Effects of constraint curvature on structural instability: tensile buckling and multiple bifurcations

Bifurcation of an elastic structure crucially depends on the curvature of the constraints against which the ends of the structure are prescribed to move, an effect which deserves more attention than it has received so far. In fact, we show theoretically and we provide definitive experimental verification that an appropriate curvature of the constraint over which the end of a structure has to slide strongly affects buckling loads and can induce: (i.) tensile buckling; (ii.) decreasing- (softening), increasing- (hardening), or constant-load (null stiffness) postcritical behaviour; (iii.) multiple bifurcations, determining for instance two bifurcation loads (one tensile and one compressive) in a single-degree-of-freedom elastic system. We show how to design a constraint profile to obtain a desired postcritical behaviour and we provide the solution for the elastica constrained to slide along a circle on one end, representing the first example of an inflexional elastica developed from a buckling in tension. These results have important practical implications in the design of compliant mechanisms and may find applications in devices operating in quasi-static or dynamic conditions.

(PDF, 406 KB) On L3 acquisition and phonological permeability: a new test case for debates on the mental representation of non-native phonological systems

The goal of this article is to make an epistemological and theoretical contribution to the nascent field of third language (L3) acquisition and show how examining L3 development can offer a unique view into longstanding debates within L2 acquisition theory. We offer the Phonological Permeability Hypothesis (PPH), which maintains that examining the development of an L3/Ln phonological system and its effects on a previously acquired L2 phonological system can inform contemporary debates regarding the mental constitution of postcritical period adult phonological acquisition. We discuss the predictions and functional significance of the PPH for adult SLA and multilingualism studies, detailing a methodology that examines the effects of acquiring Brazilian Portuguese on the Spanish phonological systems learned before and after the so-called critical period (i.e., comparing simultaneous versus successive adult English-Spanish bilinguals learning Brazilian Portuguese as an L3).

Producción de maíz bajo diferentes regímenes de riego complementario en Río Cuarto, Córdoba, Argentina. I. Rendimiento en grano de maíz y sus componentes

El maíz (Zea mays L.) es uno de los principales cultivos de la Pampa Húmeda de Argentina. El objetivo de este trabajo fue evaluar los efectos del riego complementario sobre el rendimiento de grano y sus componentes. El mismo se llevó a cabo en el ciclo agrícola 2001-2002, en el campo experimental de la Universidad Nacional de Río Cuarto. Se usó un diseño completamente al azar con 5 tratamientos y 4 repeticiones. Para efectuar la programación de los diferentes riegos se dividió el ciclo del cultivo en tres etapas: precrítico, crítico y poscrítico. Para la determinación del momento de riego se realizó un balance hídrico. El rendimiento de grano no mostró diferencias significativas en los cuatro tratamientos con riego, sin embargo, hubo diferencia significativa (α = 0,05) entre los tratamientos regados y no regados. En promedio el rendimiento en grano en los tratamientos regados fue de 72 % mayor que en el tratamiento sin riego. Los componentes del rendimiento fueron afectados significativamente (α = 0,05) por la falta de riego. La cantidad de agua aplicada varió entre 360 y 300 mm y el agua total consumida en el ciclo del cultivo (según el balance hídrico) fue para los tratamientos con riego, de 575 mm y para el testigo de 308 mm. La eficiencia del uso de agua para grano fue de 2.75 kg.m-3, en promedio.

Stage-Specific Plasticity in Ovary Size Is Regulated by Insulin/Insulin-Like Growth Factor and Ecdysone Signaling in Drosophila

Animals from flies to humans adjust their development in response to environmental conditions through a series of developmental checkpoints, which alter the sensitivity of organs to environmental perturbation. Despite their importance, we know little about the molecular mechanisms through which this change in sensitivity occurs. Here we identify two phases of sensitivity to larval nutrition that contribute to plasticity in ovariole number, an important determinant of fecundity, in Drosophila melanogaster. These two phases of sensitivity are separated by the developmental checkpoint called "critical weight"; poor nutrition has greater effects on ovariole number in larvae before critical weight than after. We find that this switch in sensitivity results from distinct developmental processes. In precritical weight larvae, poor nutrition delays the onset of terminal filament cell differentiation, the starting point for ovariole development, and strongly suppresses the rate of terminal filament addition and the rate of increase in ovary volume. Conversely, in postcritical weight larvae, poor nutrition affects only the rate of increase in ovary volume. Our results further indicate that two hormonal pathways, the insulin/insulin-like growth factor and the ecdysone-signaling pathways, modulate the timing and rates of all three developmental processes. The change in sensitivity in the ovary results from changes in the relative contribution of each pathway to the rates of terminal filament addition and increase in ovary volume before and after critical weight. Our work deepens our understanding of how hormones act to modify the sensitivity of organs to environmental conditions, thereby affecting their plasticity.