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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]

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]

Purpose: The purpose of this paper is to determine (1) the relationship between microstructure and fatigue cracking behavior and (2) effect of rolling on the process of crack initiation and propagation in FeCrAl alloys. Design/methodology/approach: The qualitative and quantitative fracture studies were performed using scanning electron microscopy and the non-contact optical measurement system (IFMG5). Findings: The results show that the formation of facets, rough facets and parallel stripes in the crack initiation and early crack propagation zones are closely related to the sensitivity of crack behavior to the microstructure of the material. Besides, the rolling process has a significant influence on the small crack initiation and propagation behavior. Quantitative analysis demonstrates that the size of the stress intensity factor and plastic zone size in the rough zone is associated with the rolling process. Originality/value: The findings of this study have the potential to enhance the understanding of the microstructural crack formation mechanisms in FeCrAl alloys and shed light on the impact of rolling on the long-term and ultra-long fatigue behavior of these alloys. This new knowledge is vital for improving manufacturing processes and ensuring the safety and reliability of FeCrAl alloys used in nuclear industry applications. [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]

The very high cycle fatigue (VHCF) problems of thin-plate structures are usually caused by high-frequency vibrations. This paper proposes an accelerated fatigue test method based on ultrasonic loading technology in order to develop a feasible bending testing method and explore the bending fatigue characteristics of thin-plate structures in the VHCF regime. A new bending fatigue specimen with an intrinsic frequency of 20 kHz was designed based on cantilever bending through finite element simulation. It was verified by the axial load test with R = −1. The results showed that the method could effectively transfer the dangerous cross-section at the first-order cantilever bending restraint to the internal part of the specimen, thereby making the fracture location independent of the complex stresses. The linear relationship between the vibration amplitude and the maximum stress was also verified using strain measurements. Furthermore, the S-N curves and fracture morphology for different loading types were consistent with conventional studies to a certain extent, which indicated that the design of the bending test model was reasonable. [ABSTRACT FROM AUTHOR]

The microstructural evolution and mechanical properties of welded joints for Q450NQR1 and Q450AWR5 alloys were investigated. The inhomogeneous microstructures of Q450NQR1 and Q450AWR5 welded joints can result in heterogeneous mechanical responses. Microhardness distributions of the two welded joints are not uniform with the highest hardness value in the FZ, followed by the HAZ, and the lowest hardness value in the BM. The tensile strength of Q450AWR5 welded joints is slightly higher than the Q450NQR1 welded joints. The tensile fracture occurred in the BM of both welded joints, probably due to the strengthening effect of bainite with high dislocation density and lattice distortion. The fatigue life is dominated by the welded defects inside the welded joint. The location of fatigue failure occurs mainly near the fusion zone and is limited by the welded defects, independent of the microstructure. Highlights: Microstructure and mechanical properties of weathering steel welded joints were studied.Microhardness values of two welded joints are nonuniform and the maximum value in FZ.High dislocation density bainite strengthening contributed to failure occurring in BM.Welded defects are insensitive to tensile behavior but extremely sensitive to fatigue behavior. [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]

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]

Highlights • Welding asymmetrical softening effects govern tensile failure mechanism. • Static mechanical property governs the fatigue crack initiation at lower fatigue life. • Porous micro-defects govern the fatigue crack initiation at higher fatigue life. Abstract The effects of microstructural inhomogeneities and micro-defects on tensile and very high cycle fatigue behaviors of a friction stir welded joint of ZK60 magnesium alloy have been investigated. The transitional zone between thermo-mechanically affected zone and heat affected zone on advancing side is prone to tensile fracture. With increasing of fatigue life, fatigue fractures transfer from the weak zone of static mechanical properties to the welding nugget zone, where softening effects of welding heat input govern fatigue failure at lower fatigue life and the porous micro-defects govern that at higher fatigue life. [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]

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]

This paper presents the results of experimental investigation on fatigue behaviors of friction stir welded joints in AA7075-T6 with ultrasonic fatigue test system (20 kHz). Two kinds of particles, Fe-rich intermetallic compounds and Mg 2 Si-based particles, governed the fatigue crack initiation. The plastic deformation and recrystallization during welding process led to the changes in particle size and micro crack occurrence between thermo-mechanically affected zone (TMAZ) and nugget zone (NZ). Therefore, the fatigue crack initiation sites leaned to be located at the TMAZ in short fatigue life, or at the NZ in very high cycle fatigue regime. [ABSTRACT FROM AUTHOR]

Highlights: [•] Very high cycle fatigue behaviors of HAZ and FZ for Q345 welded joints. [•] The association between frequency variation and crack growth under ultrasonic loading condition. [•] A nonlinear fatigue model based on natural frequency changes for very high cycle fatigue. [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]

• Microstructure inhomogeneity governs the fatigue crack initiation mechanism. • Fatigue damages prefer to concentrate in the few larger grains due to cyclic slip deformations. • Internal failures present longer fatigue lives than surface failures due to the environment effect. The VHCF behaviors of an extruded magnesium alloy are investigated. Due to the microstructure inhomogeneity, fatigue damages prefer to concentrate in the few larger grains. Basal slip deformations are supposed to be responsible for the transgranular crack initiations, and form the similar faceted morphologies. Under same stress amplitude, internal failures present longer fatigue lives than surface failures due to the requirement of larger initial crack and the deceleration rate in the vacuum-like environment at specimen interior. [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]

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]

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]

• A sound ("defect-free") Mg-FSW joint is produced. • Unique texture and banded structure of Mg-FSW joint affect crack initiations. • A semi-quantitative mechanical model verifies the different failure mechanisms. Friction stir welding (FSW) produces heterogeneous microstructure in nugget zone of its joint, which should affect the fatigue responses. This work revealed the unique texture and banded structure of the Mg-FSW joint and their decisive roles on crack initiation behaviors. At higher stress levels, failures occur along the AS NZ/TMAZ boundary (AS: advancing side, NZ: nugget zone, TMAZ: thermo-mechanically affected zone) due to the inhomogeneous deformations near the boundary. Severe plastic deformations concentrate in the softer layers of banded structure. However, at lower stress levels, plastic deformations become weak and localized, but accumulate in the alternate layers of banded structure simultaneously due to the same soft texture orientation, and induce random failures in banded structure. The failure mechanisms are verified by semi-quantitative mechanical models. [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]

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]

• 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]

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]

• 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] • 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]

Working service of structural materials at elevated temperatures generally leads to a significant decrease in their fatigue performance, thereby threatening the safety and reliability of engineering structures. In this pursuit, the very high cycle fatigue behavior and the relevant crack initiation mechanism of super austenitic stainless steel 654SMO was comparatively investigated at room temperature (RT) and high temperature (HT, 300 °C). It was found that the heterogeneous microstructure strongly influenced the fatigue failure mechanism, wherein the crack initiation sites transferred from fine grain clusters at the RT to coarse grains at HT. Furthermore, the cross-section of the crack initiation sites revealed that the micro-porosity and accumulative slip bands resulted in the crack originations at RT and HT, respectively. The differences of crack initiation were ascribed to the effect of elevated temperature, which weakened the sensibility of crack initiation to micro-defect in very high cycle fatigue regime. Our study provides new insights in the correlation of high temperature and heterogeneous microstructure, in regards to the crack initiation behavior. Unlabelled Image • Fatigue cracks initiated from the micropores within fine-grain clusters at room temperature due to the micropores shortened the crack initiation process. • Non-uniform microstructures caused strain concentration on the interior of the super coarse grains, where fatigue cracks trend to initiate at high temperature. • High temperature weakened the sensibility of crack initiation to micro-defect in very high cycle fatigue regime. [ABSTRACT FROM AUTHOR]