4,033 research outputs found
Fundamental structure of steady plastic shock waves in metals
The propagation of steady plane shock waves in metallic materials is considered. Following the constitutive framework adopted by R. J. Clifton [Shock Waves and the Mechanical Properties of Solids, edited by J. J. Burke and V. Weiss (Syracuse University Press, Syracuse, N.Y., 1971), p. 73] for analyzing elasticâplastic transient waves, an analytical solution of the steady state propagation of plastic shocks is proposed. The problem is formulated in a Lagrangian setting appropriate for large deformations. The material response is characterized by a quasistatic tensile (compression) test (providing the isothermal strain hardening law). In addition the elastic response is determined up to second order elastic constants by ultrasonic measurements. Based on this simple information, it is shown that the shock kinetics can be quite well described for moderate shocks in aluminum with stress amplitude up to 10 GPa. Under the later assumption, the elastic response is assumed to be isentropic, and thermomechanical coupling is neglected. The model material considered here is aluminum, but the analysis is general and can be applied to any viscoplastic material subjected to moderate amplitude shocks. Comparisons with experimental data are made for the shock velocity, the particle velocity and the shock structure. The shock structure is obtained by quadrature of a first order differential equation, which provides analytical results under certain simplifying assumptions. The effects of material parameters and loading conditions on the shock kinetics and shock structure are discussed. The shock width is characterized by assuming an overstress formulation for the viscoplastic response. The effects on the shock structure of strain rate sensitivity are analyzed and the rationale for the J. W. Swegle and D. E. Grady [J. Appl. Phys. 58, 692 (1985)] universal scaling law for homogeneous materials is explored. Finally, the ability to deduce information on the viscoplastic response of materials subjected to very high strain rates from shock wave experiments is discussed
On conformally recurrent manifolds of dimension greater than 4
Conformally recurrent pseudo-Riemannian manifolds of dimension n>4 are
investigated. The Weyl tensor is represented as a Kulkarni-Nomizu product. If
the square of the Weyl tensor is nonzero, a covariantly constant symmetric
tensor is constructed, that is quadratic in the Weyl tensor. Then, by Grycak's
theorem, the explicit expression of the traceless part of the Ricci tensor is
obtained, up to a scalar function. The Ricci tensor has at most two distinct
eigenvalues, and the recurrence vector is an eigenvector. Lorentzian
conformally recurrent manifolds are then considered. If the square of the Weyl
tensor is nonzero, the manifold is decomposable. A null recurrence vector makes
the Weyl tensor of algebraic type IId or higher in the Bel - Debever - Ortaggio
classification, while a time-like recurrence vector makes the Weyl tensor
purely electric.Comment: Title changed and typos corrected. 14 page
Checking Interval Properties of Computations
Model checking is a powerful method widely explored in formal verification.
Given a model of a system, e.g., a Kripke structure, and a formula specifying
its expected behaviour, one can verify whether the system meets the behaviour
by checking the formula against the model.
Classically, system behaviour is expressed by a formula of a temporal logic,
such as LTL and the like. These logics are "point-wise" interpreted, as they
describe how the system evolves state-by-state. However, there are relevant
properties, such as those constraining the temporal relations between pairs of
temporally extended events or involving temporal aggregations, which are
inherently "interval-based", and thus asking for an interval temporal logic.
In this paper, we give a formalization of the model checking problem in an
interval logic setting. First, we provide an interpretation of formulas of
Halpern and Shoham's interval temporal logic HS over finite Kripke structures,
which allows one to check interval properties of computations. Then, we prove
that the model checking problem for HS against finite Kripke structures is
decidable by a suitable small model theorem, and we provide a lower bound to
its computational complexity.Comment: In Journal: Acta Informatica, Springer Berlin Heidelber, 201
The luminosity function of cluster galaxies. II. Data reduction procedures applied to the cluster Abell 496
We initiated a large project aimed to estimate the Luminosity Function of
galaxies in clusters and to evaluate its relation to cluster morphology. With
this paper we deem necessary to outline the general procedures of the data
reduction and details of the data analysis. The cluster sample includes the
brightest southern ROSAT all-sky survey clusters with z < 0.1. These have been
observed in three colours g, r, i, and mapped up to a few core radii using a
mosaic of CCD frames. E/S0 galaxies in the cluster core are singled out both by
morphology (for the brightest galaxies), and by colour. The details of the data
reduction procedure are illustrated via the analysis of the cluster Abell 496,
which has been used as a pilot cluster for the whole program. The related
photometric catalogue consists of 2355 objects. The limiting magnitudes (the
reference Surface Brightness is given in parenthesis) in the various colours
are respectively g(25.5) = 24.14, r(25.5) = 24.46, i(25.0) = 23.75$. These
correspond to the limiting absolute magnitudes -12.28, -11.96 and -12.67
(H_0=50 km/sec/Mpc).Comment: 17 pages, 19 ps figures, aa.cl
C3N4 for CO2 photoreduction: catalyst performance and stability in batch and continuous reactor
In this study, various C3N4 samples were
prepared and characterized. CO2 photoreduction was carried out by using C3N4 as
powder and coated on glass support in a batch reactor or embedded in a Nafion
membrane in a continuous reacto
The initial conditions of stellar protocluster formation. II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15<l<55
We present a catalogue of starless and protostellar clumps associated with
infrared dark clouds (IRDCs) in a 40 degrees wide region of the inner Galactic
Plane (b<1). We have extracted the far-infrared (FIR) counterparts of 3493
IRDCs with known distance in the Galactic longitude range 15<l<55 and searched
for the young clumps using Hi-GAL, the survey of the Galactic Plane carried out
with the Herschel satellite. Each clump is identified as a compact source
detected at 160, 250 and 350 mum. The clumps have been classified as
protostellar or starless, based on their emission (or lack of emission) at 70
mum. We identify 1723 clumps, 1056 (61%) of which are protostellar and 667
(39%) starless. These clumps are found within 764 different IRDCs, 375 (49%) of
which are only associated with protostellar clumps, 178 (23%) only with
starless clumps, and 211 (28%) with both categories of clumps. The clumps have
a median mass of 250 M_sun and range up to >10^4$ M_sun in mass and up to 10^5
L_sun in luminosity. The mass-radius distribution shows that almost 30% of the
starless clumps identified in this survey could form high-mass stars, however
these massive clumps are confined in only ~4% of the IRDCs. Assuming a minimum
mass surface density threshold for the formation of high-mass stars, the
comparison of the numbers of massive starless clumps and those already
containing embedded sources suggests an upper limit lifetime for the starless
phase of 10^5 years for clumps with a mass M>500 M_sun.Comment: accepted for publication in MNRAS. Online catalogues available soon,
please contact the authors if intereste
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