290 research outputs found
Generalized Plasticity and Cyclic Pressuremeter Test Modelling
A constitutive model based on generalized elastoplasticity (Zienkiewicz et al., 1985) is used for the modelling of monotonic and cyclic pressuremeter tests in a clay. The permeability of the material is taken into account for the modelling of the excess pore water pressure generation during the test (combination of pore pressure build up and dissipation). It is shown how this type of model can simply represent the main features observed during a cyclic pressuremeter test in a clay, particularly the accumulation of excess pore water pressure during the cycles of loading, and the importance of dissipation on the excess pore water pressure build up
Résistance d'un milieu granulaire renforcé par injection de ciment
International audienceThe present Note describes an experimental study devoted to the strength of a sand reinforced by cement grouting. Through grouting, the granular medium gains a cohesion without significant change of the friction angle. The most significant experimental feature is that the cohesion is proportional to the volume fraction of cement in the grouted material. This result is interpreted within the framework of a periodic homogenization applied to yield design.On présente une étude expérimentale consacrée aux capacités de résistance d'un sable injecté par un coulis de ciment. L'effet principal de l'injection est un gain de cohésion sans modification significative de l'angle de frottement. La principale observation expérimentale réside dans le fait que la cohésion est proportionnelle à la fraction volumique de ciment réalisée dans le matériau injecté. On interprète ce résultat dans le cadre de l'homogénéisation périodique en Calcul à la Rupture
Flux emergence and coronal eruption
Our aim is to study the photospheric flux distribution of a twisted flux tube
that emerges from the solar interior. We also report on the eruption of a new
flux rope when the emerging tube rises into a pre-existing magnetic field in
the corona. To study the evolution, we use 3D numerical simulations by solving
the time-dependent and resistive MHD equations. We qualitatively compare our
numerical results with MDI magnetograms of emerging flux at the solar surface.
We find that the photospheric magnetic flux distribution consists of two
regions of opposite polarities and elongated magnetic tails on the two sides of
the polarity inversion line (PIL), depending on the azimuthal nature of the
emerging field lines and the initial field strength of the rising tube. Their
shape is progressively deformed due to plasma motions towards the PIL. Our
results are in qualitative agreement with observational studies of magnetic
flux emergence in active regions (ARs). Moreover, if the initial twist of the
emerging tube is small, the photospheric magnetic field develops an undulating
shape and does not possess tails. In all cases, we find that a new flux rope is
formed above the original axis of the emerging tube that may erupt into the
corona, depending on the strength of the ambient field.Comment: 5 pages, 3 figures, accepted for publication in A&
Nonlinear force-free and potential field models of active-region and global coronal fields during the Whole Heliospheric Interval
Between 2008/3/24 and 2008/4/2, the three active regions NOAA active regions
10987, 10988 and 10989 were observed daily by the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM) while they
traversed the solar disk. We use these measurements and the nonlinear
force-free magnetic field code XTRAPOL to reconstruct the coronal magnetic
field for each active region and compare model field lines with images from the
Solar Terrestrial RElations Observatory (STEREO) and Hinode X-ray Telescope
(XRT) telescopes. Synoptic maps made from continuous, round-the-clock Global
Oscillations Network Group (GONG) magnetograms provide information on the
global photospheric field and potential-field source-surface models based on
these maps describe the global coronal field during the Whole Heliospheric
Interval (WHI) and its neighboring rotations. Features of the modeled global
field, such as the coronal holes and streamer belt locations, are discussed in
comparison with extreme ultra-violet and coronagraph observations from STEREO.
The global field is found to be far from a minimum, dipolar state. From the
nonlinear models we compute physical quantities for the active regions such as
the photospheric magnetic and electric current fluxes, the free magnetic energy
and the relative helicity for each region each day where observations permit.
The interconnectivity of the three regions is addressed in the context of the
potential-field source-surface model. Using local and global quantities derived
from the models, we briefly discuss the different observed activity levels of
the regions.Comment: Accepted for publication in the Solar Physics Whole Heliospheric
Interval (WHI) topical issue. We had difficulty squeezing this paper into
arXiv's 15 Mb limit. The full paper is available here
ftp://gong2.nso.edu/dsds_user/petrie/PetrieCanouAmari.pd
Behavior of Soil-structure interfaces under cyclic loading for large numbers of cycles: Application to piles
Pile foundations are usually subjected to cyclic loading which can be either environmental or industrial. Loading and unloading sequences of the pile cause very significant variation in the behavior of the pile-soil system and generate degradation of the bearing capacity and an accumulation of irreversible displacement. The paper presents a study of the behavior of piles subjected to axial cyclic loading for large numbers of cycles using a physical modeling approach in a calibration chamber. To focus on the degradation of the friction in soil-pile interface, the experiments are carried out in two-way displacement–controlled tests. Firstly, The experimental set-up was presented, then the typical results of the evolution of skin friction under cyclic loading. The results indicate that the application of a large number of cycles to the pile, the skin friction decrease initially then increase continually up to the end of the cyclic sequence. It can be concluded that the phase of friction reinforcement is due to a partially constrained dilatancy phenomenon of the sand within the interface zone.Résumé :Généralement les pieux sont soumis à des sollicitations cycliques qui peuvent être de type environnemental ou industriel. Le chargement et le déchargement du pieu provoquent une évolution très significative du comportement du système pieu- sol et engendrent une dégradation de la capacité portante ainsi que l’accumulation des déplacements irréversibles. L’article présente une étude du comportement des pieux chargés axialement, par une modélisation physique en chambre d’étalonnage, sous l’effet d’un chargement cyclique à grand nombre de cycles. Les essais ont été réalisés à déplacement contrôlé de type alterné afin de maître en évidence le phénomène de dégradation du frottement à l’interface sol-pieu. On présente tout d’abord le dispositif expérimental utilisé, puis les résultats typiques sous l’effet du chargement cyclique en s’intéressant, plus particulièrement, à l’évolution du frottement au niveau de l’interface sol-pieu. L’analyse des résultats trouvés nous a permis de conclure qu’en appliquant un chargement cyclique à grand nombre de cycles sur le pieu, le frottement latéral subira au début une dégradation puis une augmentation qui continuera jusqu’à la fin de la séquence cyclique. Ce renforcement observé du frottement latéral est dû à la forte dilatance partiellement empêchée de la zone d’interface sol-pieu.Mots clés : Chambre d’étalonnage, pieu, sol, interface, frottement latéral, chargement cyclique, dégradation
Fine particles production in sand under triaxial compression
An experimental study, carried out in laboratory using a triaxial apparatus, proposes to evaluate the influence of grain size, grain shape and confining stress in fine particles production in sand specimen subjected to triaxial loading. 16 triaxial tests were conducted on specimens of an initial density index of 0.90 using the pluviation technique, with three different sands and under initial confining pressure of 100, 400, 800 and 1200 kPa. The experimental results show that the fine particles production is considerably affected by confining stresses, even at low values (100 kPa), and also by grain geometry. The experimental results show also that the fine particles production increase with the strain level during the shearing test
Re-flaring of a Post-Flare Loop System Driven by Flux Rope Emergence and Twisting
In this letter, we study in detail the evolution of the post-flare loops on
2005 January 15 that occurred between two consecutive solar eruption events,
both of which generated a fast halo CME and a major flare. The post-flare loop
system, formed after the first CME/flare eruption, evolved rapidly, as
manifested by the unusual accelerating rise motion of the loops. Through
nonlinear force-free field (NLFFF) models, we obtain the magnetic structure
over the active region. It clearly shows that the flux rope below the loops
also kept rising accompanied with increasing twist and length. Finally, the
post-flare magnetic configuration evolved to a state that resulted in the
second CME/flare eruption. This is an event in which the post-flare loops can
re-flare in a short period of 16 hr following the first CME/flare
eruption. The observed re-flaring at the same location is likely driven by the
rapid evolution of the flux rope caused by the magnetic flux emergence and the
rotation of the sunspot. This observation provides valuable information on
CME/flare models and their prediction.Comment: 11 pages, 4 figures, 1 table, accepted for publication in ApJ Lette
Influence of the cyclic loading of very large number of cycles on the pile capacity
The paper presents a study of the bearing capacity of a pile model in calibration chamber, under a cyclic axial loading, up to very large number of cycles. The experimental study is aimed to the evolution of the bearing capacity of pile obtained after a cyclic loading up to 105 cycles. Firstly, we present briefly the experimental device and then the results of cyclic tests performed on the pile model in the calibration chamber. Several parameters which have a significant influence on the cyclic behavior of piles have been investigated, such as the cyclic displacement amplitude and the density of the soil. The analysis of results indicated that the mechanism of densification of the soil around the pile leads to improve pile capacity
A "low-cost" subtractive method for freshly finished 3D concrete printed structures.
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