Cyclic Hardening Behavior Research Articles

Influence of strain amplitude on the development of dislocation structure during cyclic plastic deformation of 304 LN austenitic stainless steel

In the present study, electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) were used to document the evolution of dislocations in a 304 LN austenitic stainless steel subjected to cyclic plastic deformation at room temperature using different strain amplitudes. In particular EBSD was used to quantify the development of geometrically necessary dislocation density (GND) with varying strain amplitudes. Solution annealed multiple specimens were cyclically deformed at varying strain amplitudes ranging from ±0.25 to ±1.2% under completely reversed straining conditions until complete failure. It is found that GND density, which is independent of the character of the dislocations, increases with the increase of strain amplitude. This is consistent with the cyclic hardening characteristic of the investigated steel. It is found that the evolution of GND structure during the course of deformation is heterogeneous in nature with small grains having higher GND density than the coarse grains. The clustering of GNDs is visible near grain boundaries leaving the grain interior free. EBSD can capture the variation in GND storage patterns with strain amplitudes. The heterogeneity of the cyclic deformation induced GND structure decreased with the increase of the strain amplitude. The evolution of GND structure is also found to be sensitive to the size and orientation of the austenite grains. It has been found that the GND density is maximum in {111}<0-11> grains, those with the highest Taylor factor. The fidelity of dislocation storage pattern with strain amplitudes is assessed using ECCI. This is consistent with the GND storage pattern shown by EBSD and explains the cyclic hardening behavior of the steel.

Elevated temperature low cycle fatigue behaviour of Haynes 282 and its correlation with microstructure – Effect of ageing conditions

In this paper, total strain-controlled high temperature (760 °C) low cycle fatigue (HTLCF) behaviour (at 0.5% strain amplitude) of a new γ′-strengthened wrought Ni based superalloy (Haynes 282) for various ageing conditions is presented for the first time, emphasizing the microstructural effects. Haynes 282 is a promising material for hot gas path components of ultra-supercritical steam turbines due to its adequate fatigue property at elevated temperatures, along with high creep and oxidation resistance. Various microstructures were generated by varying ageing time (1 day, 6 days and 9 days) and temperature (650 °C and 760 °C). While continuous cyclic hardening behaviour was witnessed for lower aging temperature (650 °C), predominantly cyclic softening after initial hardening through few cycles was observed for higher aging temperature (760 °C), due to formation of more number of larger size voids and secondary cracks in the later specimen during the testing (as characterized by X-ray tomography). Underlying deformation mechanisms (characterized through TEM) and fracture modes (characterized through SEM, EDS) were correlated to alloy's microstructures under various HTLCF conditions; distinct differences were observed between the two ageing temperatures. While 650 °C, 9 days ageing condition showed primarily signatures of γ′ shearing, 760 °C, 9 days ageing condition showed primarily the Orowan mechanism of deformation. For 650 °C ageing condition, predominantly intergranular fracture mode has been observed, which is attributed to the presence of brittle primary MC particles on the boundaries. Whereas for the 760 °C ageing condition, the fracture mode remained predominantly transgranular because it involves less crack deflection through its path. Plastic strain energy density based life prediction model were used to predict fatigue life. Close agreement between calculated and measured life indicates validity of these models for the present material.

Modelling Cyclic Behaviour of Martensitic Steel with J2 Plasticity and Crystal Plasticity

In order to capture the stress-strain response of metallic materials under cyclic loading, it is necessary to consider the cyclic hardening behaviour in the constitutive model. Among different cyclic hardening approaches available in the literature, the Chaboche model proves to be very efficient and convenient to model the kinematic hardening and ratcheting behaviour of materials observed during cyclic loading. The purpose of this study is to determine the material parameters of the Chaboche kinematic hardening material model by using isotropic J2 plasticity and micromechanical crystal plasticity (CP) models as constitutive rules in finite element modelling. As model material, we chose a martensitic steel with a very fine microstructure. Thus, it is possible to compare the quality of description between the simpler J2 plasticity and more complex micromechanical material models. The quality of the results is rated based on the quantitative comparison between experimental and numerical stress-strain hysteresis curves for a rather wide range of loading amplitudes. It is seen that the ratcheting effect is captured well by both approaches. Furthermore, the results show that concerning macroscopic properties, J2 plasticity and CP are equally suited to describe cyclic plasticity. However, J2 plasticity is computationally less expensive whereas CP finite element analysis provides insight into local stresses and plastic strains on the microstructural length scale. With this study, we show that a consistent material description on the microstructural and the macroscopic scale is possible, which will enable future scale-bridging applications, by combining both constitutive rules within one single finite element model.

Fatigue-driven acceleration of abnormal grain growth in nanocrystalline wires

Molecular dynamics simulations were employed to simulate the mechanical response and grain evolution in a Ni nanowire for both static and cyclic loading conditions at both 300 and 500 K for periods of 40 ns. The loading conditions included thermal annealing with no deformation, constant 1% extension (creep loading) and cyclic loading with strain amplitudes of 0.5% and 1% for 200 cycles. Under cyclic loading, the stress–strain response showed permanent deformation and cyclic hardening behavior. At 300 K, modest grain evolution was observed at all conditions within the 40 ns simulations. At 500 K, substantial grain growth is observed in all cases, but is most pronounced under cyclic loading. This may result mechanistically from a net motion of the boundaries associated with boundary ratcheting. There is a striking qualitative consistency between the present simulation results and the experimental observation of abnormal grain growth in nanocrystalline metals as a precursor to fatigue crack initiation.

Experimental and numerical investigations of low cycle fatigue behavior of cryorolled AA 5754

The objective of the present work is to study the influence of cryogenic forming on low cycle fatigue (LCF) properties of Al-Mg (AA 5754) alloy. AA 5754 is rolled at liquid nitrogen temperature (cryorolling) for maximum of 40% thickness reduction. Post-annealing heat treatment (PAHT) at 250oC for 2 hours is adopted to enhance the mechanical and fatigue properties of the cryorolled alloy. A series of uniaxial strain controlled LCF tests are executed at three different strain amplitudes from 0.4% to 0.6% for cryorolled (CR) and annealed cryorolled (ACR) AA 5754 alloy. PAHT significantly improves the tensile strength as well as ductility of CR alloys. LCF tests reveal that cryorolled alloys show mild hardening in the initial cycles followed by noticeable cyclic softening at all strain amplitudes. In contrast, only cyclic hardening behavior is depicted by ACR specimens at all strain amplitudes. The extended finite element method (XFEM) has been utilized to simulate the tensile behavior of the specimen. Finite Element Method (FEM) coupled with Chaboche kinematic hardening cyclic plasticity model is used for simulating hysteresis loop obtained during LCF test. The present study exhibits good match between simulations and experimental results.

Improved fatigue life of the newly developed Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy

Fe-15Mn-10Cr-8Ni-4Si is a newly developed alloy for seismic damper applications. The low- and high-cycle fatigue (LCF and HCF) properties of this alloy was investigated in this study. The LCF resistance of this alloy was found to be superior to that of other materials. The inelastic component was established to be the main contributor to the superior fatigue life of Fe-15Mn-10Cr-8Ni-4Si, including at relatively low strain levels. This alloy demonstrated a gradual and mild cyclic hardening behavior at a wide range of strains. In terms of the HCF property, the fatigue limit was 262.5 MPa, which is 96% of 0.2% proof stress of 274 MPa. A surface observation of the post-fatigued specimen tested at a stress amplitude of 250 MPa for 107 cycles without failure showed that while surface relief developed, no cracks with a size of at least several grains were detected. Since Fe-high Mn alloys generally exhibit several unique deformation modes involving the deformation-induced phase transformation, the microstructure of Fe-15Mn-10Cr-8Ni-4Si after fatigue was investigated using electron-backscatter diffraction (EBSD) microscopy. As a result, ε-martensite formed via the deformation-induced γ→ε martensitic transformation was detected in all specimens tested under different conditions. Even at a stress level lower than the fatigue limit, ε-martensite formed while the amount of it was very limited. Except for the specimen tested at a low stress level of 275 MPa, a well-developed deformation-induced microstructure was found near the crack; α′-martensite formed by the two-stage phase transformation γ→ε→α′ was detected only in the region near the crack in a specimen tested at a total strain range of 2%.

Strain controlled low cycle fatigue properties of stepped aging heat treated AA5754

Abstract In present work strain controlled low cycle fatigue (SC-LCF) properties of stepped ageing heat treated(SAHT) AA5754 alloy is evaluated at three different strain amplitudes (0.4–0.6)% at fixed strain rate of 0.001 sec-1. SAHT process significantly improves the mechanical and fatigue life of the as-received alloy. The material shows cyclic hardening behavior at all the strain amplitudes. The hysteresis loop areas for SAHTed alloys are found to be significantly higher at corresponding elapsed cycles at higher strain amplitudes. Finite element simulation using Chaboche cyclic plasticity is utilized to predict the cyclic response of the alloy. Chaboche kinematic hardening material parameters are determined from stabilized hysteresis loop. Numerical results agree well with experimentally obtained results.

Cyclic deformation behavior of an Fe-Ni-Cr alloy at 700 °C: microstructural evolution and cyclic hardening model

The cyclic deformation behavior of an Fe-Ni-Cr alloy was studied based on low-cycle fatigue (LCF) experiments at 700 °C with various total strain amplitudes Δεt/2 A cyclic hardening behavior was found for Sanicro 25 alloy. The increasing rate of stress amplitude increased first, and subsequently decreased with prolonged cycles. The cyclic hardening ratio increased when Δεt/2 increased. Microstructure observations revealed that the Cr23C6 in the grain interiors were mainly distributed around the NbCrN precipitate. Additionally, the Cr23C6 precipitate aggregated and coarsened with an increasing Δεt/2 The shape of Cr23C6 precipitate also changed from rectangular to circular. The fracture surface observation showed the precipitates can be observed at the crack initiation site and in the microcavities and dimples. It indicates that the precipitates affect the fatigue fracture. In addition, the fraction of deformed grains increases as Δεt/2 increases. Furthermore, based on the modified Fisher-Hart-Pry (FHP) back-stress influenced by precipitates with different radii, the influences of the glide strain, half spacing of precipitates, and radius of precipitates on FHP back-stress were investigated. The applied stress increases with increasing glide strain and radius of precipitates. When the half spacing of precipitates increases, the applied stress decreases.

Study on cyclic constitutive model and ultra low cycle fracture prediction model of duplex stainless steel

This paper calibrates micromechanical fracture models for duplex stainless steel in order to study its fracture behaviour under seismic loading, characterized by large strain ultra low cyclic conditions. In this investigation, smooth round bars and notched round bars were subject to monotonic and cyclic loading. Monotonic and cyclic loading constitutive models were calibrated against smooth round bar test tests. The fracture toughness parameters of the void growth model (VGM), the stress modified critical strain (SMCS) model and the cyclic void growth model (CVGM) were calibrated against the notched round bar tests. Furthermore, the skeleton curve, hysteresis loop and characteristic length of duplex stainless steel were also investigated. The test results indicate that duplex stainless steel exhibits cyclic hardening behaviour under cyclic loading, and the subsequent numerical simulation successfully captured this behaviour. The toughness parameters of duplex stainless steel are generally higher than those for carbon steel grades Q345B and Q460, which demonstrates that duplex stainless steel has better toughness. All the experimental results provide useful data for the prediction of ultra low cycle fatigue (ULCF) fracture of duplex stainless steel.

Low Cycle Fatigue Behavior of Steam Generator Tubes under Axial Loading.

Compared with the fatigue properties of the material (Inconel Alloy 690), the real fatigue lives of tubes are more meaningful in the fatigue design and assessment of steam generator (SG) tube bundles. However, it is almost impossible to get a satisfactory result by conducting fatigue tests on the tube directly. A tube with a uniform and thin wall easily fails near the clamping ends under cyclic loading due to the stress concentration. In this research, a set-up for fatigue tests of real tubes is proposed to overcome the stress concentration. With the set-up, low cycle fatigue tests were conducted in accordance with an existing fatigue design curve for Alloy 690. Strain control mode was applied with fully reversed push–pull loading under different strain amplitudes (0.15%, 0.2%, 0.3%, and 0.4%). A favourable result was obtained, and the low cycle fatigue behavior was investigated. The results showed that the fatigue life tested by the real tube was below the strain–life curve of Alloy 690 which was fitted by conventional solid specimens. A cyclic hardening behavior was found by the cyclic stress–strain curve when compared with the monotonic stress–strain curve.

Life, dislocation evolution, and fracture mechanism of a 41Fe-25.5Ni-23.5Cr alloy during low cycle fatigue at 700 °C

The low-cycle fatigue behavior, dislocation evolution, and fracture mechanism of a 41Fe-25.5Ni-23.5Cr alloy, Sanicro 25, were investigated under various total strain amplitudes of 0.3%, 0.35%, 0.4%, and 0.5% at 700 °C. According to strain-controlled fatigue tests, this alloy exhibited a cyclic hardening behavior. The back stress and friction stress increased with increasing total strain amplitude, which was attributed to the increase in the interactions of the dislocations with precipitates and dislocations. The hysteresis loops showed that the serrated flow was particularly pronounced at the total strain amplitude of 0.5%. Furthermore, the plastic strain amplitude and the hysteresis loop area increased as the total strain amplitude increased. The modified relationship of fatigue life and plastic strain amplitude (Nf-Δεp/2) was established based on the energy-based prediction model of fatigue life, which was influenced by the parameters cyclic strain hardening exponent n′ and cyclic strength coefficient K′. Nf increased with the increase in n′, whereas decreased with the increase in K′ at the same Δεp/2. TEM observations showed that straight dislocation lines were transformed into zigzag dislocation lines with increasing total strain amplitude. The relationship between the deformation and misorientation was analysed, and the fraction of grains with relatively high Taylor factor increased with increasing total strain amplitude. Finally, the microstructures of the fracture surface and the fracture frontier were studied to understand the fracture mechanism. The intergranular cavities, dimples and tear ridges were observed from fracture surface. The propagation path of microcrack at the fracture frontier was also shown, which indicated a mixture of transgranular and intergranular fracture modes.

Cyclic stress responses of a newly developed nickel-base superalloy at elevated temperatures

Total strain-controlled low cycle fatigue tests were conducted on the newly designed nickel-base superalloy M951G under different testing conditions; the relationship among cyclic stress responses, microstructural degradations, deformation mechanisms and testing conditions has been established. Results show that both the cyclic hardening and softening behaviors are dependent on the testing temperature and strain amplitude. As the strain amplitude increases both at 900 and 1000 °C, M951G alloy exhibits cyclic hardening under low strain amplitudes and cyclic softening under higher strain amplitudes. At 900 °C, the initial cyclic hardening is related to the coherent γ/γ′ interface, parallel dislocation arrays and dislocation bypassing the tiny γ′ particles. At higher strain amplitudes, the initial cyclic softening is due to the higher density of shearing dislocations in γ′ precipitates. At 1000 °C, plenty of parallel dislocation arrays present in γ channels, which reduces the possibility of dislocation interactions from different slip systems and results in initial cyclic hardening. Under higher strain amplitudes, apart from microstructural degradations, dislocations shearing into γ′ precipitates and formation of dislocation networks, the dislocation annihilation and partial loss of coherency of γ′ precipitates are also responsible for the initial cyclic softening at 1000 °C.

Low cycle fatigue behavior of AlSi10Mg(Cu) alloy at high temperature

Low cycle fatigue (LCF) behavior of AlSi10Mg(Cu) alloy was investigated at 100 °C and 250 °C over a range of strain amplitude from 0.25% to 0.40%. Cycle stress response, hysteresis loops, fatigue life, as well as post-fatigue microstructure were studied in detail. Except 0.25%, the alloy performed cycle hardening behavior at 100 °C and softening behavior at 250 °C at any given strain amplitude, which corresponded well with the observed phenomena in hysteresis loops. At any given temperature, cyclic stress increases while fatigue life decreases with the increasing strain amplitude. The fatigue life of the studied alloy at 100 °C was higher than that at 250 °C under the same strain amplitude. Fractography indicated that most fatigue crack initiated from casting defects. The fatigue crack propagation was mainly characterized by fatigue striations and tear ridges in any conditions. Section inspection shows that the number of cracked eutectic Si increases with decreasing the temperature or increasing strain amplitude. Post-mortem transmission electron microscopy (TEM) investigations indicated that cyclic hardening behavior at 100 °C was due to the pinning effect of coherent precipitates (β″-Mg2Si and GP zones). Most of tiny coherency precipitates and clusters transformed to large semi-coherency Q′ and β′-Mg2Si precipitates after cyclic loading at 250 °C, leading to the sharp decrease of pinning number and dislocation density, which is a key reason for the cyclic softening. Another factor is that more dislocations can be activated and dislocation climb is easier at higher temperature.

Substructural evolution in a directionally solidified nickel-base superalloy during relaxation fatigue tests at 850 °C

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.

Effect of isothermal and non-isothermal aging on the low cycle fatigue behavior of an Al–Cu–Mg–Si forging alloy

Low cycle fatigue (LCF) behavior of an Al–Cu–Mg–Si forging alloy was investigated at room temperature. Isothermal aging (T6) and non-isothermal aging (NIA) treatments were applied to produce different microstructures. Under the same strain amplitude, the LCF life of NIA specimen exceeded that of T6 specimen. Besides, NIA specimens exhibited cyclic stability behavior in contrast to cyclic hardening behavior of T6 specimens. The different LCF behaviors could be rationalized by different deformation mechanisms under cyclic straining. T6 specimens with non-shearable θ′-phases underwent Orowan bypassing mechanism leading to the cyclic hardening behavior. While NIA specimens with shearable and non-shearable θ′-phases suffered from both particle shearing and Orowan bypassing mechanisms resulting in the cyclic stability behavior. Non-shearable θ′-phases bypassed by dislocations introduced more stress concentration, while shearable θ′-phases cut through by dislocations caused less stress concentration. Therefore, fatigue life of NIA specimen is longer than that of T6 specimen.

Enhanced low-cycle fatigue and crack propagation resistance of an Al-Cu-Mg-Si forging alloy by non-isothermal aging

Low-cycle fatigue (LCF) and fatigue crack propagation (FCP) behavior of an Al-Cu-Mg-Si forging alloy with different non-isothermal aging (NIA) degrees were investigated. The main question addressed was about the interactions between precipitates and dislocations and their influences on LCF and FCP resistance. Strain-controlled LCF and ΔK-controlled FCP were tested. The characterization of precipitates and fractography were performed on TEM/HRTEM and SEM, respectively. The results showed that under cyclic loading, shearable precipitates were suffered from particle shearing mechanism associated with slip localization, resulting in cyclic softening behavior and low FCP rates; non-shearable precipitates were suffered from bypassing mechanism associated with homogeneous slip, resulting in cyclic hardening behavior and high FCP rates. The mixed precipitation characteristics obtained by complete NIA treatment, consisting of shearable and non-shearable precipitates, led to cyclic stability behavior and low FCP rates, which means a simultaneous enhancement of LCF and FCP resistance.

Effect of aging on high temperature fatigue properties of Ni-23Cr-7W alloy for boiler pipes and tubes

This study focuses on the effect of aging on fatigue behavior of nickel-based Ni-23Cr-7W alloy for boiler pipes and tubes to reveal the fatigue properties of the alloy after long-term exposure (1000–5000 h) to high temperature (700 °C). Tensile, Charpy impact, and high temperature fatigue tests were conducted for both aged and non-aged alloys. An increase in the tensile strength and a decrease in the tensile ductility and Charpy absorbed energy caused by the aging were observed. The fatigue life of the aged alloy is slightly shorter than that of the non-aged alloy under PP (fast-fast) type of strain waveform. Under CP (slow-fast) type of strain waveform, the fatigue life of the aged alloy is slightly shorter in the high strain range region and equal to or slightly longer in the low strain range region as compared with the non-aged alloy. The non-aged alloy shows strong cyclic hardening behavior. Therefore, the stress–strain response of the non-aged alloy after fatigue loading locates in the higher stress side than that of the aged alloy, although the stress–strain curve of the non-aged alloy obtained by the tensile test locates in the lower stress side. The observations of the morphology of precipitates and dislocations for the alloy before and after the fatigue test reveal that fine carbides precipitate uniformly in the non-aged alloy during the fatigue test. Furthermore, it is revealed that the cyclic hardening caused by aging has a significant influence on the generation of inelastic strain in the non-aged alloy under low strain range conditions. Although a large influence of aging at high temperature occurs on the cyclic hardening behavior, aging does not significantly affect the fatigue life of Ni-23Cr-7W alloy.

Low cycle fatigue analysis of ferrite-martensite dual-phase steel using advanced cyclic plasticity models

A computational model has been developed to predict the low cycle fatigue (LCF) characteristics of ferrite-martensite dual phase (DP) steel. LCF results of DP steel performed at strain amplitudes varying from 0.3% to 1.5% show initial cyclic hardening followed by cyclic softening behavior. Finite element simulation employing Chaboche model fails to capture the initial cyclic hardening behavior specifically at lower strain amplitudes. To predict the observed cyclic hardening and softening characteristics of DP steel with a progressive number of cycles, Ohno-Wang cyclic plasticity model has been modified by altering the yield function, which is formulated by incorporating a memory stress function. All the material parameters have been calibrated considering hysteresis loop of the first cycle of LCF result of ±1.5% uniaxial strain. The developed material model has been integrated with ABAQUS 6.14 finite element package to perform the LCF simulation. A comparison between simulation and experimental results for all strain amplitudes reveals that the proposed model has the better capability to predict the overall LCF characteristics of DP steel.

High Temperature Fatigue Properties Research of GH4169 under Multiaxial Cyclic Loading

The high temperature (550°C and 650°C) fatigue properties of GH4169 for thin-wall tube specimen are investigated under uniaxial tension, uniaxial torsion, proportional tension-torsion and nonproportional tension-torsion. All tests are strain-controlled. The results indicate that the shape of the hysteresis loops of uniaxial tension, uniaxial torsion and proportional tension-torsion are similar, but hysteresis loop of non-proportional tension-torsion has distortion; the cyclic softening behavior is shown for GH4169 under uniaxial tension, uniaxial torsion and proportional tension-torsion, but the cyclic hardening behavior is shown for the first several cycles of nonproportional tension-torsion.

Low cycle fatigue property and fracture behavior of low yield point steels

For seismic control and isolation techniques, low yield point steels are among the most reliable and ideal energy-dissipating materials. Structures under earthquake conditions are usually subjected to cyclic loads with large strain amplitudes, where excellent low cycle fatigue and fracture performance under repeated loading is an essential requirement. In the study presented in this paper, comprehensive and systematic investigation was conducted on the low cycle fatigue properties and fracture behaviors of low yield point steels (LY100, LY160, and LY225). Axial steel coupons were tested under different constant strain amplitudes ranging from 0.5% to 6% with increments of 0.5%. Following introduction of the experimental details, observations and fatigue life are reported. Then, based on the experimental data, the cyclic hardening behavior and the cyclic stress-strain response are thoroughly analyzed. Finally, with the aim of predicting the low cycle fatigue life, the material parameters of the Coffin-Manson relationship and Kuroda model are determined and comparative analyses are performed. This paper may provide a valuable reference for engineering applications and further research into the low cycle fatigue performance of low yield point steels.