Low Cycle Fatigue Research Articles

Post-fire mechanical behavior of iron-based shape memory alloy used for structural damping

This study sheds light on the post-fire mechanical behavior of Fe-SMA by conducting a series of tests at material level, with a particular focus on the low-cycle fatigue (LCF) performance. Three temperatures (500 °C, 750 °C, 1000 °C) and three cooling methods (air cooling, water spray quenching, water quenching) are considered to simulate different fire-cooling scenarios. After the heating-cooling cycle, 10 monotonic tensile specimens and 20 LCF specimens are tested. Essential material parameters for Manson-Coffin model and combined hardening model are also calibrated based on the experimental data. The failure mechanism and possible factors affecting the post-fire LCF performance of Fe-SMA are further explored by scanning electron microscope (SEM), metallographic observation (MO), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) analysis. Results show that Fe-SMA could exhibit even better LCF resistance after undergoing the considered fire-cooling procedure, especially those heated up to 750 °C and 1000 °C. Even under rapid cooling, no signs of brittle fracture are observed, which is superior to traditional structural steels. Different heating temperatures and cooling methods affect the grain sizes and microstructure of Fe-SMA, leading to variations in its post-fire LCF behavior.

High Temperature Fatigue of Additively Manufactured Inconel 718: A Short Review

Abstract With the increasing interest in adopting additively manufactured (AM) IN718 for high-temperature applications, driven by the design and manufacturing flexibility offered by AM technologies, understanding its fatigue performance is crucial before full-scale adoption. This paper reviews the recent literature on the high-temperature fatigue behavior of AM IN718. The review focuses on two primary stages of fatigue damage: fatigue crack initiation and fatigue crack growth. Notably, most existing studies have concentrated on fatigue crack initiation, and thus, this review emphasizes this aspect. In the fatigue crack initiation stage, discrepancies in low cycle fatigue (LCF) and high cycle fatigue (HCF) life performances are observed in the literature. Some studies have shown that the average room temperature (RT) fatigue life of AM IN718 is superior or comparable to that at high temperatures in the LCF regime. Conversely, in the HCF regime, high-temperature fatigue life is sometimes found to be superior to that at room temperature. However, other studies indicate no clear trend regarding the effect of temperature on HCF life. Although various mechanisms have been proposed to either improve or degrade fatigue performance across the LCF, HCF, and very high cycle fatigue (VHCF) regimes, the underlying reasons for the distinct behaviors in these regimes remain unclear. Competing mechanisms, such as surface oxide formation and thermally driven dislocations glide, can potentially enhance or reduce fatigue life. However, the interaction and control of these mechanisms over the fatigue strength of AM IN718 are not yet fully understood. Systematic studies are required to elucidate their roles in high-temperature fatigue. Microstructural investigations have suggested that controlling the formation and precipitation of deleterious secondary phases is crucial for tailoring the high-temperature fatigue strength of AM IN718. Therefore, it is imperative to design heat treatment protocols informed by a comprehensive understanding of phase formation kinetics to improve the high-temperature fatigue performance of AM IN718 compared to their traditionally manufactured (TM) counterparts. This is particularly important for IN718 parts manufactured using directed energy deposition technology, which currently lacks standardized heat treatment procedures. The review also identifies open research areas and provides recommendations for future work to address these gaps.

Transfer-Learning Prediction Model for Low-Cycle Fatigue Life of Bimetallic Steel Bars

The prediction of the low-cycle fatigue life of bimetallic steel bars (BSBs) is essential to promote the engineering application of BSBs. However, research on the low-cycle fatigue properties of BSB is limited, and fatigue experiments are time-consuming. Moreover, considering that sufficient data are needed for model training, the lack of data hinders the leverage of typical data-driven machine learning, which is widely used in fatigue life prediction. To address this issue, a transfer learning framework was suggested to accurately predict the low-cycle fatigue life of BSBs with limited data. To achieve this goal, 54 data points obtained from low-cycle fatigue tests on BSBs and 264 data points of other metallic bars were collected. Source models based on artificial neural networks (ANNs) were first constructed using the collected source dataset. Then, the learned knowledge stored in the source models was transferred to the transfer models. After that, transfer models were further fine-tuned and then tested using the target dataset of BSBs. The ANN models, which were of the same structure as the transfer models but only trained with the target dataset without transferring deep features from the source models, were set as baseline models. Compared with baseline models, the constructed transfer models could be used to accurately predict the fatigue life of BSBs. Moreover, the influence of hidden layers of ANNs on accuracy was examined by comparing one-layer and two-layer transfer models. Furthermore, the influence of key parameters on fatigue life of metallic bars was evaluated by feature analysis.

Low cycle fatigue properties of CoCrFeNiMn high-entropy alloy with heterogeneous microstructure

Heterogeneous microstructure is a hot topic in materials science which facilitates tackling the dilemma of strength-ductility trade-off in engineering materials including high-entropy alloys. The present investigation was conducted on the impact of different microstructures including heterogeneous microstructure, constructed by thermomechanical treatment including cold rolling followed by annealing, on the monotonic and cyclic behavior of a CoCrFeNiMn high-entropy alloy. The tailored microstructure contained three features: non-recrystallized as a hard domain, ultrafine-recrystallized, and fine-recrystallized regions. A desirable combination of strength and ductility including UTS of ∼1.1 GPa together with an elongation of ∼15 % was achieved due to benefiting from hetero-deformation-induced hardening and activation of deformation twins in the fine recrystallized grains. Regarding low-cycle fatigue behavior, at the strain amplitude of 0.4 %, heterogeneous microstructure exhibits remarkable fatigue properties, including high maximum stress and extraordinary lifetime (more than 2 × 105). The compelling reasons for these desirable properties are triggering deformation twins in fine recrystallized grain, the absence of brittle sigma phase as a good location of crack initiation, and the non-recrystallized area as a hard domain for suppressing crack propagation. The results suggested that the heterogeneous microstructure has significant benefits during applying low strain amplitude, however, at high strain amplitude, grain refinement is not recommended, and coarse-grained microstructure provides better properties. The present investigation is a step forward to understand the significance of heterogeneous microstructures to improve fatigue properties of high- or medium-entropy alloys.

Digital image correlation approach for low-cycle fatigue life monitoring of 13HMF power engineering steel

In this paper, strain evolution of 13HMF power engineering steel was analysed during force-controlled low cycle fatigue (LCF). The material performance under cyclic loading was monitored by using Digital Image Correlation (DIC) technique for different values of stress amplitude exceeding the yield strength of the material significantly. Data collected from DIC was compared to the extensometer ones to confirm the effectiveness of the proposed approach. Finally, the cloud data reflecting the fatigue performance of 13HMF was generated in the form of strain-stress amplitude - service life distribution map.

An enhanced multiaxial low-cycle fatigue life model

To improve the accuracy of predicting multiaxial fatigue life, an enhanced multiaxial low-cycle fatigue life model is proposed based on the FS model. This model introduces a correction parameter for the stress-related term to consider the influence of normal stress and shear stress on the critical plane. The feasibility of the model is investigated, and it is validated using fatigue test data from eight different materials. The results indicate that the proposed model is applicable for both symmetric and asymmetric loading conditions under constant amplitude loading, with highly accurate fatigue life prediction results.

Low-cycle fatigue behavior of coated and uncoated single crystal Ni-based superalloy at various temperatures

Within this work, the effect of different oxidation-resistant coatings on the low-cycle fatigue (LCF) behavior and damage mechanisms of third-generation single crystal (SC) Ni-based superalloy was investigated. Stress-controlled LCF tests of bare alloy, MCrAlY-coated and PtAl-coated thin-walled specimens were carried out at 760 °C, 850 °C and 980 °C. A unique clamping system was meticulously designed to overcome the clamping challenge in high-temperature furnaces. The cracking behaviors and failure modes were determined by advanced microstructure characterization. The results show that the coatings are detrimental to the LCF lifetime of thin-walled specimens, especially at low-medium temperatures of 760 °C and 850 °C. For bare alloy, the tip cracks nucleate from the surface oxide layer, whereas cracks in the coating-substrate system nucleate prematurely from the coating surface and propagate to the substrate. The oxygen diffusion and the frontward propagation of cracks are synergistically enhanced in the SC substrate, resulting in oxidation-assisted crack propagation and ultimate failure. Finally, A fatigue life prediction model is proposed by introducing coating-induced crack initiation and oxidation effects, where the predictive results fit well with experimental results. This study provides a comprehensive insight into the damage mechanisms and coating-induced LCF life degradation of the coating-substrate system.

Fatigue deformation behavior and fracture mechanism of high fatigue-resistant laser directed energy deposition fabricated titanium alloy: Effect of multi-scale microstructure

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 αGB and coarsened αP+(αS+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.

Tensile and Low Cycle Fatigue Response of SS321 at Room Temperature

Tensile and Low Cycle Fatigue Response of SS321 at Room Temperature

Modeling thermo-mechanical fatigue for rolling die under continuous casting process

In the continuous casting process, the rolling die is exposed to non-steady stress conditions that can initiate thermo-mechanical fatigue damage. Understanding the rolling die failure mechanism is essential in metal-forming industries. Thermo-mechanical fatigue damage mechanism (crack growth) is governed by highly localized stresses in the crack tip region. This paper presents analytical models to determine thermo-mechanical fatigue failure mechanism based on the elastoplastic fracture mechanics and continuum damage mechanics. The model includes new proposed analytical equations for low-cyclic fatigue crack propagation. To verify the analytical models, experimental data and finite element simulation are considered. Results show good agreement with experimental data and finite element simulation.

Effect of peak stress and dwell time on the fatigue damage behavior of Ti-22Al-23Nb-2(Mo, Zr) alloy with lamellar microstructure

Ti2AlNb alloy is one of the candidate high strength titanium alloy materials under consideration for compressor disc, which will endure dwell loading at high peak stress level during service. In this work, low cycle fatigue (LCF) and low cycle dwell fatigue (LCDF) deformation behaviors and damage mechanism of Ti2AlNb alloy with lamellar microstructure under high loading level were investigated. The results show that fatigue life is highly dependent on the peak stress (σpeak) and dwell time (tdwell). With increasing the σpeak, the LCF life decreases gradually since higher loading stress can accelerate plastic strain accumulation and lead to faster crack propagation. When at the same σpeak, the LCDF life is lower than the LCF life due to greater plastic strain accumulation caused by dwell effect, indicating a dwell life debit of the Ti2AlNb alloy. Moreover, dwell fatigue sensitivity exhibits an increasing trend as the tdwell increases. Under different loading waveforms, the fractography shows remarkably different features. Crack source of LCF appears on sample surface, while crack source of LCDF appears on surface and sub-surface of sample. The research of LCDF behavior is relevant to aviation industry to prevent premature failure of rotating components made of Ti2AlNb alloys.

Fretting fatigue in dovetail assembly and bridge-flat model: Verification of a life prediction model based on stress gradient

The dovetail assembly and the bridge-flat model are two typical fretting fatigue tests. This article concentrates on the fretting fatigue of the dovetail assembly, utilizing a sub-scaled specimen tested under cyclic loading. Simultaneously, fretting tests on the bridge-flat and combined high and low cycle fatigue tests on the dovetail assembly were conducted. These tests were analyzed in conjunction with the dovetail assembly to understand the stress state characteristics in the contact region. The results from both tests and finite element analysis highlight that fretting leads to a significant reduction in fatigue life, primarily attributed to stress concentration at the edges of the contact area. Building upon the method of stress gradient correction, a life prediction model for fretting fatigue is proposed, demonstrating good accuracy in predicting fretting fatigue life.

Study on the Deformation Behavior of Two Phases during the Low Cycle Fatigue of UNS S32750 Duplex Stainless Steel.

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.

Crystal Plasticity Parameter Optimization in Cyclically Deformed Electrodeposited Copper-A Machine Learning Approach.

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.

Modeling of LCF Behaviour on AISI316L Steel Applying the Armstrong-Frederick Kinematic Hardening Model.

The combination of kinematic and isotropic hardening models makes it possible to model the behaviour of cyclic elastic-plastic steel material, though the estimation of the hardening parameters and catching the influence of those parameters on the material response is a challenging task. In the current work, an approach for the numerical simulation of the low-cycle fatigue of AISI316L steel is presented using a finite element method to study the fatigue behaviour of the steel at different strain amplitudes and operating temperatures. Fully reversed uniaxial LCF tests are performed at different strain amplitudes and operating temperatures. Based on the LCF test experimental results, the non-linear isotropic and kinematic hardening parameters are estimated for numerical simulation. On comparing, the numerical simulation results were in very good agreement with those of the experimental ones. This presented method for the numerical simulation of the low-cycle fatigue on AISI316 stainless steel can be used for the approximate prediction of the fatigue life of the components under different cyclic loading amplitudes.

Distributed-collaborative surrogate modeling approach for creep-fatigue reliability assessment of turbine blades considering multi-source uncertainty

This paper proposes a substructure-based distributed-collaborative surrogate modeling approach for improving the accuracy and efficiency in the estimation of the creep-fatigue reliability of gas turbine blades with the integration of the Least Absolute Shrinkage and Selection Operator (LASSO), kriging, Distributed Collaborative (DC) strategy and substructure simulation method, called as the substructure-based DCLKM. Further, the creep-fatigue reliability assessment framework is set by combining the proposed substructure-based DCLKM with some theoretical models, which takes into account the uncertainties of structural sizes, applied loads and material properties and the nonlinearity of creep/fatigue damage interaction. Firstly, the total strain range, mean stress, absolute temperature and maximum stress at the critical location are regarded as the first-level output responses, and their required samples are predicted through the fitted surrogate models. Secondly, the creep life and Low-Cycle Fatigue (LCF) life are regarded as the second-level output responses and predicted. Finally, the creep-fatigue damage and reliability assessment are performed for several given applied cycles, respectively. The proposed substructure-based DCLKM is validated on a case study. The results show that the proposed substructure-based DCLKM is feasible with high accuracy and efficiency in the creep-fatigue reliability assessment of turbine blades.

Comparison of Warm and Cold Forging with Friction Welding for Inner Constant Velocity Joints (CVJs)

Driveshafts are used in all vehicles, and their service life is expected to be at least three years or 100.000 km. Many driveshaft manufacturers prefer friction welding due to its relatively cheaper cost and ease of the process. However, they should meet some property-related criteria to achieve the expected lifetime. The forging technique becomes essential to succeed in these mechanical requirements. A comparative study evaluates the performance of constant velocity joints (CVJs) produced by multi-step warm–cold forging and friction welding processes. Medium carbon steels were used in both of the techniques. The microstructures, mechanical properties (i.e. hardness, strength, impact energy and shear strength), low-cycle fatigue (LCF) properties, wear resistance and cost-efficiency (number of operations, material saving, number of produced components and cost) are compared in detail for an industrial production point of view. The experimental results reveal that warm–cold forged specimens exhibit superior mechanical properties such as increased strength, hardness, relatively higher impact energy, improved shear strength, relatively longer LCF life and enhanced wear resistance (lower wear volume loss). In addition, it is also assessed that warm–cold forging is a more cost-effective manufacturing process (reduced weight, decreased number of operations and increased yield) in the production of CVJs compared to the friction welding process.

Analysing the Impact of Severe Shot Peening on the Fatigue Strength of Wire Arc Additively Manufactured Carbon Steel

The study investigates the impact of severe shot peening on the fatigue strength of wire arc additively manufactured carbon steel. Initial characterization revealed a material with prominent equiaxed grains and large grain sizes. However, the application of SSP induced a considerable reduction in grain size, particularly on the surface, consequently enhancing the surface's strength and hardness, yet leading to an inhomogeneous structure within the WAAM CS SSP part. Hardness measurements demonstrated a substantial impact on surface hardness, reaching a depth of approximately 0.4 mm, with a 64% increase observed due to SSP, elevating it from an average of 165 HV to a maximum of 270 HV near the surface. Tensile tests on WAAM CS and WAAM CS SSP displayed notable improvements in mechanical properties following SSP treatment. Yield strength increased by approximately 5%, and ultimate tensile strength rose by 2.5%, resulting in a peak tensile strength of 513 MPa. However, this enhancement was accompanied by reduced ductility, evidenced by decreased elongation from 44% in WAAM CS to 35% in WAAM CS SSP. Bending fatigue tests highlighted a significant enhancement in fatigue resistance due to SSP treatment. The fatigue limit increased by 21% from 190 MPa for WAAM CS to 230 MPa for WAAM CS SSP, indicating improved resistance in both low-cycle and high-cycle fatigue regimes. This enhancement in fatigue resistance is attributed to the heightened mechanical strength post-SSP treatment, suggesting a trade-off between increased strength and reduced ductility. The results demonstrate that SSP significantly enhances surface attributes, strength, and fatigue resistance of WAAM CS. This advancement bears implications for engineering applications where enhanced mechanical properties and fatigue resistance are vital, despite the induced trade-offs in material characteristics.

Low-cycle fatigue property of Inconel 617 alloy at 700 °C: Mechanisms of cyclic deformation, precipitation and cracking behavior

Effects of strain amplitude ranging from 0.2 % to 1.0 % on the low-cycle fatigue properties of Inconel 617 alloy at 700 °C were studied. Much attention was paid to reveal the physical mechanisms of the cyclic deformation, γ′ phase precipitate and cracking behaviors based on the methods of flow stress partitioning and microstructure evolution characterization. Results showed that the material demonstrated a two-slope cyclic hardening behavior with different hardening rates. The strain amplitude of 0.4 % was found to be the threshold value in terms of both the precipitation behavior and dynamic strain aging (DSA). The diameter of γ′ phase precipitate was decreased from 28 nm to 12 nm when the strain amplitude increased from 0.2 % to 0.4 % and became disappeared at the larger strain amplitudes. The DSA activity characterized by the DSA strain range and maximum stress drop got significantly intensified with the increasing strain amplitude, about five times larger at 1.0 % than that at 0.4 %. Further, the mutual interaction between precipitation behavior and DSA was thoroughly analyzed. The number of crack initiation sites was increased with the increasing strain amplitude, and the dominant damage mechanism responsible for the crack propagation behavior was elucidated.

Influence of friction and back stresses evolution on cyclic softening of laser powder bed fusion Ti-6Al-4V ELI alloy

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V ELI enables the fabrication of complex and individualized load-bearing patient-specific implants (PSI). These PSI are subjected to cyclic loading, which necessitates their fatigue performance evaluation. In this work, the fatigue properties of LPBF Ti-6Al-4V ELI in heat-treated conditions were characterized by studying the low cycle fatigue (LCF) behavior and underlying deformation mechanism. The fully reversed strain-controlled fatigue tests were performed at various strain amplitudes at room temperature. The ratio of fatigue life of conventional wrought alloy and LPBF Ti-6Al-4V ELI at the lowest (0.8 %) and the highest (1.8 %) strain amplitudes were found to be approximately 2.7 and 4.5, respectively. Further, the softening response during fatigue loading was correlated to the variation in the back and friction stresses. The TEM observations revealed that the dislocation pileup along α/β interfaces caused an increase in dislocation density, which drives the increase in back stress. However, the ease of slip transmission at high strain amplitude (1.8 %) reduced the heterogeneity between α and β phases, causing a decrease in the back stress. The TEM observations further suggested that the dislocation annihilation and subsequent rise in dislocation-free zones in α-phase reduced the friction stress at all strain amplitudes, which resulted in cyclic softening of LPBF Ti-6Al-4V ELI.