Cyclic Softening Behavior Research Articles

Experimental and modelling study of cyclic plasticity of high strength steel Q690

High strength steels (HSSs) have received attention in the research community over the past few years owing to their high strength-to-weight ratio. This study examined the cyclic plastic behaviour of HSS Q690 under various cyclic loading protocols via experimental investigation, and a constitutive model was subsequently proposed. The strain amplitude and mean strain were the key parameters for constructing the cyclic loading protocols. The test results indicated that the peak stress of each cycle decreased with increasing equivalent plastic strain, which confirmed the cyclic softening behaviour. The peak stress evolution was also observed to be independent of the maximum strain amplitude, but dependent on the current strain amplitude, which proved the absence of the strain range memory effect. Furthermore, the cyclic plastic response of Q690 steel was comprehensively examined within the framework of metallic cyclic plasticity, including kinematic hardening, isotropic hardening/softening, and elastic modulus evolution. The kinematic hardening of Q690 steel was proven to be dependent on the equivalent plastic strain and the current plastic strain amplitude, whereas its isotropic hardening/softening was determined only by the equivalent plastic strain. It was also observed that the elastic modulus decreased to a stable value with increasing equivalent plastic strain. A new constitutive model was proposed to quantify these cyclic plastic features. The proposed model provided an excellent prediction of the cyclic response of Q690 steel under all loading protocols. The relative error between the simulated results and the test results was within a narrow band between -5% and 5%.

Characterization of the cyclic deformation behavior of simulated HAZs and other constituent microstructural regions of P91 steel weldment

Low cycle fatigue tests were conducted on theweld metal, coarse grain heat-affected zone (CGHAZ), fine grain heat-affected zone (FGHAZ), inter-critical heat-affected zone (ICHAZ), base metaland the actual weldmentof P91 steelat 550 °C using different strain amplitudes.The P91 weldment and its constituent microstructural regions exhibited typical continuous cyclic softening behavior. In the HAZs, the FGHAZ exhibited better fatigue behavior. The CGHAZ and ICHAZ showed higher fatigue lives at the lower and higher strain amplitudes, respectively. An empirical relationship was established for predicting the fatigue life of the weldment using weighted factors of all the constituent regions.

Development of a continuum damage model to predict accumulation of sub-failure damage in tendons

Many painful and physically debilitating conditions involve sub-failure mechanical damage to seemingly intact connective tissues such as tendons and ligaments. We found that the amount of denatured collagen in rat tail tendon (RTT) fascicles increased over experiments of cyclic loading to a constant load level (creep cyclic fatigue) with fluorescently tagged collagen hybridizing peptides (CHPs) that bind to denatured collagen. To better understand tendon sub-failure damage progression, computational modeling of tendon materials via finite element analysis in FEBio has been conducted. The objective of this project was to develop, implement, and test the ability of a new continuum damage mechanics (CDM) model in FEBio to represent the sub-failure damage behavior seen in our RTT fascicle creep cyclic fatigue experimental data.There appeared to be two distinct mechanisms responsible for the creep cyclic fatigue softening behavior of RTT fascicles over the number of cycles to failure: the preconditioning effect and overall collagen damage. In our finite element (FE) models, the RTT fascicle undamaged elastic constitutive material was composed of a matrix and fibers described by the Coupled Veronda-Westmann and exponential-linear materials. This undamaged elastic material was convolved with a modified CDM model adapted from Balzani et al., in 2012.The novelty of the Balzani damage model is the inclusion of two interrelated mechanisms described as continuous and discontinuous damage. The continuous damage formulation calculates damage accumulation during the loading and reloading of each new cycle, while the discontinuous damage approach accumulates damage from the maximum strain over the loading history to the current time. We modified the Balzani damage model formulations to represent exponential and sigmoidal increases in damage marked by the preconditioning effect and collagen damage in RTT fascicles as functions of continuous and discontinuous damage. The original Balzani damage model was first verified, then the modified CDM model was implemented into FEBio and used to reproduce the sample specific experimental creep cyclic fatigue stress-strain data as well as predict incremental cyclic fatigue. The resulting model will be useful for future experimental and computational studies of damage mechanics to understand tendon pathologies.

Low Cycle Fatigue Behavior of TC21 Titanium Alloy with Bi-Lamellar Basketweave Microstructure

Low cycle fatigue (LCF) behaviors of TC21 alloy with a bi-lamellar basketweave microstructure were investigated in this paper. The strain fatigue tests were carried out at total strain amplitudes of 1.4% to 2.0%. The cyclic stress response showed the cyclic softening behavior. In addition, the shape of the hysteresis rings exhibited a non-Masing model behavior. The cyclic stress–strain as well as the strain-life equations were obtained. The fatigue life decreased significantly with an increasing total strain from 1.4% to 2.0%. The cyclic softening behavior was interpreted by cyclic back stress and friction stress. Low cycle fatigue cracks were predominantly initiated on the surface of the samples. The relationship between the fatigue sub-critical crack and microstructure was also discussed. The cyclic deformation behavior and crack initiation mechanism were revealed on the basis of the deformation microstructure under different strain amplitudes.

Fatigue improvement and residual stress relaxation of shot-peened alloy steel DIN 34CrNiMo6 under axial loading

Shot-peening treatment was applied to a quenched and tempered DIN 34CrNiMo6 steel to improve its high-cycle R: −1 axial fatigue strength. Compared with the machined condition, the increase in the fatigue limit was 21.8%. S-N curves for shot-peened and the as machined condition were presented and compared with those obtained in previous research for rotating bending fatigue, including curves for mirror-polished specimens.The applied shot-peening treatment in this work (Isp: 8A and 200% coverage) for quenched and tempered (Q + T) DIN 34CrNiMo6 steel introduced a compressive residual stress field and an increase in surface roughness, as well as minor variations in microstructure, hardness and the FWHM (full width of the diffraction peak at half maximum intensity) parameter.The introduced compressive residual stress field tended to reduce when an external stress is applied. This was due to the onset of plastic strain. In this paper, two types of quasi-static tests were conducted by applying an axial stress with six different magnitudes and in the two directions (compressive or tensile). This was in order to assess their influence on the relaxation of surface residual stresses. Due to the introduced compressive residual stresses, if the applied stress was compressive, the onset of plastic deformations was achieved with a lower stress magnitude.In addition, surface residual stress relaxation under cyclic applied stress was evaluated at four different stress magnitudes. Due to the cyclic-softening behaviour of this Q + T steel, its cyclic mechanical properties must be considered to assess the onset of plastic strains. With the experimental data, a logarithmic model to predict the evolution of surface residual stresses with the number of cycles for different applied stress magnitudes was presented.

Deformation mechanisms of selective laser melted 316L austenitic stainless steel in high temperature low cycle fatigue

To investigate the low cycle fatigue (LCF) properties of SLM (Selective laser melting) 316L at 550 °C high temperature, a series of LCF tests at different strain amplitudes is conducted on SLM 316L and traditional 316L. In contrast to the cyclic hardening behavior of traditional 316L, SLM 316L is shown to have a stable cyclic softening behavior and higher fatigue life due to its higher strength (yield strength is 1.9 times of traditional 316L). The coarsening of cellular sub-structure, evolution of geometrically necessary dislocations (GND) and texture direction contribute to the cyclic softening behavior of SLM 316L. Fractographic observations illustrate that strain amplitude has a significant influence on initiation and propagation of transgranular fatigue cracks, and lack-of-fusion defects near the surface are key initiation sites of fatigue cracks. Additionally, the life prediction method of the non-Masing model is suitable for SLM 316L.

Strain amplitude-dependent cyclic softening behavior of carbide-free bainitic rail steel: Experiments and modeling

Experimental investigations of Carbide-free bainite (CFB) rail steel under different strain amplitudes are conducted to indicate the cyclic deformation feature and hysteresis loop evolution. A modified yield function with an exponential evolution coefficient is proposed to consider the strength differential effect, and a strain range-dependent coefficient is introduced to consider the transient Bauschinger effect. Moreover, a Logistic dose–response function and a cyclic softening function are introduced into the isotropic and kinematic hardening rules. Finally, the comparison between the simulated and experimental results demonstrates that the proposed model can reasonably describe the strain amplitude-dependent cyclic softening of CFB rail steel.

Estimation of cyclic hardening/softening and ratcheting response of materials through an algorithm to optimize parameters in Chaboche's hardening rule

AbstractA methodology considering Chaboche's isotropic‐kinematic hardening (CIKH) model and genetic algorithm optimization technique is examined here to simulate the cyclic‐plastic response for materials exhibiting cyclic hardening or softening behavior. The aim of the present report is to illustrate the inherent potential of this approach by assessing the closeness of fit of the obtained predictions to the experimental results for several materials like Z2CND18.12N, and 25CDV4.11 steels, OFHC copper, and INCONEL718 alloy. The obtained results were further subjected to comparative examinations with the available predictions to demonstrate the adoptability of the suggested methodology for the cyclic hardening or softening materials. This proposed approach provides a single set of CIKH model parameters to replicate the stabilized hysteresis loops and cyclic hardening or softening behavior under symmetric strain‐controlled loading, as well as ratcheting characteristics under asymmetric stress‐controlled cycling, and is thus generalized in nature.

Thermally activated dependence of fatigue behaviour of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion

The CrMnFeCoNi high-entropy alloy demonstrates a promising potential for applications over a range of temperature. The alloy also shows excellent printability to be fabricated by additive manufacturing for complex structures. Nevertheless, there are limited studies on the thermo-mechanical behaviour of the alloy, in particular when fabricated by laser powder-bed fusion. This study provides an in-depth understanding of the relationship between as-built cellular microstructures and fatigue behaviour at a range of temperatures (22–600 °C) in particular concerning the stability of dislocation cells and thermo-mechanical dependence of the fatigue behaviour of the alloy. At all tested temperatures, the alloy exhibits a very short duration cyclic hardening with a low hardening rate followed by a cyclic softening. The high density of dislocations already existing in as-built condition were able to accommodate most of the prescribed strain. Hence, only a small number of mobile dislocations needs to be generated, causing a short cyclic hardening phase. Upon further loading, the back stress associated with the long-range stress field was dominant factor governing the cyclic softening behaviour. The similitude relationship provided insights into the stability of as-built cells, in particular it explains why the size of as-built cells did not change during cyclic loading at 22 °C. The significant reduction in dislocation density due to the increased annihilation rate and untanglement of dislocation substructures thanks mainly to thermal assistance at elevated temperatures led to a decrease in cyclic strength and related properties (yield stress, friction and back stress, hysteresis loop shape parameter and energy per cycle). The LPBF HEA shows an insignificant strain rate dependence of the primary cyclic hardening and softening in the range of 10 −3 s −1 and 10 −2 s −1 . However, the dynamic strain ageing results in a secondary cyclic hardening at 400 °C and the reversed strain sensitivity at temperatures from 200° to 400 °C. The fracture mode was transgranular at 22–400 °C but changed to more intergranular-like at 600 °C due to the decohesion of grain boundaries, resulting in a reduction in fatigue life.

Microstructure evolution and cyclic deformation behavior of Ti-6Al-4 V alloy via electron beam melting during low cycle fatigue

Low cycle fatigue (LCF) behavior of Ti-6Al-4 V alloy constructed via electron beam melting (EBM) was investigated. The relationship among fatigue property, internal stress and microstructure was established to characterize cyclic-deformation behavior and LCF life of EBM Ti-6Al-4 V alloy. The LCF life of EBM Ti-6Al-4 V is almost comparable to the wrought counterparts under low strains and higher than that of selective laser melted Ti-6Al-4 V reported. Tension-compression asymmetry and cyclic softening behavior were found at various strains. Considering the asymmetry of hysteresis loops, internal stresses during tension and compression were distinguished. The anisotropy of tension-compression enhances with increasing strain due to the increase of back stress and the decrease of friction stress. Sub-grain boundaries along with fragmented β phase were observed. The formation of sub-grains and dislocation annihilation in α grains reduce the friction stress, resulting in cyclic softening.

Experimental And Numerical Modelling of Cyclic Softening and Damage Behaviors for a Turbine Rotor Material at Elevated Temperature

In order to better understand the physical process of deformation and cyclic softening a 12% Cr martensitic stainless steel FV566 has been cyclically tested at high temperature in strain control. Increase in temperature was found to increase the cyclic life, softening rate and viscous stress magnitude. An increase in the dwell time led to the acceleration of the material degradation. The microstructure changes and dominating deformation mechanisms were investigated by means of scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results have revealed a gradual sub-grain coarsening, transformation of lath structure into fine equiaxed sub-grains, and misorientation angle development in blocks and packets until material failure. Further, 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.

Cyclic elasto-plasticity behavior and fatigue crack initiation lives of low carbon steels and their simulated HAZ.

Fatigue is one of the major causes of structural failure. In particular, the crack initiation and propagation are usually located in correspondence of the welded parts of a structure. Therefore, the correct evaluation and prediction of the fatigue life of welded components is a crucial aspect to avoid unexpected failure and to improve the design and durability of structural parts. The present study focuses on the cyclic stress-strain behavior in the weld heat-affected zone (HAZ), where mostly fatigue cracks are found. In order to investigate the cyclic plasticity behavior of HAZ including cyclic hardening and softening together, experimental and numerical campaigns are carried out. The experimental part of this study deals with the fatigue experiments conducted on test specimens that have undergone the same temperature history to which the heat-affected zone of a weld is exposed (hereafter, simulated HAZ) and the softening and hardening response of base metal, CGHAZ and FGHAZ are observed. Numerical investigations aim to reproduce the cyclic stress–strain behavior simulating the nonlinear material behavior utilizing an elasto-plasticity model called the Fatigue SS Model (hereafter, FSS model). The main feature of the FSS model is the ability to describe the cyclic softening behavior within a macroscopically elastic stress state. The cyclic stress-strain behavior predicted by numerical simulation showed good agreement with the experimental result. Finally, fatigue crack initiation life criteria of SM490A is proposed based on experimental result.

Coupling of phase field and viscoplasticity for modelling cyclic softening and crack growth under fatigue

A coupled phase field-viscoplasticity approach was developed to model the deformation and crack growth in a nickel-based superalloy under fatigue. The coupled model has an advantage in predicting the cyclic softening behavior of the alloy caused by fatigue damage, overcoming a major limitation of the original cyclic viscoplasticity model. The coupled approach is also highly effective in predicting fatigue crack propagation under varied dwell times at peak load, an important behavior for crack growth under dwell fatigue. By incorporating the stress state factor, the coupled model is further utilized to investigate the growth behavior of 3D cracks under fatigue. Both the geometrical feature of the 3D crack front and the overall crack growth rate are well captured, confirming the predicative capability of the coupled model.

Microstructure evolution of 55Ni−23Cr−13Co nickel-based superalloy during high-temperature cyclic deformation

The cyclic deformation behavior and microstructure evolution of the 55Ni−23Cr−13Co nickel-based superalloy were studied at 750 °C under the strain amplitudes from 0.35% to 0.6%. Coffin−Manson−Basquin and Smith−Watson−Topper relationships were employed, which satisfactorily predicted the fatigue life of the alloy under various strain amplitudes. The superalloy showed an initial cyclic hardening as a result of the interaction between the dislocations and the precipitates, and following cyclic softening behavior mainly due to the shearing of the γ′ phase by dislocations and dislocations recovery under all strain amplitudes. Microstructure analyses showed that the M23C6 carbides exhibited a continuous-chain distribution at lower strain amplitudes, while they showed a discontinuous distribution at higher strain amplitudes. As the strain amplitude increased, the size of the γ′ phase decreased as the consequence of repeated shearing by dislocations. Fracture mechanisms were analyzed. Under higher strain amplitudes, cavities preferred to form around grain boundaries.

Low cycle fatigue performance of DP900 steel under cyclic shear paths

AbstractIn this study, a sheet fatigue shear test device is designed and applied to the low‐cycle fatigue testing of DP900 with varying strain amplitudes within the range of 0.5%–6.0%. The microstructure is analyzed by using electron backscatter diffraction, and fracture surfaces are examined via scanning electron microscopy. Results indicate that the material exhibits cyclic softening behavior after the first two cycles of hardening, with a stable softening rate and a high damage evolution rate related to loading amplitude. The variation of the hysteresis curve in the cyclic process is shown. Total plastic strain energy absorbed increases as loading amplitude decreases and it reaches the peak at approximately 1%. The life prediction model based on plastic strain energy density and strain amplitude is verified to be suitable for the cyclic shear path. The influence of microinhomogeneity on the distribution of stress and strain, especially the deformation of martensite, is closely related to the bi‐linear region of fatigue life curve.

The Investigation of the Fracture Behavior of a Chinese 9% Cr Steel Welded Joint under Creep-Fatigue Interactive Loading

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.

Effect of environmental media on the fatigue property of Chinese A508-3 steel of AP1000

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.

Damage behaviour in different micro-regions of P92 steel weldment under sequential low cycle fatigue and creep loading

Understanding the evolution of microstructure and micro-region properties of weldments during cyclic loading is a challenge for the reliability assessment of high-temperature components. This work is devoted to quantitatively evaluating the variation of micro-region properties and corresponding responsible microstructural features of P92 steel weldments under interrupted fatigue tests and subsequent creep fracture. To achieve this target, high-resolution characterization techniques, including electron backscatter diffraction (EBSD) and nanoindentation tester were used. The results revealed that the microstructures in the inter-critical heat affected zone (ICHAZ) and adjacent regions, fine-grained HAZ (FGHAZ) and parent metal (PM), show significant sensitivity to fatigue loading. Notably, the grain size in the ICHAZ reaches saturation after fatigue cycles of 10% fatigue lifetime, and the weld metal (WM) keeps unchanged throughout the fatigue loading. After subsequent creep fracture, only the WM presents an increasing trend in the average grain size. Nanoindentation tests uncover that the reductions of the elastic modulus and microhardness in each region also present three cyclic softening behaviour during the fatigue process due to the evolution of martensite lath structure. However, due to nucleation and growth of cavities accelerated by carbide precipitation, relatively low elastic modulus and microhardness were observed after subsequent creep loading.

Fatigue‐creep interaction performance of Incoloy 825 nickel‐based superalloy at 650 °C

AbstractThe fatigue‐creep interaction performance of Incoloy 825 nickel‐based superalloy at 650 °C was investigated through introducing the tensile, compressive, and tensile‐compressive strain hold time at the controlled total strain amplitude Δϵt = 0.3 %∼0.7 %. The results show that the Incoloy 825 nickel‐based superalloy exhibits the cyclic hardening behavior, the cyclic hardening behavior followed by cyclic softening behavior and the cyclic hardening behavior followed by cyclic stability during the cyclic deformation with tensile strain hold time, while the alloy exhibits the cyclic hardening behavior and the cyclic hardening behavior followed by the cyclic stability during the cyclic deformation with compressive and tensile‐compressive strain hold time. The relationship between both plastic and elastic strain amplitudes with reversals to failure for the alloy shows a single slope linear behavior, which can be described by the Coffin‐Manson and Basquin equations, respectively. The deformation mechanism of the alloy under three loading condition of fatigue‐creep interaction is mainly the planar slip. In addition, under three loading condition of fatigue‐creep interaction, the cracks initiate and propagate in the transgranular mode.

A Study of Thermal Cyclic Softening Behavior of Hot-Deformed Die Steel

A Study of Thermal Cyclic Softening Behavior of Hot-Deformed Die Steel