Search

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

The tensile, creep, fatigue and creep-fatigue tests of the nickel-based superalloy GH4169 were carried out. According to the deformation characteristics of GH4169 alloy, the Ohno-Karim kinematic model (O-K model) can be used to describe the tensile behavior. The creep constitutive model presented in this paper can be used to predict the three-stage creep characteristics of the GH4169 alloy. The modified Ohno-Karim kinematic hardening model, combined with an isotropic hardening model, can well predict the cyclic softening behavior of the material under symmetric loads and the mean stress relaxation behavior under asymmetric loads. Based on the modified Ohno-Karim kinematic hardening model, isotropic hardening model and creep constitutive model, a non-unified constitutive model was established. The creep-fatigue behavior of the GH4169 alloy under symmetric and asymmetric loads is simulated by using the non-unified constitutive model. The simulation results are very close to the experimental results; however, the prediction results of the time-dependent relaxation load are relatively small.

Multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis.

Over the past years, Al-Mg-Si alloys have been largely applied in automotive industry, which has required a deep knowledge of their mechanical properties and the influence of precipitates distribution on their mechanical behavior. This work evaluated the main mechanical properties of AA6005, AA6063, and AA6351 alloys by means of tensile and low cycle fatigue tests with 0.005 seg-1 deformation rate and 0.3% < εat

It has been proposed that a plastic strain surface is reset to a point and re-evolves every cycle to correctly evaluate the plastic strain range under cyclic loading (Ohno et al., 2018b). This resetting scheme has the advantage of providing a definite value for the evolution parameter irrespective of the amounts of cyclic hardening, pre-straining, and ratcheting. In the present study, the resetting scheme was experimentally verified by performing the following three types of uniaxial cyclic tests of 316 stainless steel at 600 °C: constant strain range, gradual decrease, and step decrease tests. The gradual decrease tests, a new type of cyclic tests, had gradual strain range reduction with increasing number of cycles in the absence and presence of ratcheting such as found at notch roots. Thus, the effect of the strain range history on cyclic hardening was observed to be weaker than that due to stepwise decreasing the strain range. Using a constitutive model with the resetting scheme enabled good simulation of the cyclic hardening behavior in the gradual decrease tests. Cyclic softening after stepwise decreasing the strain range was also simulated well by extending the evolution equation of cyclic hardening.

International audience; This study focuses on the low cycle fatigue mechanism of the quenched 9Ni martensitic steel containing retainedaustenite films at martensite laths. HRTEM and EBSD suggested that extrusions developed along Low AngleBoundaries or along High Angle Boundaries at low and high strain respectively. Short cracks grew along LAB atlow strain while they used any kind of boundary as well as crossed the martensite laths at high strain. Retainedaustenite allowed a flowing of matter along interfaces by acting as a lubricant. Consequently, the Coffin-Mansonrelation exhibited a bilinearity with a change in curve slope at Δεt = 1%.

To understand the premature-fracture mechanisms of long-term service damage of an advanced alloy’s (Chinese P92 steel) welded joint, the creep-fatigue (CF) experiments with holding times of 30, 120, 300, 600 and 900 s were individually performed at 923 K. The cyclic softening, inelastic-strain amplitudes and stress-relaxation behaviors were compared between welded and base-metal (BM) specimens. From the results, the failure stage of the welded specimens occupies 45% of the lifetime fraction, while it only takes up 20% of the lifetime fraction in BM specimens with short holding times (30 and 120 s). Furthermore, only two softening stages were observed for both kinds of CF specimens with long holding times. The absence of a third softening stage in longer-held specimens indicates that the processes of macroscopic-crack initiation, propagation and rupture were accelerated. Based on the observation of the fracture surfaces, the fracture mechanism shifted from fatigue-dominated damage to creep-fatigue interaction when the holding period was increased.

Standards for the fatigue testing of wearable sensing technologies are lacking. The majority of published fatigue tests for wearable sensors are performed on proof-of-concept stretch sensors fabricated from a variety of materials. Due to their flexibility and stretchability, polymers are often used in the fabrication of wearable sensors. Other materials, including textiles, carbon nanotubes, graphene, and conductive metals or inks, may be used in conjunction with polymers to fabricate wearable sensors. Depending on the combination of the materials used, the fatigue behaviors of wearable sensors can vary. Additionally, fatigue testing methodologies for the sensors also vary, with most tests focusing only on the low-cycle fatigue (LCF) regime, and few sensors are cycled until failure or runout are achieved. Fatigue life predictions of wearable sensors are also lacking. These issues make direct comparisons of wearable sensors difficult. To facilitate direct comparisons of wearable sensors and to move proof-of-concept sensors from “bench to bedside”, fatigue testing standards should be established. Further, both high-cycle fatigue (HCF) and failure data are needed to determine the appropriateness in the use, modification, development, and validation of fatigue life prediction models and to further the understanding of how cracks initiate and propagate in wearable sensing technologies.

In order to investigate the effect of non-proportional loading history on cyclic deformation of 316L stainless steel, a series of multiaxial cyclic experiments with variable loading path are conducted at room temperature. It is found that the materials exhibit cyclic hardening at initial stage, followed by a long period of cyclic softening. Significant cyclic softening is observed when non-proportional loading path changes into proportional loading path and the rate of cyclic softening is related to non-proportional loading history. A visco-plasticity constitutive model based on Ohno-Wang kinematic hardening rule and Marquis isotropic hardening rule is used to characterize non-proportional cyclic behavior. Simulation results agree with experiments very well in terms of cyclic hardening effects. However, this model is found to be less effective in simulating cyclic softening effect, especially considering the influence of non-proportional loading history. A modified isotropic hardening model is thus proposed to simulate the cyclic softening behavior of 316L stainless steel by introducing a memory non-proportionality and qualifying a partial recoverable softening term of isotropic hardening rule. Simulation results of the proposed model agree much better with experimental values.

The low-cycle fatigue (LCF) behaviour of AISI 4140 steel under annealed and as-received conditions was investigated at room temperature. The annealing treatment causes a marked decrease in mechanical strength but an increase in plastic energy and ductility. The annealing treatment of AISI 4140 steel significantly increases the LCF resistance of the material. In addition, the steel exhibits a transitional behaviour from initial cyclic softening to stable cyclic hardening with increasing strain amplitude. By contrast, the as-received AISI 4140 steel undergoes progressive cyclic softening until failure. Microstructural changes induced in the annealed steel cause near-Masing-type behaviour at low and high strain amplitudes. The enhancement of the LCF resistance of the annealed steel is attributed to the high magnitude of the compressive stress, which is dependent on the applied strain amplitude. A good correlation of the total failure cycles with plastic strain and elastic strain provides a precise equation model for predicting the LCF life of annealing-treated AISI 4140 steel.

We report that the uniform and stable microstructure essentially ameliorate the cyclic softening of the ultrafine-grained (UFG) Cu prepared by friction stir processing (FSP) and then enhance its low-cycle fatigue properties. In lieu of the fatigue-induced grain coarsening, dislocation and grain boundary (GB) activities play crucial roles in accommodating the cyclic plasticity. In addition, the protrusion and small cracks near the GB regions rather than shear bands are main surface damage morphologies that lead to its final fatigue failure.

A series of ratcheting tests on quenched–tempered 42CrMo4 steel with varying combinations of mean stresses and stress amplitudes were conducted at room temperature till failure. The ratcheting lives of the steel at different stresses were estimated using a few existing stress-based life prediction equations. A stress-based life prediction model was newly proposed. The experimental results revealed that ratcheting strain monotonically increased with mean stress and stress amplitude for all employed test parameters. The steel showed predominantly cyclic softening feature. The newly proposed model predicted ratcheting life efficiently in the life range of 102–104 cycles with a life factor of 1. It also efficiently predicted the fatigue life of other materials. Fractographic examinations revealed the standard fatigue fracture features.

Cyclic softening behavior at elevated temperature of nanolath strengthened steel was rationalized under different strain amplitudes. Results indicated that the evolution of internal stress components and the coarsening of subgrains are significantly depending on the plastic strain level. This paper proposes a dislocation-based viscoplastic constitutive model to represent the cyclic deformation response especially at low plastic strain level. The heterogeneous coarsening of subgrains is explicitly formulated via employing different evolution laws for multiple dislocation populations. An additional back-stress hardening component is introduced to account for the retarded cyclic softening. The cyclic deformation response and microstructural evolution of a nanolath strengthened steel at high/low plastic strain levels are perfectly reproduced.

Modeling of material behaviour under strain-controlled and stress-controlled loading for cyclically stable materials (CSMs) is generally done using kinematic hardening, neglecting the isotropic hardening component. Although this approach is able to simulate the hysteresis loops, the quantitative ratcheting characteristics are often not of the desired accuracy; thus earlier investigators have introduced different modifications in the kinematic hardening component for case to case basis to obtain better agreement to the experimentally observed cyclic-plastic behaviour. Combined hardening models, though used for cyclic softening/hardening materials, have not been used for cyclically stable materials to the best knowledge of the authors. The primary aim of this study is to improve the ratcheting predictions for CSMs considering the well-known Chaboche's combined hardening model using a new generalized methodology. The initial estimates of the hardening components are obtained from symmetric strain-controlled tests subjected to their refinement by genetic algorithm based optimization technique. The Chaboche's parameters have been evaluated using the proposed methodology for available reported experimental data on cyclically stable steels to demonstrate the response of materials under stress-controlled and strain-controlled loading conditions.

Uniaxial strain- and stress-controlled cyclic tests were performed at five prescribed temperatures (i.e., 25, 100, 150, 200 and 250 °C) to investigate the temperature-dependent uniaxial cyclic deformation (especially for ratchetting) of extruded AZ31B magnesium alloy. The results show that AZ31B magnesium alloy presents a remarkable temperature-dependent cyclic deformation, that is: obvious tension-compression asymmetry is observed at room temperature, but it becomes weaker and weaker and eventually disappears with the increase of test temperature; significant cyclic hardening occurs at low temperatures (lower than 150 °C), but a cyclic softening is eventually observed as the test temperature increases to a certain critical value (e.g., 150 °C); the ratchetting in the tensile direction becomes more significant as the test temperature increases, while the ratchetting in the compressive direction is relatively temperature-insensitive when the test temperature is below 150 °C; the effect of different peak/valley stresses on the ratchetting also depends on the test temperature, and is more significant at the temperatures lower than 200 °C. Such a specific ratchetting of AZ31B magnesium alloy originates from different mechanisms of plastic deformation presented at different temperatures.

A novel method of generating creep-fatigue interaction diagram is presented. Creep-fatigue interaction diagram for reduced activation ferritic–martensitic steel (RAFMS) is generated at 823 K by sequential tests decoupling fatigue and creep loading modes. Prior exposure to fatigue was found to be the life limiting factor compared to prior creep under creep-fatigue interaction, on account of the extensive cyclic softening occurring during the prior fatigue exposures.

We report the effect of cell structures on the fatigue behavior of additively manufactured (AM) 316L stainless steel (316LSS). Compared with the cell-free samples, the fatigue process of fully cellular samples only consists of steady and overload stages, without an initial softening stage. Moreover, the fully cellular sample possesses higher strength, lower cyclic softening rate and longer lifetime. Microscopic analyses show no difference in grain orientations, dimensions, and shapes. However, the fully cellular samples show planar dislocation structures, whereas the cell-free samples display wavy dislocation structures. The existence of cell structures promotes the activation of planar slip, delays strain localization, and ultimately enhances the fatigue performance of AM 316LSS. Funding: Swedish Governmental Agency for Innovation Systems (Vinnova)Vinnova [2016-05175]; Science Foundation Ireland (SFI)Science Foundation Ireland [16/RC/3872]; European Regional Development FundEuropean Commission; I-Form industry partners; Ji Hua Laboratroy [X210141TL210]; Center for Additive Manufacturing-metal (CAM2)

The strain-controlled low-cycle fatigue behavior of P91 steel is investigated under various strain amplitudes ranging from 0.20% to 1.0% at 25, 538, and 566 °C. The fatigue life decreases with an increase in the test temperature; however, a clear reduction is observed at low strain amplitudes, resulting in the degradation of the fatigue limits. The amount of cyclic softening increases with an increase in the strain amplitude and is significantly higher at elevated temperatures. Based on experimental results, Basquin-Coffin-Mason models are established to express the relationship between the strain amplitude-fatigue life curves and the cyclic stress-strain. The transition fatigue life increased from 6677 reversals at 25 °C to approximately 12000 reversals at the elevated temperatures of 538 and 566 °C. The more pronounced effect of temperature at the very low strain amplitude condition is attributed to the occurrence of multiple oxidation-enhanced crack initiations after long-term exposure to elevated temperatures.

Very high cycle fatigue (VHCF) tests on metastable austenitic stainless steels using an ultrasonic fatigue testing (USFT) system are challenging due to the transient material behavior and associated pronounced self-heating effects. Because the conventional measurement of stress-strain hysteresis is not possible in USFT, the cyclic deformation behavior can’t be described in a conventional manner. Hence, an energy-based approach is proposed for the characterization of cyclic deformation behavior of austenitic stainless steels in the VHCF regime at ambient temperature (AT) and elevated temperature. Therefore, in-situ dissipated energy and temperature measurements were performed. At AT both values underwent a change during cyclic loading, while at 300 °C only the dissipated energy changed. The investigated metastable austenitic stainless steel AISI 347 (X6CrNiNb1810, 1.4550) showed cyclic softening in the high cycle fatigue (HCF) regime (Nf

The effect of temperature on tensile behavior, low cycle fatigue, and microstructure evolution considering the damage mechanisms of Cr-based heat-resisting steel was investigated. The results present that significant monotonic and cyclic softening was observed at both room temperature and 630 °C. However, the cyclic softening factor is independent of the strain amplitudes and temperatures. At room temperature, the steady fatigue behavior until 80% fatigue life was presented, by contrast, great changes were offered below 80% fatigue life at 630 °C. this is directly related to the microstructure properties during fatigue tests, where dislocation network, wall, and dynamic recovery were formed, resulting from the dislocation movement, interaction, and annihilation. The density of dislocation and intensity of slip is inversely proportional to the temperature. Besides, a prediction fatigue life model fitted by the quadratic function was established through the hysteresis energy, which can consider the effect of temperature on the different fitting parameters and offer the method to optimize the fatigue behavior of Cr-based steel at different temperatures

The low cycle fatigue behaviour of a 9Cr1MoNbV martensitic steel has been investigated at 350 °C in air and in lead–bismuth eutectic (LBE) with low and high oxygen content. Total strain controlled tests were performed from Δet = 0.40% to 1.2%. The material exhibited a pronounced cyclic softening in both environments. LBE reduced the fatigue resistance. LBE accelerated the formation of the long crack by promoting the growth of the first short cracks. A clear change in propagation mode was observed. In air, ductile fatigue striations were observed while in LBE a brittle fracture decorated by voluminous and largely spaced striations were visible. From EBSD analysis, it was concluded that in LBE, the long crack advanced quickly by repeated and discontinuous cleavage.

Accepted: 2018-06-16, 資料番号: SA1180172000

New higher strength steels are required for deep and ultra-deepwater steel catenary risers (SCRs). In this work, the cyclic elastic-plastic-damage behaviour of X100Q, a candidate next-generation SCR material is experimentally characterised and modelled. The material is shown to exhibit early life (primary) fatigue damage followed by the more conventional (secondary) fatigue damage; as a result, it is necessary to demarcate the observed cyclic softening into dynamic recovery and damage-induced softening. An automated constitutive parameter optimisation process in combination with a new two-stage cyclic damage evolution model successfully predicts the effect of strain-range on damage evolution. The model is implemented in a user material (UMAT) subroutine for multiaxial application, within a hierarchical global-local modelling methodology for dynamic fatigue analysis of an SCR girth weld geometry. The interdependency between fatigue damage-induced material degradation and cyclic plasticity at the weld is shown for a range of load cases. This publication has emanated from research conducted with the financial support of Science Foundation Ireland as part of the MECHANNICS joint project between NUI Galway and University of Limerick under Grant Number SFI/14/IA/2604. The authors would also like to thank Dr. Adrian Connaire of Wood plc for his advice and support in providing the Flexcom software used in this publication, and the Irish centre for high-end computing (ICHEC) for the provision of computational resources used in this work. peer-reviewed 2020-06-21

This paper reports new dislocation configurations responsible for cyclic hardening/softening behaviour of a directionally solidified nickel base superalloy after relaxation fatigue tests at 850 °C with hold durations of 1 and 5 min. Dislocation interactions in the gamma (γ)-matrix and the formation of new dislocation configurations during relaxation fatigue (RF) test with 1 min hold duration is responsible for the cyclic hardening behaviour, whereas, the coarsening of gamma prime (γ′)-precipitates are attributed to the cyclic softening behaviour during 5 min hold testing.

High strength structural steels have recently achieved increasing application in civil and structural engineering worldwide. For instance, the sony center tower located in Berlin were constructed through steel truss made of S460 and S690 high strength steel whose nominal yield stress is 460 MPa and 690 MPa respectively. The utilization of high strength steel considerably reduced the structure deadweight and produced larger architecture space. Besides, steel connections of one military steel bridge in Mittadalenthe, Sweden were fabricated of 960 MPa ultra-high strength steel. This application remarkably increases the construction convenience since high strength steel presents excellent ability to reduce member size as well as improve structure performance. Nevertheless, limited test data have been available on the monotonic properties and cyclic behavior of these structural steels. In this study, uniaxial coupons were tested for four types of Chinese high strength structural steels, including grades Q460D, Q550D, Q690D as well as ultra-high strength structural steel Q890D, to investigate their stress-strain response and energy-dissipation behavior under monotonic and various cyclic strain demands. The monotonic test results of the tested steels were then compared with the requirements of current seismic design codes GB 50011-2010 and Eurocode8. Besides, cyclic strength, deformability and energy dissipation capacity were evaluated through the tested strain-stress loops. The experimental observations in cyclic tests show that cyclic softening, which significantly affects the shape of cyclic loops and energy dissipation capacity, is an important hysteretic characteristic for all tested high strength structural steels. Moreover, cyclic backbones of these four steels were calibrated using Ramberg-Osgood model. To fundamentally define the cyclic stress-strain relationship, essential parameters of three commonly used constitutive models, including Chaboche model, Dong-Shen model and Giuffre-Menegotto-Pinto model were calibrated based on the experimental data under cyclic loading. Those calibrated models were then employed in simulations of aforementioned cyclic loading tests and their validity were compared. Good agreements between experimental response and simulated performance by Dong-Shen model were achieved, demonstrating that the calibrated Dong-Shen model can elaborately describe the cyclic behavior of these four types of high strength structural steels.

In this work our goal is to better understand the origin of the cyclic accumulation of the inelastic strain (often called ratcheting) observed in 304L SS subjected to uniaxial cyclic stress control at room temperature. Recent works performed in the frame of small strain assumption attribute this phenomenon essentially to creep (Taleb, 2013). However, outside this frame, it seems that creep is not the only contributor in this phenomenon (Facheris, 2014). New experiments are performed here in order to investigate the role played by creep, cyclic softening, fatigue damage, the mode of control (engineering or true stress) and ratcheting in this observation.

Effect of tempering on low cycle fatigue (LCF) behaviors of nuclear-grade deposited weld metal was investigated, and The LCF tests were performed at 350 °C with strain amplitudes ranging from 0.2% to 0.6%. The results showed that at a low strain amplitude, deposited weld metal tempered for 1 h had a high fatigue resistance due to high yield strength, while at a high strain amplitude, the one tempered for 24 h had a superior fatigue resistance due to high ductility. Deposited weld metal tempered for 1 h exhibited cyclic hardening at the tested strain amplitudes. Deposited weld metal tempered for 24 h exhibited cyclic hardening at a low strain amplitude but cyclic softening at a high strain amplitude. Existence and decomposition of martensite-austenite (M-A) islands as well as dislocations activities contributed to fatigue property discrepancy among the two tempered deposited weld metal.

Ferritic-martensitic (F/M) steels are a common choice for high-pressure, high-temperature boilers and steam piping in power plants, but cyclic softening without saturation is a major degradation mechanism of F/M steels. This degradation mechanism is more pronounced and complex under creep-fatigue loadings. For reliable, but not overly conservative design a better understanding of this phenomenon is necessary. The intent of this work is to characterize and model the softening and lifetime behavior under creep-fatigue conditions at high temperature. Therefore, low-cycle fatigue (LCF) tests with hold time were performed at 550 °C. A distinct dependence of the softening behavior on hold time was found. Additionally, detrimental effect of compressive hold time on lifetime was also observed in this study. All effects of hold time were more pronounced for small strain amplitudes. Based on these experimental observations, a unified viscoplastic deformation and damage model is further developed to describe the high-temperature cyclic softening and lifetime behavior of F/M steels under creep-fatigue loading. An optimized parameter set for the extended model is determined based on experimental results. Good agreement of experimental results and model descriptions is achieved both under pure fatigue as well as under creep-fatigue conditions. The detrimental effect of compressive hold times on lifetime of ferritic-martensitic steels is well represented.

A full-scale test model with a distinct loading system is developed to analyze the dynamic change process of ballast beds under cyclic longitudinal reciprocated loading. The test analysis methodology accounts for the service performance, longitudinal resistances, and evolution trend of ballast beds under long-term loading. The analysis shows that the ballast resistance-displacement curves under cyclic loading are a set of closed hysteretic curves, indicating obvious energy consumption. In particular, a granular ballast bed is subject to cyclic softening during the cyclic process and the ballast longitudinal resistances degenerate obviously. More particularly, the cyclic softening of a granular ballast bed is dependent on the dynamic disturbance amplitude—the higher the dynamic disturbance amplitude, the more severe the cyclic softening will become. Moreover, this methodology contributes to forming a foundation for an in-depth understanding of the dynamic service performance of ballast CWR tracks and of the stress deformation mechanism of continuously welded rail tracks.

This paper presents fatigue behavior of ultrafine-grained (UFG) 5483 Al-Mg alloy processed by hydroextrusion in comparison to its coarse-grained counterpart (CG 5483) and precipitation strengthened CG 7475 alloy. The specimens were tested at low (−50 °C), ambient (25 °C) and elevated (100 °C) temperature in tension–tension (R = 0.1) fatigue under stress control. It was found that the decrease of temperatures leads to a shift of Wohler curves to higher stress values. Despite of similar YS, UFG 5483 alloy exhibited reduced fatigue strength compared with CG 7475 alloy. The changes in fatigue life were rationalized in terms of lower stability of UFG structure under cyclic loading. UFG 5483 and CG 7475 alloys were susceptible to cyclic softening attributed to fatigue-induced recovery process, whereas CG 5483 exhibited cyclic hardening accompanied by dislocation density increase.

Fatigue specimens of A508-3 steel were irradiated in the swimming-pool test reactor in China Institute of Atomic Energy, the fluence was 3 × 10 19 n/cm 2 at 300 °C, then low-cycle fatigue tests were carried out at ambient temperature, with the fatigue strain range is 0.32–1.8%. The results indicate that, irradiated A508-3 specimens exhibit cyclic softening and instability behavior during the test, and the cyclic softening rate increased with strain range increased; fatigue life decreased from 1.7 × 10 5 to about 5 × 10 2 , as the strain range increased from 0.32% to 1.8%, the fatigue life of A508-3 steel increased after the neutron irradiation; fatigue fracture initiated at the surface of specimen, and more individual cracks formed on the specimens of higher strain range compared with the specimens of lower strain range.

This paper presents the results of high temperature strain-range controlled low cycle fatigue tests of a laser powder bead fused AlSi10Mg alloy. Following stress relief (2hrs at 300 ℃), two cyclic loading waveforms (standard triangular and dwell-type trapezoidal waveforms) and three temperatures (100 ℃, 250 ℃ and 400 ℃) were applied to investigate both the mechanical response and the related microstructural changes of this additively manufactured (AM) aluminium alloy. The bulk mechanical responses were found to exhibit a continuous cyclic softening, decreasing stress relaxation and decreased energy dissipated per cycle. The stress relaxation is strongly affected by the test temperature rising to almost complete relaxation at 400 ℃. At lower temperatures (100 ℃ and 250 ℃), the higher the temperature the more subgrains are developed during cyclic loading. Up to 250 ℃, the subgrain size increases with temperature and laser powder bead fused defects pref­erentially act as the fatigue crack initiation sites. While at 400 ℃, coarse Si particles precipitate during cyclic deformation and a high density of microvoids are nucleated from these coarse Si precipitates, which grow and link up to cause failure, resulting in a dimple dominated ductile fracture. Jianguang Bao, Professor Adil Benaarbia and Professor Wei Sun would like to acknowledge the China Scholarship Council for the sponsorship of the PhD project for Jianguang Bao. Financial support through the Joint Fund of Large-scale Scientific Facility of National Natural Science Foundation of China (U2032121) is gratefully acknowledged. Sincere thanks are also due to Professor Yanan Fu from the SSRF for her technical assistance on performing the SR- μ CT exper­imental setup, and to Mr Shane Maskill at the University of Nottingham for his support during LCF tests. The Manchester (Henry Moseley) X-ray Imaging Facility was funded in part by the EPSRC (grants EP/F007906/1, EP/F001452/1 and EP/M010619/1).

In the present work, thermomechanical fatigue behavior and life prediction of P92 steel welded joints under various mechanical strain amplitudes and phase angles are investigated. Results demonstrate that the welded joints exhibit significant cyclic softening during fatigue process and the thermomechanical fatigue life is strain amplitude and phase angle dependent. The decrease in strain amplitude and the increase in phase angle induce the increase in fatigue life. Fracture analysis indicates that the fracture position changes significantly with the strain amplitude and phase angle. Finally, a modified energy-based life prediction model is proposed by considering the effects of phase angle and mean stress.

Strain controlled fatigue of P92 steel with various strain hold dwells introduced at the peak loading point were conducted at 625 °C. Two features which depend on the cycle and strain range level were observed under the fatigue-creep condition for the viscous and cyclic softening material. The first one is the accelerated cyclic softening response which is ascribed to the accumulated inelastic strain transformation from the creep mechanism during the strain dwell period and becomes more significant with the decrease of strain ranges. The second one is the decelerated stress relaxation behavior which is caused by the reduced viscous stress related to the continuous cyclic softening and fades with the decrease of cyclic strain ranges. Accordingly, a new unified viscoplastic constitutive model within the framework of Chaboche model was developed by improving the nonlinear isotropic hardening rule and the kinematic hardening rule with a cyclic softening parameter. As a result, the accelerated cyclic softening and decelerated stress relaxation response of fatigue-creep interaction was finely reproduced by the proposed model.

The aim of the present investigation is to construct a constitutive model suitable for describing the cyclic behavior of 10Ni5CrMoV high-strength steel, which displays both cyclic softening and asymptotic ratcheting. Uniaixal tests of monotonic loading, symmetrical and asymmetrical strain cycling and stress controlled cyclic loading are conducted. Results reveal that the material exhibits non-saturated cyclic softening with combined nonlinear and linear softening rates, both isotropic and kinematic hardening are responsible for cyclic softening. A long period of steady-state ratcheting has been experimentally detected. Cyclic softening is found to have a considerable effect on the uniaxial ratcheting of the material. Significantly larger monotonic yield stress than that at cyclic loading is also observed. The dependence of cyclic softening on strain amplitude is revealed by a low-high sequence strain controlled cyclic test. Based on the experimental findings, a new constitutive model extended from Abdel-Karim–Ohno model has been constructed for 10Ni5CrMoV steel. The identification procedure of the material parameters is scrutinized critically by quantifying the contributions of the isotropic hardening and kinematic hardening on cyclic softening. Verification of the proposed model is achieved by simulating uniaxial strain and stress controlled cyclic loading tests. Fairly good modeling results are obtained. The superiority of the proposed model over other kinds of Ohno–Wang type models is also illustrated.

TMF tests were carried out on EN-GJSA-XNiSiCr35-5-2 at constant minimum temperature (400 °C) and varying maximum temperatures (Tmax = 700 °C, 800 °C, 900 °C) with hold times of 180 s at Tmax and two phase angles (in-phase (IP), 180° out-of-phase (OP)). The results showed a comparable strength under OP- and IP-TMF loading. At Tmax = 700 °C and 900 °C, the lifetime in IP-tests was slightly longer than that of OP-tests, while it is vice versa at Tmax = 800 °C. The IP-tests at Tmax = 900 °C showed a similar lifetime as OP-tests at Tmax = 700 °C and 800 °C, which was unexpected for such a high testing temperature. All IP-tests at Tmax = 900 °C showed a continuous cyclic softening from the beginning on, which was different from all other testing conditions. Complementary metallographic investigations indicated that under this test condition, intergranular creep damage is present in the volume of the test pieces.

In order to study multiaxial fatigue behavior of 2024-T4 aluminum alloy with an aircraft in-service environment, fatigue tests of multiaxial pre-corrosion and alternate corrosion are carried out. With the extension of pre-corrosion time, the multiaxial fatigue life decreases under the same equivalent stress. Pitting caused by corrosion is observed on the surface of specimens. While pre-corrosion time increases, the number and size of pits become larger and connection between adjacent corrosion pits becomes more obvious. For the multiaxial fatigue tests with alternate corrosion, for the same unit corrosion time, the fatigue life decreases with the increase of unit loading cycles. Under the same unit loading cycles, the fatigue life decreases as the unit corrosion time increases. The cyclic hardening appears in both axial and tangential directions at the initial loading stage. Cyclic softening trend becomes obvious as the number of alternation increases. It is shown that cracks initiate at specimen surface and gradually extend to inside. Exfoliation corrosion morphology can be observed on specimen surface. Based on Miner’s rule and data of pre-corroded multiaxial fatigue test, the alternate corrosion-multiaxial fatigue lives are predicted.

Dislocation structures and their evolution of 304L stainless steel and weld metal made with ER308L stainless steel welding wire subjected to uniaxial symmetric strain-controlled loading and stress-controlled ratcheting loading were observed by transmission electron microscopy (TEM). The correlation between the cyclic response and the dislocation structure has been studied. The experiment results show that the cyclic behaviour of base metal and weld metal are different. The cyclic behaviour of the base metal consists of primary hardening, slight softening and secondary hardening, while the weld metal shows a short hardening within several cycles followed by the cyclic softening behaviour. The microscopic observations indicate that in base metal, the dislocation structures evolve from low density patterns to those with higher dislocation density during both strain cycling and ratcheting deformation. However, the dislocation structures of weld metal change oppositely form initial complicated structures to simple patterns and the dislocation density gradually decrease. The dislocation evolution presented during the strain cycling and ratcheting deformation is summarized, which can qualitatively explain the cyclic behaviour and the uniaxial ratcheting behaviour of two materials. Moreover, the dislocation evolution in the two types of tests is compared, which shows that the mean stress has an effect on the rate of dislocation evolution during the cyclic loading.

The low cycle fatigue (LCF) behavior of AZ31B magnesium alloy with fiber texture was investigated in two asymmetric loading manners with strain ratios of Re=0 (named as T-T loading manner) and Re=-∞(named as C-C loading manner), respectively. It was found that {10–12} extension twinning-detwinning occurred in the samples in both the manners. Although the characteristics of twinning-detwinning alternations in the two manners exhibited little difference, the mechanical behaviors were quite different. Detwinning was induced during tensile unloading in the T-T manner and resulted in an evident Bauschginer effect, and cyclic softening in the tension was observed. The LCF life in this case was 68 cycles. In contrast, cyclic hardening occurred in both the tension and compression in the C-C manner with a life of 53 cycles. The twinning-detwinning contribution to the plastic strain-range was reduced especially in the C-C manner. The difference of twinning-detwinning contribution to the plastic strain-range between the two loading manners was responsible for the different LCF lives.

In the present investigation uniaxial ratcheting behaviour of the 42CrMo4 steel has been investigated at ambient temperature using different combinations of mean stresses (σ m ) and stress amplitudes (σ a ) in two different heat treatment conditions. The results indicate that the ratcheting strain increases with increase in number of cycles for all combinations of σ m and σ a in both annealed and normalized conditions. Further, the value is less in the normalized condition and is attributed to the rapid attainment of stable dislocation configuration with fine grained structure accompanying it. The specimens subjected to ratcheting exhibit smaller grain size than the undeformed one and it decreases with increase in stress parameter. TEM results indicate that the dislocation substructure varies from thin stripes to walls and then to cells, as the cyclic load continuously increases. The tensile strength of the ratcheted samples reduces compared to the unratcheted one owing to cyclic softening associated with the steel in both annealed and normalized conditions.

Low cycle fatigue, tensile tests, and microstructure of MarBN steel at different temperatures are conducted. The results show that tensile strength decreases with increasing temperature. Continuous cyclic softening has occurred as increasing the temperature, which is associated with the laths and dynamic recrystallization at room temperature, and grain rotation at high temperature, respectively. The relationship between cyclic softening and microstructure at different temperatures is discussed. Besides, the fatigue life prediction model with hysteresis energy method considering the temperature is fitted by the quadratic function, which presents a good method and model to optimize the low cyclic fatigue life and properties.

Results of an extended TMF test program on grade P92 steel in the temperature range of 620 °C–300 °C, comprising in-phase (IP) and out-of-phase (OP) tests, partly performed with symmetric dwells at Tmax/Tmin, are presented. In contrast to previous studies, the low-strain regime is also illuminated, which approaches flexible operation in a power plant with start/stop cycles. At all strain amplitudes, the material performance is characterized by continuous cyclic softening, which is retarded in tests at lower strains but reaches similar magnitudes in the course of testing. In the investigated temperature range, the phase angle does not affect fatigue life in continuous experiments, whereas the IP condition is more detrimental in tests with dwells. Fractographic analyses indicate creep-dominated and fatigue-dominated damage for IP and OP, respectively. Analyses of the (micro)hardness distribution in the tested specimens suggest an enhanced microstructural softening in tests with dwell times for the low- but not for the high-strain regime. To rationalize the obtained fatigue data, the fracture-mechanics-based D T M F concept, which was developed for TMF life assessment of ductile alloys, was applied. It is found that the D T M F parameter correlates well with the measured fatigue lives, suggesting that subcritical growth of cracks (with sizes from a few microns to a few millimeters) governs failure in the investigated range of strain amplitudes.

A unified viscoplastic constitutive model coupled with a physically-based damage variable, is proposed to capture the cyclic mechanical behavior and microstructural evolution of the material at elevated temperature. The mechanical strength can be reduced by the decrease in the dislocation density, the coarsening of the martensitic lath and the loss of the martensitic structure under cyclic loading. The proposed physically-based damage variable is driven by the evolutions of dislocation density and martensitic lath width. The good comparisons with test results mean that the proposed model can reasonably model the cyclic elastic-viscoplastic constitutive behavior of the material at high temperature.

The cyclic softening behavior of reduced activation ferritic-martensitic (RAFM) steel was investigated by low-cycle fatigue tests with and without dwell times at 550 °C, and a mechanistic understanding of the softening was achieved by analyzing the grain boundaries and crystal orientation in the grains using the electron backscatter diffraction (EBSD) technique. The cyclic stress response of the steel was divided into two distinct rate stages with the number of cycles, namely, the initial rapid stage and the uniform softening stage. The former stage was dependent on and the latter was independent of the input strain amplitudes and fatigue modes. The softening in the two stages was led by both the disappearance of lath and packet boundaries and the release of lattice distortion by dislocations in the initial RAFM steel; their relative contributions in each stage were different depending on the input strain ranges and fatigue modes. A new equation was proposed as a measure of the lattice distortion in grains, which showed a strong correlation with the amount of cyclic softening under different fatigue conditions.

In this paper, the analysis of the low cycle fatigue behavior of P91 steel is presented. First, experimental tests are performed to recognize different aspects of material behavior. Then an appropriate constitutive model is developed within the framework of thermodynamics of irreversible processes with internal state variables. Two different approaches to fatigue damage modeling are applied comparatively to describe the final stage of material cyclic softening. The material parameters are identified, and the model is validated based on the available experimental data.

The strain-controlled fatigue behavior of cold-drawn type 316 austenitic stainless steel has been investigated at strain amplitudes varying from 0.22% to 0.75% at room temperature. The results showed that continuous cyclic softening occurs over the whole lifetime at strain amplitude higher than 0.35% in the 20% cold-drawn steel, and 0.31% in the 30% cold-drawn steel, which implies that there is a threshold strain required for cyclic softening during fatigue. The 30% cold-drawn steel displays higher elastic strain amplitudes, resulting in higher fatigue life at strain ranges below 0.80%, while the 20% cold-drawn steel exhibits higher plastic strain amplitudes, leading to higher fatigue resistance at strain ranges above 0.80%. The interactions between dislocations and deformation twins activated during prior cold-drawing process were observed under cycle loading, which play a significant role in the evolution of dislocation structure and subsequently influence the fatigue property.

This study deals with the effect of the loading history on the cyclic behavior and the fatigue life of a 304L stainless steel at room temperature. The experiments have been performed using two specimens’ categories. The first one (virgin) has been submitted to only classical fatigue tests while in the second category, prior to the fatigue test; the specimen was subjected to a pre-hardening process under either monotonic or cyclic strain control. Cyclic softening followed by cyclic hardening are observed for the virgin specimens while only cyclic softening is exhibited by the pre-hardened specimens. The obtained results show that fatigue life is strongly influenced by the pre-hardening: the latter seems beneficial under stress control but detrimental under strain control, even in the presence of a compressive mean stress. The results are discussed regarding the cyclic evolution of the elastic modulus as well as the isotropic and kinematic parts of the strain hardening in different configurations: with or without pre-hardening, stress or strain control.

Fatigue performance of ferrite–martensite (FM) and ferrite–bainite (FB) dual-phase (DP) steels used in automotive wheels has been compared in terms of (i) high-cycle fatigue performance and failure mechanisms and (b) low-cycle fatigue performance (Δet/2 = 0.002 to 0.01) and associated deformation mechanisms. FBDP steel exhibits moderately better high-cycle fatigue performance, owing to delay in microcrack initiation. In FBDP steel, microcracks initiate predominantly along ferrite grain boundaries, while that at FB interface is significantly delayed in comparison with FMDP steel, where few microcracks appear at FM interface even below the endurance limit. During low-cycle fatigue, however, FMDP steel performs considerably better than FBDP steel till Δet/2 ≤ 0.005 attributed to initial cyclic hardening, followed by cyclically stable behaviour exhibited by FMDP steel. In sharp contrast, at all Δet/2 > 0.002, FBDP steel undergoes continuous cyclic softening. The latter may cause undesirable deformation of wheels in service.

The effects of the reduction ratio (RR) on the low-cycle fatigue (LCF) properties of the Zn-22 wt.% Al (Zn-22Al) alloy were investigated. Various grain sizes from 0.68 to 1.13 μm were obtained by controlled RRs. Tensile and LCF tests were carried out at room temperature. Superplasticity and cyclic softening were observed. Strength and ductility of the rolled Zn-22Al alloy increased with the RR, owing to the decrease in its grain size. The RR did not affect the cyclic softening behavior of the alloy. The fatigue life of the alloy decreased with increasing strain amplitude, while the fatigue life first decreased and then increased with increasing RR. The longest fatigue life was observed for the alloy rolled at a RR of 60%. A bilinear Coffin-Manson relationship was observed to hold true for this alloy.