye (BE) is a key connecting component between the insulator and transmission tower, whose fatigue characteristics concern the safety of transmission lines. To understand the fatigue mechanism and characteristics of it, the fatigue test was conducted based on the following data: r = 0.25, S = 500 MPa,then plotting of SN and Δε_axis − N, to analyze the fatigue failure of the test specimen from the macro and micro point of views. The research results show that: the life of BE significantly reduces with the increase of the stress amplitude, but the relative reduction in life is not the same; softening and strain amplitude of the specimen change differently before and after the stress amplitude of 300 MPa; when S ≤ 300 MPa, the fracture is more smooth, the fatigue crack propagation is slow; when S > 300 MPa, the rate of fatigue crack growth is faster, and the fatigue crack growth zones are not obvious. The cracks are easily detectable appear at the joint of the BE and insulator cap, and the cracks along the fracture cross section are constantly expanding, showing multiple fatigue sources and fatigue steps. The number of fatigue steps increases as the magnitude of the tensile stress increases. When S = 500 MPa, the yield strength decreases during the lifetime, the decrease rate of the tensile strength and microstructure strength in each stage are different. Axial lengthening and section shrinkage ratio decrease with the development of fatigue, fatigue evolution process is accompanied by phenomenon of crystalline slip, deformation, dislocation, at the same time, dissipation and decomposition of pearlite occur, and carbide precipitates from the matrix, growing and moving to the grain boundaries, the specific phenomenon of grain growth appears.     

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.     

Acoustic emission study of fatigue crack closure of physical short and long cracks for aluminum alloy LY12CZ

Crack closure of physical short and long cracks of LY12CZ aluminum alloy during fatigue process was investigated using acoustic emission (AE) technique. Results showed that the effective fatigue crack growth curve (da/dN vs. ΔK_eff) of physical short and long cracks obtained by the AE technique was consistent with the effective fatigue crack growth curve at high stress ratio (R = 0.8), which implied that the AE technique could measure the crack closure level, especially for physical short crack. The growth rate of physical short crack was much higher than that of long crack at the same ΔK, and the lower crack closure level of short crack was the main reason.     

Gigacycle fatigue initiation mechanism in Armco iron

Microstructure irreversibility plays a major role in the gigacycle fatigue crack initiation. Surface Persistent Slip Bands (PSB) formation on Copper and its alloy was well studied by Mughrabi et al. as typical fatigue crack nucleation in the very high cycle fatigue regime. In the present paper, Armco iron sheet specimens (1 mm thickness) were tested under ultrasonic frequency fatigue loading in tension–compression (R = −1). The test on the thin sheets has required a new design of specimen and new attachment of specimen. After gigacycle fatigue testing, the surface appearance was observed by optical and Scanning Electron Microscope (SEM). Below about 88 MPa stress, there is no PSBs even after fatigue cycle up to 5 × 10⁹. With a sufficient stress (above 88 MPa), PSBs in the ferrite grain was observed by optic microscope after 10⁸ cycles loading. Investigation with the SEM shows that the PSB can appear in the body-centered cubic crystal in the gigacycle fatigue regime. Because of the grain boundary, however, the local PSB did not continually progress to the grain beside even after 10⁹ cycles when the stress remained at the low level.     

Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes

This paper is focused on the VHCF behavior of aeronautical titanium alloy under tensile and torsion fatigue loadings. Tensile tests were carried out with two different stress ratios: R = −1 and R = 0.1. Both surface and subsurface crack initiations were observed. In the case of subsurface crack initiation several fatigue life controlling mechanisms of crack initiation were found under fully-reversed loading conditions: initiation from (1) strong defects; (2) ‘macro-zone’ borders; (3) quasi-smooth facets and (4) smooth facets. Tests with stress ratio R = 0.1, have shown that initiation from the borders of ‘macro-zones’ becomes the dominant crack initiation mechanism in presence of positive mean stress. Like for the tensile results, surface and subsurface crack initiations were observed under ultrasonic torsion in spite of the maximum shear stress location on the specimen surface. But the real reason for the subsurface crack initiation under torsion was not found.     

High- and very high-cycle plain fatigue resistance of shot peened high-strength aluminum alloys: The role of surface morphology

The present paper is aimed at investigating the effect of shot peening on the high and very-high cycle plain fatigue resistance of the Al-7075-T651 alloy. Pulsating bending fatigue tests (R = 0.05) were carried out on smooth samples exploring fatigue lives comprised between 10⁵ and 10⁸ cycles. Three peening treatments were considered to explore different initial residual stress profiles and surface microstructural conditions. An extensive analysis of the residual stress field was carried out by measuring with the X-ray diffraction (XRD) technique the residual stress profile before and at the end of the fatigue tests. Fatigue crack initiation sites were investigated through scanning electron microscopy (SEM) fractography. The surface morphology modifications induced by shot peening were evaluated using an optical profilometer. The influence of surface finishing on the fatigue resistance was quantified by eliminating the surface roughness in some peened specimens through a tribofinishing treatment. The capability of shot peening to hinder the initiation and to retard the subsequent propagation of surface cracks is discussed on the basis of a model combining a multiaxial fatigue criterion and a fracture mechanics approach.