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ABSTRACT This study aims to investigate the low cycle fatigue (LCF) behavior and fatigue life prediction method of an advanced solid solution–strengthened nickel‐based superalloy at various temperatures and stress concentrations. Cylindrical specimens as well as plates with three types of holes (including straight holes and inclined holes with different orientations) were designed and machined for fatigue tests. The test results revealed fatigue life decreases with the temperature increasing. Meanwhile, the fatigue lives of the plate specimens have decreased differently and are strongly dependent on the geometry and orientation of the holes. The key damage parameter related to the fatigue life was found on the strain gradient path. A criterion based on strain gradient is defined as the boundary of the damage process zone. The results show that the prediction was in good agreement with the experimental data, for the majority of the data are within a scatter band of ±2. [ABSTRACT FROM AUTHOR]

In this paper, the thickness debit and stress concentration effects of a nickel‐based directionally solidified superalloy were investigated through strain‐controlled low cycle fatigue (LCF) and creep–fatigue interaction (CFI) experiments. Compared to the fatigue lives of solid specimens provided in the existing literature, the results of this paper indicate that when the wall thickness of the specimen is reduced from 5 to 1.125 mm, the fatigue life with different strain amplitudes decreases by about 45%. When film‐cooling holes (FCHs) are introduced into the hollow specimens, the fatigue life is further reduced by about 25%. Based on the observation of the scanning electron microscope (SEM), the formation mechanisms of the thickness debit and stress concentration effects under LCF and CFI loads were revealed. Subsequently, two parameters, the "wall‐thickness coefficient" and "hole coefficient," were proposed to establish a new fatigue life prediction method. Highlights: LCF and CFI tests were investigated on hollow specimens with and without holes.The deformational behaviors of hollow specimens with holes were studied.Two parameters, the "wall‐thickness coefficient" and "hole coefficient," were proposed.A method for estimating the fatigue life of hollow specimens with holes was proposed. [ABSTRACT FROM AUTHOR]

A physically‐based constitutive modeling framework is proposed for the cyclic deformation and softening of Ni‐based superalloys. The constitutive model accounts for underlying mechanisms of grain size strengthening, dislocation strengthening, solid solution strengthening, and precipitate strengthening, commonly observed in these alloys. A constitutive model is proposed for the shearing of precipitates on their interaction with glide dislocations during cyclic deformation. This is the primary contributor to the experimentally observed cyclic softening behavior in Ni‐based superalloys. Model predictions of the cyclic stress–strain response and the cyclic softening (quantified in terms of the peak stress) are compared with the experimental counterparts for a range of strain amplitudes under fully‐reversed cyclic loading of Inconel 718 (IN 718). Further, model predictions of precipitate size are also compared with the experimentally measured transmission electron micrographs of precipitate sizes at the end of deformation. Concurrence in the cyclic stress–strain response, peak stress, and precipitate size provides validation for our constitutive modeling framework. Highlight: Development of a physically‐based constitutive modeling framework for cyclic deformation.Microstructure‐based strengthening mechanisms are considered in the model.Model incorporated the physical mechanisms of cyclic softening due to precipitate shearing. [ABSTRACT FROM AUTHOR]

A customized digital image correlation (DIC) system was implemented to monitor the strain produced in a cold-rolled AL-6XN stainless steel plate, 3.0 mm thick, subjected to quasi-static and cyclic loading tests. A comparison of the DIC strain measurements was made against those provided by conventional extensometers. Furthermore, the DIC system was used to monitor the fatigue crack initiation in low-cycle fatigue tests. The true stress–strain behavior for the AL-6XN material was properly captured by the DIC measurements. For low-cycle fatigue tests (strain control), the strain mapping generated by DIC allowed for identifying zones with higher strain than the nominal strain amplitude applied ( ε a ) since the first stages of the fatigue life (FL). These zones become potential fatigue crack initiation sites. [ABSTRACT FROM AUTHOR]

The 16MnCr5 alloy steel samples are evaluated in a normalized condition and put through low cycle fatigue (LCF) under constant amplitude strains of εa = 0.2%, 0.25%, 0.4%, and 0.6%. Mechanical properties are reported based on the LCF results. Fatigue damage is checked in specimens that have reached 80% of their nominal fatigue life by two methods that are different from traditional electronic microscopy: measuring the roughness of the surface and looking at it through full‐field optical microscopy. In addition, the electric resistivity is determined for the test specimens. It presents an increment from the blank value of 6.5 × 10−8 Ωm (free of fatigue damage) to 1.25 × 10−7 Ωm for the fatigue test specimen (εa = 0.6%). Finally, X‐ray diffraction patterns are determined for analyzing the residual equivalent strain at the microscale for the fatigued samples. The residual equivalent strain exhibits a trend toward increasing. For instance, it increases from 18.5 (blank value) to 34.0 με for the fatigue test specimen with εa = 0.6%. [ABSTRACT FROM AUTHOR]

The low cycle fatigue properties of Q355B structural steel subjected to different levels of corrosion were investigated in the present study. The corroded Q355B steel coupons were obtained by means of cyclic wet–dry immersion corrosion, and the low cycle fatigue tests of coupons with different corrosion degrees, featuring both constant and variable strain amplitudes, were carried out. The cyclic stress response, cyclic skeleton curve, and hysteretic curve of the corroded steel were analyzed. The low cycle fatigue life of the corroded steel was predicted, and a constitutive model of the corroded steel was proposed. The results show that corrosion reduced the low cycle fatigue life of Q355B structural steel. Finite-element (FE) models were developed, and the FE results were in good agreement with the experimental curves, indicating the accuracy of the constitutive model proposed in this paper. The findings from the present research provide a valuable reference for the accurate assessment of the low cycle fatigue properties of steel structures with varying degrees of corrosion. [ABSTRACT FROM AUTHOR]

The low cycle fatigue (LCF) properties of DD6 single crystal superalloy were investigated at 700 C and R of 0.05. SEM was used to study the fracture surface and fracture microstructure. The results show that the LCF life of the alloy decreases with the increase of strain amplitude. LCF properties of the alloy are excellent under asymmetrical cyclic loading. The alloy has no transition fatigue life during LCF tests at all total strain amplitudes. LCF fatigue damage can be dominantly contributed to elastic damage and the plastic deformation is very minimal. The plastic damage increases with the increase of total strain amplitude. The crack initiation site, the fatigue crack propagation area and the final fracture zone can be observed in the fracture surface and all specimens is similar to quasi-cleavage fracture. The fatigue cracks are initiated from the micro-pores on the surface, sub-surface or far from the surface. Far from the surface crack fractures have fish-eye feature. The fatigue crack propagates perpendicularly to main stress at first and then along (111) plane. Typical fatigue striation, cleavage steps and river pattern characteristic are formed on fatigue crack propagation zone. The cleavage plane, slip band and tearing ridge are seen in the final fracture zone. Fracture microstructure analysis shows that the γ phase morphology far from the fracture surface still maintains cubic shape and the slip bands are visible seen near the fracture surface and secondary cracks are formed along slip bands. [ABSTRACT FROM AUTHOR]

In this investigation, experiments have been carried out at room temperature (RT); 300K, and at high temperature (HT); 573K to determine the role of martensite formation in the strain-controlled low cycle fatigue of 304LN (0.11%) stainless steel. Deformation-induced martensite was significantly present (0.7 to 2.3%) for RT, however, in trace amounts (0.05 to 0.07%) at HT. The presence of deformation-induced martensite at RT had detrimental effect on low cycle fatigue life of 304LN-SS. Significant drop in stress level was observed at HT (270-407 MPa) compared to value of stress at RT (395-493 MPa) for cyclic stain amplitude values in the range (0.5 to 1.1%). The number of reversals to failure decreased with increasing strain amplitude (0.5 to 1.1%) from 17540.0 to 1046.0 at RT while at HT the number of cycles to failure decreased (15562 to 700). The maximum number of hardening cycles was found to increase from 15 to 55 for a temperature rise from 300K to 573K. The beneficial improvement in cyclic stress-strain response (hardening cycles) at 573K is attributed to absence of martensite. Effects of deformation-induced martensite and temperature have been invoked to account for changes that have been observed in the cyclic stress-strain behavior. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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]

Fatigue life is a critical property of structural materials for ensuring public safety. While the reversibility of deformation mechanisms under cyclic loading can greatly enhance fatigue life, the evolution of this reversibility during continuous fatigue has not received much attention, despite its significant impact. This study used in-situ neutron diffraction to analyze the evolution of multiple deformation mechanisms activated during the early stage of low-cycle fatigue at ±1 % strain amplitudes in fully austenitic Fe-17Mn-0.5C steel. In the steel with near-zero Δ G γ → ϵ controlled by carbon concentration, the leading partial dislocations move in { 111 } γ grains during tension and in { 200 } γ grains during compression, creating stacking faults. Under cyclic loading, these mechanisms exhibit reversible stacking faults behavior due to the activation of trailing partial dislocations, ultimately leading to the reversibility of both twinning-induced plasticity and transformation-induced plasticity. As the number of fatigue cycles increases, stacking faults become more prominent, while the contribution from twinning faults diminishes. The pronounced stacking faults cause work hardening at the intersections of newly formed ε-martensite plates and twinned areas, thus accelerating phase transformations that require higher critical resolved shear stresses. However, the negative Δ G γ → ϵ and increased stability of ε-martensite during fatigue cycles result in a quasi-reversible martensitic transformation behavior with an increase in residual ε-martensite. This work provides insights into enhancing a fundamental understanding of fatigue mechanisms by demonstrating the unusual quasi-reversibility characteristics of high-Mn steel and highlighting its potential as a fatigue-resistant structural material. [Display omitted] • In-situ neutron diffraction was used on Fe-17Mn-0.5C steel under low-cycle fatigue. • Reversible stacking faults cause reversible TWIP and TRIP effects. • Stacking faults increase with cycles; twinning faults decrease. • Work hardening from stacking faults accelerates phase transformations requiring higher stress. • Negative Δ G γ → ϵ and stable ε-martensite lead to quasi-reversible phase transformation. [ABSTRACT FROM AUTHOR]

During the service, turbine blades and vanes of aero-engine and gas-powered turbines sustain surface-initiated damage owing to the synergistic effect of cyclic stresses, high temperature, and oxidizing environment. As a result, oxidation-fatigue interaction curtails the life of these components. This research investigates the oxidation-fatigue damage mechanisms of CM 247 DS LC, at 850 °C for strain amplitudes (Δε/2) of 0.5–0.8 %. It is observed that the micro-mechanism of fatigue-oxidation interactions at 850 °C is a complex phenomenon involving multiple competing mechanisms acting simultaneously. Both the distinguishing damage features and oxygen ingress affect the strain localization and are attributed to the divergence in fatigue behavior. The strain is more uniform at Δε/2 of 0.5 % in as-received (H0) samples, accounting for the highest fatigue life. Compared to the cyclic heat-treated (H500) samples, the strain localizations during low cycle fatigue deformation are more effectively alleviated in H0 samples, resulting in the highest fatigue life. [ABSTRACT FROM AUTHOR]

Corrosion pits preferentially initiate in the bainitic regions of Q420qNH steel against the simulated marine atmospheric environment, where a high initial dislocation density is present. This occurs in addition to the initiation sites associated with nonmetallic inclusions that exhibit significant lattice distortion. The formation of corrosion the corrosion-induced non elastic buckling exacerbates the cyclic softening effect and significantly reduces fatigue strength. At a strain amplitude of 0.4 %, the fatigue life decreases by 73.2 % after 60 days of pre-corrosion treatment. Layered segregation of Cr and Cu alloying elements could develop the formation of dense rust layer to inhibit further surface degradation. • Corrosion pits of ferrite-bainite weathering steel predominantly occur in the bainite regions with high initial dislocation density upon cyclic salt spray corrosion environment. • Hierarchical non-metallic inclusions enclosed by a significantly distorted lattice shell tend to be preferential initiation sites for corrosion. • Surface degradation caused by corrosion pits, coupled with the cyclic softening, could result in inelastic buckling upon low cycle fatigue loading, leading to fatigue strength degradation. [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]

A novel fatigue life prediction model incorporating the thermal aging parameters for a low alloy steel. [Display omitted] • Tensile strength slightly increases, while the fatigue life decreases after thermal aging. • The break of bainite facilitates the fatigue crack initiation and propagation. • Plastic strain amplitude is recognized as a thermal aging indicator of the low alloy steel. • A low cycle fatigue life model considering the thermal aging parameters is established. Fatigue failure of materials has a considerable impact on the safety of equipment in service. In this study, axially tensile and low cycle fatigue tests were conducted on a low alloy steel after accelerated thermal aged at 450 °C for 10,000 h. The experimental results indicate that the ultimate and yield strengths increase moderately, while the fatigue life of specimens experience a slight decrease in this circumstance. The fracture analysis demonstrates that the bainite breaking facilitates the fatigue crack initiation and propagation after thermal aging, which is accompanied by a decrease in plastic strain amplitude. Therefore, the plastic strain amplitude is considered as an indicator of thermal aging in fatigue life modeling for the low alloy steel. Finally, a novel life model that incorporates both aging time and temperature was proposed for rapid prediction of low cycle fatigue life. It is assumed that this model promotes reliable fatigue life prediction in low alloy steels under various thermal aging circumstances, as well as the extrapolation of fatigue performance of the material in service. [ABSTRACT FROM AUTHOR]

• LCF tests on pitting corroded structural steel at various strain amplitudes and high temperatures. • Develops temperature-dependent parameters for integration into CPSED, Coffin-Manson, pit-SIF. • Proposes MLR models relating total CPSED and average SED with strain amplitude and temperature. • Corroded specimens show higher plastic deformation and reduced fatigue life and total CPSED. • The parameters with average SED predict LCF life within ±1.5 error; power-law reduces it to ±1.2. High-tension structural steels are prone to accelerated fatigue damage from pitting corrosion and high-temperature. Despite adverse effects, research on their low cycle fatigue (LCF) behavior is limited. Specifically, studies analyzing temperature-dependent pit sensitivity effects, considering pit-related material's susceptibility to surface topographic features variation and stress concentration are lacking. This study conducts LCF tests on pitting corroded killed E350 BR structural steel at multiple strain amplitudes and high temperatures. It develops temperature-dependent parameters, such as the cyclic softening pit sensitivity factor, suitable for integration into existing approaches like total cyclic plastic strain energy density (CPSED), power-law, average strain energy density (SED), Coffin-Manson, and pit stress intensity factor (pit-SIF). Further, it proposes multiple linear regression-based prediction models relating total CPSED and average SED with strain amplitude and temperature. Corroded specimens show higher plastic deformation and reduced peak stress, fatigue life, and total CPSED compared to uncorroded ones. The developed parameters, integrated with average SED approach, predicts LCF life within an error band of ±1.5, while power-law relationship reduces it to ±1.2. Moreover, pit-SIF approach estimates fatigue life within an error band of ±1.5. The findings provide critical knowledge for enhanced component design, leading to structural safety, performance, and fire resilience. [ABSTRACT FROM AUTHOR]

• The cyclic behaviour of FGH95 under asymmetrical loading with different strain amplitude is affected by the competition mechanism of back stress and effective stress. • The cyclic hardening of back stress is mainly affected by the dynamic recovery term, which need to consider continuous change term and stage change term. • A modified constitutive model based on changing dynamic recovery term is established to reflect the cyclic stress strain response. • A model parameter confirmation method based on asymmetric fatigue test is established. The asymmetric cyclic loading process occurs in aero-engine turbine discs. A constitutive model that accurately describes the cyclic elastoplastic behaviour of the material is important for structural design and low cycle fatigue life prediction of turbine discs. In this paper, the low cycle fatigue test of FGH95 was carried out at 620℃ under 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0% and 1.1% strain amplitude with strain ratio equal to 0. The material exhibited cyclic hardening followed by cyclic softening at high strain amplitudes and cyclic softening at low strain amplitudes. The mean stress relaxation rate was similar for each strain amplitude. In addition, the evolution of effective stress and back stress was obtained through the method of internal stress division. Then, the relationship between the change in stress amplitude and the change in internal stress was discussed. The results showed that with cyclic loading, cyclic hardening/softening of FGH95 was affected by the competition mechanism of back and effective stresses. Considering that slip deformation and crystal lattice rotation coexist in plastic deformation, the dynamic recovery term of the Abdel-Karim and Ohno model was used. In order to characterize the different magnitudes of back stress change in materials at different plastic strain intervals, a dynamic recovery term coefficient was introduced to the dynamic recovery term and the critical surface of the back stress. The modified model was used to compare with experimental results. Then, it gives a good description of the material's mean stress relaxation and strain amplitude variation and gives good agreement on the hysteresis loop. [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]

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]

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]

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel. [ABSTRACT FROM AUTHOR]

The effects of anisotropy and pre-strain on the low-cycle-fatigue properties of DP600 steel sheet under shear path were investigated. Fully reversed strain-controlled fatigue tests were performed along the 0° and 90° directions with the initial strain direction. The cyclic-loading amplitude determined whether the pre-strained material had a hardening process. The high dislocation density in the crystals of pre-strained material can promote the formation of low-energy substructures, obviously accelerating the cyclic-softening rate and reducing the fatigue life. Average plastic strain energy density was affected by the pre-straining state and loading direction and had a linear relationship with the strain amplitude. Overall, these results can be used to verify the calculation model of structural parts. [ABSTRACT FROM AUTHOR]

Laser shock peening (LSP) experiments, low cycle fatigue experiments, microhardness and residual stress measurements, and scanning electron microscopy (SEM) analyses were conducted on FV520B steel cylindrical specimen s. Additionally, ABAQUS software was employed to predict fatigue life following the LSP process. The experimental results indicate that the surface compressive residual stresses induced by LSP inhibit both the initiation and propagation of fatigue cracks effectively, thereby prolonging the fatigue life of FV520B steel. The fatigue life of specimens at strain amplitudes of ± 0.5 %, ±0.6 %, ±0.7 %, and ± 1.0 % is improved by 132.2 %, 0.4 %, 18.0 %, and 88.8 %, respectively. A residual stress of −90 MPa is measured on the surface of the specimen and the surface hardness value increase by 48 % after LSP. The SEM results reveal that the characteristic of single crack sources for crack initiation is presented after LSP. The results of single point LSP simulation using ABAQUS demonstrate that the circumferential surface experiences a lower compressive residual stress of 30 MPa compared to a higher value of 45 MPa on the planar surface, this discrepancy arises due to rapidly decay associated with reduced rebound tensile strain on convex surfaces. A "multi-point continuous shock" simulation strategy was implemented for modeling LSP effects on the circumferential surface. The average compressive residual stress achieved was found to be 92 MPa, additionally, tensile residual stresses ranging from 8 MPa to 29 MPa were detected within secondary surfaces and inside the cylinder. Notably, discrepancies between simulated and experimental fatigue lives for LSP specimens across strain amplitudes of ± 0.5 %, ±0.6 %, ±0.7 %, and ± 1.0 % were minimal-ranging only between 10.4 % and 22.3 %. [ABSTRACT FROM AUTHOR]

• The SU-263 alloy exhibited higher fatigue life at 1123 K compared to 1073 and 1173 K. • Extended cyclic softening was observed with an increase in temperature in the alloy. • Bi-linear Coffin-Manson behavior was observed only at 1123 K. • The mode of failure changed from intergranular to transgranular with temperature. • Dislocation-dislocation interactions increased with temperature due to the activation of multiple slips. The LCF behavior of SU-263 was investigated at various temperatures (1073, 1123, and 1173 K) and strain amplitudes of ± 0.4 to 0.8% at a constant strain rate of 3 × 10-3 s−1. The alloy displayed initial hardening followed by extensive cyclic softening until failure. It is observed that the presence of dislocation networks absorbs mobile dislocation, and the shearing of γ′ precipitates were responsible for cyclic softening at 1073 and 1123 K, whereas dissolution of γ′ precipitate and dislocation annihilation were responsible for cyclic softening at 1173 K. At elevated temperatures, the LCF behavior is significantly influenced by time-dependent processes such as dynamic strain aging (DSA), and oxidation. The occurrence of DSA manifests in the form of serrated plastic flow in stress–strain hysteresis loops, reduced half-life plastic strain amplitude, and increased cyclic work hardening. The alloy exhibits linear behavior in the Coffin-Manson (C-M) plot at 1073 and 1173 K. However, the C-M plot shows bi-linear behavior at 1123 K with the corresponding shift in the deformation mechanism at ± 0.5% strain amplitude. This study focuses on understanding the effects of temperature on fatigue behavior and the associated deformation mechanisms by using characterization techniques, such as scanning and transmission electron microscopy. [ABSTRACT FROM AUTHOR]

• The pre-cracked aluminum plate is patched with CFRP, including different wt.% of B4C. • The samples were subjected to fatigue tests in two different strain ratios. • Improved fatigue life of specimen with 0.6 wt% of B4C in CF patch. Composite patch over the cracked metal surface plays a vital role in repairing the damaged part of the lightweight structures, as it is one of the most efficient and could be done with the reduced cost. This manuscript documents the repair efficacy of pre-cracked aluminum alloy AA6061 under low cycle fatigue (LCF). As a novel attempt, the alloy is patched using carbon fiber composite, with and without nano ceramic boron carbide (B 4 C) particles infused. In this regard, B 4 C nanoparticles are added in 0.2%, 0.4%, and 0.6% wt. composition, referred herein as 2BCF, 4BCF, and 6BCF, respectively. Each sample, either patched and non-patched, are tested for LCF loading at two strain ratios, 0.1 and 0.3. LCF-test outcomes reveal that adding B 4 C nanoparticles effectively reduces the plastic strain amplitude (PSA) and energy dissipation. As a result, the fatigue life of the patched AA6061 alloys is significantly improved. Notably, the sample 6BCF exhibits higher fatigue resistance at strain ratio of 0.1, resulting in 6.5-fold improvement in fatigue life compared to other patched specimens indicating a strong adhesion between the patch and the aluminum surface. Therefore, more stable response to fatigue loading is observed due to less dissipated energy. [ABSTRACT FROM AUTHOR]

• Transition from the non-Masing behavior to Masing behavior at the strain amplitude of 0.6%. • Explaining cyclic hardening, stabilization and softening based on flow stress decomposition. • Characterization of the local plastic deformation, grain classification and carbide precipitation. • Evolution of crack initiation and propagation characteristics and dominant damage mechanism. Low-cycle fatigue tests were conducted at 650 °C to investigate the cyclic deformation behavior and damage mechanism of 316H stainless steel at various strain amplitudes ranging from 0.2 % to 1.0 %. The relationship between macroscopic deformation properties and microscopic physical mechanisms was revealed based on the comprehensive analysis of local deformation misorientation and dislocation microstructure. Results showed that the 316H stainless steel presented non-Masing behavior at strain amplitudes lower than 0.6 %, and Masing behavior at strain amplitudes larger than 0.6 %, which was primarily attributed to the evolution of dislocation configurations. The crack initiation and propagation behavior at 0.2 % strain amplitude, characterized by the transgranular mode, was dominated by fatigue. However, the intergranular damage became dominant at the strain amplitude larger than 0.2 %, due to the intensified strain localization at grain boundaries. Moreover, the plastic strain energy was used for life assessment with the consideration of non-Masing behavior. [ABSTRACT FROM AUTHOR]

• The tensile properties and LCF life of LDED-ed Ti6Al4V alloys are improved through microstructure regulation. • The cyclic softening is attributed to the competition effect of back stress and friction stress. • The grain orientation strongly affects the fatigue crack propagation process. The application of the laser directed energy deposition (LDED) titanium (Ti) alloys has been severely impeded by their poor low cycle fatigue (LCF) properties in aerospace industry. Herein, we propose to improve the LCF properties of LDED-ed Ti6Al4V alloys via microstructure adjustment. A solid solution aging 850AA treatment is made to regulate the microstructure into a multi-scale composition dominated by discontinuous α GB and coarsened α P +(α S +β) phases. The LCF properties of LDED-ed Ti6Al4V with a multi-scale microstructure even outperform the wrought Ti6Al4V at high strain amplitudes. Furthermore, this multi-scale microstructure has special local plastic deformation behavior and dislocation activities under cyclic stress loading, which strongly affects the internal stress evolution and crack propagation. The fatigue softening behavior is attributed to the combined effect of back stress and friction stress. Fatigue cracks are deflected by the α/β boundary when the angle (φ) between the long axis of α lamellae and the crack direction is less than 35°. The transgranular fracture occurs when φ is close to 90° (70°–90°). Meanwhile, the fatigue crack usually propagates along the basal or prismatic planes with high Schmid factors. These findings provide novel insights into the microstructure design of LDED-ed Ti alloys with high fatigue damage tolerance. [ABSTRACT FROM AUTHOR]

• The cyclic deformation behavior and fatigue fracture mechanism of TC17 alloy under different strain amplitudes were investigated. • The microscopic causes of the cyclic softening/hardening phenomenon were analyzed. • The relationship between defects (secondary cracks, holes) and the basket-weave microstructure was explored. • The variations in microstructure surrounding the crack propagation path were elucidated. Room-temperature low-fatigue tests were conducted on TC17 titanium alloy, and the low-fatigue damage behavior of TC17 titanium alloy with a basket-weave microstructure at room temperature was investigated. The results show that TC17 titanium alloy exhibits different cyclic hardening/softening phenomena at different strain amplitude (ε ta ) levels, which was closely related to the magnitude of strain amplitude, the crack sources of all specimens generated on the surface, and the number of crack sources increases with the increase of strain amplitude level. The competitive relationship between back stress hardening and frictional stress softening caused by changes in different microstructures is the reason for the occurrence of different cyclic hardening/softening phenomena. The relationship between the holes and secondary cracks near the fracture surface and the basket-weave microstructure was further discussed. The secondary cracks/holes were more likely to initiate in the primary α phase where the solid solution strengthening effect was weak. In addition, the interaction between the secondary crack propagation path and the nearby α lamellae was studied by the Electron Backscatter Diffraction (EBSD) method. [ABSTRACT FROM AUTHOR]

Thermo-mechanical creep-fatigue (TMCF) behaviors of a TiAl alloy were investigated by conducting various creep, creep-fatigue, and thermo-mechanical tests and performing detailed microstructural analysis. A new creep model was proposed to unify primary strain hardening and tertiary accelerating damage effects. The TMCF failure of TiAl alloys is accompanied by the crack initiation from surface oxides/carbides and mixed transgranular and intergranular cracking. It revealed that introducing thermo-mechanical cycles significantly reduces the creep damage rate during the creep dwell stage and the fatigue process, i.e., the creep-delaying effect, which is attributed to the non-proportional strengthening effect induced by thermal cycles. A life model based on the average minimum creep strain rate was proposed to predict the creep-dominated TMCF life. This model incorporates the complex creep-fatigue interactions on creep damage and demonstrated good agreement with experimental results under varying loading conditions. The present work provides new insights into understanding the creep-dominant thermo-mechanical damage evolution of TiAl alloys. [ABSTRACT FROM AUTHOR]

The cyclic uniaxial stress-controlled ratcheting behaviour of GH3536 formed by selective laser melting (SLM) was tested at 800 °C. The effects of different stress rates and building orientations of 0°, 45° and 90° were investigated. Results present that the ratcheting behaviour of SLM GH3536 at 800 °C exhibits significant stress rate dependence. Moreover, the accumulated ratcheting strain increasing with increasing the stress amplitude and stress rate. It is noted that the building orientation angle of the specimen has an important influence on the mechanical performance. The 45° specimen in the experimental conditions has the greatest low cycle fatigue life. The ratcheting evolution over the whole cyclic load history can be divided into three stages. The relationship between steady ratcheting strain range and low cycle fatigue life was established. [ABSTRACT FROM AUTHOR]

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Low cycle fatigue (LCF) behaviors of TC21 alloy with a bi-lamellar basketweave microstructure were investigated in this paper. The strain fatigue tests were carried out at total strain amplitudes of 1.4% to 2.0%. The cyclic stress response showed the cyclic softening behavior. In addition, the shape of the hysteresis rings exhibited a non-Masing model behavior. The cyclic stress–strain as well as the strain-life equations were obtained. The fatigue life decreased significantly with an increasing total strain from 1.4% to 2.0%. The cyclic softening behavior was interpreted by cyclic back stress and friction stress. Low cycle fatigue cracks were predominantly initiated on the surface of the samples. The relationship between the fatigue sub-critical crack and microstructure was also discussed. The cyclic deformation behavior and crack initiation mechanism were revealed on the basis of the deformation microstructure under different strain amplitudes. [ABSTRACT FROM AUTHOR]

The fatigue tests of Chinese RPV steel (A508-3) were carried out under the simulated primary coolant environment of AP1000 which is a kind of 3rd generation nuclear power plant. The effects of temperature, strain rate and dissolved oxygen (DO) on fatigue life were analyzed. During the fatigue test, the material exhibited a cyclic hardening followed by cyclic softening behavior, with no obvious cyclic stability region. In logarithmic coordinates, the S–N curve showed a bilinear relationship. As the strain amplitude increased from 0.2 to 0.6%, the fatigue life approximately decreased from 105 to 3 × 102. The environmental correction factor (Fen), which is defined as the ratio of life in air at room temperature to that in the primary coolant environment, decreases as the temperature decreases from 321 to 150 °C. The Fen also decreases as the DO decreases from 0.6 to 0.03 ppm while it increases as the strain rate decreases from 1 to 0.0004%/s. Compared with the NUREG6909 model, the presented model is more applicable for fatigue life prediction of Chinese A508-3 steel. [ABSTRACT FROM AUTHOR]

The aim of this study was to identify cyclic deformation behavior of a high‐pressure die‐cast Silafont®‐36 automotive cast aluminum alloy consisting of heterogeneous composite‐like microstructures of nearly circular eutectic silicon particles embedded in Al matrix, with particular attention to fatigue life prediction. The alloy exhibited superior fatigue resistance at higher strain amplitudes, where cyclic hardening occurred. The fatigue life of the alloy could be well predicted via a strain‐energy density based accumulative damage model using a new method proposed to calculate the intrinsic fatigue toughness. The predicted fatigue life was in good agreement with the experimental life, and also lay in‐between the results obtained via other methods. Fatigue crack initiation was observed to occur from the sample surface or near‐surface defects and crack propagation was characterized by fatigue striations along with tear ridges in the primary Al‐grains. [ABSTRACT FROM AUTHOR]

Dual phase (DP) steels have high strength, while maintaining outstanding elongation capacities. This is possible using a well-controlled thermomechanical process that produces a perfect phase combination in the DP microstructures. However, automotive makers are required to weld the DP steels, which generates a soft zone in the microstructure. In this work, 1.6 mm-thick DP980 steel sheets were welded by gas metal arc welding process to analyze the response of the welded soft zone to cyclic loading conditions. Conducted macrographic and metallography analyses revealed good quality in the appearance of the welded joints, with a complete fusion of the DP980 joint and without the presence of discontinuities. Low cycle fatigue tests of the DP welded joints were conducted under a constant amplitude strain control mode. The welded joints experienced a fatigue life reduction with respect to the DP980 steel of ~16% at strain amplitudes of 0.2, 0.3, and 0.4%. For strain amplitudes larger than 0.6%, the fatigue life of the welded joint was reduced by 39%. Weld thermal cycles combined with metallography analysis indicated that a tempered process of the martensite during the welding was responsible for the soft-zone formation and the poor fatigue response. [ABSTRACT FROM AUTHOR]

In this study, a sheet fatigue shear test device is designed and applied to the low‐cycle fatigue testing of DP900 with varying strain amplitudes within the range of 0.5%–6.0%. The microstructure is analyzed by using electron backscatter diffraction, and fracture surfaces are examined via scanning electron microscopy. Results indicate that the material exhibits cyclic softening behavior after the first two cycles of hardening, with a stable softening rate and a high damage evolution rate related to loading amplitude. The variation of the hysteresis curve in the cyclic process is shown. Total plastic strain energy absorbed increases as loading amplitude decreases and it reaches the peak at approximately 1%. The life prediction model based on plastic strain energy density and strain amplitude is verified to be suitable for the cyclic shear path. The influence of microinhomogeneity on the distribution of stress and strain, especially the deformation of martensite, is closely related to the bi‐linear region of fatigue life curve. [ABSTRACT FROM AUTHOR]

The paper studies fatigue and fracture behavior of aluminum alloy AA 7085. Fatigue crack growth, low cycle fatigue (LCF), and High cycle fatigue tests have been performed at room temperature. Material's performance against the crack growth has been evaluated at load ratio of 0.1 and 0.3, parameters that govern crack growth are calculated using Paris and Walker model. XFEM has been used for the simulation of fatigue crack growth. Heaviside function is used for the modeling of crack surface while crack front is modeled by branch enrichment function. The simulation result presents a good match with experimental results. In LCF, cyclic softening was observed at higher strain amplitude. Fractographic features of the fracture surfaces obtained from the tests are examined by scanning electron microscopy. [ABSTRACT FROM AUTHOR]

• Multiaxial low cycle fatigue behaviors of 308L SS fabricated by WAAM and hot-rolling technology are studied comparatively. • WAAM 308L SS exhibits significant non-proportional additional hardening phenomenon similar to the hot-rolled material. • Under uniaxial loading, WAAM 308L SS has longer fatigue life under high strain amplitudes. • Under torsional loading, WAAM 308L SS exhibits a significantly shorter fatigue life under each strain amplitude. • The CXH model can predict the fatigue life of both hot-rolled and WAAM 308L SS over a range of loading conditions. Wire arc additive manufacturing (WAAM) is a technique that has gained popularity in various industrial sectors due to its high productivity and flexibility. However, the comprehension of cyclic deformation and fatigue behaviors in WAAM austenitic stainless steel (SS), particularly under complex multiaxial loading conditions, is much less advanced compared to conventional manufacturing methods. This study aims to fill some of these gaps by performing fatigue tests of WAAM 308L SS under axial, torsional and axial–torsional multiaxial loadings. Similar tests were carried out on the hot-rolled 308L SS as a basis for comparison. WAAM 308L SS exhibits notable distinctions in cyclic deformation, fatigue life, cracking mode and fatigue failure mechanisms compared to its hot-rolled counterparts as a result of its distinct manufacturing technologies and resulting microstructures. To assess all fatigue life data, three critical plane criteria, namely FS, SWT and CXH criteria, were employed. The CXH model, which considers the effects of both tensile and shear damage on fatigue properties, has the ability to predict the fatigue life of both hot-rolled and WAAM 308L SS. [ABSTRACT FROM AUTHOR]

In this study, the low cycle fatigue behavior and failure mechanism of WAAM 16MND5 bainitic steel, which is commonly used in the nuclear reactor, were investigated thoroughly via a series of strain-controlled and stress-controlled low cycle fatigue tests. Results show that the microstructure of WAAM 16MND5 steel is granular bainite with a certain number of banded pro-eutectoid ferrite. WAAM 16MND5 steel exhibits similar cyclic deformation behavior under different strain amplitudes, i.e., a long stage of continuous cyclic softening and rapid cyclic softening till failure. The fatigue life decreases with the increasing strain amplitude. Moreover, the persistent slip markings in ferrite grains, the interface of pro-eutectoid ferrite and large Al2O3 inclusions are the preferred locations for microcrack initiation. The crack initiated from the edge of the specimen is observed to develop into a main crack. The main crack propagates mainly in transgranular mode and can also propagate along the pro-eutectoid ferrite interface. [ABSTRACT FROM AUTHOR]