Cyclic Hardening Research Articles

The strain ratio-dependent multiaxial low cycle fatigue behaviour and life prediction of 316L stainless steel based on critical plane at elevated temperature

The study mainly investigates the effect of strain ratio on the multiaxial low cycle fatigue behaviour of 316L at 550 °C. The evolution of both axial and torsional stress amplitude is observed to go through three stages: cyclic hardening, softening and fracture. What's different is that the torsional loading experiences fewer cyclic hardening cycles compared to the axial loading. Regarding the interaction between axial and torsional stresses, it is observed that as the torsional strain increases, the axial deformation response presents a lower peak stress and a higher softening rate, while the torsional peak stress and softening rate hardly changes with the increase of axial strain amplitude. The fracture pattern is obtained through three-dimensional (3D) depth-of-field observation. It is discovered that as the strain ratio increases, the fracture pattern transits from tensile failure mode to shear failure mode. Furthermore, the orientation angle of the fracture surface increases and lies between the critical plane angle of maximum shear strain and maximum normal strain, indicating a mixed failure mode. Additionally, a comparison of nine representative critical plane models reveals that the energy-based models can better predict the multiaxial fatigue life influenced by the strain ratio. Finally, based on the analysis of deformation and failure mechanisms, a normal damage parameter weakening factor (NDWF) is introduced into the KBM-R model, and an improved normal stress component (INSC) is introduced into the Wu-R model. The two modified models well capture the impact of strain ratio and accurately predict the multiaxial fatigue life of both 316L and P92 steel.

Cyclic behavior and damage mechanism of 304 austenitic stainless steel under different control modes

In this work, low cycle fatigue experiments under both strain and stress control modes with different loading levels were conducted on 304 austenitic stainless steel at ambient temperature to discuss the influence of loading mode and level on the cyclic response and physical mechanism. The results demonstrated that the cyclic response generally exhibited three stages: primary cyclic hardening, subsequent cyclic softening, and the secondary hardening finally. The degree of each hardening and softening stage was depended on not only the loading modes but also the strain/stress level. Accelerated cyclic softening and retarded secondary hardening occurred under the stress mode, which made a higher low-cycle fatigue life during stress cyclic loading than that for the strain control mode, especially at relatively high strain/stress level. The secondary hardening stage, occupying the main portion of the lifetime, was proved to be associated with the development of martensitic phase and the extension and intersecting of stacking faults or micro-twin.

Static-dynamic damage mechanism and self-heating effect of a clean elastic polyurethane grouting material for trenchless rehabilitation under high stresses

Compared with traditional excavation rehabilitation, trenchless grouting rehabilitation technology has the advantages of fast, precise, and low-carbon, which has a broad application prospect in wind turbine foundation rehabilitation. In this work, a non-toxic, environment-friendly, fast-setting, high-strength, and high-toughness elastic polyurethane (PU) grouting material was proposed, and the static-dynamic damage mechanism and self-heating effect under high stress were studied by DMA, SEM, static-dynamic compression, and self-heating monitoring tests. The results show that the static compressive properties increased in a power function relationship with increasing strain rate. The dynamic compression fatigue process can be divided into adjustment stage (Stage I), viscoelastic development stage (Stage II) and failure stage (Stage III) or cyclic hardening stage (Stage III*). At 41.57 MPa and 36.95 MPa, fatigue damage parameters increased slowly in Stage I and Stage II, and sharply in Stage III. The stiffness damage, loss factor, cumulative strain, and strain rate increased by more than 100% in Stage III. Fatigue failure dominated by vertical/slanting through-cracks, shed debris, drum deformation, etc. was closely related to temperature and loading. The failure temperature about 59.1 °C, which was higher than the glass transition temperature. At 32.33 MPa, fatigue damage parameters changed slightly and then stabilized, showing exponential relationships with cycles. The max temperatures were less than 15 °C. The slight temperature rise facilitated the adjustment of structure and cyclic hardening under cyclic loading. This work can guide the further engineering application and fatigue durability evaluation of PU grouting materials.

On the influence of crystallographic texture on cyclic hardening of an AlMgSi alloy

The influence of crystallographic texture on hardening achieved by cyclic loading at room temperature, was tested by comparing samples from an axisymmetric round bar and a flat profile, both made of non-recrystallized, extruded profiles of similar AlMgSi alloys (AA6082). It was found that the plastic strain accumulated during the cyclic loading was significantly (nine times) larger for the flat profile. Tensile testes of the cyclically hardened samples revealed that in spite of the large accumulated plastic strain during cyclic hardening of the flat sample, the local tensile strain at fracture in the necking region was similar to the peak aged conditions obtained by artificial aging.

Combined anisotropic and cyclic constitutive model for laser powder bed fusion fabricated aluminum alloy

This study presents new methods to effectively model the anisotropic yielding and hardening behavior of laser powder bed fusion fabricated aluminum alloy under both monotonic and cyclic loading conditions. The proposed model combines the yield surface-interpolation method to accurately describe the anisotropic hardening rates in various directions, with the Chaboche kinematic hardening rule to precisely reflect the cyclic characteristics. For numerical implementation of the combined anisotropic and cyclic constitutive model, a fully implicit stress integration algorithm based on return mapping method is provided. Moreover, the multiple parameters associated with the model are categorized and identified in an uncoupled manner. The isotropic and cyclic hardening parameters are determined by an inverse method, and the stability of the optimization outcomes is validated by applying different starting points for the parameters. Particularly, the back-stress effect on the identification of anisotropic parameters associated with the stress invariant-based Hill48 yield function is considered for the first time. This consideration leads to an improved prediction accuracy compared to the identification of anisotropic parameters without considering back-stress effect. The combined anisotropic and cyclic constitutive model, along with the calibrated parameters, are proven capable of accurately reproducing the intricate deformation behavior of laser powder bed fusion fabricated AlSi10Mg.

A rolling contact fatigue life prediction model for bearing steel considering its gradient structure due to cyclic hardening

This paper presents a rolling contact fatigue life prediction model for bearing steel. In the initial stage of rolling contact, the gradient structure appears in the subsurface region of bearing steel and the hardness shows a gradient distribution along the contact depth direction due to the roller compaction effect. This indicates that the fatigue resistance of bearing steel varies in the subsurface region. Besides, each subsurface material volume element is subject to different stress cycles. Based on Weibull theory, the survival possibility of subsurface volume elements is formulated. Then the rolling contact fatigue life is evaluated considering the stress state and anti-fatigue performance of each elementary volume of the subsurface material. According to the phenomenon and assumption, the model proposed for gradient material was applied in the rolling contact fatigue life prediction of the bearing steel GCr15 and validated with the fatigue experiment data in the open literature. Furthermore, the accuracy of the proposed model results was compared with the traditional empirical rolling contact fatigue life prediction models.

A crystal plasticity model for multiaxial cyclic deformation of U75V rail steel

The theoretical investigations of cyclic deformation behavior and deformation mechanism of the rail steels under multiaxial loadings are of great significance to the study of rolling contact fatigue. Based on the classical crystal plasticity framework, a new nonproportional parameter at the mesoscale is proposed, and the isotropic and kinematic hardening models are modified. A series of finite element simulations of U75V rail steel under uniaxial and multiaxial cyclic loading are carried out using the proposed model. The simulation results show that the proposed model can reasonably reflect the cyclic softening under strain-controlled uniaxial cyclic loading, cyclic hardening under strain-controlled multiaxial cyclic loadings, and ratcheting behavior under stress-controlled uniaxial and multiaxial cyclic loadings of U75V rail steel. Moreover, the influence of multiaxial loading paths on the plastic strain is explained from a mesoscopic perspective. The values of nonproportional parameters of different slip systems under uniaxial and multiaxial paths are compared, and the difference of nonproportional parameters at the macroscale and mesoscale are discussed. Finally, the simulation results of the proposed model and the existing model with modified kinematic hardening rules are compared. The results show that the proposed model has better predictions on the evolution of ratcheting strain and nonproportionally additional hardening effect caused by nonproportionally multiaxial loadings.

Effects of pre-compression modes on mechanical properties and fatigue behaviors of rolled ZK60 magnesium alloy

The research investigated the microstructure, monotonic mechanical properties, and fatigue behaviors of ZK60 magnesium alloy treated with single-pass pre-compression and combined pre-compression. The study found that combined pre-compression significantly improved mechanical strength in both the transverse and rolling directions compared to the single-pass method. Due to the randomized texture and intricate interwoven grain structures, the combined pre-compression method significantly mitigated the anisotropy observed in samples treated with single-pass pre-compression. Both pre-compression techniques enhanced the fatigue life of the ZK60 magnesium alloy. Samples subjected to combined pre-compression exhibited fatigue performance comparable to those treated with single-pass pre-compression. Additionally, there was a clear negative correlation between the fatigue life of the three samples and the magnitude of the mean stress. The abundance of pre-existing dislocations in the combined pre-compression processed samples helped in reducing the trend towards cyclic hardening. In contrast to single-pass pre-compression, the combined pre-compression method effectively reduced twinning activities and promoted the participation of basal slip during cyclic loading, leading to a more symmetrical hysteresis loop.

Continuous damage model for structural steels and weld metals under ultra-low-cyclic loading

For the fracture problem of structural steel under strong earthquakes, a continuous damage model which comprehensively considers the influence of stress triaxiality and Lode parameter was proposed in this paper. According to the variation law of Mises stress under cyclic loading, this model decomposes the effective cumulative plastic strain into isotropic hardening and kinematic hardening components, assuming a linear proportional relationship between the fatigue damage induced by these two components. The basic mechanical properties and constitutive relationships of five common Chinese structural steels and four corresponding weld metals were investigated. The fracture failure tests for both notched round bar specimens and pure shear specimens subjected to monotonic tensile and ultra-low-cyclic loading were thoroughly examined on these materials. The continuous damage model parameters were calibrated by means of test results and complementary finite element simulations. Experimental and numerical simulation results indicate that Q690 high strength steel exhibits cyclic softening behavior, whereas the other steels show cyclic hardening characteristics. For pure shear specimens, plane strain specimens and shear energy dissipation cover plates, the continuous damage model was adopted to simulate and analyze the damage development and fracture failure process of the specimens. The predictions for crack initiation, crack propagation and fatigue life are consistent with experimental results, lending further credibility to the model. The calibrated continuous damage model of various steels provides an effective means for analyzing the ULCF fracture failure of structural steel under intense earthquakes.

Evaluations and enhancements of fatigue crack initiation criteria for steels subjected to large shear deformations

While large accumulated plastic deformations occur in the rail surface layer where rolling contact fatigue cracks initiate, many available Low Cycle Fatigue (LCF) crack initiation criteria focus on small plastic strains. Accordingly, this paper evaluates available fatigue crack initiation criteria for highly shear-deformed R260 steels, reflecting the conditions in the surface layer of rails. Furthermore, modified crack initiation criteria are suggested. The evaluation is based on three different experiments: Large shear strain increments under varying axial loading (predeformation), strain-controlled LCF tests after some predeformation, and axial High Cycle Fatigue (HCF) experiments. For the predeformation, Finite Element (FE) simulations, with a large-strain plasticity model for cyclic and distortional hardening, provide predictions of the local stress and strain histories. A cross-validation procedure is used to assess the accuracy and reliability of both established and modified fatigue crack initiation criteria. The proposed modifications to one of the criteria show an improved fit to the experimental data. However, there is a tendency to overfitting, which can be improved by including more experimental data.

Molecular dynamics simulation of effects of loading parameters on fatigue crack growth behavior in titanium single crystal

AbstractThe fatigue crack growth behavior in single‐crystal titanium with a prismatic crack was simulated using molecular dynamics (MD) considering the effects of applied temperature, strain ratio, and strain rate. The results indicate that the material exhibits transient cyclic hardening behavior and dominant cyclic softening behavior. The peak tensile stress decreases with increasing temperature or decreasing strain rate. The deformation mechanism for crack growth involves prismatic dislocation emission and the formation of vacancy defects at different loading conditions. Deformation twinning occurs at the highest temperature, and a secondary crack emerges at the highest strain rate. The Mises stress concentration at the twin boundary and the coalescence between the initial and secondary cracks may accelerate crack propagation. The ΔJ shows good linear relationships with fatigue crack growth rate (FCGR), indicating that ΔJ possesses the potential to assess fatigue crack growth behavior at the atomic scale for ductile metallic materials.

Insights into thermomechanical fatigue behavior of nitrogen alloyed 316LN stainless steel under various temperature cycling ranges

Thermomechanical fatigue (TMF) failure has become one of the most important life-limiting factors of components exposure to the high temperature in the fourth-generation sodium-cooled fast reactor (SFR) nuclear power plant. Therefore, TMF tests at different temperature cycling ranges are performed to study the cyclic deformation behavior and damage mechanism of a nitrogen alloyed 316LN stainless steel. Moreover, the isothermal fatigue (IF) tests at the maximum temperature of TMF cycling are conducted for comparison. The cyclic hardening and softening response are analyzed, taking into account the phenomenon of dynamic strain aging which occurs at the temperature range of 350 °C–650 °C. Furthermore, the critical strain corresponding to the occurrence of serrated stress flow in engineering stress-strain curves keeps decreasing from 28.94% at 350 °C to 0.09% at 650 °C. The distribution of plastic deformation and the degree of strain localization are evaluated by the characterization results of kernel average misorientation map, high/low angle grain boundary map and dislocation configuration. The dominant damage mechanism and its corresponding cracking behavior in both IF and TMF tests are analyzed based on the observation of fracture surface morphology. In addition, the sequence order of fatigue life is determined as IF < IP-TMF < OP-TMF. Moreover, the maximum difference of fatigue life between IF test and TMF test occurs at the temperature range of 400 °C–600 °C, in which the IP and OP life is about 2.91 and 3.65 times larger than the IF life. The results in this work provide reliable data support and theoretical guidance for the fatigue design of high-temperature components in SFR.

Cyclic plasticity behavior and deformation mechanism of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings

Fatigue performance and durability application have always been the key issues impeding the understanding of aero-engine pressure pipe due to its non-standardized shape, making tests more challenging. This paper constructs an optimized testing system to elucidate the plastic shakedown and ratcheting behavior of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings. Moreover, microstructural evolution's role in regulating the cyclic plasticity behavior is qualitatively established. Experimental results indicate that the pipe deformation exhibits compressive ratcheting behavior in plastic shakedown state, and the initial cyclic hardening followed by cyclic saturation occurs during the whole cyclic response. This corresponds to a decreasing ratcheting displacement rate followed by a steady state appears. Except for the first two similar deformation stages mentioned above, the abrupt cyclic softening and the resultant increasing ratcheting displacement rate are obtained at the final fracture stage of the pipe deformation in ratcheting state. A series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and acoustic emission (AE) are combined to reveal the underlying microstructural mechanisms during cyclic deformation. The volume fraction of austenite and ferrite and the average grain size change during different stages of plastic shakedown and ratcheting deformation. The synergistic effects of dislocation structure multiplication, destruction, and dissolution at different stages of plastic shakedown and ratcheting deformation dominate the cyclic plasticity behavior.

Investigating the deformation and microstructural evolution of laser powder-bed fusion of Hastelloy X during high temperature fatigue loading

This study investigates the fatigue behaviour of samples made by laser powder-bed fusion of Hastelloy X (LPBF-HX) with as-built and machined surface conditions at 700 °C under fully reversed strain-controlled cyclic loading. Samples with both surface conditions exhibited initially cyclic hardening followed by cyclic softening under large strain amplitude testing, where a slight continuous hardening was observed for tests with smaller strain amplitudes. The samples with machined surfaces showed longer endurance and higher stress ranges than those with as-built surfaces. Post-fatigue-test EBSD analysis showed the formation of the Goss texture and extensive local strain accumulation in the samples tested under high strain amplitude at 700 °C. Fractography investigations revealed that early crack initiation in the samples with as-built surfaces was from stress concentrations induced by valleys on the rough surface. No evidence of crack initiation induced by pre-existing defects was observed in the machined samples, and the excessive slip activity at the surface was found to be responsible for the crack initiation.

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

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

Test and constitutive modelling of wire arc additively manufactured stainless steel

Wire arc additive manufacturing (WAAM) is a promising metal 3D printing technology with high deposition efficiency and low cost, which makes it more applicable to the cost-sensitive construction industry. Although many studies have been carried out on the material properties of WAAM metals under monotonic loading, there are few studies on the constitutive model under cyclic behaviors. Therefore, this paper aims to study the mechanical behavior of WAAM austenitic stainless steel S30408 under monotonic and cyclic loadings, and propose a constitutive model to describe its cyclic response. A total of 36 coupons extracted at three different angles from WAAM plates were tested. Based on the results of the monotonic tensile tests and metallographic analysis, the mechanical properties anisotropy of WAAM austenitic stainless steel and effect of heat treatment on its ductility were reported. Based on the results of cyclic loading test, cyclic hardening was observed, and the existing isotropic/nonlinear-kinematic hardening model was employed to describe the cyclic plasticity behavior, which exhibited low accuracy. Subsequently, the evolutions of the peak stress, backstress, and yield surface radius with equivalent plastic strain in each cyclic loop were discussed. Finally, to accurately describe the cyclic hardening behavior of WAAM austenitic stainless steel, a constitutive model which considered the degradation of the Young's modulus, the hardening behavior of the backstress, and the strain memory effect of the yield surface radius was proposed and calibrated. Comparation with test results demonstrated the accuracy of the proposed model.

Influence of Loading Waveform on the Fatigue Life of 34CrNi3MoVA Steel

Mechanical components often experience fatigue loading from various waveform conditions during their operational lifespan. However, the underlying mechanisms through which variations in loading waveform affect the fatigue life of components remain unclear. Thus, this study conducted tension–compression fatigue experiments on 34CrNi3MoVA steel specimens under the same stress amplitude with different waveforms (cosine, triangular, sawtooth, and reverse sawtooth) to investigate the effects of loading waveform variations on the cyclic strain hardening behaviors, the fatigue fracture failure, and the fatigue life. The results indicated that specimens under different waveforms all exhibited cyclic strain hardening. The fatigue cyclic hardening level progressively increased in the order of cosine, triangular, and sawtooth waveforms, resulting in a continuous increase in cyclic saturation strain amplitude. The analysis of fatigue fractures demonstrated a consistent increase in both the initiation and propagation zone areas in the order of cosine, triangular, and sawtooth waveforms, and the boundary between the propagation and final fracture zones gradually shifted from a straight to a curved shape. The influence mechanisms of cyclic loading waveforms on the fatigue life of specimens were analyzed based on the energy dissipation, leading to the development of a universal fatigue life prediction model applicable to different waveform conditions, the model was then verified with the reverse sawtooth wave specimens and resulted in a prediction error less than 15%. The study is expected to serve as a significant guide for predicting and evaluating the fatigue life of mechanical components under various fatigue loading conditions.

A continuum damage coupled unified viscoplastic model for simulating the mechanical behaviour of a ductile cast iron under isothermal low-cycle fatigue, fatigue-creep and creep loading

A novel continuum damage coupled unified viscoplastic model is used for simulating the stress–strain responses of SiMo 4.06 cast iron under Low-Cycle Fatigue (LCF), fatigue-creep and creep loading for temperatures up to 650 °C. The advanced constitutive model that is developed within the framework of the Chaboche model allows the simulation of various effects including strain rate sensitivity, cyclic hardening and softening, static recovery and strain range dependency. The hyperbolic sine flow rule and the static recovery of kinematic hardening rule are proposed in order to effectively simulate both stress relaxation and creep strains. The isotropic damage variable is introduced into the constitutive equations to represent the effects of material degradation. Three main damage mechanisms are considered: fatigue, creep and ductile damage. In addition, the model is further modified to take into account the progressive microdefects closure effect. Finally, the prediction capability of the proposed model is illustrated for the experimental data obtained from the various uniaxial material tests. A good correlation was achieved between the simulated and the experimental results.

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

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

Cryogenic Deformation Behaviour of Aluminium Alloy 6061-T6

Aluminium alloy 6061-T6 (AA6061-T6) shows a promising potential for cryogenic structural applications. This alloy exhibits remarkable monotonic tensile properties at low temperatures. However, there is a limited number of studies on the cryogenic deformation behaviour. In this study, both monotonic and cyclic loading were conducted, and various microstructure characterisation techniques were performed to understand influence of cryogenic temperatures on microstructure evolution and deformation behaviour of this alloy. At cryogenic temperatures, the aluminium alloy exhibited superior mechanical properties over those at room temperature. Yield stress, UTS and elongation at failure increased by 18%, 33%, and 53% at 77 K compared to those at room temperature. Such increase in mechanical properties was attributed by the stronger resistance to dislocation movement due to the reduced thermal assistance. Work hardening rate also increased as dynamic recovery was suppressed at lower temperatures. As a result, a high density of dislocations was evenly distributed within grain interior and led to a homogeneous deformation. The test temperature appeared to have a significant influence on fatigue performance; maximum stress response increased by 23% at 108 K with respect to those at room temperature. During cyclic loading, a high number of dislocations was generated to accommodate prescribed strain because of the resistance to dislocation movement including the pinning of dislocations by β’’ precipitates which are known to be sheared at room temperature. Thus, the alloy exhibited an enhanced cyclic hardening behaviour without a noticeable cyclic softening phase. Fatigue life improved by 143% at 108 K with respect to that at room temperature as the homogeneous deformation prohibited localised slip activity and delayed formation of slip bands which act as crack initiation sites. Moreover, the initiation and propagation of secondary cracks at 108 K retarded the propagation of main crack to improve fatigue life.Graphical