345 research outputs found
Nonlinear pre-stress for cloaking from antiplane elastic waves
A theory is presented showing that cloaking of objects from antiplane elastic
waves can be achieved by elastic pre-stress of a neo-Hookean nonlinear elastic
material. This approach would appear to eliminate the requirement of
metamaterials with inhomogeneous anisotropic shear moduli and density. Waves in
the pre-stressed medium are bent around the cloaked region by inducing
inhomogeneous stress fields via pre-stress. The equation governing antiplane
waves in the pre-stressed medium is equivalent to the antiplane equation in an
unstressed medium with inhomogeneous and anisotropic shear modulus and
isotropic scalar mass density. Note however that these properties are induced
naturally by the pre-stress. Since the magnitude of pre-stress can be altered
at will, this enables objects of varying size and shape to be cloaked by
placing them inside the fluid-filled deformed cavity region.Comment: 21 pages, 4 figure
The Hill and Eshelby tensors for ellipsoidal inhomogeneities in the Newtonian potential problem and linear elastostatics
In 1957 Eshelby showed that a homogeneous isotropic ellipsoidal inhomogeneity
embedded in a homogeneous isotropic host would feel uniform strains and
stresses when uniform strains or stresses are applied in the far-field. Of
specific importance is the uniformity of Eshelby's tensor S. Following this
paper a vast literature has been generated using and developing Eshelby's
result and ideas, leading to some beautiful mathematics and extremely useful
results in a wide range of application areas. In 1961 Eshelby conjectured that
for anisotropic materials only ellipsoidal inhomogeneities would lead to such
uniform interior fields. Although much progress has been made since then, the
quest to prove this conjecture is still not complete; numerous important
problems remain open. Following a different approach to that considered by
Eshelby, a closely related tensor P=S D^0 arises, where D^0 is the host medium
compliance tensor. The tensor P is associated with Hill and is of course also
uniform when ellipsoidal inhomogeneities are embedded in a homogeneous host
phase. Two of the most fundamental and useful areas of applications of these
tensors are in Newtonian potential problems such as heat conduction,
electrostatics, etc. and in the vector problems of elastostatics.
Micromechanical methods established mainly over the last half-century have
enabled bounds on and predictions of the effective properties of composite
media. In many cases such predictions can be explicitly written down in terms
of the Hill, or equivalently the Eshelby tensor and can be shown to provide
excellent predictions in many cases. Here this classical problem is revisited
and a large number of results for problems that are felt to be of great utility
in a wide range of disciplines are derived or recalled
Band Gap Formation and Tunability in Stretchable Serpentine Interconnects
Serpentine interconnects are highly stretchable and frequently used in
flexible electronic systems. In this work, we show that the undulating geometry
of the serpentine interconnects will generate phononic band gaps to manipulate
elastic wave propagation. The interesting effect of `bands-sticking-together'
is observed. We further illustrate that the band structures of the serpentine
interconnects can be tuned by applying pre-stretch deformation. The discovery
offers a way to design stretchable and tunable phononic crystals by using
metallic interconnects instead of the conventional design with soft rubbers and
unfavorable damping.Comment: 12 pages, 8 figure
Hyperelastic antiplane ground cloaking
Hyperelastic materials possess the appealing property that they may be
employed as elastic wave manipulation devices and cloaks by imposing
pre-deformation. They provide an alternative to microstructured metamaterials
and can be used in a reconfigurable manner. Previous studies indicate that
exact elastodynamic invariance to pre-deformation holds only for neo-Hookean
solids in the antiplane wave scenario and the semi-linear material in the
in-plane compressional/shear wave context. Furthermore, although ground cloaks
have been considered in the acoustic context they have not yet been discussed
for elastodynamics, either by employing microstructured cloaks or hyperelastic
cloaks. This work therefore aims at exploring the possibility of employing a
range of hyperelastic materials for use as antiplane ground cloaks (AGCs). The
use of the popular incompressible Arruda-Boyce and Mooney-Rivlin nonlinear
materials is explored. The scattering problem associated with the AGC is
simulated via finite element analysis where the cloaked region is formed by an
indentation of the surface. Results demonstrate that the neo-Hookean medium can
be used to generate a perfect hyperelastic AGC as should be expected.
Furthermore, although the AGC performance of the Mooney-Rivlin material is not
particularly satisfactory, it is shown that the Arruda-Boyce medium is an
excellent candidate material for this purpose
Acoustic scattering from a strained region
A composite material consists of a rubber filled with gas-filled microspheres. In underwater applications it is compressed hydrostatically by a pressure that may be not insignificant compared with the shear modulus of the rubber, so large strains are produced around each spherical inclusion. When these spherical inclusions scatter an incident acoustic wave, the strained region around an inclusion has had its elastic properties altered by the large static strain. Thales Underwater Systems asked the Study Group to address the question of how this strained region affects the elastic scattering, bearing in mind that the dynamic shear modulus differs from its static value
Loss Compensation in Time-Dependent Elastic Metamaterials
Materials with properties that are modulated in time are known to display
wave phenomena showing energy increasing with time, with the rate mediated by
the modulation. Until now there has been no accounting for material
dissipation, which clearly counteracts energy growth. This paper provides an
exact expression for the amplitude of elastic or acoustic waves propagating in
lossy materials with properties that are periodically modulated in time. It is
found that these materials can support a special propagation regime in which
waves travel at constant amplitude, with temporal modulation compensating for
the normal energy dissipation. We derive a general condition under which
amplification due to time-dependent properties offsets the material
dissipation. This identity relates band-gap properties associated with the
temporal modulation and the average of the viscosity coefficient, thereby
providing a simple recipe for the design of loss-compensated mechanical
metamaterials
Image of the Teacher in Educational Leadership
Is the early childhood teacher a babysitter and the banker of knowledge lling empty-headed children with meaningless information? Or, is the teacher of young children a facilitator, guide and co-learner at the round-table of learning with competent children and educational community participants? Ex- amining teaching and learning, organizational leadership, politics and policy, and educational research, this paper explores the image of the teacher as a theoretical discussion in educational leadership with a fundamental question: How can teaching and learning, organizational leadership, politics and policy, and educational research inform and bolster the image of the early childhood teacher? Solutions are suggested to combat teacher image problems which include utilizing a constructivist paradigm; engaging parents, teachers, sta , and community to collaborate on the subject of teacher image and classroom learning practices; adopting the teacher, parent and child\u27s voice in the political arena; and, supporting teachers as researchers through documentation, collaboration, re ection and narration
Studies on the Scottish Mountain Blackfaced sheep: with special reference to the inheritance of certain breed characteristics
GENERAI,:
Available evidence seems to show that the
Scottish Blackface breed is not an indigenous breed
of sheep in Scotland. It has been introduced from
the south and has spread northwards. The breed has
never become successfully established in Caithness
and Sutherland: otherwise it has become the most
numerous and important breed in the remaining mountain-
ous parts of the country. It is probable that its
characters have been influenced by crossing with the
native "dun" - or with the. "white-faced" sheep and the
infusion of this and possibly other blood may explain
in part the differing characters in various districts.
FLEECE CHARACTERS:
The character of the fleece varies in diftferent
districts in Scotland: so much so that definite
areas are associated with the production of various
grades of Blackface wool required for tweeds, carpets
mattresses and so on. The reasons for the difference
between the fleeces of these districts are not obvious
it is suggested however that there may be some areas
where a larger amount of unimproved blood enters into
the compositon of the sheep stocks than elsewhere
and that parasitic infestation and variation in the
mineral and other content of the pasture may collectively
or individually play a large part in this
differentiation.
Climatic conditions per se do not appear
to exert that considerable influence on fleece which
is usually attributed to them.
CROSSING:
The results of crossing with modern breeds
of sheep has been considered and it seems evident
that some of the characters are sex -linked, (for
example the fleece characters of the Blackface x
Gritstone.) Of the other crosses that with the
Merino appears to be of the greatest interest, since
increased density of fleece could still be demonstrated
in the fourth generation of back- crossing to the
Blackface. In practically all crosses rams of other
breeds have been crossed with Blackface ewes.
Crossing with Border Leicesters and Wensleydale are
important commercial crosses, producing sheep, specially
useful for the butcher.
FACE COLOUR:
The relationship between face-colour and
various other factors has been considered, mainly
from data accumulated on the writer's own farm in
Perthshire during the past ten years. These results
indicate that breeders can, by selection of their
stock, with due regard to face colour, improve commercially
important points.
Sufficient data has been collected to show
that by selection of the parents for face colours, a
considerable proportion of the progeny will have those
colours which this work has shown to be desirable, i.e.
the extremes of face colour both black and white in
ewes have been shown to be associated with lambs of
inferior size and dead weights. That the face colour
of the sire is important because it is shown that
different colours are c-o,rrelat.ed with different decrees
of hardiness in his lambs. It has been demonstrated
that blackspotting in the fleece can be materially
improved by using sires free from this fault and that
black spotting and face colour are correlated.
FERTILITY.
It is shown that under a constant environment
bath high and low fertility is transmitted from
the sire to his daughters and is characteristically
exhibited by thee: subsequently. It has not however
been possible to test whether females may transmit
fertility factors to their sons, and only very limited
figures are given to show the influence of rams on
their grand - daughters and on their sons, since only
a few home -bred rams were used. Evidence is advanced
which suggests potential economic importance of the
influence of rams on the fertility of a Blackfaced
breeding stock; for although the average for barrenness
in this flock was 14%, one ram left daughters who were
never barren, another ram's daughters only had a figure
of 5% for barrenness, but four rams left daughters
whose barrenness figures were 22 %, 23%, 24% and 29 %.
FLOCK RECORDING:
It is very evident that much valuable information
on matters intimately concerned with breeding
and inheritance under practical conditions is lost
through the absence of a general system of marking and
flock recording. The experience of marking and recording
on which this work is based shows that the advantages
gained compensate for the expenditure of such time
and money as is involved
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