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Quasi-static uniaxial tension and compression, three-point bending, and Hopkinson bar measurements have been carried out on low-porosity Al/Ni compacts (pressed powders) as well as compacts of the constituent Al and Ni. Concurrently, mesoscale material point method (MPM) simulations of fully resolved Al, Ni, and Al/Ni compact microstructures and engineering scale MPM simulations of homogenized Al/Ni compacts have been conducted. These simulations, in conjunction with the experimental measurements, have allowed us to parameterize mesoscale particle/particle contact models and engineering scale anisotropic damage mechanics models for the materials. The goal of this coordinated experimental and simulation study of Al/Ni and its constituent materials was to better understand their mechanical properties and gain insight into the nature of Al–Ni and Al–Al interfaces in Al/Ni compacts and their impact on mechanical properties of the compacts. We found that brittle tensile failure of Al/Ni compacts was dominated by the strength of the Al–Al interface while ductile compressive behavior of Al/Ni compacts was largely determined by the behavior of the Al phase. The properties of both Al–Al interface and Al phase appear to be altered by the presence of Ni. Finally, it was found that softening resulting from shear damage offset underlying strain hardening of the Al and Ni phases during compressive loading of the Al/Ni compacts. [ABSTRACT FROM AUTHOR]

We explore the use of an aluminum–silicon eutectic powder to produce reactive materials that fragment and combust following high-velocity impact. Unlike the pure aluminum powders traditionally used in these materials, the eutectic sinters readily and has a reduced melting point. This allows the tensile strength/ductility and fracture toughness to be tuned with partial sintering; increases in these properties, however, generally result in a reduction in combustion energy release on impact. Three heat treatments were studied for isostatically compressed aluminum–silicon materials, and two baseline pure aluminum materials using fine and coarse powders were also fabricated for comparison. Mechanical measurements and impact calorimetry tests showed that partial sintering at the nominal melting point increases tensile strength and toughness by approximately 350% but does reduce combustion energy output; however, the reduction is only prominent above 1300 m/s for the conditions studied here. Below this point, the partially sintered Al–Si has similar impact-induced combustion as an extremely brittle, unannealed pressed Al powder. Pure aluminum and alloy materials with similar mechanical properties and starting particle size show comparable combustion energy release, suggesting that powder size and fragmentation properties are more important than the variation in melt point. [ABSTRACT FROM AUTHOR]

Reactive materials are otherwise inert solids designed to generate combustion energy upon dynamic impact via fragmentation and combustion. We show that the impact fragmentation of a prototypical aluminum reactive material is very sensitive to mild annealing. Cold isostatically pressed compacts of micrometer-scale Al powder were annealed at 200 ° C and launched through thin plates or into thick steel targets, where they fragmented heavily before being recovered in a soft-catch medium. Though the heat treatment causes no sintering, grain growth, or recrystallization, it has a significant effect on the fragment distribution. The distribution can be tuned from brittle power-law behavior to one with a clear characteristic size. The effect is presumed to arise from dislocation recovery, affecting interlocking of Al particles during the multiaxial stress loading of impact. A similar material made partially with aluminum alloy powder does not show this extreme sensitivity to annealing. This strong variation in impact fragmentation is particularly important for reactive materials, which rely on the production of fine fragments for efficient combustion. [ABSTRACT FROM AUTHOR]

Cylindrical reactive material cases produced by the consolidation of an aluminum powder were tested via explosive launch in a closed chamber. One configuration measured the quasistatic overpressure generated by the case and explosive, and two further tests focused on soft‐catch of fragments before and after striking the chamber walls. On a volumetric basis, the reactive material cases produced two to three times the combustion energy of an aluminum 6061 alloy case or a bare nitromethane explosive that was tested as comparisons. The metal combustion primarily occurs after case fragments impact the walls. Increasing the reactive material case thickness produces a higher pressure but lower combustion efficiency per unit mass, despite producing comparable or slightly more fine fragments on a per gram basis. Though the brittle, pressed aluminum cases have low toughness and tensile strength, recovered fragment patterns show a range of fragment sizes up to 1 mm, with approximately one‐third of the mass below 100 μm. [ABSTRACT FROM AUTHOR]

This study examined the effects of silica addition on electrochemical behavior of rebar in native pH 12.5 saturated solutions prepared by leaching of Portland Type-II cement, and in solutions reduced by CO2 to pH 9. Cyclic polarizations showed that at pH 12.5 the silica additive increased the threshold concentration required for pitting from 100 to 105mM, and from 0.3 mM to 0.5 mM at pH 9. Passivation kinetic exponents were consistently larger for solutions with either pH when silica was present. The silica additive provided the passive film with generally increased resistance and lower capacitance, as confirmed by electrochemical impedance spectroscopy. Charge carrier densities calculated from Mott-Schottky plots were on the order of 1020 (cm-3), increasing significantly after the passive film breakdown. Raman spectroscopy of rebar samples immersed in saturated cement solutions showed films predominantly composed of iron oxyhydroxides with a new shoulder attributed to amorphous silica. [ABSTRACT FROM AUTHOR]

The article discusses a mathematical solution to the problem of assigning radio channel frequencies that maximize efficiency and maintain the quality of signals. This can be obtained by assigning the frequencies in a particular pattern. The planar surface can be divided into hexagonal cells with sides of length s. The first concentric cell lies within 2s and the second concentric cell lies within 4s of the central cell. An analysis of transmitters that lie within 4s and 2s was made. Different levels of interference were taken. Certain constraints were defined and a method to satisfy these conditions was derived. One can also find optimum number of channels for a given area, based on the likelihood of interference between channels.