Stress-controlled Low Cycle Fatigue Research Articles

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.

Crystallographic micro-mechanism of faceted crack initiation in near-α titanium alloy Ti6321 under room-temperature fatigue and dwell fatigue loadings

Crack initiation mechanism of dwell fatigue has always been a key problem in rationalizing the dwell effect, and it is not completely understood yet. This study conducted stress-controlled low-cycle fatigue and dwell fatigue tests on Ti-6Al-3Nb-2Zr-1Mo alloy with bimodal microstructure to reveal its microstructural characteristics and crack initiation mechanisms. The study demonstrated that the faceted primary α nodules located near the specimen surface acted as crack initiation sites during both fatigue and dwell fatigue tests. Slip trace analysis revealed that faceted cracking occurred at (0001) basal plane with the maximum Schmid factor value through a special cracking mode referred to as (0001) twist boundary cracking. Innovative criteria of parameters C1 and C2 were proposed based on experimental observation and molecular dynamics simulations, which well identify candidates for (0001) twist boundary crack nucleation. It demonstrated that grain pairs combining a moderately high Schmid factor for basal slip and a well-orientated Burgers vector in the out-of-surface plane was the preferable location for surface (0001) twist-boundary crack initiation, and grain pairs combining a high Schmid factor for basal slip and a high normal stress on basal plane are perfect candidates for subsurface cracking. Based on this, phenomenological models are proposed to explain the surface (0001) twist-boundary cracking mechanism from the perspective of surface extrusion-intrusion-induced micro-notches.

Influence of build direction on the ratchetting-fatigue interaction of heat-treated additively manufactured 316L stainless steel

This study has investigated the cyclic deformation and ratchetting-fatigue interaction of 316L stainless steel fabricated by laser powder bed fusion (LPBF) and subjected to a 900 °C/2 h post-heat treatment, considering both the vertical and horizontal orientations. Strain- and stress-controlled low cycle fatigue (LCF) tests are conducted to explore the cyclic deformation and the effects of the mean stress and stress amplitude on the fatigue life and ratchetting deformation. A detailed analysis of the cyclic deformation mechanism is conducted through X-ray microtomography and transmission electron microscope (TEM) observations. The results show that post-heat treated LPBF 316L steel exhibits cyclic hardening followed by a non-saturated cyclic softening stage. This behavior is attributed to the evolution of dislocation density and dislocation patterns, and the formation of surface cracks. The ratchetting strain and its rate are sensitive to the mean stress and stress amplitude. The fatigue lives of vertically built specimens are slightly higher under strain-controlled loading conditions, but significantly lower under stress-controlled loading conditions than those of horizontally built ones. Furthermore, ratchetting deformation can promote fatigue damage, resulting in a reduction in fatigue life.

The Stress-Controlled Low Cycle Fatigue Properties of HK40 and HP40 Heat-Resistant Fe–Ni Base Alloys

The Stress-Controlled Low Cycle Fatigue Properties of HK40 and HP40 Heat-Resistant Fe–Ni Base Alloys

Low cycle fatigue lifetime and deformation behaviour prediction of nickel-based single crystal superalloy considering thickness debit effect

Through stress-controlled low cycle fatigue (LCF) experiments, LCF performances for thickness debit effect of nickel-based single crystal superalloy DD6 with [001] orientation are investigated. Based on scanning electron microscope (SEM) observation, the formation mechanism of thickness debit effect and the damage mechanism under LCF loads are revealed. Then, considering the thickness debit effect of nickel-based single crystal superalloy DD6, an improved slip-based damage model is developed. Predicted LCF lifetimes are mainly within a scatter band of factor 2, and the predicted laws between LCF lifetimes and wall-thicknesses are in good agreement with the experimental data, which verifies the rationality and accuracy of the thickness-sensitive LCF damage model established in this paper. Furthermore, a damage-coupled crystallographic constitutive model reflecting thickness debit effect is developed. For DD6 specimens with different wall-thicknesses, the predicted results of the stress-controlled LCF deformation behaviours including the whole-lifetime ratcheting behaviours show good agreement with the experimental data.

Investigation on uniaxial ratcheting fatigue behaviors and microstructure evolution of ultrafine-grained 6061 aluminum alloy

The motivation of the present study is to explore ratcheting-fatigue behavior and the corresponding micro-deformation mechanism of ultrafine-grained aluminum alloys 6061 (UFG AA6061) under two stress-controlled low-cycle fatigue (LCF) loading modes, i.e., mean tension stress (MTS) and mean compressive stress (MCS). The uniaxial ratcheting tests demonstrate that the fatigue-ratcheting damage of UFG AA6061 is prone to occur under MCS-LCF loading, rather than MTS-LCF loading. Detailed microstructure characterizations by Electron backscatter diffraction (EBSD) techniques under these two loading modes show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries increase remarkably in the case of MCS-LCF loading. These observations are mainly ascribed to the room temperature continuous dynamic recrystallization (CDRX), in which low angle boundaries evolve into high angle grain boundaries and the formation of equiaxed grains is accelerated. In contrast, there were no obvious change in the grain aspect ratios or the fractions of high angle boundaries at MTS-LCF loading condition, while the Schmid factor of non-octahedral slip system {100} <110> increases and the texture evolve gradually. The deformation mechanism is grain boundary sliding (GBS) and grain rotation would accommodate the process of GBS. An analysis of the microstructure-properties relationship showed the superposition effect of residual shear deformation and uniaxial LCF ratcheting deformation is responsible for the CDRX at room temperature. And the occurrence of GBS is mainly caused by the activation of non-octahedral slip system.

Crystal plasticity theory coupled with meso-damage to predict the ratchetting behavior of nickel-based single crystal superalloy

Stress-controlled low cycle fatigue tests with different mean stress of a second-generation nickel-based single crystal superalloy DD6 with [001] orientation is investigated in the current study. The observation of fracture surface and microstructure show that although the phase structure does not change for different conditions, the damage failure mode changes for different mean stress. It is found that ratcheting strain exhibits a significant dependency on mean stress and microstructure damage. Ratcheting strain cannot be simulated only considering the evolution of back stress. To simulate the whole process evolution of ratcheting strain, crystal plasticity constitutive model coupled with a new anisotropic damage is developed. The simulated results showed that the anisotropic damage model could accurately predict the ratcheting strain for different mean stress. Different damage evolution modes are well identified by a damage index in the proposed model.

Damage evolution of extruded magnesium alloy from deformation twinning and dislocation slipping in uniaxial stress-controlled low cycle fatigue

The damage evolution of an extruded AZ31 magnesium alloy in uniaxial stress-controlled low cycle fatigue is investigated using a quasi-in-situ method. In the three studied samples, that is, slipping dominated (SD), twinning/detwinning/basal slipping (TDBS), and twinning/detwinning/slipping (TDS), the fractions of slip bands and/or twins keep increasing till a critical cyclic number, and are essentially unchanged in the subsequent cycles. Inter-granular micro-crack initiation is found to be favorable in the TDS and SD samples. While both the inter-granular and intra-granular micro-cracks are detected in the TDBS sample. Moreover, the non-Schmid behavior in the crack trans-granular propagation path is investigated.

Development of a new ratcheting fatigue life prediction model of a nickel-based superalloy based on ratcheting strain rate

The stress-controlled low cycle fatigue and ratcheting fatigue behaviors of a nickel-based superalloy were proposed under a series of symmetric and asymmetric cyclic loadings at 650 °C to explore the effects of stress amplitude and mean stress on fatigue life. The results show that ratcheting strain increases with cycle numbers increasing. Ratcheting strain rate curves exhibit three different stages including deceleration, steady state and acceleration growth stage. A fatigue life prediction model based on the ratcheting strain rate considering stress amplitude and mean stress was conducted to predict the fatigue life. The predicted fatigue life agrees with the measured fatigue life very well, and all the data points are located in 1.25X scatter band based on the model.

Effect of nano-clay addition and heat-treatment on tensile and stress-controlled low-cycle fatigue behaviors of aluminum-silicon alloy: Effect of nano-clay addition and heat-treatment

The objective of the present paper is to investigate the stress-controlled low-cycle fatigue behavior of piston aluminum-silicon (AlSi) alloy reinforced with nano-clay particles and T6 heat-treatment. The piston aluminum-silicon alloy strengthened by 1 wt.% nano-clay particles were prepared by the stir casting method and then subjected to the heat-treatment. The optical microscopy analysis demonstrates that heat-treatment changed the size, morphology, and distribution of silicon phases through the microstructure of the aluminum matrix. In addition to tensile tests, stress-controlled low-cycle fatigue experiments at different loading conditions including the variation of the mean stress, the stress rate, and the stress amplitude were conducted at room temperature. The obtained experimental results showed no clear improvement in either mechanical or fatigue properties of the material. Moreover, the density measurements using the Archimedes method reveal a higher content of the porosity in nano-composite. It was observed that the reinforcement (nano-particles and heat-treatment) can change the cyclic behavior of the AlSi alloy, significantly. The cyclic hardening feature of the AlSi alloy changed to cyclic softening and also the fatigue lifetime and the ratcheting resistance decreased after the nano-particles addition and heat-treatment. Through the microstructural analysis, it was indicated that the neglecting of higher kinematics of age hardening in nano-composite was the major source of mechanical properties reduction. In the end, it was shown that the fatigue lifetime of samples can be described adequately utilizing a modified plastic strain energy technique considering the mean stress effect.

Effect of shot peening on notched fatigue performance of powder metallurgy Udimet 720Li superalloy

The tensile and stress-controlled LCF tests of a powder metallurgy (PM) superalloy, Udimet 720Li have been conducted at 650 °C in order to investigate the effect of shot peening (SP) on notched low cycle fatigue (LCF) performance. The measurements of surface tomography, Micro-Vickers hardness, residual stress and failure mechanism analysis were conducted in detail. The results show that there are many regularly shaped machining traces on as-received (AS) surface while lots of highly disorganized dimples are mainly distributed on SP surface. A larger microhardness gradient is produced along the path from SP surface to the bulk of material compared with AS specimen. The maximum compressive residual stress of SP specimen is not more located at surface like AS one but at a depth of approximately 50–90 μm from surface. Compared with the applied stress, the effective stress curve of SP specimen becomes “flat” and the effective stress gradient decreases accordingly. The LCF lifetime of the notched specimen is significantly improved after SP process and the LCF lifetime increment is between 2.05 and 7.66 times. Finally, there are obvious differences on the failure mechanisms, such as fatigue crack initiation, propagation and final fracture between AS and SP specimens.

Low Cycle Fatigue Behavior and Failure Mechanism of Wire Arc Additive Manufacturing 16MND5 Bainitic Steel

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.

Stress-controlled LCF experiments and ratcheting behaviour simulation of a nickel-based single crystal superalloy with [001] orientation

Uniaxial ratcheting behaviour and low cycle fatigue (LCF) failure mechanism of nickel-based single crystal superalloy DD6 with [001] orientation are investigated through the stress-controlled LCF tests with stress ratio of −1. Then the deformation behaviour during the whole-lifetime from the beginning of the experiment to the fracture of the specimen, as well as the fractographic/metallographic morphology, are compared with the strain-controlled LCF experimental results. Through the scanning electron microscope (SEM) observations, it is shown that the failure characteristics under stress-controlled LCF loading are similar with those under strain-controlled loading. Nevertheless, unlike strain-controlled LCF loading, even under fully reversed cycle loading for stress-controlled LCF, DD6 shows significant ratcheting behaviour due to the tension-compression asymmetry. In addition, the LCF lifetimes under stress control are significantly shorter than the LCF lifetimes under strain control, and the culprit might be the detrimental effect of ratcheting strain on LCF lifetime. Based on these phenomena, an improved crystal plasticity constitutive model on the basis of slip-based Walker constitutive model is developed through modifying the kinematic hardening rule in order to overcome the inaccurate prediction of decelerating stage and stable stage of ratcheting behaviour. Furthermore, combining the continuum damage mechanics, a damage-coupled crystal plasticity constitutive model is proposed to reflect the damage behaviour of DD6 and the accelerating stage of ratcheting behaviour. The simulation results for the stress-controlled LCF deformation behaviour including the whole-lifetime ratcheting behaviour show good agreement with the experimental data.

A low-cycle fatigue life-prediction model for SUS301L stainless steel butt-welded joint with considering ratchetting

According to the existing experimental investigation on the ratchetting-fatigue interaction of SUS301L stainless steel butt-welded joint, a new life-prediction model addressing the important role of ratchetting is developed based on the ductility exhaustion theory (DET) and continuum damage mechanics (CDM). The total ductility damage is assumed to consist of fatigue ductility and ratchetting ductility ones, and the ductility and corresponding damage variables are defined in a strain-based form in the proposed life-prediction model. The proposed model is employed to predict the stress-controlled low-cycle fatigue life of SUS301L stainless steel butt-welded joint based on the local strain in its fusion zone FZ-1 measured directly by using the digital image correlation (DIC) technology, and the predicted results show a good agreement with the experimental ones.

Evaluation of Fatigue Behavior of Silicon Carbide Particles Reinforced with Magnesium Alloy Metal Matrix Composites

To evaluate the fatigue behaviors of AZ61 magnesium alloy with different weight percentages (0, 1 and 2) of silicon carbide particles (SiCp) were fabricated through gravity casting method. In addition, stress-controlled low-cycle fatigue test of SiCp reinforced magnesium alloys AZ61 were performed in ambient atmosphere at room temperature using ASTM 606 standard specimens. Fatigue measurement results proved, that the fatigue life of SiCp reinforced metal matrix composites (MMCs) decreased with increasing SiCp content. However, the results of the cyclic ductility decreased owing to the presence of significant amount of SiCp, which induces the brittleness of fatigue properties. This is probably occurring because of increasing the SiCp content in the matrix causes highly localized plastic strain. In addition, a high concentration of stress results around the reinforcements particles regions initiate the crack leading to rapid failure of MMCs. Therefore, the SiCp did not act as a stress reliever and it behaves in a brittle manner for the crack propagation through the particles.

Experimental Investigation on the LCF Behavior Affected by Manufacturing Defects and Creep Damage of One Selective Laser Melting Nickel-Based Superalloy at 815 °C

Uniaxial tensile tests and stress-controlled low-cycle fatigue (LCF) and creep–fatigue interaction (CFI) tests of Inconel 625 alloy manufactured by selective laser melting (SLM) were performed at 815 °C in air environments. The microstructure was characterized by optical microscopy and scanning electron microscopy after testing. The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects. The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain; however, the selected dwell time (i.e., 60 s and 300 s) exhibits low sensitivity to the fracture time and elongation to failure. The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries. Due to the quality of the SLM process, the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles, which result in the large dispersion of LCF life. Meanwhile, due to the accumulation of creep damage, cracks in the CFI test are initiated along the grain boundaries and then linked together, contributing to a significant decline in fatigue life.

The difference in low cycle fatigue behavior of CP-Ti under fully reversed strain and stress controlled modes along rolling direction

In order to understand the effects of cyclic asymmetry and control mode on low cycle fatigue (LCF) behavior of CP-Ti along rolling direction (RD), symmetrical strain and stress-controlled fatigue tests were conducted in this paper. LCF behavior of CP-Ti along RD under different control modes was systematically compared from the aspects of cyclic response, microstructure evolution and fatigue life with the technique of electron backscatter diffraction (EBSD) and scanning electron microscope (SEM). It was found that cyclic softening resulted by dislocation slip dominates strain-controlled fatigue. On the contrary, cyclic hardening dominates stress-controlled fatigue at high stress amplitude, leading to a higher cyclic stress-strain curve. Owing to asymmetrical hysteresis loop, compressive ratcheting strain is accumulated in stress-controlled LCF test despite of the fully reversed stress cycling. EBSD results show there is a transition of deformation mechanism from dislocation slip to dislocation slip and twinning dominated cyclic deformation with the increase of compressive ratcheting strain. Meanwhile, GOS analysis and SEM observation indicate that the appearance of massive twins results in the heterogeneous plastic deformation and micro-cracks are favorable to be initiated in the twin abundant area. Besides, compressive ratcheting strain has significant effects on fatigue crack growth as the fracture surface presents the serious abrasion of cleavage facets and fatigue striations. Finally, strain-life curves under different control modes are identical, indicating that fatigue life along RD is insensitive to compressive ratcheting strain under the interaction of these factors.

Methodology for Stress-Controlled Fatigue Test Under In-Air and Pressurized Water Reactor Coolant Water Condition and to Evaluate the Effect of Pressurized Water Reactor Water and Loading Rate on Ratcheting

This work investigates the behavior of 316 stainless steel (SS) under stress-controlled low cycle fatigue loading. Several fatigue experiments are conducted under different environment such as in air at 300 °C and primary loop water conditions for a pressurized water reactor (PWR). Two different loading conditions are also employed to examine the effect of stress rate on material hardening and ratcheting. During PWR water test, actuator position measurements are used to determine the strain of the specimen. Under PWR environment, 316 SS is found to ratchet to a significantly greater degree compared with in air. At slow stress rate, higher amount of cyclic hardening is observed in 316 SS, and slow stress rate increases the rate of ratcheting. Results also indicate that 316 SS exhibits asymptotic strain response at higher stress loading which can cause material to behave very differently under same stress cyclic loading.

Ratcheting strain and its effect on low cycle fatigue behavior of Al 7075-T6 alloy

The aim of this investigation was to examine the role of accumulation of ratcheting strain on low cycle fatigue behavior of aluminum 7075-T6 alloy. For this purpose, first ratcheting behavior of the alloy was studied using asymmetric stress-controlled low cycle fatigue tests up to 2000 cycles. Post-ratcheting stain-controlled low cycle fatigue tests were done on ratcheted specimens up to failure. Further, stain-controlled low cycle fatigue tests without any previous ratcheting were also conducted up to failure in order to compare the behavior with and without previous ratcheting. A few ratcheted specimens were studied using transmission electron microscope to assess the dislocation features generated due to ratcheting. The results indicated that increase of either stress amplitude or mean stress increased the ratcheting strain accumulated in the material. Ladder-like dislocation structure prevailed in the specimens deformed at lower stress levels while dislocation tangles were predominantly present in specimens deformed at higher stresses. Analysis of post-ratcheting low cycle fatigue behavior showed that fatigue life of the alloy decreased with the increase of previously imposed ratcheting strain. Comparison of low cycle fatigue behavior of the investigated alloy with and without ratcheting indicated that ratcheting strain was detrimental to fatigue life of aluminum 7075-T6 alloy.

Behavior of 321 stainless steel under engineering stress and strain controlled fatigue

Cyclic deformation and fracture behavior of the 321 stainless steel are investigated at 633K under engineering-stress and strain controlled low cycle fatigue (LCF) cycling, and the fatigue life data are compared with ASME section-III Division-1 S-N curve. In stress-controlled LCF tests, ratcheting-strain accumulation is observed even under zero mean-stress, and the direction of ratcheting is found to strongly depend on initial-ramp direction of first-cycle. In addition, plastic-strain range and striation width are observed to be higher than those in strain-controlled tests. The stress-life curve obtained from stress-controlled tests is 2–3 orders of magnitude lower in comparison to stress-life curve derived from strain-controlled LCF tests as per the ASME procedure. The discrepancy is attributed to the differences in deformation and damage development in stress and strain-controlled cycling.