298 research outputs found
Effect of Prior Exposure at Elevated Temperatures on Tensile Properties and Stress-Strain Behavior of Three Oxide/Oxide Ceramic Matrix Composites
Thermal stability of three oxide-oxide ceramic matrix composites was studied. The materials studied were NextelTM610/aluminosilicate (N610/AS), NextelTM720/aluminosilicate (N720/AS), and NextelTM720/Alumina (N720/A), commercially available oxide-oxide ceramic composites (COI Ceramics, San Diego, CA). The N610/AS composite consists of a porous aluminosilicate matrix reinforced with laminated woven alumina N610 fibers. The N720/AS and N720/A composites consist of a porous oxide matrix reinforced with laminated, woven mullite/alumina (NextelTM720) fibers. The matrix materials are aluminosilicate in N720/AS and alumina in N720/A. All three composites have no interface between the fibers and matrix, and rely on the porous matrix for flaw tolerance. The N610/AS and N720/AS CMCs were heat treated in laboratory air for 100 h at 1100°C and for 10, 20, 40 and 100 h at 1200°C. The N720/A CMC was heat treated in laboratory air for 100 h at 1200°C and for 10, 20, 40 and 100 h at 1300°C. The room-temperature tensile properties of all composites were measured after each type of heat treatment. Effects of prior heat treatment on tensile strength were evaluated. Heat treatment at 1100°C had little effect on tensile strength of the N610/AS and N720/AS composites, while heat treatment at 1200°C caused dramatic loss of tensile strength. Poor strength retention after heat treatment at 1200°C is attributed to degradation of the aluminosilicate matrix. The N720/A composite exhibited excellent thermal stability, retaining about 90% of its tensile strength after heat treatment at 1300°C. Results indicate that the aluminosilicate matrix is considerably more susceptible to localized densification and coarsening of the porosity than the alumina matrix
Einstein Supergravity and New Twistor String Theories
A family of new twistor string theories is constructed and shown to be free
from world-sheet anomalies. The spectra in space-time are calculated and shown
to give Einstein supergravities with second order field equations instead of
the higher derivative conformal supergravities that arose from earlier twistor
strings. The theories include one with the spectrum of N=8 supergravity,
another with the spectrum of N=4 supergravity coupled to N=4 super-Yang-Mills,
and a family with supersymmetries with the spectra of self-dual
supergravity coupled to self-dual super-Yang-Mills. The non-supersymmetric
string with N=0 gives self-dual gravity coupled to self-dual Yang-Mills and a
scalar. A three-graviton amplitude is calculated for the N=8 and N=4 theories
and shown to give a result consistent with the cubic interaction of Einstein
supergravity.Comment: LaTeX, 69 pages, no figures; v2: minor corrections made, footnotes
and references adde
A new maximally supersymmetric background of IIB superstring theory
We present a maximally supersymmetric IIB string background. The geometry is
that of a conformally flat lorentzian symmetric space G/K with solvable G, with
a homogeneous five-form flux. We give the explicit supergravity solution,
compute the isometries, the 32 Killing spinors, and the symmetry superalgebra,
and then discuss T-duality and the relation to M-theory.Comment: 17 page
Flux Compactifications of M-Theory on Twisted Tori
We find the bosonic sector of the gauged supergravities that are obtained
from 11-dimensional supergravity by Scherk-Schwarz dimensional reduction with
flux to any dimension D. We show that, if certain obstructions are absent, the
Scherk-Schwarz ansatz for a finite set of D-dimensional fields can be extended
to a full compactification of M-theory, including an infinite tower of
Kaluza-Klein fields. The internal space is obtained from a group manifold
(which may be non-compact) by a discrete identification. We discuss the
symmetry algebra and the symmetry breaking patterns and illustrate these with
particular examples. We discuss the action of U-duality on these theories in
terms of symmetries of the D-dimensional supergravity, and argue that in
general it will take geometric flux compactifications to M-theory on
non-geometric backgrounds, such as U-folds with U-duality transition functions.Comment: Latex, 47 page
Conformal topological Yang-Mills theory and de Sitter holography
A new topological conformal field theory in four Euclidean dimensions is
constructed from N=4 super Yang-Mills theory by twisting the whole of the
conformal group with the whole of the R-symmetry group, resulting in a theory
that is conformally invariant and has two conformally invariant BRST operators.
A curved space generalisation is found on any Riemannian 4-fold. This
formulation has local Weyl invariance and two Weyl-invariant BRST symmetries,
with an action and energy-momentum tensor that are BRST-exact. This theory is
expected to have a holographic dual in 5-dimensional de Sitter space.Comment: 34 pages, AMSTex, Reference adde
Generalised Geometry for M-Theory
Generalised geometry studies structures on a d-dimensional manifold with a
metric and 2-form gauge field on which there is a natural action of the group
SO(d,d). This is generalised to d-dimensional manifolds with a metric and
3-form gauge field on which there is a natural action of the group .
This provides a framework for the discussion of M-theory solutions with flux. A
different generalisation is to d-dimensional manifolds with a metric, 2-form
gauge field and a set of p-forms for either odd or even on which there is a
natural action of the group . This is useful for type IIA or IIB
string solutions with flux. Further generalisations give extended tangent
bundles and extended spin bundles relevant for non-geometric backgrounds.
Special structures that arise for supersymmetric backgrounds are discussed.Comment: 31 page
Job creation in small and medium sized enterprises: Federal Republic of Germany, France, Netherlands, Belgium, Luxembourg. Vol. II: Main report. Programme of research and actions on development of the labour market
Global Aspects of T-Duality, Gauged Sigma Models and T-Folds
The gauged sigma-model argument that string backgrounds related by T-dual
give equivalent quantum theories is revisited, taking careful account of global
considerations. The topological obstructions to gauging sigma-models give rise
to obstructions to T-duality, but these are milder than those for gauging: it
is possible to T-dualise a large class of sigma-models that cannot be gauged.
For backgrounds that are torus fibrations, it is expected that T-duality can be
applied fibrewise in the general case in which there are no globally-defined
Killing vector fields, so that there is no isometry symmetry that can be
gauged; the derivation of T-duality is extended to this case. The T-duality
transformations are presented in terms of globally-defined quantities. The
generalisation to non-geometric string backgrounds is discussed, the conditions
for the T-dual background to be geometric found and the topology of T-folds
analysed.Comment: Minor corrections and addition
Compactifications with S-Duality Twists
We consider generalised Scherk Schwarz reductions of supergravity and
superstring theories with twists by electromagnetic dualities that are
symmetries of the equations of motion but not of the action, such as the
S-duality of D=4, N=4 super-Yang-Mills coupled to supergravity. The reduction
cannot be done on the action itself, but must be done either on the field
equations or on a duality invariant form of the action, such as one in the
doubled formalism in which potentials are introduced for both electric and
magnetic fields. The resulting theory in odd-dimensions has massive form fields
satisfying a self-duality condition . We construct such theories
in D=3,5,7.Comment: Latex, 26 pages. References adde
Geometric Second Order Field Equations for General Tensor Gauge Fields
Higher spin tensor gauge fields have natural gauge-invariant field equations
written in terms of generalised curvatures, but these are typically of higher
than second order in derivatives. We construct geometric second order field
equations and actions for general higher spin boson fields, and first order
ones for fermions, which are non-local but which become local on gauge-fixing,
or on introducing auxiliary fields. This generalises the results of Francia and
Sagnotti to all representations of the Lorentz group.Comment: 34 pages, LaTeX. Reference adde
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