59,558 research outputs found
The assessment of metal fiber reinforced polymeric composites
Because of their low cost, excellent electrical conductivity, high specific strength (strength/density), and high specific modulus (modulus/density) short metal fiber reinforced composites have enjoyed a widespread use in many critical applications such as automotive industry, aircraft manufacturing, national defense, and space technology. However, little data has been found in the study of short metal fibrous composites. Optimum fiber concentration in a resin matrix and fiber aspect ratio (length-to-diameter ratio) are often not available to a user. Stress concentration at short fiber ends is the other concern when the composite is applied to a load-bearing application. Fracture in such composites where the damage will be initiated or accumulated is usually difficult to be determined. An experimental investigation is therefore carefully designed and undertaken to systematically evaluate the mechanical properties as well as electrical properties. Inconel 601 (nickel based) metal fiber with a diameter of eight microns is used to reinforce commercially available thermoset polyester resin. Mechanical testing such as tensile, impact, and flexure tests along with electrical conductivity measurements is conducted to study the feasibility of using such composites. The advantages and limitations of applying chopped metal fiber reinforced polymeric composites are also discussed
Improved Lignin Polyurethane Properties with Lewis Acid Treatment
Chemical modification strategies to improve the mechanical properties of lignin-based polyurethanes are presented. We hypothesized that treatment of lignin with Lewis acids would increase the concentration of hydroxyl groups
available to react with diisocyanate monomers. Under the conditions used, hydrogen bromide-catalyzed modification resulted in a 28% increase in hydroxyl group content. Associated increases in hydrophilicity of solvent-cast thin films were also recorded as evidenced by decreases in water contact angle. Polyurethanes were then prepared by first preparing a prepolymer based on mixtures of toluene-2,4-diisocyanate (TDI) and unmodified or modified lignin, then polymerization was completed through addition of polyethylene glycol (PEG), resulting in mass ratios of TDI:lignin:PEG of 43:17:40 in the compositions investigated
here. The mixture of TDI and unmodified lignin resulted in a lignin powder at the bottom of the liquid, suggesting it did not react directly with TDI. However, a homogeneous solution resulted when TDI and the hydrogen bromide-treated lignin were mixed, suggesting demethylation indeed increased reactivity and resulted in better integration of lignin into the urethane network. Significant improvements in mechanical properties of modified lignin polyurethanes were observed, with a 6.5-fold increase in modulus, which were attributed to better integration of the modified lignin into the covalent polymer network due to
the higher concentration of hydroxyl groups. This research indicates that chemical modification strategies can lead to significant improvements in the properties of lignin-based polymeric materials using a higher fraction of an inexpensive lignin monomer from renewable resources and a lower fraction an expensive, petroleum-derived isocyanate monomer to achieve the required material properties
Constraints on the Inner Mass Profiles of Lensing Galaxies from Missing Odd Images
Most gravitational lens systems consist of two or four observable images. The
absence of detectable odd images allows us to place a lower limit on the
power-law slope of the inner mass profile of lensing galaxies. Using a sample
of six two-image radio-loud lens systems and assuming a singular power-law
surface density (Sigma proportional to r^{-beta}) for the inner several kpc of
the mass distribution, we find that there is less than a 10% probability that
the data are consistent with profile slopes beta < 0.80. Furthermore,
individual mass modeling yields beta > 0.85 for B0739+366 and beta > 0.91 for
B1030+074. Modeling central black holes as additional point masses changes the
constraints in these systems to beta > 0.84 and beta > 0.83, respectively. The
inner mass profiles of lensing galaxies are therefore not much shallower than
isothermal.Comment: Final published version, minor typos corrected, 13 page
Modal analysis using a Fourier analyzer, curve-fitting, and modal tuning
The proposed modal test program differs from single-input methods in that preliminary data may be acquired using multiple inputs, and modal tuning procedures may be employed to define closely spaced frquency modes more accurately or to make use of frequency response functions (FRF's) which are based on several input locations. In some respects the proposed modal test proram resembles earlier sine-sweep and sine-dwell testing in that broadband FRF's are acquired using several input locations, and tuning is employed to refine the modal parameter estimates. The major tasks performed in the proposed modal test program are outlined. Data acquisition and FFT processing, curve fitting, and modal tuning phases are described and examples are given to illustrate and evaluate them
Ultra-High Energy Cosmic Rays: The Annihilation of Super-Heavy Relics
We investigate the possibility that ultra-high energy cosmic rays (UHECRs)
originate from the annihilation of relic superheavy (SH) dark matter in the
Galactic halo. In order to fit the data on UHECRs, a cross section of
is
required if the SH dark matter follows a Navarro--Frenk--White (NFW) density
profile. This would require extremely large- contributions to the
annihilation cross section. An interesting finding of our calculation is that
the annihilation in sub-galactic clumps of dark matter dominates over the
annihilations in the smooth dark matter halo, thus implying much smaller values
of the cross section needed to explain the observed fluxes of UHECRs.Comment: To appear in the proceedings of the TAUP conference, September 8-12,
2001 - Gran Sasso Laboratory, Ital
Crossing Patterns in Nonplanar Road Networks
We define the crossing graph of a given embedded graph (such as a road
network) to be a graph with a vertex for each edge of the embedding, with two
crossing graph vertices adjacent when the corresponding two edges of the
embedding cross each other. In this paper, we study the sparsity properties of
crossing graphs of real-world road networks. We show that, in large road
networks (the Urban Road Network Dataset), the crossing graphs have connected
components that are primarily trees, and that the remaining non-tree components
are typically sparse (technically, that they have bounded degeneracy). We prove
theoretically that when an embedded graph has a sparse crossing graph, it has
other desirable properties that lead to fast algorithms for shortest paths and
other algorithms important in geographic information systems. Notably, these
graphs have polynomial expansion, meaning that they and all their subgraphs
have small separators.Comment: 9 pages, 4 figures. To appear at the 25th ACM SIGSPATIAL
International Conference on Advances in Geographic Information Systems(ACM
SIGSPATIAL 2017
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