5,749 research outputs found
Prediction of ductile fracture in anisotropic steels for pipeline applications
Large diameter steel pipelines for gas transportation may experience extreme overloads due to external actions such as soil sliding, faults movements, third part interactions. In these scenarios the material undergoes severe plastic strains which locally may reach the fracture limits. Due to the manufacturing process, the steels used in such applications have an anisotropic behavior both for plasticity and fracture. In this paper two steel grades have been characterized in view of anisotropic plastic fracture. Fracture tests have been planned to characterize the fracture behavior under different stress states and in different directions to define the anisotropic sensitivity. Finite element modelling, incorporating an anisotropic plasticity formulation, has been used to calculate the local fracture parameters in the specimens and to define the complete ductile fracture locus. An uncoupled damage evolution law has been finally used to evaluate the fracture limits on real pipelines failed in full scale laboratory tests. The strain to fracture prediction has been verified by local strain measurements on the fractured pipes. The model robustness has been also verified on global parameter predictions, such us the burst pressur
Prosthetic abdominal wall hernia repair in emergency surgery: from polypropylene to biological meshes
The use of nonabsorbable prosthetic materials such as polypropylene, polyester, and ePTFE, have expanded and are now widely used in reparative surgery for abdominal wall hernias
Implementation of a lode angle sensitive yield criterion for numerical modelling of ductile materials in the large strain range
Ductile metallic materials are usually mechanically characterized in the large strain range by means of conventional laboratory tests, namely tensile and torsion test. The identification of the elastic-plastic equivalent stress-strain relation is made by using the widely accepted J2 hypothesis and isotropic behaviour, but results of dedicated tensile and torsion tests on different steel grades showed that it does not seem possible to identify a unique stress-strain curve, as expected on theoretical ground. Curves obtained from dissimilar tests, even though perfectly matching at small plastic strains, start differing significantly from medium strains. Starting from the experimental observations, a general yield surface has been developed based on a well established framework, including beside the J2 invariant also the J3 one. The new yield function has been coded inside a general purpose finite element code by means of dedicated user routines. Implications on the parameters identification and examples of the new yield surface application are discussed in the paper
Reconstruction of Black Hole Metric Perturbations from Weyl Curvature
Perturbation theory of rotating black holes is usually described in terms of
Weyl scalars and , which each satisfy Teukolsky's complex
master wave equation and respectively represent outgoing and ingoing radiation.
On the other hand metric perturbations of a Kerr hole can be described in terms
of (Hertz-like) potentials in outgoing or ingoing {\it radiation
gauges}. In this paper we relate these potentials to what one actually computes
in perturbation theory, i.e and . We explicitly construct
these relations in the nonrotating limit, preparatory to devising a
corresponding approach for building up the perturbed spacetime of a rotating
black hole. We discuss the application of our procedure to second order
perturbation theory and to the study of radiation reaction effects for a
particle orbiting a massive black hole.Comment: 6 Pages, Revtex
Po River Plume Influence on Marine Biochemical Properties along the Western Adriatic Coast
An international research program was devoted to understanding the dynamic properties of the mesoscale circulation in the Adriatic Sea such as fronts, eddies, coastal filaments, river inflow and study the effects of forcing by winds and river run-offs. The present work focuses specifically on the impact of the Po river outflow on the biochemical properties along the western Adriatic coast under different conditions of river discharge and wind stress in winter and spring. In the winter cruise, the satellite images show a strong front in the northern part of the Adriatic extending from the Italian coast to the Istrian Peninsula. In situ measures showed that the northern water mass was characterized by low temperature and salinity, and high nutrient and chlorophyll concentrations due to a strong Bora event which expanded the Po River plume towards the Istrian Peninsula instead of southwards in the West Adriatic Current. During the spring cruise, wind forcing was quite weak and the volume flux from the Po River was about one third of its mean discharge for this period of the year. Off the Po River, the water column was characterized by a surface layer with low salinity and high dissolved inorganic nitrogen and chlorophyll. This water mass did not extend as far eastwards as in winter because the wind forcing was not nearly as important and the alongshore plume extended southward along the Italian Coast
Gravitational-Wave Recoil from the Ringdown Phase of Coalescing Black Hole Binaries
The gravitational recoil or "kick" of a black hole formed from the merger of
two orbiting black holes, and caused by the anisotropic emission of
gravitational radiation, is an astrophysically important phenomenon. We combine
(i) an earlier calculation, using post-Newtonian theory, of the kick velocity
accumulated up to the merger of two non-spinning black holes, (ii) a
"close-limit approximation" calculation of the radiation emitted during the
ringdown phase, and based on a solution of the Regge-Wheeler and Zerilli
equations using initial data accurate to second post-Newtonian order. We prove
that ringdown radiation produces a significant "anti-kick". Adding the
contributions due to inspiral, merger and ringdown phases, our results for the
net kick velocity agree with those from numerical relativity to 10-15 percent
over a wide range of mass ratios, with a maximum velocity of 180 km/s at a mass
ratio of 0.38.Comment: 9 pages, 5 figures; to appear in Class. Quant. Gra
Black hole puncture initial data with realistic gravitational wave content
We present improved post-Newtonian-inspired initial data for non-spinning
black-hole binaries, suitable for numerical evolution with punctures. We
revisit the work of Tichy et al. [W. Tichy, B. Bruegmann, M. Campanelli, and P.
Diener, Phys. Rev. D 67, 064008 (2003)], explicitly calculating the remaining
integral terms. These terms improve accuracy in the far zone and, for the first
time, include realistic gravitational waves in the initial data. We investigate
the behavior of these data both at the center of mass and in the far zone,
demonstrating agreement of the transverse-traceless parts of the new metric
with quadrupole-approximation waveforms. These data can be used for numerical
evolutions, enabling a direct connection between the merger waveforms and the
post-Newtonian inspiral waveforms.Comment: 13 pages, 7 figures; replaced with published versio
The Yamabe invariant for axially symmetric two Kerr black holes initial data
An explicit 3-dimensional Riemannian metric is constructed which can be
interpreted as the (conformal) sum of two Kerr black holes with aligned angular
momentum. When the separation distance between them is large we prove that this
metric has positive Ricci scalar and hence positive Yamabe invariant. This
metric can be used to construct axially symmetric initial data for two Kerr
black holes with large angular momentum.Comment: 14 pages, 2 figure
Modeling Gravitational Recoil Using Numerical Relativity
We review the developments in modeling gravitational recoil from merging
black-hole binaries and introduce a new set of 20 simulations to test our
previously proposed empirical formula for the recoil. The configurations are
chosen to represent generic binaries with unequal masses and precessing spins.
Results of these simulations indicate that the recoil formula is accurate to
within a few km/s in the similar mass-ratio regime for the out-of-plane recoil.Comment: corrections to text, 11 pages, 1 figur
Comparisons of binary black hole merger waveforms
This a particularly exciting time for gravitational wave physics.
Ground-based gravitational wave detectors are now operating at a sensitivity
such that gravitational radiation may soon be directly detected, and recently
several groups have independently made significant breakthroughs that have
finally enabled numerical relativists to solve the Einstein field equations for
coalescing black-hole binaries, a key source of gravitational radiation. The
numerical relativity community is now in the position to begin providing
simulated merger waveforms for use by the data analysis community, and it is
therefore very important that we provide ways to validate the results produced
by various numerical approaches. Here, we present a simple comparison of the
waveforms produced by two very different, but equally successful
approaches--the generalized harmonic gauge and the moving puncture methods. We
compare waveforms of equal-mass black hole mergers with minimal or vanishing
spins. The results show exceptional agreement for the final burst of radiation,
with some differences attributable to small spins on the black holes in one
case.Comment: Revtex 4, 5 pages. Published versio
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