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Different micro-regions of weldment are expected to present various cyclic softening behaviours during creep–fatigue interaction (CFI), which has a significant influence on the remaining mechanical properties. The present work aims to investigate the cyclic softening and remaining creep behaviour of different micro-regions of P92 steel weldment using electron backscattered diffraction (EBSD) and nanoindentation technology. CFI tests were performed and interrupted at various pre-set fatigue cycles at elevated temperature of 650 °C and hold time of 180 s, and thereafter EBSD analyses, nanoindentation tests and high-temperature creep tests were conducted on these interrupted specimens. Experimental results reveal that the microstructure of the parent metal and fine austenite grain heat-affected zone, which is adjacent to the intercritical heat affected zone, are sensitive to CFI loading. Notably, the microstructural evolution in intercritical heat affected zone almost reaches saturated after fatigue cycles of 20%Nf. Nanoindentation results uncover that the intercritical heat affected zone exhibits the lowest elastic modulus and microhardness, and with increasing the number of fatigue cycles, the weakest region changes. Accordingly, creep fracture location transits and creep life was degraded. Finally, a fatigue damage parameter based on the microhardness was developed to capture the effects of CFI on remaining creep life.

Motion capture is the current gold standard for assessing movement of the human body, but laboratory settings do not always mimic the natural terrains and movements encountered by humans. To overcome such limitations, a smart sock that is equipped with stretch sensors is being developed to record movement data outside of the laboratory. For the smart sock stretch sensors to provide valuable feedback, the sensors should have durability of both materials and signal. To test the durability of the stretch sensors, the sensors were exposed to high-cycle fatigue testing with simultaneous capture of the capacitance. Following randomization, either the fatigued sensor or an unfatigued sensor was placed in the plantarflexion position on the smart sock, and participants were asked to complete the following static movements: dorsiflexion, inversion, eversion, and plantarflexion. Participants were then asked to complete gait trials. The sensor was then exchanged for either an unfatigued or fatigued plantarflexion sensor, depending upon which sensor the trials began with, and each trial was repeated by the participant using the opposite sensor. Results of the tests show that for both the static and dynamic movements, the capacitive output of the fatigued sensor was consistently higher than that of the unfatigued sensor suggesting that an upwards drift of the capacitance was occurring in the fatigued sensors. More research is needed to determine whether stretch sensors should be pre-stretched prior to data collection, and to also determine whether the drift stabilizes once the cyclic softening of the materials comprising the sensor has stabilized.

Lateral spreading of an interlayered deposit adjacent to a meandering stream channel in Wufeng, Taiwan, during the 1999 Chi-Chi Earthquake is evaluated using two-dimensional (2D) nonlinear dynamic analyses (NDAs) with geostatistical modeling of the subsurface to assess their ability to approximate the observed magnitude and spatial extent of ground deformations, as well as identify the key factors and mechanisms that most contributed to the overall system response. In-situ data from borings and cone penetration tests (CPTs) depict thinly stratified overbank deposits of low-plasticity silty sands, silts, and clays, interlayered with laterally discontinuous channel-deposited sands. The three-dimensional (3D) subsurface is simulated using transition probability-based indicator geostatistics, conditioned on available CPT data and geological inferences. The NDAs are performed using the PM4Sand and PM4Silt constitutive models, within the FLAC finite difference program. Sensitivity analyses are performed to understand the influence of uncertainties in the stratigraphy, channel conditions, soil properties, input ground motions, constitutive model calibration protocols, and numerical boundary conditions, as well as the performance of alternate channel transects. Most analysis cases generally matched the maximum displacements observed near the channel but overestimated the extent of displacements away from the channel. The most favorable results were largely influenced by nonstationary stratigraphic trends and cyclic softening of fine-grained soils, in addition to the liquefaction of coarse-grained soils. This case history demonstrates the capabilities and limitations of current subsurface and NDA modeling procedures for predicting ground deformation patterns.

The fatigue tests of Chinese RPV steel (A508-3) were carried out under the simulated primary coolant environment of AP1000 which is a kind of 3rd generation nuclear power plant. The effects of temperature, strain rate and dissolved oxygen (DO) on fatigue life were analyzed. During the fatigue test, the material exhibited a cyclic hardening followed by cyclic softening behavior, with no obvious cyclic stability region. In logarithmic coordinates, the S–N curve showed a bilinear relationship. As the strain amplitude increased from 0.2 to 0.6%, the fatigue life approximately decreased from 105 to 3 × 102. The environmental correction factor (Fen), which is defined as the ratio of life in air at room temperature to that in the primary coolant environment, decreases as the temperature decreases from 321 to 150 °C. The Fen also decreases as the DO decreases from 0.6 to 0.03 ppm while it increases as the strain rate decreases from 1 to 0.0004%/s. Compared with the NUREG6909 model, the presented model is more applicable for fatigue life prediction of Chinese A508-3 steel.

The softening of a hot-deformed die steel under thermocycling used in thermal fatigue tests is studied. The changes in the microstructure, in the dislocation structure and in the hardness of the surface layer and of the core of the thermocycled sample are analyzed. It is shown that the thermocycling process is accompanied by softening of the hot-deformed die steel, because the total concentration of structural flaws grows or decreases in turns at a general tendency to lowering.

We describe two experiments performed on a 9-m-radius geotechnical centrifuge to evaluate dynamic soil–structure interaction effects on the cyclic failure potential of fine-grained soil. Each experiment incorporated three different structures with a range of mass and stiffness properties. Structures were founded on strip footings embedded in a thin layer of sand overlying lightly overconsolidated low-plasticity fine-grained soil. Shaking was applied to the base of the model container, consisting of scaled versions of recorded earthquake ground motions, sweep motions, and step waves. Data recorded during testing were processed and published on the platform DesignSafe. We describe the model configuration, sensor information, shaking events, and data processing procedures and present selected processed data to illustrate key model responses and to provide a benchmark for data use.

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

The methods for low-cycle fatigue testing and cyclic crack growth testing of 10GN2MFA steel at 20 and 270°C are described. Based on test results, fatigue crack growth rate curves are plotted and the characteristics of low-cycle plastic deformation and fracture are found. A difference in the kinetics of low-cycle plastic deformation is shown – the cyclic softening at 20°C and hardening at 270°C – which has an influence on fatigue life and fatigue crack growth resistance of this steel. The last-mentioned characteristic has been studied using two test methods: (i) with a decreasing stress intensity factor range and (ii) with a constant magnitude of load.

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.

Cohesive soil undergoes cyclic softening when experiences the dynamic loading conditions. Evaluation of strength loss under such loading conditions is essential for soil stability analysis. The pre...

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.

The present study investigates the hysteresis response, stiffness degradation/cyclic softening and energy dissipation features of Ahmedabad cohesive soil. Saturated specimens of Ahmedabad cohesive soil were reconstituted using slurry consolidation technique at different void ratios to perform the following series of tests, strain-controlled cyclic triaxial tests, shear strength, and compressibility tests. Load repetition exhibited significant degradation in shear modulus of slurry consolidated cohesive soil specimens under cyclic loading conditions. A maximum stiffness degradation of 78% was observed for the specimen possessing minimum initial void ratio. Hysteresis response of Ahmedabad soil revealed alteration in the rotation angle of hysteresis loops and their sizes with varying initial void ratios of the Ahmedabad cohesive soil. A substantial influence of initial void ratio on the energy dissipation behavior of cohesive soil was depicted in the present study. The cumulative energy dissipation of the specimen prepared with lower initial void ratio was obtained to be 7.1 times higher than the specimen possessing maximum initial void ratio.

In the present study effect of mean strain on cyclic plastic deformation characteristics of P91 steel is experimentally evaluated and compared with results of finite element simulation considering cyclic plasticity models. P91 steels being used in various components in fossil fired and nuclear power plants are subjected to cyclic loading with mean strains/stresses. Symmetrical and asymmetrical strain controlled tests of P91 steel conducted at room temperature revealed cyclic softening nature during the fatigue cycling process. The influence of mean strain imposed showed strong dependence on stress relaxation behavior and fatigue life of P91 steel. Tensile mean stress was found to relax steeply in initial cycles followed by stabilization during the asymmetric strain cycling. A reduction in fatigue life is observed with increase in mean strain for particular strain amplitude. An attempt has also been made to simulate asymmetric strain controlled behavior of P91 steel through cyclic plastic modeling in the framework of rate independent plasticity theory. Ohno-Wang material model is employed to predict the influence of mean strain on stress relaxation behavior of the investigated steel. The simulated results depicted that Ohno-Wang model captures the cyclic plastic deformation behavior of P91 steel reasonably well.

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.

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.

In this work, improved creep-fatigue life prediction models based on time fraction (TF) and ductility exhaustion (DE) rule are established for creep-fatigue life prediction of components at elevated temperatures. Cyclic softening/hardening and stress relaxation models for cyclic softening/hardening material are incorporated in the model. Materials data conducted by authors and collected from references are employed for validations. Results indicated that the improved models proposed could present more realistic evolution behavior of materials than the half-life models. The differences of the predicted lives based on the improved models and half-life models are dependent on cyclic behaviors of materials. The predicted life by the improved model is lower than that gained by the half-life model for cyclic softening steels, while it is higher than that by the half-life model for cyclic hardening steels. In addition, the DE rule could overcome the influence of cyclic softening/hardening on creep-fatigue life prediction more significantly than the TF rule.

The seismic performance of Balboa Boulevard during the 1994 MW 6.7 Northridge earthquake was examined through nonlinear deformation analyses (NDAs) using advanced tools to (1) investigate the failure mechanism leading to ground deformations at this site; (2) evaluate the accuracy of the adopted analysis methods, engineering procedures, and state-of-the-art tools to reasonably estimate horizontal ground displacements; and (3) identify key factors and parameters contributing to earthquake-induced ground deformations at this site. One-dimensional (1D) liquefaction vulnerability indexes (LVIs) and permanent displacements using Newmark sliding block analyses were also estimated and compared against ground deformations observed after the earthquake. The geotechnical characterization of Balboa Boulevard was assessed based on field and laboratory data obtained from two investigation campaigns. Transitional probability geostatistics were used to develop stratigraphic models that capture the heterogeneity and the spatial variability patterns of sand-like and clay-like soils present at this site. The stratigraphic models were implemented in the finite difference software FLAC and the behavior of sand-like and clay-like soils simulated using the PM4Sand and PM4Silt constitutive models, respectively. Sensitivity analyses were performed to address uncertainties associated with the spatial variability of soils, input ground motions, the proportion of sand-like and clay-like soils within the soil deposit, and the strength properties of these materials. Results from NDAs suggest that a compounded effect of both liquefaction of sand-like soils and cyclic softening of clay-like soils led to the excessive ground deformations at Balboa Boulevard. This study sheds light on the importance of using appropriate engineering procedures and numerical modeling protocols in the prediction of deformation patterns, the selection of key input parameters, as well as the applicability of LVIs in complex sites.

Low cycle fatigue (LCF) tests of a low-carbon microallyed steel have been performed to assess its response under cyclic loading for automobile application. Material exhibits significant initial cyclic hardening afterward prolong cyclic softening until failure at higher strain ranges (≥1.50%); however, sustained cyclic softening from second cycle onwards is observed at lower strain amplitudes (≤1.00%). The selected steel is found to follow non-Masing behavior but its fatigue life can well be described by strain-life relationship. Various monotonic and cyclic properties are determined and discussed together with mechanism of fatigue failure.

This chapter is devoted to hysteretic behavior of high strength steel (HSS) and the combined isotropic and kinematic hardening hysteretic model for predicting the cyclic behavior of HSS, with consideration of the Bauschinger effect, cyclic softening/hardening behavior, and cumulative damage.

Cyclic softening of fine-grained soils poses a significant hazard to geotechnical infrastructure in many parts of the world. Methods to numerically evaluate the cyclic softening and subsequ...

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

Component design of rubber-based anti-vibration devices remains a challenge, since there is a lack of predictive models in the typical regimes encountered by anti-vibration devices that are deformed to medium dynamic strains (0.5 to 3.5) at medium strain rates (0.5/s to 10/s). An approach is proposed that demonstrates all non-linear viscoelastic effects such as hysteresis and cyclic stress softening. As it is based on a free-energy, it is fast and easily implementable. The fitting parameters behave meaningfully when changing the filler volume fraction. The model was implemented for use in the commercial finite element software ABAQUS. Examples of how to fit experimental data and simulations for a variety of carbon black filled natural rubber compounds are presented.

The fatigue process of multi-axial low cycle fatigue in F82H steels was investigated via multi-axial fatigue tests under non-proportional loading at 573 K in air. Various strain paths and strain waveforms were employed and the microstructure of each fatigue-tested F82H steel was examined. Sudden cyclic softening occurred at the initial stage of 50 cycles and the failure life associated with the circle loading test was significantly lower than those corresponding to push–pull and reversed torsion loading tests. Dislocation formation increased rapidly under all loading conditions. At failure, the dislocation density generated by circle loading at Δεeq = 1.0% was tens of times larger than those of the other loadings.Sudden cyclic softening results from the formation of mobile dislocations in grain interiors during the initial stage of loading. The significant reduction in the lifetime is directly correlated with the high density of dislocations generated during circle loadings. Keywords: Fatigue, F82H ferritic steel, Microstructure, Multi-axial loading

The fatigue test for rock salt is conducted to investigate the effects of stress amplitude, loading frequency and loading rate on the plastic strain energy, from which the evaluation rule of the plastic strain energy is analyzed, which is divided into three stages: cyclic hardening, saturation and cyclic softening. The total accumulated plastic strain energy only depends on the mechanical behavior of rock salt, but is immune to the loading conditions. A novel model for fatigue life prediction is proposed based on the invariance of the total plastic dissipation energy and the stability of the plastic energy per cycle.

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.

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).

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.

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.

Experimental investigations have been conducted on the cyclic-plastic response of a structural mild steel (Grade YST210) under varying strain amplitudes (ea) under symmetric strain-controlled cycling. The results on the investigated YST210 steel and the reported results on four similar grades of steels (E250A, Q235, SS400, and GR345 steel) under symmetric strain-controlled cyclic loading are analyzed using a suggested approach based on Chaboche isotopic-kinematic hardening (CIKH) model. The parameters of the CIKH model are estimated from the stabilized hysteresis loops, and the obtained parameters are subsequently optimized using genetic algorithm to obtain closer predictions for the experimental cyclic-plastic response. The experimental fatigue response of YST210 steel reveals cyclic softening, and the estimated softening factor (SF) varies from 10% to 22% when ea alters from 0.25% to 2%; the magnitude of SF at ea = 2% decreases from 22% to 17.5% as the strain rate increases from 5 × 10−4 s−1 to 5 × 10−2 s−1. The efficacy of the suggested approach was estimated using the maximum root mean square error (Ferror) between predictions and experimental results. For the YST210 steel at different ea, the magnitude of Ferror is

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.

The lack of in-depth understanding on the low-temperature fatigue cracking performance has limited the extensive application of the Q345qD bridge steel in cold and severely cold regions. This study investigates the resistance to the fatigue crack initiation of the Q345qD steel through a series of strain-controlled fatigue tests of smooth coupons at room and low temperatures. Experimental results present the cyclic stress-strain relationship, non-Masing properties and Coffin-Manson-type fatigue crack initiation resistance. Thereafter, this study analyzes the effect of low temperatures on the stain-controlled fatigue life of the studied Q345qD steel using the different energy approaches. Results indicate that the low temperatures within -60 °C increase the resistance to the fatigue crack initiation and the cyclic strain-hardening coefficients. A comparison with other structural steels demonstrates the excellent fatigue performance of the Q345qD steel at room and low temperatures. Since the corresponding stress amplitudes of the Q345qD steel under each cyclic strain range exhibit some strain-range-dependent cyclic softening/hardening behavior, the plastic strain energy density and the concept of fatigue toughness are more suitable and effective to predict the fatigue life of the Q345qD steel at room and low temperatures. Finally, a simple and accurate method for calculating the plastic strain energy density has been proposed.

In the paper, the influence of the yield-point phenomenon (YPP) on cyclic plasticity of the console beam is presented with the objective to demonstrate the impact of the YPP on the local cyclic plasticity. The influence of the YPP and its dependence on cyclic material hardening or softening was studied through experiments and numerical simulations. Console beams are made from the low-alloy EN 42 CrMo 4 steel in its normalized state (184 HV), which exhibits cyclic hardening, and in its tempered state (296 HV), which is subject to cyclic softening. Numerical simulations were performed on constitutive model of cyclic plasticity taking into account the kinematic hardening, isotropic hardening or softening and formulations of the YPP which are based on the change of the elastic region surface in the stress space at first transition into the stress plateau. Analysis of the results shows the importance of taking into account the YPP equations in constitutive models of cyclic plasticity as well as the influence of the YPP on cyclic plasticity of the console beam. Fenomen tečenja je na napetostno-deformacijskih krivuljah enoosno obremenjenih preizkušancev viden ob prvem prehodu iz elastičnega v elasto-plastično področje kot ostra konica (točka tečenja), kateri sledijo padec napetosti, napetostna planota (t. i. Lüders-ova planota) ter nadaljnje utrjevanje. Padec napetosti in napetostna planota sta posledici hitrega večanja števila mobilnih dislokacij. Posledica takšnega odziva je nehomogena plastična deformacija materiala tekom tvorjenja napetostne planote. Pojav je značilen za železo, nizkolegirana jekla, titanove in aluminijeve zlitine ter nekatere druge kovinske zlitine, kot tudi za polimere, kompozite in laminate ter nekatere kamenine. Fenomen tečenja je bil v povezavi z enoosnim obremenjevanjem podrobneje raziskan že v preteklosti. Za popis fenomena tečenja so bile izpeljane različne formulacije konstitutivnih modelov, ki pa so se v večji meri osredotočale na monotone obremenitvene primere. Večina obstoječih konstitutivnih modelov za popis ciklične plastifikacije omenjenega fenomena ne upošteva, dejanski vpliv upoštevanja fenomena tečenja na ciklično plastifikacijo pa do sedaj ni bil poznan. Avtorji prispevka so v preteklosti predstavili konstitutivni model ciklične plastifikacije, ki vsebuje tudi popis fenomena tečenja. Z namenom verifikacije razvitega konstitutivnega modela in prikaza doprinosa enačb popisa fenomena tečenja na rezultate numeričnih simulacij ciklične plastifikacije je v prispevku predstavljena analiza ciklične plastifikacije konzolnega nosilca. Nosilec je izdelan iz materiala EN 42 CrMo 4 v normaliziranem stanju (184 HV), ki izkazuje ciklično utrjevanje, in v poboljšanem stanju (296 HV), ki izkazuje ciklično mehčanje. Analiza zajema primerjavo numeričnih simulacij z rezultati eksperimentalnega dela. Zaradi dolgih računskih časov je primerjava omejena na prvih 100 obremenitvenih ciklov. Eksperimentalno delo je izvedeno na enoosnem dinamičnem preizkuševališču Instron 8802, za ustrezno vpetje konzolnega nosilca pa je izdelano posebno vpetje, ki zagotavlja ustrezno togost. Konzolni nosilci so obremenjeni z različnimi robnimi pogoji (različne amplitude pomika), tekom preizkusa pa so bili opazovani sila, pomik ter deformacije na mestu največjih pričakovanih deformacij konzolnega nosilca. Za merjenje deformacij je uporabljen optični merilni sistem GOM Aramis 5M, medtem ko sta sila in pomik merjena s pripadajočo opremo preizkuševališča. Za izvedbo numeričnih simulacij po metodi končnih elementov je generirana in uporabljena simbolno- numerična koda končnega elementa, simulacije pa so izvedene za primer konstitutivnega modela z upoštevanjem enačb popisa fenomena tečenja in brez njih. Na podlagi predhodnih meritev so določeni parametri konstitutivnega modela za obe stanji materiala. Primerjava sile konzolnega nosilca v odvisnosti od pomika in števila ciklov kaže na dobro ujemanje rezultatov eksperimentalnega dela z rezultati numeričnih simulacij v primeru upoštevanja fenomena tečenja, medtem ko v primeru neupoštevanja fenomena tečenja rezultati izraziteje odstopajo. Podobne ugotovitve glede doprinosa popisa fenomena tečenja potrjuje tudi primerjava deformacijskih polj konzolnega nosilca, kjer je izrazita razlika vidna predvsem v začetni fazi obremenjevanja, medtem ko v nadaljevanju nekoliko izzveni. Za samo razumevanje in prikaz doprinosa popisa fenomena tečenja v konstitutivnih modelih so prikazane največje primerjalne napetosti (Von Mises). Primerjava posameznih rezultatov prikazuje popolnoma drugačen potek primerjalnih napetosti v primeru upoštevanja fenomena tečenja in kaže na to, da je v primeru upoštevanja fenomena tečenja cona plastične deformacije manjša, iz česar je možno sklepati, da je ciklična plastifikacija v najbolj obremenjenem delu izrazitejša. Analiziran je tudi vpliv cikličnega utrjevanja/mehčanja na ciklično plastifikacijo. Analiza sprememb posameznih parametrov konstitutivnega modela, prikazanih v delu, omogoča razumevanje vpliva fenomena tečenja na ciklično plastifikacijo konzolnega nosilca. Predstavljeni rezultati potrjujejo ustreznost in smiselnost popisa fenomena tečenja v konstitutivnem modelu ciklične plastifikacije.