Low Cycle Fatigue Behavior Research Articles

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Low-cycle fatigue behaviour of magnesium alloy thick plate joints fabricated via differential double-shoulder friction stir welding

Magnesium (Mg) alloy thick plates welded by friction stir welding (FSW) often exhibit poor formability and uneven microstructures, leading to a significant deterioration in the mechanical properties. In this work, AZ31 Mg alloy plates with a thickness of 20 mm were successfully welded by differential double-shoulder friction stir welding (DDS-FSW). The low-cycle fatigue response behaviour was characterised, and the damage mechanism of the joints were studied. The results indicated that the fatigue life of the DDS-FSW joints in both parameters exceeded that of the FSW joint at 0.3 % strain amplitude, with increases of 200 % and 82 %, respectively. The predominant deformation mechanisms are basal slip and {10 1¯ 2} twinning. Basal slip is more likely to be activated in the FSW joint, whereas {10 1¯ 2} twinning is more easily activated in the DDS-FSW joints. Furthermore, the fatigue toughness (W0) and damage transition exponent (β) could be increased by improving the asymmetric distribution of the stir zone in the joint, the strength of “orientated micro-interface”, and the proportion and distribution uniformity of twins.

Multiaxial low cycle fatigue behavior and life prediction of wire arc additive manufactured 308L stainless steel

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.

Effects of pressurized water reactor environment and cyclic loading parameters on the low cycle fatigue behavior of 304L stainless steel

Austenitic stainless steels used in light water reactor coolant environments are susceptible to environmentally assisted fatigue due to non-monotonic loading conditions, primarily associated with load-follow, thermal transients, or intermittent plant shutdowns and start-ups. This study investigates the effects of a high-temperature pressurized water reactor (PWR) water environment and cyclic loading parameters on the low cycle fatigue behavior of austenitic 304L stainless steel. Prolonged exposure to a PWR environment and cyclic loading conditions such as a lower strain rate or a higher fraction of slow strain rate enhances the initiation and accelerates the crack growth rate of fatigue cracks, resulting in decreased fatigue life. The deformation-induced α'-martensite is observed in proximity to fatigue crack tips primarily in specimens tested in simulated PWR primary water, while cellular dislocation structures are more frequently observed near crack tips in specimens tested in high-temperature air. The deformation-induced martensitic transformation from γ-austenite to α'-martensite, occurring via the precursor ε-martensite phase, contributes to the accelerated fatigue crack growth rate in a PWR environment with hydrogen.

Low-cycle fatigue behaviour of extruded 7075 aluminium alloy bar: Competition of grain sizes and textures

The low-cycle fatigue (LCF) behaviour of a large 7075 aluminium alloy bar was investigated, and the results indicated that the microstructure at the center (0R) and surface section of the bar (R) were different in terms of grain sizes and textures. The microstructure of the 0R specimen was characterised by coarser elongated grain and higher <111> texture intensity compared to that of the R specimen. The LCF life of the 0R specimen was higher than that of the R specimen at a strain amplitude range of 0.6%–0.9%, thereby indicating the dominant effect of higher texture intensity. However, the 0R specimen sustained short LCF life at low strain amplitudes of 0.4% and 0.5%, indicating the predominant effect of grain refinement. The fracture analysis results indicated that there were wider striations for the 0R specimen at a strain amplitude of 0.4%. Interestingly, the yield strength, ultimate tensile strength and elongation of the 0R specimen were higher than that of the R specimen. The higher mechanical properties of the 0R specimen was consistent with its longer low cycle fatigue life at strain amplitude range of 0.6%–0.9%. Moreover, the features of the fracture, including fracture initiation and fast fracture regions, were also elaborated. The competition of grain sizes and texture evolutions determined the LCF life of the alloy depending on the strain amplitude range: the grain size effect gradually overshadowed that of the texture with decreasing strain in terms of fatigue life.

Low cycle fatigue behavior of Incoloy 800H weld joint at 800 °C

The fatigue behavior of the 800H weld joint at the temperature of 800 °C was investigated, taking into account the effects of residual stress and microstructure. To model the residual stress in the welded joint, a finite element method was employed and its accuracy was verified using a hole drilling method, which was also used to study the evolution of residual stress during cutting and heating processes. Additionally, the weld region was characterized by measuring the distributions of elements and micro-hardness via energy dispersive X-ray spectroscopy and Vickers hardness tester. Fatigue lives, curves of displacement versus the number of cycles were obtained, and the fracture morphology was analyzed. The results reveal that the calculated residual stress in the weld region agrees well with the experimental results, and the transverse residual stress reduces in magnitude due to cutting and heating. Element distribution and micro-hardness exhibit obvious variations from the base metal to the weld metal. The micro-hardness has a minor effect on residual stress but a significant impact on local stress amplitudes at the weld toe. Furthermore, fatigue fractures primarily occur at the weld toe when the maximum cyclic stress exceeds 200 MPa, while the fracture location varies within the middle weld metal under smaller loads.

Microstructure, fatigue crack growth rate and low-cycle fatigue behavior of an Al[sbnd]Li alloy sheet with different aging

The microstructures, fatigue crack growth (FCG) rates and low-cycle fatigue (LCF) behaviors of 1445 Al-Li alloy sheet with T6 aging at 170 °C, single-step T8 (S-T8) aging at 150 °C and double-step T8 (D-T8) aging at 170 °C following aging at 150 °C for 4 h were investigated. The main precipitates in T6 and S-T8 samples are δ′ phases, but strain strengthening causes 40 MPa enhancement in the S-T8 sample yield strength (YS). Due to coherent shearable δ′ precipitates, the FCG rate curves of S-T8 samples are highly coincident. With the 2nd-step aging time extension, δ′ precipitates in D-T8 sample gradually coarsen, semi-coherent S′ phases form and their amount increases. The interaction between δ′ precipitates with dislocations transites from shearing to by-passing, leading to the YS enhancement and the FCG resistance decrease. Meanwhile, cyclic-hardening decreases and transfers to cyclic-softening during LCF at starin amplitude of 0.6% ∼ 0.9%. At 0.9% strain amplitude, due to greater cyclic-softening and cleavage fracture of S′ precipitates, the D-T8 sample with 2nd-step aging time of 120 h has a shorter LCF life. The D-T8 sample with 2nd-step aging time of 24 h possesses a comprehensive property of higher strength, lower FCG rate and better LCF life.

Effect of long-term aging on the low cycle fatigue behavior and microstructure of a 10% Cr martensitic steel with low nitrogen and high boron contents at 650 °C

The effect of thermal aging for 10,000 h at 650 °C on the low cycle fatigue (LCF) behavior and microstructural changes of the 10% Cr steel at 650 °C was studied. A series of uniaxial strain-controlled LCF tests were carried out at 650 °C on the aged steel under fully reversed tension-compression loading conditions. Comparison of the fatigue results of the aged and unaged steels revealed that long-term aging did not affect significantly the number of cycles to failure. After aging, the slightly lower stress amplitude and slightly higher rate of cyclic softening during LCF testing provided an approximately 10% reduction in the cyclic strength coefficient K′ of the Morrow’s equation, whereas the cyclic strain hardening exponent n′ did not change due to the remaining effect of dynamic strain aging. The parameters of the Basquin-Coffin-Manson relationship between fatigue life and strain amplitude slightly changed compared to the unaged steel. Larger cyclic softening of the aged steel was associated with faster annihilation of dislocations and lath boundaries, respectively.

Experimental study on low cycle fatigue behavior of TWIP steel under cyclic shear loading condition

This study investigated the low-cycle fatigue (LCF) behavior of Twinning induced plasticity (TWIP) steel based on the twin bridge shear specimen and cyclic shear loading condition. The cyclic shear experiments were conducted with different strain amplitudes, and we compared their softening and hardening behaviors, cyclic stress-strain curves, and fatigue life. Through studying the crack morphology and the damage mechanism of shear specimens the role of cyclic shear stress on the damage evolution of TWIP under cyclic shear conditions was shown. The results indicate that the cyclic hardening and softening capacities of TWIP steel are highly affected by the strain amplitude. With increasing the load amplitude, the hardening rate increases, and its fatigue life is negatively correlated with the strain amplitude. Meanwhile, the hardening rate is positively correlated with the strain amplitude, and the softening rate is negatively correlated. The magnitude of strain amplitude affects the ratio of cyclic softening and cyclic hardening periods. Through analyzing the obtained microstructure characteristics, it is found that there is a high density of dislocations around the crack zone which causes the final failure of TWIP.

Effect of thermal aging on the low cycle fatigue behavior of Z3CN20.09M stainless steel in high-temperature pressurized water

The effect of thermal aging (0 h- 15,000 h at 400 °C) on the low cycle fatigue properties of Z3CN20.09M stainless steel was investigated. The micro-hardness of the ferrite increases with thermal aging time, while that of the austenite is almost unchanged. The fatigue life of Z3CN20.09M stainless steel in high-temperature pressurized water decreases with thermal aging time. The fatigue cracks formed everywhere, mainly including at slip bands in austenite, at the δ/γ phase boundary and in ferrite. Thermal aging promotes crack initiation and propagation in ferrite. The effect of thermal aging on the fatigue damage process is discussed.

Effects of temperature and strain amplitude on low-cycle fatigue behavior of 12Cr13 martensitic stainless steel

12Cr13 martensitic stainless steel has been selected as the structural material for steam turbine blade which is frequently subjected to low-cycle fatigue (LCF) damage. The study involved conducting low-cycle fatigue (LCF) tests under strain control at various temperatures (25 °C, 250 °C, 350 °C, 450 °C) and strain amplitudes (±0.3 %, ±0.4 %, ±0.5 %, ±0.6 %). The cyclic deformation behavior, primarily indicating cyclic stress response and the Massing effect is thoroughly analyzed based on the characterization of dislocation microstructure and electron backscatter diffraction maps. The mechanism of crack initiation and the propagation behavior are discussed based on the SEM observations of microcracks on the specimen surface and the fracture surface morphology. The influence of temperature and strain amplitude on fatigue life behavior is determined, and the underlying mechanisms are revealed. Moreover, the life prediction is performed by using the classical Basquin-Coffin-Manson model at ambient temperature.

Statistical analysis of the effect of micro-defect size on the low cyclic fatigue and creep fatigue response of 316FR stainless steel using crystal plasticity finite element method

The main purpose of this study is to explore and discuss the effect of micro-defects of different sizes on the low cyclic fatigue (LCF) and creep fatigue (CF) behavior in 316FR stainless steel. From the crystal plasticity finite element (CPFE) perspective, representative volume element (RVE) models including different micro-defect sizes are systematically developed, and an extended Armstrong-Frederick (A-F) nonlinear kinematic hardening rule is used to capture the LCF and CF behavior of the 316FR stainless steel. Statistical probability analysis shows that the relative errors between the macroscopic stress–strain simulation results and the experimental data ranged from 0.24% to 16.52%, indicating that the model has high accuracy. Three fatigue index parameters (FIPs) are used to characterize the degree of fatigue of the material: the standard deviation of strain increases with increasing micro-defect size, and the standard deviation of the maximum strain at CF loads exceeds that at LCF loads by 0.0173; the larger the size of the micro-defects are, the coarser the persistent slip band (PSB) around the defects is; the strain distribution under CF loading is wider than that of LCF, and the strain at 99% of the strain accumulation frequency at CF loading exceeds that at LCF loading by 0.078. These results indicate that the larger the defect sizes are, the higher the degree of fatigue accumulation in 316FR stainless steel is, and that the deformation behavior of the material is more complex under CF loading.

Zone-wise low cycle fatigue behavior of AA6061-T6 similar friction stir welding

Friction stir welding (FSW) introduces variations in temperature and strain distribution, resulting in diverse microstructures in and around the weld-nugget zone. This impacts the original strengthening mechanisms of the base material, especially in precipitate-hardening aluminum alloys. This study focuses on the low-cycle fatigue (LCF) behavior of base material (BM) AA6061-T6 and its FSW joint, revealing lower fatigue life in various zones of FSW joints compared to the BM. The loss of precipitate hardening in the FSW joint leads to increased fatigue damage due to process-induced softening. Specimens from the heat-affected zone (HAZ) on the advancing and retreating sides exhibit lower fatigue life than the nugget/stir zone (SZ) due to crack-susceptible boundaries. All zone-wise specimens of AA6061 FSW joints display notable cyclic strain hardening compared to the base material. SZ specimens show slightly higher stress amplitude and plastic strain energy density than HAZ specimens, attributed to larger grains in the HAZ.

LCF behavior of 2024AA under uni- and biaxial loading taking into account creep pre-deformation

This study presents the results of experimental low-cycle fatigue (LCF) tests of aluminum 2024 alloy T3511 temper in uni- and biaxial loading states. Tests were carried out on both the as-received material (hardened extruded rods) and material with different pre-deformation histories. These deformations were carried out in the creep process at 200 °C and 300 °C for two different levels of at each temperature. The pre-deformed material’s basic fatigue characteristics were determined and compared with the appropriate characteristics of the as-received material. In-depth macro- and microscopic analysis (SEM) of fracture surfaces was done. The effect of preliminary creep on LCF behavior of investigated alloy was characterized for both uni- and biaxial loading. An increase of fatigue life occurs for large plastic strains in the case of cyclic tension/compression. For in-phase biaxial loading, improvement of life is observed only for material with pre-deformation at 300 °C. Crack initiates in the plane of maximum shear strains for both biaxial loading and pure torsion. For tension/compression – in the plane of maximum principal stress (strain).

Influence of dynamic strain aging on low-cycle fatigue behavior in nano-sized κ-carbide strengthened FeMnAlC lightweight steel

The effects of dynamic strain aging (DSA) on the cyclic response and deformation behavior of a hot rolled Fe–22Mn–8Al–0.9C–0.02Nb (wt.%) lightweight steel have been investigated during strain-controlled low-cycle fatigue (LCF) tests at the temperature range of 25–400 °C. The initial microstructure consisted of a complete γ-matrix with nano-sized κ-carbides. During LCF testing, the cyclic stress amplitude displayed a monotonous softening until fracture at 25 °C, 100 °C, and 400 °C in all the investigated strain ranges (Δεt) of 0.8–2.2 %. However, at 200 °C and 300 °C, cyclic softening was followed by pronounced cyclic hardening. Notably, hysteresis loops at the cycle of half-life exhibited serrations in the flow stress at 200 °C and 300 °C. This indicated that cyclic hardening was attributed to the DSA phenomenon, which was confirmed by a negative strain rate sensitivity at 200 °C and 300 °C. A significant reduction in LCF life was observed under the DSA regime. During LCF testing, sequential transmission electron microscopic (TEM) observation indicated that deformation led to the formation of shear bands in multiple directions. Gliding of wavy dislocations was frequently observed at 200 °C compared to that at 25 °C, which resulted from the dragging effect by carbon solute atoms.

Combined high and low cycle fatigue analysis of FGH96 alloy under high temperature conditions

The FGH96 alloy as a common material for preparing turbine discs, can withstand high- and low-cycle combined loading during service, which significantly affects its fatigue performance. In this work, the combined high and low cycle fatigue (CCF) behavior of the FGH96 alloy was explored at various low cycle stress amplitudes (600, 630, 650, 680, and 700 MPa) using a self-improved experimental platform at a high temperature of 650 °C. The results indicate that increasing the amplitude of the low-cycle stress decreases the service endurance of the FGH96 alloy at CCF condition. Under the same low cycle stress amplitude, its CCF life of alloy is lower than the pure low cycle fatigue life. A fatigue service life prediction model, grounded in the theory of crystal plasticity, has been developed to assess the combined fatigue service life across different low-cycle stress amplitudes. A comparison between the predicted and actual lifespan showed that the predicted lifetime fell within a range three times the scattering band.

Fatigue Life and Crack Growth Rate Prediction of Additively Manufactured 17-4 PH Stainless Steel using Machine Learning

The present work is focused on machine learning-assisted predictions of the low cycle fatigue behaviour and fatigue crack growth rate (FCGR) of 17- 4 PH SS processed through L-PBF and post-processing. Various machine learning techniques reported in the literature provided a flexible approach for explaining the complex mathematical interrelationship among processing-structure-property of the materials. In the present work, four machine learning (ML) algorithms, such as K- Nearest Neighbor (KNN), Decision Trees (DT), Random Forests (RF), and Extreme Gradient Boosting (XGB) algorithms, are implemented to analyze the Fatigue Crack growth rate (FCGR) of 17-4 PH SS alloy. After optimizing the hyper parameters for these algorithms, the trained models were found to estimate the unseen data as equally well as the trained data. The four tested ML models are compared among each other over the training as well as the testing phase based on their mean squared error and R2 scores. Extreme Gradient Boosting model has performed better for the FCGR predictions providing the least mean squared errors and higher R2 scores compared to other models.

An Overview of the mechanism-based thermomechanical fatigue method (DTMF) in the design of automotive components

The DTMF damage Parameter described in the paper is the generalization of the creep fatigue parameter (DCF) proposed by Riedel, which quantifies the amount of damage produced by a non-isothermal loading cycle. The entire design methodology is outlined in the text, starting with the calibration of Chaboche's viscoplastic model and then the determination of the thermomechanical fatigue (TMF) parameters. Chaboche is generally a good choice for its ability to describe well both kinematic and isotropic hardening behaviors, also enabling stress relaxation and strain rate dependency to be considered. The DTMF method is based on the fracture mechanics concept of cyclic crack tip opening displacement (ΔCTOD) where the effective stress range takes the crack closure effect into account. This method is typically used to describe the thermomechanical low cycle fatigue behavior of cast irons, cast steels, stainless steels, Inconel, HiSiMo and nickel-based alloys. These materials are used in components that need to withstand high temperatures (higher than half of the melting point) in service as exhaust manifolds, cylinder heads, valves, turbochargers and disk brakes. Three failure mechanisms are detailed here: oxidation, creep and fatigue. Oxidation is a complex phenomenon that may occur when the material is hot under tensile in-phase loading or hot under compressive out-of-phase loading. Creep is a time and temperature dependent diffusion process which is incorporated into the DTMF parameter as a multiplicative factor. In this context fatigue life is defined as the number of cycles to propagate an existent microcrack up to an arbitrary size that is defined on a case-by-case basis.

Low-Cycle Fatigue Behavior of Hydrostatic Extruded AZ80 Mg Alloy

The fatigue properties of conventional and hydrostatic extruded AZ80 Mg alloy were fully studied under strain-controlled mode. The hydrostatic extrusion can effectively increase the tensile strength, but decrease the elongation. Consequently, the low cycle fatigue lifespan of the hydrostatic extruded specimens was shorter than that of the conventional extruded ones. The Manson-Coffin-Basquin relationship was adopted to describe the fatigue lifespan of the extruded AZ80 alloys. Finally, the fracture surfaces were observed by scanning electron microscope. The fatigue crack stable propagation zone of the hydrostatic extruded sample has decreased significantly compared with that of the conventional extruded sample. The hydrostatic extruded specimen has small reverse plastic zone size, which leads to microscopically rough fatigue crack propagation zone.

Low cycle fatigue and creep-fatigue interaction behavior of 2.25CrMoV steel at high temperature

This paper presents a comprehensive investigation into the low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of 2.25CrMoV steel at an elevated temperature of 455 °C. The LCF tests are conducted using a triangular waveform with varying strain amplitudes ranging from 0.4 % to 0.7 %. Furthermore, the CFI tests are conducted using a trapezoidal waveform with different hold times and hold directions. To gain further insights, the microstructure and fracture behavior of the steel are characterized using optical, scanning, and transmission electron microscopy techniques. The results reveal that the softening behavior and fatigue life degradation of the steel are dependent on the applied strain amplitude, hold time, and hold direction. Higher strain amplitudes in the LCF tests lead to an increased number of crack initiation sources. During the CFI tests, fatigue fracture is identified as the primary failure mechanism under tensile hold conditions, and an increase in hold time promotes crack propagation. The interaction between fatigue and creep is found to be more significant in compressive hold and tensile compressive combination hold conditions. Microstructure analysis demonstrates that the lath structure recovers during cyclic loading, resulting in cyclic softening. Finally, a modified energy-based model is proposed to distinguish the different roles of tensile energy and compressive energy in the fatigue process. The proposed model accurately predicts the fatigue life of the 2.25CrMoV steel, as demonstrated by the good agreement between the experimental and predicted results.