6,311 research outputs found
Application of quasi-homogeneous anisotropic laminates in grid-stiffened panel design
Composite laminates are derived for standard configurations with quasi-homogeneous anisotropic properties, whereby in-plane and out-of-plane stiffness properties are concomitant. Dimensionless parameters, and their relationship to the well-known ply- orientation-dependent lamination parameters, are also developed from which the elements of the extensional and bending stiffness matrices are readily calculated for any fiber/resin properties. The definitive list of laminate configurations for up to 21 plies is presented, together with graphical representations of the lamination parameter design space for standard ply orientations +45, -45, 0 and 90 degrees. Finally, the potential of quasi-homogeneous anisotropic laminates as an optimum design solution for anisogid structures is explored for cases where buckling and strength constraints are both active
Properties of the Optokinetic Motor Fibres in the Rock Lobster: Build-Up, Flipback, Afterdischarge and Memory, Shown by Their Firing Patterns
The properties of sets of motor fibres responding to both clockwise and anticlockwise rotation have been studied in the oculomotor nerve of the rock lobster. There are probably three, but perhaps four, units in each set.
None of these fibres has statocyst input, but there is weak input onto the tonic fibres from the antennal joints such that the eye turns in the direction toward which the antenna points.
Many preparations show bilateral visual input onto all fibres but the degree of coupling between the eyes is very variable, and at times can be nearly totally absent.
Depending on the speed of rotation the fibres show a gradual build-up in frequency, during rotation in the preferred direction, interrupted by flipbacks. During the fast stage of the resulting nystagmic movements all agonistic fibres can be completely inhibited and all antagonistic ones can be activated, usually for a period of about 0.5 sec.
Fibre activity is demonstrated which appears to underlie an ‘optokinetic memory’ of contrasting target position in the visual field. It consists of (a) very prolonged after-discharges for a stationary striped pattern (b) resumption of discharges at an appropriate frequency after dark periods up to 2 min, and (c) adjustment of such frequencies to changes in stripe position during the dark period.
The fibres show habituation to repeated stripe movement but the response can be dishabituated by passive rotation of the animal.
The largest visual responses were obtained to intermediate speeds of stripe rotation (about 2°/sec)
Characterization of ply mixing rules for non-symmetric forms of fully orthotropic laminates
Stacking sequence listings are presented for fully orthotropic angle-ply
laminates, with up to 21 plies, together with rules for mixing these
sequences to form laminates containing any number of plies. The mixing
rules are demonstrated through an abridged set of sequences, which are
characterized in terms of angle- and cross-ply sub-sequence symmetries. The
abridged set of sequences is derived from a new definitive list that
supersedes previously published listings. Stacking sequences are presented
together with dimensionless parameters from which the bending stiffness
terms are readily calculated and an assessment of the bending stiffness
efficiency made for angle- and cross-ply sub-sequences. Expressions
relating the dimensionless parameters to the well-known lamination
parameters are also given, together with graphical representations of
feasible domains for all sub-sequence symmetries contained in the definitive
list. Feasible domains for extensionally isotropic and fully isotropic
laminates are also presented as important sub-sets of fully orthotropic
laminates. Finally, examples are given for tapered laminates with fully
orthotropic properties, derived from compatible sequences in the definite
list
Input Sources and Properties of Position-Sensitive Oculomotor Fibres in the Rock Lobster, Panulirus Interruptus (Randall)
Sets of head-up, head-down, eye-up and eye-down motor fibres were studied in the oculomotor nerve of the rock lobster. An eye-withdrawal fibre was also investigated.
Apart from the statocyst input, light distribution on the eyes has the strongest influence on the position-sensitive fibres. Weaker optokinetic input from moving targets is also present.
Strongly habituating input is obtained from the antennal joints. This input causes orientation of the eye toward the direction in which the antenna points.
The same antennule movement in the vertical plane can result in either excitation or inhibition of the head-down fibre, suggesting the presence of two opposing inputs, presumably from the statocysts and basal joint receptors of the antennule.
The inputs on to the position-sensitive fibres which indicate body position are such as to stabilize the eye position in space during body movement. The optokinetic and antennal joint inputs are probably involved in tracking and antennal pointing reactions.
The eye-withdrawal fibre is stimulated by touch of the head and around the eye, but is inhibited by the excited state
The warp drive: hyper-fast travel within general relativity
It is shown how, within the framework of general relativity and without the
introduction of wormholes, it is possible to modify a spacetime in a way that
allows a spaceship to travel with an arbitrarily large speed. By a purely local
expansion of spacetime behind the spaceship and an opposite contraction in
front of it, motion faster than the speed of light as seen by observers outside
the disturbed region is possible. The resulting distortion is reminiscent of
the ``warp drive'' of science fiction. However, just as it happens with
wormholes, exotic matter will be needed in order to generate a distortion of
spacetime like the one discussed here.Comment: 10 pages, 1 figure. Not previously available in gr-q
A Study on Buckling Response of FML Members of ‘Classical’ Versus Thin-Ply Design
The paper is devoted to the buckling response and load carrying capacity of thin-walled open cross-section profiles made of hybrid, multi-layered materials; specifically Fiber Metal Laminates (FML) subjected to static compression loading. The objective of the study is a comparison of ‘classical’ Fibre Reinforced Plastic (FRP) layers versus thin-ply technology design. Analytical, finite element method and experimental investigations are performed to investigate different material combinations and sub-laminate tailoring strategies. These are compared with more common FML designs for thin-walled columns and rectangular plates
Excision boundary conditions for black hole initial data
We define and extensively test a set of boundary conditions that can be
applied at black hole excision surfaces when the Hamiltonian and momentum
constraints of general relativity are solved within the conformal thin-sandwich
formalism. These boundary conditions have been designed to result in black
holes that are in quasiequilibrium and are completely general in the sense that
they can be applied with any conformal three-geometry and slicing condition.
Furthermore, we show that they retain precisely the freedom to specify an
arbitrary spin on each black hole. Interestingly, we have been unable to find a
boundary condition on the lapse that can be derived from a quasiequilibrium
condition. Rather, we find evidence that the lapse boundary condition is part
of the initial temporal gauge choice. To test these boundary conditions, we
have extensively explored the case of a single black hole and the case of a
binary system of equal-mass black holes, including the computation of
quasi-circular orbits and the determination of the inner-most stable circular
orbit. Our tests show that the boundary conditions work well.Comment: 23 pages, 23 figures, revtex4, corrected typos, added reference,
minor content changes including additional post-Newtonian comparison. Version
accepted by PR
Local and global properties of conformally flat initial data for black hole collisions
We study physical properties of conformal initial value data for single and
binary black hole configurations obtained using conformal-imaging and
conformal-puncture methods. We investigate how the total mass M_tot of a
dataset with two black holes depends on the configuration of linear or angular
momentum and separation of the holes. The asymptotic behavior of M_tot with
increasing separation allows us to make conclusions about an unphysical
``junk'' gravitation field introduced in the solutions by the conformal
approaches. We also calculate the spatial distribution of scalar invariants of
the Riemann tensor which determine the gravitational tidal forces. For single
black hole configurations, these are compared to known analytical solutions.
Spatial distribution of the invariants allows us to make certain conclusions
about the local distribution of the additional field in the numerical datasets
Robust evolution system for Numerical Relativity
The paper combines theoretical and applied ideas which have been previously
considered separately into a single set of evolution equations for Numerical
Relativity. New numerical ingredients are presented which avoid gauge
pathologies and allow one to perform robust 3D calculations. The potential of
the resulting numerical code is demonstrated by using the Schwarzschild black
hole as a test-bed. Its evolution can be followed up to times greater than one
hundred black hole masses.Comment: 11 pages, 4 figures; figure correcte
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