5,781 research outputs found
Physical aging and solvent effects on the fracture of LaRC-TPI adhesives
When amorphous materials are quenched below their glass transition temperature, excess enthalpy is trapped in the glassy material because the viscosity is too great to allow the material to remain in volumetric equilibrium. Over time, this excess free volume is reduced as the material slowly approaches its equilibrium configuration. This process, known as physical aging, leads to substantial changes in the constitutive behavior of polymers, as has been widely discussed in the literature. Less is known about the effects of this physical aging process on fracture and fatigue properties of aged materials. The original goal of the summer was to investigate the effects of physical aging on the fracture and fatigue behavior of LaRC-TPI, a thermoplastic polyimide developed at NASA-Langley. Preliminary results are reported, although a lack of equipment availability prevented completion of this task. In the process of making specimens, the current LaRC-TPI was observed to be extremely susceptible to environmental stress cracking. A study of the unique failure patterns resulting from this degradation process in bonded joints was conducted and is also reported herein
Probing the Enhancon via Calculations in Supersymmetric Gauge Theory
We consider the N=2 gauge theory on N D7-branes wrapping K3, with D3-brane
probes. In the large N limit, the D7-branes blow up to form an enhancon shell.
We probe the region inside and outside the enhancon shell using the D3-branes,
and compute the probe metric using the Seiberg-Witten formalism. Supergravity
arguments suggest a flat interior up to 1/N corrections, and indeed our results
for the D3-brane probes are consistent with that. By including the dynamics of
the branes, these results, together with those of hep-th/0204050, demonstrate
the robustness of the enhancon mechanism beyond patching together of
supergravity solutions with D-brane source junction conditions.Comment: 20 pages, 2 figures, minor correction
STRUCTURAL CHANGE IN AN ERA OF INCREASED OPENNESS: A BACKGROUND PAPER ON THE STRUCTURE OF U.S. AGRICULTURE
Agribusiness,
Degenerate four-wave mixing in triply-resonant Kerr cavities
We demonstrate theoretical conditions for highly-efficient degenerate
four-wave mixing in triply-resonant nonlinear (Kerr) cavities. We employ a
general and accurate temporal coupled-mode analysis in which the interaction of
light in arbitrary microcavities is expressed in terms a set of coupling
coefficients that we rigorously derive from the full Maxwell equations. Using
the coupled-mode theory, we show that light consisting of an input signal of
frequency can, in the presence of pump light at
, be converted with quantum-limited efficiency into an output shifted
signal of frequency , and we derive expressions for
the critical input powers at which this occurs. We find that critical powers in
the order of 10mW assuming very conservative cavity parameters (modal volumes
cubic wavelengths and quality factors . The standard
Manley-Rowe efficiency limits are obtained from the solution of the classical
coupled-mode equations, although we also derive them from simple
photon-counting "quantum" arguments. Finally, using a linear stability
analysis, we demonstrate that maximal conversion efficiency can be retained
even in the presence of self- and cross-phase modulation effects that generally
act to disrupt the resonance condition.Comment: 13 pages, 8 figures. To appear in Physical Review
Water-tunnel study results of a TF/A-18 and F/A-18 canopy flow visualization
A water tunnel study examining the influence of canopy shape on canopy and leading edge extension flow patterns was initiated. The F/A-18 single-place canopy model and the TF/A-18 two place canopy model were the study subjects. Plan view and side view photographs showing the flow patterns created by injected colored dye are presented for 0 deg and 5 deg sideslip angles. Photographs taken at angle of attack and sideslip conditions correspond to test departure points found in flight test. Flight experience has shown that the TF/A-18 airplane departs in regions where the F/A-18 airplane is departure-resistant. The study results provide insight into the differences in flow patterns which may influence the resulting aerodynamics of the TF/A-18 and F/A-18 aircraft. It was found that at 0 deg sideslip, the TF/A-18 model has more downward flow on the sides of the canopy than the F/A-18 model. This could be indicative of flow from the leading edge extension (LEX) vortexes impinging on the sides of the wider TF/A-18 canopy. In addition, the TF/A-18 model has larger areas of asymmetric separated and unsteady flow on the LEXs and fuselage, possibly indicating a lateral and directional destabilizing effect at the conditions studied
Achieving a Strongly Temperature-Dependent Casimir Effect
We propose a method of achieving large temperature sensitivity in the Casimir
force that involves measuring the stable separation between dielectric objects
immersed in fluid. We study the Casimir force between slabs and spheres using
realistic material models, and find large > 2nm/K variations in their stable
separations (hundreds of nanometers) near room temperature. In addition, we
analyze the effects of Brownian motion on suspended objects, and show that the
average separation is also sensitive to changes in temperature . Finally, this
approach also leads to rich qualitative phenomena, such as irreversible
transitions, from suspension to stiction, as the temperature is varied
Classical and fluctuation-induced electromagnetic interactions in micronscale systems: designer bonding, antibonding, and Casimir forces
Whether intentionally introduced to exert control over particles and
macroscopic objects, such as for trapping or cooling, or whether arising from
the quantum and thermal fluctuations of charges in otherwise neutral bodies,
leading to unwanted stiction between nearby mechanical parts, electromagnetic
interactions play a fundamental role in many naturally occurring processes and
technologies. In this review, we survey recent progress in the understanding
and experimental observation of optomechanical and quantum-fluctuation forces.
Although both of these effects arise from exchange of electromagnetic momentum,
their dramatically different origins, involving either real or virtual photons,
lead to different physical manifestations and design principles. Specifically,
we describe recent predictions and measurements of attractive and repulsive
optomechanical forces, based on the bonding and antibonding interactions of
evanescent waves, as well as predictions of modified and even repulsive Casimir
forces between nanostructured bodies. Finally, we discuss the potential impact
and interplay of these forces in emerging experimental regimes of
micromechanical devices.Comment: Review to appear on the topical issue "Quantum and Hybrid Mechanical
Systems" in Annalen der Physi
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
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