484 research outputs found
Time-independent Anisotropic Plastic Behavior by Mechanical Subelement Models
The paper describes a procedure for modelling the anisotropic elastic-plastic behavior of metals in plane stress state by the mechanical sub-layer model. In this model the stress-strain curves along the longitudinal and transverse directions are represented by short smooth segments which are considered as piecewise linear for simplicity. The model is incorporated in a finite element analysis program which is based on the assumed stress hybrid element and the iscoplasticity-theory
On Hybrid and mixed finite element methods
Three versions of the assumed stress hybrid model in finite element methods and the corresponding variational principles for the formulation are presented. Examples of rank deficiency for stiffness matrices by the hybrid stress model are given and their corresponding kinematic deformation modes are identified. A discussion of the derivation of general semi-Loof elements for plates and shells by the hybrid stress method is given. It is shown that the equilibrium model by Fraeijs de Veubeke can be derived by the approach of the hybrid stress model as a special case of semi-Loof elements
Recent advances in hybrid/mixed finite elements
In formulations of Hybrid/Mixed finite element methods respectively by the Hellinger-Reissner principle and the Hu-Washizu principle, the stress equilibrium equations are brought in as conditions of constraint through the introduction of additional internal displacement parameters. These two approaches are more flexible and have better computing efficiencies. A procedure for the choice of assumed stress terms for 3-D solids is suggested. Example solutions are given for plates and shells using the present formulations and the idea of semiloof elements
Solutions of contact problems by the assumed stress hybrid model
A method was developed for contact problems which may be either frictional or frictionless and may involve extensive sliding between deformable bodies. It was based on an assumed stress hybrid approach and on an incremental variational principle for which the Euler's equations of the functional include the equilibrium and compatibility conditions at the contact surface. The tractions at an assumed contact surface were introduced as Lagrangian multipliers in the formulation. It was concluded from the results of several example solutions that the extensive sliding contact between deformable bodies can be solved by the present method
Advanced stress analysis methods applicable to turbine engine structures
Advanced stress analysis methods applicable to turbine engine structures are investigated. Constructions of special elements which containing traction-free circular boundaries are investigated. New versions of mixed variational principle and version of hybrid stress elements are formulated. A method is established for suppression of kinematic deformation modes. semiLoof plate and shell elements are constructed by assumed stress hybrid method. An elastic-plastic analysis is conducted by viscoplasticity theory using the mechanical subelement model
VLT observations of GRB 990510 and its environment
We present BVRI photometry and spectrophotometry of GRB990510 obtained with
the ESO VLT/Antu telescope during the late decline phase. Between days 8 and 29
after the burst, the afterglow faded from R=24.2 to ~26.4. The spectral flux
distribution and the light curve support the interpretation of the afterglow as
synchrotron emission from a jet. The light curve is consistent with the optical
transient alone but an underlying SN with maximum brightness R>27.4 or a galaxy
with R>27.6 (3-sigma upper limits) cannot be ruled out. To a 5-sigma detection
threshold of R=26.1, no galaxy is found within 6'' of the transient. A very
blue V~24.5 extended object which may qualify as a starburst galaxy is located
12'' SE, but at unknown redshift.Comment: 5 pages A&A Latex, accepted for publication in A&A Letter
The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB
The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any
event localized by BeppoSAX as yet but it did not have a detected optical
afterglow. It is therefore one of the events recently classified as dark GRBs
or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear.
Chandra observations allowed us to localize this GRB within ~1" and a radio
transient was detected with the VLA. We identify the likely (P=0.01) host
galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows
intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the
optical flux of this object is consistent with the above HI column density,
given a dust-to-gas ratio similar to that of our Galaxy. We do not find
evidence for a partially ionized absorber expected if the absorption takes
place in a Giant Molecular Cloud. We therefore conclude that either the gas is
local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3)
clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy.
Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be
substantially different from that observed in GRBs with optical afterglows.Comment: 29 pages, 7 fig.s, some revisions, ApJ, in pres
GRB host galaxies with VLT/X-Shooter: properties at 0.8 < z < 1.3
Long gamma-ray bursts (LGRBs) are associated with the death of massive stars.
Their host galaxies therefore represent a unique class of objects tracing star
formation across the observable Universe. Indeed, recently accumulated evidence
shows that GRB hosts do not differ substantially from general population of
galaxies at high (z > 2) redshifts. However, it has been long recognised that
the properties of z < 1.5 hosts, compared to general star-forming population,
are unusual. To better understand the reasons for the supposed difference in
LGRB hosts properties at z < 1.5, we obtained VLT/X- Shooter spectra of six
hosts lying in the redshift range of 0.8 < z < 1.3. Some of these hosts have
been observed before, yet we still lack well constrained information on their
characteristics such as metallicity, dust extinction and star formation rate.
We search for emission lines in the VLT/X-Shooter spectra of the hosts and
measure their fluxes. We perform a detailed analysis, estimating host average
extinction, star-formation rates, metallicities and electron densities where
possible. Measured quantities of our hosts are compared to a larger sample of
previously observed GRB hosts at z < 2. Star-formation rates and metallicities
are measured for all the hosts analyzed in this paper and metallicities are
well determined for 4 hosts. The mass-metallicity relation, the fundamental
metallicity relation and SFRs derived from our hosts occupy similar parameter
space as other host galaxies investigated so-far at the same redshift. We
therefore conclude that GRB hosts in our sample support the found discrepancy
between the properties of low-redshift GRB hosts and the general population of
star- forming galaxies.Comment: 13 pages, 6 figures, accepted for publication in MNRA
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