861 research outputs found
The dynamics of cracks in torn thin sheets
Motivated by recent experiments, we present a study of the dynamics of cracks
in thin sheets. While the equations of elasticity for thin plates are well
known, there remains the question of path selection for a propagating crack. We
invoke a generalization of the principle of local symmetry to provide a
criterion for path selection and demonstrate qualitative agreement with the
experimental findings. The nature of the singularity at the crack tip is
studied with and without the interference of nonlinear terms.Comment: 7 pages, 11 figure
Acute nephritis in the canine Due to Leptospirosis (A Case Report)
The case report presented demonstrates the protean nature of leptospirosis and the absolute dependence on either cultural or serological evidence o the disease for definitive diagnosis
Frictional sliding without geometrical reflection symmetry
The dynamics of frictional interfaces play an important role in many physical
systems spanning a broad range of scales. It is well-known that frictional
interfaces separating two dissimilar materials couple interfacial slip and
normal stress variations, a coupling that has major implications on their
stability, failure mechanism and rupture directionality. In contrast,
interfaces separating identical materials are traditionally assumed not to
feature such a coupling due to symmetry considerations. We show, combining
theory and experiments, that interfaces which separate bodies made of
macroscopically identical materials, but lack geometrical reflection symmetry,
generically feature such a coupling. We discuss two applications of this novel
feature. First, we show that it accounts for a distinct, and previously
unexplained, experimentally observed weakening effect in frictional cracks.
Second, we demonstrate that it can destabilize frictional sliding which is
otherwise stable. The emerging framework is expected to find applications in a
broad range of systems.Comment: 14 pages, 5 figures + Supplementary Material. Minor change in the
title, extended analysis in the second par
Cloudbus Toolkit for Market-Oriented Cloud Computing
This keynote paper: (1) presents the 21st century vision of computing and
identifies various IT paradigms promising to deliver computing as a utility;
(2) defines the architecture for creating market-oriented Clouds and computing
atmosphere by leveraging technologies such as virtual machines; (3) provides
thoughts on market-based resource management strategies that encompass both
customer-driven service management and computational risk management to sustain
SLA-oriented resource allocation; (4) presents the work carried out as part of
our new Cloud Computing initiative, called Cloudbus: (i) Aneka, a Platform as a
Service software system containing SDK (Software Development Kit) for
construction of Cloud applications and deployment on private or public Clouds,
in addition to supporting market-oriented resource management; (ii)
internetworking of Clouds for dynamic creation of federated computing
environments for scaling of elastic applications; (iii) creation of 3rd party
Cloud brokering services for building content delivery networks and e-Science
applications and their deployment on capabilities of IaaS providers such as
Amazon along with Grid mashups; (iv) CloudSim supporting modelling and
simulation of Clouds for performance studies; (v) Energy Efficient Resource
Allocation Mechanisms and Techniques for creation and management of Green
Clouds; and (vi) pathways for future research.Comment: 21 pages, 6 figures, 2 tables, Conference pape
Drying Patterns: Sensitivity to Residual Stresses
Volume alteration in solid materials is a common cause of material failure.
Here we investigate the crack formation in thin elastic layers attached to a
substrate. We show that small variations in the volume contraction and
substrate restraint can produce widely different crack patterns ranging from
spirals to complex hierarchical networks. The networks are formed when there is
no prevailing gradient in material contraction whereas spirals are formed in
the presence of a radial gradient in the contraction of a thin elastic layer.Comment: 4 pages, 4 figure
Velocity Fluctuations in Dynamical Fracture: the Role of Microcracks
We address the velocity fluctuations of fastly moving cracks in stressed
materials. One possible mechanism for such fluctuations is the interaction of
the main crack with micro cracks (irrespective whether these are existing
material defects or they form during the crack evolution). We analyze carefully
the dynamics (in 2 space dimensions) of one macro and one micro crack, and
demonstrate that their interaction results in a {\em large} and {\em rapid}
velocity fluctuation, in qualitative correspondence with typical velocity
fluctuations observed in experiments. In developing the theory of the dynamical
interaction we invoke an approximation that affords a reduction in mathematical
complexity to a simple set of ordinary differential equations for the positions
of the cracks tips; we propose that this kind of approximation has a range of
usefulness that exceeds the present context.Comment: 7 pages, 7 figure
Crack growth by surface diffusion in viscoelastic media
We discuss steady state crack growth in the spirit of a free boundary
problem. It turns out that mode I and mode III situations are very different
from each other: In particular, mode III exhibits a pronounced transition
towards unstable crack growth at higher driving forces, and the behavior close
to the Griffith point is determined entirely through crack surface dissipation,
whereas in mode I the fracture energy is renormalized due to a remaining finite
viscous dissipation. Intermediate mixed-mode scenarios allow steady state crack
growth with higher velocities, leading to the conjecture that mode I cracks can
be unstable with respect to a rotation of the crack front line
Very static enforcement of dynamic policies
Security policies are naturally dynamic. Reflecting this, there has been a growing interest in studying information-flow properties which change during program execution, including concepts such as declassification, revocation, and role-change.
A static verification of a dynamic information flow policy, from a semantic perspective, should only need to concern itself with two things: 1) the dependencies between data in a program, and 2) whether those dependencies are consistent with the intended flow policies as they change over time. In this paper we provide a formal ground for this intuition. We present a straightforward extension to the principal flow-sensitive type system introduced by Hunt and Sands (POPL’06, ESOP’11) to infer both end-to-end dependencies and dependencies at intermediate points in a program. This allows typings to be applied to verification of both static and dynamic policies. Our extension preserves the principal type system’s distinguishing feature, that type inference is independent of the policy to be enforced: a single, generic dependency analysis (typing) can be used to verify many different dynamic policies of a given program, thus achieving a clean separation between (1) and (2).
We also make contributions to the foundations of dynamic information flow. Arguably, the most compelling semantic definitions for dynamic security conditions in the literature are phrased in the so-called knowledge-based style. We contribute a new definition of knowledge-based progress insensitive security for dynamic policies. We show that the new definition avoids anomalies of previous definitions and enjoys a simple and useful characterisation as a two-run style property
Start of the 2014/15 influenza season in Europe: drifted influenza A(H3N2) viruses circulate as dominant subtype
Members of the WHO European Region and European Influenza Surveillance Network: Portugal: Raquel Guiomar, Pedro Pechirra, Paula Cristovão, Inês Costa, Baltazar Nunes, Ana Rodrigues.The influenza season 2014/15 started in Europe in week 50 2014 with influenza A(H3N2) viruses predominating. The majority of the A(H3N2) viruses characterised antigenically and/or genetically differ from the northern hemisphere vaccine component which may result in reduced vaccine effectiveness for the season. We therefore anticipate that this season may be more severe than the 2013/14 season. Treating influenza with antivirals in addition to prevention with vaccination will be important
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