This paper presents a study on the dynamic stability of the steady frictional sliding of a linear elastic or viscoelastic half-space compressed against a rigid plane which moves with a prescribed nonvanishing tangential speed. The system of differential equations and boundary conditions that govern the small plane oscillations of the body about the steady-sliding state of deformation is established. It is shown that for large coefficient of friction and large Poisson’s ratio the steady-sliding of the elastic body is dynamically unstable. This instability manifests itself by growing surface oscillations which necessarily propagate from front to rear and which in a short time lead to situations of loss of contact or stick. Similarly to what has been found with various finite dimensional frictional systems, these flutter type surface instabilities result from the intrinsic nonsymmetry of dry friction contact laws. The effect of viscous dissipation within the deformable body is also assessed: when viscous dissipation is present larger coefficients of friction are required for the occurrence of surface solutions propagating and growing from front to rear.

Surveys of orchid bees at the Brazilian Atlantic forest have been restricted to a few regions, making difficult to understand latitudinal patterns of distribution and diversity of these bees. For this reason we sampled the euglossine fauna at Atlantic forest areas at the coastal region of São Paulo (Sete Barras, Faz. Morro do Capim: SP3) and state of Paraná (Antonina, Reserva Natural do Rio Cachoeira: PR3), in southern Brazil. In PR3, we also evaluated the efficiency of collecting methods for sampling the fauna, comparing bait traps with direct collecting using entomological nets on fragrance baits. The diversity and abundance of bees was very low: we caught only 39 males of eight species in SP3 (Euglossa iopoecila, Euglossa roderici, Eulaema nigrita, Euglossa annectans, Eulaema cingulata, Euglossa pleosticta, Euglossa viridis and Exaerete smaragdina) and 254 males of six species in PR3 (Euglossa iopoecila, Euglossa annectans, Euglossa stellfeldi, Euglossa roderici, Euglossa pleosticta and Eulaema nigrita). Comparing the sampling methodologies, use of insect nets on fragrance baits (six species; 221 specimens) was more efficient than bait traps (three species; 33 specimens). When comparing the faunas of these two areas with other surveys at the Atlantic forest sites, through a DCA analysis, we found that the two surveys presented in this paper were placed relatively close to each other, but apart from the other sites analyzed, not clustering with the southernmost survey at the subtropical Atlantic forest of Rio Grande do Sul or with the remaining surveys carried out at northern lowland sites of this biome.

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Abstract:The composition of local orchid-bee faunas (Hymenoptera, Apidae, Euglossina) in open-vegetation domains is poorly known, making the ecology and biogeography of the group difficult to understand. The aim of this work was to answer the following questions: i) Is the orchid-bee fauna composition, species richness and abundance in the cerrado (Brazilian savanna) sensu stricto similar to that of riparian forests immersed in that domain? ii) Do species from neighboring forest domains use riparian forests as mesic corridors into the Cerrado? Two sites in cerrado s.s. and two in riparian forests were sampled monthly, one day per month, during one year (Nov/2003-Oct/2004) in northwestern Minas Gerais state, Brazil. Six aromatic compounds (β-ionone, 1,8-cineole, eugenol, methyl trans-cinnamate, methyl salicilate and vanillin) were exposed from 8:00 AM to 4:00 PM to attract orchid bees. The results suggest that: i) The composition of the orchid bee fauna in the two kinds of environments is the same; ii) Riparian forests apparently have no role as mesic corridors for penetration of forest-dependent euglossine species into the core of the Cerrado Domain. Resumo: A composição das faunas locais de Euglossina (Hymenoptera, Apidae) em formações vegetais abertas é pouco conhecida, o que dificulta o entendimento da ecologia e biogeografia do grupo. O objetivo deste trabalho foi responder às seguintes perguntas: i) No domínio do Cerrado, a composição da fauna, riqueza em espécies e abundância de euglossinas em áreas de cerrado s.s. e em mata ciliar são semelhantes entre si? ii) Espécies de domínios florestais adjacentes utilizam as matas ciliares como corredores mésicos para dentro do Cerrado? Duas áreas de cerrado s.s. e duas de mata ciliar foram amostradas mensalmente, um dia por mês, durante um ano (nov/2003-out/2004), no noroeste do estado de Minas Gerais, Brasil. Seis compostos aromáticos (β-ionona, cineol, cinamato de metila, eugenol, salicilato de metila e vanilina) eram expostos das 8 às 16 horas para atrair as euglossinas. Os resultados encontrados sugerem que: i) As composições das faunas em áreas de cerrado s.s. e matas ciliares são iguais; ii) As matas ciliares aparentemente não atuam como corredores mésicos para a penetração, em áreas centrais do Cerrado, de espécies de euglossinas dependentes de florestas. Palavras-chave: Euglossina, mata ciliar, mata de galeria, savana, ecologia de paisagem.

ABSTRACT. Spatial distribution of trophic guilds and community structure of Ephemeroptera (Insecta) in streams of the Cerrado region in Mato Grosso, Brazil. We analyzed the structure of the functional feeding groups of Ephemeroptera in five different types of substrates (litter in riffles, backwater litter, wood, stones and roots) from first to fifth orders at the Pindaíba River basin, state of Mato Grosso, Brazil. We also studied the specificity and frequency of the organism, and if the functional approach reflects changes within the community through ordination analysis. Our results assemble information about the functional classification of mayfly nymphs and show that scrapers were the most abundant, and gathering -collectors are the more diverse. Third order streams were the most richest and most abundant while functional feeding group distribution corroborated the predictions of The River Continuum Concept. Besides, communities were much more taxonomically structured than functionally structured. This fact underlies the importance of the biology of organisms, and not only the functional feeding group, for substrate selection. The preference of some genera for specific substrates confirms this value. KEYWORDS.Taxonomic composition, aquatic insect, mesodistribution, indicator species.RESUMO. Analisamos a estrutura dos grupos funcionais alimentares de Ephemeroptera em cinco tipos de substratos (folhiço em corredeira, folhiço em remanso, madeira, pedras e raízes) de trechos de primeira a quinta ordens da bacia do rio Pindaíba, estado do Mato Grosso, Brasil. Verificamos, ainda, a preferência de táxons por substratos específicos através da especificidade e frequência dos organismos, e se a abordagem funcional reflete as variações dentro da comunidade. Nossos resultados agregam informações a respeito da classificação funcional dos efemerópteros e mostram que os raspadores foram os mais abundantes e os coletores-apanhadores comportam uma maior riqueza de táxons. Os córregos de terceira ordem foram os mais ricos e abundantes enquanto que as distribuições dos grupos funcionais alimentares corroboram as predições do Conceito do Continuum fluvial. Além disso, as comunidades estudadas foram muito mais estruturadas taxonomicamente do que funcionalmente. Esse fato atesta a importância da biologia dos organismos para a seleção dos substratos. A preferência de alguns gêneros por substrato específico corrobora esta importância. PALAVRAS-CHAVE. Composição taxonômica, insetos aquáticos, mesodistribuição, espécies indicadoras.O conceito de guilda alimentar foi proposto originalmente por Root (1967), definido como um grupo de espécies, indiferente da afiliação taxonômica, que exploram a mesma classe de recursos alimentares de uma maneira similar. Quando insetos aquáticos são divididos em guildas tróficas, essas categorias são conhecidas como grupos funcionais alimentares (Functional Feeding Groups -FFG), e são baseadas não só no tipo de recurso consumido, mas também nos mecanismos morfológicos e comportamentais de aquisição dess...

Recent advances in the development of a general three-dimensional finite element methodology for modeling large deformation steady state behavior of tire structures is presented. The new developments outlined here include the extension of the material modeling capabilities to include viscoelastic materials and a generalization of the formulation of the rolling contact problem to include special nonlinear constraints. These constraints include normal contact load, applied torque, and constant pressure-volume. Several new test problems and examples of tire analysis are presented.

We report an enhanced X-ray shielding effect related to graphene. The mass attenuation coefficients measured for nanocomposites made of poly­(vinylidene fluoride) (PVDF) filled with 1.88 wt % functionalized graphene oxides (GO), pyrolytic graphite (PG), multiwalled carbon nanotubes (MWCNT), and amorphous carbon (soot) have been compared. For 6.9 keV photons, the value measured for graphene-based nanocomposite was found to be four times higher than that encountered for the other graphitic-based nanocomposites. The mass attenuation coefficients were measured for X-ray photons with 6.9, 8.1, 17.5, and 22.1 keV, respectively. Fourier transform infrared data revealed that all graphitic composites casted from solution are in the ferroelectric β-phase of PVDF. It is demonstrated that thin films of ferroelectric PVDF/1.88 wt % GO nanocomposite, with thickness of only 0.1 mm, can attenuate 82.9% and 48.5% of X-ray beams with energies of 6.9 and 8.1 keV, respectively. Thus, lightweight, very thin, and lead-free PVDF/GO radiopaque films can be manufactured, offering efficient protection against X-ray radiation for patients and devices in radiology procedures.