13,140 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
Quasiperiodic Solutions of the Fibre Optics Coupled Nonlinear Schr{\"o}dinger Equations
We consider travelling periodical and quasiperiodical waves in single mode
fibres, with weak birefringence and under the action of cross-phase modulation.
The problem is reduced to the ``1:2:1" integrable case of the two-particle
quartic potential. A general approach for finding elliptic solutions is given.
New solutions which are associated with two-gap Treibich-Verdier potentials are
found. General quasiperiodic solutions are given in terms of two dimensional
theta functions with explicit expressions for frequencies in terms of theta
constants. The reduction of quasiperiodic solutions to elliptic functions is
discussed.Comment: 24 page
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