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A combined experimental-numerical study to tensile behaviour of limestone
In this paper, a combined experimental-computational study of double-edge notched stone specimen subject to cyclic tensile loading is presented. In the experimental part, the load-deformation response and the local displacement field are recorded. Both experimental results are used to validate a numerical model for the description of fracture within finite elements. The model uses displacement discontinuities to model cracks. These discontinuities are implemented using the partition of unity property of finite element shape functions. In the discontinuity, a combined damage-plasticity cohesive law is used. Numerical simulations are compared with experimental observations
A discrete model for cyclic mode I loading
AbstractThe cyclic behaviour of a double-edge notched specimen loaded in tension is studied. Cracks in the material are modelled by displacement discontinuities that can propagate during computation. Within these discontinuities, a cohesive zone model is used. The model assumes an additive split of the inelastic jump into a recoverable and an unrecoverable part. The influence of model parameters and discretisation is studied and the results have been compared with experimental data
Coulomb Distortion Effects for (e,e'p) Reactions at High Electron Energy
We report a significant improvement of an approximate method of including
electron Coulomb distortion in electron induced reactions at momentum transfers
greater than the inverse of the size of the target nucleus. In particular, we
have found a new parametrization for the elastic electron scattering phase
shifts that works well at all electron energies greater than 300 . As an
illustration, we apply the improved approximation to the reaction
from medium and heavy nuclei. We use a relativistic ``single particle'' model
for as as applied to and to recently measured data
at CEBAF on to investigate Coulomb distortion effects while
examining the physics of the reaction.Comment: 14 pages, 3 figures, PRC submitte
Type Ia Supernovae and Accretion Induced Collapse
Using the population synthesis binary evolution code StarTrack, we present
theoretical rates and delay times of Type Ia supernovae arising from various
formation channels. These channels include binaries in which the exploding
white dwarf reaches the Chandrasekhar mass limit (DDS, SDS, and helium-rich
donor scenario) as well as the sub-Chandrasekhar mass scenario, in which a
white dwarf accretes from a helium-rich companion and explodes as a SN Ia
before reaching the Chandrasekhar mass limit. We find that using a common
envelope parameterization employing energy balance with alpha=1 and lambda=1,
the supernova rates per unit mass (born in stars) of sub-Chandrasekhar mass SNe
Ia exceed those of all other progenitor channels at epochs t=0.7 - 4 Gyr for a
burst of star formation at t=0. Additionally, the delay time distribution of
the sub-Chandrasekhar model can be divided in to two distinct evolutionary
channels: the `prompt' helium-star channel with delay times < 500 Myr, and the
`delayed' double white dwarf channel with delay times > 800 Myr spanning up to
a Hubble time. These findings are in agreement with recent
observationally-derived delay time distributions which predict that a large
number of SNe Ia have delay times < 1 Gyr, with a significant fraction having
delay times < 500 Myr. We find that the DDS channel is also able to account for
the observed rates of SNe Ia. However, detailed simulations of white dwarf
mergers have shown that most of these mergers will not lead to SNe Ia but
rather to the formation of a neutron star via accretion-induced collapse. If
this is true, our standard population synthesis model predicts that the only
progenitor channel which can account for the rates of SNe Ia is the
sub-Chandrasekhar mass scenario, and none of the other progenitors considered
can fully account for the observed rates.Comment: 6 pages, 1 figure, 1 table, to appear in proceedings for "Binary Star
Evolution: Mass Loss, Accretion and Mergers
Systematic study of Coulomb distortion effects in exclusive (e,e'p) reactions
A technique to deal with Coulomb electron distortions in the analysis of
(e,e'p) reactions is presented. Thereby, no approximations are made. The
suggested technique relies on a partial-wave expansion of the electron wave
functions and a multipole decomposition of the electron and nuclear current in
momentum space. In that way, we succeed in keeping the computational times
within reasonable limits. This theoretical framework is used to calculate the
quasielastic (e,e'p) reduced cross sections for proton knockout from the
valence shells in O, Ca, Zr and Pb. The
final-state interaction of the ejected proton with the residual nucleus is
treated within an optical potential model. The role of electron distortion on
the extracted spectroscopic factors is discussed.Comment: 45 pages, 10 encapsulated postscript figures, Revtex, uses epsfig.sty
and fancybox.sty, to be published in Physical Review
Electroinduced two-nucleon knockout and correlations in nuclei
We present a model to calculate cross sections for electroinduced two-nucleon
emission from finite nuclei. Short-range correlations in the wave functions and
meson-exchange contributions to the photoabsorption process are implemented.
Effects of the short-range correlations are studied with the aid of a
perturbation expansion method with various choices of the Jastrow correlation
function. The model is used to investigate the relative importance of the
different reaction mechanisms contributing to the A(e,epn) and A(e,epp)
process. Representative examples for the target nuclei C and O
and for kinematical conditions accessible with contemporary high-duty cycle
electron accelerators are presented. A procedure is outlined to calculate the
two-nucleon knockout contribution to the semi-exclusive (e,ep) cross
section. Using this technique we investigate in how far semi-exclusive
(e,ep) reactions can be used to detect high-momentum components in the
nuclear spectral function.Comment: 51 pages, Latex, uses epsf.sty and elsart.sty, 17 figures (in eps
format
Parameter estimation of spinning binary inspirals using Markov-chain Monte Carlo
We present a Markov-chain Monte-Carlo (MCMC) technique to study the source
parameters of gravitational-wave signals from the inspirals of stellar-mass
compact binaries detected with ground-based gravitational-wave detectors such
as LIGO and Virgo, for the case where spin is present in the more massive
compact object in the binary. We discuss aspects of the MCMC algorithm that
allow us to sample the parameter space in an efficient way. We show sample runs
that illustrate the possibilities of our MCMC code and the difficulties that we
encounter.Comment: 10 pages, 2 figures, submitted to Classical and Quantum Gravit
Random template placement and prior information
In signal detection problems, one is usually faced with the task of searching
a parameter space for peaks in the likelihood function which indicate the
presence of a signal. Random searches have proven to be very efficient as well
as easy to implement, compared e.g. to searches along regular grids in
parameter space. Knowledge of the parameterised shape of the signal searched
for adds structure to the parameter space, i.e., there are usually regions
requiring to be densely searched while in other regions a coarser search is
sufficient. On the other hand, prior information identifies the regions in
which a search will actually be promising or may likely be in vain. Defining
specific figures of merit allows one to combine both template metric and prior
distribution and devise optimal sampling schemes over the parameter space. We
show an example related to the gravitational wave signal from a binary inspiral
event. Here the template metric and prior information are particularly
contradictory, since signals from low-mass systems tolerate the least mismatch
in parameter space while high-mass systems are far more likely, as they imply a
greater signal-to-noise ratio (SNR) and hence are detectable to greater
distances. The derived sampling strategy is implemented in a Markov chain Monte
Carlo (MCMC) algorithm where it improves convergence.Comment: Proceedings of the 8th Edoardo Amaldi Conference on Gravitational
Waves. 7 pages, 4 figure
Stability of helium accretion discs in ultracompact binaries
Stellar companions of accreting neutron stars in ultra compact X-ray binaries
(UCXBs) are hydrogen-deficient. Their helium or C/O accretion discs are
strongly X-ray irradiated. Both the chemical composition and irradiation
determine the disc stability with respect to thermal and viscous perturbations.
At shorter periods, UCXBs are persistent, whereas longer-period systems are
mostly transient. To understand this behaviour one has to derive the stability
criteria for X-ray irradiated hydrogen-poor accretion discs. We use a modified
and updated version of the Dubus et al. code describing time-dependent
irradiated accretion discs around compact objects. We obtained the relevant
stability criteria and compared the results to observed properties of UCXBs.
Although the general trend in the stability behaviour of UCXBs is consistent
with the prediction of the disc instability model, in a few cases the
inconsistency of theoretical predictions with the system observed properties is
weak enough to be attributed to observational and/or theoretical uncertainties.
Two systems might require the presence of some amount of hydrogen in the donor
star.Comment: Astronomy & Astrophysics, in pres
Compact Binary Coalescences in the Band of Ground-based Gravitational-Wave Detectors
As the ground-based gravitational-wave telescopes LIGO, Virgo, and GEO 600
approach the era of first detections, we review the current knowledge of the
coalescence rates and the mass and spin distributions of merging neutron-star
and black-hole binaries. We emphasize the bi-directional connection between
gravitational-wave astronomy and conventional astrophysics. Astrophysical input
will make possible informed decisions about optimal detector configurations and
search techniques. Meanwhile, rate upper limits, detected merger rates, and the
distribution of masses and spins measured by gravitational-wave searches will
constrain astrophysical parameters through comparisons with astrophysical
models. Future developments necessary to the success of gravitational-wave
astronomy are discussed.Comment: Replaced with version accepted by CQG
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