10,867 research outputs found
Strength and toughness of monolithic and composite silicon nitrides
The strength and toughness of two composite and two monolithic silicon nitrides were measured from 25 to 1400 C. The monolithic and composite materials were made from similar starting powders. Both of the composite materials contained 30 vol percent silicon carbide whiskers. All measurements were made by four point flexure in surrounding air and humidity. The composite and monolithic materials exhibited similar fast fracture properties as a function of temperature
Fracture technology for brittle materials
Ceramics materials have the potential for use in high-temperature, fuel-efficient engines. However, because these materials are brittle, their fracture characteristics must be well documented prior to their application. Thus Lewis is working to understand the fracture and strength properties of brittle ceramic and ceramic matrix materials. An understanding of fracture properties aids both designers who are attempting to design high-temperature structures and materials scientists who seek to design more temperature-resistant materials. Both analytical and experimental approaches to fracture analysis are being taken. Methods for testing fracture toughness, crack growth resistance, and strength are being developed. The failure mechanisms at both room and elevated temperatures are also being investigated. Such investigations aid materials scientists in developing better high-temperature materials. Of concern is the anisotropy of ceramic materials and the experimental verification of ceramic design codes that will allow brittle material behavior to be accurately predicted at high temperature
A Model of an E-Learning Web Site for Teaching and Evaluating Online
This research is endeavoring to design an e-learning web site on the internet
having the course name as "Object Oriented Programming" (OOP) for the students
of level four at Computer Science Department (CSD). This course is to be taught
online (through web) and then a programme is to be designed to evaluate
students performance electronically while introducing a comparison between
online teaching , e-evaluation and traditional methods of evaluation. The
research seeks to lay out a futuristic perception that how the future online
teaching and e-electronic evaluation should be the matter which highlights the
importance of this research
Manipulating light at distance by a metasurface using momentum transformation
A momentum conservation approach is introduced to manipulate light at
distance using metasurfaces. Given a specified field existing on one side of
the metasurface and specified desired field transmitted from the opposite side,
a general momentum boundary condition is established, which determines the
amplitude, phase and polarization transformation to be induced by the
metasurface. This approach, named momentum transformation, enables a systematic
way to synthesize metasurfaces with complete control over the reflected and
transmitted fields. Several synthesis illustrative examples are provided: a
vortex hypergeometric-Gaussian beam and a "delayed-start" accelerated beam for
Fresnel region manipulation, and a pencil beam radiator and a holographic
repeater for Frauenhofer region manipulation
Onset of phase correlations in YBa2Cu3O{7-x} as determined from reversible magnetization measurements
Isofield magnetization curves are obtained and analyzed for three single
crystals of YBa2Cu3O{7-x}, ranging from optimally doped to very underdoped, as
well as the BCS superconductor Nb, in the presence of magnetic fields applied
both parallel and perpendicular to the planes. Near Tc, the magnetization
exhibits a temperature dependence \sqrt{M} [Ta(H)-T]^m. In accordance with
recent theories, we associated Ta(H) with the onset of coherent phase
fluctuations of the superconducting order parameter. For Nb and optimally doped
YBaCuO, Ta(H) is essentially identical to the mean-field transition line Tc(H).
The fitting exponent m=0.5 takes its mean-field value for Nb, and varies just
slightly from 0.5 for optimally doped YBaCuO. However, underdoped YBCO samples
exhibit anomalous behavior, with Ta(H)>Tc for H applied parallel to the c axis,
suggesting that the magnetization is probing a region of temperatures above Tc
where phase correlations persist. In this region, the fitting exponent falls in
the range 0.5 < m < 0.8 for H\parallel c, compared with m~0. for $H\parallel ab
planes. The results are interpreted in terms of an anisotropic pairing symmetry
of the order parameter: d-wave along the ab planes and s-wave along the c axis.Comment: 5 pages, 4 figure
A Robust Solution Procedure for Hyperelastic Solids with Large Boundary Deformation
Compressible Mooney-Rivlin theory has been used to model hyperelastic solids,
such as rubber and porous polymers, and more recently for the modeling of soft
tissues for biomedical tissues, undergoing large elastic deformations. We
propose a solution procedure for Lagrangian finite element discretization of a
static nonlinear compressible Mooney-Rivlin hyperelastic solid. We consider the
case in which the boundary condition is a large prescribed deformation, so that
mesh tangling becomes an obstacle for straightforward algorithms. Our solution
procedure involves a largely geometric procedure to untangle the mesh: solution
of a sequence of linear systems to obtain initial guesses for interior nodal
positions for which no element is inverted. After the mesh is untangled, we
take Newton iterations to converge to a mechanical equilibrium. The Newton
iterations are safeguarded by a line search similar to one used in
optimization. Our computational results indicate that the algorithm is up to 70
times faster than a straightforward Newton continuation procedure and is also
more robust (i.e., able to tolerate much larger deformations). For a few
extremely large deformations, the deformed mesh could only be computed through
the use of an expensive Newton continuation method while using a tight
convergence tolerance and taking very small steps.Comment: Revision of earlier version of paper. Submitted for publication in
Engineering with Computers on 9 September 2010. Accepted for publication on
20 May 2011. Published online 11 June 2011. The final publication is
available at http://www.springerlink.co
Landau and Ott scaling for the kinetic energy density and the low conventional superconductors, and Nb
The scaling approach recently proposed by Landau and Ott for isothermal
magnetization curves is extended to the average kinetic energy density of the
condensate. Two low superconductors, Nb and are studied
and their isothermal reversible magnetization shown to display Landau and Ott
scaling. Good agreement is obtained for the upper critical field ,
determined from the Abrikosov approximation for the reversible region (standard
linear extrapolation of the magnetization curve), and from the maximum of the
kinetic energy curves. For the full range of data, which includes the
irreversible region, the isothermal curves for show an
impressive collapse into a single curve over the entire range of field
measurements. The Nb isothermal curves exhibit the interesting
feature of a constant and temperature independent minimum value
A signature of anisotropic bubble collisions
Our universe may have formed via bubble nucleation in an eternally-inflating
background. Furthermore, the background may have a compact dimension---the
modulus of which tunnels out of a metastable minimum during bubble
nucleation---which subsequently grows to become one of our three large spatial
dimensions. When in this scenario our bubble universe collides with other ones
like it, the collision geometry is constrained by the reduced symmetry of the
tunneling instanton. While the regions affected by such bubble collisions still
appear (to leading order) as disks in an observer's sky, the centers of these
disks all lie on a single great circle, providing a distinct signature of
anisotropic bubble nucleation.Comment: 10 pages, 5 figures; v2: crucial error corrected, conclusions revise
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