5,306 research outputs found
Development of testing and analysis methodology to assess the long term durability of polymeric composites at high temperatures
A workshop was held to help assess the state-of-the-art in evaluating the long term durability of polymeric matrix composites (PMCs) and to recommend future activities. Design and evaluation of PMCs at elevated temperatures were discussed. The workshop presentations, the findings of the workshop sessions are briefly summarized
Stress-strain analysis of a (0/90)sub 2 symmetric titanium matrix laminate subjected to a generic hypersonic flight profile
Cross ply laminate behavior of Ti-15V-3Cr-3Al-3Sn (Ti-15-3) matrix reinforced with continuous silicon carbide fibers (SCS-6) subjected to a generic hypersonic flight profile was evaluated experimentally and analytically. Thermomechanical fatigue test techniques were developed to conduct a simulation of a generic hypersonic flight profile. A micromechanical analysis was used. The analysis predicts the stress-strain response of the laminate and of the constituents in each ply during thermal and mechanical cycling by using only constituent properties as input. The fiber was modeled using a thermo-viscoplastic constitutive relation. The fiber transverse modulus was reduced in the analysis to simulate the fiber matrix interface failure. Excellent correlation was found between measured and predicted laminate stress-strain response due to generic hypersonic flight profile when fiber debonding was modeled
The influence of microstructure on the tensile behavior of an aluminum metal matrix composite
The relationship between tensile properties and microstructure of a powder metallurgy aluminum alloy, 2009 was examined. The alloy was investigated both unreinforced and reinforced with 15 v/o SiC whiskers or 15 v/o SiC particulate to form a discontinuous metal matrix composite (MMC). The materials were investigated in the as-fabricated condition and in three different hot-rolled sheet thicknesses of 6.35, 3.18, and 1.8 mm. Image analysis was used to characterize the morphology of the reinforcements and their distributions within the matrix alloy. Fractographic examinations revealed that failure was associated with the presence of microstructural inhomogeneities which were related to both the matrix alloy and to the reinforcement. The results from these observations together with the matrix tensile data were used to predict the strengths and moduli of the MMC's using relatively simple models. The whisker MMC could be modeled as a short fiber composite and an attempt was made to model the particulate MMC as a dispersion/dislocation hardened alloy
Evaluation of several micromechanics models for discontinuously reinforced metal matrix composites
A systematic experimental evaluation of whisker and particulate reinforced aluminum matrix composites was conducted to assess the variation in tensile properties with reinforcement type, volume fraction, and specimen thickness. Each material was evaluated in three thicknesses, 1.8, 3.18, and 6.35 mm, to determine the size, distribution, and orientation of the reinforcements. This information was used to evaluate several micromechanical models that predict composite moduli. The longitudinal and transverse moduli were predicted for reinforced aluminum. The Paul model, the Cox model and the Halpin-Tsai model were evaluated. The Paul model gave a good upper bound prediction for the particulate reinforced composites but under predicted whisker reinforced composite moduli. The Cox model gave good moduli predictions for the whisker reinforcement, but was too low for the particulate. The Halpin-Tsai model gave good results for both whisker and particulate reinforced composites. An approach using a trigonometric projection of whisker length to predict the fiber contribution to the modulus in the longitudinal and transverse directions was compared to the more conventional lamination theory approach
The effectiveness of thin films in lieu of hyperbolic metamaterials in the near field
We show that the near-field functionality of hyperbolic metamaterials (HMM),
typically proposed for increasing the photonic local density of states (LDOS),
can be achieved with thin metal films. Although HMMs have an infinite density
of internally-propagating plane-wave states, the external coupling to nearby
emitters is severely restricted. We show analytically that properly designed
thin films, of thicknesses comparable to the metal size of a hyperbolic
metamaterial, yield a LDOS as high as (if not higher than) that of HMMs. We
illustrate these ideas by performing exact numerical computations of the LDOS
of multilayer HMMs, along with their application to the problem of maximizing
near-field heat transfer, to show that thin films are suitable replacements in
both cases.Comment: 5 pages, 3 figure
Radiative heat transfer in nonlinear Kerr media
We obtain a fluctuation--dissipation theorem describing thermal
electromagnetic fluctuation effects in nonlinear media that we exploit in
conjunction with a stochastic Langevin framework to study thermal radiation
from Kerr () photonic cavities coupled to external environments at
and out of equilibrium. We show that that in addition to thermal broadening due
to two-photon absorption,the emissivity of such cavities can exhibit
asymmetric,non-Lorentzian lineshapes due to self-phase modulation. When the
local temperature of the cavity is larger than that of the external bath, we
find that the heat transfer into the bath exceeds the radiation from a
corresponding linear black body at the same local temperature. We predict that
these temperature-tunable thermal processes can be observed in practical,
nanophotonic cavities operating at relatively small temperatures
Fluctuating surface-current formulation of radiative heat transfer: theory and applications
We describe a novel fluctuating-surface current formulation of radiative heat
transfer between bodies of arbitrary shape that exploits efficient and
sophisticated techniques from the surface-integral-equation formulation of
classical electromagnetic scattering. Unlike previous approaches to
non-equilibrium fluctuations that involve scattering matrices---relating
"incoming" and "outgoing" waves from each body---our approach is formulated in
terms of "unknown" surface currents, laying at the surfaces of the bodies, that
need not satisfy any wave equation. We show that our formulation can be applied
as a spectral method to obtain fast-converging semi-analytical formulas in
high-symmetry geometries using specialized spectral bases that conform to the
surfaces of the bodies (e.g. Fourier series for planar bodies or spherical
harmonics for spherical bodies), and can also be employed as a numerical method
by exploiting the generality of surface meshes/grids to obtain results in more
complicated geometries (e.g. interleaved bodies as well as bodies with sharp
corners). In particular, our formalism allows direct application of the
boundary-element method, a robust and powerful numerical implementation of the
surface-integral formulation of classical electromagnetism, which we use to
obtain results in new geometries, including the heat transfer between finite
slabs, cylinders, and cones
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
Growth Econometrics
This paper provides a survey and synthesis of econometric tools that have been employed to study economic growth. While these tools range across a variety of statistical methods, they are united in the common goals of first, identifying interesting contemporaneous patterns in growth data and second, drawing inferences on long-run economic outcomes from cross-section and temporal variation in growth. We describe the main stylized facts that have motivated the development of growth econometrics, the major statistical tools that have been employed to provide structural explanations for these facts, and the primary statistical issues that arise in the study of growth data. An important aspect of the survey is attention to the limits that exist in drawing conclusions from growth data, limits that reflect model uncertainty and the general weakness of available data relative to the sorts of questions for which they are employed.
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