Strain-controlled Low Cycle Fatigue Research Articles

High-temperature low-cycle fatigue and fatigue–creep behaviour of Inconel 718 superalloy: Damage and deformation mechanisms

In this article, strain-controlled Low-Cycle Fatigue (LCF) and fatigue–creep tests were performed on Inconel 718 nickel-based superalloy at temperatures of 650 °C and 730 °C. LCF tests at elevated temperatures were performed with a mechanical strain rate of 1×10−3/s, while fatigue–creep tests involved either tensile or compressive strain dwell. Both the LCF and fatigue–creep tests revealed cyclic softening, with the mean stress evolving oppositely to the applied strain dwell in the fatigue–creep tests. Investigations into the damage mechanisms identified intergranular cracking as the predominant failure mode. Fatigue–creep loading with a compressive dwell resulted in multiple crack initiations from transgranular oxide intrusions, along with multiple creep cavities during loading at 730 °C. Deformation features such as persistent slip bands and deformation nanotwins were observed during cycling at 650 °C. In addition, fatigue–creep tests at 730 °C exhibited δ phase precipitation and a coarsening of strengthening precipitates, contributing to additional softening that increased over prolonged test durations. Finally, the observed lifetime during LCF tests decreased with increasing temperatures, and fatigue–creep loading was observed to be more damaging than LCF. On the other hand, fatigue–creep loading with a tensile strain dwell demonstrated a higher lifetime compared to LCF at 730 °C.

Application of Instrumented Indentation Procedure in Assessing the Low-Cycle Fatigue Properties of Selected Heat-Treated Steels.

The paper presents an analysis of the low-cycle fatigue (LCF) properties of C45, X20Cr13, and 34CrNiMo6 steels subjected to various heat treatment processes. Strain-controlled LCF tests were carried out with a total cyclic strain amplitude equal to 0.5, 1 and 1.5%. Fatigue life, cyclic stress-strain behavior and hardness were analyzed. Qualitative and quantitative relationships between material LCF properties resulting from the heat treatment processes, were related to the indentation force P*, which was derived experimentally by applying an instrumented indentation procedure with the use of the Vickers indenter. The proposed parameter P* and its changes ΔP* seem to be promising for the identification of the structural stress parameter σ* that is necessary for deriving values of the fatigue strength coefficients σf' corresponding to different tempering temperatures. The common feature of all steels analyzed in this paper is that the elastic parts of the strain-life characteristics remain parallel after being subjected to different tempering temperatures.

Influence of Severe Plastic Deformation and Aging on Low Cycle Fatigue Behavior of Al-Mg-Si Alloys.

Strain-controlled low cycle fatigue (LCF) tests were conducted on conventionally grained (CG) and ultrafine-grained (UFG) Al-Mg-Si alloys treated under various aging conditions. In the cyclic stress response (CSR) curves, CG peak-aged (PA) alloys showed initial cyclic hardening and subsequent saturation, whereas CG over-aged (OA) alloys displayed cyclic softening behavior close to saturation. The UFG materials exhibited continuous cyclic softening except for UFG 3; it originates from the microstructural stability of the UFG materials processed by severe plastic deformation (SPD). Using a strain-based criterion, the LCF behavior and life of the CG and UFG materials were analyzed and evaluated; the results are discussed in terms of strengthening mechanisms and microstructural evolution. In the CG materials, the LCF life changed markedly owing to differences in deformation inhomogeneity depending on the precipitate state. However, the UFG materials displayed a decreasing LCF life as cyclic softening induced by dynamic recovery became more severe; additionally, a relationship between the microstructural stability of the UFG materials and the cyclic strain hardening exponent n' was suggested.

Comparison of low cycle fatigue behaviour of additively manufactured and wrought Inconel625 alloys

The present investigation highlights a comparison of strain-controlled low cycle fatigue (LCF) behaviour between the additively manufactured (AM) and wrought (WR) Inconel625 alloys at the total strain amplitude levels of ±0.4, ±0.5, ±0.6 and ±0.8 % at 650 °C. The cyclic life has been found to be lower in the AM alloy as compared to the WR alloy. The evolution of precipitates such NbC and γ″ are found in both type of alloys after LCF tests. However, the AM alloy has shown a significant presence of (Cr,Mo)23C6 along the grain boundaries, which acts as a potential source of generation of the intergranular cracks during cyclic deformation. Although, a relatively fine grain structure and enhanced number density of high-angle boundaries have been identified in the LCF-tested samples of AM alloy through EBSD analyses, the formation of intergranular cracks in the AM alloy has resulted in the reduction of cyclic life of the AM alloy.

Effect of Testing Conditions on Low-Cycle Fatigue Durability of Pre-Strained S420M Steel Specimens.

S420M steel subjected to strain-controlled low-cycle fatigue does not exhibit a period of cyclic properties stabilization. The maximum stress on a cycle continuously drops until fracture. For this reason, it is difficult to plan experimental research for different types of control in such a way that their results can be considered comparable. The aim of this paper is to present and discuss the results of tests conducted in various conditions of low-cycle fatigue of S420M steel specimens, both undeformed and pre-strained. In both loading conditions, after initial deformation, a significant change in the cyclic properties of steel described by the parameters of the hysteresis loop was observed. Also, the fatigue life of the pre-strained specimens appeared to be different from unstrained specimens and was affected by the test loading conditions. The reduction in life under controlled stress conditions was attributed to the increase in the extent of plastic deformation and stress and the occurrence of creep.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

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.

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.

Low cycle fatigue behavior of additive manufactured 316LN stainless steel at 550 °C: Effect of solution heat treatment

Heat treatment is used to change the microstructure and improve the low-cycle fatigue (LCF) performance of additive manufactured (AM) 316LN stainless steel (SS) in this work. Strain-controlled low cycle fatigue tests were performed on the as-built (AB) and heat-treated (HT) AM 316LN SS at 550 °C with strain amplitudes ranging from 0.4 % to 1.0 %. The AB material presents a cyclic softening behavior while HT material demonstrates an initially cyclic hardening followed by softening behavior, which is thoroughly analyzed based on the cyclic stress decomposition and microstructure characterizations. The crack nucleates at the specimen surface or near-surface defects in both AB and HT materials. The HT material presents a stronger resistance to crack propagation in comparison to AB material due to the modified microstructures after heat treatment. Correspondingly, the fatigue life of HT material is longer than AB material.

Isothermal low-cycle fatigue and fatigue–creep behaviour of 2618 aluminium alloy

A series of strain-controlled Low-Cycle Fatigue (LCF) and fatigue–creep tests were performed on a 2618 aluminium alloy for various strain amplitudes at temperatures of 20 °C, 160 °C and 195 °C. LCF tests at elevated temperatures were performed for three different strain rates between 1×10−4/s and 1×10−2/s, while fatigue–creep tests were performed with either tensile or compressive strain dwell. At 20 °C, cyclic hardening was associated with a high dislocation density. In contrast, a localization of plastic deformation into slip bands and cyclic softening were observed at 195 °C. The investigation of the damage mechanisms revealed transgranular cracking as the predominant failure mode, significantly influenced by Al9FeNi intermetallic particles. Finally, fatigue–creep tests were found to be more damaging than equivalent LCF tests, particularly at 195 °C, where a negative strain rate sensitivity on fatigue lifetime was evident.

Low-Cycle Fatigue Behavior of Wire and Arc Additively Manufactured Ti-6Al-4V Material.

Additive manufacturing (AM) techniques, such as wire arc additive manufacturing (WAAM), offer unique advantages in producing large, complex structures with reduced lead time and material waste. However, their application in fatigue-critical applications requires a thorough understanding of the material properties and behavior. Due to the layered nature of the manufacturing process, WAAM structures have different microstructures and mechanical properties compared to their substrate counterparts. This study investigated the mechanical behavior and fatigue performance of Ti-6Al-4V fabricated using WAAM compared to the substrate material. Tensile and low-cycle fatigue (LCF) tests were conducted on both materials, and the microstructure was analyzed using optical microscopy and scanning electron microscopy (SEM). The results showed that the WAAM material has a coarser and more heterogeneous grain structure, an increased amount of defects, and lower ultimate tensile strength and smaller elongation at fracture. Furthermore, strain-controlled LCF tests revealed a lower fatigue strength of the WAAM material compared to the substrate, with crack initiation occurring at pores in the specimen rather than microstructural features. Experimental data were used to fit the Ramberg-Osgood model for cyclic deformation behavior and the Manson-Coffin-Basquin model for strain-life curves. The fitted models were subsequently used to compare the two material conditions with other AM processes. In general, the quasi-static properties of WAAM material were found to be lower than those of powder-based processes like selective laser melting or electron beam melting due to smaller cooling rates within the WAAM process. Finally, two simplified estimation models for the strain-life relationship were compared to the experimentally fitted Manson-Coffin-Basquin parameters. The results showed that the simple "universal material law" is applicable and can be used for a quick and simple estimation of the material behavior in cyclic loading conditions. Overall, this study highlights the importance of understanding the mechanical behavior and fatigue performance of WAAM structures compared to their substrate counterparts, as well as the need for further research to improve the understanding of the effects of WAAM process parameters on the mechanical properties and fatigue performance of the fabricated structures.

Low cycle fatigue behavior and fatigue life prediction of 2195 Al-Li alloy at warm temperatures

The strain-controlled low cycle fatigue (LCF) tests were firstly carried out to investigate the LCF behavior of the 2195 Al-Li alloy at warm temperatures (100 and 200 °C). All initial and mid-life hysteresis loops showed a centrosymmetric characteristic. The cyclic stress response curve at 100 °C and strain of 0.6% exhibited complete cyclic hardening, while response curves under other conditions showed cyclic hardening initially followed by cyclic softening. Then, various fatigue life prediction models were employed to evaluate LCF life. The prediction model based on the total strain energy density could present the best prediction accuracy. Finally, the fatigue fracture under different conditions was observed to reveal the fatigue fracture mechanism. The fatigue striations were visible at 100 °C and strain of 0.6%, but gradually diminished with increasing temperature and strain. Furthermore, the fatigue fracture zone at 200 °C and strain of 1.0% presented an evident intergranular fracture characteristic.

Fatigue Behavior of the FGH96 Superalloy under High-Temperature Cyclic Loading.

Strain-controlled low-cycle fatigue (LCF) tests and stress-controlled creep-fatigue interaction (CFI) tests on the FGH96 superalloy were carried out at 550 °C to obtain the cyclic softening/hardening characteristics at different strain amplitudes and ratcheting strain characteristics under different hold time. The failure mechanism of the FGH96 superalloy under different loading conditions was analyzed through fracture observations. The results show that the FGH96 superalloy exhibits different cyclic softening/hardening characteristics at different strain amplitudes, and the introduction of the hold time at peak stress exacerbates the ratcheting strain of the FGH96 superalloy under asymmetric stress cycles. Fracture observations show that the magnitude of the strain amplitude, high-temperature oxidation, and the introduction of the hold time will affect the mechanical properties of the FGH96 superalloy and change its fracture mode.

Low cycle fatigue behavior of additively manufactured Haynes 282: Effect of post-processing and test temperature

This study investigated the fully reversed (R = -1), strain-controlled low cycle fatigue behavior of laser powder bed fused Haynes 282 at temperatures ranging from −195 to 871 °C and at strain amplitudes of 0.010 and 0.005 mm/mm. By comparing the fatigue behaviors of hot isostatically pressed (HIP) specimens in machined surfaces (HIP/M) and the Non-HIP ones in un-machined surfaces (Non-HIP/UM), the combined effect of surface condition and hot isostatic pressing was analyzed and discussed. The Non-HIP/UM specimens showed shorter fatigue lives compared to HIP/M ones from −195 to 649 °C, which was attributed to the presence of surface micro-notches and volumetric defects. This difference attenuated above 649 °C due to the combined effects of increased plastic deformation, which mitigated the negative impact of surface anomalies, and oxidation, which accelerated crack initiation and growth. The tests at −195 °C tended to suppress cyclic plastic deformation, especially at the lower strain amplitude, which led to improved fatigue lives. Lastly, both HIP/M and Non-HIP/UM specimens exhibited similar stress responses with respect to test temperature; at 21, 204, 760, and 871 °C, cyclic softening occurred due to shearing of γ′ precipitates by persistent slip bands, while at other temperatures, the softening was balanced or overcome by cyclic hardening due to deformation twinning (i.e., at −195 °C) and dislocation multiplication during ordinary slip (i.e., at 427 and 649 °C).

Theoretical and experimental analysis of the correlation between magnetic memory signals and cumulative ductile damage of butt weld under low cycle fatigue

Welded structures experiencing superimposed cyclic loads with large plastic strain during a strong earthquake can easily lead to ductile or fatigue fracture. A newly developed self-magnetic flux leakage (SMFL) nondestructive detection technology named metal magnetic memory (MMM) is a valid approach to monitoring the development of fatigue damage in ferromagnetic materials. To investigate the correlation between MMM signals and cumulative ductile damage for butt weld under low cycle fatigue (LCF), combining strain-based Jiles-Atherton hysteresis model, magnetic charge model, and cumulative damage mechanics model, the theoretical relations between normal component HSF(z) of MMM signals and cumulative ductile damage D for single V groove butt weld were established in this paper. Relevant monotonic tensile tests and strain-controlled LCF tests were conducted, and the HSF(z) signals on the surface of welding specimens under different cyclic loads were measured. Comparing with experimental results verified the feasibility and accuracy of the theoretical relations. The results indicated that the peak-valley gradient Kp-v on the HSF(z) signals curve was decreased monotonously as the cumulative ductile damage D increased and the rate of decrease also diminished gradually. A possible exponential descending tendency of Kp-v with increasing D was demonstrated. Besides, the influences of weld width and weld reinforcement on the correlation between Kp-v and D were discussed theoretically in detail. This research provided a potential possibility for the assessment of the cumulative ductile damage at the welding joints under LCF based on MMM technology.

Investigation of low cycle fatigue crack propagation behavior of 316H steel at 550℃ based on cyclic response and damage accumulation: experiment and modelling

In the framework of the unified elastic-plastic constitutive theory, the cyclic response and damage evolution of 316H steel in the low cycle fatigue (LCF) regime was analyzed and modeled. A unified damage-coupled elastic-plastic constitutive model was established based on the results of uniaxial strain-controlled LCF tests, which includes a novel strain-based continuum damage model in the framework of the critical plane theory, an accumulative plastic strain and fatigue damage-related kinematic hardening model, and an isotropic hardening model. The comparison between the simulated and the experimental results indicated that the proposed model is efficient to precisely characterize the cyclic response throughout the whole fatigue process. The model was also used to simulate the fatigue crack growth behavior under different maximum loads and load ratios via finite element (FE) simulation. The effects of the load conditions on the crack growth rate and crack tip parameters were analyzed systematically.

Isothermal low-cycle fatigue, fatigue–creep and thermo-mechanical fatigue of SiMo 4.06 cast iron: Damage mechanisms and life prediction

A series of strain-controlled Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on silicon-molybdenum ductile cast iron SiMo 4.06. LCF tests were performed for various mechanical strain amplitudes at a high strain rate of 3×10−3/s for various temperatures between 20 °C and 750 °C. In addition, several LCF tests were performed for a low strain rate of 1×10−4/s and as fatigue–creep tests with a tensile strain dwell. TMF tests were performed as out-of-phase and fully constrained between 100 °C and 650 °C. The investigation of the damage mechanisms revealed that the predominant failure mode is a combination of graphite-matrix debonding and transgranular crack propagation through ferritic matrix. Finally, a novel damage model based on the dissipated hysteresis energy is proposed in order to effectively predict the lifetime of SiMo 4.06 under LCF and TMF loading. Creep and fatigue damage are each calculated separately and the oxidation effect is taken into account indirectly.

On the low-cycle fatigue behavior of a multi-phase high entropy alloy with enhanced plasticity

A multi-phase non-equiatomic FeCrNiMnCo high entropy alloy (HEA) was fabricated using vacuum induction melting. Thermo-mechanical treatments consisting of cold rolling and annealing at 750 °C and 850 °C were employed to improve the mechanical properties of the HEA in focus. Tensile experiments revealed that yield strength and ultimate tensile strength levels can be enhanced significantly after thermo-mechanical processing (TMP). At the same time, ductility remains at an adequate level. Strain-controlled low-cycle fatigue (LCF) experiments were carried out in order to assess the mechanical properties of this HEA under cyclic loading conditions. At the same strain amplitude, the stress levels of the processed samples were considerably higher than that of the as-received counterpart. Similarly, fatigue lives of the former could surpass the base condition at the strain amplitudes of 0.2% and 0.4%; however, at the higher strain amplitudes, cyclic softening was observed. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) results revealed that phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC/B2) took place at a higher occurrence with increasing strain amplitude (0.2% to 0.6%). Furthermore, transmission electron microscopy (TEM) studies confirm that upon tensile deformation additional plasticity mechanisms, i.e., deformation twinning and phase transformation, contribute to the overall mechanical behavior of the multi-phase HEA.

Low-temperature fatigue behavior and damage analysis of Al-7Zn-2Mg-1.5Cu alloy based on hysteretic energy

The fatigue performance of Al-7Zn-2Mg-1.5Cu alloy was investigated using strain-controlled low-cycle fatigue (LCF) tests at various low temperatures focusing on the effects of hysteresis energy on fatigue life and the analysis of corresponding parameters. The results show that the alloy exhibits different cyclic hardening ratio depended on the applied total strain amplitude. The cyclic hysteresis energy displays cyclic stabilization at different temperatures. The initiation of fatigue crack occurs on the surface of specimens and many fatigue striations can be found on fatigue crack propagation region. In addition, the hysteresis energy model can be used to evaluate fatigue life at different temperatures. Wherein, intrinsic fatigue toughness (W0) decreases obviously with the decrease of temperature, while damage transition exponent (β) increases slightly. Comparing with low temperature tensile test results, Wo is related to the static toughness and β has a relation with the yield strength ratio. The relationships between the dislocation configuration, fatigue crack propagation mode and two fatigue parameters are discussed from microscopic viewpoint.

An experimental and numerical study on strain controlled fatigue response of TRIP steel under influence of hydrogen

The present work aims to study the influence of hydrogen charging on the transformation induced plasticity steel under strain-controlled low cycle fatigue (LCF). Hydrogen charging had detrimental effect on strain-controlled LCF lives. Cyclic hardening in un-charged specimen due to strain-induced martensitic transformation (SIMT) dominated over cyclic softening. In contrast, generation of large fraction of low angle grain boundaries in hydrogen charged specimen leads to cyclic softening and supresses cyclic hardening due to SIMT. Additionally, Cyclic plasticity modelling using Ohno-Wang kinematic hardening model is validated by simulating cyclic stress–strain hysteresis loop and cyclic softening curve of uncharged and hydrogen charged specimens.