Low Cycle Fatigue Loading Research Articles

Crack initiation mechanisms in Ti-6Al-4V subjected to cold dwell-fatigue, low-cycle fatigue and high-cycle fatigue loadings

Although Ti-6Al-4V is the most commonly used titanium alloy in the aerospace industry, the mechanisms governing crack initiation in the different fatigue regimes are not well understood yet. This situation partly pertains to a competition between multiple crack initiation mechanisms. In particular, applied loading conditions were identified as a key parameter governing the transition between mechanisms. The fatigue behavior of Ti-6Al-4V with a bi-modal microstructure was investigated in the present study using different waveforms, load ratios and frequencies to clarify this feature. A detailed characterization of the main crack initiation sites was carried out to identify microstructural configurations governing crack nucleation in low-cycle dwell-fatigue, low-cycle fatigue and high-cycle fatigue regimes. While lifetimes and fracture surfaces showed a good consistency with data from prior studies, a single microstructural configuration was found involved with the formation of crack initiation facets. All investigated cracks leading to specimen failure were nucleated along (0001) twist grain boundaries exhibiting similar features. Based on this finding, a criterion is proposed to identify candidates for crack initiation. This also demonstrates no major sensitivity of the critical microstructural configurations to environment, free surface, loading conditions and, as shown in previous studies, microstructure and composition. However, conventional fatigue failure results from one, or a few, surface or subsurface crack initiation sites while dwell-fatigue failure involves the formation of multiple internal cracks. This is accompanied by a significant dwell-fatigue life debit at peak stress magnitudes close to the yield strength. Features of microstructural configurations prone to crack nucleation are finally compared with data reported in existing literature to propose a mechanism of crack formation in Ti alloys.

On the microstructural evolution and failure mechanism in laser powder bed fusioned Ti–6Al–4V during low cycle fatigue at room and elevated temperatures

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti–6Al–4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti–6Al–4V. The evolution of α′ grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti–6Al–4V has greater tensile strength than conventionally manufactured wrought Ti–6Al–4V due to its microstructure, with fine α′ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increases the ability of material to overcome the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.

Combined high and low cycle fatigue life prediction model based on damage curve method considering loading interaction effect

Through analyzing the characteristics of the Miner’s rule and the Manson-Halford (M-H) model, a nonlinear fatigue damage accumulation model is established based on the damage curve method under combined high and low cycle fatigue loading. In this model, an interaction factor related to the maximum stress and high cycle stress range is proposed to consider the loading interaction effect. Furthermore, the proposed model does not require any additional material parameters except for the S-N curve parameters. The experimental data of five materials, including TC11 alloy, 2024-T3 aluminum alloy, 45 steel, LY12-CZ aluminum alloy, and GH4033 Ni-based alloy, are introduced to verify the proposed model. Compared with the Miner’s rule and the M-H model, most of the prediction results by the proposed model are within the life factor range of 2, implying the established model has higher prediction accuracy.

Investigation on uniaxial ratcheting fatigue behaviors and microstructure evolution of ultrafine-grained 6061 aluminum alloy

The motivation of the present study is to explore ratcheting-fatigue behavior and the corresponding micro-deformation mechanism of ultrafine-grained aluminum alloys 6061 (UFG AA6061) under two stress-controlled low-cycle fatigue (LCF) loading modes, i.e., mean tension stress (MTS) and mean compressive stress (MCS). The uniaxial ratcheting tests demonstrate that the fatigue-ratcheting damage of UFG AA6061 is prone to occur under MCS-LCF loading, rather than MTS-LCF loading. Detailed microstructure characterizations by Electron backscatter diffraction (EBSD) techniques under these two loading modes show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries increase remarkably in the case of MCS-LCF loading. These observations are mainly ascribed to the room temperature continuous dynamic recrystallization (CDRX), in which low angle boundaries evolve into high angle grain boundaries and the formation of equiaxed grains is accelerated. In contrast, there were no obvious change in the grain aspect ratios or the fractions of high angle boundaries at MTS-LCF loading condition, while the Schmid factor of non-octahedral slip system {100} <110> increases and the texture evolve gradually. The deformation mechanism is grain boundary sliding (GBS) and grain rotation would accommodate the process of GBS. An analysis of the microstructure-properties relationship showed the superposition effect of residual shear deformation and uniaxial LCF ratcheting deformation is responsible for the CDRX at room temperature. And the occurrence of GBS is mainly caused by the activation of non-octahedral slip system.

A generalized physical-based failure indicator parameter used in crystal plasticity model to predict fatigue life under low cycle fatigue and creep-fatigue loadings

A dislocation movement-based physical failure indicator parameter (FIP) was proposed and incorporated into crystal plasticity finite element to assess the damage for components subjected to low cycle fatigue (LCF) and creep-fatigue interaction (CFI) loadings. The concept of net slip was introduced through the distance of dislocation movement whose evolution was analyzed and compared with other FIPs. The creep damage can only be captured by the proposed physical FIP. Life prediction based on net slip was developed and validated for different materials under LCF and CFI loadings at different temperatures. Most of the predicted life falls in the ±1.5 error bands.

Generalized evaluation of the effects of wet/dry cycling on CFRP-concrete bond subjected to low-cycle fatigue loading

This paper presents the results of an investigation into the effects of wet/dry (W/D) cycling on the bond performance of carbon fiber reinforced polymer (CFRP)-concrete interfaces subjected to low-cycle fatigue loading. CFRP-concrete specimens were subjected to fatigue loading, W/D cycling and single shear tests. The bond characteristics were evaluated using a generalized approach based on single shear test results. Control specimens exhibited cohesive failure within the concrete, while fatigue loading and W/D cycling led to a mixed failure pattern featuring a combination of cohesive and adhesive failure. Fatigue loading reduces the local bond strength while inducing little change in the fracture energy. Following fatigue loading, exposure to W/D cycling improves the local bond strength and the fracture energy. Compared to control specimens, the effective bond length becomes higher for CFRP-concrete interfaces after fatigue loading and W/D cycling. Besides, it can be found that the use of piezoelectric transducers (PZTs) allowed the monitoring of the interfacial damage that occurred during fatigue loading and single shear tests.

Experimental investigations and damage growth modeling of EN‐AW 2024 aluminum alloy under LCF loading accounting creep pre‐deformation

AbstractThis article presents the results of experimental tests of creep rupture and of low‐cycle fatigue (LCF) of EN‐AW 2024 aluminum alloy devoid of damage and having preliminary damage. The preliminary damage was dealt in the process of creep at elevated temperature 100°C, 200°C, and 300°C until achievement of two different strain values at each temperature. Samples with preliminary damage were subjected to fatigue tests at room temperature. Based on the results of experimental tests, a simple damage accumulation model was proposed for creep at different temperatures. In this model, growth of the damage state variable was made dependent on the current value of axial stress and on growth of plastic strain. This model was also adapted for description of damage accumulation in the process of LCF for both as‐received material and material with creep pre‐deformation. In both cases, that is, creep and LCF, the model's parameters were determined and it was experimentally verified.

Using machine learning to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading

In this article, machine learning is used to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading, both of which represent the most complex loadings that couple creep, fatigue and oxidation damage. A uniaxial fatigue and fatigue–creep dataset, which was obtained for temperatures of between 300°C and 600°C for a low-alloy martensitic steel, is utilized in this study. Two different machine learning based approaches to lifetime prediction are demonstrated. The first approach is based only on a shallow neural network, whereas the second approach is proposed as a combination of a sequence learning based model – either long short-term memory network or gated recurrent unit – with the shallow neural network. A good correlation between the experiment and the prediction suggests that lifetime under complex thermo-mechanical loading can be reasonably predicted via the proposed machine learning based damage models.

Probabilistic fatigue life prediction of an aero-engine turbine shaft

PurposeAero-engine components endure combined high and low cycle fatigue (CCF) loading during service, which has attracted more research attention in recent years. This study aims to construct a new framework for the prediction of probabilistic fatigue life and reliability evaluation of an aero-engine turbine shaft under CCF loading if considering the material uncertainty.Design/methodology/approachTo study the CCF failure of the aero-engine turbine shaft, a CCF test is carried out. An improved damage accumulation model is first introduced to predict the CCF life and present high prediction accuracy in the CCF loading situation based on the test. Then, the probabilistic fatigue life of the turbine shaft is predicted based on the finite element analysis and Monte Carlo analysis, where the material uncertainty is taken into account. At last, the reliability evaluation of the turbine shaft is conducted by stress-strength interference models based on an improved damage accumulation model.FindingsThe results indicate that predictions agree well with the tested data. The improved damage accumulation model can accurately predict the CCF life because of interaction damage between low cycle fatigue loading and high cycle fatigue loading. As a result, a framework is available for accurate probabilistic fatigue life prediction and reliability evaluation.Practical implicationsThe proposed framework and the presented testing in this study show high efficiency on probabilistic CCF fatigue life prediction and can provide technical support for fatigue optimization of the turbine shaft.Originality/valueThe novelty of this work is that CCF loading and material uncertainty are considered in probabilistic fatigue life prediction.

Concrete splitting and tip-bearing effect in the bond of anchored bars tested under fatigue loading in the push-in mode: An experimental investigation

The load scenario with a compressive force applied to an anchored bar (push-in mode) has not been sufficiently addressed so far with reference to fatigue, since most of the studies available in the literature are focused on bond behavior under tensile forces (pull-out mode). However, special structures like the towers of wind turbines subjected to alternating moments (and tensile-compressive forces) due to the variable wind direction, are fostering the interest for the fatigue behavior of concrete-bar bond under compressive forces, all the more because several millions of load cycles may be applied. An extensive experimental investigation has been carried out in this research project using a modified beam-end test in the push-in mode. Fifty beam-end specimens were tested under monotonic loads, as well as under low-cycle and high-cycle fatigue loads, with different bar diameters and bonded lengths. The failure modes and bond behavior with the end sections of the anchored bars either unloaded (free ends) or pushing against the concrete (compression ends exhibiting the well-known tip-bearing effect) were investigated as well. The results of the cyclic tests in the push-in mode are compared with those available in the literature in the pull-out mode, through the usual Wöhler curves and fatigue-induced creep curves. Based on this comparative analysis, the failure mechanisms of bond under fatigue loading are identified, with specific attention to the interaction between bond behavior and splitting cracks.

Use of Prandtl operators in simulating the cyclic softening of Inconel 718 under isothermal low-cycle fatigue loading

In this article, a new approach is proposed for modelling the stress–strain response of the Inconel 718 super-alloy under isothermal Low-Cycle Fatigue (LCF) loading. The proposed constitutive model is based on the Prandtl operator approach, in which a set of modifications is introduced in order to simulate strain range dependent cyclic softening. A new simulation capability is introduced by evolving the yield strains of the individual hysteresis operators with an accumulated plastic strain. In addition, the effect of the strain range dependency of cyclic softening is introduced into the proposed constitutive model by coupling its parameters with the concept of the plastic strain memory surface. These introduced modifications preserve the main advantages of the Prandtl operators, such as a small number of model parameters, their fast determination from the cyclic stress–strain curve, and a high computational speed, when used to simulate complex non-linear mechanical behaviour. Finally, the prediction capability of the proposed model is illustrated by various strain controlled tests performed at 500 ∘C, including block spectrum loading and variable strain amplitude loading.

Micromechanical constitutive modeling of tensile and cyclic behaviors of nano-clay reinforced metal matrix nanocomposites

Metal matrix nanocomposites are emerging rapidly among both research and industry communities. Nonetheless, the studies for their use in Low-Cycle Fatigue (LCF) applications are rarely documented. Prior to tailoring the microstructure for the desired LCF performance, it is a fundamental task to understand the structure-property correlations of the nanocomposites. In the present paper, a continuum-based micromechanical model starting from the nanostructure is proposed to fulfill such a task. The proposed model is applied to a piston aluminum alloy reinforced with 1 wt% nano-clay subjected to fully-reversed strain-controlled LCF loading at room temperature. The model, formulated within Eshelby's inclusion framework, considers the active clay/matrix interactions, the intercalation/exfoliation effect via the equivalent stiffness method, and the size effect through the introduction of the interphase thickness as a characteristic length scale. To render the model able to reproduce the cyclic hysteretic behavior of the nanocomposites, the isotropic/kinematic hardening rule is considered to describe the aluminum alloy plasticity. The model results are favorably compared to the experimental observations in terms of monotonic and cyclic elastoplastic stress-strain responses of the aluminum/clay nanocomposite. A parametric study is then carried out in order to understand the separate and synergic effects of clay weight fraction, clay geometrical parameters (e.g., shape and size), and clay structural parameters (e.g., number of silicate layers and interlayer spacing) on the overall monotonic and cyclic responses of the present nanocomposite. The reinforcing effect of each parameter is discussed concerning the micromechanical model.

Strain localization and crack initiation behavior of a PM Ni‐based superalloy: SEM‐DIC characterization and crystal plasticity simulation

AbstractThe strain localization and crack initiation behavior of a powder metallurgy (PM) nickel‐based superalloy FGH4098 under low cycle fatigue (LCF) loads at room temperature is investigated by combining scanning electron microscope (SEM)‐digital image correlation (DIC) characterization with crystal plasticity simulation. The elastic anisotropy near annealing twin boundaries contributes to strain localization and results in high propensity of LCF failure. The effect of local microstructure on the fatigue crack initiation at {111} slip bands and tortuous crack propagation is revealed. Meanwhile, local crystal plasticity finite element cyclic deformation simulation is carried out based on explicit microstructure reconstructed from electron backscatter diffraction (EBSD) characterization. Four metrics of accumulated shear strain, effective plastic strain, shear strain energy dissipation density, and Fatemi–Socie parameter are selected as fatigue indicator parameters to predict crack initiation site. Fatemi–Socie parameter is proved to be a promising metric for crack initiation prediction.

Effects of temperature and microstructure on low cycle fatigue behaviour of a PM Ni-based superalloy: EBSD assessment and crystal plasticity simulation

The macro and mesoscale deformation and damage of the PM nickel-based superalloy FGH4098 under low cycle fatigue loading at elevated temperatures were investigated using both SEM/EBSD characterization and crystal plasticity finite element (CPFE) simulations. The results indicate that higher temperature leads to more severe macroscale damage i.e. larger areas of hysteresis loops and shorter fatigue life with transgranular failure mode at both 650 °C and 750 °C. The misorientation parameters derived from EBSD characterization show similar deformation at both temperatures, but more intense localized deformation in the crack initiation region at 750 °C. Based on the accumulated shear-strain energy dissipation density and Fatemi-Socie fatigue indicator parameter, the distribution and evolution of fatigue damage are shown to be strongly affected by local stress–strain state and temperatures, and more severe and homogeneous damage is found at 750 °C. The CPFE simulation results also show higher damage propensity, as indicated by the fatigue indicator parameters used here, in the vicinity of annealing twin boundaries with high difference in elastic modulus.

On the Compression Instability during Static and Low-Cycle Fatigue Loadings of AA 5083 Welded Joints: Full-Field and Numerical Analyses

The aim of this scientific work was to evaluate the compression instability effects during static and low-cycle fatigue loadings of AA 5083 welded joints, commonly used in marine structures. Low-cycle fatigue assessment in marine structures is of utmost importance since high levels of plastic deformation can arise in the proximity of high-stress concentration areas. Displacement ratios equal to minus one and zero were used to perform experimental low-cycle fatigue tests. The tests were monitored by means of the Digital Image Correlation technique in order to detect the strain patterns, with particular attention paid to stress concentration areas, indicating that a specimen tends to buckle during high compression loads, for tests with a displacement ratio of minus one. The tests at displacement ratios equal to −1 showed a lowering of the strain–life curve revealing a considerable effect on compression instability. A nonlinear finite element modelling procedure, depending only on hardness measurements, was developed. The hardness measurements were used in order to assess the distinct mechanical properties of the different zones that were included in the finite element model. The finite element model results were compared to the data achieved by means of the digital image correlation technique, demonstrating that hardness measurements can help predict the low-cycle fatigue behaviour of welded joints and consider compression instability phenomena.

Slip activity during low-stress cold creep deformation in a near-α titanium alloy

Near-α titanium alloys are known to be susceptible to cold dwell fatigue (CDF) debit, which has been linked to the occurrence of cold creep during high-load dwell times superimposed onto low cycle fatigue loading. In order to shed new light on the deformation mechanisms during cold dwell and to understand better the role of the microstructure, two different bimodal microstructures (fine and coarse transformation product) of TIMETAL®834 were investigated at stress levels below the 0.2% proof stress using a combination of grain orientation mapping and in-situ electron microscopy imaging. This enabled in-depth analysis of 2D slip patterns and slip system activity using High-Resolution Digital Image Correlation (HRDIC), showing that in both microstructures basal slip is initially the dominant slip mode before prismatic slip activity increases approaching the 0.2% proof stress. Comparing the two constituents in the bimodal microstructure, first slip bands are localised predominantly in primary α grains, indicating higher strength of secondary α colonies, particularly for finer transformation products. During 10 min load holds at stresses below 0.2% proof stress, more plastic strain and longer connected slip traces across several grains were observed in the sample with coarse transformation product, indicating higher susceptibility to cold creep deformation.Full-field crystal deformation modelling was used to determine local stresses in individual grains at the onset of plasticity and test the hypothesis that the dominance of basal slip at low-stress levels can be explained by the elastic anisotropy in Ti alloys. While consideration of elastic anisotropy increased resolved shear stress (RSS) values for basal slip relative to prismatic slip, it did not unambiguously explain the early activation of basal slip. Furthermore, thermal residual stresses at the crystal level, due to the anisotropy of coefficients of thermal expansion (CTE), were included in the simulation, which created a wider spread of the RSS data but did not preferentially promote high RSS values for grains well aligned for basal slip. In the absence of an unambiguous conclusion, it is hypothesised that basal slip might display lower critical resolved shear stress values than typically reported but high work hardening rates compared to prismatic slip.

Effects of exposure temperature on low–cycle fatigue properties of Q690 high–strength steel

Fire brings lasting effects on mechanical performance of structural steel. For the high strength steel (HSS) structures experiencing fire while do not collapse, it is necessary to evaluate the service performance after fire, including the ability to resist earthquake. During earthquake, the low–cycle fatigue load is applied on the structural steels. In this paper, the low–cycle fatigue properties of Q690 HSS exposed to different elevated temperatures are investigated experimentally. Totally, five exposure temperature levels (25, 600, 700, 800 and 900 °C) and four fatigue strain amplitudes (1%, 1.5%, 2% and 4%) are considered. After the heating test, the appearance characteristics and microstructures of Q690 HSS specimens are discussed. Later, the fatigue test is conducted and results show that the hysteretic curve tends to be fuller due to the increase in plastic strain when the exposure temperature ranges from 700 to 900 °C. Based on the test results, effects of exposure temperature and strain amplitude on cycles to failure and total dissipated energy density are analyzed. Besides, stress and strain coefficients are selected to analyze the stress and strain properties of post–fire Q690 HSS under low–cycle fatigue loading. Finally, a fatigue model is proposed to quantify the relation between total strain amplitude and cycles to failure, where the coefficient β and exponent a of Q690 HSS exposed to different elevated temperatures are determined by test results, after which the predictive equations are suggested to the quantify effects of exposure temperature on β and a.

On the mechanism of fatigue crack initiation in high-angle grain boundaries

Fatigue crack initiation in high-angle grain boundaries is often observed experimentally, however, its mechanism is a subject of controversy. Low cycle fatigue loading has been applied to super-austenitic stainless steel Sanicro 25 and surface features developed during cyclic loading with high strain amplitudes were studied and analyzed using SEM, EBSD and FIB cuts. Dense array of persistent slip markings developed on the surface of grains was found. Extrusions and intrusions emerged not only in the direction of the surface but also normal to the grain boundaries. Growing extrusions were bent and started to push two neighbor grains locally apart leading to production of void-like discontinuities. Deepening intrusions produced void-like defects or microcracks on the grain boundary. Both these processes led to the substantial weakening of the grain boundary. Coalescence of void-like discontinuities and microcracks on the grain boundary during cyclic loading led to the production of the grain boundary crack. Presented and discussed experimental findings lead to novel mechanism of high-angle grain boundary crack initiation and raise serious doubts about the theories of brittle-like origin of grain boundary cracks.

Cohesive zone modeling of fatigue crack propagation in slab track interface under cyclic temperature load

The ballasless track has been massively applied nowadays, while the interface damage evolution of ballastless tracks is still rarely investigated. This work aims to investigate the damage evolution property of ballastless track under low-cycle fatigue load condition. The application of cyclic cohesive model as a key approach to simulated the damage evolution of ballastless track interface. Therefore, a low-cycle fatigue cohesive model (LCFC) that follows a bilinear law is established by combining and modifying existed cohesive models. The LCFC model is then implemented by a secondary development in Abaqus through implicit user-defined material subroutine (UMAT), and the validity of the model is verified though a double cantilever beam (DCB) model. To obtain the key parameters of the LCFC model for analyzing the fatigue damage of CRTS III (Chinese Railway Track System type III) ballastless track, the split tension test of specimen that is composed of the self-compacting concrete and the slab concrete is conducted, and the damage evolution process is captured through digital image correlation (DIC) technique. Further, a simulation model that involves the LCFC model is created for simulating and analyzing the interface fatigue damage of CRTS III- ballastless track under cyclic temperature gradient for the first time. The results show that the established LCFC model is suitable for studying the interface fatigue damage of CRTS III ballastless track under extreme temperature gradient cycles. The interface near the slab edge will be completely damaged within only several cycles when the cyclic temperature gradient is ± 61℃/m. The failure mode of the interface element is a kind of mixed mode failure which gradually changes from normal-dominated failure mode to shear-dominated failure mode with the increase of temperature gradient loading cycles.

Analysis of biaxial proportional low-cycle fatigue crack propagation for hull inclined-crack plate based on accumulative plasticity

Fracture failures of ship plates subjected to in-plane biaxial low-cycle fatigue loading are generally the coupling result of accumulative plasticity and biaxial low-cycle fatigue damage. A biaxial low-cycle fatigue crack growth analysis of hull structure that accounts for the accumulative plasticity effect can be more suitable for the actual evaluation of the overall fracture performance of the hull structure in severe sea conditions. An analytical model of biaxial low-cycle fatigue crack propagation with a control parameter for ∆CTOD is presented for hull inclined-crack plate. A test was conducted for cruciform specimens made of Q235 steel with an inclined crack to validate the presented analysis. The biaxial accumulative plasticity behavior and the effects of biaxiality and stress ratios were investigated. The results of this study reveal a strong dependence of biaxial low-cycle fatigue crack propagation on biaxial accumulated plasticity.