17,708 research outputs found
The effect of grain size on workhardening and superplasticity in Zn/0.4% Al Alloy
Superplasticity*requires, amongst other things, a metal with a
grain-size in the range 0.5-5μ. Theories of SP invoking dynamic recovery
require that the cell-Size of the substructure for the alloy in question is
larger than the SP grain-size, so that gliding dislocations are always
annihilated in the grain boundaries and workhardening cannot occur (1,2).
.Thus the grain-size is critical, and for a given set of conditions, there
must be a grain-size greater than which SP cannot be achieved
Extended plasticity in commercial-purity zinc
90% rolling-reduction of annealed commercial-purity zinc sheet
(grain size 100 - 150 μ) results in the fragmentation of the large grains
into, finally, stable micro-grains, 1 - 211 in diameter. The stability of
the micro-grains is due to the presence of soluble and insoluble impurities
which prevent recrystallization.
This micro-grain material is strain-rate sensitive, and elongations of
200% have been obtained at room temperature.
Although this as-rolled, 90% reduction zinc sheet is not super-plastic
according to the current definition, its behaviour has led to the coining of
the phase 'extended plasticity'.
Evidence of grain-boundary sliding is found on examination of the surface
by scanning electron microscopy, while the examination of thin foils and
activation energy measurements support the dynamic softening (recovery) theory;
thus, both these mechanisms must be operating, to a greater or less extent,
to confer on this material the observe mechanical behaviour.
It is finally concluded that it is dangerous to draw conclusions regarding
the mechanism of plastic deformation from surface observations alone
Hydra: An Adaptive--Mesh Implementation of PPPM--SPH
We present an implementation of Smoothed Particle Hydrodynamics (SPH) in an
adaptive-mesh PPPM algorithm. The code evolves a mixture of purely
gravitational particles and gas particles. The code retains the desirable
properties of previous PPPM--SPH implementations; speed under light clustering,
naturally periodic boundary conditions and accurate pairwise forces. Under
heavy clustering the cycle time of the new code is only 2--3 times slower than
for a uniform particle distribution, overcoming the principal disadvantage of
previous implementations\dash a dramatic loss of efficiency as clustering
develops. A 1000 step simulation with 65,536 particles (half dark, half gas)
runs in one day on a Sun Sparc10 workstation. The choice of time integration
scheme is investigated in detail. A simple single-step Predictor--Corrector
type integrator is most efficient. A method for generating an initial
distribution of particles by allowing a a uniform temperature gas of SPH
particles to relax within a periodic box is presented. The average SPH density
that results varies by \%. We present a modified form of the
Layzer--Irvine equation which includes the thermal contribution of the gas
together with radiative cooling. Tests of sound waves, shocks, spherical infall
and collapse are presented. Appropriate timestep constraints sufficient to
ensure both energy and entropy conservation are discussed. A cluster
simulation, repeating Thomas andComment: 29 pp, uuencoded Postscrip
Do early-life exposures explain why more advantaged children get eczema? Findings from the U.K. Millennium Cohort Study
Background:
Atopic dermatitis (eczema) in childhood is socially patterned, with higher incidence in more advantaged populations. However, it is unclear what factors explain the social differences.
Objectives:
To identify early-life risk factors for eczema, and to explore how early-life risk factors explain any differences in eczema.
Methods:
We estimated odds ratios (ORs) for ever having had eczema by age 5 years in 14 499 children from the U.K. Millennium Cohort Study (MCS), with a focus on maternal, antenatal and early-life risk factors and socioeconomic circumstances (SECs). Risk factors were explored to assess whether they attenuated associations between SECs and eczema.
Results:
Overall 35·1% of children had ever had eczema by age 5 years. Children of mothers with degree-level qualifications vs. no educational qualifications were more likely to have eczema (OR 1·52, 95% confidence interval 1·31–1·76), and there was a gradient across the socioeconomic spectrum. Maternal atopy, breastfeeding (1–6 weeks and ≥ 6 months), introduction of solids under 4 months or cow's milk under 9 months, antibiotic exposure in the first year of life and grime exposure were associated with an increased odds of having eczema. Female sex, Pakistani and Bangladeshi ethnicity, smoking during pregnancy, exposure to environmental tobacco smoke and having more siblings were associated with reduced odds for eczema. Controlling for maternal, antenatal and early-life characteristics (particularly maternal smoking during pregnancy, breastfeeding and number of siblings) reduced the OR for eczema to 1·26 (95% confidence interval 1·03–1·50) in the group with the highest educational qualifications compared with the least.
Conclusions:
In a representative U.K. child cohort, eczema was more common in more advantaged children. This was explained partially by early-life factors including not smoking during pregnancy, breastfeeding and having fewer siblings
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