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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]

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation, thus potentially causing unanticipated catastrophic fractures. The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous, which obstructs the understanding of fretting fatigue issues, and is unable to guarantee the reliability of structures for long-term operation. Here, fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of an α/β titanium alloy up to 108 cycles. Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone. The grain size in the topmost nanostructured layer could be refined to ∼40 nm. The grain refinement process involves the initial grain rotation, the formation of low angle grain boundary (LAGB; 2°–5°), the in-situ increments of the misorientation angle, and the final subdivision, which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays. The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary (HAGB) and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer, thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress. Eventually, it facilitates surface degradation and the formation of catastrophic fractures. [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]

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]

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]

This paper describes and analyzes the appearance, phase, and photocurrent of titanium film surfaces after exposure to water vapor at 100-1000 °C. The experimental results show that water vapor at higher than 300 °C can cause measurable oxidation of the titanium surface, and the oxidation product is changed into rutile from anatase in a gradual phase transformation with temperature. In the reaction, the product produced at 500-900 °C is a mixed phase of rutile and anatase. The titanium film surface appearance can change regularly with temperature, and an acicular rutile oxidation product can be produced on the surface at 800 °C. The photocurrent density on titanium surface increases with reaction temperature, and comes to a maximum value at 800 °C. However, with further increase in reaction temperature, the density decreases. [ABSTRACT FROM AUTHOR]

The electron beam welding process is widely used in the connection among titanium alloy material parts of aero-engines. Its mechanical properties need to meet the requirements of long life and high reliability. In this paper, the static strength and the fatigue failure behavior of the electron beam weldments of TC17 titanium alloy were investigated experimentally under low amplitude high frequency (20 kHz), and the mechanical response and failure mechanism under different external loading conditions were analyzed. In summary, the samples were found to have anisotropic microstructure. The tensile strength of the PWHT of TC17 EBW joint was ~4.5% lower than that of the base metal. Meanwhile, compared with the base metal, the fatigue strength was reduced by 45.5% at 109 cycles of fatigue life. The fracture analysis showed that the fatigue failure of the welded joint of TC17 alloy was caused by the welded pores and the fatigue cracks initiated from the welded pores. A fine granular area (FGA) was observed around the crack initiation region. The existence of pores caused the stress intensity factor of the fine granular area (KFGA) to be inversely proportional to the fatigue life. The KFGA calculation formula was modified and the fatigue crack propagation threshold of the welded joint of TC17 alloy was calculated (3.62 MPa·m1/2). Moreover, the influences of the effective size and the relative depth of the pores on the very long fatigue life of the electron beam welded joint of TC17 titanium alloy were discussed. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests up to 1010 cycles were performed on a turbine engine titanium alloy (Ti-8Al-1Mo-1V) at the stress ratio (R) of −1 with smooth specimens and at R = −1, 0.1 and 0.5 with notched specimens. As a result, with increase of fatigue life, the source of reduced fatigue life caused by multi-point surface crack initiation changes from crack propagation stage to crack initiation stage in the high cycle fatigue regime. Notch effect further promotes the degeneration of high cycle and very high cycle fatigue strength at R > −1. The bilinear model, extended from the Goodman method, can better estimate the mean stress sensitivity of this titanium alloy. The fatigue mean stress sensitivity and fatigue-creep mean stress sensitivity of this material increased with the increase of fatigue life. The new model, based on the Murakami model, can provide more appropriate predictions for notch fatigue strength. [ABSTRACT FROM AUTHOR]

The pursuit of strong and ductile structures for cryogenic applications has fueled a persistent interest in the microstructure design of metals and alloys. While heterostructure design has recently demonstrated efficacy in achieving superior strength-ductility combinations at room temperature, its potential remains less explored in the cryogenic regime. In this report, we unveil anomalously improved cryogenic mechanical properties of a pure titanium with harmonic heterostructure that matches or even surpasses those of commercial titanium alloys. Through detailed comparison to room-temperature deformation behavior, we reveal that the superior performance of heterostructure at cryogenic temperature is fundamentally underpinned by the amplified inter-zone mechanical incompatibility and the suppression of dislocation cross-slip. The former promotes the generation of abundant geometrically necessary dislocations, and the latter optimally configures them. Collectively, these two factors culminate in the hetero-deformation-induced effect, delivering the anomalous improvement of cryogenic mechanical properties in titanium. These findings not only significantly advance our understanding of cryo-deformation behaviors in heterostructured materials but also offer the potential for developing high-performance materials for cryogenic applications. [Display omitted] [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]

• TMAZ consists of needle-like α precipitates and elongated α p within β grains. • Fine β grains in SZ contribute to the highest strength in the 3rd layer. • Cracks tend to nucleate from HAGBs and {1 1 2} 〈1 1 1〉 slip deformation of β structure. • Fatigue cracks initiate from the 2nd layer of SZ below 210 MPa. • Crack growth exhibits a higher crack growth rate in HAZ than SZ. Heterogeneities of microstructure, tensile and fatigue behavior of FSWed titanium alloy joint were in-situ studied by utilizing replica, DIC and electron back-scattered diffraction methods. Banded TMAZ consisting of needle-like α precipitates and elongated α p within DRXed β grains bridges the HAZ and SZ. Fine β grains in SZ contribute to the highest tensile strength in the 3rd layer. Fatigue cracks prefer to nucleate from HAGBs and {1 1 2} 〈1 1 1〉 slip deformation of β structure in the 2nd layer of SZ below 210 MPa and 3rd layer of HAZ otherwise. Transgranular crack propagation along the {1 1 2} planes exhibits a higher crack growth rate in HAZ than SZ. [ABSTRACT FROM AUTHOR]

In the present study, Ti-24Nb-4Zr-3Mn alloys were prepared by mechanical alloying and spark plasma sintering. With the increase of milling time, the particle size of alloy powder decreases, while the oxygen content shows the opposite trend. The excess O content will dramatically deteriorate the mechanical properties of Ti-24Nb-4Zr-3Mn alloy and should be strictly controlled in future studies. Among all the investigated alloys, Ti-24Nb-4Zr-3Mn alloy prepared by 20-hour milling powder (TNZM-20) shows excellent comprehensive mechanical properties with a preferable compressive strength (∼2786 MPa) and constant strain (∼ 55.3 %), as well as a highest tensile strength (∼1148 MPa) and elongation (∼11.2 %), which attribute to the combined effect of almost full density, fine grain strengthening and solid solution strengthening together with dispersion strengthening provided by FCC phase. Interestingly, the excellent combination of high strength and low elastic modulus of metastable β alloy can be obtained by manipulating the content of the FCC phase, providing a new approach for fabricating high-performance metastable β Ti alloys. • Ti-24Nb-4Zr-3Mn alloys were prepared by SPS from alloy powders milled for various times. • Ti-24Nb-4Zr-3Mn alloy prepared by 20-h milling powder shows excellent comprehensive properties. • The origination of the FCC phase has been discussed. • The strengthening mechanisms have been clarified. [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]

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]

• Very long life fatigue behavior of TC17 welded joints at 400℃ was investigated. • Intergranular welded pores are responsible for fatigue failure. • The unique facets at 400℃ were discovered for TC17 welded joints. • The formation of facets is related to columnar grain and is driven by shear stress. The very long life fatigue failure mechanism of welded joints for titanium alloy at 400℃ has been investigated. The results show that the welded joint has a nonhomogeneous microstructure. The welded defects are mainly welded pores, and maximum pore sizes are evaluated by statistical methods. The increase in temperature reduces the fatigue strength and changes the shape of S-N curves for the welded joints. Crack initiation is attributed to the existence of welded pores. Crystallographic facets are found around welded pores on the crack initiation zone, and their formation is sensitive to columnar grains and is driven by shear stress at 400℃. [ABSTRACT FROM AUTHOR]

The non-uniform temperature field and cooling rates in the molten pool during solidification process fabricate microstructural and mechanical heterogeneities in fusion zone (FZ) and heat-affected zone (HAZ) of thick titanium alloy weldment. In this study, electron beam welding was used to assemble thick Ti-5Al-2Sn-2Zr-4Mo-4Cr (TC17) alloy plates. Microstructural heterogeneity concerning recrystallized martensitic structures with different morphology and distribution features in FZ and retained microstructures after experiencing thermal exposure in HAZ were analyzed to unravel their effects on multiscale mechanical behavior. It was found that needle-shaped martensite (α') exhibited a comparative high nano hardness and the precipitation of needle-shaped martensite cluster contributed to a more severe tensile strain localization in FZ and premature brittle fracture of the joint. The bottom layer of weldment, where the FZ was verified to be martensitic free, exhibited a better combination of strength and ductility. Mo and Al exhibited a better thermostability than Cr, which held the responsibility for the microstructural stability in HAZ during severe thermal exposure after welding. • Effects of heterogeneous microstructure on nanomechanical and tensile behavior were studied. • The formation of needle-shaped martensite was verified to damage the ductility of the weld joint. • A comparatively lower diffusion rate of Al than Cr contributes to a better thermostability of α P than α L. [ABSTRACT FROM AUTHOR]

Friction stir butt welding was carried out to assemble 6 mm-thick TC17 titanium alloy plates with a W–Re weld tool. Inhomogeneous microstructures were obtained in stir zone (SZ) and heat affected zone (HAZ) of the weldment. A series of tests were conducted to reveal the influence of inhomogeneity of microstructure on mechanical behavior. Comparative refined β phase without precipitating secondary metastable microstructure generated a soft SZ and favored on the ductility of weldment. Strain localization during tension was promoted by the interface of HAZ and SZ that formed by thermal-mechanical coupling effects. Coincidentally, fatal fatigue cracks were verified to nucleate around the same region, and presented in a stress dependent mode. • Comparative refined β grains without lamellar α generate a soft SZ. • W-contaminate traces exhibit higher nano hardness than intermediate banded regions. • Strain localization promoted by the HAZ/SZ interface causes tensile fracture. • Fatal fatigue cracks nucleate around HAZ/SZ interfaces. [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]

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]

• Microstructural evolution along the depth direction presents a gradient feature. • Four types of deformation twins are introduced into primary α phases. • High-angle grain boundaries are increased by deformation twins (~13%). • Strengthening mechanisms of extra-long life fatigue resistance are revealed. Ultrasonic fatigue tests were performed on notched specimens to investigate high and very high cycle fatigue behaviors in the laser shock peened region of a compressor blade titanium alloy (Ti–8Al–1Mo–1V). The plastic deformation and compressive residual stress, induced by the laser shock peening (LSP), present gradient distributions along the depth direction. High-angle grain boundaries (HAGB, >15°) are increased by deformation twins, which are introduced into primary α phases during the plastic deformation. The enhancement in fatigue resistance is ascribed to the comprehensive retardation effects of the compressive residual stress, high-density dislocations, and HAGBs. [ABSTRACT FROM AUTHOR]

• The very high cycle fatigue strength of titanium alloy fabricated by additive manufacturing is much lower than that of the traditional process. • Combining with the weak zone, Δ K FGA is analyzed, and it is found that the size of the FGA plastic zone was similar to the width of the martensitic laths. • Failure critical dimension is obtained and a new fatigue life relationship is established based on defect characteristics. • Based on probability statistics, the influence of maximum defect size on fatigue behavior is discussed. Tensile and fatigue behaviors of selective laser melted titanium alloy were investigated in the as-built condition. The alloy has a high defect density, and its tensile properties are comparable to those of wrought alloys, but the fatigue strength is significantly reduced. The FGA plastic zone is similar to the martensite laths width. The critical size of fatigue failure is analyzed by the K-T diagram. A new value is obtained based on the defect characteristics. Using the probability statistical, the maximum effective size of the defect considering the shape was calculated, and the competitive relationship of the defects was analyzed by volume density. [ABSTRACT FROM AUTHOR]

• Ultrasonic torsional fatigue tester with the capacity to modify the mean stress is developed. • VVery high cycle tensile and torsional fatigue behavior of turbine blade titanium alloy is comparatively studied. • The short crack nucleation and propagation character of interior initiated fatal crack induced by tensile and torsional loadings are analyzed using FIB milling method. Very high cycle tensile and torsional fatigue of TC17 titanium alloy with bimodal microstructure have been firstly and comparatively studied at the stress ratio of 0.1. Like the tensile cracks, torsional cracks were substantiated to nucleate from specimen surface or interior sites and presented in a fatigue life-dependent mode. Internal tensile and torsional fatigue cracks with faceted morphology were analyzed using focus ion beam and unraveled to initiate in modes II + III that driven by maximum shear stress. Particularly, short crack branching and deflection beneath the crack initiation rough area can be observed for torsional loading condition. [ABSTRACT FROM AUTHOR]

Supersaturated coarsening β phase and submicron acicular martensitic α′ phase originated by the high cooling rate during the welding process resulted in the electron beam welded TC17 titanium weldment to be poor in strength and ductility. Post-weld heat treatment was performed at 630 °C for 2 h to improve the mechanical properties of electron beam welded TC17 titanium alloy joint through modifying the microstructures of the weldment. Results showed that undesired α′ martensites in the fusion zone and heat affected zone, which inhibited the mechanical response of the weldment, were completely eliminated. The supersaturated β phase with martensitic α′ structure in the as-welded joint was recrystallized into α lamella with a width in nanoscale embedded in the β matrix after the post-weld heat treatment, which conspicuously enhanced the mechanical performances of welded joints. Micro-hardness of the FZ presented to be more stable around HV410. Tensile strength was regained from 868 MPa to 1138 MPa, which was very closed to the raw material. Intrinsic metallic deficiency inducing fatigue failure of the as-welded joint was eliminated after the post-weld heat treatment accompanied with the fatigue strength enhancing from about 140 MPa of the as-welded joint to 240 MPa with an equal fatigue life of 107 cycles under the stress ratio of 0.1. Image 1 • Post-weld heat treatment was performed on the electron beam welded TC17 titanium alloy joint. • Martensitic α′ phase was sufficiently disintegrated and recrystallized into β phase with nano α lamellas. • The joint showed great micro-hardness stability after post-weld heat treatment. • Tensile and fatigue strength was significantly enhanced. [ABSTRACT FROM AUTHOR]

• Formation of the equiaxed and columnar coarsening β grains was analyzed. • The tensile fracture was caused by the strain concentration at the equiaxed grain region. • Fatigue crack initiation mechanisms presented in a stress-dependent mode. • Effect of extrinsic welding pores was only revealed at a low-stress amplitude condition. The tensile fracture was originated from the strain concentration in equiaxed grain region adjacent to the fusion center, where a lower micro-hardness occurred when compared to the columnar grain region in the fusion zone. A higher stress level was more potential to fabricate a fatal fatigue crack initiating from the intrinsic metallic deficiencies. At a lower stress of 180 MPa, fatigue crack appeared to initiate from the intergranular welding pores other than the intrinsic metallic deficiencies. Furthermore, intergranular welding pores induced fatal crack initiation became the major fatigue crack initiation mechanisms when the fatigue stress amplitude down to 140 MPa. [ABSTRACT FROM AUTHOR]