A computational framework is developed to investigate the process-structure-property relationship for additive manufacturing (AM) of Ti–6Al–4V alloy. The proposed model incorporates experimentally ...informed two-phase α+β morphologies within prior β-grains, which are widely observed in the as-built AM components. Specifically, the temperature-dependent phase-field model (PFM) is used to simulate the evolution of various grain morphologies, e.g., columnar and equiaxed grain structures. The proposed PFM taking into account both of the epitaxial grain growth and the constitutional cooling-driven heterogeneous nucleation enables us to capture the columnar to equiaxed transition (CET) of grain structures. The thermal fields concerned with the scanning strategies and manufacturing parameters are simulated using a finite-element model (FEM). The Burgers orientation relation (BOR) is further utilized to generate two-phase α+β morphologies within prior β-grains, accompanied by the transformation of crystal orientations, i.e., (0001)α//{101}β and <112‾0>α// β. Finally, a fast Fourier transform-based elasto-viscoplastic (EVP-FFT) model is employed to predict the micromechanical behaviors and properties for the two-phase α+β microstructures. The presented PFM-based formulation is generally applicable to predict the process-structure-property relationship for additive manufacturing of a variety of alloy systems, e.g., titanium alloys, aluminum alloys and nickel-based superalloys.
•Two-phase lamellar α+β morphology for Ti–6Al–4V are considered in the computational framework.•High travel speed and low power provide the equiaxed β-grain structures and fine α-lath microstructures.•Equiaxed β-grain structures provide higher tensile strength than that of columnar β-grain structures due to shorter effective slip length.•Fine α-lath microstructure and small colony size reduce the effective slip length, and thus provide high tensile strength.•Stress concentrations due to dislocation pile-ups induce the crack and thus reduce the ductility of columnar β-grains in Rx-direction.
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A multiscale model is developed to investigate the evolution mechanisms of site-specific grain structures during additive manufacturing (AM) of metallic alloys, using the selective ...electron beam melting (SEBM) fabrication of Ti-6Al-4V as an example. Specifically, finite-element method is utilized to predict the thermal response at macroscale during SEBM, and the extracted thermal information is then input into a temperature-dependent phase-field model to simulate the grain growth at mesoscale. The grain epitaxial growth, grain selection, grain nucleation and layer-by-layer manufacturing fashion are incorporated, in order to accurately predict grain structure development and relevant physical processes during AM. It is found that, the development of the predominant grain structures in the thick and thin walls, i.e., the large vertical columnar β//Nz grains and slanted columnar grains with various grain orientations, respectively, can be attributed to the competition and collaboration between the thermal gradient and the crystallographically preferred grain growth, as shown from the different growth stages in the simulations. Good agreements in the final grain structures and textures are achieved between the experimental observations and numerical simulations. The present study potentially offers valuable insights and guidance toward designing AM conditions to tailor the grain structures and textures.
Electrospinning of collagen and chitosan blend solutions in a 1,1,1,3,3,3-hexafluoroisopropanol/trifluoroacetic acid (v/v, 90/10) mixture was investigated for the fabrication of a biocompatible and ...biomimetic nanostructure scaffold in tissue engineering. The morphology of the electrospun collagen–chitosan nanofibers was observed by scanning electron microscopy (SEM) and stabilized by glutaraldehyde (GTA) vapor via crosslinking. Fourier transform infrared spectra analysis showed that the collagen–chitosan nanofibers do not change significantly, except for enhanced stability after crosslinking by GTA vapor. X-ray diffraction analysis implied that both collagen and chitosan molecular chains could not be crystallized in the course of electrospinning and crosslinking, and gave an amorphous structure in the nanofibers. The thermal behavior and mechanical properties of electrospun collagen–chitosan fibers were also studied by differential scanning calorimetry and tensile testing, respectively. To assay the biocompatibility of electrospun fibers, cellular behavior on the nanofibrous scaffolds was also investigated by SEM and methylthiazol tetrazolium testing. The results show that both endothelial cells and smooth muscle cells proliferate well on or within the nanofiber. The results indicate that a collagen–chitosan nanofiber matrix may be a better candidate for tissue engineering in biomedical applications such as scaffolds.
The restrictions associated with the 2020 COVID-19 pandemic has resulted in changes to young children's daily routines and habits. The impact on their participation in movement behaviours (physical ...activity, sedentary screen time and sleep) is unknown. This international longitudinal study compared young children's movement behaviours before and during the COVID-19 pandemic.
Parents of children aged 3-5 years, from 14 countries (8 low- and middle-income countries, LMICs) completed surveys to assess changes in movement behaviours and how these changes were associated with the COVID-19 pandemic. Surveys were completed in the 12 months up to March 2020 and again between May and June 2020 (at the height of restrictions). Physical activity (PA), sedentary screen time (SST) and sleep were assessed via parent survey. At Time 2, COVID-19 factors including level of restriction, environmental conditions, and parental stress were measured. Compliance with the World Health Organizations (WHO) Global guidelines for PA (180 min/day ≥60 min moderate- vigorous PA), SST (≤1 h/day) and sleep (10-13 h/day) for children under 5 years of age, was determined.
Nine hundred- forty-eight parents completed the survey at both time points. Children from LMICs were more likely to meet the PA (Adjusted Odds Ratio AdjOR = 2.0, 95%Confidence Interval CI 1.0,3.8) and SST (AdjOR = 2.2, 95%CI 1.2,3.9) guidelines than their high-income country (HIC) counterparts. Children who could go outside during COVID-19 were more likely to meet all WHO Global guidelines (AdjOR = 3.3, 95%CI 1.1,9.8) than those who were not. Children of parents with higher compared to lower stress were less likely to meet all three guidelines (AdjOR = 0.5, 95%CI 0.3,0.9).
PA and SST levels of children from LMICs have been less impacted by COVID-19 than in HICs. Ensuring children can access an outdoor space, and supporting parents' mental health are important prerequisites for enabling pre-schoolers to practice healthy movement behaviours and meet the Global guidelines.
The impact of impurity ions on a pedestal has been investigated in the HL-2A Tokamak, at the Southwestern Institute of Physics, Chengdu, China. Experimental results have clearly shown that during the ...H-mode phase, an electromagnetic turbulence was excited in the edge plasma region, where the impurity ions exhibited a peaked profile. It has been found that double impurity critical gradients are responsible for triggering the turbulence. Strong stiffness of the impurity profile has been observed during cyclic transitions between the I-phase and H-mode regime. The results suggest that the underlying physics of the self-regulated edge impurity profile offers the possibility for an active control of the pedestal dynamics via pedestal turbulence.
Abstract
With the increase of warhed charge and the development of new energetic materials, a variety of high-energy warheads have been invented, and the power of high-energy warheads has been ...greatly enhanced. The power distribution of the warhead explosion is extremely complicated in the space-time dimension, which lead to problems for accurately test and characterize the warhead damage power. According to the characteristics of the high energy warhead, this paper summarizes the research status and progress of the damage testing techniques at home and abroad in recent years, such as shock wave field, fragment field and temperature field. It analyzes the new requirements of the high energy warhead for the power field test technology, and provides a reference for further improving the test capability of the high energy warhead.
Aims
To determine whether HbA1c mismatches (HbA1c levels that are higher or lower than expected for the average glucose levels in different individuals) could lead to errors if diagnostic ...classification is based only on HbA1c levels.
Methods
In a cross‐sectional study, 3106 participants without known diabetes underwent a 75‐g oral glucose tolerance test (fasting glucose and 2‐h glucose) and a 50‐g glucose challenge test (1‐h glucose) on separate days. They were classified by oral glucose tolerance test results as having: normal glucose metabolism; prediabetes; or diabetes. Predicted HbA1c was determined from the linear regression modelling the relationship between observed HbA1c and average glucose (mean of fasting glucose and 2‐h glucose from the oral glucose tolerance test, and 1‐h glucose from the glucose challenge test) within oral glucose tolerance test groups. The haemoglobin glycation index was calculated as observed – predicted HbA1c, and divided into low, intermediate and high haemoglobin glycation index mismatch tertiles.
Results
Those participants with higher mismatches were more likely to be black, to be men, to be older, and to have higher BMI (all P<0.001). Using oral glucose tolerance test criteria, the distribution of normal glucose metabolism, prediabetes and diabetes was similar across mismatch tertiles; however, using HbA1c criteria, the participants with low mismatches were classified as 97% normal glucose metabolism, 3% prediabetes and 0% diabetes, i.e. mostly normal, while those with high mismatches were classified as 13% normal glucose metabolism, 77% prediabetes and 10% diabetes, i.e. mostly abnormal (P<0.001).
Conclusions
Measuring only HbA1c could lead to under‐diagnosis in people with low mismatches and over‐diagnosis in those with high mismatches. Additional oral glucose tolerance tests and/or fasting glucose testing to complement HbA1c in diagnostic classification should be performed in most individuals.
What's new?
HbA1c mismatches, that is, HbA1c levels that are higher or lower than expected for average glucose levels, can limit the accuracy of HbA1c measurements in diabetes diagnosis and management, but their potential impact is not widely appreciated.
In a diabetes screening cohort using the 75‐g oral glucose tolerance test (OGTT) as the ‘gold standard’, use of the HbA1c test alone led to under‐diagnosis in participants with low HbA1c mismatches and over‐diagnosis with high HbA1c mismatches.
Because HbA1c mismatches could lead to errors, diagnoses of diabetes/prediabetes should include measurement of glucose levels (fasting, OGTT) in addition to HbA1c in most individuals.
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