155 research outputs found
Progressive damage in stitched composites: Static tensile tests and tension-tension fatigue
The paper describes progressive damage in static tensile tests and tension-tension fatigue in structurally stitched carbon/epoxy NCF composites, in comparison with their non-stitched counterparts. Analogies between damage development in quasi-static tension and tension-tension fatigue are analyzed and links between the damage initiation thresholds in quasi-static tests and fatigue life are established
An experimental investigation of calibration techniques for imbalanced data
Calibration is a technique used to obtain accurate probability estimation for classification problems in real applications. Class imbalance can create considerable challenges in obtaining accurate probabilities for calibration methods. However, previous research has paid little attention to this issue. In this paper, we present an experimental investigation of some prevailing calibration methods in different imbalance scenarios. Several performance metrics are considered to evaluate different aspects of calibration performance. The experimental results show that the performance of different calibration techniques depends on the metrics and the degree of the imbalance ratio. Isotonic Regression has better overall performance on imbalanced datasets than parametric and other complex non-parametric methods. However, it performs unstably in highly imbalanced scenarios. This study provides some insights into calibration methods on imbalanced datasets, and it can be a reference for the future development of calibration methods in class imbalance scenarios
A new regional climate model for POLAR-CORDEX : evaluation of a 30-year hindcast with COSMO-CLM2 over Antarctica
Continent-wide climate information over the Antarctic Ice Sheet (AIS) is important to obtain accurate information of present climate and reduce uncertainties of the ice sheet mass balance response and resulting global sea level rise to future climate change. In this study, the COSMO-CLM2 Regional Climate Model is applied over the AIS and adapted for the specific meteorological and climatological conditions of the region. A 30-year hindcast was performed and evaluated against observational records consisting of long-term ground-based meteorological observations, automatic weather stations, radiosoundings, satellite records, stake measurements and ice cores. Reasonable agreement regarding the surface and upper-air climate is achieved by the COSMO-CLM2 model, comparable to the performance of other state-of-the-art climate models over the AIS. Meteorological variability of the surface climate is adequately simulated, and biases in the radiation and surface mass balance are small. The presented model therefore contributes as a new member to the COordinated Regional Downscaling EXperiment project over the AIS (POLAR-CORDEX) and the CORDEX-CORE initiative
Rubber Impact on 3D Textile Composites
A low velocity impact study of aircraft tire rubber on 3D textile-reinforced composite plates was performed experimentally and numerically. In contrast to regular unidirectional composite laminates, no delaminations occur in such a 3D textile composite. Yarn decohesions, matrix cracks and yarn ruptures have been identified as the major damage mechanisms under impact load. An increase in the number of 3D warp yarns is proposed to improve the impact damage resistance. The characteristic of a rubber impact is the high amount of elastic energy stored in the impactor during impact, which was more than 90% of the initial kinetic energy. This large geometrical deformation of the rubber during impact leads to a less localised loading of the target structure and poses great challenges for the numerical modelling. A hyperelastic Mooney-Rivlin constitutive law was used in Abaqus/Explicit based on a step-by-step validation with static rubber compression tests and low velocity impact tests on aluminium plates. Simulation models of the textile weave were developed on the meso- and macro-scale. The final correlation between impact simulation results on 3D textile-reinforced composite plates and impact test data was promising, highlighting the potential of such numerical simulation tools
Natural and sail-displaced doubly-symmetric Lagrange point orbits for polar coverage
This paper proposes the use of doubly-symmetric, eight-shaped orbits in the circular restricted three-body problem for continuous coverage of the high-latitude regions of the Earth. These orbits, for a range of amplitudes, spend a large fraction of their period above either pole of the Earth. It is shown that they complement Sun-synchronous polar and highly eccentric Molniya orbits, and present a possible alternative to low thrust pole-sitter orbits. Both natural and solar-sail displaced orbits are considered. Continuation methods are described and used to generate families of these orbits. Starting from ballistic orbits, other families are created either by increasing the sail lightness number, varying the period or changing the sail attitude. Some representative orbits are then chosen to demonstrate the visibility of high-latitude regions throughout the year. A stability analysis is also performed, revealing that the orbits are unstable: it is found that for particular orbits, a solar sail can reduce their instability. A preliminary design of a linear quadratic regulator is presented as a solution to stabilize the system by using the solar sail only. Finally, invariant manifolds are exploited to identify orbits that present the opportunity of a ballistic transfer directly from low Earth orbit
A uniform treatment of the orbital effects due to a violation of the Strong Equivalence Principle in the gravitational Stark-like limit
We analytically work out several effects which a violation of the Strong
Equivalence Principle (SEP) induces on the orbital motion of a binary system
constituted of self-gravitating bodies immersed in a constant and uniform
external field. We do not restrict to the small eccentricity limit. Moreover,
we do not select any specific spatial orientation of the external polarizing
field. We explicitly calculate the SEP-induced mean rates of change of all the
osculating Keplerian orbital elements of the binary, the perturbation of the
projection of the binary orbit onto the line-of-sight, the shift of the radial
velocity, and the range and range-rate signatures and as well. We find that the
ratio of the SEP precessions of the node and the inclination of the binary
depends only on and the pericenter of the binary itself, being independent on
both the magnitude and the orientation of the polarizing field, and on the
semimajor axis, the eccentricity and the node of the binary. Our results, which
do not depend on any particular SEP-violating theoretical scheme, can be
applied to quite general astronomical and astrophysical scenarios. They can be
used to better interpret present and future SEP experiments, especially when
several theoretical SEP mechanisms may be involved, and to suitably design new
dedicated tests.Comment: LaTex2e, 14 pages, no figures, no tables, 42 references. To appear in
Classical and Quantum Gravity (CQG
The evolution of the orbit distance in the double averaged restricted 3-body problem with crossing singularities
We study the long term evolution of the distance between two Keplerian
confocal trajectories in the framework of the averaged restricted 3-body
problem. The bodies may represent the Sun, a solar system planet and an
asteroid. The secular evolution of the orbital elements of the asteroid is
computed by averaging the equations of motion over the mean anomalies of the
asteroid and the planet. When an orbit crossing with the planet occurs the
averaged equations become singular. However, it is possible to define piecewise
differentiable solutions by extending the averaged vector field beyond the
singularity from both sides of the orbit crossing set. In this paper we improve
the previous results, concerning in particular the singularity extraction
technique, and show that the extended vector fields are Lipschitz-continuous.
Moreover, we consider the distance between the Keplerian trajectories of the
small body and of the planet. Apart from exceptional cases, we can select a
sign for this distance so that it becomes an analytic map of the orbital
elements near to crossing configurations. We prove that the evolution of the
'signed' distance along the averaged vector field is more regular than that of
the elements in a neighborhood of crossing times. A comparison between averaged
and non-averaged evolutions and an application of these results are shown using
orbits of near-Earth asteroids.Comment: 29 pages, 8 figure
Planets in habitable zones: A study of the binary Gamma Cephei
The recently discovered planetary system in the binary GamCep was studied
concerning its dynamical evolution. We confirm that the orbital parameters
found by the observers are in a stable configuration. The primary aim of this
study was to find stable planetary orbits in a habitable region in this system,
which consists of a double star (a=21.36 AU) and a relatively close (a=2.15 AU)
massive (1.7 Mjup sin i) planet. We did straightforward numerical integrations
of the equations of motion in different dynamical models and determined the
stability regions for a fictitious massless planet in the interval of the
semimajor axis 0.5 AU < a < 1.85 AU around the more massive primary. To confirm
the results we used the Fast Lyapunov Indicators (FLI) in separate
computations, which are a common tool for determining the chaoticity of an
orbit. Both results are in good agreement and unveiled a small island of stable
motions close to 1 AU up to an inclination of about 15 deg (which corresponds
to the 3:1 mean motion resonance between the two planets). Additionally we
computed the orbits of earthlike planets (up to 90 earthmasses) in the small
stable island and found out, that there exists a small window of stable orbits
on the inner edge of the habitable zone in GamCep even for massive planets.Comment: 4 pages, 5 figures, changed 2 references made minor changes due to
referees advic
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