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•Defect-induced cracking behavior in VHCF of SLMed AlSi10Mg alloy is investigated.•The influence of MPBs on the VHCF crack propagation is studied.•Short crack propagation modes and ...grain refinement characteristics are analyzed.•The correlation between grain refinement and crack propagation path is revealed.•The inhomogeneous fine grain distribution mechanism is proposed.
The evolution of defects during very-high-cycle fatigue (VHCF) is important for assessing the lifetime of AlSi10Mg alloy produced by laser powder bed fusion. In the study, VHCF experiments were carried out with an ultrasonic fatigue machine at a stress ratio of −1. Detailed characteristics of short cracks in VHCF were investigated by metallographic serial sections using scanning electron microscope and electron backscatter diffraction. Results show that the defects with nominal stress of 49–60 MPa have different degrees of cracking. As a result of the difference in defect shape, nominal stress and microstructure, fatigue cracks exhibit different cracking modes and propagation paths. Grain refinement behavior occurs at short cracks under VHCF, and the heterogeneous distribution of fine grains underneath the crack surfaces is strongly related to the as-printed microstructure and crack propagation path. Statistical analysis suggests that regardless of the crystal orientation, the misorientation between fine grains and matrix grains tends to be 40–50°. The average crack growth rate in VHCF is approximately in the range of 10−12 ∼ 10−10 m/Cycle.
This paper describes a microstructure‐based multiaxial nonproportional fatigue life prediction model applied to A319 alloy. The materials made with different casting cooling rates and Sr modification ...are characterized and quantified in terms of secondary dendrite arm spacing, size, and aspect ratio of eutectic Si particles. Multiaxial nonproportional fatigue tests have been performed on six groups of A319 alloys to systematically analyze the effect of microstructure and loading path on the fatigue properties of Al–Si cast alloy. The first part of the paper is focused on microstructure quantitative characterization to determine the influence of different casting conditions, followed by stress response behavior and fatigue fracture analysis. Finally, a new multiaxial fatigue life prediction model for Al–Si alloy is proposed, for which 56% of the data points fall in the bound lines of factor of 2 and 83% of the data points fall in the bound lines of factor of 3.
Vibration induced fatigue is a well-known problem in oil and gas piping systems. However the use of vibration data to detect damage is not an easy task without a priori knowledge of the undamaged ...condition. In this paper, level 1: Detection and level 2: Localization of damage from structural health monitoring strategies is adapted for damage identification. An experimental trial of acoustic emission monitoring was run to monitor fatigue damage during a full-scale resonance fatigue test of a girth-welded steel pipe from healthy condition until failure. The pipe was excited into the first mode of vibration using a resonance fatigue testing machine in order to determine the high-cycle fatigue strength of the weld. The information provided by acoustic emission monitoring is useful in evaluating the condition of the pipe during the test and the occurrence of cracking before failure. However, the acoustic emission signals are embedded in noise. To overcome this problem, the signals from different combinations of sensors were recursively cross-correlated, which provides for the derivation of a new effective coefficient (EC) parameter for Bayesian estimation. This parameter is useful for evaluating uncertainty arising from the signals that contribute to source localization errors. The estimation finds the most probable parameters corresponding to cracks using prior knowledge derived from standard pencil lead break tests. The proposed method demonstrates a strong relationship between the acoustic emission energy and the estimated coefficients. A high correlation between signals was found to be associated with cracking, and a low correlation between signals was found to be associated with random signals or noise. The method will be useful for monitoring the condition of piping to manage the risk of vibration induced fatigue failure.
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.
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.
To describe the prevalence, nature and risk factors for the main clinical sequelae in coronavirus disease 2019 (COVID-19) survivors who have been discharged from the hospital for more than 3 months.
...This longitudinal study was based on a telephone follow-up survey of COVID-19 patients hospitalized and discharged from Renmin Hospital of Wuhan University, Wuhan, China before 1 March 2020. Demographic and clinical characteristics and self-reported clinical sequelae of the survivors were described and analysed. A cohort of volunteers who were free of COVID-19 and lived in the urban area of Wuhan during the outbreak were also selected as the comparison group.
Among 538 survivors (293, 54.5% female), the median (interquartile range) age was 52.0 (41.0–62.0) years, and the time from discharge from hospital to first follow-up was 97.0 (95.0–102.0) days. Clinical sequelae were common, including general symptoms (n = 267, 49.6%), respiratory symptoms (n = 210, 39%), cardiovascular-related symptoms (n = 70, 13%), psychosocial symptoms (n = 122, 22.7%) and alopecia (n = 154, 28.6%). We found that physical decline/fatigue (p < 0.01), postactivity polypnoea (p= 0.04) and alopecia (p < 0.01) were more common in female than in male subjects. Dyspnoea during hospitalization was associated with subsequent physical decline/fatigue, postactivity polypnoea and resting heart rate increases but not specifically with alopecia. A history of asthma during hospitalization was associated with subsequent postactivity polypnoea sequela. A history of pulse ≥90 bpm during hospitalization was associated with resting heart rate increase in convalescence. The duration of virus shedding after COVID-19 onset and hospital length of stay were longer in survivors with physical decline/fatigue or postactivity polypnoea than in those without.
Clinical sequelae during early COVID-19 convalescence were common; some of these sequelae might be related to gender, age and clinical characteristics during hospitalization.
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.
•The 2D residual stress on specimen surface prior to and post to fatigue crack growth tests was measured.•A significant crack retardation was observed only at high residual stresses levels.•The ...fatigue lives of LSP specimens with square and circular spots were increased by 1.68 and 2.38 times, respectively.•The superposition principle combined with weight function method was used to predict the fatigue crack growth rate.
This study investigates fatigue crack growth behavior in residual stress fields induced by laser shock peening (LSP) and its effect on the fatigue life of Ti6Al4V titanium alloy. The two-dimensional residual stress distribution on the specimen surface prior to and post to fatigue crack growth tests was measured by X-ray diffraction. Laser peening with square spot and circular spot was performed on the fatigue specimens. The fatigue lives of LSPed specimens with square spot and circular spot were 1.68 and 2.38 times those of the unpeened specimens, respectively. The result showed that significant crack retardation was observed in specimens only at high compressive residual stresses levels, which was attributed to the reduction of local stress. The superposition principle combined with the weight function method was used to predict the fatigue crack growth rate in residual stress fields.
ABSTRACT
The present paper discusses the evolution of a critical plane‐based multiaxial high‐cycle fatigue criterion, known as Carpinteri–Spagnoli criterion. By proposing appropriate changes to the ...original formulation, the extended versions of the aforementioned criterion are able to assess smooth and notched metallic structural components subjected to different fatigue loading conditions, such as multiaxial in‐phase and out‐of‐phase synchronous cyclic loading, asynchronous cyclic loading and random loading. The results obtained through this criterion are compared with some experimental results related to relevant data reported in the literature.
•Design S-N curves for CFRP retrofitted steel details are suggested.•Fatigue curves combining physical and statistical evidence are generated.•Several statistical distribution functions are ...explored.•P-S-N curves derived using the CFC model are proposed.•A fatigue endurance limit can be predicted using the CFC model.
Experimental fatigue data of non-cracked open-hole steel specimens retrofitted with bonded carbon fiber reinforced polymer (CFRP) are collected from the literature and assessed in this paper. The specimens are representative of steel plates in tensile riveted or bolted connections. The CFRP substantially extends the fatigue life of specimens at stress range Δσ<220MPa where the fatigue data points converge towards a fatigue limit. These are promising results since most of the ancient metallic riveted assemblies are subjected to stress levels below 150 MPa. The fatigue life extension ratio decreases when the stress level increases due to the fatigue degradation of the bonding joint and/or the CFRP. The fatigue data of specimens retrofitted with CFRP follow a nonlinear trend and code-based linear S-N curves with a slope m = 3 do not capture this behavior. The commonly used methods that assume the fatigue life extension ratio as constant are not adequate to predict the full fatigue life of CFRP retrofitted specimens. To assess the fatigue behavior of the CFRP retrofitted specimens, probabilistic methods are adopted in this study using: the Normal distribution; the two-parameter Weibull; the Gumbel; and the three-parameter Weibull model developed by Castillo & Fernández-Canteli (CFC). Regardless of the assumed distribution function, linear probabilistic S-N curves (P-S-N) are not adequate to model the complex behavior of retrofitted specimens with CFRP. The nonlinear CFC model fitted fairly well the different trends of the fatigue data including the fatigue life at high cycle regime and captured the transition between medium–high cycle range. P-S-N curves derived using the CFC model are proposed for CFRP retrofitted specimens.
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