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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]

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]

This study utilized transmission electron microscopy (TEM) and on-axis transmission Kikuchi diffraction (TKD) to investigate the fatigue-induced HCP-to-FCC phase transformation in industrial pure zirconium under a stress ratio of R = 0.1. The results show that fatigue damages result from phase deformations during cyclic loadings. The fatigue-induced FCC-Zr phases exhibit a B-type orientation relationship with the HCP-Zr matrix. Notedly, due to the different growth directions of Shockley partial dislocations relative to nucleation points, there are two FCC-Zr variants after the HCP-to-FCC phase transformation. The content of these two variants accounts for 65% and 35% of the total FCC-Zr, respectively, appearing as lamellae morphology embedded parallelly within the matrix. The distribution of the two variants includes isolated distribution and adjacent distribution. For the adjacent distribution, a twinning relationship is observed between the two variants. Meanwhile, as an intermediate transition stage of the HCP-to-FCC phase transformation, stacking faults are observed at the boundaries of the FCC-Zr lamellae. These findings offer insights into the microstructural features and formation mechanisms of fatigue-induced HCP-to-FCC phase transformation. [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]

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]

Purpose: Most supersonic aircraft were manufactured using 2A70 aluminum alloy. The purpose of this paper is to study the corrosion mechanism and fatigue behavior of an aircraft in a semi-industrial atmospheric corrosive environment, alternating effects of corrosion and fatigue were used to simulate the aircraft's ground parking corrosion and air flight fatigue. Design/methodology/approach: For this purpose, the aluminum alloy samples were subjected to pre-corrosion and alternating corrosion-fatigue experiments. The failure mechanisms of corrosion and corrosion fatigue were analyzed using microscopic characterization methods of electrochemical testing, X-ray diffraction and scanning electron microscopy. Miner's linear cumulative damage rule was used to predict the fatigue life of aluminum alloy and to obtain its safe fatigue life. Findings: The results showed that the corrosion damage caused by the corrosive environment was gradually connected by pitting pits to form denudation pits along grain boundaries. The deep excavation of chloride ions and the presence of intergranular copper-rich phases result in severe intergranular corrosion morphology. During cyclic loading, alternating hardening and softening occurred. The stress concentration caused by surface pitting pits and denudation pits initiated fatigue cracks at intergranular corrosion products. At the same time, the initiation of multiple fatigue crack sources was caused by the corrosion environment and the morphology of the transient fracture zone was also changed, but the crack propagation rate was not basically affected. The polarization curve and impedance analysis results showed that the corrosion rate increases first, decreases and then increases. Fatigue failure behavior was directly related to micro characteristics such as corrosion pits and microcracks. Originality/value: In this research, alternating effects of corrosion and fatigue were used to simulate the aircraft's ground parking corrosion and air flight fatigue. To study the corrosion mechanism and fatigue behavior of an aircraft in a semi-industrial atmospheric corrosive environment, the Miner's linear cumulative damage rule was used to predict the fatigue life of aluminum alloy and to obtain its safe fatigue life. [ABSTRACT FROM AUTHOR]

Numerical simulation based on extended finite element method was employed to investigate the fatigue crack propagation of nickel-based superalloy at room temperature. Experimental tests on compact tension specimens have performed to obtain fatigue crack propagation parameters in Paris region. The extended finite element method has presented a new approach to solve the stress intensity factors and can effectively predict crack propagation without re-meshing at crack tip. The simulation results are in good accordance with experimental data in real 3D cases. [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]

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]

• 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]

This work investigated the fatigue behaviors of Mg 96.32 Gd 2.5 Zn 1 Zr 0.18 alloy under both solution and aging conditions. The main microstructural difference between the two conditions is characterized by the dense β′ nano-precipitates under aging condition. The fatigue strengths are comparable under both conditions, whereas the fatigue lives at medium stress levels tend to be improved apparently under aging condition. The improved fatigue lives under aging condition are attributed to the dense β′ nano-precipitates, which delay the processes of crack initiation and early propagation. Under solution condition, fatigue damage exhibits a continuous pattern along the damage bands. However, under aging condition, fatigue damage shows a scattered pattern along the damage bands due to the pinning effect of β′ nano-precipitates and their inhomogeneous distribution. • Dense β′ nano-precipitates delaying the crack initiation and early propagation process. • Inhomogeneous distribution of β′ nano-precipitates influence the damage accumulation. • Fatigue damage showing a high concentration of oxygen. [ABSTRACT FROM AUTHOR]

This study focuses on the fatigue durability of recycled concrete corrugated steel web composite box girders (RAC-CSWs). The parameters of recycled coarse aggregate (RCA) replacement ratios, i.e. 0%, 25%, 50%, 75%, 100%, and shear-to-span ratio (0.97, 1.46, 1.96, 2.43) were investigated on the nine specimens of RAC-CSWs subjected to four-point bending variable amplitude fatigue. The variable fatigue life, deformation properties, and fatigue damage mechanisms were explored. The results indicated that the shear-to-span ratio is the key parameter of the fatigue survival of RAC-CSWs. The RAC-CSWs exhibited a more complex crack distribution and a relatively larger deformation than the CSWs with natural coarse aggregate (NAC) during the test. For the specimens under variable amplitude fatigue life, the maximum fatigue life decrease of 31.4% was found on the RAC-CSWs with a 100% RCA replacement ratio, and a minimum of 10.2% was found on the specimens of RAC-CSWs with a 75% RCA replacement ratio. In addition, the effect of the shear-to-span ratio on the stress concentration of RAC-CSWs was investigated, which showed that a decrease of 57.5%, 79.4%, and 87.7% on fatigue life of RAC-CSWs was found when the shear-to-span ratio increased from 0.97 to 1.46, 1.96, and 2.43, respectively. Finally, the fatigue life of RAC-CSWs was evaluated based on the research of fatigue damage and finite element analysis (FEA), which shows a good agreement compared to the experimental results. • Proposed Recycled Concrete-Corrugated Steel Web Composite Girders, Pioneering Sustainable Use of Recycled Concrete. • Studied the Impact of Variable Amplitude Fatigue Loading on Recycled Concrete Web Girders. • Evaluated Fatigue Life through Combined Experimental, Finite Element Simulation, and Fatigue Damage Theory. • Emphasized the Necessity of Fatigue-Resistant Design in Green Infrastructure. [ABSTRACT FROM AUTHOR]

• The strain mismatch between coarse grains and small grain cluster regions leads to the initiation of fatigue cracks. • The surrounding microstructure influences the crack propagation mode, thereby intervening in the crack propagation rate. • Multiple slip will trigger cross slip, causing uneven local strain within the grains, thus altering the crack propagation path. • The retarding effect of small grain cluster regions on grain growth is weaker compared to high-angle grain boundaries and pinning by small grains. The objective of this study was to investigate the cyclic deformation mechanisms at the microstructural level and their correlation with the initiation and early propagation of small fatigue cracks in super austenitic stainless steel 654SMO. The findings revealed that localized microstructural variations in grain size and orientation led to strain concentration, which subsequently triggered crack initiation. The behavior of surrounding microstructural evolution influences the crack propagation mode, further affecting the crack propagation rate. The retarding effect of small grain cluster regions on grain growth is weaker compared to high-angle grain boundaries and pinning by small grains, and multiple slip will trigger cross slip, causing uneven local strain within the grains, thus altering the crack propagation path. [ABSTRACT FROM AUTHOR]

• A dual-scale modelling approach combining sub-modelling approach with crystal plasticity constitutive model. • Efficient procedure for fatigue life prediction of anotch parts using the Tanaka-Mura-Wu model and Paris law. • The microstructural size effect on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Fatigue performances of critical structures are strongly affected by the microstructural (e.g. grain, defect, inclusion, etc.) size effect, and it is thus important to quantify their detrimental effect. In this work, a numerical procedure is constructed to quantify the influence of microstructure on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Specifically, by combining sub-modelling approach with crystal plasticity constitutive model, a dual-scale modelling approach is developed for studying grain-level mechanical behavior of Ni-based superalloy GH4169 notched components. In addition, the dislocation-based Tanaka-Mura-Wu model is applied for fatigue crack initiation life prediction. The Paris law is then utilized for fatigue crack propagation analysis based on the simulated short crack. To study the microstructural size effect on fatigue crack initiation and propagation behaviors, sub-models containing various grain orientation, grain size, defect size/shape are built and analysed. Finally, a series of fatigue tests on notched specimens of Ni-based superalloy GH4169 were carried out for method validation. Results indicate that the established dual-scale modelling approach and fatigue life prediction framework yields good agreement with experimental results. [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]

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]

• A special kind of fatigue damage in the form of scattered oxide nodules. • Hierarchical anisotropic nanostructure leads to the high localization of damage. • LPSO lamellae suppress the growth of the fatigue-induced oxide nodules. Hierarchical anisotropic nanostructured Mg-RE alloys, containing LPSO lamellae and dense nano-precipitates, exhibit superior mechanical properties. However, their reliability as structural materials for long-term service, especially in terms of fatigue, is still a matter of concern. Here, scattered oxide nodules are detected as a special kind of fatigue damage that assists crack initiation, rather than the typical slip-band structure. The LPSO lamellae and dense β′ nano-precipitates lead to the high localization of fatigue damage. The fatigue-induced oxide nodules form and grow within the soft α-Mg layer, but their growth is limited by the adjacent strong LPSO lamellae, resulting in microcrack nucleation. Finally, clusters of microcracks along the LPSO/oxide interface converge to form the trans-granular crack initiation. [ABSTRACT FROM AUTHOR]

• Applied surface replica technique to study small cracks in dual-phase Mg alloy, revealing relations between crack growth and microstructure. • Rolling process led to distinct microstructures and deformation textures in specimens sampled along the transverse and extrusion directions. • Higher proportion of α-Mg grains with favorable orientation reduced fatigue crack initiation forms. • Denser grain boundaries and strong cylindrical deformation texture of the α-Mg phase severely impeded early growth of small fatigue cracks. This study investigated the high-cycle fatigue behavior of an extruded dual-phase magnesium-lithium (Mg-Li) alloy using ultrasonic fatigue experiments and surface replica techniques. Specimens along the extrusion and transverse directions were compared. The α-Mg phase was found to significantly affect fatigue crack behaviors, with different deformation textures and microstructures in ED and TD specimens. ED specimens exhibited a stronger cylindrical texture, which impeded crack initiation within the basal plane of the α-Mg grain, resulting in higher fatigue strength than TD specimens. This study highlights the importance of microstructure and deformation texture in determining the fatigue behavior of dual-phase Mg-Li alloys. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Short fatigue crack initiation and propagation were related to deformation mechanism. • Basal slip was the preferential site for crack initiation and propagation in Mg alloy. • Microstructural inhomogeneities caused a fluctuant crack growth rate within ∼200 μm. Abstract Microstructural cyclic deformation mechanisms and their relation to small fatigue crack initiation and early propagation behavior were experimentally investigated in a rare earth-containing magnesium alloy (Mg-Gd-Y-Zr). The results indicate that basal slip is the dominant deformation mechanism, especially in coarse grains, and eventually leads to fatigue crack initiation. Early crack propagation behavior was strongly influenced by local microstructural heterogeneities in grain size and orientation. Three kinds of microstructures—favorably-oriented coarse grains, fine grain clusters and unfavorably-oriented coarse grains—are discussed in terms of their deformation mechanisms and resistance to crack propagation. These microstructural effects caused highly variable crack propagation rates within the first ∼200 μm of cracks. [ABSTRACT FROM AUTHOR]

• A comprehensive review on phase-field models for fracture and fatigue is presented. • The theory fundamentals and implementation approaches are provided and compared. • All the phase field models for fatigue failure are summarized and discussed. • The future research directions of phase field fatigue models are pointed out. Phase field fracture models have demonstrated great capacities in simulating crack nucleation, propagation, branching, and joining in brittle and ductile materials subjected to external stimulations. Because of their great flexibility, phase field fracture models can incorporate various material properties including anisotropy, elastoplasticity, viscoelasticity, hyperelasticity, piezoelectricity, etc. Recently, the models have been extended to fatigue, one of the most common material failure mechanisms in structural engineering. The purpose of this paper is to provide a comprehensive review of recent work on phase field models for fatigue damage and failures. Following a brief introduction to the development of phase field fracture models as well as the theories and models of fatigue, the fundamentals of the models for elastic and elasto-plastic cases are presented, including basic theories, formulas, tension–compression splits, and numerical implementations. Then, the emphasis is placed on different aspects of the phase field models for fatigue fracture involving the approaches of introducing fatigue damage, the descriptions of crack nucleation and propagation, the coupling of cyclic plasticity and phase field models, and the acceleration algorithms. Finally, the future research directions of phase field models for fatigue fracture and the major challenges to be conquered in their engineering applications are discussed. [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]

• Two physics-guided machine learning (ML) frameworks for fatigue life prediction of AM materials; • The hybrid ML model based on the morphological details of critical defects for fatigue life prediction; • In serial physics-guided ML framework, ANN presents better prediction accuracy compared with SVR; • Under limited data condition, the serial physics-guided ML framework with ANN is recommended. Introducing random defects is a type of the dominant causes of fatigue scatter of additive manufacturing (AM) materials. The fracture mechanics-based models oversimplify the fatigue damage process and ignore the key influencing factors. Given data-driven nature of machine learning (ML), sufficient samples are required to train, which generally cannot be met in practice. In view of this, this work develops two physics-guided machine learning frameworks which combine physics-based models and ML algorithms, for improving the prediction ability. In the proposed framework, ML models can consider those factors ignored by physics-based models and physics-based models can ensure the consistency with physical results. Finally, fatigue data of three AM materials are used for model evaluation and comparison. The results show that, physics-based models lack sufficient explanatory power for scatter of fatigue life. Moreover, compared with the purely data-driven methods, the proposed framework can maintain high accuracy and alleviate the over fitting phenomenon under insufficient fatigue data sources. [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]

• 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]

• A new definition of non-proportional degree on generalized plane for non-proportional loading; • A new model without additional material constants for fatigue life assessment under non-proportional loadings; • Experimental data of three materials under different loading paths verified model predictions accuracy. Engineering components often operate under complex loadings, in which the variable amplitude multiaxial stresses are raised by geometric discontinuities including holes, grooves, fillets and shoulders, etc. Besides, the non-proportional loading will lead to the rotation of maximum principal stress/strain and additional fatigue damage of structural elements in service. Consequently, the multiaxial and non-proportional loading have attracted increasing attentions. In this study, for distinguishing the effects of different load paths on fatigue life, a simple and applicable method to quantify the non-proportionality is defined, which comprehensively considers the influence of loadings on all material planes. A new equivalent strain damage parameter based on critical plane is proposed in this study. Specifically, to quantify the effect of non-proportional loading on fatigue damage, a non-proportional degree of loading on the generalized plane is developed. Coupling with the existing non-proportional coefficient, a novel fatigue damage parameter is derived by combining a non-proportional factor. Experimental data of 304 stainless steel, sintered porous iron and CuZn37 brass are utilized for model validation and comparison. Fatigue lives with different load paths are evaluated respectively. In comparison with the Fatemi-Socie (FS), Chen and Itoh models, proposed method more effectively evaluate the multiaxial fatigue life of materials under non-proportional loadings. [ABSTRACT FROM AUTHOR]

• Recent advances on stress gradient effect in metal fatigue are reviewed. • Fatigue methods considering stress gradient effect are compared using experimental data of two materials with different notch types. • Further directions on notch fatigue assessment under stress gradient effect are given. Under external loadings, notched components always show varying degrees of stress concentration. Accordingly, different kinds of approaches have been presented to characterize notch fatigue behaviours under complex loading histories. Among them, the considering of the stress gradient effect is a popular perspective, together with the fatigue damage zone and critical distance concepts. This work systematically reviews recent advances on notch fatigue analysis considering the stress gradient effect. Specifically, four approaches are summarized and explored, namely the local stress/strain-based approaches, critical distance-based approaches, critical plane-based approaches, and weighting control parameters-based approaches. This review aims to provide a systematic review and comparison on these methods, therefore to offer a reference for subsequent research on notch fatigue. [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]

In most cases, welding processes promote the application of structural materials at the expense of strength and ductility. Post-weld treatment is proposed to regain mechanical properties of weldment, which necessitates that microstructural and the associated mechanical behavior evolutions being clearly understood. In this study, post-weld heat treatment was conducted to enhance the mechanical properties of the electron beam welded Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy joint. Microstructural evolutions and monotonic mechanical properties were characterized to optimize the post-weld heat treatment parameters. It was found that direct aging at 630 °C for 2 h can upraise the strength and ductility of the joint by ∼31% and ∼511% respectively. Metastable needle-shaped martensite α′ within rapidly solidified fusion zone had been completely decomposed, and thin recrystallized α lamella precipitated in the prior β phase without forming consecutive coarse grain boundary α after above direct aging treatment. Such microstructural characteristics enable the direct-aged joint to exhibit a higher fatigue strength from high to very high cycle region than the as-welded joint. Moreover, the fatigue crack nucleation resistance was enhanced significantly through direct aged treatment. • Post weld heat treatment was carried out to optimize the parameter on regaining mechanical properties of electron beam welded TC17 alloy joint. • The optimized parameter was obtained by precipitating recrystallized nano α lamella in FZ without coarsening in grain boundary α phase. • Fatigue strength and fatigue crack nucleation resistance were enhanced significantly by direct aging treatment. [ABSTRACT FROM AUTHOR]

• Method for calculating the creep damage during LCF based on the equivalent creep stress was proposed. • Creep damage model considering a continuously changing stress was established. • Engineering application of this model was discussed. A method for calculating the creep damage during low cycle fatigue (LCF) (tensile plasticity reversed by compressive plasticity, PP) controlled by strain rate ε ̇ was proposed. According to the equivalent principle of creep damage with the same reduction in cross section, a creep damage model considering a continuously changing stress was established based on the equivalent creep stress σ en , which was determined by the total creep damage, the actual creep time (the total time of tensile loading stage in the whole process of fatigue), and the fatigue life during LCF (PP). For Sanicro 25 alloy, σ en first increases quickly, and then increases linearly and slowly with increasing ε ̇. Based on the creep damage model, the total creep damage and fatigue life under different ε ̇ were predicted accurately. To study the application of the method for calculating the creep damage, the creep damage model for 316 stainless steel was established and verified. At a constant ε ̇ , when γ decreases or K increases, the creep damage decreases and the fatigue life increases. Based on the variation trend, the engineering application of the creep damage model was discussed. [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]

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]

• 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]

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]

• 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]

• Fatigue behaviour of slender trapezoidal profiled web is experimentally tested. • Two typical crack manners and post-buckling out-of-plane deflection are examined. • Analytical models for shear resistances versus slenderness ratios are evaluated. • Life prediction allowing for tension field and geometric features is justified. Slender trapezoidal corrugated web does generally possess notable post-buckling capacity with certain reserve of shear resistance. Repeated secondary bending stresses due to post-buckling at the boundaries of the subpanel of the trapezoidal corrugated web, however, may result in potential local fatigue failure. A series of fatigue experimental tests on the slender profiled web girders with stiff flanges subjected to the action of predominant shear is outlined. The fatigue test results are presented in terms of stiffness deterioration and fatigue failure induced by out-of-plane deflection as the applied load exceeds the elastic critical load. It was found that fatigue cracks due to predominant shear action propagate much faster than these due to combined shear and tearing. Recommendations are prescribed for evaluating the fatigue resistance of post-buckled slender trapezoidal corrugated webs with appropriate detail categories codified in the Eurocode 3. Previously published analytical models correlating normalized shear resistances with relative plate slenderness ratios are compared against test data and discussed for their applicability. Based on the test results, a new mathematical representation is derived based on von Kármán's effective width concept for ultimate loads. The derived formula is demonstrated to correlate well with test data since reasonable allowances are given for the tension field theory and geometric characteristics of the trapezoidal corrugated webs. [ABSTRACT FROM AUTHOR]