681 research outputs found
Теоретичний аналіз динаміки розвитку творчої активності дошкільника
(uk) У статті проаналізовано динаміку розвитку творчої активності дошкільника. Зазначається, що процес формування особистості дошкільника досягається під час самореалізації її в творчій діяльності. Особлива увага приділена характеристиці етапів розвитку свідомості та творчій взаємодії «дитина – дорослий». Обґрунтовується необхідність створення сприятливих умов щодо творчої активності дошкільника.(ru) В статье проанализирована динамика развития творческой активности дошкольника. Отмечается, что процесс формирования личности дошкольника достигается во время самореализации ее в творческой деятельности. Особое внимание уделено характеристике этапов развития сознания и творческому взаимодействию «ребенок – взрослый». Обосновывается необходимость создания благоприятных условий относительно творческой активности дошкольника
Dislocation interactions mediated by grain boundaries
The dynamics of dislocation assemblies in deforming crystals indicate the
emergence of collective phenomena, intermittent fluctuations and strain
avalanches. In polycrystalline materials, the understanding of plastic
deformation mechanisms depends on grasping the role of grain boundaries on
dislocation motion. Here the interaction of dislocations and elastic, low angle
grain boundaries is studied in the framework of a discrete dislocation
representation. We allow grain boundaries to deform under the effect of
dislocation stress fields and compare the effect of such a perturbation to the
case of rigid grain boudaries. We are able to determine, both analytically and
numerically, corrections to dislocation stress fields acting on neighboring
grains, as mediated by grain boundary deformation. Finally, we discuss
conclusions and consequences for the avalanche statistics, as observed in
polycrystalline samples.Comment: 13 pages, 5 figure
Plate-impact loading of cellular structures formed by selective laser melting
Porous materials are of great interest because of improved energy absorption over their solid counterparts. Their properties, however, have been difficult to optimize. Additive manufacturing has emerged as a potential technique to closely define the structure and properties of porous components, i.e. density, strut width and pore size; however, the behaviour of these materials at very high impact energies remains largely unexplored. We describe an initial study of the dynamic compression response of lattice materials fabricated through additive manufacturing. Lattices consisting of an array of intersecting stainless steel rods were fabricated into discs using selective laser melting. The resulting discs were impacted against solid stainless steel targets at velocities ranging from 300 to 700 m s-1 using a gas gun. Continuum CTH simulations were performed to identify key features in the measured wave profiles, while 3D simulations, in which the individual cells were modelled, revealed details of microscale deformation during collapse of the lattice structure. The validated computer models have been used to provide an understanding of the deformation processes in the cellular samples. The study supports the optimization of cellular structures for application as energy absorbers. © 2014 IOP Publishing Ltd
Modelling the small punch tensile behaviour of an aerospace alloy
The small punch (SP) test is a widely accepted methodology for obtaining mechanical property information from limited material quantities. Much research has presented the creep, tensile and fracture responses of numerous materials gathered from small-scale testing approaches. This is of particular interest for alloy down selection of next-generation materials and in situ mechanical assessments. However, to truly understand the evolution of deformation of the miniature disc specimen, an accurate and detailed understanding of the progressive damage is necessary. This paper will utilise the SP test to assess the tensile properties of several Ti–6Al–4V materials across different temperature regimes. Fractographic investigations will establish the contrasting damage mechanisms and finite element modelling through DEFORM software is employed to characterise specimen deformation
Atomic-scale modeling of the deformation of nanocrystalline metals
Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display
technologically interesting properties, such as dramatically increased
hardness, increasing with decreasing grain size. Due to the small grain size,
direct atomic-scale simulations of plastic deformation of these materials are
possible, as such a polycrystalline system can be modeled with the
computational resources available today.
We present molecular dynamics simulations of nanocrystalline copper with
grain sizes up to 13 nm. Two different deformation mechanisms are active, one
is deformation through the motion of dislocations, the other is sliding in the
grain boundaries. At the grain sizes studied here the latter dominates, leading
to a softening as the grain size is reduced. This implies that there is an
``optimal'' grain size, where the hardness is maximal.
Since the grain boundaries participate actively in the deformation, it is
interesting to study the effects of introducing impurity atoms in the grain
boundaries. We study how silver atoms in the grain boundaries influence the
mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in
Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see
http://www.fysik.dtu.dk/~schiotz/publist.htm
Non-Equilibrium Evolution Thermodynamics Theory
Alternative approach for description of the non-equilibrium phenomena arising
in solids at a severe external loading is analyzed. The approach is based on
the new form of kinetic equations in terms of the internal and modified free
energy. It is illustrated by a model example of a solid with vacancies, for
which there is a complete statistical ground. The approach is applied to the
description of important practical problem - the formation of fine-grained
structure of metals during their treatment by methods of severe plastic
deformation. In the framework of two-level two-mode effective internal energy
potential model the strengthening curves unified for the whole of deformation
range and containing the Hall-Petch and linear strengthening sections are
calculated.Comment: 7 pages, 1 figur
Optimization of the Strength-Fracture Toughness Relation in Particulate-Reinforced Aluminum Composites via Control of the Matrix Microstructure
The article of record as published may be found at http://dx.doi.org/10.1007/s11661-998-0119-9The evolution of the microstructure and mechanical properties of a 17.5 vol. pct SiC particulatereinforced
aluminum alloy 6092-matrix composite has been studied as a function of postfabrication
processing and heat treatment. It is demonstrated that, by the control of particulate distribution, matrix
grain, and substructure and of the matrix precipitate state, the strength-toughness combination in the
composite can be optimized over a wide range of properties, without resorting to unstable, underaged
(UA) matrix microstructures, which are usually deemed necessary to produce a higher fracture toughness
than that displayed in the peak-aged condition. Further, it is demonstrated that, following an
appropriate combination of thermomechanical processing and unconventional heat treatment, the
composite may possess better stiffness, strength, and fracture toughness than a similar unreinforced
alloy. In the high- and low-strength matrix microstructural conditions, the matrix grain and substructure
were found to play a substantial role in determining fracture properties. However, in the intermediate-
strength regime, properties appeared to be optimizable by the utilization of heat treatments
only. These observations are rationalized on the basis of current understanding of the grain size
dependence of fracture toughness and the detailed microstructural features resulting from thermomechanical
treatments.United States Army Research OfficeArmy Research LabratoryUnited States Air Force Office of Scientific ResearchWright Materials LabratoryDWA Composite
Infrared seeded parametric four-wave mixing for sensitive detection of molecules
We have developed a sensitive resonant four-wave mixing technique based on two-photon parametric four-wave mixing with the addition of a phase matched ''seeder'' field. Generation of the seeder field via the same four-wave mixing process in a high pressure cell enables automatic phase matching to be achieved in a low pressure sample cell. This arrangement facilitates sensitive detection of complex molecular spectra by simply tuning the pump laser. We demonstrate the technique with the detection of nitric oxide down to concentrations more than 4 orders of magnitude below the capability of parametric four-wave mixing alone, with an estimated detection threshold of 10(12) molecules/cm(3)
Environmental DNA reveals links between abundance and composition of airborne grass pollen and respiratory health
This is the final version. Available on open access from Elsevier via the DOI in this recordData and Code Availability Statement:
Data collected using qPCR is archived and on NERC EIDC [https://doi.org/10.5285/28208be4-0163-45e6-912c-2db205126925]. Standard pollen monitoring ‘count’ data were sourced from the
MEDMI database, with the exception of data from Bangor which were produced as part of the
present study and are available on request. Prescribing datasets are publicly available, as are
weather, air pollution, deprivation (IMD) and rural-urban category data. Hospital
episode statistics (HES) datasets are sensitive, individual-level health data, which are subject to
strict privacy regulations and are not publicly available. The study did not generate any unique
codeGrass (Poaceae) pollen is the most important outdoor aeroallergen, exacerbating a range of respiratory conditions,
including allergic asthma and rhinitis (‘hay fever’). Understanding the relationships between respiratory diseases and airborne grass pollen with view to improving forecasting has broad public health and socioeconomic relevance. It
is estimated that there are over 400 million people with allergic rhinitis and over 300 million with asthma, globally, often comorbidly
. In the UK, allergic asthma has an annual cost of around US$ 2.8 billion (2017). The relative
contributions of the >11,000 (worldwide) grass species to respiratory health have been unresolved, as grass
pollen cannot be readily discriminated using standard microscopy. Instead, here we used novel environmental DNA
(eDNA) sampling and quantitative PCR (qPCR) , to measure the relative abundances of airborne pollen from
common grass species, during two grass pollen seasons (2016 and 2017), across the UK. We quantitatively
demonstrate discrete spatiotemporal patterns in airborne grass pollen assemblages. Using a series of generalised
additive models (GAMs), we explore the relationship between the incidences of airborne pollen and severe asthma
exacerbations (sub-weekly) and prescribing rates of drugs for respiratory allergies (monthly). Our results indicate that
a subset of grass species may have disproportionate influence on these population-scale respiratory health responses
during peak grass pollen concentrations. The work demonstrates the need for sensitive and detailed biomonitoring of
harmful aeroallergens in order to investigate and mitigate their impacts on human health.Natural Environment Research Council (NERC)National Institute for Health Research (NIHR)Public Health EnglandUniversity of ExeterUniversity College LondonMet Offic
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