16,556 research outputs found
The nonlinear viscoelastic behavior of polypropylene
A series of tensile relaxation tests is performed on isotactic polypropylene
in the sub-yield and post-yield regions at room temperature. Constitutive
equations are derived for the time-dependent response of a semicrystalline
polymer at isothermal loading with small strains. Adjustable parameters in the
stress-strain relations are found by fitting experimental data. It is
demonstrated that the growth of the longitudinal strain results in an increase
in the relaxation rate in a small interval of strains in the sub-yield domain.
When the strain exceeds some critical value which is substantially less than
the apparent yield strain, the relaxation process becomes strain-independent.Comment: 20 pages, 6 figure
Modelling the linear viscoelastic behavior of silicate glasses near the glass transition point
A model is derived for the viscoelastic response of glasses at isothermal
uniaxial deformation with small strains. A glass is treated as an ensemble of
relaxing units with various activation energies for rearrangement. With
reference to the energy-landscape concept, the rearrangement process is thought
of as a series of hops of relaxing units (trapped in their potential wells on
the energy landscape) to higher energy levels. Stress-strain relations are
developed by using the laws of thermodynamics. Adjustable parameters are found
by fitting experimental data in torsional dynamic tests on a multicomponent
silicate glass at several temperatures near the glass transition point.Comment: 17 pages, 17 figure
The effect of strain rate on the viscoplastic behavior of isotactic polypropylene at finite strains
Two series of uniaxial tensile tests are performed on isotactic polypropylene
with the strain rates ranging from 5 to 200 mm/min. In the first series,
injection-molded specimens are used without thermal pre-treatment, whereas in
the other series, the samples are annealed for 51 h at 160C prior to testing.
A constitutive model is developed for the viscoplastic behavior of isotactic
polypropylene at finite strains. A semicrystalline polymer is treated as an
equivalent heterogeneous network of chains bridged by permanent junctions
(physical cross-links and entanglements). The network is thought of as an
ensemble of meso-regions connected with each other by links (lamellar blocks).
In the sub-yield region of deformations, junctions between chains in
meso-domains slide with respect to their reference positions (which reflects
sliding of nodes in the amorphous phase and fine slip of lamellar blocks).
Above the yield point, sliding of nodes is accompanied by displacements of
meso-domains in the ensemble with respect to each other (which reflects coarse
slip and fragmentation of lamellar blocks).
Stress-strain relations for a semicrystalline polymer are derived by using
the laws of thermodynamics. The constitutive equations are determined by 5
adjustable parameters that are found by matching observations. Fair agreement
is demonstrated between the experimental data and the results of numerical
simulation.Comment: 27 pages, 20 figure
Discovery of TUG-770: a highly potent free fatty acid receptor 1 (FFA1/GPR40) agonist for treatment of type 2 diabetes
Free fatty acid receptor 1 (FFA1 or GPR40) enhances glucose-stimulated insulin secretion from pancreatic β-cells and currently attracts high interest as a new target for the treatment of type 2 diabetes. We here report the discovery of a highly potent FFA1 agonist with favorable physicochemical and pharmacokinetic properties. The compound efficiently normalizes glucose tolerance in diet-induced obese mice, an effect that is fully sustained after 29 days of chronic dosing
Lattice stretching bistability and dynamic heterogeneity
A simple one-dimensional lattice model is suggested to describe the
experimentally observed plateau in force-stretching diagrams for some
macromolecules. This chain model involves the nearest-neighbor interaction of a
Morse-like potential (required to have a saturation branch) and an harmonic
second-neighbor coupling. Under an external stretching applied t o the chain
ends, the intersite Morse-like potential results in the appearance of a
double-well potential within each chain monomer, whereas the interaction
between the second neighbors provide s a homogeneous bistable (degenerate)
ground state, at least within a certain part of the chain.
As a result, different conformational changes occur in the chain under the
external forcing. The transition regions between these conformations are
described as topological solitons. With a strong second-neighbor interaction,
the solitons describe the transition between the bistable ground states.
However, the key point of the model is the appearance of a heterogenous
structure, when the second-neighbor coupling is sufficiently weak. In this
case, a part of the chain has short bonds with a single-well potential, whereas
the complementary part admits strongly stretched bonds with a double-well
potential. This case allows us to explain the existence of a plateau in the
force-stretching diagram for DNA and alpha-helix protein. Finally, the soliton
dynamics are studied in detail.Comment: Submitted to Phys. Rev. E, 13 figure
The origin of channels and associated deposits in the Elysium region of Mars
Photogeological studies of the Elysium volcanic province of Mars show that its sinuous channels are part of a large deposit which probably was emplaced as a series of huge volcanic debris flows or lahars. The suggestion is based on evidence that the lahars were : (1) gravity-driven mass flow deposits (lobate outlines, steep snouts, smooth medial channels and rough lateral deposits--the deposits narrow and widen in accord with topography, and they extend downslope); (2) wet (channeled surfaces, drainage features); and (3) associated with volcanism (the deposits and channels extend from a system of fractures which fed lava flows). It is conceivable that heat associated with magmatism melted ground ice below the Elysium volcanoes, formed a muddy slurry which issued out of regional fractures and spread over the adjoining plain
Volatile reservoirs below the surface of the Elysium region of Mars: Geomorphic evidence
The Elysium volcanic province contains a variety of geomorphic evidence for the existence of large volatile reservoirs of subsurface volatiles. Study of these landforms yields insight into the distribution and size of these reservoirs and how they interact with the surface environment and will ultimately place constraints on the geometry, constitution, origin, time of formation, and temporal evolution of these important components of the Martian crust. Three principal types of landforms appear to be related to subsurface volatile reservoirs in the Elysium region of Mars: small outflow channels; large lahars; and vast expanses of knobby terranes around the margins of the Elysium dome. The evidence provided by these landforms is internally consistent with the presence of a large relatively shallow volatile reservoir in the Elysium region. If the geologic features described are reliable indicators of subsurface volatiles, they imply that: volatile reservoirs lie relatively close to the surface and underlie millions of sq km in this region; there is no apparent latitudinal variation in the depth or thickness of the volatile reservoirs; the precursors of the knobby terranes are or were important volatile reservoirs; volatiles may be lost in a variety of ways from these reservoirs; and volatiles were incorporated in an easily eroded surficial deposit in the middle history of Mars. The ultimate origin of water in this reservoir is uncertain. A model to explain the preferential entrapment of volatiles into the region's surface materials may be required
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