40,795 research outputs found
Determining Cosserat constants of 2D cellular solids from beam models
We present results of a two-scale model of disordered cellular materials
where we describe the microstructure in an idealized manner using a beam
network model and then make a transition to a Cosserat-type continuum model
describing the same material on the macroscopic scale. In such scale
transitions, normally either bottom-up homogenization approaches or top-down
reverse modelling strategies are used in order to match the macro-scale
Cosserat continuum to the micro-scale beam network. Here we use a different
approach that is based on an energetically consistent continuization scheme
that uses data from the beam network model in order to determine continuous
stress and strain variables in a set of control volumes defined on the scale of
the individual microstructure elements (cells) in such a manner that they form
a continuous tessellation of the material domain. Stresses and strains are
determined independently in all control volumes, and constitutive parameters
are obtained from the ensemble of control volume data using a least-square
error criterion. We show that this approach yields material parameters that are
for regular honeycomb structures in close agreement with analytical results.
For strongly disordered cellular structures, the thus parametrized Cosserat
continuum produces results that reproduce the behavior of the micro-scale beam
models both in view of the observed strain patterns and in view of the
macroscopic response, including its size dependence
Continuous direct compression as manufacturing platform for sustained release tablets
This study presents a framework for process and product development on a continuous direct compression manufacturing platform. A challenging sustained release formulation with high content of a poorly flowing low density drug was selected. Two HPMC grades were evaluated as matrix former: standard Methocel CR and directly compressible Methocel DC2. The feeding behavior of each formulation component was investigated by deriving feed factor profiles. The maximum feed factor was used to estimate the drive command and depended strongly upon the density of the material. Furthermore, the shape of the feed factor profile allowed definition of a customized refill regime for each material. Inline NIRs was used to estimate the residence time distribution (RTD) in the mixer and monitor blend uniformity. Tablet content and weight variability were determined as additional measures of mixing performance. For Methocel CR, the best axial mixing (i.e. feeder fluctuation dampening) was achieved when an impeller with high number of radial mixing blades operated at low speed. However, the variability in tablet weight and content uniformity deteriorated under this condition. One can therefore conclude that balancing axial mixing with tablet quality is critical for Methocel CR. However, reformulating with the direct compressible Methocel DC2 as matrix former improved tablet quality vastly. Furthermore, both process and product were significantly more robust to changes in process and design variables. This observation underpins the importance of flowability during continuous blending and die-filling. At the compaction stage, blends with Methocel CR showed better tabletability driven by a higher compressibility as the smaller CR particles have a higher bonding area. However, tablets of similar strength were achieved using Methocel DC2 by targeting equal porosity. Compaction pressure impacted tablet properties and dissolution. Hence controlling thickness during continuous manufacturing of sustained release tablets was crucial to ensure reproducible dissolution. (C) 2017 Elsevier B.V. All rights reserved
Universal Compressed Text Indexing
The rise of repetitive datasets has lately generated a lot of interest in
compressed self-indexes based on dictionary compression, a rich and
heterogeneous family that exploits text repetitions in different ways. For each
such compression scheme, several different indexing solutions have been
proposed in the last two decades. To date, the fastest indexes for repetitive
texts are based on the run-length compressed Burrows-Wheeler transform and on
the Compact Directed Acyclic Word Graph. The most space-efficient indexes, on
the other hand, are based on the Lempel-Ziv parsing and on grammar compression.
Indexes for more universal schemes such as collage systems and macro schemes
have not yet been proposed. Very recently, Kempa and Prezza [STOC 2018] showed
that all dictionary compressors can be interpreted as approximation algorithms
for the smallest string attractor, that is, a set of text positions capturing
all distinct substrings. Starting from this observation, in this paper we
develop the first universal compressed self-index, that is, the first indexing
data structure based on string attractors, which can therefore be built on top
of any dictionary-compressed text representation. Let be the size of a
string attractor for a text of length . Our index takes
words of space and supports locating the
occurrences of any pattern of length in
time, for any constant . This is, in particular, the first index
for general macro schemes and collage systems. Our result shows that the
relation between indexing and compression is much deeper than what was
previously thought: the simple property standing at the core of all dictionary
compressors is sufficient to support fast indexed queries.Comment: Fixed with reviewer's comment
Modeling of asphalt durability and self-healing with discrete particles method
Asphalt is an important road paving material. Besides an acceptable price, durability, surface conditions (like
roughening and evenness), age-, weather- and traffic-induced failures and degradation are relevant aspects. In the
professional road-engineering branch empirical models are used to describe the mechanical behaviour of the material
and to address large-scale problems for road distress phenomena like rutting, ravelling, cracking and roughness. The
mesoscopic granular nature of asphalt and the mechanics of the bitumen layer between the particles are only partly
involved in this kind of approach. The discrete particle method is a modern tool that allows for arbitrary (self-
)organization of the asphalt meso-structure and for rearrangements due to compaction and cyclic loading. This is of
utmost importance for asphalt during the construction phase and the usage period, in forecasting the relevant distress
phenomena and understand their origin on the grain-, contact-, or molecular scales. Contact models that involve viscoelasticity,
plasticity, friction and roughness are state-of-the art in fields like particle technology and can now be
modified for asphalt and validated experimentally on small samples. The ultimate goal is then to derive micro- and
meso-based constitutive models that can be applied to model behaviour of asphalt pavements on the larger macroscale.
Using the new contact models, damage and crack formation in asphalt and their propagation can be modelled, as
well as compaction. Furthermore, the possibility to trigger self-healing in the material can be investigated from a
micro-mechanical point of view
Anomalous compliance and early yielding of nanoporous gold
We present a study of the elastic and plastic behavior of nanoporous gold in
compression, focusing on molecular dynamics simulation and inspecting
experimental data for verification. Both approaches agree on an anomalously
high elastic compliance in the early stages of deformation, along with a quasi
immediate onset of plastic yielding even at the smallest load. Already before
the first loading, the material undergoes spontaneous plastic deformation under
the action of the capillary forces, requiring no external load. Plastic
deformation under compressive load is accompanied by dislocation storage and
dislocation interaction, along with strong strain hardening.
Dislocation-starvation scenarios are not supported by our results. The
stiffness increases during deformation, but never approaches the prediction by
the relevant Gibson-Ashby scaling law. Microstructural disorder affects the
plastic deformation behavior and surface excess elasticity might modify elastic
response, yet we relate the anomalous compliance and the immediate yield onset
to an atomistic origin: the large surface-induced prestress induces elastic
shear that brings some regions in the material close to the shear instability
of the generalized stacking fault energy curve. These regions are elastically
highly compliant and plastically weak
Spread spectrum-based video watermarking algorithms for copyright protection
Merged with duplicate record 10026.1/2263 on 14.03.2017 by CS (TIS)Digital technologies know an unprecedented expansion in the last years. The consumer can
now benefit from hardware and software which was considered state-of-the-art several years
ago. The advantages offered by the digital technologies are major but the same digital
technology opens the door for unlimited piracy. Copying an analogue VCR tape was certainly
possible and relatively easy, in spite of various forms of protection, but due to the analogue
environment, the subsequent copies had an inherent loss in quality. This was a natural way of
limiting the multiple copying of a video material. With digital technology, this barrier
disappears, being possible to make as many copies as desired, without any loss in quality
whatsoever. Digital watermarking is one of the best available tools for fighting this threat.
The aim of the present work was to develop a digital watermarking system compliant with the
recommendations drawn by the EBU, for video broadcast monitoring. Since the watermark
can be inserted in either spatial domain or transform domain, this aspect was investigated and
led to the conclusion that wavelet transform is one of the best solutions available. Since
watermarking is not an easy task, especially considering the robustness under various attacks
several techniques were employed in order to increase the capacity/robustness of the system:
spread-spectrum and modulation techniques to cast the watermark, powerful error correction
to protect the mark, human visual models to insert a robust mark and to ensure its invisibility.
The combination of these methods led to a major improvement, but yet the system wasn't
robust to several important geometrical attacks. In order to achieve this last milestone, the
system uses two distinct watermarks: a spatial domain reference watermark and the main
watermark embedded in the wavelet domain. By using this reference watermark and techniques
specific to image registration, the system is able to determine the parameters of the attack and
revert it. Once the attack was reverted, the main watermark is recovered. The final result is a
high capacity, blind DWr-based video watermarking system, robust to a wide range of attacks.BBC Research & Developmen
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