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Rolling can result in anisotropy in the microstructure and mechanical properties of the sheet material. This study focuses on the very high cycle fatigue (VHCF) performance of magnesium alloys in the rolling direction (RD), normal direction (ND), and the bisecting angle between the ND and RD (ND–RD). The findings reveal that RD specimens demonstrate superior fatigue performance, while the ND specimens exhibit the lowest fatigue resistance. During the crack initiation stage, the primary influential factor is the maximum shear stress. Due to c‐axis alignment with ND direction in grains, the deformation mode for ND and RD specimens primarily involves first‐order pyramidal slip, while ND–RD specimens primarily undergo basal slip and prismatic slip. In early stable crack propagation, grain size and texture cause variations in the morphology of the rough area among the three directional specimens. Notably, the RD specimens show faster crack propagation during crack initiation than early stable propagation stage. Highlights: The rolled Mg alloys have the highest very high cycle fatigue strength along rolling direction.Texture and grain boundaries significantly affect crack initiation and early propagation.Maximum shear stress dominates slip‐induced crack initiation within primary grains. [ABSTRACT FROM AUTHOR]

This article predicts bending very high cycle fatigue (VHCF) life of three typical nickel‐based alloys SM2550, BG2532, and G3 used for completion strings. Fatigue tests were conducted on the three alloys using an ultrasonic fatigue system at a frequency of 20 kHz. The results showed that the fatigue strength ranges of the three alloys were markedly different, reflecting their different sensitivities to fatigue loading. Scanning electron microscope observations revealed numerous fatigue crack origins with internal decohesion in the fatigue source region. To achieve unified prediction of the fatigue life for the three alloys, a prediction model based on deep learning was built with inputs including fatigue initiation quantity, cleavage facet size, and other fatigue fracture characteristics. It was found that single source feature was insufficient to obtain satisfactory prediction accuracy for all alloys, while multifeature coupling integration could significantly improve the prediction precision, enabling reliable prediction of alloy fatigue life. This study provides new insights into bending VHCF life prediction. Highlights: This article predicts bending VHCF life for three completion strings.Bending VHCF life model utilizing deep wise separable convolution was established.Deep learning can effectively integrate with bending VHCF analyses. [ABSTRACT FROM AUTHOR]

The microstructure, mechanical properties, and high/very‐high cycle fatigue behaviors of 9Cr3W3Co turbine rotor steel and its welded joint were investigated. The welded nugget zone (WNZ) exhibited the highest microhardness value, followed by the heat‐affected zone (HAZ), while the base material (BM) showed the lowest hardness value. The WNZ exhibited a higher yield strength compared to the BM with reduced elongation, while maintaining similar ultimate tensile strength. The fatigue strength of the BM at 5 × 107 cycles is roughly 53.7% of the yield strength, whereas that of the WNZ increases to approximately 55.0%. In comparison to the fatigue strength of the BM at 5 × 107 cycles, the WNZ exhibits a notable increase of 10.5%. Smaller grain size and higher dislocation density were observed in the WNZ, which contributed to improved microhardness, yield strength, and fatigue resistance through grain boundary strengthening and dislocation hardening. Highlights: Microstructure and mechanical properties of 9Cr3W3Co steel welded joints were studied.Microhardness values of various zones are non‐uniform and the maximum value in WNZ.Dislocation and grain boundary strengthening contributed to strengthening in WNZ. [ABSTRACT FROM AUTHOR]

This study investigated the high cycle fatigue crack initiation and propagation behaviors in coarse‐grained FeCrAl alloys, with a focus on the relationship between the path of small cracks and the local microstructure. The results show that the small cracks initiate along the {100} slip plane due to the synthetical influence of local microstructure and shear stress. The initiation zone forms inclined areas with layered fine‐grain areas caused by cyclic shear stress. Quantitative analysis of the stress intensity factor (SIF) indicates that in the realm of mixed‐mode loading, the SIF of Mode I takes the lead in determining the intensity of stress and strain fields at the crack tip. As the crack propagates further, the SIF steadily increases. Once it reaches a specific threshold, the propagation mode within a coarse grain shifts from Mode II to Mode I. Moreover, fatigue life increases with a tendency for decreased SIF during crack deflection. Highlights: The influence of microstructure on crack initiation and early propagation behavior was studied.The cyclic shear stress leads to the formation of layered fine‐grain areas.The quantitative analysis elucidates the reason for the deviation of crack direction within a coarse grain. [ABSTRACT FROM AUTHOR]

In the present study, a physics-informed neural network model based on Bayesian hyperparameter optimization is proposed for the prediction of short crack growth paths. A large number of cyclic loadings at a lower amplitude were applied to an α titanium sample by an ultrasonic fatigue machine to ensure a sufficient amount of data for machine learning. The grain size, grain orientation and grain boundary direction on the path, as well as crack growth direction, were selected as feature data for training the prediction model. The optimizations of the size ratio and the angle operation were conducted to compare different data processing methods, respectively. After evaluation, eventually, a model for predicting crack growth path is obtained with a reliable performance of 10% tolerance on the path angle at each grain boundary. And the prediction effect of the proposed model is better than that of some classic machine learning models and slip trace analysis. This article is part of the theme issue 'Physics-informed machine learning and its structural integrity applications (Part 1)'. [ABSTRACT FROM AUTHOR]

During steam turbine rotor operation, the startup and showdown processes can cause asymmetric loading stress, leading to fatigue damage. To research the cyclic deformation behavior and fatigue failure mechanisms of 9% Cr‐based steel under asymmetric loading conditions, the strain ratios including −1, 0, and 0.1 are introduced into the low cyclic fatigue tests at 430°C. The results show the cyclic softening behavior is observed under three strain ratios at 430°C. As the strain ratio increases, the crack initiation sources, plastic strain, and consumed energy present an increasing trend. Comparing the microstructure morphology at different strain ratios, the coarsening lath and carbides affect fatigue crack initiation. Besides, the grain rotation and dynamic recrystallization can be observed, resulting in the cyclic softening behavior during the fatigue test. Finally, a new fatigue life prediction model is proposed at 430°C. Highlights: The low cycle fatigue tests are conducted under R = −1, 0, and 0.1 at 430°C.The dynamic recrystallization is observed, resulting in the cyclic softening behavior.A fatigue life prediction model considering the strain ratio is proposed at 430°C. [ABSTRACT FROM AUTHOR]

The behavior of small cracks in magnesium alloy is key to understanding very high cycle fatigue (VHCF), which is influenced by microstructure and deformation mechanism. This study explores how basal texture affects VHCF failure mechanisms in extruded magnesium alloys. Findings reveal that faceted and rough morphologies characterize fracture surfaces during crack initiation and early propagation, respectively. Basal texturing inhibits basal slipping during faceted‐like crack initiation while compression twinning dominates deformation during early propagation, contributing to rough zones. Stress intensity factor and plastic zone size analysis indicates significant influence of basal texture on VHCF failure mechanisms but with limited effect on stable propagation. Highlights: The influence of basal texture on fatigue failure in Mg alloys is investigated.SEM, EBSD, FIB, and TEM characterize mate texture, faceted/ribbon microstructure.TEM analyzes the formation mechanism of facet and rough fracture surface. [ABSTRACT FROM AUTHOR]

The materials with grain size gradient variation on the surface, which were prepared with mechanical-induced severe plastic deformation, always show high resistance to high and low cycle fatigue and frictional wear because of their good strength–ductility synergy. The ultrasonic surface rolling treatment (USRT) has the advantages of high processing efficiency, good surface quality, and large residual compressive stress introduced to the surface after treatment. The USRT was used to prepare aluminum alloy (AA7075) samples with a surface gradient structure; meanwhile, the microstructural evolution mechanism of the deformation layers on the gradient structure was studied with XRD, SEM, and TEM. The microstructure with gradient distribution of grain size and dislocation density formed on the surface of AA7075 aluminum alloy after USRT. The surface layer consists of nanocrystals with random orientation distribution, and high-density dislocation cells and subgrains formed in some grains in the subsurface layer, while the center of the material is an undeformed coarse-grained matrix. The results show that the dislocation slip dominates the grain refinement process, following the continuous cutting and refinement of dislocation cells, subgrains, and fragmentation of the second precipitates. This study systematically clarified the mechanism of grain refinement and nanocrystallization on the surface of high-strength aluminum alloys and laid a theoretical foundation for further research on mechanical behavior and surface friction and wear properties of high-strength non-ferrous materials with gradient structure. [ABSTRACT FROM AUTHOR]

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]

In this study, the main purpose is to analyze the fatigue failure of thermoplastic polyurethane (TPU) plate under tension-tension load control tests (frequency = 5 Hz, stress ratio = 0.1) and consider the change in hydrogen bond content. The results show that the S-N curve of TPU material shows a downward trend before reaching the fatigue limit (10.25 MPa), and the energy is continuously consumed during the cyclic creep process and undergoes three stages of the hard segment and the soft segment changes. The infrared spectrum study shows that the increase in fatigue life will lead to more physical crosslinking, resulting in the reduction of hydrogen bond content, and the increase in microphase separation, leading to the occurrence of fatigue fracture. In addition, the scanning electron microscope and three-dimensional confocal analysis showed that the crack originated from the aggregation of micropores on the surface of the material and was accompanied by the slip of the molecular chain, the crack propagation direction was at an angle of about 45°. [ABSTRACT FROM AUTHOR]

The very high cycle fatigue (VHCF) failure of in-service components is mainly caused by the vibration of thin-wall elements at a high frequency. In this work, a novel model of ultrasonic fatigue test was developed to test thin-wall material in bending up to VHCF with an accelerated frequency. The theoretical principle and finite element analysis were introduced for designing a sample that resonated at the frequency of 20 kHz in flexural vibration. In the advantage of the second-order flexural vibration, the gauge section of the sample was in the pure bending condition which prevented the intricate stress condition for thin-wall material as in the root of cantilever or the contact point of three points bending. Moreover, combining the constraint and the loading contact in one small section significantly reduced heating that originated from the friction at an ultrasonic frequency. Both strain gauge and deflection angle methods were applied to verify the controlling of stress amplitude. The fractography observation on Ti6Al4V samples indicated that the characterized fracture obtained from the novel model was the same as that from the conventional bending test. [ABSTRACT FROM AUTHOR]

A non-destructive thermographic methodology based on the temperature field is utilized to determine the crack tip position during the very high cycle fatigue (VHCF) test of pure iron and deduce the corresponding fatigue crack growth rate (FCGR). To this end, a piezoelectric fatigue machine is employed to test 1 mm thick pure iron samples at 20 kHz in push–pull fatigue loading. Two cameras are placed on each side of the plate sample, an infrared one for measuring the temperature fields on the specimen surface and an optical one for visualizing the crack tip verification. The centre section of the specimen is notched to initiate the crack. The temperature field is converted into intrinsic dissipation to quantify the inelastic strain energy according to energy conservation. The maximum value of intrinsic dissipation in each thermal image is related to the position of the crack tip and thus allows monitoring of the crack evolution during the fatigue test. The obtained results show that one specific specimen broke at 7.25 × 107 cycles in the presence of a very low-stress amplitude (122 MPa). It is observed that the intrinsic dissipation has a low-constant level during the initiation and the short cracking, then sharply grows during the long cracking. This transition is visible on the polished surface of the sample, where the plasticity appears during the long cracking and slightly before. The material parameters in the Paris equation obtained from the intrinsic dissipation in the short crack growth are close to the results available in the literature as well as those obtained by the optical camera. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were performed on Ti60 titanium alloy up to a very high cycle fatigue (VHCF) regime at various stress ratios to investigate the characteristics. The S-N curves showed continuous declining trends with fatigue limits of 400, 144 and 130 MPa at 109 cycles corresponding to stress ratios of R = −1, 0.1 and 0.3, respectively. Fatigue cracks found to be initiated from the subsurface of the specimens in the VHCF regime, especially at high stress ratios. Two modified fatigue life prediction models based on fatigue crack initiation mechanisms for Ti60 titanium alloy in the VHCF regime were developed which showed good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

• Complete change of crack initiation site. • Quantitative evaluation of grain-boundary retarding effect. • Crack type affects the formation of FGA. Ultrasonic fatigue tests are performed on a magnesium alloy with and without ultrasonic peening treatment (UPT). Surface enhancement layer leads to the complete change of crack initiation sites. However, crack initiation mechanism keeps the same and results in a single-faceted morphology at crack initiation site. Microcracks initiate as Mode II crack within the original grain, but deflect to Mode I crack outside of the original cracked grain. A threshold SIF value is proposed to evaluate the retarding effect of grain boundary on microcrack propagation. Outside of the original cracked grain, Mode I crack propagation below the threshold ΔK σ-th is responsible for the formation of fine granular area (FGA, a nano-grain layer). Based on the Numerous Cyclic Pressing (NCP) model, it is proposed that crack type should be another necessary condition for the formation of FGA. [ABSTRACT FROM AUTHOR]

Ultralight Mg–Li alloys are promising aerospace materials as they are the lightest structural alloys at present; however, their fatigue behaviors remain to be explored. This work focuses on the fatigue strength and crack initiation behavior of an extruded dual‐phase Mg–Li alloy (LZ91) under high‐cycle and very‐high‐cycle fatigue regimes. The fatigue limit of LZ91 alloy at 109 cycles was determined to be 78 MPa, and the fatigue ratio is 0.46. Microstructural characterization demonstrates that fatigue cracks tend to initiate at β‐Li phase‐enriched regions. The α‐Mg phase presents a < 101¯0> fiber texture with a basal plane that has low deformation in the extrusion direction and acts as an enhanced phase in relation to the β‐Li phase. Deformation discrepancies cause localized cyclic plasticity at the Li phase that leads to fatigue crack initiation. Highlights: The influence of local microstructure on the fatigue behavior was studied.Fatigue strength of LZ91 is 78 MPa at 109 cycles, and the fatigue ratio is 0.46.The α‐Mg phase exhibits strong textures and acts as an enhancing phase in the material.The local proportion of α‐Mg phase significantly affects the crack initiation behavior. [ABSTRACT FROM AUTHOR]

A turbine engine blade alloy (Ti‐8Al‐1Mo‐1V) under different heat treatment conditions was employed to conduct ultrasonic fatigue tests to investigate the influence of the colony content of bimodal titanium alloys on the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) behavior. As a result, the increase in the content of colonies or the decrease in the content of primary α grains (αp) can be achieved by a double‐annealed treatment below the α + β to β transformation temperature. A higher colony content will result in a higher HCF and VHCF properties. The crack initiation life has a lower sensitivity to the microstructure change in the αp/colony content than the fatigue crack growth life. With the fatigue life increasing from the HCF regime to the VHCF regime, the impact of the microstructure evolution on fatigue resistances gradually decreases. Irreversibly, slips mainly along the maximum shearing stress direction make the plastic strain accumulate within some favorably oriented αp grains and further cause the nucleation of fatigue micro‐cracks. The formation of the fine granular area is attributed to the comprehensive effects of the vacuum‐enhanced plasticity and the numerous cyclic pressing. [ABSTRACT FROM AUTHOR]

When high‐strength steels are subjected to very high‐cycle fatigue loading, crack initiation site shifts from surfaces to the interior, and a fish‐eye forms on the fracture surface. Majority of the fatigue life is estimated to be associated with the formation of this internal crack morphology. In the present work, features of such internal cracks in two high‐strength steels are studied. Specifically, three initiation patterns are investigated. A general internal crack initiating scenario is proposed base on an understanding of dislocation slip in the materials. A simplified threshold is calculated from Young's modulus and interatomic spacing, defining the transition from the initiation stage to the crack propagation. The relationship between internal crack initiation and slower descending S‐N curves is discussed. [ABSTRACT FROM AUTHOR]

Effect of microstructure on the crack initiation and early propagation mechanism in the very high cycle fatigue (VHCF) regime was studied in 316L stainless steel (316L SS) by atomic force microscope (AFM) and electron back scattered diffraction (EBSD). The results show that small fatigue cracks initiate from the slip band near the grain boundaries (GBs) or the twin boundaries (TBs). Early crack propagation along or cross the slip band is strongly influenced by the local microstructure such as grain size, orientation, and boundary. Besides, the gathered slip bands (SBs) are presented side by side with the damage grains of the run-out specimen. Finally, it is found that dislocations can either pass through the TBs, or be arrested at the TBs. [ABSTRACT FROM AUTHOR]

Effects of the stress ratio on the very high cycle fatigue behaviors of 9%Cr turbine steel have been investigated at 630 °C. The experimental results show that the S–N curve has a continuous downward trend and has no fatigue limit with the increasing in the cycles at 630 °C. Meanwhile, according to the analysis of microstructure, there are two failure modes that were observed at different stress ratios (R = −1 and 0.1), including surface crack failure and internal crack failure, respectively. Besides, the theoretical threshold value of the crack growth is compared with the calculated value of the fracture surface. To decrease the difference between the threshold value of internal crack initiation and the corresponding theoretical value, a new model for the crack growth threshold of the interior-induced fracture at different stress ratios is proposed and discussed. [ABSTRACT FROM AUTHOR]

• Gigacycle fatigue behavior of a high-entropy alloy is firstly studied. • Fatigue micro-cracks trend to initiate near a triple junction grain boundary. • The {1 1 0} slip planes may also lead to the fatigue crack initiation, not only preferential {1 1 1} slip planes. Widespread interest has been generated by the high-entropy alloys (HEAs) due to their exceptional ability to combine high plasticity and high strength. However, there is still no comprehensive explanation for their fatigue behavior and crack initiation mechanism in the very high cycle fatigue region up to 109 cycles to the author's best knowledge. This work studied the gigacycle fatigue behavior of a single-phase CoCrFeMnNi HEA. It is found that fatigue failure is caused by casting defects and crystallographic facet. For the case of crystallographic facet just originating from matrix, the fatigue micro-cracks tend to initiate near a triple junction grain boundary. The activation of the {1 1 0} slip planes may also lead to the fatigue crack initiation, not only preferential {1 1 1} slip planes for this alloy. [ABSTRACT FROM AUTHOR]

The article offers information on the sixth international conference on Very High Cycle Fatigue (VHCF-6) held in Chengdu, China from October 15-18, 2014.

• A novel neural network model is designed to predict the low-cycle fatigue life under thermal–mechanical loading. • The model leverages the advantage of CNN and Transformer from taking binarized hysteresis images as input. • The proposed model is not sensitive to the crystal structure (BCC and FCC) and has a good accuracy. In this study, a novel neural network model is designed to predict the low-cycle fatigue (LCF) life under thermal–mechanical loading. The model is composed of three sub-networks: (1) a Convolutional Neural Network (CNN), (2) a Transformer, and (3) a Fully Connected Neural Network (FCNN). These three sub-networks function as the Backbone, Neck, and Head of the model, respectively. The model, named ConTrans (an abbreviation for Con volutional Neural Network- Trans former), utilizes binarized hysteresis images as input. The predictive capability of the ConTrans model for LCF life has been validated using experimental data from four different materials. The results demonstrate that almost all predictions fall within the 2-factor band, confirming the model's accuracy and effectiveness. [ABSTRACT FROM AUTHOR]

• The formation of FGA is attributed to continuous dynamic recrystallization. • The formation of fine grains promotes the extension of microcracks. • A mechanism for VHCF crack initiation in Q450NQR1 weathering steel is proposed. The fatigue behavior of Q450NQR1 high-strength weathering steel under a loading ratio of R = 0.1 was experimentally investigated in the very high cycle fatigue (VHCF) regime, elucidating the formation mechanism of the fine granular area (FGA). The results indicated that Q450NQR1 high-strength weathering steel mainly fails due to non-inclusion-induced crack initiation (NIICI) in VHCF regime. Slip in {1 1 0} and {1 1 2} plane families dominates in ferrite. Intergranular cracking results in fatigue microcracking, which in turn leads to the formation of NIICI in Q450NQR1 high-strength weathering steel. The formation of FGA was attributed to the localized severe plastic strain-induced continuous dynamic recrystallization at the tip of microcrack propagation. Moreover, it can be found that the formation of fine grains promotes the extension and aggregation of microcracks. Based on all experimental findings, a mechanism governing crack initiation in Q450NQR1 high-strength weathering steel during VHCF loading was proposed. [ABSTRACT FROM AUTHOR]

• To improve the computational efficiency and accuracy of SP simulation, a coupled DEM-FEM model was established. • Dislocations are plugged in the γ matrix to form subgranular boundaries that can fix γ' to form small-sized grains, and in addition some of the dislocations shear γꞌ further leading to grain refinement. • The SP treatment produced a high density of dislocations, work hardening, and residual compressive stresses, also reduces porosity defects left on the surface. The effects of shot peening on the surface microstructure, tensile properties, and high-cycle fatigue properties of MAR-M247 nickel-based superalloy were investigated. The predicted surface roughness, magnitude of residual stress, and depth of plastic strain in the surface layer were simulated by the numerical simulations coupled Finite Element Method-Discrete Element Method (FEM-DEM). The results show that the shot peening treatment induced significant plastic deformation in the surface layer, leading to the accumulation of a large number of dislocations in the γ matrix, forming dislocation walls, resulting in the formation of fine grains. A high density of dislocations, work hardening, and residual compressive stresses occur after the shot peening treatment. These beneficial changes enhanced the resistance to crack initiation and propagation, thereby inhibiting surface crack initiation, and increasing the fatigue life and tensile strength of the alloy. [ABSTRACT FROM AUTHOR]

• Back stress hardening improves the fatigue performance at room temperature. • Stress-assisted second phase precipitation inhibiting the back stress hardening. • Life prediction model considering grain orientation shows high accuracy. During hot end components service, i.e., startup and shutdown processes, the cyclic stress caused by the asymmetry loading conditions is introduced. To study the effect of mean stress on the cyclic response, the low cycle fatigue, failure mechanism and life prediction model of additively manufactured (AMed) Inconel 625 superalloy at room temperature (RT) and service temperature (6 0 0 ° C) with strain ratio R = −1 and 0.1 are conducted. Experimental results indicate that the different cyclic softening and hardening behavior at RT and 600 ° C is the result of competition between dislocation slip, back stress hardening and second phase precipitation. According to the evaluation of the internal stress variables, the long-range back stress plays a crucial role to control the cyclic behavior of the alloy. At RT, the heterogeneous grain structure introduced by additive manufacturing causes the back stress hardening, making the material exhibit high fatigue performance. But this hetero-deformation induced (HDI) strengthening is inhibited at high temperatures due to the uniform distribution of dislocations. In addition, a low cycle fatigue life prediction model based on critical plane method is established which presents an easily assess to evaluate the low cyclic fatigue life of an AMed material under various test condition. [ABSTRACT FROM AUTHOR]

• Very high cycle fatigue behavior of titanium alloy welded joints under asymmetric loading was investigated. • Mean stress causes the fatigue failure to transfer from the FZ to the BM. • The size of the plastic zone at the crack tip for FGA is related to the lamella α structures in FZ. • The crack non-propagation region and critical safe size of welded pores are obtained. Very high cycle fatigue behavior of electron beam welded TC17 titanium alloy joints under different stress ratios was investigated. The results revealed that the welded joints exhibited different failure modes depending on the stress ratios applied. As the stress ratio increased, the failure probability attributed to welded pores in the fusion zone (FZ) gradually decreased, shifting the failure mode towards fatigue failure induced by the primary α phase in the base metal (BM). Moreover, the stress intensity factor at the edge of fine granular area (FGA) was determined to be the threshold of long crack growth. It was observed that the size of the plastic zone at the crack tip of FGA was related to the acicular α phase structures precipitated through recrystallization in the FZ. The influence of effective size and location of welded pores on fatigue life was further discussed. To gain further insights, improvements were made to the K-T diagram. Consequently, the non-crack propagation zone and the critical safety size of welded pores responsible for initiating cracks in the welded joint were identified. Additionally, the Weibull probability function was employed to determine the average size of welded pores that led to fatigue failure in the welded joints. [ABSTRACT FROM AUTHOR]

Preventing or mitigating very high cycle fatigue (VHCF) damage and failure of completion string is essential for ensuring their long-term stable operation. Therefore, a comprehensive understanding of the string's fatigue performance in tension, compression, and bending over an extended service life is necessary. This study investigated the evolution, microscopic mechanisms, fracture behavior, and life prediction of VHCF damage in completion string under axial and bending conditions. It was found that the intragranular twin structures on the critical interface of VHCF source directly participate in lattice reconstruction, enhancing the grain's strain coordination ability, and increasing the proportion of facet area on the fatigue fracture surface. Based on the fracture characteristics of VHCF, the Mayer empirical formula was improved, and a novel convolutional neural network (CNN) based VHCF life prediction model was proposed. Utilizing the CNN's powerful fracture feature extraction and nonlinear modeling capabilities, the VHCF fatigue life prediction accuracy was significantly enhanced, with prediction errors all within 2 times the experimental results' error range. Based on the axial and bending VHCF fracture mechanisms of completion string and CNN, a more advanced method for predicting axial and bending VHCF life is provided, offering theoretical supports for the engineering safety service and life assessment of completion string. • The structural reconstruction in VHCF fatigue source region will delay fatigue initiation and propagation. • Lifetime prediction of VHCF damage in completion string under axial and bending conditions was investigated. • Convolutional neural networks provide a more advanced VHCF lifetime prediction method. [ABSTRACT FROM AUTHOR]

• The separate S - N curves are firstly detected in additively manufactured metals. • Fatigue cracks originate from gas pores, lack of fusions, and columnar grains. • Δ K at the border of rough area corresponds to propagation threshold for long cracks. • Fatigue sensitivity of maximal micro-crack type, size, and location increase successively. Ultrasonic fatigue tests were performed to investigate high and very high cycle fatigue behaviors of a laser additively manufactured Inconel 718 (IN718) alloy in the as-deposited condition. The results indicate that the competition failure behavior between the surface and interior crack initiation results in the separate S - N curve. Both manufacturing defects (e.g., gas pore, lack of fusion) and columnar grains (matrix) observed in the microstructure could act as the original fatigue micro-cracks due to the effective restriction on manufacturing defects. The fatigue sensitivity levels increase successively in terms of the type, size, and location of the maximal micro-crack. [ABSTRACT FROM AUTHOR]

High-speed railway wheels often experience harsh cyclic loadings in service. To date, plenty of works have been performed to investigate the on-axis high cycle fatigue (HCF) behavior of high-speed railway wheel rim steel. However, the effect of off-axis loading on the HCF behavior is generally not discussed in literature. In this work, a self-designed multi-axis fatigue test fixture was used to realize biaxial tension-bending/shear fatigue loading in the direction of 30° to the axis. For comparison, the on-axis HCF behavior is studied experimentally as well. Based on multiaxial mechanics analysis, a new composite elastic-plastic model for off-axis fatigue was presented. Under almost the same life, although the difference in equivalent stress was not significant between on-axis and off-axis fatigue, experiments found that there were some clear distinct damage mechanisms behind the two fatigue modes in different stress dimensions. Combining with fatigue tests, evolutionary characteristics of macro fracture surface deflection, micro fracture morphology, crack propagation path, dislocation and lattice microstructure beneath fracture critical region were analyzed in depth to make connections with each other, and it was vitally affected by lattice structural evolution, sub-/grains reconstruction, slips shearing at boundaries, and fracture toughness in various degrees. The present research would help us to better understand the law of micro-damage and its evolution of wheel steels under off-axis fatigue. [ABSTRACT FROM AUTHOR]

In the present work, the fretting fatigue behaviors of GCr15 steel (Chinese GB/T-standard) have been studied up to the very high cycle fatigue (VHCF) regime by using an ultrasonic fatigue system. The main aims of the investigations were the analyses of S–N curves, worn morphology, as well as fracture morphology. As expected, the results of the fretting fatigue tests show a reduction in the lifetime compared to plain fatigue tests without fretting. 2-D and 3-D morphologies of the fretting worn surfaces are combined to study the evolution of the stick zone. With the increasing fatigue life, the stick zone will fully occupy the whole worn zone. Besides, fretting causes severe oxidation and debris in the worn surface. The crack initiation depth increases with the increase in fatigue life, especially in the VHCF regime, the depth increases drastically because of fretting delamination. • There exists no "traditional fatigue strength" for GCr15 steel in the vicinity of 107 cycles in fretting fatigue.. • Fretting fatigue strength is much lower than plain fatigue's and the strength reduction factor varied from HCF to VHCF. • Fatigue cracks nucleate in the stick zone in the VHCF regime, which is different to the previous two existing boundaries. • Fretting promote the crack initiation, especially in the VHCF regime. [ABSTRACT FROM AUTHOR]

Characterizing the transition from a crack-free to a cracked state remains a challenging topic in fatigue. Mg-rare earth alloys, containing the long-period stacking ordered (LPSO) phase, exhibit superior mechanical properties. Here, microcracks are found to nucleate at the soft α-Mg nano-layers, away from the LPSO lamellae. Notably, severe oxidation is observed along the damage bands. Based on the detailed characterizations, it is suggested that dislocation motions continuously bring the new α-Mg matrix to oxidation transformation, resulting in the thickening MgO layer. However, once the fatigue-induced oxide reaches a certain thickness, it starts acting as a barrier for the further dislocation motions. As a result, dislocation accumulation and cumulative damage occur in the region ahead of the thick oxide, causing microcrack nucleation. This fatigue-induced oxidation, assisting microcrack nucleation, is distinct from the existing fatigue mechanisms. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract High cycle fatigue testing was performed on an as-extruded Mg-10Gd-3Y-0.5Zr alloy to investigate its fatigue crack initiation and early propagation behaviors. Experimental results showed that only basal slip was activated at a low cyclic stress amplitude. Fatigue crack initiated from the slip bands along the basal plane, leading to the formation of cleavage-like facets on the fracture surface. The formation of facets around the crack initiation sites consumed a vast majority of the cyclic loadings in high cycle fatigue regime, and small fatigue crack propagation was significantly retarded by local microstructure heterogeneity of neighboring grains. This can be ascribed to the incompatible deformation at low cyclic stresses, resulting in the localization of slip bands within isolated grains. [ABSTRACT FROM AUTHOR]

Highlights • The unique morphology of "surface and subsurface crack initiation" appeared. • The preferred micro-texture of α p grains, instead of the α p cluster, acts as the crack origin. • Multi-points surface crack initiation was more likely to happen at low stress ratio. • The varying dominance among cyclic creep failure and fatigue failure were proposed. Abstract In order to investigate the stress ratio effect on notched fatigue behavior of a turbine engine blade titanium alloy (Ti-8Al-1Mo-1V), notched specimens were employed to perform ultrasonic fatigue experiments up to 1010 cycles at the stress ratios (R) of −1, 0.1 and 0.5. Three typical shapes of S - N curves were clearly observed at three different stress ratios. Moreover, the unique morphology of "surface and subsurface crack initiation" appeared in the very high cycle fatigue (VHCF) regime at R = 0.1 and 0.5. With the increase of stress ratios, the occurrence probability of multi-points surface crack initiation became lower. The fatigue crack originates from the preferred micro-texture, rather than cluster, of two types of α p grains. The quantitative feature of crack initiation region was presented by means of the facets density, and it was used to explain the life variability at the nominal maximum stress of 333.3 MPa. Furthermore, the varying dominance among cyclic creep and fatigue on the failure of specimens was proposed to expound the variation of S - N curve shapes at different stress ratios. [ABSTRACT FROM AUTHOR]

• With a small part of online data, the final fatigue failure was predicted with reasonable accuracy. • The crack growth rate was practically estimated for an inner crack at a very high cycle regime. • The initiation site of an inner crack with eccentricity was detected from the thermographic analysis. • A combined approach of simulation and measurement was introduced to study the propagation rate of internal crack. This paper presents a thermodynamic characterization method for estimating the internal crack growth rate, which has been a puzzle in very high cycle fatigue research. A theoretical approach of surface temperature is discussed with crack size, initiation site, and time for thin sheet material. Infrared thermography is used to study the inner crack behavior and the heat dissipation phenomenon under 20 kHz vibration loading on high-strength stainless steel. A numerical simulation reveals the consequent temperature elevation on the surfaces by the heat generation at the crack tip and the heat conduction. Ultimately, the internal crack growth rate and final fatigue failure prediction are obtained by combining the calculation of heat dissipation and the observed evolution of the surface temperature field. [ABSTRACT FROM AUTHOR]

This paper investigates the high cycle fatigue behavior of industrial pure zirconium at room temperature under a stress ratio of R = 0.1, focusing on the HCP-to-FCC phase transformation and its role in fatigue crack nucleation. The results show the presence of fatigue damage in the form of the HCP-to-FCC phase transformation, which is responsible for the crack nucleation. The fatigue-induced phase transformation is triggered by the activation and slip of partial dislocations. This phase transformation results in lattice expansion, local strain redistribution, and macroscopic strain. Notably, the strain concentration along the HCP/FCC phase interfaces is identified as a critical factor contributing to fatigue crack nucleation. These findings provide new insights into the mechanisms underlying fatigue-induced phase transformations and the nucleation of fatigue crack. [ABSTRACT FROM AUTHOR]

• 1. Very high cycle fatigue behavior of LPBF TC4 was investigated at 400℃. • 2. Crack initiation competitions among defects at different locations was analyzed. • 3. Multiple slip systems activated at HT, even those with low Schmid factors. • 4. The formation of FGA is related to grain refinement caused by strain localization. The crack initiation mechanism of laser powder bed fusion (LPBF) Ti6Al4V was investigated at elevated temperature up to very high cycle fatigue (VHCF) regime. The competition concerning defect location is elaborated using the stress intensity factor range and Z-parameter model. Additionally, localized high stress near the defects is responsible for plastic strain localization in the non-prior α' region, following the grain refinement and fragmentation in the larger grains nearby, which is strongly associated with the formation of microcracks and fine granular area. Meanwhile, the process is facilitated by reducing dislocation resistance and activating multiple slip systems due to high temperature. [ABSTRACT FROM AUTHOR]

• The high cyclic fatigue tests of 9% Cr steel are conducted under R = −1, 0.1, 0.5, and 0.7 at 630 °C. • The mean stress sensitivity factor (M R) is proposed and related to the stress ratio. • A new fatigue life empirical relationship considering the mean stress sensitivity factor (M R) is proposed with reasonable confidence. During the startup and shutdown processes of the steam turbine rotor, the fatigue failure behavior caused by asymmetric stress is introduced. To study the effect of mean stress on the fatigue failure behavior of 9% Cr steel at high temperatures, the high cyclic fatigue tests are conducted under different stress ratios (R = −1, 0.1, 0.5, and 0.7) at 630 °C. The results indicate three fatigue failure modes: surface crack initiation failure, sub-surface crack initiation failure, and necking induced failure. With the increasing stress ratio, the surface and sub-surface crack initiation failure modes gradually change to necking induced failure mode. Moreover, the oxide intrusive-extrusion mechanism primarily determines the surface crack initiation failure and promotes surface defect formation. Meanwhile, the inhomogeneous microstructure induces sub-surface crack initiation failure, wherein the crack propagates along the martensitic lath boundary or through the martensitic lath. The attenuation of mechanical properties and the increase in mean stress level are the critical factors affecting the necking induced failure under 630 °C. In addition, the mean stress sensitivity factor (M R) is proposed to explain the effect of the stress ratio on fatigue performance. Furthermore, the mean stress sensitivity factor (M R) can be used to quantitatively analyze the effect of mean stress on fatigue properties under different stress ratios. Finally, the fatigue life empirical relationship considering the mean stress sensitivity factor (M R) is constituted. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were carried out to investigate the effect of stress ratio on the very high cycle fatigue (VHCF) behaviors of titanium alloy (Ti-8Al-1Mo-1V). The fatigue strength at stress ratios of 0.1 and 0.5 were lower than the safely modified Goodman approximation, and a significant deviation was observed in the VHCF regime. Based on fractography and fracture mechanics, the fatigue failure process was congruously divided into four stages: (1) crack initiation induced by cleavage of primary α grains and its coalescence; (2) microstructure-sensitive slow crack propagation; (3) microstructure-insensitive fast crack propagation; and (4) final fatigue failure. Meanwhile, for surface crack initiation, fatigue crack propagation life only occupied less than 3% of total life in the VHCF regime, and over 95% of the fatigue crack propagation life was expended in the microstructure-sensitive crack propagation stage. The effect of stress ratio on crack propagation life was not distinct. [ABSTRACT FROM AUTHOR]

Subsurface cracking is a common feature at N > 107 cycles [i.e., in very high cycle fatigue (VHCF) regime]. Widely reported literatures focus mainly on the subsurface cracking at inclusions. However, it can initiate at the inner matrix as well, and the physic origin remains unclear. Here, we systematically explore the subsurface cracking at the inner matrix from mesoscale to atomic-scale. The results show that the subsurface cracking initiates from a single coarse lath, and dislocation tangles and cells generate at the extremely localized plastic deformation zones in proximity to the nanoprecipitates being of α-Fe. We suggest that the dislocations with high dense are blocked at the nanoprecipitate/α-Fe interfaces resulting in the nucleation of interfacial microcracks, which then coalesce into a trans-granular crack along the sub-boundaries covered by nanoprecipitates, finally the crack initiates within the coarse lath. These findings provide fundamental insights into the mechanism of precipitation-strengthening in anti-fatigue designs. [Display omitted] [ABSTRACT FROM AUTHOR]

• Slip bands initiated inside martensitic laths, are oriented parallel to the lath. • Internal slip bands of the martensitic laths cause cracks to initiate. • Crack are more likely to propagation along the martensitic laths. • The complex structure of martensite has a greater resistance to small crack growth. Martensitic steel presents high fatigue crack resistance due to its complex and hierarchical microstructure, but its mechanisms are unclear. This work experimentally investigated effect of the lath and the prior austenitic grain (PAGs) on crack initiation and the early propagation behavior in high cycle fatigue. It is found that micro cracks initiated from the slip bands in the interior of thin monocrystalline laths, in which the dislocation intensity zones deflected the crack path. Subsequently, the micro cracks coalesced through the lath boundaries, and resulted in an intermittent and staggered appearance of crack path in the PAGs. Finally, the crack propagation across the boundaries of the primary PAG are extremely slow and presents significant zigzag manners due to the misorientation of the laths in neighboring PAGs. The above observations demonstrate that the crack initiation and early propagation are significantly hindered by the multi-scaled microstructure of the martensitic steel. [ABSTRACT FROM AUTHOR]

• Crack initiation from the microstructural inhomogeneity was investigated in very high cycle fatigue. • Mode II cracking along the martensite lath dominant the crack initiation behavior. • Fatigue cracking prefers to occur at the boundaries of favorably oriented martensite lath due to slip-cracking process. Super martensitic stainless steel is known for its outstanding mechanical properties, which lead to its widely application in structural component bearing cyclic loadings. This study investigates the effect of local microstructure on the crack initiation behavior of a super martensitic stainless steel in the very high cycle fatigue (VHCF) regime. Results show that the non-inclusion-induced crack initiation is the dominating pattern of VHCF failure. Fatigue cracks initiate from the boundaries of the martensite lath due to the mode II cracking process, resulting in the formation of a slip area located at the core of the fine granular area (FGA). The size and the orientation of the martensite lath determine the morphology of the slip area but exert limited influence on the formation of the FGA at the fracture surface. These results may effectively improve the service safety and reliability of super martensitic stainless steel. [ABSTRACT FROM AUTHOR]

[Display omitted] • Very high cycle fatigue behavior of TC17 with stress ratios was investigated at 400℃. • Crack initiation depends on facets formation and is related to maximum shear stress. • Formation of facets is induced by basal or prismatic slips with high Schmid factor. • GNDs density is accumulated along with basal or prismatic slips, forming sub-GBs. Fatigue behavior of titanium alloy with various stress ratios was investigated at 400℃ in VHCF regime. Cracks initiation depends on the formation of facets and is related to the maximum shear stress. The formation of facets is induced by basal or prismatic slips with the high Schmid factor inside α p grains. The morphology of α p grains will affect the crack initiation. The high-density GNDs are found to be accumulated along with the basal or prismatic slips, then form subgrain boundaries (sub-GBs) inside α p grains. With the cyclic loading, the created sub-GBs will further form facets, thus initiating the microcracks. [ABSTRACT FROM AUTHOR]

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

Very high cycle fatigue failure of bimodal titanium alloy at 400 °C and high mean stress has been investigated. Many circular creep-fatigue voids (CFVs) were found underneath the crack initiation zone, which were distributed in the β transformed matrix and near the grain boundaries (GBs) of primary α grains. By the energy dispersive spectroscopy (EDS) element mapping, the oxygen element was found to be enriched around the CFVs. In addition, many sub-GBs of small-angle GBs could be observed inside the primary α grains. Based on these, a new mechanism of CFVs and sub-GBs assisted crack initiation and early propagation in bimodal titanium alloys was proposed in very high cycle fatigue regime. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fatigue behavior of the thick electron beam welded titanium alloy joint was studied from high cycle to very high cycle domain. • Site specific deformation microstructures were analyzed using FIB and TEM. • Cracking grain boundary and slip-driven fatigue crack formation mechanisms were unraveled. • Critical parameters of porosity defects in assisting very high cycle fatigue crack initiation were clarified. High cycle and very high cycle fatigue (VHCF) crack initiation mechanism of TC17 alloy joint with heterogeneous microstructures in FZ has been studied at the stress ratio of 0.1. Intrinsic deficiencies of slip deformation in β grains with few martensites and cracking coarse grain boundary between prior β grains with martensite colonies account for the high cycle fatigue (HCF) of joints. The average diameter of intergranular welding pores regarding VHCF crack nucleation is measured to be ∼36 μm. The driving force governing the crack nucleation from intrinsic deficiencies in HCF to porosity defects in VHCF is ∼1.66 MPa·m1/2. [ABSTRACT FROM AUTHOR]

• Low-stress cyclic deformation is dominated by basal slip at grain scale. • Gliding of 〈c + a〉 dislocation accommodates heterogeneous deformation at grain boundaries. • 〈c + a〉 dislocation activities contribute to dislocation intensity zones within the basal slip bands. The underlying low-stress cyclic deformation and associated dislocation activities within the slip bands (SBs) in a rare earth-containing magnesium (RE-Mg) alloy were characterized. The results show that basal slip is the predominant deformation at grain scale, but the gliding of 〈c + a〉 dislocation is also visible near grain boundaries to accommodate localized inhomogeneous deformation. The interaction between basal 〈a〉 dislocations and non-basal 〈c + a〉 dislocations results in the nucleation of dislocation intensity zones (DIZs), which subsequently hinders the development of the basal slip bands. Therefore, it is suggested that the DIZs contributed to the enhanced high cycle fatigue resistance of RE-Mg alloys. [ABSTRACT FROM AUTHOR]

Tensile and fatigue behaviors of a compressor blade titanium alloy TC17 at room and high temperatures (RT and HT) have been investigated up to very high cycle fatigue (VHCF) regime. Compared to RT, the HT tensile strength of TC17 titanium alloy decreases, but the elongation remains basically unchanged at HT. The fatigue S–N curves are changed from single linear mode at RT to bilinear mode at HT owing to the temperature effect, and the failure mode is transformed from the surface and subsurface crack initiations at RT to the surface crack initiation at HT. The brittle oxygen-enriched subsurface (BOES) layer on the specimen surface is responsible for the crack initiation at HT. The failure models of oxide shedding and oxide intrusion are proposed. Dislocation distribution indicates that the fatigue failure at HT is insensitive to the intrinsic material microstructure and is only related to the BOES layer on the specimen surface in the VHCF regime. • Very high cycle fatigue behavior of titanium alloy at room temperature and high temperature was investigated. • The oxide layer on the specimen surface is responsible for the fatigue cracks initiation at high temperature. • The dislocation distribution revealed that the material is insensitive to microstructure at high temperature. [ABSTRACT FROM AUTHOR]

When turbine steel services, i.e., startup and shutdown processes, the cyclic mechanical stress caused by the asymmetry loading conditions is introduced. To study the effect of mean strain on the cycle response and microstructure-sensitive at room temperature, the low cycle fatigue under two strain ratios was used. Experimental results indicate that significant cyclic softening was observed for both modes. The cyclic strain ratio is independent of strain amplitude and strain ratio. The plastic strain, plastic strain energy density, and cumulative strain energy density at strain ratio of 0.1 are higher than that at strain ratio of −1. Therefore, applied stress amplitude and fatigue life fitted by Manson-Coffin formula under strain ratio of 0.1 is lower than that at strain ratio of −1. According to the evaluation of the internal stress variables, i.e., back stress, isotropic stress and viscous stress under both modes, the back stress is a critical part of the cyclic stress, which can directly control the cycle softening behavior. Besides, the fatigue life prediction model considering the strain ratio is constituted through hysteresis energy method. Finally, the mechanical results are linked to the microstructure observed by transmission electron microscopy. The microstructure sensitive was evaluated and deduced. Unlabelled Image • The cyclic softening behavior exhibits at both strain ratios in low cycle fatigue. • The intragranular back stress is the key factor to control the cyclic softening behavior under different strain ratios. • A transition process of dislocation pattern occurs, resulting from incompatibilities of plastic strain due to mean strain. • Low cycle fatigue life prediction model considering the strain ratio has been constituted and confirmed. [ABSTRACT FROM AUTHOR]

The mechanical behavior of electron beam welded joints for titanium alloy has been investigated in static and cyclic loading up to very high cycle fatigue. It can be concluded that a higher welding voltage parameter and optimal heat treatment condition in combination will produce higher static and cyclic properties of titanium alloy welded joints. The metastable martensite α′ structures in the β grains of the fusion and heat-affected zones are wholly decomposed into small lamellar α phases, forming α + β phases through the post-weld heat treatment (PWHT). Hence, the welded joints by PWHT could transform the "concave" shape microhardness distribution into "convex" shape distribution. The fatigue cracks are initiated from the internal pores of the material in very high cycle fatigue regime. The higher voltage parameter applied to the welding process could reduce the density of pores in the material, which is in favour of the fatigue life of the welded joints. • A higher voltage and optimal heat treatment in combination produce higher mechanical properties of welded joints. • Microstructure and microhardness of welded joints were analyzed before and after heat treatment. • The higher voltage reduced the pores density in the welded joints, which improved fatigue life. [ABSTRACT FROM AUTHOR]