3,324 research outputs found
Attenuation of stress waves in single and multi-layered structures
Analytical and experimental studies were made of the attenuation of the stress waves during passage through single and multilayer structures. The investigation included studies on elastic and plastic stress wave propagation in the composites and those on shock mitigating material characteristics such as dynamic stress-strain relations and energy absorbing properties. The results of the studies are applied to methods for reducing the stresses imposed on a spacecraft during planetary or ocean landings
Optimum design of structures of composite materials in response to aerodynamic noise and noise transmission
Elastic wave propagation and attenuation in a model fiber matrix was investigated. Damping characteristics in graphite epoxy composite materials were measured. A sound transmission test facility suitable to incorporate into NASA Ames wind tunnel for measurement of transmission loss due to sound generation in boundary layers was constructed. Measurement of transmission loss of graphite epoxy composite panels was also included
A Multivariate GARCH Model with Time-Varying correlations
In this paper we propose a new multivariate GARCH model with time- varying correlations. We adopt the vech representation based on the conditional variances and the conditional correlations. While each conditional-variance term is assumed to follow a univariate GARCH formulation, the conditional-correlation matrix is postulated to follow an autoregressive moving average type of analogue. By imposing some suitable restrictions on the conditional-correlation-matrix equation, we construct a MGARCH model in which the conditional-correlation matrix is guaranteed to be positive definite during the optimisation. Thus, our new model retains the intuition and interpretation of the univariate GARCH model and yet satisfies the positive-definite condition as found in the constant-correlation and BEKK models. We report some Monte Carlo results on the finite-sample distributions of the MLE of the varying- correlation MGARCH model. The new model is applied to some real data sets. It is found that extending the constant-correlation model to allow for time-varying correlations provides some interesting time histories that are not available in a constant-correlation model.BEKK model, constant correlation, Monte Carlo method, multivariate GARCH model, maximum likelihood estimate, varying correlation
Juncture stress fields in multicellular shell structures. Volume IV - Stresses and deformations of fixed-edge segmental spherical shells Final report
Equations for thin elastic spherical shells and digital program for analysis of stresses and deformation of fixed edge segmental spherical shells - solution by finite difference techniqu
Magnetically-induced reconstructions of the ground state in a few-electron Si quantum dot
We report unexpected fluctuations in the positions of Coulomb blockade peaks
at high magnetic fields in a small Si quantum dot. The fluctuations have a
distinctive saw-tooth pattern: as a function of magnetic field, linear shifts
of peak positions are compensated by abrupt jumps in the opposite direction.
The linear shifts have large slopes, suggesting formation of the ground state
with a non-zero angular momentum. The value of the momentum is found to be well
defined, despite the absence of the rotational symmetry in the dot.Comment: 5 pages, 4 figures, accepted to PR
Double-dot charge transport in Si single electron/hole transistors
We studied transport through ultra-small Si quantum dot transistors
fabricated from silicon-on-insulator wafers. At high temperatures, 4K<T<100K,
the devices show single-electron or single-hole transport through the
lithographically defined dot. At T<4K, current through the devices is
characterized by multidot transport. From the analysis of the transport in
samples with double-dot characteristics, we conclude that extra dots are formed
inside the thermally grown gate oxide which surrounds the lithographically
defined dot.Comment: 4 pages, 5 figures, to appear in Appl. Phys. Let
Plasma-Like Negative Capacitance in Nano-Colloids
A negative capacitance has been observed in a nano-colloid between 0.1 and
10^-5 Hz. The response is linear over a broad range of conditions. The
low-omega dispersions of both the resistance and capacitance are consistent
with the free-carrier plasma model, while the transient behavior demonstrates
an unusual energy storage mechanism. A collective excitation, therefore, is
suggested.Comment: 3 pages, 3 figure
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