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This study explores the impact of microstructural features on fatigue crack initiation in poly‐crystalline materials, emphasizing hydrogen‐induced complexities. Grain anisotropy, misorientations, grain size variations, and elastic–plastic inhomogeneities concentrate stress at grain boundaries, making them susceptible to crack initiation during fatigue loading. The presence of hydrogen compounds this process, due to complications of characterization of local hydrogen content and activating embrittling mechanisms. Building upon a model for nickel, this research investigates 316L austenitic stainless steel specimens with varying grain sizes, both uncharged and hydrogen‐charged. In situ low‐cycle fatigue loading experiments establish correlations between fatigue crack initiation and microstructural features. The study reveals specific combinations of features crucial in the initiation process, undergoing alterations in the presence of hydrogen. A proposed qualitative model links microstructural features with accumulated plastic shear strain during fatigue and prevalent hydrogen embrittlement mechanisms like hydrogen‐enhanced local plasticity and hydrogen‐enhanced decohesion. Highlights: The effects of hydrogen on fatigue crack initiation (FCI) sites in stainless steel 316L are studied.Two different grain sizes were analyzed by in‐situ scanning electron microscopy (SEM) and electron back‐scattered diffraction (EBSD) and high‐resolution digital image correlation (HR‐DIC).Hydrogen‐assisted FCI is correlated with initial features of the microstructure.A model is proposed linking FCI with microstructure and accumulated plastic shear strain. [ABSTRACT FROM AUTHOR]

Jacket structures are widely used as foundation structures for offshore wind energy. The jacket structure as a truss shape structure is not deformed easily and has the advantage of keeping the structure stable under wind load. The offshore jacket structures are subjected to unstable repetitive loads such as wind, and wave loads in the deep sea. These harsh environments lead to the deterioration of material performance and fatigue failure with cracks in the structure. It is crucial to locate fatigue cracks since they might compromise a structure's overall stability. In this study, fatigue life and crack initiation for different braced jacket structures were investigated. First, 3D non-steady heat conduction analysis and thermal elastic-plastic analysis were performed to reproduce the initial state of the weld joint in the jacket structure. The heat history obtained from the 3D non-steady heat conduction analysis was enforced as an input in thermal elastic-plastic analysis to calculate welding deformation and residual stress. Next, the fatigue FE analysis of the jacket welding structure was carried out using the residual stress and welding deformation as initial values along with the external cyclic loadings. The 3D fatigue FE analysis employed cyclic hysteresis constitutive equations and fatigue damage theory to calculate the fatigue life and crack initiation. The 3D fatigue FE results were compared with the S-N curve by European code 3 and the hot spot stress (HSS) method. The results show that the 3D fatigue FE method effectively calculates the fatigue life and finds crack initiation. [ABSTRACT FROM AUTHOR]

Wire and arc additive manufacturing (WAAM) offers promise in creating large complex structures due to its flexibility and high material deposition rates. The nickel-based alloy IN718 is favoured for WAAM due to its weldability and compatibility. However, WAAM can introduce issues like anisotropic grain structure, porosity, and residual stresses which can lead to directional variations in tensile, fatigue, and fracture behaviour. This paper studied the WAAM process of IN718, utilising cold metal transfer (CMT). The optimised CMT-WAAM parameters for IN718 were identified to as a wire feed speed of 8–10 m/min and a torch travel speed of 0.5–0.7 m/min, resulting in stable deposition and minimal defects. Nevertheless, columnar grain structures were observed in the build direction (BD), with coarse grains in the wall-length direction (WD). This anisotropic microstructure coupled with stress concentrators, contributes to the directional dependence observed in tensile properties, fatigue endurance, and crack growth. The investigation revealed superior ductility in the BD compared to the WD. Interestingly, the fatigue endurance testing showed a longer life in the WD compared with the BD, attributed to stronger stress concentrators in the BD specimens. However, when examining a cracked specimen, the fatigue crack propagated faster in the WD rather than the BD. [ABSTRACT FROM AUTHOR]

In this study, plane bending fatigue tests were conducted on Ti–22V–4Al alloy, a β-type titanium alloy, to examine the fatigue crack initiation behavior in detail. In addition, the prediction of fatigue crack initiation points was investigated from the perspectives of the Schmidt factor (SF) and crystal plasticity finite element method (CP-FEM). The slip system contributing to fatigue crack initiation can be accurately predicted by assessing the magnitude relationship of SF. Also, this prediction is already indicated in a lot of paper by using out of component of slip activity. However, the location where the fatigue crack will occur can be not estimated by SF on polycrystalline. Therefore, prediction of grains where fatigue cracks will occur could be achieved with high accuracy by constructing a CP-FEM that considers the mechanical interaction of polycrystals and grain boundary. Utilizing advanced methodologies such as CP-FEM and numerical calculation techniques, it is strictly investigated that the factors influencing fatigue crack initiation in polycrystalline materials. Our research concluded the understanding of fatigue crack initiation on polycrystal grains by considering the mechanical interaction of polycrystals and grain boundary.

Based on a very fast numerical procedure for the determination of the subsurface stress field beneath frictional contacts of axisymmetric elastic bodies under arbitrary 2D oblique loading, the contact mechanical influences of loading parameters and contact profile geometry on the Smith–Watson–Topper (SWT) fatigue crack initiation parameter in elastic fretting contacts with superimposed normal and tangential oscillations are studied in detail. The efficiency of the stress calculation allows for a comprehensive physical analysis of the multi-dimensional parameter space of influencing variables. It is found that a superimposed normal oscillation of the contact can significantly increase or decrease the SWT parameter, depending on the initial phase difference and frequency ratio between the normal and tangential oscillation. Written in proper non-dimensional variables, the rounded flat punch always exhibits smaller values of the SWT parameter, compared to a full paraboloid with the same curvature, while the truncated paraboloid exhibits larger values. A small superimposed profile waviness also significantly increased or decreased the SWT parameter, depending on the amplitude and wave length of the waviness. While both the load protocol and the profile geometry can significantly alter the SWT parameter, the coupling between both influencing factors is weak. [ABSTRACT FROM AUTHOR]

Based on a very fast numerical procedure for the determination of the subsurface stress field beneath frictional contacts of axisymmetric elastic bodies under arbitrary 2D oblique loading, the contact mechanical influences of loading parameters and contact profile geometry on the Smith–Watson–Topper (SWT) fatigue crack initiation parameter in elastic fretting contacts with superimposed normal and tangential oscillations are studied in detail. The efficiency of the stress calculation allows for a comprehensive physical analysis of the multi-dimensional parameter space of influencing variables. It is found that a superimposed normal oscillation of the contact can significantly increase or decrease the SWT parameter, depending on the initial phase difference and frequency ratio between the normal and tangential oscillation. Written in proper non-dimensional variables, the rounded flat punch always exhibits smaller values of the SWT parameter, compared to a full paraboloid with the same curvature, while the truncated paraboloid exhibits larger values. A small superimposed profile waviness also significantly increased or decreased the SWT parameter, depending on the amplitude and wave length of the waviness. While both the load protocol and the profile geometry can significantly alter the SWT parameter, the coupling between both influencing factors is weak.

Film cooling holes (FCHs) geometric structure significantly affects low‐cycle fatigue (LCF) of the single crystal turbine blade. This study investigates the LCF behavior of Ni‐based single crystal plate specimens with two different structure FCHs, including elliptical and fan‐shaped FCHs, by high temperature fatigue tests and crystal plasticity simulation. The results show that the LCF life of the fan‐shaped FCHs is shorter than those of the elliptical FCHs. The fracture characteristics and microstructure evolution of different structure FCHs are dissimilar. Oxidation affects the fatigue crack nucleation process at the FCH edge. The resolved shear stress (RSS) around fan‐shaped FCHs is greater than elliptical FCHs under one cyclic stress, and the area with larger RSS is consistent with fatigue crack nucleation position. The early LCF failure is mainly attributed to the high temperature oxidation of the local slip deformation induced by RSS concentration around the FCHs. Highlights: Based on real‐shaped structure, the elliptical and fan‐shaped FCHs were designed.Impact of FCH shape on low‐cycle fatigue behavior was investigated.Failure mechanism of shaped FCHs was studied by fatigue test and simulation.Coupling effect of stress oxidation on fatigue crack initiation is discussed. [ABSTRACT FROM AUTHOR]

In a wind energy system, the safety and stability of substructures plays an important role during the in service of offshore structure. Offshore structures are continuously subjected to high cyclic fatigue loads and may experience fatigue cracks due to the continuous accumulation of plastic strain and stress concentrations at welded joints. The fatigue life of welded tubular joints is one of the most important factors determining the life of offshore structures. In this study, the fatigue analysis of tubular joints of the tripod and jacket support structure was performed using 3D fatigue FEM to estimate the fatigue life and predict the positions of crack initiation. The 3D fatigue FE is based on constitutive equations and continuum damage mechanics. The welding state of tubular joints were reproduced to calculate the welding residual stresses and welding deformation. The residual stresses and weld deformation were used as input together with cyclic loading in the 3D fatigue FE to calculate the fatigue life and predict the crack initiation positions. The S-N curve calculated by the 3D fatigue FE analysis were compared with the SN curves of Eurocode 3. The results show that 3D fatigue FE analysis is an effective tool to analyze large and complex structures before installation to ensure the safety and stability of the structure. [ABSTRACT FROM AUTHOR]

Due to their distinct physical, chemical, and mechanical features, high-entropy alloys have significantly broadened the possibilities of designing metal materials, and are anticipated to hold a crucial position in key engineering domains such as aviation and aerospace. The fatigue performance of high-entropy alloys is a crucial aspect in assessing their applicability as a structural material with immense potential. This paper provides an overview of fatigue experiments conducted on high-entropy alloys in the past two decades, focusing on crack initiation behavior, crack propagation modes, and fatigue life prediction models. [ABSTRACT FROM AUTHOR]

A fracture mechanics model is developed to simulate cracking events that may have taken place at the root of a notch in a uniaxially loaded plate made of a two‐phase alloy during the fatigue crack initiation life. The roughness of the notch surface resembles microcracks of different sizes and locations at the notch root along the plate thickness, where material grains of different phases, sizes, and strengths are distributed at random. Potential notch surface cracking activities are anticipated during loading cycles until the emergence of a through‐thickness crack that propagates from the notch root on the plate surface along its width. Thus, the duration of the experimentally defined fatigue crack initiation at the notch root can be calculated. The model is applied to previously published experimental data on symmetrically U‐shaped ferritic‐pearlitic steel notched plates subjected to constant amplitude cyclic uniaxial stresses with different stress concentration factors. Plates with a random configuration were virtually tested. The comparison of experimental results with corresponding predictions validates the model. Highlights: This work develops a fracture mechanics model for the FCI at uniaxially loaded notches.Potential notch root surface cracking activities during the FCI period are considered.The notch FCI life is spent propagating microcracks resembling its surface roughness.The model predicts the effect of some factors that control the notch FCI behavior. [ABSTRACT FROM AUTHOR]

Wire and arc additive manufacturing (WAAM) offers promise in creating large complex structures due to its flexibility and high material deposition rates. The nickel-based alloy IN718 is favoured for WAAM due to its weldability and compatibility. However, WAAM can introduce issues like anisotropic grain structure, porosity, and residual stresses which can lead to directional variations in tensile, fatigue, and fracture behaviour. This paper studied the WAAM process of IN718, utilising cold metal transfer (CMT). The optimised CMT-WAAM parameters for IN718 were identified to as a wire feed speed of 8–10 m/min and a torch travel speed of 0.5–0.7 m/min, resulting in stable deposition and minimal defects. Nevertheless, columnar grain structures were observed in the build direction (BD), with coarse grains in the wall-length direction (WD). This anisotropic microstructure coupled with stress concentrators, contributes to the directional dependence observed in tensile properties, fatigue endurance, and crack growth. The investigation revealed superior ductility in the BD compared to the WD. Interestingly, the fatigue endurance testing showed a longer life in the WD compared with the BD, attributed to stronger stress concentrators in the BD specimens. However, when examining a cracked specimen, the fatigue crack propagated faster in the WD rather than the BD.

Magnesium alloys with a long-period stacking ordered (LPSO) structure usually possess excellent static strength, but their fatigue behaviors are poorly understood. This work presents the effect of the LPSO structure on the crack behaviors of Mg alloys in a very high cycle fatigue (VHCF) regime. The LPSO lamellas lead to a facet-like cracking process along the basal planes at the crack initiation site and strongly prohibit the early crack propagation by deflecting the growth direction. The stress intensity factor at the periphery of the faceted area is much higher than the conventional LPSO-free Mg alloys, contributing higher fatigue crack propagation threshold of LPSO-containing Mg alloys. Microstructure observation at the facets reveals a layer of ultrafine grains at the fracture surface due to the cyclic contact of the crack surface, which supports the numerous cyclic pressing model describing the VHCF crack initiation behavior.

In the fatigue design of metallic components using the safe-life approach, fatigue crack initiation as a development of slip systems at the nanoscale, followed by microstructurally short crack growth, is critical for the onset of structural failure. The development of reliable analytical tools for the prediction of crack initiation, although very complex due to the inherent multiscale fatigue damage processes involved, is important for promoting a more sophisticated design but, more importantly, enhancing the safety in regard to fatigue. The assessment of fatigue crack initiation life at the root of a V-shaped notch is performed by implementing a local strain and a fracture mechanics concept. In the low cycle fatigue analysis, the finite element method is used to determine the local stress–strain response at the notch root, which takes into account elastoplastic material behavior. Fatigue crack initiation is treated as the onset of a short corner crack by incremental damage accumulation and failure of a material element volume at the notch root. The finite element results are compared against established methodologies such as the Neuber and strain energy density methods. In the fracture mechanics approach, fatigue crack initiation is treated as the onset and propagation of a corner crack to a finite short crack. Fatigue experiments in two different transformation-induced plasticity (TRIP) steels were conducted to evaluate the analytical predictions and to determine the physical parameters for the definition of crack initiation. The analytical results show that the finite element method may be successfully implemented with existing fatigue models for a more accurate determination of the local stress–strain behavior at the notch tip in order to improve the assessment of fatigue crack initiation life compared to the established analytical methodologies. [ABSTRACT FROM AUTHOR]

Notch high cycle fatigue (HCF) properties and microcrack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure under various notch radii were systematically investigated. Results indicate that the reduction of notch root radius significantly promotes fatigue microcrack initiation, and then dramatically reduces the HCF life and strength of the alloy. Cyclic deformation of the alloy is mainly controlled by the slipping and deformation twinning in α plates. The primary fatigue crack initiation micro-mechanism is α/β interface cracking induced by slipping and twinning at all notch HCF specimens. Moreover, the volume fraction of twinning would increase with decreasing of the notch root radius. Interestingly, when the notch root radius is the smallest (R = 0.14 mm) and the stress concentration factor is the largest (Kt = 4), in addition to slipping and twinning, basal stacking faults promoting the cracking of α/β interface could be another crucial HCF microcrack initiation mechanism of the alloy. Furthermore, with decreasing of notch root radius and increasing of stress concentration factor, the size of cycle plastic deformation zone at notch root gradually reduces to the size of α colony and even the α plate. Therefore, the order of influential degree of three different levels microstructure on the crack initiation mechanism of notched HCF can be arranged as α plate > α colony > β GB.

Magnesium alloys with a long-period stacking ordered (LPSO) structure usually possess excellent static strength, but their fatigue behaviors are poorly understood. This work presents the effect of the LPSO structure on the crack behaviors of Mg alloys in a very high cycle fatigue (VHCF) regime. The LPSO lamellas lead to a facet-like cracking process along the basal planes at the crack initiation site and strongly prohibit the early crack propagation by deflecting the growth direction. The stress intensity factor at the periphery of the faceted area is much higher than the conventional LPSO-free Mg alloys, contributing higher fatigue crack propagation threshold of LPSO-containing Mg alloys. Microstructure observation at the facets reveals a layer of ultrafine grains at the fracture surface due to the cyclic contact of the crack surface, which supports the numerous cyclic pressing model describing the VHCF crack initiation behavior. [ABSTRACT FROM AUTHOR]

The mechanical properties of Cu alloyed steels are influenced significantly by the Cu content and the respective state of Cu precipitations as well as the C content. In this context, the effect of an increased C content on the fatigue crack initiation and growth of differently aged Cu alloyed steels with 0.005 (X0.5CuNi2-2: X0.5) and 0.21 wt.-% C (X21CuNi2-2: X21) was investigated in this study. Notched specimens were examined via SEM in interrupted fatigue tests to detect the location of crack initiation and growth. The results showed that fatigue crack initiation and growth occurred for both steels at grain boundaries, and within ferrite grains. However, a higher C content increased the incidence of crack initiation and growth at grain boundaries. This is caused by the smaller grains of X21 and especially by the presence of cementite on the grain boundaries. This explains why, in contrast to X0.5, no influence of the Cu precipitation state on the defect-based failure was observed for X21, as the precipitates are located within the ferrite grains and, thus, only have a minor impact on the fatigue failure mechanisms of X21. [ABSTRACT FROM AUTHOR]

The construction of new renewable energy infrastructures and the development of new ocean resources continues to proceed apace. In this regard, the increasing size and capacity of offshore wind turbines demands that the size of their accompanying supporting marine structures likewise increase. The types of marine structures utilized for these offshore applications include gravity base, monopile, jacket, and tripod structures. Of these four types, monopile structures are widely used, given that they are comparatively easy to construct and more economical than other structures. However, constant exposure to harsh cyclic environmental loads can cause material deterioration or the initiation of fatigue cracks, which can then lead to catastrophic failures. In this paper, a 3D fatigue finite element analysis was performed to predict both the fatigue life and the crack initiation of a welded monopile substructure. The whole analysis was undertaken in three steps. First, a 3D non-steady heat conduction analysis was used to calculate the thermal history. Second, a thermal load was induced, as an input in 3D elastoplastic analysis, in order to determine welding residual stresses and welding deformation. Finally, the plastic strain and residual stress were used as inputs in a 3D fatigue FE analysis in order to calculate fatigue crack initiation and fatigue life. The 3D fatigue finite element analysis was based on continuum damage mechanics (CDM) and elastoplastic constitutive equations. The results obtained from the 3D fatigue finite element analysis were compared with hot spot stresses and Det Norske Veritas (DNV-GL) standards. [ABSTRACT FROM AUTHOR]

The present study envisaged the behavior and relevant crack initiation mechanism of super martensitic stainless steel (0Cr13Ni5Mo) at elevated temperature (ET) under very‐high‐cycle fatigue (VHCF). Fractographic analysis revealed that all the VHCF cracks of the failure specimens initiated from the surface and formed fine granular area (FGA) morphology at ET in an absence of vacuum. The initiation characteristic area (ICA) consisted of an initiation site, a relatively smooth area, and the annular continuous FGA. FGA was formed in the peaks and valleys of local microstructures, and the smooth regions connected these peaks and valleys dominated by shear stress. It was observed that the FGA was unevenly distributed on the crack surface related to the local microstructure and below the grain surface relevant to local strain distribution. The stress intensity factor (SIF) and the crack tip plastic zone of the ICA slightly changed with temperature. Highlights: The vacuum environment was no longer necessary for the formation of FGA.FGA was unevenly distributed related to local microstructure and local strain condition.The fatigue cracks' initiation and early propagation were affected by local microstructure. [ABSTRACT FROM AUTHOR]

Due to their distinct physical, chemical, and mechanical features, high-entropy alloys have significantly broadened the possibilities of designing metal materials, and are anticipated to hold a crucial position in key engineering domains such as aviation and aerospace. The fatigue performance of high-entropy alloys is a crucial aspect in assessing their applicability as a structural material with immense potential. This paper provides an overview of fatigue experiments conducted on high-entropy alloys in the past two decades, focusing on crack initiation behavior, crack propagation modes, and fatigue life prediction models.

The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and is characterized by alternating dislocation-rich and dislocation-poor regions. The mechanisms of the dislocation motion in the PSBs and the formation of the point defects and their migration are quantitatively described. It is shown that, due to localized cyclic plastic straining in the PSBs, persistent slip markings (PSMs) are produced where the PSBs emerge on the surface. They typically consist of a central extrusion accompanied by one or two parallel intrusions. The deep intrusion is equivalent to the crack-like surface defect. The concentration of the cyclic strain in the tip of an intrusion leads to intragranular fatigue crack initiation. The mechanism of the early crack growth in the primary slip plane is proposed and discussed. Numerous PSMs are produced on the surface of the cyclically loaded materials. PSMs contribute to the formation of the surface relief, as well as the relief on the grain boundary. PSMs from one grain impinging the grain boundary are sufficient to create sharp relief on the grain boundary. Void-like defects weaken the grain boundary cohesion and extra material push both grains locally apart. The conditions necessary for the weakening of the grain boundary are enumerated and examples of grain boundary crack initiations are shown. The relevant parameters affecting grain boundary initiation are identified and discussed. The collected experimental evidence and analysis is mostly based on the papers published by the author and his colleagues in the Institute of Physics of Materials in Brno. [ABSTRACT FROM AUTHOR]

Nucleation of in‐service cracks leads to detrimental consequences for structural components of near‐α titanium alloys subjected to fatigue loads. Experimental observations show that the fatigue initiation facets usually form in certain crystallographic orientation ranges of "hard" primary α grains which differ between pure and dwell fatigue loads. In this manuscript, a comparative study has been performed using several fatigue indicator parameters (FIPs) to assess their ability to predict the location of fatigue crack nucleation in near‐α titanium alloy microstructures. All selected FIPs are implemented within the same polycrystalline plasticity finite element modeling framework to facilitate one‐to‐one comparisons. Comparison on predictability of critical initiation locations and their crystallographic orientations is studied for incorporated FIPs under pure and dwell fatigue. The critical locations predicted by some FIPs were found to be close to each other, and consistent with the crystallographic orientation ranges from fractography measurements, in addition to the range transition from pure to dwell fatigue loads. Critical locations from slip driven FIPs are obtained to be several grains away from that of the former ones and are inclined to capture orientations of slip traces from experiments. Highlights: Comparative study of is performed on different fatigue indicator parameters in the same CPFE framework.Some FIPs show similar critical sites and crystallography thereof, consistent with experimental data.CP results of these FIPs capture the experimental orientation range transition of critical sites from pure to dwell fatigue load.Proposed FIP, solely based on dislocation density, is validated through comparison with other FIPs and experiments. [ABSTRACT FROM AUTHOR]

Corroded surfaces of offshore mooring chains typically show a very irregular shape with different pit geometries. Fatigue tests were performed in three‐point bending on specimens with hemispherical notches, representing an idealized geometry of a corrosion pit. Digital image correlation was used to detect crack initiation lives and locations. The aim of the study was to quantify mean stress relaxation (MSR) and its influence on fatigue initiation life. MSR data were obtained from strain‐controlled fatigue tests on R4‐grade offshore mooring chain steel. Interestingly, MSR was observed even at a stress amplitude below the cyclic yield stress. For initiation lives below 100,000 cycles, significant improvement in predictions was obtained if cyclic softening and MSR were accounted for. Using only the monotonic stress‐strain curve led to less accurate predictions. For longer lives, scatter in the base material S‐N data and residual stresses from machining of the notch made predictions less accurate. [ABSTRACT FROM AUTHOR]

Offshore wind power became an important source of renewable energy that is rapidly growing over the past several years. Large offshore wind turbines require strong and large support structures. The support substructures are exposed to diverse dynamic and static loads such as waves, snow, wind ice, earthquakes, and tides. These unstable, destructive repeated loads lead to material degradation or damage accumulation which in turn leads to the failure of offshore structures. Fatigue is considered one of the significant modes of failure in offshore structures. The constant exposure of welded multiplanar tubular joints of tripod structure under cyclic loading or the presence of flaws due to fabrication and construction induces high-stress concentration at these joints. The material degradation or damage accumulation at welded joints gives rise to the formation or propagation of cracks, and ultimately leads to a collapse of giant steel structures. This research aimed at investigating the origin of crack positions and fatigue life in tripod offshore structures. The 3-D fatigue FE analysis was carried out in two steps, Firstly, the welding deformation and welding residual stress were calculated in thermal and mechanical analysis. Secondly, 3D fatigue crack analysis based on cyclic hysteresis constitutive equation and fatigue damage theory was used to calculate the fatigue life and crack initiation in tripod offshores structure. Results obtained from the analysis were compared with the results from hot-spot stress method and Eurocode 3. From the results, it was considered that 3D-fatigue FEM is a useful method to estimate fatigue life and crack initiation. [ABSTRACT FROM AUTHOR]

• CPFE simulations identify twin boundaries as key sites for fatigue crack initiation in IN718. • Δ ε eps (increment of equivalent plastic strain) is proposed as an effective parameter for predicting fatigue life. • A material-level CPFE model effectively predicts the fatigue life of notched IN718 specimens. The aim of this study was to establish a method for evaluating notch fatigue behavior through crystal plasticity finite element (CPFE) simulations based on the actual microstructure of the nickel-based alloy Inconel 718 (IN718). Initially, the equivalent plastic strain, ε eps , which reflects the comprehensive slip at the grain scale, was employed to analyze the fatigue crack initiation mechanism of IN718, revealing that twinning and triple junctions of grain boundaries were high-risk locations for fatigue crack initiation. Next, fatigue simulations were performed on notched specimens using the CPFE model, with a material-level CPFE model particularly employed at the notch root. The increment of ε eps , Δ ε eps , in a stable cycle was used as the fatigue damage control parameter and correlated with the fatigue life, N f , revealing that the Δ ε eps - N f relationship at material-level satisfied the form of the Mason-Coffin model. Finally, fatigue life prediction of IN718 notched specimens was carried out based on the Δ ε eps - N f relationship, with the predicted results falling within the 2-fold scatter band, demonstrating good prediction accuracy. [ABSTRACT FROM AUTHOR]

To investigate the influence of different modification gradient fields on the fatigue crack initiation mechanisms of the Ti6Al4V alloy, this study employed two surface treatments, polishing (P) and polishing combined with ultrasonic rolling process (P+USRP), to generate distinct modification gradient fields for the examination of crack initiation mechanisms, as illustrated in the figure. [Display omitted] • In our investigation, we employed polishing (P) and polishing combined with ultrasonic rolling process (P + USRP) as surface strengthening treatments for the Ti6Al4V alloy, leading to different modification gradient fields. • The specimens with different modification gradient fields underwent high-cycle fatigue tests, showing different levels of enhancement in the alloy's fatigue performance. • The differences in modification gradient fields affected the fatigue crack initiation mechanisms of the Ti6Al4V alloy. In this paper, the high-cycle fatigue (HCF) properties and microcrack initiation mechanism of Ti6Al4V alloy after polishing (P) and after polishing followed by the ultrasonic surface rolling process (P + USRP) treatments are investigated. The results indicate that variations in modified gradient fields would alter the crack initiation location and mode. Specifically, after P treatment, the fatigue crack initiation source occurs approximately 205 μm from the surface, owing to localized strain formation within the internal α p /α p grain boundaries under cyclic loading, thereby promoting rapid cleavage at the (0001) twist interface and evolving into microcracks. This result aligns with the crack initiation mechanism of an untreated specimen. Conversely, fatigue crack initiation after P + USRP treatment occurs approximately 425 μm from the surface, as a result of the deeper modified gradient field, where the crack initiation mechanism results from the competing effects of rapid cleavage at the (0001) twist interface of α p /α p and the accumulation of numerous microvoids. The difference in strain between the α and β phases causes the α/β interface to become a weak region of dislocation accumulation, leading to the formation of microvoids induced by microvoid aggregation. Both P and P + USRP are beneficial for improving the fatigue performance of the alloy. [ABSTRACT FROM AUTHOR]

The indentation of bearing raceways frequently contributes to bearing failures, as in gearboxes. The dents act as surface stress raisers through the Rolling Contact Fatigue (RCF) cycles, leading to crack initiation and spalling. The microstructural evolution mechanism occurring is not completely understood. Hence, to enhance bearing lifetime, it is crucial to identify microstructural parameters that influence crack initiation and propagation. Therefore, this study presents a multi-scale characterization of martensitic 100Cr6 bearings before and after RCF. It includes SEM, SEM/FIB, EBSD, TKD and ACOM-ASTAR observations. It showed that, before RCF, the bearing raceway features a 0.5 µm thick surface layer consisting in refined martensite and spread primary carbides, resulting from finishing operations. After RCF, no further grain refinement was observed along the raceway, except beneath the dent shoulder where a material plastic flow, nano-grains and ultra-fine grains of martensite, and sheared and spread primary carbides were detected up to 1.4 µm deep. Below and up to 3 µm deep, a fine grain layer is observed. The martensite morphology, size and disorientation suggest that it undergoes refinement through continuous Dynamic Recrystallisation and twinning. Crack initiation was preferentially observed in the nano-grains layer, at primary carbides/matrix interfaces. Above the crack, another refined region is observed resulting from high deformation and stresses through crack propagation. Finally, even in severely deformed areas, nanosized retained austenite islands were detected, indicating that it is not completely transformed. [Display omitted] [ABSTRACT FROM AUTHOR]

• Advanced in-situ torsional fatigue testing discloses the early occurrence of short cracks. • Short cracks form a continuously growing network leading to coalescence of cracks. • The crack length distribution and the crack densities are discussed in detail. • Complementary, load increase tests disclose the fatigue damage based on material reactions. • Validation of in-situ results by constant amplitude testing in terms of the S-N curve. This study investigates the fatigue behaviour and damage mechanisms of the martensitic spring steel SAE 9254, with emphasis on short crack propagation. Shear stress-controlled torsional fatigue tests are performed at stress ratio R = -1 and test frequency f = 79 Hz using structure-sensitive measurement techniques to detect the relevant damage mechanisms. An in-situ torsional fatigue testing device is applied for shear-controlled fatigue tests at two amplitudes of the torsion angle using R = –1 and f = 5 Hz, and the microstructural-dependent short crack propagation path is documented by means of confocal laser microscopy and electron backscattering analysis. It is found that the early development of fatigue damage is characterised by the formation of slip bands, which serve as crack initiation sites. In this process, crack initiation occurs preferentially at or near prior austenite grain boundaries. Furthermore, prior austenite grain boundaries were identified as obstacles for the propagation of short cracks, because of the change of the crystallographic orientation leading to an oscillating propagation rate of short cracks. The short crack density ρ CD , which is an important parameter for fatigue damage, is rapidly growing with increasing shear stress amplitude Δτ T /2 and reveals a stress type specific behaviour. [ABSTRACT FROM AUTHOR]

In the fatigue design of metallic components using the safe-life approach, fatigue crack initiation as a development of slip systems at the nanoscale, followed by microstructurally short crack growth, is critical for the onset of structural failure. The development of reliable analytical tools for the prediction of crack initiation, although very complex due to the inherent multiscale fatigue damage processes involved, is important for promoting a more sophisticated design but, more importantly, enhancing the safety in regard to fatigue. The assessment of fatigue crack initiation life at the root of a V-shaped notch is performed by implementing a local strain and a fracture mechanics concept. In the low cycle fatigue analysis, the finite element method is used to determine the local stress–strain response at the notch root, which takes into account elastoplastic material behavior. Fatigue crack initiation is treated as the onset of a short corner crack by incremental damage accumulation and failure of a material element volume at the notch root. The finite element results are compared against established methodologies such as the Neuber and strain energy density methods. In the fracture mechanics approach, fatigue crack initiation is treated as the onset and propagation of a corner crack to a finite short crack. Fatigue experiments in two different transformation-induced plasticity (TRIP) steels were conducted to evaluate the analytical predictions and to determine the physical parameters for the definition of crack initiation. The analytical results show that the finite element method may be successfully implemented with existing fatigue models for a more accurate determination of the local stress–strain behavior at the notch tip in order to improve the assessment of fatigue crack initiation life compared to the established analytical methodologies.

In the present paper, the excavation of the energetic approach that estimates the fatigue crack initiation life of metal is conducted for H62 brass. The benefit of the energetic approach is the division of the actual applied strain range Δε into two parts, that is, a damage strain range Δεd that induces fatigue damage within the metal, and an undamaged strain range Δεc, which does not produce fatigue damage of the metal and corresponds to theoretical strain fatigue limit. The brightness of this approach is that the undamaged strain range Δεc can be estimated by the fundamental conventional parameters of metal in tensile test. The result indicated that the fatigue crack initiation life of H62 brass can be estimated by this approach successfully.

Several studies have been conducted on the fatigue behavior of copper and 7-3, and 6-4 brasses. However, there have been fewer studies on the fatigue behavior and fatigue crack growth (FCG) properties of free-cutting brass, primarily because emphasis has been placed on the development of lead-free free-cutting brass. In this study, fatigue experiments were performed in the atmosphere at room temperature using three types of free-cutting, two types of bismuth (Bi)-based (with different grain sizes), and lead (Pb)-based brasses. It was found that lead-free Bi-based free-cutting brass had approximately the same fatigue performance as that of Pb-based free-cutting brass. It was also clarified that the addition of Bi or Pb initiated fatigue cracks, and that the crack growth period occupied most of the fatigue life. Differences in the FCG behavior of the three free-cutting brasses were observed in the low ΔK range. The modified linear fracture mechanics parameter M was used to quantitatively analyze the fatigue life and FCG behavior (short surface cracks). A comparison between the calculated and experimental results showed that M was useful. [ABSTRACT FROM AUTHOR]

A vibration‐based bending fatigue test based on electrodynamic shaker is conducted in order to study the high cycle and very high cycle fatigue (VHCF) behaviors of a titanium alloy TC17 with a sort of novel specimens. After obtaining a full S‐N curve covering the lifetime up to 109 cycles, several fatigue crack initiation and failure process modes are revealed based on the observation of the fracture surfaces. Especially, the statistical distribution characteristics of the fatigue lifetime and some relevant factors are discussed. The results indicate that the obtained S‐N curve does not exhibit a plateau region in the VHCF regime. And the present crack initiation mechanism involves the facets formed by the primary α‐phase cleavage and the development of microcracks. Moreover, the scatter of the fatigue lifetime can be obviously influenced by the resonance frequency range of the specimens. [ABSTRACT FROM AUTHOR]

The crack initiation and short crack propagation in a martensitic spring steel were investigated by means of in-situ fatigue testing. Shot peened samples as well as untreated samples were exposed to uniaxial alternating stress to analyze the impact of compressive residual stresses. The early fatigue damage started in both sample conditions with the formation of slip bands, which subsequently served as crack initiation sites. Most of the slip bands and, correspondingly, most of the short fatigue cracks initiated at or close to prior austenite grain boundaries. The observed crack density of the emerging network of short cracks increased with the number of cycles and with increasing applied stress amplitudes. Furthermore, the prior austenite grain boundaries acted as obstacles to short crack propagation in both sample conditions. Compressive residual stresses enhanced the fatigue strength, and it is assumed that this beneficial effect was due to a delayed transition from short crack propagation to long crack propagation and a shift of the crack initiation site from the sample surface to the sample interior. [ABSTRACT FROM AUTHOR]

• Crack propagation slows during the transition from early to steady stages. • Cracks tend to propagate along-boundaries during the early propagation stage. • Multiscale martensite interfaces can cause varying degrees of crack deflection. • Martensite laths are the fundamental units influencing small crack propagation. The objective of this study is to investigate the influence mechanism of martensite multi-scale interfaces on the fatigue small cracks propagation behavior in super martensite stainless steel. The findings revealed that crack propagation slows during the transition from early to steady stages. Multiscale martensite interfaces can cause varying degrees of crack deflection, and cracks tend to propagate along-boundaries during the early propagation stage. Martensite lath is the basic unit that affects the fatigue performance and the behavior of small crack propagation. [ABSTRACT FROM AUTHOR]

• Fatigue performance of nitinol wire was studied to 2 billion cycles. • UHCF crack initiating inclusion fractures grew via a Region of Reduced Roughness. • RRR growth models may enable nitinol fatigue prediction beyond 100 million cycles. Fatigue fracture of small nitinol components commonly used in medical devices is currently dominated by the initiation and/or growth of small cracks at non-metallic inclusions preceding conventional fatigue crack growth. Therefore, an understanding of the threshold and growth of small cracks is critical to inform fatigue performance of devices. In this paper, we conduct rotary bend fatigue experiments of nitinol wire to 2 billion cycles, measure the inclusion from which the crack initiated, and calculate the stress intensity threshold. Inclusion size is compared to the size of a unique feature observable with a scanning electron microscope which appears as a smooth area surrounding the inclusion and only on specimens that fractured after > 10 million cycles. The hypothesis presented is that the smooth feature around the inclusion is the growth of a small crack which continues until it reaches a size large enough to cause conventional fatigue crack growth. Relating inclusion size to that of the smooth feature creates a damage curve that can be written as a function of cycles to fracture. This damage curve may be useful to estimate the critical size of the largest allowable defect based on the design life and applied loading of the nitinol component. [ABSTRACT FROM AUTHOR]

The TC21 alloy has been extensively utilized in the aerospace structural components, making it crucial to investigate the fatigue damage mechanism of this alloy. In this study, a comprehensive investigation was conducted into the high-cycle fatigue damage mechanism of TC21 alloy under rotation bending, considering variations in primary equiaxed α (α p) phase contents and size in bimodal microstructure (BM). Results demonstrate that the TC21 alloy exhibits optimal strength-ductility and fatigue life at approximately 25 percent of the α p phase content. The content and dimension of the α p phase play a significant role in regulating the onset of fatigue fractures. Dislocation pile-up around the α p phase promotes the formation of microvoids and microcracks. Notably, when the content of the α p phase falls below 15 %, there is an increased influence from secondary α phase (α s) on crack initiation. Moreover, excessive amounts of α p coupled with decreased size lead to a reduction in fatigue life. The aforementioned factors contribute to an intensified concentration of stress, facilitate a more uniform path for crack extension, and expedite the rate of fatigue crack propagation. [ABSTRACT FROM AUTHOR]

The mechanical properties of Cu alloyed steels are influenced significantly by the Cu content and the respective state of Cu precipitations as well as the C content. In this context, the effect of an increased C content on the fatigue crack initiation and growth of differently aged Cu alloyed steels with 0.005 (X0.5CuNi2-2: X0.5) and 0.21 wt.-% C (X21CuNi2-2: X21) was investigated in this study. Notched specimens were examined via SEM in interrupted fatigue tests to detect the location of crack initiation and growth. The results showed that fatigue crack initiation and growth occurred for both steels at grain boundaries, and within ferrite grains. However, a higher C content increased the incidence of crack initiation and growth at grain boundaries. This is caused by the smaller grains of X21 and especially by the presence of cementite on the grain boundaries. This explains why, in contrast to X0.5, no influence of the Cu precipitation state on the defect-based failure was observed for X21, as the precipitates are located within the ferrite grains and, thus, only have a minor impact on the fatigue failure mechanisms of X21.

High-cycle fatigue (HCF) behaviors of medium-carbon bainitic steels with various inclusion sizes and microstructural features were studied using the rotating–bending fatigue test. Here, the medium-carbon bainitic steels with different melting processes were treated by three heat treatment routes incorporating bainite formation, namely bainite-based quenching plus partitioning (BQ&P), bainite austempering (BAT) and "disturbed bainite austempering, DBAT". The interior inclusion-induced crack initiation (IICI) and noninclusion-induced crack initiation (NIICI) modes were found after fatigue failure. The fracture surface of IICI is characterized by a "fish-eye" surrounding a "fine granular area, FGA" in the vicinity of an inclusion. In contrast, a microfacet, instead of an inclusion, is found at the center of FGA for the NIICI fracture surface. The predications of fatigue strength and life were performed on the two crack initiation modes based on fracture surface analysis. The results showed that a majority of fatigue life is consumed within the FGA for both the IICI and NIICI failure modes. The fatigue strength of the NIICI-fatigued samples can be conveniently predicted via the two parameters of the hardness of the sample and the size of the microfacet. [ABSTRACT FROM AUTHOR]

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In the present paper, the high cycle fatigue (HCF) of a novel 211Z.X aluminum alloy with high strength was studied under hot-rolling and as-cast states at room temperature. The effects of microstructure and distribution of precipitated phases and impurities on the mechanical properties, HCF performances, fatigue microcrack initiation, and propagation behavior of the 211Z.X alloy were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The HCF S–N curves, P–S–N curves and Goodman fatigue diagrams of 211Z.X alloy consisting of two microstructures were drawn. The results suggested that the fine and dispersive distribution of the second phases improved the strength of the alloy. The formation of short-bar and spherical precipitates promoted coordinated deformation of the alloy. This promoted higher microcrack initiation resistance of 211Z.X alloy with a hot rolling state than in the cast state. As a result, the HCF properties of the hot-rolling alloy were better than those of the cast alloy. In sum, these results look promising for future reliable design of engineering structures and application of new aluminum alloys. [ABSTRACT FROM AUTHOR]

CONCLUSIONS The fatigue simulation based on crystal plasticity theory considers the characteristics of WAAM microstructure. Keywords: additive materials; fatigue crack initiation; microstructure; numerical simulation; porosity EN additive materials fatigue crack initiation microstructure numerical simulation porosity 620 624 5 01/18/22 20220201 NES 220201 INTRODUCTION The lamellar structure with porosity defect fabricated by wire and arc additive manufacturing (WAAM) have an effect on the initiation and propagation of fatigue cracks.1-4 The crystal plasticity simulation considering metal microstructure has been widely used in the study of fatigue behavior.5,6 Fatigue crack behavior of WAAM 5356 aluminum alloy was studied by combining experimental work and crystal plasticity simulation. [Extracted from the article]