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The ultra-fine grained (UFG) pure titanium was prepared by equal channel angular pressing (ECAP) and rotary swaging (RS). The strain controlled low cycle fatigue (LCF) test was carried out at room temperature. The fatigue life prediction model and mean stress relaxation model under asymmetrical stress load were discussed. The results show that the strain ratio has a significant effect on the low cycle fatigue performance of the UFG pure titanium, and the traditional Manson-coffin model can not accurately predict the fatigue life under asymmetric stress load. Therefore, the SWT mean stress correction model and three-parameter power curve model are proposed, and the test results are verified. The final research shows that the three-parameter power surface model has better representation. By studying the mean stress relaxation phenomenon under the condition of R≠−1 it is revealed that the stress ratio and the strain amplitude are the factors that significantly affect the mean stress relaxation rate, and the mean stress relaxation model with the two variables is calculated to describe the mean stress relaxation phenomenon of the UFG pure titanium under different strain ratios. The fracture morphology of the samples was observed by SEM, and it was concluded that the final fracture zone of the fatigue fracture of the UFG pure titanium was a mixture of ductile fracture and quasi cleavage fracture. The toughness of the material increases with the increase of strain ratio at the same strain amplitude. [ABSTRACT FROM AUTHOR]

Turbine blades made of Ni‐based single crystal superalloys (NBSXs) have long‐strip shaped cross sections and rectangular‐sectional structures, where the secondary orientation produces potential effects even the primary orientation is fixed at [001]. Low cycle fatigue behaviors between [010] and [110] transversely oriented rectangular‐sectional NBSX specimens were compared. Obvious differences existed under 600°C but disappeared under 850°C, with the deformation mechanism and fracture mode transitions. Secondary orientation effects on stress asymmetry and fatigue life cannot be described by the conventional LCP model and critical plane method but were well explained by dislocation path length‐dependent back‐stress model and A.N. May's random slip model. Highlights: Secondary orientation effects on rectangular‐sectional NBSXs under 600°C are more obvious than those under 850°C;[110] case shows higher stress asymmetry and longer fatigue life than [010] case under 600°C;The secondary orientation effect on stress asymmetry is attributed to the different dislocations paths;The secondary orientation effect on fatigue life is attributed to the different dislocations intrusions' depths. [ABSTRACT FROM AUTHOR]

A dual phase (DP500/800) steel sheet (1.6 mm thickness) was evaluated by means of low cycle fatigue (LCF). The fatigue properties of the material were assessed by means of the cyclic elastic modulus determination at half-life as well as by the plastic strain amplitude measurements. Both methods show a good correlation and high reliability, however, the plastic strain amplitude approach tends to fit better to the experimental results. From a parametric analysis performed, it was observed that the plastic strain amplitude approximation could be more suitable to assess the fatigue properties for LCF of the material due to an improved correlation of the computed cyclic elastic modulus in comparison to the cyclic elastic modulus measurements at half-life. Both methods showed that, for strain amplitudes higher than εa = 0.004, the cyclic elastic modulus tends to be lower than 210 GPa, meanwhile for lower strain amplitudes, the cyclic elastic modulus tends to increase. An evaluation by X-ray Diffraction technique allowed to determine that the increment of residual equivalent strain contributes to the cyclic elastic modulus diminution.

The performance of formed components is significantly influenced by the initiation of ductile damage. Preceding forming operations, for instance, affect the service life determined in fatigue tests. In the current investigation, the effect of ductile damage in forming is isolated by changing the shoulder opening angle in forward rod extrusion. Forming-induced ductile damage is then related to measurements of void area fraction, density and Young’s modulus. Subsequent fatigue tests in the low cycle range indicate that the service life of the extruded components can be improved through a reduction of the forming-induced damage. A novel constitutive model considering forming-induced damage and fatigue damage is proposed to account for the observed behavior in axial fatigue tests of extruded components. The non-local ductile damage formulation is formulated in the framework of Generalized Standard Materials. Kinematic and isotropic hardening are considered. Based on earlier work of Lemaitre and Desmorat, the fatigue damage initiation criterion is extended to take the observed mechanical behavior in low cycle axial fatigue tests of formed components into account. The extended model is able to capture the effect of forming-induced damage on the service life.

Plain bend bars made from FV566 martensitic stainless steel were extracted from the root of ex‐service power plant turbine blades and several industry‐relevant notch geometries were introduced. Some of the samples were shot peened. The notched bend bars were loaded plastically in the low‐cycle fatigue regime and finite element (FE) modeling carried out to investigate the effects of changing notch geometry, combined with shot peening, on fatigue behaviors such as crack initiation, short crack growth, and coalescence. Shot peening damaged the notch surface, accelerating initiation behaviors, but had a lifetime‐extending effect by retarding short crack growth in all tested notch geometries. At a total strain range higher than 1.2%, the lifetime extension benefit from shot peening was diminished due to compressive residual stress relaxation in the notch stress field. Notch geometry (and the associated varying constraint levels and stress/strain gradients) was found to have no notable difference on fatigue life when tested at identical notch‐root strain ranges. Highlights: Initiation activity increased with increasing strain range regardless of notch geometry. Surface damage from the shot peening increased initiation activity and lowered a/c ratios. Shot peening reduced the early short crack growth rate at lower ΔK by a factor of 10. Shot peening increased lifetimes up to a strain range of 0.8%–1.2%. [ABSTRACT FROM AUTHOR]

The mechanical properties and low-cycle fatigue behavior of the hot-rolled AA2195 Al-Li alloy taken along the longitudinal direction and transverse direction were investigated. The anisotropy ductility of the alloy with the short-term natural aging (T4-SN), long-term natural aging (T4-LN), and T6 artificial aging alloy is mainly related to the grain structure and grain boundaries, whereas the weakened anisotropy of ductility is attributed to the appearance of PFZs. The T4-SN and T4-LN specimens present the cyclic hardening phenomenon due to the interaction of dislocations with dislocations or the Guinier–Preston zone. The cyclic softening phenomenon in T6 specimens is attributed to that the dislocations cut through the T1 precipitates, and some dislocations bypass the non-shearable precipitates, which activates more potential slip systems to shear the T1 precipitates. [ABSTRACT FROM AUTHOR]

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To investigate the effect of temperature on microcracking mechanisms of modified 9Cr‐1Mo steel, low cycle fatigue tests were performed at 350°C and room temperature (RT). At RT, the cyclic stress response at low strain amplitude (<0.5%) is controlled by friction stress while dominated by back stress at high strain amplitude. By comparison, the disappearance of sub‐grain boundaries induced by dislocation annihilation occurs at 350°C, which contributes to the decrease in back stress, resulting in more significant effects of back stress compared to RT. As a result of the accumulated fatigue damage, the distance between two parallel extrusions at 350°C is larger than that at RT, which is ascribed to the disappearance of low angle sub‐grain boundaries due to dislocation annihilation at 350°C. Due to the disappearance of small‐angle grain boundaries at 350°C, microcracks initiate along high‐angle grain boundaries at 350°C instead of along low‐angle grain boundaries at RT. Highlights: The temperature‐dependent fatigue microcrack initiation mechanisms are revealed.Cyclic stress response of modified 9Cr‐1Mo steel is controlled by back stress at 350°C.Increased width of slip bands at 350°C is induced by disappearance of lath boundaries.Microcracks initiate along HAGB at 350°C while along laths at RT. [ABSTRACT FROM AUTHOR]

The traditional study on fatigue strength for ship structures usually focuses on high cycle fatigue and ignores low cycle fatigue. However, given the recent trend towards large-scale ship development, the stress and deformation experienced by ship structures are becoming increasingly significant, leading to greater attention being paid to low cycle fatigue damage. Therefore, experimental and numerical studies on crack propagation behavior of cracked plates under low cycle fatigue loads were carried out in this paper, in order to explain the fatigue crack propagation mechanism. The effect of the stress ratio and maximum applied load on the crack propagation behavior was investigated by conducting experimental research on the cracked plate of AH32 steel. The experimental results show that an increasing maximum applied load and decreasing stress ratio will shorten the fatigue life of the cracked plate. Meanwhile, based on the finite element method, the distribution of the stress–strain field at the crack tip and the effect of crack closure were evaluated. The influencing factors such as the stress ratio and crack length were considered in numerical studies, which provided a new way to study the low cycle fatigue crack propagation behavior. [ABSTRACT FROM AUTHOR]

In recent decades, the effect of Fe element addition on titanium alloy has been investigated extensively due to the development of low-cost titanium alloys, as well as B microalloying, which could decrease the grain size of titanium alloys during the casting process. As a key structural material, the study of the fatigue behavior of titanium alloys is crucial and always attractive for scientists. Hence, in this paper, the low cycle fatigue (LCF) behavior and fracture characteristics of a low-cost Ti-2Fe-0.1B alloy with a lamellar structure were investigated systematically, five different strain amplitudes (Δεt/2) in the range from 0.6% to 1.4% were selected to control the LCF process. It was found that the Ti-2Fe-0.1B alloy exhibits continuous cyclic softening behavior in the cycle as a whole at Δεt/2 ≤ 1.2%, while at Δεt/2 = 1.4%, it exhibits slight cyclic hardening at the initial stage of the cycle, then shows cyclic softening. Compared with pure titanium and other typical titanium alloys, the Ti-2Fe-0.1B alloy indicated maximum fatigue life under the same strain amplitude, it can be attributed to the fine grain size result from the effect of Fe element and trace B, which could hinder the dislocation movement and crack propagation. [ABSTRACT FROM AUTHOR]

A sustained load holding period imposed during fatigue loading is detrimental to material performances, causing a sharp decline in the fatigue life of near-α titanium alloys. Therefore, the deformation discrepancies of dwell fatigue (DF) and low cycle fatigue (LCF) were studied for Ti834 titanium alloy with bimodal structures in this work. The fractographies after dwell fatigue and low cycle fatigue testing were characterized using scanning electron microcopy (SEM), and the crack propagation paths at the subsurface were investigated using an optical microscope (OM). In order to reveal the mechanism of fatigue damage, detailed dislocation structures were observed using transmission electron microcopy (TEM). The crack propagation paths in microscales and the dislocation distributions were observed in the LCF and DF. The reasons for the discrepancies are also discussed in this work, which effectively enhances the understanding of the dwell failure procedures. The results show that the near basal cracks are formed under dwell fatigue, and the deformation is highly localized at the boundary of αp grains under dwell fatigue. In contrast, during low cycle fatigue, the sample tends to deform homogenously. An intergranular fracture along the primary αp grains is formed due to the localized deformation during dwell fatigue. However, a transgranular fracture is formed in the primary αp grains under low cycle fatigue. [ABSTRACT FROM AUTHOR]

Low cycle fatigue (LCF) regime was experimentally studied for a 316L steel additively manufactured by laser‐powder bed fusion (L‐PBF), a material widely used in sectors that require a reliable durability analysis. Material cyclic elastoplastic behavior is described by the Chaboche–Voce combined plasticity model, which displayed a great degree of accuracy. The fatigue life was modeled by both invoking the Manson–Coffin curve and other simplified models derived from static properties of the material; some of which showed remarkably good accuracy. A quantitative comparison with a wrought‐processed 316L steel displayed a markedly different cyclic elastoplastic response but comparable fatigue strengths. Highlights: Chaboche–Voce model is calibrated on experimental cyclic response of laser‐powder bed fusion (L‐PBF) 316L stainless steel (SS).L‐PBF 316L SS experiences higher stress than wrought material during low cycle fatigue (LCF) loading.Midlife cycles of L‐PBF and wrought 316L SS have similar plastic strain energy density.L‐PBF 316L SS has a lower fatigue strength at lower strain ranges than wrought material. [ABSTRACT FROM AUTHOR]

This paper aims to demonstrate a novel technique enabling the accurate visualization and fast mapping of deformation-induced α′-martensite produced during cyclic straining of a metastable austenitic stainless steel, refined by reversion annealing to different grain sizes. The technique is based on energy and angular separation of the signal electrons in a scanning electron microscope (SEM). Collection of the inelastic backscattered electrons emitted under high take-off angles from a sample surface results in the acquisition of micrographs with high sensitivity to structural defects, such as dislocations, grain boundaries, and other imperfections. The areas with a high density of lattice imperfections reduce the penetration depth of the primary electrons, and simultaneously affect the signal electrons leaving the specimen. This results in an increase in the inelastic backscattered electrons yielded from the vicinity of α′-martensite, and a bright halo surrounds this phase. The α′-martensite phase can thus be separated from the austenitic matrix in SEM micrographs. In this work, we propose a deep learning method for a precise α′-martensite mapping within a large area. Various deep learning-based methods have been tested, and the best result measured by both Dice loss and IoU scores has been achieved using the U-Net architecture extended by the ResNet encoder. [ABSTRACT FROM AUTHOR]

The divertor is a key component of plasma facing components (PFCs). Its servicing lifetime under high heat flux (HHF) loadings seriously affects the operating time of the nuclear fusion reactor. Therefore, it is of great significance to study the thermomechanical behavior of the divertor and its fabricating materials under HHF loadings to optimize its design. However, it lacks benchmark measurements on the thermal deformation, strain and fractures of components under servicing conditions to validate simulations. In this paper, a thermomechanical benchmark model of a flat-type divertor mock-up is established, which is based on a series of HHF loading experiments with heat flux magnitudes of 1 ∼ 15 MW/m2. The thermomechanical measurements, including surface temperature, surface displacements and strain components, under HHF loading conditions are obtained with both infrared thermal imaging and digital image correlation (DIC) techniques. With postprocessing of decoupling thermal/mechanical strains, the cumulative equivalent mechanical strain at points of interest and thus their low cycle fatigue lifetime is evaluated according to measured data, which can be used as benchmark datasets for related numerical simulations. Several interesting findings involving heat transfer modes, thermomechanical responses and the corresponding fatigue lifetime change patterns were also discussed.

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti–6Al–4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti–6Al–4V. The evolution of α′ grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti–6Al–4V has greater tensile strength than conventionally manufactured wrought Ti–6Al–4V due to its microstructure, with fine α′ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increases the ability of material to overcome the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.

The non-metallic inclusions are one of the main defects in powder metallurgy (PM) superalloy, which may promote crack initiation under the cyclic loading and seriously affect the safety of aero engines. To quantify the effects of inclusions to the low cycle fatigue (LCF) lifetime, the LCF tests on PM FGH96 superalloy are firstly carried out to observe the inclusions on fracture. By equating the inclusion to an ellipsoid, the relationship between the inclusion's feature and the stress field is established based on Eshelby's theory and finite element method (FEM). Then, the ellipse-equated fracture is determined according to the maximum shear stress's location. Subsequently, key inclusion-related parameters influencing the LCF lifetime are introduced to construct a modified SWT model. Finally, the model's accuracy is validated by comparing with the LCF experimental data. It is demonstrated that the modified SWT model has a good accuracy, with an improvement of scattering band from ±15 to ±4 compared with the original SWT model.

The motivation of the present study is to explore ratcheting-fatigue behavior and the corresponding micro-deformation mechanism of ultrafine-grained aluminum alloys 6061 (UFG AA6061) under two stress-controlled low-cycle fatigue (LCF) loading modes, i.e., mean tension stress (MTS) and mean compressive stress (MCS). The uniaxial ratcheting tests demonstrate that the fatigue-ratcheting damage of UFG AA6061 is prone to occur under MCS-LCF loading, rather than MTS-LCF loading. Detailed microstructure characterizations by Electron backscatter diffraction (EBSD) techniques under these two loading modes show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries increase remarkably in the case of MCS-LCF loading. These observations are mainly ascribed to the room temperature continuous dynamic recrystallization (CDRX), in which low angle boundaries evolve into high angle grain boundaries and the formation of equiaxed grains is accelerated. In contrast, there were no obvious change in the grain aspect ratios or the fractions of high angle boundaries at MTS-LCF loading condition, while the Schmid factor of non-octahedral slip system {100} increases and the texture evolve gradually. The deformation mechanism is grain boundary sliding (GBS) and grain rotation would accommodate the process of GBS. An analysis of the microstructure-properties relationship showed the superposition effect of residual shear deformation and uniaxial LCF ratcheting deformation is responsible for the CDRX at room temperature. And the occurrence of GBS is mainly caused by the activation of non-octahedral slip system.

Low cycle fatigue (LCF) tests were carried out on Ti6Al4V alloy at room temperature for different strain amplitudes ranging from 0.5% to 2%. The microstructure and texture evolutions as a function of strain amplitude have been investigated in the present study. Cyclic softening behaviour was observed at all strain amplitudes, and it was found to increase with increasing the strain amplitude. The fatigue transition life was observed to be at 1.25% strain amplitude. Only (10 1 ¯ 2) type tensile twins were formed during the deformation, and this was found to be significant at higher strain amplitude of 2%. The grains along [ 10 1 ¯ 0 ] were appeared to be the twinning parent, and the twinned products were appeared along [0001]. The [ 10 1 ¯ 0 ] grains were further found to have substantial cyclic deformation than the other grains, such as [ 2 1 ¯ 1 ¯ 0 ] and [0001]. The dominant texture was found along [ 10 1 ¯ 0 ] during the LCF tests. However, the sample at higher strain amplitude of 2% had also prominent [0001] texture along with [ 10 1 ¯ 0 ] texture. Fractographic examination revealed a dimple fracture surface at lower strain amplitude (0.5%), whereas a quasi-cleavage type fracture was observed at higher strain amplitudes (1% & 2%). [ABSTRACT FROM AUTHOR]

In the present study, a novel strengthening mechanism that does not impair the plasticity has been ascertained in a directionally solidified nickel-based superalloy (DS alloy) during monotonic and cyclic deformations. The alloy exhibited excellent mechanical properties at 750°C. Sub-structural examinations revealed that the activation of the stacking fault-induced micro-twinning mechanism is responsible for the improved fatigue performance at 750°C. This work may lead us to design advanced nickel-based superalloys having good synergy of strength and plasticity for turbine blade applications. [ABSTRACT FROM AUTHOR]

The substructure of an integral abutment bridge is designed to accommodate the lateral demand induced by the expansion and contraction of the bridge deck. The assembly of H-piles oriented about their weak axis of bending and aligned in a single row with a rigid connection to the abutment is most preferred for supporting the integral abutment bridges. The weak axis orientation of H-pile allows for higher displacement capacity, thus accommodating the cyclic thermal demand induced by a superstructure. The hysteretic response of H-piles is suggestive of the early onset of the plastic hinge formation, which helps in accommodating the cyclic demand. The H-piles in the integral abutment bridge experience low cycle fatigue induced by cyclic thermal variations, resulting in the buckling of flanges at the critical zone. This study assesses the impact of perforation made on H-pile flanges at the critical buckling zone. In this study, six unique geometries of perforations are modelled and analysed using combined nonlinear kinematic and isotropic hardening formulation. The hysteretic response of the perforated H-pile models has been compared with an unperforated model to assess the proposed perforation geometry's impact on the H-pile's hysteretic response. [ABSTRACT FROM AUTHOR]

Since fatigue cracks nucleate and initiate generally at the surface of the rotary components such as blades and discs, the surface condition is the most important factor affecting the fatigue life. Surface scratches are suitable sites for stress concentrations and therefore the nucleation stage of fatigue cracks will be shortened. In the present work, the influence of surface roughness on the low cycle fatigue life behavior of nickel-based superalloy Rene®80 at the temperature of 900°C was evaluated. Results of low cycle fatigue tests (LCF) under strain-controlled condition at 900°C for R = emin/emax = 0 and strain rate of 2×10-3 s-1, at a total strain range of 1.2% showed an inverse relationship between fatigue strength and surface roughness of the specimens. In this study, increasing the surface roughness of Rene®80 from 0.2 µm to 5.4 µm led to the decline in the final LCF life from 127 cycles to 53 cycles which indicated a 58.3% reduction in fatigue life at the same condition. Fractography evaluation also exhibited that fatigue cracks initiated from the notch in the rough specimens, whereas in the smooth specimen fatigue cracks nucleated from the internal imperfections and carbides. [ABSTRACT FROM AUTHOR]

Fatigue models using the strain-life method do not show exact conformity with the empirical results. Therefore, the use of the mean-stress correction approach is to be evaluated, with a particular focus on mild and higher-strength steel. The influence of the ductility parameters will be studied. A potential favorable development of structural steels with regard to ductility will be checked. The paper will focus on two types of structural steel: S355 and S700. Initially, the mechanical properties of the steel test specimens were measured via a tensile testing rig. In addition, a fatigue test was carried out by applying various mean-stresses. Surface roughness was measured at the notch and introduced into the initial model. The strain amplitudes were determined using the Ramberg-Osgood and Masing material models. Subsequently, a curve fitting was applied to the strain-life data for the fatigue ductility exponent. The multiparameter model was fitted with only one parameter. The resulting model showed a good fit between the strain-life curve and the test results. During the course of the optimization, the error and the scatter were calculated separately for steel types S355 and S700. Based on the ductility exponent, a favorable behavior of the materials was determined. [ABSTRACT FROM AUTHOR]

In the current study, three different combinations of dual-phase (DP) steels,i.e., ferrite–pearlite, ferrite–bainite, and ferrite–martensite containing the nearly same amount of second phase/phase mixture, have been developed following suitable heat-treatment schedules using a low carbon steel. Tensile and strain-controlled low cycle fatigue (LCF) tests at different strain amplitudes have been performed to understand the role of microstructure on the static and cyclic plasticity characteristics of DP steels. The experimental results reveal wide variations in the nature of cyclic hardening–softening behavior depending on the nature of the second phase. For the isotropic hardening, a new formulation considering the variation of yield stress as a function of memory stress and accumulated plastic strain has been incorporated, and Ohno–Wang kinematic hardening model has been used to simulate the observed fatigue response of DP structures. The developed model is capable of predicting the fatigue characteristics such as the stress amplitude versus the number of cycles, maximum hardening stress, and the progressive hysteresis loops. Further, the LCF simulations have been performed for the developed DP steels based on the real microstructure-based models adopting a sub-modeling technique. The stress and strain levels at different phases in the DP steels and their influence on the fatigue properties are quantified and discussed. [ABSTRACT FROM AUTHOR]

Over the last 50 years, many researchers have contributed significantly towards understanding the Masing/non‐Masing behavior of materials. However, there has been no review article or scientific report published to date that highlights the progress made in this domain. This article presents a state‐of‐the‐art comprehensive review and analysis of the Masing/non‐Masing behavior of materials under low cycle fatigue loading. Understanding of Masing/non‐Masing behavior is important for the development of fatigue‐resistant materials, prediction of fatigue life, and constitutive modeling and simulation. The influence of various factors such as stacking fault energy, temperature, strain amplitude, and loading conditions on Masing/non‐Masing behavior has been summarized in this article. The literature pertaining to the internal microstructural changes observed in different materials by various researchers has been collected and compiled. The different methods of analyzing Masing/non‐Masing behavior reported in the open literature are assessed and presented. The importance/significance of Masing/non‐Masing behavior in constitutive modeling and fatigue life prediction has also been highlighted. Finally, conclusions based on the review and the potential problems required to be resolved in the future are also pointed out. Highlights: A comprehensive review of Masing/non‐Masing behavior of materials is presented.Various factors that influence the Masing/non‐Masing behavior have been summarized.Different assessment methods of Masing/non‐Masing behavior are discussed in detail.Masing/non‐Masing is important for new material development and constitutive modeling. [ABSTRACT FROM AUTHOR]

This study explores the dual‐slope Coffin–Manson (C–M) behavior of a polycrystalline nickel‐based superalloy, EA, used in turbine engine applications with an emphasis on discerning the micro‐mechanisms responsible for it. The motivation for distinguishing the micro‐mechanisms responsible for bi‐linear C–M behavior stemmed from the earlier evolved comprehensions that state that the fatigue life estimation based on the extrapolation of any single line gives inaccurate results. Transmission electron microscopy (TEM) investigations of fatigue fractured specimens at low strain amplitudes, Δε/2, revealed that dislocations are homogeneously distributed in the γ‐channels and occasionally form networks at γ/γ′ interface. Whereas deformation is heterogeneous at high Δε/2 owing to the complex dislocation reactions. Cr23C6 carbides are precipitated during the high Δε/2 fatigue tests, which act as obstacles for dislocations. The deformation heterogeneity resulting from the dislocation–γ′ precipitate interactions and the dislocation–M23C6 carbide interactions accounts for the dual‐slope C–M behavior. Highlights: The EA alloy exhibited dual‐slope C–M behavior.Cr23C6 carbides are precipitated during high Δε/2 fatigue test.Deformation is mostly homogeneous at low Δε/2.At high Δε/2, dislocation interactions and carbides cause strain localization.Deformation heterogeneity at high Δε/2 is attributed to bilinearity in C–M plot. [ABSTRACT FROM AUTHOR]

The present study investigates the phase transformation processes in SS316L during Low Cycle Fatigue (LCF) and reversion heat treatment. The LCF tests were carried out at different strain amplitudes (0.6%, 0.8%, 1%, and 1.2%) at −80°C and 10−3 s−1 strain rate, which transforms γ-austenite phase to deformation induced martensite (DIM). Subsequently, all the deformed samples were annealed at 700°C for an hour to retransform DIM to the γ-austenite phase. Extensive microstructure analysis was performed using Electron Backscatter Diffraction (EBSD) and X-ray Diffraction (XRD) techniques to understand the phase transformation and reversion processes. Molecular dynamics simulated the DIM formation during LCF, revealing fcc to hcp (ε-martensite) and bcc (α’-martensite) phases in compression but no phase transformation during the tensile cycle. [ABSTRACT FROM AUTHOR]

The past studies in the low cycle fatigue (LCF) behavior of a γ′ strengthened nickel-based superalloy SU-263 were restricted to a maximum temperature of 923 K, though their service conditions far exceed this limit. In the present work, the LCF behavior of SU-263 is investigated at 1023 K with strain amplitude varying from ± 0.3% to ± 0.8% at 1023 K. During fatigue, the alloy displayed initial cyclic hardening followed by softening. The fatigue life decreased drastically with increasing strain amplitude. The alloy depicted gradual initial hardening at low strain amplitudes and sharp initial hardening at higher strain amplitudes, along with extensive softening until fracture. Significant increases in slip band density, stacking faults, dislocation network formation, and dislocation-dislocation and dislocation-precipitate interactions were identified as the deformation mechanisms responsible for cyclic hardening. In contrast, dislocation annihilation and shearing of γ′ precipitates were found to control cyclic softening. Dislocation-precipitate interactions were associated with looping at low strain amplitudes, while precipitate shearing occurred at high strain amplitudes. The alloy exhibited a tendency bilinear strain-life behavior in the Coffin-Manson plot with the corresponding shift in the fracture mode at ± 0.5% strain amplitude. At strain amplitudes below ± 0.5%, the alloy showed a mixed mode of failure, predominantly transgranular in nature, while at high strain amplitudes, the failure becomes predominantly intergranular. [Display omitted] • SU-263 exhibited a bi-linear Coffin-Manson behavior at 1023 K. • Transition from transgranular to intergranular mode of failure resulted in dual slope. • Increasing strain amplitude, shifted γ′ - dislocation interaction from looping to shear. • The width of the stacking fault increased with increasing strain amplitude. [ABSTRACT FROM AUTHOR]

The crystal orientation significantly affects the low-cycle fatigue (LCF) properties of single crystal (SC) superalloys. However, the orientation-dependent LCF life model with precise mechanisms and strong applicability is still lacking. This investigation aims at establishing an energy-based LCF life evaluation method that could consider the orientation effect. First, the influencing factors of anisotropy were identified through the literature review. Secondly, the multiaxial formula of the Ramberg-Osgood (R–O) equation was established to describe the anisotropic cyclic deformation characteristics. Furthermore, the strain energy density of SC superalloys was determined based on this equation, and the effective strain energy density was introduced to account for the effect of orientation. Finally, the energy-based method was validated by its application to several SC superalloys. Results showed that the crystallographic orientation with a lower Young's modulus usually exhibits better LCF resistance. This phenomenon could be attributed to the different values of strain energy density dissipated in one cycle. The multiaxial R–O relationship could capture the anisotropic cyclic deformation response of DD6. Compared with the classical methods, the energy-based model is favored by its precise mechanism and strong applicability. And it also exhibited better prediction accuracy. Most data points of different crystallographic orientations lay within the ±3 error band.

In this study, high-temperature properties of two newly developed Al-Si-Cu alloys (2Cu and 3.5Cu alloys) were investigated and compared to the commercial-grade A356 + 0.5Cu alloy (R alloy). 3.5Cu alloy exhibited the highest strength, outperforming R alloy by over 50 MPa at room temperature and by more than 20 MPa at elevated temperatures in ultimate tensile strength. However, R alloy demonstrated three to four times higher elongation than 3.5Cu alloy at room temperature, though this difference diminished at high temperatures. The minimum creep rate of 3.5Cu alloy was 2.4 times lower than that of 2Cu alloy and 14.5 times lower than that of R alloy, showing the superior creep resistance. Under low cycle fatigue loadings, the fatigue lifetimes of R and 2Cu alloys were similar, and slightly longer than that of 3.5Cu alloy. Conversely, in the high cycle fatigue regime, 3.5Cu alloy exhibited the highest fatigue resistance, followed by 2Cu and R alloys. The superior high-temperature performances of 3.5Cu alloy were attributed to the enhanced thermal stability of θ' precipitates compared to β' precipitates in R alloy, as confirmed during long-term thermal exposures at 250 and 300 °C. These findings suggest that 3.5Cu alloy is a promising candidate to replace the traditional A356 + 0.5Cu alloy for cylinder head applications. • High-temperature performances of two newly developed Al-Si-Cu alloys were investigated. • Tensile and creep properties, low and high cycle fatigue properties at elevated temperatures were studied in details. • Long-term thermal stability of strengthening precipitates at 300 °C was characterized. • Al-9Si-3.5Cu alloy exhibited the best comprehensive properties for cylinder head applications. [ABSTRACT FROM AUTHOR]

Efficiency of Digital Image Correlation (DIC) technique for fatigue analysis can be significantly increased by capturing images at periodic intervals for correlation. However, the scarcity of images to capture the history of deformation can cause inaccuracies in full-field measurements. In this study, a comprehensive analysis is conducted to quantitatively assess this error by considering various scenarios. These include skipping of frames within every cycle and across cycles, as well as deviations between intended and captured frames. DIC measurements based on a high density of frames in every cycle is assumed to be true solution to evaluate error for all the cases. Analysis indicates that the skipping frames within every cycle introduces error in full-field strain, which increases with the propagation of crack. Interestingly, magnitude of error for periodic skipping of frames across cycles is observed to be more than skipping of frames within cycle. Furthermore, error shows an increasing trend as the gap between periodic frames becomes larger. Analysis also shows that random deviations disturbing periodicity of skipped frames for DIC can further increase the error in full-field strains. Overall, accuracy of the accelerated approach is seen to strongly depend on presence of discontinuous features and local gradients in response. • Accuracy of DIC results for fatigue using images taken at periodic intervals is analyzed. • Uniform and single edged notch samples are considered for analysis. • Error in full-field strain due to skipping of frames across cycles by different magnitude considered. • Effect of random deviations from periodic skipping emulating mismatch between image capture and loading frequency is considered. [ABSTRACT FROM AUTHOR]

• Quasi-intermediate temperature brittleness (ITB) is observed during fatigue at 700 °C. • Dislocations pile up around Lomer-Cottrell locks leading to premature failure. • W and Ta increase the APB energy hindering dislocation shearing at 700 °C. • Life prediction model considering strain amplitude shows high accuracy. Low cycle fatigue behaviors of MAR-M 247 superalloy were investigated at 500 °C, 700 °C and 900 °C. The detailed deformation mechanisms were characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) and the critical resolved stress to active different deformation mechanisms were calculated. The results show that crack propagates along favorable {111} plane at 500 °C, and anti-phase boundary (APB) shearing is the main deformation mechanism. At intermediate temperatures, the cracking mode is similar to 500 °C. The dislocation pinning at L-C lock leads to premature failure. It is difficult for {111} 〈110〉 slip system to shear hard oriented grain with low Schmid factor at 900 °C. Dislocation movement is achieved by Orowan by pass process and thermal activated dislocation climb at high temperature. In addition, the improved life prediction model based on energy method has been constituted and discussed at different temperatures. [ABSTRACT FROM AUTHOR]

[Display omitted] • Y addition significantly increases the fatigue life of the IN713C alloy. • The decrease of porosity is attribute to the decrease of solidification temperature range. • Fatigue cracks in IN713C alloy mainly originate from pores and carbides. In order to improve the high temperature and low cycle fatigue performance of the IN713C alloy, trace amounts of yttrium (Y) element were added to such an alloy. The effect of Y on the microstructure and fatigue performance under push-pull loading condition of the IN713C alloy at 650 °C was investigated, and the fracture mechanisms of the IN713C alloy with different Y additions at strain amplitudes of 0.3%, 0.4% and 0.5% were also investigated. The results showed that addition of Y significantly increased the fatigue life of the IN713C alloy under different strain amplitude values. Compared with the traditional IN713C, the fatigue life of the alloy with 300 ppm Y was increased by 44.4%, 213.3%, and 61.4%, and that of the alloy with 500 ppm Y was increased by 184.6%, 66.1%, and 172.1%, respectively, at the strain amplitudes of 0.3%, 0.4%, and 0.5%. Shrinkage pores were found to be the main crack source at low strain amplitude (0.3%), while carbides and other interdendritic precipitates were the source of cracks at high strain amplitude (0.4% and 0.5%). The addition of Y increased the fatigue life of the IN713C alloy by reducing shrinkage and refining carbides and grain size. [ABSTRACT FROM AUTHOR]

• New insights in fatigue life of T91 steel in LBE are obtained using existing data. • A plastic LME factor is proposed to quantitatively evaluate the effect of LME and oxide film. • The LME effect is related to temperature instead of the plastic strain amplitude. • A fatigue life prediction model considering different factors is proposed with high accuracy regardless of data from different sources. The fatigue life of modified 9Cr-1Mo steel in liquid lead bismuth eutectic (LBE) at different strain amplitudes, temperatures and oxygen concentrations are analyzed. A liquid metal embrittlement (LME) factor of plastic strain is proposed to account for the reduced fatigue life induced by LME effect, which is also found to correlate well with tensile elongation in LBE. In low oxygen content LBE, the LME effect is influenced by temperature instead of plastic strain amplitude. While in high oxygen content LBE, the plastic LME factor is found to decrease exponentially with increasing plastic strain amplitude. Based on these findings, a fatigue life prediction model is proposed taking into account of different environmental influencing factors. In total, 86 data points are used with 70 % data points for independent validation only. Regardless of the discrepancy in fatigue life from different sources, good prediction results are still achieved with 98 % data points fall within ± 3 error band and 75 % data points fall within ± 2 error band. [ABSTRACT FROM AUTHOR]

• Low cycle fatigue behaviour of 304L at 300 °C and various strain ranges was captured. • Stainless steel 304L exhibits cyclic softening behaviour under cyclic loading. • The combined isotropic-kinematic hardening described the cyclic behaviour. • The model based on FEM was able to predict well the LCF behaviour. The integrity of the components of nuclear power plants must be guaranteed under operating and emergency conditions. It is necessary to evaluate all possible degradation mechanisms that could impact the structural integrity of nuclear power plant structures such as the pipelines and other components of the cooling systems. Environmental fatigue significantly influences the degradation mechanisms of steel components operating in water, which eventually affects the operational lifetime of the components. Exploring non-codified methods for more precise assessment of fatigue phenomena can pave the way for developing safer nuclear power plants with longer operational lifetimes. This is very important as the global demand for cleaner energy is increasing. This research study deals with a numerical investigation of the low-cycle fatigue behaviour of steel used for those nuclear reactor components where environmental fatigue plays a major role. The numerical simulation methodology is proposed to study the dependence of the kinematic hardening parameter on the strain amplitude to investigate the low-cycle fatigue behaviour of AISI 304L steel for the prediction of the fatigue curves that can be used for the estimation of nuclear power plant safety and its lifetime. The experimental data was used to estimate the parameters required to define the material model for the numerical simulation and to validate the results of the numerical simulation of the low cycle fatigue behaviour. The presented methodology can be used for fully reversed constant-amplitude strain loading low cycle fatigue simulations for various strain ranges to predict the low-cycle fatigue behaviour of steel under repetitive loading. [ABSTRACT FROM AUTHOR]

A mechanical assessment was conducted to characterise the near β titanium alloy Ti-10V-2Fe-3Al (Ti 10-2-3), which was heat-treated to provide two strength variants. Low cycle fatigue and crack propagation tests were performed under standard laboratory air plus a salt fog environment. The differences in static strength were also demonstrated under fatigue conditions utilising plain specimens. However, the alloy was essentially insensitive to the test environment when comparing LCF performance in air and salt fog. Salt fog also provided no effect on crack growth behaviour. A double edge notch specimen geometry was employed to measure free initiation and the growth of cracks from a stress-raising feature. The current data now supplement previous studies aimed at expanding the mechanical database for Ti 10-2-3 component design and in-service life predictions.

Molecular dynamics simulations are carried out to investigate the effects of rhenium (Re) on low cycle fatigue behaviors and deformation mechanisms of Ni-based single crystal superalloys. The low cycle fatigue behaviors and deformation mechanisms at different temperatures are discussed. The results show that Re has a positive effect on the low cycle fatigue behaviors of Ni-based single crystal superalloys and the cyclic deformation mechanism is strongly dependent on temperature, which are in good agreement with the previous experimental results. The addition of Re can increase the saturation stress amplitude, reduce the cyclic hysteresis energy and dislocation density, and improve the plastic deformation resistance of the superalloys. During cyclic loading, Re atoms in γ matrix phase gradually move to the γ/γʹ interface, these enrichment of Re atoms at the γ/γʹ interface have pinning effect on dislocations, improving the microstructure stability. At the later of the saturation cycle, segregations of Re atoms prevent the dislocations from cutting into the γ′ phase by dragging dislocations, thus improving the fatigue resistance of superalloys. The simulation results also show that the deformation mechanism gradually changes from dislocations and stacking faults shearing γʹ precipitate phase to Orowan bypassing and climbing mechanism with temperature increases.

In this study, 7075 Al alloy was prepared by solution treatment + natural aging. The anisotropic behavior of 7075 Al alloy in extrusion direction (ED), 45° direction (45°) and radial direction (RD) was studied by quasi-static tensile test and strain controlled low cycle fatigue (LCF) test. The specimens of ED, 45° and RD were 0°, 45° and 90° to the extrusion direction of the bar, respectively. The mechanical properties of the specimens in three directions showed strong anisotropy. The yield strength (YS) and ultimate tensile strength (UTS) of ED specimens were better than 45° and RD specimens, but 45° specimens had better plasticity. With the extension of aging time, the YS and UTS increase significantly. In the quasi-static tensile process, the specimens would have dynamic strain aging (DSA), and obvious serrated flow behavior could be observed on the curve. For the fatigue performance of specimens in three directions, when the strain amplitude was low, the fatigue life of ED and 45° specimens was high. When the strain amplitude was high, the fatigue life of RD specimens was high. The experimental results showed that after cyclic loading, the second phase size of solution treatment + cyclic loading specimens was larger than T6 (480 °C × 3 h + 130 °C × 24 h). Therefore, the fatigue fracture mode was multi-source fracture at low strain amplitude.

The present work deals with the fatigue and corrosion-fatigue crack initiation in cold-pilgered Sanicro 28 super austenitic stainless steel. Toward this end, the stress-controlled symmetric reverse bending fatigue tests were conducted in air and mixed acid aqueous solution under the stress amplitude of 200–570 MPa with the stress ratio of −1 and frequencies of 1 and 15 Hz at ambient temperature. The fatigue study was supplemented with electrochemical experiments and advanced microstructural analyses to explore the damage characteristics in detail. The planar character of dislocation substructure owing to the low stacking fault energy of the material, and formation of parallel slip bands led to a more homogenous distribution of the imposed strain which retarded the progress of damage accumulation. According to the X-ray photoelectron spectrometer results, the formation of the insoluble and stable MoO3 oxide increased the rate of repassivation. This sticky oxide resisted against breakdown rendered by the attack of aggressive chloride ions. The corrosion fatigue behavior of the experimented alloy under the various test frequencies was discussed relying on the synergistic effects of mechanical and environmental damages.

Low cycle fatigue (LCF) behavior of 2- and 3-step-aged Inconel 706 (IN706) specimens that were prepared from the center and the periphery of forged disc was examined at 650 °C and an R ratio of −1. The resistance to LCF of IN706 alloy was greater in approximately half order at 650 °C than that of IN718 alloy for the same aging condition. Elevated temperature Coffin–Manson relationship between 2- and 3-step-aged IN706 specimens was similar with each other, despite a considerable difference in microstructure. The fractographic analysis suggested that cluster of carbides provided the sites for crack initiation for both 2- and 3-step-aged specimens. The mode of fatigue crack growth for each specimen was intergranular and transgranular, but with different magnitude for each mode. Depending on the location of specimen preparation form the forged block of IN706 alloy, no notable difference in LCF resistance was found. The role of grain boundaries, clusters of carbides and bands of acicular η platelets on the initiation and propagation mechanism of IN706 alloy was discussed based on detailed fractographic and micrographic analyses.

In many engineering structural applications, including car body structures, sheet metal formed components are used. A systematic investigation of the impact of forming strain on fatigue life is necessary for an accurate prediction of a formed component's fatigue life. The present work aims to determine the impact of diverse types of tensile pre-straining on high cycle fatigue (HCF), low cycle fatigue (LCF) and notch fatigue performance of automotive grade dual phase steels. In all examined pre-straining conditions, HCF life improves, LCF life deteriorates, and little change in notch fatigue life are observed. The most significant improvement in HCF life and deterioration in LCF life are observed for equi-biaxial and orthogonal tensile pre-straining conditions due to the rotation of maximum shear stress plane and the rotation of the deformation concentrated region around the hard martensite. As the strength improves during pre-straining, there is a corresponding increase in stress concentration around a notch, and as a result, no significant change in notch fatigue life is observed.

Metallic foils are crucial to many applications ranging from printed circuit boards, aviation, aerospace, packaging, photovoltaics to batteries. In these applications they are often part of multifunctional composites, for which mechanical integrity is a prerequisite for the functionality. To ensure precise simulations and lifetime prediction, exact numerical material models are needed. In this work, material parameters of the Lemaitre-Chaboche model were fitted for electrodeposited copper foils. By using nanoindentation and microstructural characterisation, good agreement with literature was achieved. Additionally, it was found that the Tabor rule to estimate yield stress from indentation may not be applicable for metallic foils.

Ti2AlNb alloy is a three-phase alloy, which consists of O phase, β phase and α2 phase. Because of the difference in the mechanical characteristics between phases, Ti2AlNb alloy often exhibits deformation heterogeneity. Based on EBSD images of the Ti2AlNb alloy, a crystal plasticity finite element model (CPFEM) was built to study the effects of O phase and β phase (dominant phases) on stress and strain distribution. Four types of fatigue experiments, and the Chaboche model with 1.2%~1.6% total strain range were conducted to verify the CPFEM. The simulation results showed that the phase boundary was the important position of stress concentration. The main reason for the stress concentration was the inconsistency deformation of grains which resulted from the different deformation abilities of the O and β phases. [ABSTRACT FROM AUTHOR]

The present study addressed the cyclic deformation behavior and fatigue properties of Cu‐0.69Cr‐0.07Zr alloy with different cold deformation (ε = 64%, 75%, and 84%) using low cycle fatigue test. Low cycle fatigue tests were conducted under fully‐reversed conditions at different total strain amplitudes. The microstructure changes and fatigue fracture characteristics were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The main findings suggest that the Bauschinger effect was significantly stronger with larger deformation at low total strain amplitude. And it was proved that the relationship between the total strain amplitude and the low cycle fatigue life of Cu‐Cr‐Zr alloy with different deformation can be expressed by the Manson–Coffin–Basquin formula. Further, the reason for the fatigue life was shorter and the cyclic softening rate decreased faster at high applied total strain amplitude was that the dislocation density decreased due to the rearrangement of the dislocations. Highlights: Bauschinger effect was found in LCF behavior of Cu‐Cr‐Zr alloys.The microstructure changes before and after LCF test were observed.The LCF life of Cu‐Cr‐Zr alloy may not be improved by large cold deformation.The LCF behavior of Cu‐Cr‐Zr alloy was consistent with Manson–Coffin–Basquin formula. [ABSTRACT FROM AUTHOR]

During the service life of offshore pipelines, many start-up and shut-down cycles take place, possibly leading to significant cyclic loads. Fatigue failure may occur, resulting in serious environmental pollution and loss of property. The study aims to assess the fatigue strength of single-sided girth welds in offshore pipelines under these specific fatigue loads. The longitudinal stress range caused by the variation of the pipeline's internal pressure and temperature is calculated. The effective notch strain approach is used to assess the fatigue strength of welds. The plastic behaviour of the weld root is investigated for a study case to justify the use of low-cycle fatigue assessment approaches. The effect of weld root geometry on the notch stress factor is studied to identify the dominant geometrical parameters. The fatigue strength of the study case is assessed, and some limitations of the assessment are discussed. The results show that the plastic behaviour of the weld root is only significant for severe local stress concentrations, which is mainly governed by the axial misalignment, weld root angle and the weld root bead width. If the fatigue damage at failure is 0.1, a limited number of start-up and shut-down cycles are allowed during the service life of the pipeline for the study case, indicating the necessity of fatigue strength assessment. [ABSTRACT FROM AUTHOR]

As a new highly damage-tolerant structural material, the TC21 titanium alloy has been widely used in aerospace applications. Inertial friction welding (IFW) is a form of pressure welding technology with less welding parameters and high welding joint performance, which is especially suitable for the connection of rotors of aero-compressors and engines. In this paper, inertia friction welding of TC21 titanium alloys was successfully carried out, and the microhardness, tensile properties and low cycle fatigue (LCF) behaviors of IFW joints were studied. Based on the mechanical parametric results of the tensile test, the true stress–strain curves of the IFW joint of TC21 titanium alloys are obtained by further calculation. Based on the LCF test results under different strain amplitudes, life prediction of IFW joints was investigated. The results of the LCF test show that there is no obvious cyclic hardening and cyclic softening of the IFW joints. Moreover, the fracture morphology of LCF samples under high strain amplitude (0.9%) and low strain amplitude (0.6%) was observed. The results show that the fatigue cracks initiate and propagate at multiple points in the LCF samples, and the transient fracture zone is larger under high strain amplitude. However, under low strain amplitude, a fatigue crack nucleates and propagates at a single point, and the crack propagation zone is larger. [ABSTRACT FROM AUTHOR]

An approach to determine the fatigue behaviour in the heat-affected zone (HAZ) of AA 2014 has not been investigated exhaustively since the HAZ has been treated as one region. In this study, fatigue crack growth (FCG), fatigue strength and low cycle fatigue (LCF) properties of the base metal (BM) and two regions of the HAZ were studied. The two regions of the HAZ were thermally simulated at 590°C and 650°C representing regions of the weldment 4 mm and 5 mm from the weld centreline respectively. The hardness tests and microstructure characterisation of the base metal and the two regions of the HAZ were also carried out. The HAZ region thermally simulated at 590°C was found to offer the least resistance to FCG, had shorter LCF life and lowest value of fatigue strength compared to both the BM and the HAZ region thermally simulated at 650°C. The cause of weakness in this region was associated with the dissolution of strengthening phases. The degradation of the fatigue properties of AA2014 occurred severely at 590°C due to the dissolution of CuAl2 precipitates in the aluminium matrix. The HAZ region thermally simulated at 590°C was confirmed to be the weakest link in the HAZ. [ABSTRACT FROM AUTHOR]