507 research outputs found
Maia and Mandos: Tools for Integrity Protection on Arbitrary Files
We present the results of our dissertation research, which focuses on practical means of protecting system data integrity. In particular, we present Maia, a language for describing integrity constraints on arbitrary file types, and Mandos, a Linux Security Module which uses verify-on-close to enforce mandatory integrity guarantees. We also provide details of a Maia-based verifier generator, demonstrate that Maia and Mandos introduce minimal delay in performing their tasks, and include a selection of sample Maia specifications
Electrical conductance of a 2D packing of metallic beads under thermal perturbation
Electrical conductivity measurements on a 2D packing of metallic beads have
been performed to study internal rearrangements in weakly pertubed granular
materials. Small thermal perturbations lead to large non gaussian conductance
fluctuations. These fluctuations are found to be intermittent and gathered in
bursts. The distributions of the waiting time between to peaks is found to be a
power law inside bursts. The exponent is independent of the bead network, the
intensity of the perturbation and external stress. these bursts are interpreted
as the signature of individual bead creep rather than collective vaults
reorganisations. We propose a simple model linking the exponent of the waiting
time distribution to the roughness exponent of the surface of the beads.Comment: 7 pages, 6 figure
Fracture through cavitation in a metallic glass
The fracture surfaces of a Zr-based bulk metallic glass exhibit exotic multi-affine isotropic scaling properties. The study of the mismatch between the two facing fracture surfaces as a function of their distance shows that fracture occurs mostly through the growth and coalescence of damage cavities. The fractal nature of these damage cavities is shown to control the roughness of the fracture surfaces
A study of the rates of heat transfer and bubble site density for nucleate boiling on an inclined heating surface
Experimental study of granular surface flows via a fast camera: a continuous description
Depth averaged conservation equations are written for granular surface flows.
Their application to the study of steady surface flows in a rotating drum
allows to find experimentally the constitutive relations needed to close these
equations from measurements of the velocity profile in the flowing layer at the
center of the drum and from the flowing layer thickness and the static/flowing
boundary profiles. The velocity varies linearly with depth, with a gradient
independent of both the flowing layer thickness and the static/flowing boundary
local slope. The first two closure relations relating the flow rate and the
momentum flux to the flowing layer thickness and the slope are then deduced.
Measurements of the profile of the flowing layer thickness and the
static/flowing boundary in the whole drum explicitly give the last relation
concerning the force acting on the flowing layer. Finally, these closure
relations are compared to existing continuous models of surface flows.Comment: 20 pages, 11 figures, submitted to Phys. FLuid
Understanding fast macroscale fracture from microcrack post mortem patterns
Dynamic crack propagation drives catastrophic solid failures. In many
amorphous brittle materials, sufficiently fast crack growth involves
small-scale, high-frequency microcracking damage localized near the crack tip.
The ultra-fast dynamics of microcrack nucleation, growth and coalescence is
inaccessible experimentally and fast crack propagation was therefore studied
only as a macroscale average. Here, we overcome this limitation in
polymethylmethacrylate, the archetype of brittle amorphous materials: We
reconstruct the complete spatio-temporal microcracking dynamics, with
micrometer / nanosecond resolution, through post mortem analysis of the
fracture surfaces. We find that all individual microcracks propagate at the
same low, load-independent, velocity. Collectively, the main effect of
microcracks is not to slow down fracture by increasing the energy required for
crack propagation, as commonly believed, but on the contrary to boost the
macroscale velocity through an acceleration factor selected on geometric
grounds. Our results emphasize the key role of damage-related internal
variables in the selection of macroscale fracture dynamics.Comment: 9 pages, 5 figures + supporting information (15 pages
Reconfiguring Independent Sets in Claw-Free Graphs
We present a polynomial-time algorithm that, given two independent sets in a
claw-free graph , decides whether one can be transformed into the other by a
sequence of elementary steps. Each elementary step is to remove a vertex
from the current independent set and to add a new vertex (not in )
such that the result is again an independent set. We also consider the more
restricted model where and have to be adjacent
Cleaved surface of i-AlPdMn quasicrystals: Influence of the local temperature elevation at the crack tip on the fracture surface roughness
Roughness of i-AlPdMn cleaved surfaces are presently analysed. From the
atomic scale to 2-3 nm, they are shown to exhibit scaling properties hiding the
cluster (0.45 nm) aperiodic structure. These properties are quantitatively
similar to those observed on various disordered materials, albeit on other
ranges of length scales. These properties are interpreted as the signature of
damage mechanisms occurring within a 2-3 nm wide zone at the crack tip. The
size of this process zone finds its origin in the local temperature elevation
at the crack tip. For the very first time, this effect is reported to be
responsible for a transition from a perfectly brittle behavior to a nanoductile
one.Comment: 8 page
Block to granular-like transition in dense bubble flows
We have experimentally investigated 2-dimensional dense bubble flows
underneath inclined planes. Velocity profiles and velocity fluctuations have
been measured. A broad second-order phase transition between two dynamical
regimes is observed as a function of the tilt angle . For low
values, a block motion is observed. For high values, the velocity
profile becomes curved and a shear velocity gradient appears in the flow.Comment: Europhys. Lett. (2003) in pres
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