Modeling dislocation sources and size effects at initial yield in continuum plasticity

Size effects at initial yield (prior to stage II) of idealized micron-sized specimens are modeled within a continuum model of plasticity. Two different aspects are considered: specification of a density of dislocation sources that represent the emission of dislocation dipoles, and the presence of an initial, spatially inhomogeneous excess dislocation content. Discreteness of the source distribution appears to lead to a stochastic response in stress-strain curves, with the stochasticity diminishing as the number of sources increases. Variability in stress-strain response due to variations of source distribution is also shown. These size effects at initial yield are inferred to be due to physical length scales in dislocation mobility and the discrete description of sources that induce internal-stress-related effects, and not due to length-scale effects in the mean-field strain-hardening response (as represented through a constitutive equation)

The Dynamics of Entry and Exit

The relation between profits and the number of firms in a market is one of the essential topics in the field of industrial organization. Usually, the relation is modeled in an error-correction framework where profits and/or the number of firms respond to out-of-equilibrium situations. In an out-of-equilibrium situation one or both of these variables deviate from some long-term sustainable level. These models predict that in situations of equilibrium, the number of firms does not change and hence, entry equals exit. Moreover, in equilibrium entry and exit are expected to be equal to zero. These predictions are at odds with real life observations showing that entry and exit levels are significantly positive in all markets of substantial size and that entry and exit levels often differ drastically. In this paper we develop a new model for the relation between profit levels and the number of firms by specifying not only an equation for the equilibrium level of profits in a market but also equations for the equilibrium levels of entry and exit. In our empirical application we show that our entry and exit equations satisfy the usual errorcorrection conditions. We also find that a one-time positive shock to entry or profits has a small but permanent positive effect on both the number of firms and total industry profits.

The super-oscillating superlens

We demonstrate a lens that creates a sub-wavelength focal spot beyond the near-field by exploiting the phenomenon of super-oscillation

Prey Selection by Larval Fishes as Influenced by Available Zooplankton and Gape Limitation

Feeding success during the first weeks of life is critical to determining survival and ultimate year-class strength of fishes. To compare the relative influence of gape limitation and available zooplankton on prey size selection among the larvae of three species of freshwater fishes, we gathered data on fish gape size, prey size, and size-specific prey selection in lakes and reservoirs. These variables were compared among black crappies Pomoxis nigromaculatus from a lake that contained large zooplankton as prey and white crappies P. annularis and gizzard shad Dorosoma cepedianum (a potential competitor of white crappie) from reservoirs that contained small zooplankton. In three Ohio reservoirs (i.e., small-zooplankton systems), available zooplankton and larval stages of white crappies and gizzard shad were collected once per week during April through September 1987 and 1988. Although mean prey size of white crappies continued to increase with fish size, mean prey size of smaller-gaped gizzard shad did not. However, as documented for black crappies in north-temperate lakes, white crappies in reservoirs continued to consume prey that were smaller than other available prey, even when they were no longer gape limited. Thus, although the potential for gape limitation differed between large- and small-zooplankton assemblages, prey selection did not differ as expected. Given between-species prey size selection, gizzard shad (that prefer small zooplankton) should be relatively more successful in reservoirs with small zooplankton, whereas white and black crappies (that prefer large zooplankton) should have better success in lakes with large zooplankton.This work was supported in part by DEB- 9108986 and DEB-9410323 to D.R.D., and by NSF BSR-8705518, DEB-9107173, DEB-9407859, and Federal Aid in Fish Restoration, project F-57-R to R.A.S., administered through the Ohio Division of Wildlife

Snapshot 2005: Where the American Public Stands on Terrorism and Preparedness Four Years after September 11

The National Center for Disaster Preparedness (NCDP) 2005 survey of the American public’s attitudes and views on terrorism, preparedness, and associated issues is the latest in a series of national surveys administered annually beginning in the months after September 11, 2001. The survey was completed in July 2005, just after the London Underground bombings and just before Hurricane Katrina. NCDP commissioned national and New York City public opinion polls in the immediate aftermath of September 11, 2001 to gauge the American public’s views and attitudes on a wide range of topics germane to disaster preparedness and emergency events. Since the first polls which were completed 3 and 6-months after September 11, 2001, NCDP has annually commissioned a survey which goes to the field in July-August, just before the anniversary of September 11. The 2005 survey was the fourth annual. Each survey includes trended questions as well as “one-off” questions appropriate to the given time period. Trended questions include confidence in government; willingness and ability to evacuate; personal and family preparedness plans; personal sacrifice; community preparedness; perceptions and engagement of all-hazard preparedness; and other questions thematic with the afore listed.. All questions are cross-tabulated with a variety of demographics including race, age, gender, income, region, size of community, political affiliation, and education. Further, select questions establishing a division of respondent (e.g. those having personal and family preparedness plans versus those who do not) are cross-tabulated with other selected questions to observe correlations. (e.g. awareness of community preparedness plans). The surveys are developed by NCDP investigators in conjunction with Marist, who administers the survey, codes the data, and produces the frequency tables. Full data and trend tables are available on request