125 research outputs found
Morphologies of three-dimensional shear bands in granular media
We present numerical results on spontaneous symmetry breaking strain
localization in axisymmetric triaxial shear tests of granular materials. We
simulated shear band formation using three-dimensional Distinct Element Method
with spherical particles. We demonstrate that the local shear intensity, the
angular velocity of the grains, the coordination number, and the local void
ratio are correlated and any of them can be used to identify shear bands,
however the latter two are less sensitive. The calculated shear band
morphologies are in good agreement with those found experimentally. We show
that boundary conditions play an important role. We discuss the formation
mechanism of shear bands in the light of our observations and compare the
results with experiments. At large strains, with enforced symmetry, we found
strain hardening.Comment: 6 pages 5 figures, low resolution figures
Critical packing in granular shear bands
In a realistic three-dimensional setup, we simulate the slow deformation of
idealized granular media composed of spheres undergoing an axisymmetric
triaxial shear test. We follow the self-organization of the spontaneous strain
localization process leading to a shear band and demonstrate the existence of a
critical packing density inside this failure zone. The asymptotic criticality
arising from the dynamic equilibrium of dilation and compaction is found to be
restricted to the shear band, while the density outside of it keeps the memory
of the initial packing. The critical density of the shear band depends on
friction (and grain geometry) and in the limit of infinite friction it defines
a specific packing state, namely the \emph{dynamic random loose packing}
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Condensed tannins in the gastrointestinal tract of cattle after sainfoin (Onobrychis viciifolia) intake and their possible relationship with anthelmintic effects
The fate of condensed tannins (CTs) along the digestive tract of ruminants is not well known and may account for the variable efficacy of CTs against gastrointestinal nematodes in different locations. Here, we analyzed sainfoin CTs in the digesta of cattle from two separate experiments. When using the acetone-butanol-HCl assay, the total CTs concentrations in the digestive tract were close to those in the diets (6.3 and 1.5% of DM in Expt. 1 and 2, resp.) indicating that CTs remained largely undegraded and unabsorbed. Yet with the thiolysis assay in Expt. 1, CTs concentration was much higher in the abomasum (2.3 ± 0.4 % of DM) compared with the rumen, small and large intestines, along with increases of mean size and percentage of prodelphinidins within CTs. This corroborates the anthelmintic efficacy reported only against Ostertagia ostertagi in the abomasum. In Expt. 2, no anthelmintic effect was observed against the larval exsheathment in the rumen, probably because the dietary level of CTs was too low. Overall, the level of CTs accessible to thiolysis in the gut appears to be critical for anthelmintic activity, which is favored under the acidic conditions of the abomasum
Feeding chicory (Cichorium intybus) selectively reduces Ostertagia ostertagi infection levels in cattle
Objectives: Studies were conducted to test the potential use of chicory against gastrointestinal nematode infections in cattle. Methods: In study 1, fifteen 2-4 months-old dairy calves were allocated into a chicory (CHI, n=9) or control (CTL, n=6) group. CHI and CTL were stabled and fed with chicory silage or hay, resp., ad lib for 56 days. Protein/energy intakes were equalized between groups throughout the study. After 14 days on the diet all calves were infected with 10,000 Ostertagia ostertagi and 66,000 Cooperia oncophora third-stage (L3) larvae. In study 2, twenty 4-6 months-old dairy calves grazed a second-year, pure chicory sward (CHI, n=10) or a ryegrass/white clover pasture (CTL, n=10) for 43 days. After 7 days on the diet all calves were infected with 20,000 O. ostertagi L3. In both studies, individual live weights were recorded and faecal egg counts were calculated as number of eggs per g of dried feces (FECDM). At day 56 (study 1) calves were killed for worm recovery. Live weights and log-transformed FECDM were analysed by ANOVA using repeated measurements. Log-transformed worm counts were analysed by t-test. Results: In study 1 daily live weight gains were 500 and 329 g/day in CHI and CTL animals, resp. (p=0.02). Mean FECDM were not significantly different between groups (p=0.19). O. ostertagi geo mean worm counts were 1599 (± 296) and 3752 (± 258) in CHI and CTL groups, resp. (p0.05). From this point, egg excretion in CHI calves was significantly reduced and by day 36 post-infection FECDM was decreased by 48-65% compared to CTL (P<0.05). Discussion: Feeding on a chicory diet demonstrated a marked anthelmintic effect against O. ostertagi in both trials, whereas C. oncophora in study 1 was unaffected. Apparently, chicory does not interfere with worm establishment of O. ostertagi but significantly reduces egg excretion and adult worm counts. The lower weight gains in study 2 probably reflect lower energy consumption in this group and suggest that duration of grazing of pure chicory should be limited to selectively target established O. ostertagi adult populations
Contact dynamics in a gently vibrated granular pile
We use multi-speckle diffusive wave spectroscopy (MSDWS) to probe the
micron-scale dynamics of a granular pile submitted to discrete gentle taps. The
typical time-scale between plastic events is found to increase dramatically
with the number of applied taps. Furthermore, this microscopic dynamics weakly
depends on the solid fraction of the sample. This process is strongly analogous
to the aging phenomenon observed in thermal glassy systems. We propose a
heuristic model where this slowing down mechanism is associated with a slow
evolution of the distribution of the contact forces between particles. This
model accounts for the main features of the observed dynamics.Comment: 4 pages, 4 figure
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Impact of chemical structure of flavanol monomers and condensed tannins on in vitro anthelmintic activity against bovine nematodes
Plants containing condensed tannins (CT) may have potential to control gastrointestinal nematodes (GIN) of cattle. The aim was to investigate the anthelmintic activities of four flavan-3-ols, two galloyl derivatives and 14 purified CT fractions, and to define which structural features of CT determine the anti-parasitic effects against the main cattle nematodes. We used in vitro tests targeting L1 larvae (feeding inhibition assay) and adults (motility assay) of Ostertagia ostertagi and Cooperia oncophora. In the larval feeding inhibition assay, O. ostertagi L1 were significantly more susceptible to all CT fractions than C. oncophora L1. The mean degree of polymerization of CT (i.e. average size) was the most important structural parameter: large CT reduced larval feeding more than small CT. The flavan-3-ols of prodelphinidin (PD)-type tannins had a stronger negative influence on parasite activity than the stereochemistry, i.e. cis- vs trans-configurations, or the presence of a gallate group. In contrast, for C. oncophora high reductions in the motility of larvae and adult worms were strongly related with a higher percentage of PDs within the CT fractions while there was no effect of size. Overall, the size and the percentage of PDs within CT seemed to be the most important parameters that influence anti-parasitic activity
Anomalous acoustic reflection on a sliding interface or a shear band
We study the reflection of an acoustic plane wave from a steadily sliding
planar interface with velocity strengthening friction or a shear band in a
confined granular medium. The corresponding acoustic impedance is utterly
different from that of the static interface. In particular, the system being
open, the energy of an in-plane polarized wave is no longer conserved, the work
of the external pulling force being partitioned between frictional dissipation
and gain (of either sign) of coherent acoustic energy. Large values of the
friction coefficient favor energy gain, while velocity strengthening tends to
suppress it. An interface with infinite elastic contrast (one rigid medium) and
V-independent (Coulomb) friction exhibits spontaneous acoustic emission, as
already shown by M. Nosonovsky and G.G. Adams (Int. J. Ing. Sci., {\bf 39},
1257 (2001)). But this pathology is cured by any finite elastic contrast, or by
a moderately large V-strengthening of friction.
We show that (i) positive gain should be observable for rough-on-flat
multicontact interfaces (ii) a sliding shear band in a granular medium should
give rise to sizeable reflection, which opens a promising possibility for the
detection of shear localization.Comment: 13 pages, 10 figure
Plastic Flow in Two-Dimensional Solids
A time-dependent Ginzburg-Landau model of plastic deformation in
two-dimensional solids is presented. The fundamental dynamic variables are the
displacement field \bi u and the lattice velocity {\bi v}=\p {\bi u}/\p t.
Damping is assumed to arise from the shear viscosity in the momentum equation.
The elastic energy density is a periodic function of the shear and tetragonal
strains, which enables formation of slips at large strains. In this work we
neglect defects such as vacancies, interstitials, or grain boundaries. The
simplest slip consists of two edge dislocations with opposite Burgers vectors.
The formation energy of a slip is minimized if its orientation is parallel or
perpendicular to the flow in simple shear deformation and if it makes angles of
with respect to the stretched direction in uniaxial stretching.
High-density dislocations produced in plastic flow do not disappear even if
the flow is stopped. Thus large applied strains give rise to metastable,
structurally disordered states. We divide the elastic energy into an elastic
part due to affine deformation and a defect part. The latter represents degree
of disorder and is nearly constant in plastic flow under cyclic straining.Comment: 16pages, Figures can be obtained at
http://stat.scphys.kyoto-u.ac.jp/index-e.htm
Sheared Solid Materials
We present a time-dependent Ginzburg-Landau model of nonlinear elasticity in
solid materials. We assume that the elastic energy density is a periodic
function of the shear and tetragonal strains owing to the underlying lattice
structure. With this new ingredient, solving the equations yields formation of
dislocation dipoles or slips. In plastic flow high-density dislocations emerge
at large strains to accumulate and grow into shear bands where the strains are
localized. In addition to the elastic displacement, we also introduce the local
free volume {\it m}. For very small the defect structures are metastable
and long-lived where the dislocations are pinned by the Peierls potential
barrier. However, if the shear modulus decreases with increasing {\it m},
accumulation of {\it m} around dislocation cores eventually breaks the Peierls
potential leading to slow relaxations in the stress and the free energy
(aging). As another application of our scheme, we also study dislocation
formation in two-phase alloys (coherency loss) under shear strains, where
dislocations glide preferentially in the softer regions and are trapped at the
interfaces.Comment: 16pages, 11figure
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