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Simulation of subseismic joint and fault networks using a heuristic mechanical model
Flow simulations of fractured and faulted reservoirs require representation of subseismic structures about which subsurface data are limited. We describe a method for simulating fracture growth that is mechanically based but heuristic, allowing for realistic modelling of fracture networks with reasonable run times. The method takes a triangulated meshed surface as input, together with an initial stress field. Fractures initiate and grow based on the stress field, and the growing fractures relieve the stress in the mesh. We show that a wide range of bedding-plane joint networks can be modelled simply by varying the distribution and anisotropy of the initial stress field. The results are in good qualitative agreement with natural joint patterns. We then apply the method to a set of parallel veins and demonstrate how the variations in thickness of the veins can be represented. Lastly, we apply the method to the simulation of normal fault patterns on salt domes. We derive the stress field on the bedding surface using the horizon curvature. The modelled fault network shows both radial and concentric faults. The new method provides an effective means of modelling joint and fault networks that can be imported to the flow simulator
Annular cracks in thin films of nanoparticle suspensions drying on a fiber
We report an experimental study of the crack pattern formed during the drying
of a colloidal suspension. A horizontal fiber, which provides a one
dimensional, boundary-free substrate, is coated by a film of micronic
thickness. The geometry imposes a remarkable annular crack pattern and allowing
precise measurements of the crack spacing over a short range of film thickness
(between 2 and 10 m) which varies linearly with the film height. We
compare our experimental data with a model proposed by Kitsunezaki which
suggests that the variation of the crack spacing with the film thickness
depends on the ratio between a critical stress at cracking and a critical
stress for slipping on the substrate. By measuring the friction force of the
colloidal gels on a hydrophobic surface through a cantilever technique, we can
deduce the critical crack stress for these colloidal gels simply by measuring
the crack spacing of the pattern.Comment: Accepted in EP
On the puzzling feature of the silence of precursory electromagnetic emissions
It has been suggested that fracture-induced MHz-kHz electromagnetic (EM)
emissions, which emerge from a few days up to a few hours before the main
seismic shock occurrence permit a real-time monitoring of the damage process
during the last stages of earthquake preparation, as it happens at the
laboratory scale. Despite fairly abundant evidence, EM precursors have not been
adequately accepted as credible physical phenomena. These negative views are
enhanced by the fact that certain 'puzzling features' are repetitively observed
in candidate fracture-induced pre-seismic EM emissions. More precisely, EM
silence in all frequency bands appears before the main seismic shock
occurrence, as well as during the aftershock period. Actually, the view that
'acceptance of 'precursive' EM signals without convincing co-seismic signals
should not be expected' seems to be reasonable. In this work we focus on this
point. We examine whether the aforementioned features of EM silence are really
puzzling ones or, instead, reflect well-documented characteristic features of
the fracture process, in terms of: universal structural patterns of the
fracture process, recent laboratory experiments, numerical and theoretical
studies of fracture dynamics, critical phenomena, percolation theory, and
micromechanics of granular materials. Our analysis shows that these features
should not be considered puzzling.Comment: arXiv admin note: text overlap with arXiv:cond-mat/0603542 by other
author
A quadtree-polygon-based scaled boundary finite element method for image-based mesoscale fracture modelling in concrete
A quadtree-polygon scaled boundary finite element-based approach for image-based modelling of concrete fracture at the mesoscale is developed. Digital images representing the two-phase mesostructure of concrete, which comprises of coarse aggregates and mortar are either generated using a take-and-place algorithm with a user-defined aggregate volume ratio or obtained from X-ray computed tomography as an input. The digital images are automatically discretised for analysis by applying a balanced quadtree decomposition in combination with a smoothing operation. The scaled boundary finite element method is applied to model the constituents in the concrete mesostructure. A quadtree formulation within the framework of the scaled boundary finite element method is advantageous in that the displacement compatibility between the cells are automatically preserved even in the presence of hanging nodes. Moreover, the geometric flexibility of the scaled boundary finite element method facilitates the use of arbitrary sided polygons, allowing better representation of the aggregate boundaries. The computational burden is significantly reduced as there are only finite number of cell types in a balanced quadtree mesh. The cells in the mesh are connected to each other using cohesive interface elements with appropriate softening laws to model the fracture of the mesostructure. Parametric studies are carried out on concrete specimens subjected to uniaxial tension to investigate the effects of various parameters e.g. aggregate size distribution, porosity and aggregate volume ratio on the fracture of concrete at the meso-scale. Mesoscale fracture of concrete specimens obtained from X-ray computed tomography scans are carried out to demonstrate its feasibility
Thermomechanical surface instability at the origin of surface fissure patterns on heated circular MDF samples
When a flat sample of medium density fibreboard (MDF) is exposed to radiant
heat in an inert atmosphere, primary crack patterns suddenly start to appear
over the entire surface before pyrolysis and any charring occurs. Contrary to
common belief that crack formation is due to drying and shrinkage, it was
demonstrated for square samples that this results from thermomechanical
instability.
In the present paper, new experimental data are presented for circular
samples of the same MDF material. The sample was exposed to radiant heating at
20 or 50 kW/m2, and completely different crack patterns with independent
Eigenmodes were observed at the two heat fluxes. We show that the two patterns
can be reproduced with a full 3-D thermomechanical surface instability model of
a hot layer adhered to an elastic colder foundation in an axisymmetric domain.
Analytical and numerical solutions of a simplified 2-D formulation of the same
problem provide excellent qualitative agreement between observed and calculated
patterns.
Previous data for square samples together with the results reported in the
present paper for circular samples confirm the validity of the model for
qualitative predictions, and indicate that further refinements can be made to
improve its quantitative predictive capability.Comment: 9 pages, 13 figures. New title and abstract, added experimental and
simulation details and figures, conclusions unchanged. Matches the version
published in Fire And Material
Thermomechanical surface instability at the origin of surface fissure patterns on heated circular MDF samples
When a flat sample of medium density fibreboard (MDF) is exposed to radiant
heat in an inert atmosphere, primary crack patterns suddenly start to appear
over the entire surface before pyrolysis and any charring occurs. Contrary to
common belief that crack formation is due to drying and shrinkage, it was
demonstrated for square samples that this results from thermomechanical
instability.
In the present paper, new experimental data are presented for circular
samples of the same MDF material. The sample was exposed to radiant heating at
20 or 50 kW/m2, and completely different crack patterns with independent
Eigenmodes were observed at the two heat fluxes. We show that the two patterns
can be reproduced with a full 3-D thermomechanical surface instability model of
a hot layer adhered to an elastic colder foundation in an axisymmetric domain.
Analytical and numerical solutions of a simplified 2-D formulation of the same
problem provide excellent qualitative agreement between observed and calculated
patterns.
Previous data for square samples together with the results reported in the
present paper for circular samples confirm the validity of the model for
qualitative predictions, and indicate that further refinements can be made to
improve its quantitative predictive capability.Comment: 9 pages, 13 figures. New title and abstract, added experimental and
simulation details and figures, conclusions unchanged. Matches the version
published in Fire And Material
Evolving fracture patterns: columnar joints, mud cracks, and polygonal terrain
When cracks form in a thin contracting layer, they sequentially break the
layer into smaller and smaller pieces. A rectilinear crack pattern encodes
information about the order of crack formation, as later cracks tend to
intersect with earlier cracks at right angles. In a hexagonal pattern, in
contrast, the angles between all cracks at a vertex are near 120.
However, hexagonal crack patterns are typically only seen when a crack network
opens and heals repeatedly, in a thin layer, or advances by many intermittent
steps into a thick layer. Here it is shown how both types of pattern can arise
from identical forces, and how a rectilinear crack pattern evolves towards a
hexagonal one. Such an evolution is expected when cracks undergo many opening
cycles, where the cracks in any cycle are guided by the positions of cracks in
the previous cycle, but when they can slightly vary their position, and order
of opening. The general features of this evolution are outlined, and compared
to a review of the specific patterns of contraction cracks in dried mud,
polygonal terrain, columnar joints, and eroding gypsum-sand cementsComment: 19 pages, 9 figures, accepted for publication in Phil. Trans. R. Soc.
A; theme issue on Geophysical Pattern Formation (to appear 2013
Pattern formation and selection in quasi-static fracture
Fracture in quasi-statically driven systems is studied by means of a discrete
spring-block model. Developed from close comparison with desiccation
experiments, it describes crack formation induced by friction on a substrate.
The model produces cellular, hierarchical patterns of cracks, characterized by
a mean fragment size linear in the layer thickness, in agreement with
experiments. The selection of a stationary fragment size is explained by
exploiting the correlations prior to cracking. A scaling behavior associated
with the thickness and substrate coupling, derived and confirmed by
simulations, suggests why patterns have similar morphology despite their
disparity in scales.Comment: 4 pages, RevTeX, two-column, 5 PS figures include
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