On the formation of adiabatic shear bands in textured HCP polycrystals

AbstractAdiabatic shear band (ASB) formation in textured HCP polycrystals has been investigated under regimes of high rate compression and shear loading using dynamic thermo-mechanically coupled, dislocation-based crystal plasticity modelling. The balance between rate of plastic dissipation leading to internal heat generation versus rate of thermal diffusion at a crystallographic length scale has been shown to be pivotal for the formation or otherwise of ASBs. Micro-texture has been found to have a key role in both advancing and inhibiting shear band growth, and its control offers the possibility of new alloys with higher impact strength over strain rate range1 × 10−2 to 1 × 105 s−1. Texture has been found to lead to wide variations in applied macroscopic strain at which ASB formation occurs, such that strain level in isolation is inappropriate as a universal indicator of ASB onset.High-rate shear loading is found to lead to lower onset strains for ASBs compared to high rate compression, but the dependence of both on texture leads to considerable variation in strain level for ASB formation. A preliminary map demarcating ASB onset has been established over regimes of applied strain and texture for dynamic shear and compression

Mercury flux to sediments of Lake Tahoe, California-Nevada

Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Water, Air, & Soil Pollution 210 (2010): 399-407, doi:10.1007/s11270-009-0262-y.We report estimates of mercury (Hg) flux to the sediments of Lake Tahoe, California-Nevada: 2 and 15-20 µg/m2/yr in preindustrial and modern sediments, respectively. These values result in a modern to preindustrial flux ratio of 7.5-10, which is similar to flux ratios recently reported for other alpine lakes in California, and greater than the value of 3 typically seen worldwide. We offer plausible hypotheses to explain the high flux ratios, including (1) proportionally less photoreduction and evasion of Hg with the onset of cultural eutrophication and (2) a combination of enhanced regional oxidation of gaseous elemental Hg and transport of the resulting reactive gaseous Hg to the surface with nightly downslope flows of air. If either of these mechanisms is correct, it could lead to local/regional solutions to lessen the impact of globally increasing anthropogenic emissions of Hg on Lake Tahoe and other alpine ecosystems.Funding was provided by Miami University, EPA-STAR, the Postdoctoral Scholar Program at Woods Hole Oceanographic Institution, and the USGS

The role of homogeneous nucleation in planar dynamic discrete dislocation plasticity

Homogeneous nucleation of dislocations is the dominant dislocation generation mechanism at strain rates above 108 s−1; at those rates, homogeneous nucleation dominates the plastic relaxation of shock waves in the same way that Frank–Read sources control the onset of plastic flow at low strain rates. This article describes the implementation of homogeneous nucleation in dynamic discrete dislocation plasticity (D3P), a planar method of discrete dislocation dynamics (DDD) that offers a complete elastodynamic treatment of plasticity. The implemented methodology is put to the test by studying four materials—Al, Fe, Ni, and Mo—that are shock loaded with the same intensity and a strain rate of 1010 s−1. It is found that, even for comparable dislocation densities, the lattice shear strength is fundamental in determining the amount of plastic relaxation a material displays when shock loaded.</jats:p

Investigation of the dynamic fragmentation process in ceramics by using ultra-high speed x-ray imaging with synchrotron radiation

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