179 research outputs found
Rifts in Spreading Wax Layers
We report experimental results on the rift formation between two freezing wax
plates. The plates were pulled apart with constant velocity, while floating on
the melt, in a way akin to the tectonic plates of the earth's crust. At slow
spreading rates, a rift, initially perpendicular to the spreading direction,
was found to be stable, while above a critical spreading rate a "spiky" rift
with fracture zones almost parallel to the spreading direction developed. At
yet higher spreading rates a second transition from the spiky rift to a zig-zag
pattern occurred. In this regime the rift can be characterized by a single
angle which was found to be dependent on the spreading rate. We show that the
oblique spreading angles agree with a simple geometrical model. The coarsening
of the zig-zag pattern over time and the three-dimensional structure of the
solidified crust are also discussed.Comment: 4 pages, Postscript fil
Self healing slip pulses along a gel/glass interface
We present an experimental evidence of self-healing shear cracks at a
gel/glass interface. This system exhibits two dynamical regimes depending on
the driving velocity : steady sliding at high velocity (> Vc = 100-125 \mu
m/s), caracterized by a shear-thinning rheology, and periodic stick-slip
dynamics at low velocity. In this last regime, slip occurs by propagation of
pulses that restick via a ``healing instability'' occuring when the local
sliding velocity reaches the macroscopic transition velocity Vc. At driving
velocities close below Vc, the system exhibits complex spatio-temporal
behavior.Comment: 4 pages, 6 figure
Codage d'objets de type VOP par représentation en régions auto-extractibles
Le nouveau standard multimédia Mpeg-4 traite aussi bien des aspects compression que de fonctionnalités de haut niveau telles que la manipulation d'objets. Notre approche propose un cadre unique permettant une compression efficace tout en offrant une segmentation à coût « quasi-nul » à travers le principe de représentation en régions auto-extractibles. Aussi montrons-nous ici que notre méthode peut constituer une alternative au mode classique de codage intra de Mpeg-4. En particulier, l'approche réussi à unifier le codage de la forme et de la texture d'un VOP
Color LAR codec : a color image representation and compression scheme based on local resolution adjustment and on self-extracting region representation
This LAR (Locally Adaptive Resolution) color image coding scheme yields to an efficient progressive compression with a
better subjective quality than Jpeg2000. Additionally, it offers region functionalities for low bit rate coding and decoding.
From highly compressed luminance, a region description, without contours encoding, can be obtained through a
segmentation process performed at both coder and decoder. Considering color results, controlled chrominance
components segmentation provides a better data consistency simultaneously with a low bit rate compression. As regions
and their encoding are based on a same representation grid, enhancement of image quality can be global, or only
restricted to a Region Of Interest.Cet article présente un schéma original de codage progressif d'images couleur apportant à la fois une
efficacité en termes de compression (meilleure qualité subjective que Jpeg2000) et des fonctionnalités au
niveau région à bas débits pour le codeur et le décodeur. À partir de l'image des luminances codée à bas
débit par le codec LAR (Locally Adaptive Resolution), une description en régions, sans codage des contours,
est obtenue à travers un procédé de segmentation effectué au codeur et au décodeur. Cette segmentation
peut être contrôlée par les composantes chromatiques pour une meilleure cohérence du résultat d'un point de
vue couleur. Un codage basé régions appliqué sur les images de chrominance produit alors une compression
de ces composantes à très bas débit. Comme les régions et le codage de leur contenu partagent une même
grille de représentation, l'amélioration de la qualité de l'image peut être globale, ou limitée à une zone
d'intérêt
Theory of dynamic crack branching in brittle materials
The problem of dynamic symmetric branching of an initial single brittle crack
propagating at a given speed under plane loading conditions is studied within a
continuum mechanics approach. Griffith's energy criterion and the principle of
local symmetry are used to determine the cracks paths. The bifurcation is
predicted at a given critical speed and at a specific branching angle: both
correlated very well with experiments. The curvature of the subsequent branches
is also studied: the sign of , with being the non singular stress at the
initial crack tip, separates branches paths that diverge from or converge to
the initial path, a feature that may be tested in future experiments. The model
rests on a scenario of crack branching with some reasonable assumptions based
on general considerations and in exact dynamic results for anti-plane
branching. It is argued that it is possible to use a static analysis of the
crack bifurcation for plane loading as a good approximation to the dynamical
case. The results are interesting since they explain within a continuum
mechanics approach the main features of the branching instabilities of fast
cracks in brittle materials, i.e. critical speeds, branching angle and the
geometry of subsequent branches paths.Comment: 41 pages, 15 figures. Accepted to International Journal of Fractur
Stable propagation of an ordered array of cracks during directional drying
We study the appearance and evolution of an array of parallel cracks in a
thin slab of material that is directionally dried, and show that the cracks
penetrate the material uniformly if the drying front is sufficiently sharp. We
also show that cracks have a tendency to become evenly spaced during the
penetration. The typical distance between cracks is mainly governed by the
typical distance of the pattern at the surface, and it is not modified during
the penetration. Our results agree with recent experimental work, and can be
extended to three dimensions to describe the properties of columnar polygonal
patterns observed in some geological formations.Comment: 8 pages, 4 figures, to appear in PR
P53 germline mutations in childhood cancers and cancer risk for carrier individuals
The family history of cancer in children treated for a solid malignant tumour in the Paediatric Oncology Department at Institute Gustave-Roussy, has been investigated. In order to determine the role of germline p53 mutations in genetic predisposition to childhood cancer, germline p53 mutations were sought in individuals with at least one relative (first- or second-degree relative or first cousin) affected by any cancer before 46 years of age, or affected by multiple cancers. Screening for germline p53 mutation was possible in 268 index cases among individuals fulfilling selection criteria. Seventeen (6.3%) mutations were identified, of which 13 were inherited and four were de novo. Using maximum likelihood methods that incorporate retrospective family data and correct for ascertainment bias, the lifetime risk of cancer for mutation carriers was estimated to be 73% for males and nearly 100% for females with a high risk of breast cancer accounting for the difference. The risk of cancer associated with such mutations is very high and no evidence of low penetrance mutation was found. These mutations are frequently inherited but de novo mutations are not rare. © 2000 Cancer Research Campaig
Thermal fracture as a framework for quasi-static crack propagation
We address analytically and numerically the problem of crack path prediction
in the model system of a crack propagating under thermal loading. We show that
one can explain the instability from a straight to a wavy crack propagation by
using only the principle of local symmetry and the Griffith criterion. We then
argue that the calculations of the stress intensity factors can be combined
with the standard crack propagation criteria to obtain the evolution equation
for the crack tip within any loading configuration. The theoretical results of
the thermal crack problem agree with the numerical simulations we performed
using a phase field model. Moreover, it turns out that the phase-field model
allows to clarify the nature of the transition between straight and oscillatory
cracks which is shown to be supercritical.Comment: 19 pages, 8 figure
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