304 research outputs found
A traceless photocleavable linker for the automated glycan assembly of carbohydrates with free reducing ends
We report a traceless photocleavable linker for the automated glycan assembly
of carbohydrates with free reducing ends. The reductive-labile functionality
in the linker tolerates all commonly used reagents and protocols for automated
glycan assembly, as demonstrated with the successful preparation of nine plant
cell wall-related oligosaccharides, and is cleaved by hydrogenolysis
Mesoscale theory of grains and cells: crystal plasticity and coarsening
Solids with spatial variations in the crystalline axes naturally evolve into
cells or grains separated by sharp walls. Such variations are mathematically
described using the Nye dislocation density tensor. At high temperatures,
polycrystalline grains form from the melt and coarsen with time: the
dislocations can both climb and glide. At low temperatures under shear the
dislocations (which allow only glide) form into cell structures. While both the
microscopic laws of dislocation motion and the macroscopic laws of coarsening
and plastic deformation are well studied, we hitherto have had no simple,
continuum explanation for the evolution of dislocations into sharp walls. We
present here a mesoscale theory of dislocation motion. It provides a
quantitative description of deformation and rotation, grounded in a microscopic
order parameter field exhibiting the topologically conserved quantities. The
topological current of the Nye dislocation density tensor is derived from a
microscopic theory of glide driven by Peach-Koehler forces between dislocations
using a simple closure approximation. The resulting theory is shown to form
sharp dislocation walls in finite time, both with and without dislocation
climb.Comment: 5 pages, 3 figure
Maximal möglicher Niederschlag ĂŒber einem Mittelgebirge
Hier wird ein Verfahren beschrieben, dass den Einfluss der GelĂ€ndestruktur auf die Niederschlagsmenge, speziell bei Starkniederschlagsereignissen, abbildet. Die physikalischen Prozesse, die in der AtmosphĂ€re zu einer Niederschlagsbildung fĂŒhren, sind an die Hebung der feuchtegesĂ€ttigten Luftmassen u.a. an GebirgshĂ€ngen gebunden. Diese Niederschlagsbildungsprozesse weisen aus, dass es umso mehr regnet, je stĂ€rker das GelĂ€nde an Höhe gewinnt und nicht je höher das GelĂ€nde liegt. Das Ziel des beschriebenen Verfahrens ist die AbschĂ€tzung des flĂ€chengemittelten Maximal Möglichen Niederschlages (fMMN) bei vorgegebenen meteorologischen Bedingungen und einer variablen Orografie einer Landschaft. Dieser fMMN setzt sich aus einem an die synoptischen Prozesse gebundenen Niederschlagsanteil (sMMN) und einem aus den orografischen VerhĂ€ltnisse resultierenden Anteil (oMMN) zusammen. Um die VariabilitĂ€t der Niederschlagsmengen in Bezug auf die Orografie der Landschaft zu zeigen wird hier ein NiederschlagsverstĂ€rkungsfaktor FRR als eine VerhĂ€ltniszahl zwischen fMMN und sMMN eingefĂŒhrt. Diese VerhĂ€ltniszahl kann mit realen Niederschlagsereignissen verglichen werden. Das Verfahren wird exemplarisch fĂŒr eine Region des mittleren Erzgebirges validiert und steht so fĂŒr andere Mittelgebirgsregionen zur VerfĂŒgung
Micro-plasticity and intermittent dislocation activity in a simplified micro structural model
Here we present a model to study the micro-plastic regime of a stress-strain
curve. In this model an explicit dislocation population represents the mobile
dislocation content and an internal shear-stress field represents a mean-field
description of the immobile dislocation content. The mobile dislocations are
constrained to a simple dipolar mat geometry and modelled via a dislocation
dynamics algorithm, whilst the shear-stress field is chosen to be a sinusoidal
function of distance along the mat direction. The latter, defined by a periodic
length and a shear-stress amplitude, represents a pre-existing micro-structure.
These model parameters, along with the mobile dislocation density, are found to
admit a diversity of micro-plastic behaviour involving intermittent plasticity
in the form of a scale-free avalanche phenomenon, with an exponent for the
strain burst magnitude distribution similar to those seen in experiment and
more complex dislocation dynamics simulations.Comment: 30 pages, 12 figures, to appear in "Modelling and Simulation in
Materials Science and Engineering
Stress-free states of continuum dislocation fields: Rotations, grain boundaries, and the Nye dislocation density tensor
We derive general relations between grain boundaries, rotational
deformations, and stress-free states for the mesoscale continuum Nye
dislocation density tensor. Dislocations generally are associated with
long-range stress fields. We provide the general form for dislocation density
fields whose stress fields vanish. We explain that a grain boundary (a
dislocation wall satisfying Frank's formula) has vanishing stress in the
continuum limit. We show that the general stress-free state can be written
explicitly as a (perhaps continuous) superposition of flat Frank walls. We show
that the stress-free states are also naturally interpreted as configurations
generated by a general spatially-dependent rotational deformation. Finally, we
propose a least-squares definition for the spatially-dependent rotation field
of a general (stressful) dislocation density field.Comment: 9 pages, 3 figure
Do experienced physiotherapists apply equal magnitude of force during a grade I central pa on the cervical spine?
Background and purpose: Physiotherapists frequently use central posterior-anterior (PA) joint mobilization techniques for assessing and managing spinal disorders. Manual examination findings provide the basis for the selection of treatment techniques. From the literature it is evident that the level of reliability varies when physiotherapists perform different mobilization techniques. Repeatability of mobilization techniques is important for better physiotherapy management. The aim of the study was to determine whether experienced physiotherapists apply equal magnitude of force during a grade I central PA mobilisation technique on the cervical spine. Another aim was to determine the variation in the magnitude of force
applied by each individual physiotherapist.
Subjects: Sample of convenience, consisting of sixteen (n=16) selected qualified physiotherapists with experience in
Orthopaedic Manual Therapy.
Methods: A grade I central PA was performed on the Flexiforce TM sensors positioned on C6 of the same asymptomatic model to measure the applied magnitude of force. Two separate measurements, each lasting 30-seconds, were obtained.
Results: The average maximum peak force applied by the majority of physiotherapists (87.5%) was between 10.95g
and 72g. The difference in the forces applied for the two measurements ranged between 0.64g and 24.4g. The Bland
Altman scatterplot determined the mean of the difference between measurement one and two, calculated for the group, was zero. When comparing the two measurements, little variation was noted in the forces applied, as well as the coefficient of variation for each physiotherapist.
Conclusion and Discussion: Current results demonstrated good intra-therapist and moderate to good inter-therapist repeatability. Further research is required to generalize results
"Cold Melting" of Invar Alloys
An anomalously strong volume magnetostriction in Invars may lead to a
situation when at low temperatures the dislocation free energy becomes negative
and a multiple generation of dislocations becomes possible. This generation
induces a first order phase transition from the FCC crystalline to an amorphous
state, and may be called "cold melting". The possibility of the cold melting in
Invars is connected with the fact that the exchange energy contribution into
the dislocation self energy in Invars is strongly enhanced, as compared to
conventional ferromagnetics, due to anomalously strong volume magnetostriction.
The possible candidate, where this effect can be observed, is a FePt disordered
Invar alloy in which the volume magnetostriction is especially large
Melting as a String-Mediated Phase Transition
We present a theory of the melting of elemental solids as a
dislocation-mediated phase transition. We model dislocations near melt as
non-interacting closed strings on a lattice. In this framework we derive simple
expressions for the melting temperature and latent heat of fusion that depend
on the dislocation density at melt. We use experimental data for more than half
the elements in the Periodic Table to determine the dislocation density from
both relations. Melting temperatures yield a dislocation density of (0.61\pm
0.20) b^{-2}, in good agreement with the density obtained from latent heats,
(0.66\pm 0.11) b^{-2}, where b is the length of the smallest
perfect-dislocation Burgers vector. Melting corresponds to the situation where,
on average, half of the atoms are within a dislocation core.Comment: 18 pages, LaTeX, 3 eps figures, to appear in Phys. Rev.
Influence of head size on the development of metallic wear and on the characteristics of carbon layers in metal-on-metal hip joints
Background and purpose Particles originating from the articulating surfaces of hip endoprostheses often induce an inflammatory response, which can be related to implant failure. We therefore analyzed the metal content in capsular tissue from 44 McKee-Farrar metal-on-metal hip prostheses (with 3 different head sizes) and we also analyzed the morphological structure of layers located on articulating surfaces
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