LCF Tests Research Articles

Modeling of LCF Behaviour on AISI316L Steel Applying the Armstrong-Frederick Kinematic Hardening Model.

The combination of kinematic and isotropic hardening models makes it possible to model the behaviour of cyclic elastic-plastic steel material, though the estimation of the hardening parameters and catching the influence of those parameters on the material response is a challenging task. In the current work, an approach for the numerical simulation of the low-cycle fatigue of AISI316L steel is presented using a finite element method to study the fatigue behaviour of the steel at different strain amplitudes and operating temperatures. Fully reversed uniaxial LCF tests are performed at different strain amplitudes and operating temperatures. Based on the LCF test experimental results, the non-linear isotropic and kinematic hardening parameters are estimated for numerical simulation. On comparing, the numerical simulation results were in very good agreement with those of the experimental ones. This presented method for the numerical simulation of the low-cycle fatigue on AISI316 stainless steel can be used for the approximate prediction of the fatigue life of the components under different cyclic loading amplitudes.

Numerical parameter sensitivity analysis of residual stresses induced by deep rolling for a 34CrNiMo6 steel railway axle

To optimise the benefits of the deep-rolling process in the service life context of treated components, the process application must be investigated. In addition to the reduction in surface roughness and near-surface material strengthening, compressive residual stresses are introduced, which are primarily responsible for the increase in service life for components, especially in the case of high-strength steel materials. A numerical parameter sensitivity analysis is performed in order to investigate the introduced residual stresses in detail. For this purpose, a validated deep-rolling simulation model is used, which replicates the deep rolling of a railway axle made of the high-strength steel material 34CrNiMo6. The model is based on an elastic-plastic Chaboche material model parameterised on uniaxial tensile and LCF test results and validated with residual stress measurements. Using this model as a basis, the effect of the main process parameters deep-rolling force, feed rate, friction coefficient, number of overruns, tool geometry, and shaft geometry on the resulting residual stress state are investigated. The results reveal that the deep-rolling force has the most significant influence on the introduced residual stress state and should therefore be highlighted. In the case of applying a deep-rolling force of more than 10 kN, maximum compressive residual stresses of around − 1000 MPa are introduced, and a strong saturating behaviour is shown. Maximum compensating tensile residual stresses of + 100 MPa occur below the surface. The main influence of the deep-rolling force is the effective depth achieved, which is determined by the depth of the zero crossing. This varies from 1 mm with an applied force of 2 kN to more than 3.5 mm with 20 kN. Furthermore, the results are analysed to conclude suggestions for the process’s applicability, and a proposal for an optimised deep-rolling treatment is presented. There multiple deep rolling with decreased deep-rolling forces is used to achieve a comparably optimised residual stress state. In summary, with the presented results, a contribution to a deeper understanding of the deep-rolling process can be achieved; the influence of the most important process parameters on the residual stress in-depth profiles is established; an optimisation proposal is presented; and correlations are found. Thus, the base work for further fatigue strength assessments and the optimisation of the deep-rolling process regarding the increase of service is laid.

Constitutive modelling and damage prediction of AlSi10Mg alloy manufactured by SLM technology with emphasis on ratcheting in LCF regime

The present work has been aimed at developing an improved cyclic plasticity model in the framework of Ohno-Wang kinematic hardening formulation and evaluating the performance of the model with reference to the critical experimental investigation. LCF test specimens of the AlSi10Mg aluminium alloy have been fabricated through selective laser melting technology. The material shows strain-range dependent variation of modulus of elasticity under symmetric strain-controlled loading. The modulus of elasticity decreases with increasing strain amplitude. Stress–strain responses have been critically examined under multistep uniaxial ratcheting in the LCF regime with incremental mean stress and stress amplitude. Damage calculation considering ratcheting and LCF mechanisms as independent gives unconservative predictions. The linear damage accumulation rule taking the largest contribution of both is bringing much more accurate estimates. The proposed model incorporates effects of mean stress and stress amplitude under uniaxial multistep ratcheting in the LCF regime. All fatigue tests in the LCF regime have been simulated using the proposed improved model. Evolution of strain-range dependent elastic modulus has been incorporated in the formulation of the improved model through the memory history dependent parameter. The ratcheting parameter is newly formulated in this present work to account for the effects of accumulated mean plastic strain on the opening of stress strain hysteresis loops that results in a better prediction of the behaviour of cyclic plastic deformation. The proposed model has been validated by comparing simulated results with experimental observations and reference published simulation results.

Low-cycle fatigue performance and life prediction of a P/M nickel-based superalloy with artificial surface defect at elevated temperature

P/M nickel-based superalloys are widely used in turbine disks, defects such as inclusions and surface scratches inevitably arisen from manufacturing process will exert negative influences on the low-cycle fatigue performance of superalloys and make traditional life prediction method no longer reliable. The residual fatigue life in presence of defect is an important performance index for disc alloy in damage tolerance design criteria. In this work, LCF tests of FGH96 specimens with artificial surface defect were conducted at 600℃, 650℃, 700℃ and room temperature, the size of the defect is determined in accordance with the minimum flaw size which can be detected by NDT technique. The results show that the LCF life of the specimens with artificial surface defect decreases significantly with the increase of temperature and size of defect. Microscopic characterization technique SEM and EDS were executed to investigate the failure mechanism of the specimens, different mechanisms of crack initiation from artificial surface defect at room and elevated temperature were discussed. And then life prediction based on fracture mechanics was carried out by ABAQUS-FRANC3D co-simulation. This method allows that SIF can be conveniently calculated by finite element method and it is applicable to analysis of arbitrary engineering structure. Good life prediction results which fall within double dispersion band of experimental lives were obtained at elevated temperature, but the predicted life is excessively conservative at room temperature because the crack initiation process accounts for a large proportion of fatigue life. It seems that life prediction method based on fracture mechanics is more applicable to specimens with surface defect at elevated temperature due to the shorter crack initiation process. This work has reference significance to the damage tolerance analysis of turbine disks.

Low cycle fatigue behavior of 316LN stainless steel: Effects of temperature, strain rate and strain amplitude

Continuous and interrupted LCF tests were conducted to reveal the effects of temperature, strain rate and strain amplitude on the LCF behavior of a nitrogen-alloyed 316LN stainless steel. The cyclic hardening and softening behavior at high and low strain amplitudes were elaborately elucidated based on the method of stress decomposition and characterization of dislocation configuration evolution. In addition, the dominant damage mechanism responsible for the different cracking modes under various loading conditions was determined based on the observations of fracture surface and secondary cracks on the longitudinal section.

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.

Early Detection of Damage of Inconel 718 with the Use of Strain-Hardening Cross-Effect

Indentation tests were used for early detection of deformation-induced damage of Inconel 718. Damage was produced in a controlled manner using specially designed specimens with a variable width of the gage part. Static tension, LCF, and HCF tests were performed to induce material damage. Accumulated plastic strain intensity was used as the reference measure of deformation-induced damage. Detection of damage progress in its early phase was based on the use of strain-hardening cross-effect. Experimentally determined correlation of hardness and indentation work with reference damage parameter was analyzed. The results of our analysis lead to the conclusion that the proposed experimental procedure is useful for the detection of deformation-induced damage in the early stage of the process.

A Walker-based mean strain correction models for low-cycle fatigue life prediction

A Walker-based mean strain correction model of low-cycle fatigue (LFC) life prediction is proposed for high loaded parts. The model is based on a function depending on the strain range and strain ratio controlled in the strain-controlled LCF test of fatigue specimens and a constant reflecting the material sensitivity to strain ratio. The independence from the stress cycle parameters which can change during the strain-controlled LCF test is an obvious advantage of the model. The model was verified using the results of strain-controlled LCF tests of smooth titanium alloy Ti-6A1-4V ELI and iron-based alloy specimens conducted at room temperature. The proposed model was compared to the Smith - Watson - Topper and Walker models that take into account the mean stress effect. The proposed model provided the best prediction accuracy for titanium alloy. For Iron-based alloys the results obtained by the Walker model and the model proposed are close to each other. A simplified model based on the analysis of model parameters tailing into account the mean strain effect for predicting fatigue life of aeroengine critical parts is developed using a limited amount of experimental data when only the results of Rε = 0 tests are known. A comparison of the predicted life with the number of cycles to failure showed satisfactory results of fatigue life prediction for Ti-6A1-4V ELI and Iron-based alloys specimens.

Low-Cycle Fatigue Behavior of Hot V-Bent Structural Components Made of AZ31B Wrought Magnesium Alloy

Recent studies have shown the change of microstructure during hot-bending in uniaxial specimens made of AZ31B alloy. They also investigated the influence of the changed microstructure on the quasi-static and cyclic material behavior under uniaxial stress states. However, studies on the fatigue behavior of hot-bent structural components in which a multiaxial inhomogeneous stress state occurs are still lacking. For this purpose, a novel hot-bent V-shaped specimen was developed, of which three different variants, each with a different bending radius, were fabricated and investigated. Microstructural analyses reveal that band-like accumulations of twinned grains are already formed in the compressively stressed area of the specimen during the bending process. Force-controlled low-cycle fatigue tests were performed to investigate the twinning evolution after cyclic loading. Subsequent microstructure analyses show that bands of twinned grains are no longer visible but also that the occurrence of twins is evenly distributed. Due to the specimen shape, the specimens are subjected to a multiaxial stress state. During LCF tests, the strain was measured using 3D digital image correlation and fatigue life was modeled successfully with the application of the concept of highly strained volume.

Low cycle fatigue behaviour study of a nano precipitate strengthened ferrite-bainite 780 steel

Low-cycle fatigue behaviour of a newly developed ferrite-bainite 780 steel with the ferrite phase, strengthened by nanosized precipitates was investigated. The LCF tests were conducted in fully reversed strain-controlled mode to determine the cyclic stress response and strain life. The steel showed predominantly cyclic softening behaviour at lower strain amplitudes (up to 0.25%). However, at higher strain amplitudes (≥ 0.35%), cyclic hardening was observed with increase in the number of loading cycles. EBSD study done on the fatigue tested samples revealed grain fragmentation and texture weakening during cyclic deformation. TEM study elucidated the evolution of dislocation structure responsible for cyclic softening/hardening behaviour during cyclic deformation. At lower strain amplitude (0.2%), concurrent presence of random arrangement of dislocations and dislocation tangles were observed with some grains having weakly developed persistent slip bands (PSBs) and veins. At high strain amplitude (0.55%), subgrain structure was found to be the dominant feature along with occurrence of PSBs and veins at few locations. Further to this the role of nano-precipitates in influencing the fatigue behaviour was also investigated.

Recent studies to the impact of a ceramic breeder environment on the mechanical properties of EUROFER97 under operating conditions

The reduced activation ferritic-martensitic steel EUROFER is a prime candidate for fusion structural applications. In the advanced helium-cooled pebble bed blanket concept of DEMO, this steel is in direct contact with Li-ceramics pebbles and experiences cyclic thermomechanical loading under operating conditions. In this work, the effect of ceramic breeder environment on both the microstructure and particularly the fatigue properties of the RAFM steel was investigated. For this purpose, EUROFER samples were embedded in ceramic pebbles and annealed at 550 °C under a purge gas atmosphere for durations ranging 8–128 days. Microstructural studies show that annealing at the given conditions leads to the formation of a complex corrosion multilayer consisting of ferrite and iron oxide in an outer layer, as well as chromium oxide in both an intermediate and an inner layers. For annealing durations up to 32 days, the thickness of the oxide layer is continuously increasing. However, for more prolonged annealing, the thickness of the corrosion layer reaches saturation at about 20 µm. Similar to the growth behavior of the oxide layer, the strain-controlled LCF tests showed a remarkable reduction of the LCF lifetime with increasing in the annealing duration up to 32 days. However, beyond an annealing duration of 64 days, an additional decrease of lifetime is not observed. When performing additional experiments (annealing with ceramic pebbles in vacuum and annealing in purge gas without pebbles) it is found that corrosion is mainly caused by purge gas impurities, namely oxygen and water. Furthermore, the decrease in fatigue lifetime is related to the degradation of surface quality, causing an accelerated progress of damage. Finally, Charpy impact tests combined with carrier gas hot extraction investigations revealed that hydrogen embrittlement of the samples, annealed in a breeder environment containing 0.1 vol.% H2, can be excluded.

LOW-CYCLE FATIGUE OF COLD-DRAWN TYPE 304 AUSTENITIC STAINLESS STEEL IN ANNEALING CONDITIONS

Type 304 austenitic stainless steel, or better known as SS304, generally has alloying elements: C < 0.1%, Cr 18 – 20%, Fe 66 – 74%, Mn <2%, Ni 8 – 10.5%, P <0.045%, Si <1%, and S <0.030%. In general, this material has good ductility, high tensile strength, and excellent corrosion resistance. In application, type SS304 will be subjected to repeated loading, and eventually, the material will undergo plastic deformation, which leads to structural failure in a short life. The failure of SS304 is generally due to the inability of the material to repeat loading, which results in large amounts of plastic deformation so that the SS304 will experience fatigue and then fracture. Based on the description above, this study aims to evaluate the LCF properties of 304 CDS stainless steel with annealing heat treatment. The parameter used was the strain amplitude 0.003 – 0.013 mm/mm. The results of this study revealed that the highest fatigue life in the LCF test was experienced by steel with heat treatment at an amplitude of 0.003 mm/mm with 48367 cycles. In contrast, at the amplitude condition of 0.013 mm/mm, the fatigue life of the steel decreased drastically with the resulting plastic strain being larger, namely 0.0094 mm/mm and elastic strain of 0.0035 mm/mm, with an average modulus of elasticity of 194 GPa. Annealing treatment conditions experienced decreased mechanical strength but tended to be ductile. Using Basquin-Coffin-Manson Equation, empirical equations to predict LCF of 304 stainless steels can be determined.

The influence of tool traverse speed on the low cycle fatigue properties of AZ31 friction stir welded joints

This paper describes the basic low cycle fatigue properties of AZ31 friction stir welded joints obtained by different welding velocities: 150, 300, 600, and 900 mm/min with the constant value of 1600 rpm tool rotation speed. The performed LCF tests at various strain amplitudes: 0.25, 0.35, 0.5 (for R=-1) together with fractography analysis allowed to state that the value of welding velocity has no significant impact on the LCF behavior of AZ31 FSW in the investigated range. The tests also revealed that the AZ31 FSW joint is characterized by three stages of fatigue life: a very short period of cyclic hardening (up to 100 cycles), cyclic stabilization, and a relatively long period of cyclic softening until failure. All joints are free of imperfections and tend to fail in the heat-affected zones.

Stress analysis and lifetime prediction for Ti–6Al–4V welding joint under fatigue loading

A welded joint has a complex gradient microstructure. The effects of local deformation in the fusion zone, heat-affected zone (HAZ), and other areas, on the macroscopic fatigue properties of an inhomogeneous welded joint, are not fully understood. Thus, it is necessary to correlate the microstructural features and their related local deformation mechanisms with macroscopic fatigue properties. In this study, cyclic stress–strain curves and lifetime model parameters of materials in different HAZs were determined by hardness distribution and base–metal material properties. A user element subroutine was developed in the elastic–plastic finite element analysis to obtain the local stress and strain in the joint. ‘Danger points’ were selected to predict fatigue lifetime with the Manson–Coffin damage equation. The prediction results agreed closely with experimental observations within the low-cycle fatigue lifetime regime. Highlights A new LCF life analysis method, for coated specimens, was developed based on the fracture mechanics. Life prediction method for coated specimens was proposed and verified by LCF tests. Coating-induced lifetime degradation was analysed quantitatively at different temperatures. Effects of stress amplitude on the surface crack initiation were investigated.

Notch energy-based low and high cycle fatigue assessment of load-carrying cruciform welded joints considering the strength mismatch

This study examines the Low and High Cycle Fatigue failure (LCF and HCF) of Load-carrying Cruciform Welded Joints (LCWJ) made of 10CrNi3MoV steel through a combined investigation of experimental, simulation, and analytical methods. LCF and HCF tests were conducted on LCWJ specimens with various weldment strength matching conditions and geometric configurations. Different fatigue failure modes can be observed from potential fracture points due to the discrepancy of strength and geometry in weldments, which exhibits the discrepancy of fatigue life. New uniform energy analytical formulations for Weld Toe (WT) and Weld Root (WR) are established to predict the LCF and HCF indicator of LCWJ considering the plasticity and mechanical heterogeneity effects of the weldments based on the generalized Neuber concept of Fictitious Notch Rounding (FNR). Furthermore, an effective notch energy indicator is introduced and implemented to evaluate fatigue behaviors of welded components in both LCF and HCF regimes. According to the proposed analytical solutions, experimental data from WT and WR failure under force and displacement-controlled cyclic loadings are evaluated and verified. The results show that the notch-energy indicator can express the unique relationship with fatigue life regardless of the strength mismatch ratios and geometry discrepancy.

The crack surface morphology investigation of S355J2 steel after bending-torsion fatigue

The paper describes the analysis of crack surface morphology of S355J2 steel specimens after bending-torsion fatigue. These experimental investigations of the surface topography were carried out using the focus variation microscope, an optical 3D measurement device. Selected results of measured fracture surfaces for S355J2 steel were analysed according to the surface texture ISO 25178 standard. Differences in roughness values for different loadings were demonstrated. For profile Rx and areal Sx parameters, characteristic relationships of fracture zones have been demonstrated. It has been shown that roughness profile Ra for the rupture area is higher than for the propagation area, as well as increases after both the LCF and HCF tests and increases with the torsional loading level. However, Sa in the propagation area increases and the rupture area decreases.

Comparative evaluation of LCF behavior of dissimilar steels and welds in an ultra-supercritical turbine rotor at 280 ℃

Buttering layer (BL) was used to weld two ultra-supercritical turbine rotor steels. Low-cycle fatigue behavior of different regions in the dissimilar metal welded joint (DMWJ) was compared and evaluated at 280 °C. Microstructure, fracture and weak link were observed by OM, SEM, EDS and TEM. BL had lower fatigue life than weld metal (WM) and base metals (BMs). Weld zone showed cyclic softening characteristic. Microstructures evolved into cell structures after LCF tests. Fatigue crack in weld zone propagated mainly through coarse carbides. The interface near BL was the weakest link of the DMWJ due to ferrite layer and strength mismatch.

Evaluation of component‐similar rotor steel specimens with a local probabilistic approach for LCF

AbstractLately, a successful validation of a combined size and gradient effect modelling approach in a local probabilistic framework for LCF was published. A second validation study is here presented using LCF data of component‐similar specimens made of forged X12CrMoWVNbN10‐1‐1. It is found that a reasonable LCF crack initiation life prediction can be achieved by the local probabilistic model using single batch material parameters. However, the material parameters obtained from mixed batch LCF test data overestimate the actual material inherent LCF life scatter. Consequently, the size effect shift is pronounced too much, and the LCF life is predicted beyond crack initiation. Estimating the material parameters from a single batch data set instead results in a much less dispersed LCF life distribution and an approximately correct size effect shift. The predicted median Wöhler curve then corresponds to borehole‐specimen lives for initial cracks at the detection threshold. A reasonable prediction of technically relevant crack sizes in such inhomogeneous components requires additional modelling of stress gradient effects.

Low Cycle Fatigue Behavior of Superalloy GH4169 in High Temperature Gas Environment

Turbine components (blades, guides and casing) of gas turbine engine usually suffer from fatigue load in company with a HTG environment in service. The corrosive substances in HTG can deposit on the surface of turbine components and induce accelerated damage known as hot corrosion. In this study, an experimental system is designed and built up to conduct LCF tests of superalloy in HTG environment. The influence of HTG on the LCF behavior of the nickel-base superalloy GH4169 at 650°C is studied. According to the test results, the average fatigue life of the specimens in HTG environment (22270 cycles) is about 31.5% less than that in air (32496 cycles). The protective oxide film on the surface of the specimen can be destroyed by the electrochemical reaction between HTG and oxide film. Compared with the specimen tested in air, there are more fatigue sources in the specimen tested in HTG environment, and the transgranular–intergranular transition occurs in the crack growth area.

Ratcheting and fatigue behavior of Eurofer97 at high temperature, part 1: Experiment

Eurofer97 is considered as one of the structural material candidates for in-vessel components of future fusion reactor. Due to frequent startups, shutdowns and load changes in reactor, the material is under complex cyclic loadings. This work investigates the mechanical behavior of Eurofer97 under various cyclic loadings, including strain-controlled LCF tests and stress-controlled ratcheting tests at high temperatures 450°C and 550°C. Various influencing factors on ratcheting, such as stress magnitude, stress ratio and stress rate are evaluated. Since Eurofer97 is developed based on conventional grade 91 ferritic martensitic (FM) steels, corresponding data of mod.9Cr-1Mo FM steel P91 are also presented for comparison.