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Continuous and interrupted low cycle fatigue tests were conducted on nuclear-grade S30408 stainless steel under different stress conditions at room temperature. Vickers hardness testing and microstructure characterization were performed on the fatigue samples with different fatigue states. The evolutionary mechanism of the microstructure defects in materials under fatigue cyclic loading was discussed. The traditional Basquin formula was used to predict the fatigue life of these fatigue samples. At the same time, a quantitative mechanical model related to the characteristic micro-defects parameter KAM and the Vickers hardness (Hv) was established for the S30408 stainless steel during the low cycle fatigue damage process, and the prediction accuracy of the Vickers hardness is greater than 90%, which is significant and useful for the fatigue life prediction of the 304 stainless steels used in nuclear systems and the safe operation of the reactors. [ABSTRACT FROM AUTHOR]

This study utilizes the rank correlation coefficient to examine the multi‐site failure correlation of turbine discs. Drawing from the stress‐strength interference model, reliability models both with and without factoring in the multi‐site failure correlation are constructed. Furthermore, the weakest link theory (WLT) within the context of the Weibull distribution function is invoked to develop a model for predicting the fatigue life of turbine discs, taking into account the quantity of critical sections. The variability in the low cycle fatigue (LCF) of turbine discs is scrutinized, leading to the formulation of a probabilistic fatigue life prediction method for these discs. When comparing theoretical values with experimental ones, it becomes evident that factoring in the multi‐site failure correlation significantly enhances the accuracy of turbine disc life predictions. [ABSTRACT FROM AUTHOR]

With the rapid development of hydrogen pipelines, their safety issues have become increasingly prominent. In order to evaluate the properties of pipeline materials under a high-pressure hydrogen environment, this study investigates the hydrogen embrittlement sensitivity of X70 welded pipe in a 10 MPa high-pressure hydrogen environment, using slow strain rate testing (SSRT) and low-cycle fatigue (LCF) analysis. The microstructure, slow tensile and fatigue fracture morphology of base metal (BM) and weld metal (WM) were characterized and analyzed by means of ultra-depth microscope, scanning electron microscope (SEM), electron backscattering diffraction (EBSD), and transmission electron microscope (TEM). Results indicate that while the high-pressure hydrogen environment has minimal impact on ultimate tensile strength (UTS) for both BM and WM, it significantly decreases reduction of area (RA) and elongation (EL), with RA reduction in WM exceeding that in BM. Under the nitrogen environment, the slow tensile fracture of X70 pipeline steel BM and WM is a typical ductile fracture, while under the high-pressure hydrogen environment, the unevenness of the slow tensile fracture increased, and a large number of microcracks appeared on the fracture surface and edges, with the fracture mode changing to ductile fracture + quasi-cleavage fracture. In addition, the high-pressure hydrogen environment reduces the fatigue life of the BM and WM of X70 pipeline steel, and the fatigue life of the WM decreases more than that of the BM as well. Compared to the nitrogen environment, the fatigue fracture specimens of BM and WM in the hydrogen environment showed quasi-cleavage fracture patterns, and the fracture area in the instantaneous fracture zone (IFZ) was significantly reduced. Compared with the BM of X70 pipeline steel, although the effective grain size of the WM is smaller, WM's microstructure, with larger Martensite/austenite (M/A) constituents and MnS and Al-rich oxides, contributes to a heightened embrittlement sensitivity. In contrast, the second-phase precipitation of nanosized Nb, V, and Ti composite carbon-nitride in the BM acts as an effective irreversible hydrogen trap, which can significantly reduce the hydrogen embrittlement sensitivity. [ABSTRACT FROM AUTHOR]

In high-rise buildings, large member sections are often required, particularly for columns in their lower stories. It is difficult sometime to find adequate member sections provided by steel manufacturers for such members. The use of high strength steel could tackle such difficulties. The aim of this study is to simulate the cyclic behavior of welded unreinforced flange-welded web (WUF-W) connections comprising of high strength steel columns and mild steel beams using numerical analyses. The (WUF-W) connection is a prequalified connection for steel special moment frames (SMFs). Nonlinear three-dimensional finite element (FE) models are constructed for the WUF-W connections. Combining hardening models are also constructed to simulate the low cycle fatigue (LCF) behavior of high strength and mild steel materials based on measured material test data, which are implemented in the FE model. The accuracy of the FE model is verified by comparing measured and calculated cyclic curves of the WUF-W connections. [ABSTRACT FROM AUTHOR]

The integration of additive manufacturing with traditional processes, termed hybrid additive manufacturing, has expanded its application domain, particularly in the repair of gas turbine blade tips. However, process-related defects in additively manufactured materials, interface formation, and material property mismatches in dual-material structures can significantly impact the fatigue performance of components. This investigation examines the low cycle fatigue and fatigue crack growth behaviors in dual-material specimens of nickel-based alloys, specifically the additively manufactured STAL15 and the cast alloy 247DS, at elevated temperatures. Low cycle fatigue experiments were conducted at temperatures of 950 °C and 1000 °C under a range of strain levels (0.3%–0.8%) and fatigue crack growth tests were conducted at 950 °C with stress ratios of 0.1 and −1. Fractographic and microscopic analyses were performed to comprehend fatigue crack initiation and crack growth mechanisms in the dual-material structure. The results consistently indicated crack initiation and fatigue fracture in the additively manufactured STAL15 material. Notably, fatigue crack growth retardation was observed near the interface when the crack extended from the additively manufactured STAL15 material to the perpendicularly positioned interface. This study highlights the importance of considering yield strength mismatch, as well as the potential effects of residual stresses and grain structure differences, in the interpretation of fatigue crack growth behavior at the interface.

ABSTRACT This study investigates the impact of various surface finishes on the low cycle fatigue (LCF) properties of laser powder bed fusion GRCop‐42. The evaluated surfaces include as‐built, machined, and chemically polished finishes (1.0% and 2.0% ranges). LCF life of polished GRCop‐42 was assessed at cryogenic (−195°C), ambient, and elevated temperatures (200°C, 400°C, 600°C, and 800°C) across three strain ranges. Results indicate that surface finish has minimal impact on LCF life. Stress across different strain levels showed minimal effect of surface finish on cyclic hardening/softening. Cryogenic temperatures led to cyclic hardening followed by stabilization, while ambient and 200°C temperatures showed initial hardening followed by softening. At 400°C and above, specimens displayed continuous cyclic softening. Fractography showed that surface finish impacts plastic deformation: as‐printed and polished surfaces had brittle fractures, while machined specimens were ductile. Specimens at cryogenic and ambient temperatures exhibited brittle fractures, whereas those at elevated temperatures showed plastic deformation and microcracks. [ABSTRACT FROM AUTHOR]

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]

This study, aims to weld the 5.2 mm thick AZ31 magnesium alloy with conventional friction stir welding at the highest joining efficiency. As a result of the experiments, 88% joining efficiency in tensile strength has been obtained at 1250 rpm, 400 mm.min−1 welding parameter. As a result of micro–macrostructure photographic examinations of the samples joined with these parameters, it is seen that the joining is fully realized. Samples joined with these parameters have been used in fatigue tests. According to the strain‐controlled low cycle fatigue test results performed on welded and base metal samples, the base metal samples have exceeded the 50,000‐cycle limit without failure, with an elongation rate of 0.3%, and the welded samples with an elongation rate of 0.2%. Low cycle fatigue parameters of welded and base metal samples have been obtained according to the Coffin‐Manson‐Basquin equation. Highlights: In joining AZ31 alloy with FSW, a high joining efficiency of 88% was achieved.Similar fatigue test results showed that the weld quality was retained along the joint.Coffin‐Manson‐Basquin equation constants were determined. [ABSTRACT FROM AUTHOR]

In this study, we propose an innovative approach that enhances the performance of the backpropagation (BP) neural network in predicting the low‐cycle fatigue life of Ti‐6Al‐4V alloy by improving the dung beetle optimization (DBO) algorithm with the maximin Latin hypercube design (MLHD) strategy. To address the challenges posed by complex geometric components under different temperature conditions, this research employs finite element simulation to expand the limited experimental dataset and utilizes these data to further guide and optimize the MLHD_DBO_BP model. Test results indicate that the proposed MLHD_DBO_BP model significantly outperforms the traditional finite element method (FEM) and other neural network models in terms of fatigue life prediction performance. This research demonstrates the effectiveness of machine learning models that combine experimental and simulation data in predicting the low‐cycle fatigue life of Ti‐6Al‐4V alloy. Highlights: Maximin Latin hypercube design enhances dung beetle optimization for neural network tuning.Improved dung beetle algorithm outperforms finite element method in predicting fatigue life.High‐precision fatigue life predictions for complex geometries are achieved.Dung beetle‐optimized neural network validated across various engineering structures. [ABSTRACT FROM AUTHOR]

Current nickel-based single crystal superalloys (SX) are mainly designed to increase significant mechanical loading at high temperatures. Therefore, the mechanical resistance is greatly dependant on the microstructure and on the potential metallurgical defects. Corrosion and oxidation at high temperatures may further induce a loss of load-bearing section and lower the overall mechanical performance of such single crystals. While the yet complex mechanical and corrosion mechanisms are relatively well established separately, little is known on their combined effects, let alone on as-cast (AC) versus fully heat-treated (FHT) microstructures. This paper shows that the low cycle fatigue (LCF) at 0.5 Hz, Rσ = 0.05 and 950 °C is lowered when the AM1 nickel-based single crystal superalloy is pre-corroded with 1 mg/cm2 Na2SO4 at 950 °C. The degradation increases with increasing pre-corrosion time due to the formation of a porous, brittle corrosion layer that favours the number of crack initiation sites, which are subsequently assisted by hot corrosion and oxidation. In addition, AM1 FHT shows better LCF fatigue resistance than AM1 AC, due to a better creep resistance of the FHT microstructure under these conditions. [ABSTRACT FROM AUTHOR]

The load history exerts a considerable impact on the low cycle fatigue (LCF) life of turbine discs. Thus, oversimplifying the load history leads to substantial errors in fatigue life prediction. This study introduces a probabilistic fatigue life prediction method for turbine discs, accounting for the randomness inherent in LCF load history. The method involves quantifying the randomness of load history through numerical simulation and employing a surrogate model enhanced with learning functions to balance computational efficiency and accuracy. The probabilistic LCF life prediction of full‐scale turbine disc was conducted, demonstrating that the fatigue life scatter predicted by the proposed method more closely aligns with experimental data compared to the original approach. By refining the numerical simulation process, the proposed method better accounts for uncertainties in load history while maintaining computational efficiency, offering significant insights for the fatigue reliability design of turbine discs. [ABSTRACT FROM AUTHOR]

This study entails comprehensive analyses of the temperature dependence of tensile properties and strain-controlled low cycle fatigue behavior exhibited by ASTM A302 grade C steel, which is specifically a manganese-molybdenum-nickel alloy. Tensile strength at elevated temperature ranges from 400 to 428 °C closely approximates those measured under ambient conditions, while the ductility properties show a noteworthy increase compared to ambient temperature testing conditions. Cyclic stress responses at elevated temperatures are characterized by two discernible stress cyclic hardening phases. Fatigue lives of the materials tested at 400, and 428 °C reveals a significantly prolonged fatigue life compared to testing under ambient temperature conditions. The transition of fatigue life for specimens tested under ambient temperature conditions is approximately two times shorter than that observed under elevated temperature conditions. The substantial extension of fatigue lives is attributed to the strengthening effect of the peak dynamic strain aging within the test temperature range predominates over the adverse effects of elevated temperature conditions. Drawing from the experimental findings, comprehensive derivations of cyclic stress–strain responses and fatigue properties have been formulated to elucidate the effects of elevated temperature conditions. [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]

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This study seeks an early prediction method of crack failure location and orientation due to low cycle fatigue in additively manufactured metallic cellular materials by leveraging experimentally observed accumulation of plastic deformation. To study this, a novel spatial-temporal approach for analyzing Infrared Thermographic (IRT) video was developed to detect heat generated by local plastic deformation. The method was validated experimentally by conducting fully reversed low cycle fatigue tests of Inconel 718 (IN718) honeycomb specimens manufactured using Laser Powder Bed Fusion (LPBF). Using the approach developed, results showed that localized heating due to plastic work could be detected and used for early prediction of the most probable path, and orientation of crack propagation. Furthermore, the method developed was found to be able to predict these results within the first 1.5 % of the total life of the specimen apriori to crack initiation.

Compared to the traditional alloys, high entropy alloys exhibit exceptional strength and outstanding ductility, making them highly attractive for use in demanding engineering applications. However, the atomic-scale deformation behavior of HEAs with precipitate under the low-cycle loading conditions has not been well studied. Here, we utilize molecular dynamics simulations to investigate the low cycle fatigue behavior of AlCoCrFeNi HEAs with AlNi-rich phase, in order to better understand the cyclic deformation, work hardening, and damage mechanisms. In the stress-strain hysteresis loops, the stress in the elastic stage exhibits a gradual linear increase, followed by fluctuations at yielding and plastic deformation. The strain hardening depends on the cycle number after the yielding stage. With an increase in the number of cycles, the activation mode of stacking faults gradually transitions from a multi-slip system to a single-slip system, attributed to the gradual phase transformation. A thorough examination of dislocation evolution is crucial in understanding the strengthening and plastic behavior of materials under cyclic loading. The generation of more stair-rod dislocations further suppresses the movement of dislocations. The combined effects of element diffusion, structural transformation, and incoherent precipitation play a critical role in enhancing the mechanical properties of AlCoCrFeNi HEAs. The strength of high entropy alloys is improved through interface strengthening caused by element diffusion and structural transformation, along with dispersion induced by incoherent precipitation. This work provides a detailed atomic-level understanding of the cyclic deformation-induced strengthening mechanism, in order to design highstrength and ductile HEAs with specific desired properties. [ABSTRACT FROM AUTHOR]

In this paper, the cyclic elastic–plastic behaviors of the centrifugal steam compressor impeller (CSCI) structure under practical operating conditions are discussed employing the linear matching method (LMM). The three‐stage strategy of the LMM is employed to determine the shakedown and ratchet boundaries by conducting a series of iterative direct steady cyclic analysis, and based on this, the load regions corresponding to different types of damage behavior that may occur during the operation are identified. Computational results reveal slight contraction in the structural response diagram when considering actual material constitutive relationships, compared to ideal elastic–plastic models. Meanwhile, by integrating the unified ratchet and fatigue program with fatigue experimental data, a series of constant low‐cycle fatigue life curves has been presented for different service conditions. Furthermore, a comparison with detailed step‐by‐step inelastic analyses confirms the applicability and great efficiency of the LMM in practical engineering problems. Highlights: The cyclic elastic‐plastic behavior of impeller were investigated using the three‐stage procedure.The effect of temperature‐dependent and strain hardening on structural response of impeller were evaluated.The constant low‐cycle life fatigue curves of the impeller structure were generated.The applicability and great efficiency of linear matching method have been confirmed. [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 additive manufacturing (AM) is a relative new manufacturing process that belongs to the fourth industrial revolution, thus yielding new challenges that are worth the study in terms of reliability. In that sense, this work presents a comparative study of as printed and optimally heat-treated 1.2709 maraging steel manufactured by laser powder bead fusion (LPBF) AM process. The results suggest that parts were sound in terms of visual inspection and microstructural analysis. In the case of mechanical properties, the results suggest that the highest variability occurred after the heat treatment. The high cycle fatigue samples revealed a fracture without exhibiting a considerable amount of plastic deformation. On the other hand, the low cycle fatigue results revealed that at 1% strain the optimally heat-treated specimens have lower fatigue life due to the presence of surface discontinuities and the reduction of ductility after the heat treatment. [ABSTRACT FROM AUTHOR]

This paper describes an application of a machine learning approach for parameter optimization. The method is demonstrated for the elasto-viscoplastic model with both isotropic and kinematic hardening. It is shown that the proposed method based on long short-term memory networks allowed a reasonable agreement of stress–strain curves to be obtained for cyclic deformation in a low-cycle fatigue regime. The main advantage of the proposed approach over traditional optimization schemes lies in the possibility of obtaining parameters for a new material without the necessity of conducting any further optimizations. As the power and robustness of the developed method was demonstrated for very challenging problems (cyclic deformation, crystal plasticity, self-consistent model and isotropic and kinematic hardening), it is directly applicable to other experiments and models. [ABSTRACT FROM AUTHOR]

In this paper, the deformation behavior of UNS S32750 (S32750) duplex stainless steel during low cycle fatigue was studied by controlling the number of cycles. The microstructure of the specimens under different cycles was characterized by optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM). The microhardness of the two phases was measured by a digital microhardness instrument. The results showed that the microhardness of ferrite increases significantly after the first 4000 cycles, while the austenite shows a higher strain hardening rate after fatigue fracture, and the microhardness of ferrite and austenite increases by 23 HV and 87 HV, respectively. The two-phase kernel average misorientation (KAM) diagram showed that the continuous accumulation of plastic deformation easily leads to the initiation of cracks inside the austenite and at the phase boundaries. The evolution of dislocation morphology in the two phases was obviously different. With the increase in cycle number, the dislocation in ferrite gradually transforms from dislocation bundles and a dislocation array to a sub-grain structure, while the dislocation in austenite gradually develops from dipole array to an ordered Taylor lattice network structure. [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]

This paper was attempted to investigate the microstructure degradation and low cycle fatigue (LCF) performance of a serviced K465 Ni‐based superalloy turbine blade. LCF tests were carried out with small‐scale plate specimens sampled from the blades. Relationship between residual LCF life and microstructure state was estimated. The results indicate that the coarsening of γ/γ′ phases was the most significant microstructure degradation mode for the serviced blades. Both the γ matrix width and the γ′ precipitate diameter increased with the increase of service duration, while the γ′ precipitate volume fraction slightly decreased. The most severe microstructure degradation occurred at the leading edge along the chord direction, particularly at 50–70 % airfoil spans. The residual LCF life exhibited an accelerated decrease characteristic as increases of microstructure degradation degree. The coarsened microstructure diminished shear resistance of the superalloy, which resulted in additional accumulated inelastic deformation and a corresponding reduction in LCF life. Highlights: Service induced microstructure evolution and LCF life decay of a blade were investigated.Coarsening was identified as the main degradation form of γ/γ' phases in serviced blade.Accelerated decrease of residual LCF life was identified as increases of coarsening.Reduced plastic deformation resistance owing to coarsening is crucial to decay LCF life. [ABSTRACT FROM AUTHOR]