184,125 research outputs found
Evaluation of energy and failure parameters in composite structures via a Component-Wise approach
This paper deals with the static analysis of fiber reinforced composites via the Component-Wise approach (CW). The main aim of this work is the investigation of the CW capabilities for the evaluation of integral quantities such as the strain energy, or integral failure indexes. Such quantities are evaluated in the global structures and local volumes. The integral failure indexes, in particular, are proposed as alternatives to point-wise failure indexes. The CW approach has been recently developed as an extension of the 1D Carrera Unified Formulation (CUF). The CUF provides hierarchical higher-order structural models with arbitrary expansion orders. In this work, Lagrange-type polynomials are used to interpolate the displacement field over the element cross-sections. The CW makes use of the 1D CUF finite elements to model simultaneously different scale components (fiber, matrix, laminae and laminates) with a reduced computational cost. CW models do not require the homogenization of the material characteristics nor the definition of mathematical lines or surfaces. In other words, the material characteristics of each component, e.g. fibers and matrix, are employed, and the problem unknowns are placed above the physical surface of the body. In the perspective of failure analyses, the integral evaluation of failure parameters is introduced to determine critical portions of the structure where failure could take place. Integral quantities are evaluated using 3D integration sub-domains that may cover macro- and micro-volumes of the structure. The integral quantities can be evaluated directly on fiber and matrix portions. Numerical results are provided for different configurations and compared with solid finite element models. The results prove the accuracy of the CW approach and its computational efficiency. In particular, 3D local effects can be detected. The use of the integral failure index provides qualitatively reliable results; however, experimental campaigns should be carried out to relate such
indexes to the failure occurrence
On the origin of the large scale structures of the universe
We revise the statistical properties of the primordial cosmological density
anisotropies that, at the time of matter radiation equality, seeded the
gravitational development of large scale structures in the, otherwise,
homogeneous and isotropic Friedmann-Robertson-Walker flat universe. Our
analysis shows that random fluctuations of the density field at the same
instant of equality and with comoving wavelength shorter than the causal
horizon at that time can naturally account, when globally constrained to
conserve the total mass (energy) of the system, for the observed scale
invariance of the anisotropies over cosmologically large comoving volumes.
Statistical systems with similar features are generically known as glass-like
or lattice-like. Obviously, these conclusions conflict with the widely accepted
understanding of the primordial structures reported in the literature, which
requires an epoch of inflationary cosmology to precede the standard expansion
of the universe. The origin of the conflict must be found in the widespread,
but unjustified, claim that scale invariant mass (energy) anisotropies at the
instant of equality over comoving volumes of cosmological size, larger than the
causal horizon at the time, must be generated by fluctuations in the density
field with comparably large comoving wavelength.Comment: New section added; final version to appear in Physical Review D;
discussion extended and detailed with new calculations to support the claims
of the paper; statistical properties of vacuum fluctuations now discussed in
the context of FRW flat universe; new important conclussions adde
Quantum Gravity from Causal Dynamical Triangulations: A Review
This topical review gives a comprehensive overview and assessment of recent
results in Causal Dynamical Triangulations (CDT), a modern formulation of
lattice gravity, whose aim is to obtain a theory of quantum gravity
nonperturbatively from a scaling limit of the lattice-regularized theory. In
this manifestly diffeomorphism-invariant approach one has direct, computational
access to a Planckian spacetime regime, which is explored with the help of
invariant quantum observables. During the last few years, there have been
numerous new and important developments and insights concerning the theory's
phase structure, the roles of time, causality, diffeomorphisms and global
topology, the application of renormalization group methods and new observables.
We will focus on these new results, primarily in four spacetime dimensions, and
discuss some of their geometric and physical implications.Comment: 64 pages, 28 figure
Semiclassical Universe from First Principles
Causal Dynamical Triangulations in four dimensions provide a
background-independent definition of the sum over space-time geometries in
nonperturbative quantum gravity. We show that the macroscopic four-dimensional
world which emerges in the Euclidean sector of this theory is a bounce which
satisfies a semiclassical equation. After integrating out all degrees of
freedom except for a global scale factor, we obtain the ground state wave
function of the universe as a function of this scale factor.Comment: 15 pages, 4 figure
Foliations and 2+1 Causal Dynamical Triangulation Models
The original models of causal dynamical triangulations construct space-time
by arranging a set of simplices in layers separated by a fixed time-like
distance. The importance of the foliation structure in the 2+1 dimensional
model is studied by considering variations in which this property is relaxed.
It turns out that the fixed-lapse condition can be equivalently replaced by a
set of global constraints that have geometrical interpretation. On the other
hand, the introduction of new types of simplices that puncture the foliating
sheets in general leads to different low-energy behavior compared to the
original model.Comment: v2: 9 pages, 3 figures, published versio
The No-Boundary Measure in the Regime of Eternal Inflation
The no-boundary wave function (NBWF) specifies a measure for prediction in
cosmology that selects inflationary histories and remains well behaved for
spatially large or infinite universes. This paper explores the predictions of
the NBWF for linear scalar fluctuations about homogeneous and isotropic
backgrounds in models with a single scalar field moving in a quadratic
potential. We treat both the space-time geometry of the universe and the
observers inhabiting it quantum mechanically. We evaluate top-down
probabilities for local observations that are conditioned on the NBWF and on
part of our data as observers of the universe. For models where the most
probable histories do not have a regime of eternal inflation, the NBWF predicts
homogeneity on large scales, a specific non-Gaussian spectrum of observable
fluctuations, and a small amount of inflation in our past. By contrast, for
models where the dominant histories have a regime of eternal inflation, the
NBWF predicts significant inhomogeneity on scales much larger than the present
horizon, a Gaussian spectrum of observable fluctuations, and a long period of
inflation in our past. The absence or presence of local non-Gaussianity
therefore provides information about the global structure of the universe,
assuming the NBWF.Comment: 29 pages, 8 figure
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