238 research outputs found
A numerical method for rapid estimation of drawbead restraining force based on non-linear, anisotropic constitutive equations
AbstractNumerical procedures to predict drawbead restraining forces (DBRF) were developed based on the semi-analytical (non-finite-element) hybrid membrane/bending method. The section forces were derived by equating the work to pull sheet material through the drawbead to the work required to bend and unbend the sheet along with frictional forces on drawbead radii. As a semi-analytical method, the new approach was especially useful to analyze the effects of various constitutive parameters with less computational cost. The present model could accommodate general non-quadratic anisotropic yield function and non-linear anisotropic hardening under the plane strain condition. Several numerical sensitivity analyses for examining the effects of process parameters and material properties including the Bauschinger effect and the shape of yield surface on DBRF were presented. Finally, the DBRFs of SPCC steel sheet passing a single circular drawbead were predicted and compared with the measurements
Kinetic Inflation in Stringy and Other Cosmologies
An inflationary epoch driven by the kinetic energy density in a dynamical
Planck mass is studied. In the conformally related Einstein frame it is easiest
to see the demands of successful inflation cannot be satisfied by kinetic
inflation alone. Viewed in the original Jordan-Brans-Dicke frame, the obstacle
is manifest as a kind of graceful exit problem and/or a kind of flatness
problem. These arguments indicate the weakness of only the simplest
formulation. {}From them can be gleaned directions toward successful kinetic
inflation.Comment: 26 pages, LaTeX, CITA-94-2
Gravity-Driven Acceleration of the Cosmic Expansion
It is shown here that a dynamical Planck mass can drive the scale factor of
the universe to accelerate. The negative pressure which drives the cosmic
acceleration is identified with the unusual kinetic energy density of the
Planck field. No potential nor cosmological constant is required. This suggests
a purely gravity driven, kinetic inflation. Although the possibility is not
ruled out, the burst of acceleration is often too weak to address the initial
condition problems of cosmology. To illustrate the kinetic acceleration, three
different cosmologies are presented. One such example, that of a bouncing
universe, demonstrates the additional feature of being nonsingular. The
acceleration is also considered in the conformally related Einstein frame in
which the Planck mass is constant.Comment: 23 pages, LaTex, figures available upon request, (revisions include
added references and comment on inflation) CITA-94-1
WMAP constraints on scalar-tensor cosmology and the variation of the gravitational constant
We present observational constraints on a scalar-tensor gravity theory by
test for CMB anisotropy spectrum. We compare the WMAP temperature
power spectrum with the harmonic attractor model, in which the scalar field has
its harmonic effective potential with curvature in the Einstein
conformal frame and the theory relaxes toward Einstein gravity with time. We
found that the present value of the scalar coupling, i.e. the present level of
deviation from Einstein gravity , is bounded to be smaller than
(), and () for . This constraint is much stronger than the bound from the solar
system experiments for large models, i.e., and 0.3 in
and limits, respectively. Furthermore, within the framework
of this model, the variation of the gravitational constant at the recombination
epoch is constrained as , and
.Comment: 7 page
Gravitational radiation from a particle in circular orbit around a black hole. V. Black-hole absorption and tail corrections
A particle of mass moves on a circular orbit of a nonrotating black
hole of mass . Under the restrictions and , where
is the orbital velocity, we consider the gravitational waves emitted by such a
binary system. We calculate , the rate at which the gravitational
waves remove energy from the system. The total energy loss is given by , where denotes that part of the
gravitational-wave energy which is carried off to infinity, while
denotes the part which is absorbed by the black hole. We show that the
black-hole absorption is a small effect: . We
also compare the wave generation formalism which derives from perturbation
theory to the post-Newtonian formalism of Blanchet and Damour. Among other
things we consider the corrections to the asymptotic gravitational-wave field
which are due to wave-propagation (tail) effects.Comment: ReVTeX, 17 page
Observational Consequences of Evolution of Primordial Fluctuations in Scalar-Tensor Cosmology
Evolution of primordial fluctuations in a Brans-Dicke type scalar-tensor
gravity theory is comprehensively investigated. The harmonic attractor model,
in which the scalar field has its harmonic effective potential in the Einstein
conformal frame and the theory relaxes toward Einstein gravity with time, is
considered. The evolution of adiabatic initial perturbations in flat SCDM
models is examined from the radiation-dominated epoch up to the present. We
discuss how the scalar-tensor gravity affects the evolution of metric and
matter perturbations, mainly focusing on the observational consequences, i.e.,
the matter power spectrum and the power spectrum of cosmic microwave background
temperature. We find that the early time deviation is characterized only by the
large static gravitational constant while the late time behavior is
qualitatively different from that in Einstein gravity because the time
variation of the gravitational constant and its fluctuation have non-negligible
effects. The attracting scalar-tensor gravity affects only small scale modes
due to its attracting nature, the degree of which is far beyond the
post-Newtonian deviation at the present epoch.Comment: 18 page
Constraints from Inflation on Scalar-Tensor Gravity Theories
We show how observations of the perturbation spectra produced during
inflation may be used to constrain the parameters of general scalar-tensor
theories of gravity, which include both an inflaton and dilaton field. An
interesting feature of these models is the possibility that the curvature
perturbations on super-horizon scales may not be constant due to non-adiabatic
perturbations of the two fields. Within a given model, the tilt and relative
amplitude of the scalar and tensor perturbation spectra gives constraints on
the parameters of the gravity theory, which may be comparable with those from
primordial nucleosynthesis and post-Newtonian experiments.Comment: LaTeX (with RevTex) 19 pages, 8 uuencoded figures appended, also
available on WWW via http://star.maps.susx.ac.uk/index.htm
On the stability of scalar-vacuum space-times
We study the stability of static, spherically symmetric solutions to the
Einstein equations with a scalar field as the source. We describe a general
methodology of studying small radial perturbations of scalar-vacuum
configurations with arbitrary potentials V(\phi), and in particular space-times
with throats (including wormholes), which are possible if the scalar is
phantom. At such a throat, the effective potential for perturbations V_eff has
a positive pole (a potential wall) that prevents a complete perturbation
analysis. We show that, generically, (i) V_eff has precisely the form required
for regularization by the known S-deformation method, and (ii) a solution with
the regularized potential leads to regular scalar field and metric
perturbations of the initial configuration. The well-known conformal mappings
make these results also applicable to scalar-tensor and f(R) theories of
gravity. As a particular example, we prove the instability of all static
solutions with both normal and phantom scalars and V(\phi) = 0 under spherical
perturbations. We thus confirm the previous results on the unstable nature of
anti-Fisher wormholes and Fisher's singular solution and prove the instability
of other branches of these solutions including the anti-Fisher "cold black
holes".Comment: 18 pages, 5 figures. A few comments and references added. Final
version accepted at EPJ
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