The small fatigue crack growth (FCG) behavior in Ti6Al4V alloy manufactured by selective laser melting (SLM) was experimentally investigated on an in situ fatigue testing machine. A semi‐elliptical ...artificial defect was introduced into the specimen to simulate the pore defects induced during additive manufacturing (AM) process. The results indicated that the fatigue life was mainly consumed in the small FCG stage. The FCG rate was significantly affected by microstructure at the early small FCG stage with great fluctuations. With the increase of crack length, the effect of microstructure decreased. Compared with long crack growth data, it was found that small crack can still grow below the threshold value of long crack, and at the same stress intensity factor (SIF) ΔK, the FCG rate of small crack was higher than that of long crack. Therefore, the small crack growth behavior should be considered in fatigue life prediction for SLM Ti6Al4V.
Highlights
The typical “small crack effect” of SLM Ti6Al4V alloy was verified by experiments.
The fatigue life of SLM Ti6Al4V is mainly consumed in the stage of small crack growth.
The effect of microstructure on small FCG behaviors were examined and identified.
At the stage of small crack growth, crack closure effect still exists to a certain extent.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
This paper investigates the transverse crack initiation behaviors of carbon fiber‐reinforced plastics (CFRPs) in the high‐cycle and very high cycle regimes. To accelerate the fatigue test up to the ...very high cycle fatigue (VHCF) regime, an ultrasonic fatigue testing system that can apply the specimen ultrasonic load at a frequency of 20 kHz was used. The fatigue behavior of cross‐ply CFRP laminates formed with T800S/2592 prepreg was investigated. The fatigue behaviors at N = 103–107 cycles and N = 108–109 cycles were obtained by hydraulic and ultrasonic fatigue tests, respectively. The slope of the S–N diagram for transverse crack initiation showed a linear decrease in the region of N = 103–106 cycles and leveled off over N = 106 cycles. Furthermore, no cracks occurred in any specimen tested under the 90°‐layer stress σ90°max = 44 MPa up to a maximum of N = 6.56 × 109 cycles.
Highlights
Ultrasonic fatigue tests over N = 109 cycles were successfully performed.
Fatigue behavior was obtained over a wide stress range using three different frequencies.
The existence of a stress threshold for transverse crack initiation was suggested.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
•The fatigue behaviour of aluminium matrix composites reinforced by alumina particles or short-fibres was investigated in the very high cycle fatigue regime.•Fatigue tests were complemented by in ...situ thermographic measurements and by recording the resonance frequency (fres) and the nonlinearity parameter (βrel).•No influence of test frequency was found.•In VHCF-region marginally higher fatigue life can be measured for the two reinforced materials.•Both reinforced materials show multiple crack initiation and damage after fatigue tests. High deformation and microcracks are often localized in areas of fibre/particle agglomeration.•Short-fibre reinforced specimens with an overaged heat treatment show in all tests more than one hot spot.
The fatigue behaviour of aluminium matrix composites (Al-MMCs) reinforced by alumina particles (15 vol% Al2O3) or short-fibres (20 vol% Saffil), respectively, was investigated in the very high cycle fatigue (VHCF) regime at stress amplitudes ≤140 MPa to reach fatigue lives of up to 1010 cycles using an ultrasonic fatigue testing device. Fatigue tests were complemented by in situ thermographic measurements to record the temperature of the whole specimen and to find “hot spots” indicating changes in microstructure and, therefore, the initiation or growth of cracks. Results obtained in the VHCF-region fit well with results obtained in LCF/HCF-region. A significant influence of test frequency was not found. Surprisingly, the reinforced Al-MMCs are capable to bear up to 109 cycles, although damage starts early during cyclic loading. In addition, although in the LCF/HCF regime the fatigue life and fatigue strength of the short-fibre reinforced Al-MMCs are significantly smaller compared to particle reinforced and unreinforced material, no significant differences to the particle-reinforced and unreinforced Al-MMC were observed at stresses below 140 MPa. Moreover, at stresses below 80 MPa not fatigue failure occurred at all up to 1010 cycles.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
As nickel-based alloys are more and more widely used in engineering fields for bearing cyclic loadings, it is necessary to study their very-high-cycle fatigue (VHCF) properties. In this paper, the ...fatigue properties of nickel-based alloy 625 were investigated using an ultrasonic fatigue test apparatus. The fracture microscopy shows that around the crack initiation site there are two characteristic zones, a rough area (RA) and a fine granular area (FGA). Inclusions caused the interior fatigue crack initiation, and the coalescence of neighboring micro cracks was strongly influenced by the local microstructure, resulting in the RA morphology. Subsequently, the contact and compressing of the crack surfaces contributed to the formation of the FGA. Finally, the stress intensity factors of the RA and FGA were quantitatively evaluated for further discussion of the crack initiation and propagation processes.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
•A strain level of 15% should be included in comparing and/or ranking of binders with respect to fatigue resistance.•The fatigue lives (Nf) at various aging levels are described by a simple function ...of aging at any strain level.•The effect of aging on asphalt binder LAS model parameters can relate to changes in complex modulus.•The proposed G*-related prediction model can used to estimate the fatigue life any aging and strain level.
The linear amplitude sweep (LAS) test is considered a useful tool for evaluating fatigue of asphalt binders. It has been confirmed that a good correlation exists between fatigue performance of binders and mixtures in a few studies. While the change in binder fatigue with load or strain level is well studied and modeled, the effects of oxidative aging remain difficult to explain or to model in a simple format. In this paper, the combined effects of strain and aging are investigated and a method for estimating binder fatigue behavior after different aging durations from limited measurements is introduced. LAS tests were conducted on eight binders at three different aging periods in the PAV. It is found that binder fatigue damage parameters C1 and C2 have a good linear relationship with aging level when using the dissipated pseudo-strain energy approach. The results also show that LAS parameters A and B at different aging duration maintain a linear relationship with the binder complex modulus (G*) measured at corresponding aging conditions. Therefore, a new fatigue life (Nf) model accounting for strain level and aging effect is proposed using a power function of the binder G*. To verify the model, its parameters are fitted using test data at first two aging conditions and used to extrapolate and predict the fatigue life at extended aging conditions. This study findings offer a simpler and reliable method to predicted values of fatigue life at new aging condition using both 35% damage level and maximum stress failure criteria that are recommended in AASHTO TP101-12 and AASHTO TP101-14, respectively. The new model will allow use of the LAS procedure to a wider range of aging and strain level conditions for which binders could be subjected to in the pavement.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
To accurately predict the fatigue life of cast aluminum alloy wheels, based on the results of finite element calculation and simulation, this work selected the rim area, inner flange and spoke areas ...to conduct systematic tensile and high‐cycle fatigue tests. The results show that the tensile properties of A356‐T6 cast aluminum alloy wheels are similar in various areas, but the fatigue properties are significantly different. Plastic deformation can improve the fatigue strength of the aluminum alloy, but the fatigue properties are also affected by deformation uniformity and other factors. Based on the difference of mechanical properties in different areas, this study also selected the characteristic loading spectrum to predict the fatigue life of wheels. A linear and a nonlinear damage accumulation were employed. The above methods are not only applicable to the wheel, but also provides a new idea for the life prediction of key parts in the engineering field.
Highlights
Systematically test and characterize the difference of fatigue performance of each part of casting and spinning aluminum alloy wheel.
The smaller the SDAS is, the better the fatigue performance is not necessarily. The coordinated deformation ability and plastic deformation will have a significant impact on the fatigue performance.
Based on LBF load spectrum, the fatigue life of casting and spinning wheel is predicted, and the influence of different damage models on the fatigue life prediction results is analyzed.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
This article aims to analyze the influence of high‐cycle fatigue damage on the mechanical properties and low‐cycle fatigue performance of Q690D. Monotonic tensile and cyclic loading tests were ...performed on Q690D specimens with different degrees of high‐cycle fatigue damage. Degradation models were established to describe the declining trend of mechanical properties with the increase of pre‐fatigue damage. Manson–Coffin models for Q690D steels of different pre‐damage levels were established. Besides, a comparison was presented between Q690D and Q355B. The research work in this article provides a fundamental reference for the appropriate assessment of the mechanical performance of Q690D high‐strength steel structures after long‐term alternating loading.
Highlights
Degradation models of Q690D mechanical behavior with different pre‐damages were proposed.
Manson–Coffin models for Q690D steels of different pre‐damage levels were established.
A cyclic constitutive model of Q690D was established and validated.
A comparison of the influence of high‐cycle fatigue (HCF) was presented between Q690D and Q355B.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The effect of the time of operation, of the surface roughness and of the corrosive environment on the number of cycles to failure in fatigue tests of steel drill pipes of strength groups S-135 and ...G-105 of various diameters is studied. Tests for high-cycle fatigue by a “bending with torsion” loading scheme imitating the service conditions of drill pipes are performed. It is shown that the fracture resistance of the pipes is reduced in service by 20% due to accumulation of internal defects. The scale factor affects substantially the fatigue endurance of the steels, and this effect decreases with increase in the pipe diameter. Growth of the surface roughness and action of aggressive media lower the cyclic endurance considerably due to formation of stress concentrators on the surface of the pipes.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
•Baseline fatigue is not associated with the development of persistent fatigue after IFN-α.•IFN-α-induced persistent fatigue is associated with increased baseline IL-10.•Patients who develop ...persistent fatigue experience greater increases in IL-6 and 10 in response to IFN-α.•Persistently fatigued patients recover at a similar rate, but from a more severe acute response to the initial trigger.•Once established, neither the persistent fatigue phenotype, nor CFS, are associated with peripheral immune activation.
The role of immune or infective triggers in the pathogenesis of Chronic Fatigue Syndrome (CFS) is not yet fully understood. Barriers to obtaining immune measures at baseline (i.e., before the trigger) in CFS and post-infective fatigue model cohorts have prevented the study of pre-existing immune dysfunction and subsequent immune changes in response to the trigger.
This study presents interferon-alpha (IFN-α)-induced persistent fatigue as a model of CFS. IFN-α, which is used in the treatment of chronic Hepatitis C Virus (HCV) infection, induces a persistent fatigue in some individuals, which does not abate post-treatment, that is, once there is no longer immune activation. This model allows for the assessment of patients before and during exposure to the immune trigger, and afterwards when the original trigger is no longer present.
Fifty-five patients undergoing IFN-α treatment for chronic HCV were assessed at baseline, during the 6–12 months of IFN-α treatment, and at six-months post-treatment. Measures of fatigue, cytokines and kynurenine pathway metabolites were obtained. Fifty-four CFS patients and 57 healthy volunteers completed the same measures at a one-off assessment, which were compared with post-treatment follow-up measures from the HCV patients.
Eighteen patients undergoing IFN-α treatment (33%) were subsequently defined as having ‘persistent fatigue’ (the proposed model for CFS), if their levels of fatigue were higher six-months post-treatment than at baseline; the other 67% were considered ‘resolved fatigue’. Patients who went on to develop persistent fatigue experienced a greater increase in fatigue symptoms over the first four weeks of IFN-α, compared with patients who did not (Δ Treatment Week (TW)-0 vs. TW4; PF: 7.1 ± 1.5 vs. RF: 4.0 ± 0.8, p = 0.046). Moreover, there was a trend towards increased baseline interleukin (IL)-6, and significantly higher baseline IL-10 levels, as well as higher levels of these cytokines in response to IFN-α treatment, alongside concurrent increases in fatigue. Levels increased to more than double those of the other patients by Treatment Week (TW)4 (p = 0.011 for IL-6 and p = 0.001 for IL-10). There was no evidence of an association between persistent fatigue and peripheral inflammation six-months post-treatment, nor did we observe peripheral inflammation in the CFS cohort. While there were changes in kynurenine metabolites in response to IFN-α, there was no association with persistent fatigue. CFS patients had lower levels of the ratio of kynurenine to tryptophan and 3-hydroxykynurenine than controls.
Future studies are needed to elucidate the mechanisms behind the initial exaggerated response of the immune system in those who go on to experience persistent fatigue even if the immune trigger is no longer present, and the change from acute to chronic fatigue in the absence of continued peripheral immune activation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Freeze-thaw-fatigue (FTF) testing was carried out on Tibet marble at lab-scale by considering various Freeze-Thaw (FT) cycles and multi-level cyclic loading. The aim is to investigate the evolution ...of deformation and damage related parameters as well as the hysteresis behavior. The test results show that an increase of FT cycles and fatigue load level both accelerate the damage rate of marble. A rock suffering more FT cycles shows a much larger axial strain rate, radial strain rate and increasing Poisson's ratio. The relation of axial strain rate, radial strain rate and Poisson's ratio rate versus fatigue load level can be fitted by an exponential function. A warning level is defined according to the evolution of radial strain and Poisson's ratio which can inform before dilation starts. Moreover, the hysteresis behavior of stress-strain is investigated. It has been proven, that the proposed two indexes, “advance ratio” and “lag ratio” are effective precursors for rock failure prediction if subjected to FTF actions. The drastic drop of the “advance ratio” and the increase of the “lag ratio” are the indicators for forthcoming failure. The recommended warning limits for “advance ratio” and “lag ratio” for the tested marble are 5% and 70%, respectively.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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