40,086 research outputs found
Boundary integral equation methods for the elastic and thermoelastic waves in three dimensions
In this paper, we consider the boundary integral equation (BIE) method for
solving the exterior Neumann boundary value problems of elastic and
thermoelastic waves in three dimensions based on the Fredholm integral
equations of the first kind. The innovative contribution of this work lies in
the proposal of the new regularized formulations for the hyper-singular
boundary integral operators (BIO) associated with the time-harmonic elastic and
thermoelastic wave equations. With the help of the new regularized
formulations, we only need to compute the integrals with weak singularities at
most in the corresponding variational forms of the boundary integral equations.
The accuracy of the regularized formulations is demonstrated through numerical
examples using the Galerkin boundary element method (BEM).Comment: 24 pages, 6 figure
Calculations of three-nucleon reactions with N3LO chiral forces: achievements and challenges
We discuss the application of the chiral N3LO forces to three-nucleon
reactions and point to the challenges which will have to be addressed. Present
approaches to solve three-nucleon Faddeev equations are based on a partial-wave
decomposition. A rapid increase of the number of terms contributing to the
chiral three-nucleon force when increasing the order of the chiral expansion
from N2LO to N3LO forced us to develop a fast and effective method of
automatized partial wave decomposition. At low energies of the incoming nucleon
below about 20MeV, where only a limited number of partial waves is required,
this method allowed us to perform calculations of reactions in the
three-nucleon continuum using N3LO two- and three-nucleon forces. It turns out
that inclusion of consistent chiral interactions, with relativistic 1/m
corrections and short-range 2pi-contact term omitted in the N3LO three-nucleon
force, does not explain the long standing low energy Ay-puzzle. We discuss
problems arising when chiral forces are applied at higher energies, where large
three-nucleon force effects are expected. It seems plausible that at higher
energies, due to a rapid increase of a number of partial waves required to
reach convergent results, a three-dimensional formulation of the Faddeev
equations which avoids partial-wave decomposition is desirable.Comment: 31 pages, 13 figure
Transient Propagation and Scattering of Quasi-Rayleigh Waves in Plates: Quantitative comparison between Pulsed TV-Holography Measurements and FC(Gram) elastodynamic simulations
We study the scattering of transient, high-frequency, narrow-band
quasi-Rayleigh elastic waves by through-thickness holes in aluminum plates, in
the framework of ultrasonic nondestructive testing (NDT) based on full-field
optical detection. Sequences of the instantaneous two-dimensional (2-D)
out-of-plane displacement scattering maps are measured with a self-developed
PTVH system. The corresponding simulated sequences are obtained by means of an
FC(Gram) elastodynamic solver introduced recently, which implements a full
three-dimensional (3D) vector formulation of the direct linear-elasticity
scattering problem. A detailed quantitative comparison between these
experimental and numerical sequences, which is presented here for the first
time, shows very good agreement both in the amplitude and the phase of the
acoustic field in the forward, lateral and backscattering areas. It is thus
suggested that the combination of the PTVH system and the FC(Gram)
elastodynamic solver provides an effective ultrasonic inspection tool for
plate-like structures, with a significant potential for ultrasonic NDT
applications.Comment: 46 pages, 16 figures, corresponding author Jos\'e Carlos
L\'opez-V\'azquez, [email protected]. Changes: 1st, 4th, 5th paragraphs
(intro), 3rd, 4th paragraphs (sec. 4); [59-60] cited only in appendixes; old
ref. [52] removed; misprints corrected in the uncertainty of c_L (subsec.
3.1), citation to fig. 10 (sec. 4), size of images (caption fig.15);
reference to Lam\'e constants removed in subsec. 3.
On the problem of the relation between phason elasticity and phason dynamics in quasicrystals
It has recently been claimed that the dynamics of long-wavelength phason
fluctuations has been observed in i-AlPdMn quasicrystals. We will show that the
data reported call for a more detailed development of the elasticity theory of
Jaric and Nelsson in order to determine the nature of small phonon-like atomic
displacements with a symmetry that follows the phason elastic constants. We
also show that a simple model with a single diffusing tile is sufficient to
produce a signal that (1) is situated at a "satellite position'' at a distance
q from each Bragg peak, that (2) has an intensity that scales with the
intensity of the corresponding Bragg peak, (3) falls off as 1/q-squared and (4)
has a time decay constant that is proportional to 1/(D q-squared). It is thus
superfluous to call for a picture of "phason waves'' in order to explain such
data, especially as such "waves'' violate many physical principles.Comment: 36 pages, 0 figures, discussion about vacancies, fluctuating Fourier
components, and difference between static and dynamical structure factors
added, other addition
On the scattering of torsional elastic waves from axisymmetric defects in coated pipes
This is the post-print version of the Article - Copyright @ 2012 ElsevierLong range ultrasonic testing is now a well established method for examining in-service degradation in pipelines. In order to protect pipelines from the surrounding environment it is common for viscoelastic coatings to be applied to the outer surface. These coatings are, however, known to impact on the ability of long range ultrasonic techniques to locate degradation, or defects, within a coated pipe. The coating dissipates sound energy travelling along the pipe, attenuating both the incident and reflected signals making responses from defects difficult to detect. This article aims to investigate the influence of a viscoelastic coating on the ability of long range ultrasonic testing to detect a defect in an axisymmetric pipe. The article focuses on understanding the behaviour of the fundamental torsional mode and quantifying the effect of bitumen coatings on reflection coefficients generated by axisymmetric defects. Reflection coefficients are measured experimentally for coated and uncoated pipes and compared to theoretical predictions generated using numerical mode matching and a hybrid finite element technique. Good agreement between prediction and measurement is observed for uncoated pipes, and it is shown that the theoretical methods presented here are fast and efficient making them suitable for studying long pipe runs. However, when studying coated pipes agreement between theory and prediction is observed to be poor for predictions based on those bulk acoustic properties currently reported in the literature for bitumen. Good agreement is observed only after conducting a parametric study to identify more appropriate values for the bulk acoustic properties. Furthermore, the reflection coefficients obtained for the fundamental torsional mode in a coated pipe show that significant sound attenuation is present over relatively short lengths of coating, thus quantifying those problems commonly encountered with the use of long range ultrasonic testing on coated pipes in the field
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