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| Titel |
Brittle onset of monodispersed magmatic suspensions: from spheres to spheroid |
| VerfasserIn |
B. Cordonnier, B. Kaus, M. Manga, L. Caricchi, M. Pistone, J. Castro, K.-U. Hess, S. Gottschaller, D. B. Dingwell, L. Burlini |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069839
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| Zusammenfassung |
| This abstract describes one of the last projects engaged by Dr. Luigi Burlini. It highlights his
wish to make a close link between experimental and numerical studies, and push
even further our understanding of rock mechanics. His students, engaged in this
study, wish to credit these results to the legacy left by him owing to his constant
involvement in Science and in educating the next generation of rheologists. While
he could not see this project to fruition, his constant support and help during the
conception of the project made it possible. The brittle-ductile transition remains a central
question of modern geology as rock failure is the main parameter in mitigating
geological risks, such as, for volcanic eruptions, the transitions from effusive to
explosive eruptive style. Although numerical simulations are the only way to fully
understanding the physical processes involved, we are in a strong need of an experimental
validation of the proposed models. We first recall some experimental results obtained
under torsion and uni-axial compression on both pure melts and crystal-bearing
magmas. Torsion experiments were performed at high temperature (600 to 900
degC) and high pressure (200 to 300 MPa) using a Paterson-type rock deformation
apparatus (ETH Zurich). We characterized the brittle onset of two phases magmas
from 0 to 65 vol% crystals. The strain-rates span 5 orders of magnitude, with a
change in the behavior of the material from viscous to brittle (10^-5- 10^0 s^-1).
The materials tested are a standard borosilicate glass (NIST717), a natural crystal
bearing rhyolitic melt (Mt Unzen volcano) and a suspension of haplogranitic synthetic
sample with corundum particles. To characterize the physical processes leading to
failure in the experiments, we performed 2D and 3D numerical simulations on
monodispersed rigid spheroids with eccentricities ranging from 10^-2 to 10^2. The model is
numerically solved with Finite Elements Methods. The pre-processing, processing and
post-processing are all performed under MATLAB. For the largest meshes, the
computation has been performed with the help of the BRUTUS cluster at ETH Zurich.
For solving the system of equation we used the MILAMIN solver and extended it
from 2-D to 3-D by the use of Crouzeix-Raviart type elements. MILAMIN is a
native MATLAB implementation, which takes advantage of Tim Davi’s SuiteSparse
package. Here we solve the incompressible Stokes equations. We tested random to
structured configurations (Simple Cubic, Body Centered Cubic and Face Centered
Cubic) for different particle orientations from random to aligned. These numerical
simulations allow us to estimate the stress concentration in magmas due to the
presence of the crystals. Our results first confirm the hydrodynamics effects on the flow
of elongated particles. The calculated apparent viscosity of the material versus
the crystal fraction confirms an early increase for the suspensions viscosity with
elongated particles. More importantly, the stress localization due to the particles
suggests that the melt will start cracking for a relatively lower bulk stress. Finally, the
experimental trend is supported by the numerical simulations, which highlight the
importance of the critical packing fraction in addition to the maximum packing
fraction. The combination of experimental results and numerical modeling allow us to
characterize the physical processes responsible for the failure of particle bearing
suspensions and characterize the effect of fraction and shape on the brittle-ductile
transition. |
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