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.     

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.     

Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10 V-2 Fe-3 Al alloy components are expected to endure cyclic loading with cycles above 109. To assess their operating safety, S-N relations of Ti-10 V-2 Fe-3 Al alloy in very high cycle fatigue(VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S-N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from α_p cleavage to α_s/βdecohesion, occur when the stress ratio(R) and stress level are reduced. Fatigue limits exist at N_f = 6 × 10~7 cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of α_p facets and α_s/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from-0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.     

Very high cycle fatigue of nitrided 18Ni maraging steel sheet

Thin sheets of nitrided 18Ni maraging steel are tested under cyclic tension (load ratio R = 0.1) in the very high cycle fatigue (VHCF) regime. The ultrasonic fatigue testing method with a cycling frequency of about 20 kHz has been further developed for these experiments. Sheet specimens with 0.35 mm thickness are mounted on a carrier specimen, they are pre-stressed and are forced to vibrate jointly. Between 10⁷ and 10⁹ cycles, fatigue cracks are initiated exclusively at internal TiN inclusions. The areas of the crack initiating inclusions projected perpendicular to the applied tensile stress are evaluated. The square root of inclusion areas, (area_INC)^1/2 lies between 2.5 μm and 5.3 μm. Considering inclusions as cracks, their stress intensity range is between ΔK_INC = 1.3 MPa m^1/2 and 2.4 MPa m^1/2. The sizes of crack initiating inclusions influence fatigue lifetimes. This is considered in a crack propagation model and by presenting lifetimes versus the stress amplitudes multiplied by (area_INC)^1/12. A mean lifetime of 10⁹ cycles is found at a stress amplitude of 22% of the tensile strength, which is comparable to other high strength steels tested under cyclic tension.     

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.     

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.     

Very high cycle fatigue behavior of a low strength welded joint at moderate temperature

Very high cycle fatigue (VHCF) behavior of a low strength weldment was investigated by fully reversed axial tests in air at room temperature and 370 °C. The role of non-metallic inclusions in the VHCF was addressed in terms of experimental results and finite element simulations. The higher potential for interior crack nucleation at higher temperature was ascribed to matrix softening, surface oxidation and surface compressive residual stress. A new model for interpretation of the role of inclusion in the transition of crack initiation modes was developed.     

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.     

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.     

Threshold crack growth behavior of shear and tensile cracks

Crack-face interference-free mode I and mode II crack-growth data was combined with smooth axial (λ = ε_xy/ε_xx = 0) and torsional (λ = ∞) endurance limit data to develop unified crack growth models that incorporate both shear and tensile cracking. The crack growth models incorporated growth from a slip band (including short crack behavior) size crack until the final failure of a long crack, and the ability to switch between crack growth on shear planes to growth on tensile planes. The models successfully predicted smooth specimen crack-face interference-free fatigue lives and gave reasonable estimates of the smooth specimen endurance limits of crack-face interference free tubular tests run at intermediate strain ratios (λ = 3/4, 3/2, and 3). The series of Kitigawa–Takahashi (threshold fatigue) diagrams developed from the models help illustrate the competition between shear and tensile cracking at the fatigue limit under crack-face interference-free crack growth.     

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.     

Effect of combined plasma-carburizing and deep-rolling on notch fatigue property of Ti-6Al-4V alloy

This paper discusses the effects of a combination of plasma-carburizing and deep-rolling on notch fatigue properties of a Ti-6Al-4V alloy. Circumferentially V-notched cylindrical Ti-6Al-4V alloy specimens were plasma-carburized at a relatively low temperature for the improvement of wear resistance, and then, deep-rolled at the notch root for inducing compressive residual stress. Scanning electron microscopy, optical microscopy, laser scanning microscopy, surface roughness tester, and micro-hardness tester were used to characterize the modified surface layer at the notch root. Axial loading fatigue tests (R = 0.1) were performed using a servo-hydraulic testing machine in a laboratory atmosphere at an ambient temperature. The notch fatigue life of the specimen was reduced by plasma-carburizing due to the brittleness caused by the higher hardness in addition to the disappearance of compressive residual stress on the notched surface, but remarkably improved by the subsequent deep-rolling. The surface layer containing the compressive residual stress and the work hardening induced by deep-rolling effectively prevented and delayed the fatigue crack initiation and propagation of deep-rolled carburized specimen.     

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.     

A micromechanical cyclic void growth model for ultra-low cycle fatigue

The influence of stress triaxiality and Lode parameter on microvoid growth phase of ductile fracture under ultra-low cycle fatigue (ULCF) (N_f < 100, N_f = cycles to failure) loading is investigated using micromechanical analyses. A new micromechanical cyclic void growth model (MM-CVGM) to predict the ULCF life of ASTM A992 steels is presented. The MM-CVGM is calibrated and validated from the experiments conducted on axisymmetrically notched specimens. Number of cycles to failure (N_f) and the fracture initiation locations predicted by the model closely matched the experimental observations.     

3D modeling of subsurface fatigue crack nucleation potency of primary inclusions in heat treated and shot peened martensitic gear steels

A computational strategy is developed to characterize the driving force for fatigue crack nucleation at subsurface primary inclusions in carburized and shot peened C61^® martensitic gear steels. Experimental investigation revealed minimum fatigue strength to be controlled by subsurface fatigue crack nucleation at inclusion clusters under cyclic bending. An algorithm is presented to simulate residual stress distribution induced through the shot peening process following carburization and tempering. A methodology is developed to analyze potency of fatigue crack nucleation at subsurface inclusions. Rate-independent 3D finite element analyses are performed to evaluate plastic deformation during processing and service. The specimen is subjected to reversed bending stress cycles with R = 0.05, representative of loading on a gear tooth. The matrix is modeled as an elastic–plastic material with pure nonlinear kinematic hardening. The inclusions are modeled as isotropic, linear elastic. Idealized inclusion geometries (ellipsoidal) are considered to study the fatigue crack nucleation potency at various subsurface depths. Three distinct types of second-phase particles (perfectly bonded, partially debonded, and cracked) are analyzed. Parametric studies quantify the effects of inclusion size, orientation and clustering on subsurface crack nucleation in the high cycle fatigue (HCF) or very high cycle fatigue (VHCF) regimes. The nonlocal average values of maximum plastic shear strain amplitude and Fatemi–Socie (FS) parameter calculated in the proximity of the inclusions are considered as the primary driving force parameters for fatigue crack nucleation and microstructurally small crack growth. The simulations indicate a strong propensity for crack nucleation at subsurface depths in agreement with experiments in which fatigue cracks nucleated at inclusion clusters, still in the compressive residual stress field. It is observed that the gradient from the surface of residual stress distribution, bending stress, and carburized material properties play a pivotal role in fatigue crack nucleation and small crack growth at subsurface primary inclusions. The fatigue potency of inclusion clusters is greatly increased by prior interfacial damage during processing.     

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.     

Fatigue lifetime and tearing resistance of AA2198 Al–Cu–Li alloy friction stir welds: Effect of defects

The fatigue strength and failure mechanisms of defect-free (“sound”) and flaw bearing friction stir butt-welds of 3.1 mm-thick AA2198-T8 Al–Li–Cu alloy have been investigated via S–N curves at R = 0.1 using cross weld specimens. The fatigue strength of sound welds is only reduced by 10–15% at the aimed lifetime of 10⁵ cycles compared to the base material. Joint Line Remnant (JLR) bearing welds have a similar fatigue strength as sound welds and the JLR is not the crack initiation site. Kissing Bond (KB) bearing welds that have undergone a weld root polishing show a reduction in fatigue strength by 17% compared to sound welds. For specimens loaded at or above yield strength of the weld nugget the crack systematically initiates from the KB during the first cycle, which is interpreted further using fracture mechanics. The strongest reduction, about 28% in fatigue strength, is found for welds with an initial gap between the parent sheets (GAP welds) along with initiation at intergranular surface microcracks. Kahn tear tests show a reduction in tearing resistance for the flaw bearing welds with a similar ranking as for the fatigue strength.     

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.     

Rotating bending fatigue properties of two case hardening steels after nitriding treatment

Fatigue properties of two Al-containing steels have been investigated by rotating bending fatigue tests. Results show that the fatigue limits (the fatigue strength at 10⁷ cycles) were improved remarkably by plasma nitriding due to the high hardness of 1000 Hv and compressive stress of 400 MPa in the nitrided layer. Scanning electron microscopic observations show that after nitriding the fatigue crack initiation sites moved from the surface flaws or near-surface matrix into the AlN inclusions at around the case-core interface. Degassing treatment can increase the fatigue limit because it prevented fatigue crack initiation at AlN inclusions due to the reduced [N] contents and refined inclusion size.     

Fatigue behavior and failure mechanism of basalt FRP composites under long-term cyclic loads

This paper studies the fatigue behavior of basalt fiber reinforced epoxy polymer (BFRP) composites and reveals the degradation mechanism of BFRP under different stress levels of cyclic loadings. The BFRP composites were tested under tension–tension fatigue load with different stress levels by an advanced fatigue loading equipment combined with in-situ scanning electron microscopy (SEM). The specimens were under long-term cyclic loads up to 1 × 10⁷ cycles. The stiffness degradation, S–N curves and the residual strength of run-out specimens were recorded during the test. The fatigue strength was predicted with the testing results using reliability methods. Meanwhile, the damage propagation and fracture surface of all specimens were observed and tracked during fatigue loading by an in-situ SEM, based on which damage mechanism under different stress levels was studied. The results show the prediction of fatigue strength by fitting S–N data up to 2 × 10⁶ cycles is lower than that of the data by 1 × 10⁷ cycles. It reveals the fatigue strength perdition is highly associated with the long-term run-out cycles and traditional two million run-out cycles cannot accurately predict fatigue behavior. The SEM images reveal that under high level of stress, the critical fiber breaking failure is the dominant damage, while the matrix cracking and interfacial debonding are main damage patterns at the low and middle fatigue stress level for BFRP. Based on the above fatigue behavior and damage pattern, a three stage fracture mechanism model under fatigue loading is developed.