Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility

We present a systematic study of the effect of crack blunting on subsequent crack propagation and dislocation emission. We show that the stress intensity factor required to propagate the crack is increased as the crack is blunted by up to thirteen atomic layers, but only by a relatively modest amount for a crack with a sharp 60$^\circ$ corner. The effect of the blunting is far less than would be expected from a smoothly blunted crack; the sharp corners preserve the stress concentration, reducing the effect of the blunting. However, for some material parameters blunting changes the preferred deformation mode from brittle cleavage to dislocation emission. In such materials, the absorption of preexisting dislocations by the crack tip can cause the crack tip to be locally arrested, causing a significant increase in the microscopic toughness of the crack tip. Continuum plasticity models have shown that even a moderate increase in the microscopic toughness can lead to an increase in the macroscopic fracture toughness of the material by several orders of magnitude. We thus propose an atomic-scale mechanism at the crack tip, that ultimately may lead to a high fracture toughness in some materials where a sharp crack would seem to be able to propagate in a brittle manner. Results for blunt cracks loaded in mode II are also presented.Comment: 12 pages, REVTeX using epsfig.sty. 13 PostScript figures. Final version to appear in Phys. Rev. B. Main changes: Discussion slightly shortened, one figure remove

Calculation of Elastic Green's Functions for Lattices with Cavities

In this Brief Report, we present an algorithm for calculating the elastic Lattice Greens Function of a regular lattice, in which defects are created by removing lattice points. The method is computationally efficient, since the required matrix operations are on matrices that scale with the size of the defect subspace, and not with the size of the full lattice. This method allows the treatment of force fields with multi-atom interactions.Comment: 3 pages. RevTeX, using epsfig.sty. One figur

The possible explanation of electric-field-doped C60 phenomenology in the framework of Eliashberg theory

In a recent paper (J.H. Schon, Ch. Kloc, R.C. Haddon and B. Batlogg, Nature 408 (2000) 549) a large increase in the superconducting critical temperature was observed in C60 doped with holes by application of a high electric field. We demonstrate that the measured Tc versus doping curves can be explained by solving the (four) s-wave Eliashberg equations in the case of a finite, non-half-filled energy band. In order to reproduce the experimental data, we assume a Coulomb pseudopotential depending on the filling in a very simple and plausible way. Reasonable values of the physical parameters involved are obtained. The application of the same approach to new experimental data (J.H. Schon, Ch. Kloc and B. Batlogg, Science 293 (2001) 2432) on electric field-doped, lattice-expanded C60 single crystals (Tc=117 K in the hole-doped case) gives equally good results and sets a theoretical limit to the linear increase of Tc at the increase of the lattice spacing.Comment: latex2e, 6 pages, 7 figures, 1 table, revised versio

Local density of states in the vortex lattice in a type II superconductor

Local density of states (LDOS) in the triangular vortex lattice is investigated based on the quasi-classical Eilenberger theory. We consider the case of an isotropic s-wave superconductor with the material parameter appropriate to NbSe_2. At a weak magnetic field, the spatial variation of the LDOS shows cylindrical structure around a vortex core. On the other hand, at a high field where the core regions substantially overlap each other, the LDOS is sixfold star-shaped structure due to the vortex lattice effect. The orientation of the star coincides with the experimental data of the scanning tunneling microscopy. That is, the ray of the star extends toward the nearest-neighbor (next nearest-neighbor) vortex direction at higher (lower) energy.Comment: 10 pages, RevTex, 32 figure

Patrones de distribución de dos especies de Corynosoma (Acanthocephala: Polymorphidae) en peces del Atlántico sudoccidental

Corynosoma australe Johnston, 1937 y C. cetaceum Johnston and Best, 1942 son los acantocéfalos más frecuentemente reportados en peces del Mar Argentino. Sus hospedadores definitivos son otáridos y cetáceos respectivamente, aunque también han sido reportados en otros mamíferos, aves e, incluso, en humanos. Los peces, por su parte, actúan como hospedadores intermediarios de los mismos alojando a las larvas cistacantas. Dada su amplia distribución, el género Corynosoma es útil para la determinación de stocks y estudios zoogeográficos.Asociación Parasitológica Argentin

A Negative Feedback Loop Regulates Integrin Inactivation and Promotes Neutrophil Recruitment to Inflammatory Sites

International audienc

Institutionalisation of Social Movements: Co-option And Democratic Policy-making

Over the past 30 years, urban policy in Brazil has undergone a major transformation, both in terms of regulatory frameworks and the involvement of citizens in the process of policy-making. As an intense process of institutional innovation and mobilisation for decent publicservices took place, academics started to consider the impact of institutionalisation on the autonomy of social movements. Using empirical evidence from a city in the northeast of Brazil, this article addresses the wider literature on citizen participation and social movements to examine specifically the problem with co-optation. I examine the risks linked to co-optation, risks that can undermine the credibility of social movements as agents of change, and explore the tensions that go beyond the ‘co-optation versus autonomy’ divide, an issue frequently found in the practices of social movements, in their dealings with those in power. In particular, this article explores the learning processes and contentious relationships between mainly institutionally oriented urban movements and local government. This study found that the learning of deliberative skills not only led to changes in the objectives and repertoires of housing movements, but also to the inclusion of new components in their objectives that provide room for creative agency and which, in some cases, might allow them to maintain their autonomy from the state

The leadership component of Kelly’s Mobilisation Theory : contribution, tensions, limitations and further development

This reassessment of Kelly’s analysis of the relationship of activist leadership to collective action within the overall jigsaw of mobilisation theory draws on social movement literature, studies by industrial relations scholars utilising aspects of Kelly’s approach – including this author’s own work – and related research on union leadership within collective mobilisation. In the process, it identifies and celebrates how Kelly’s work, whilst contributing a distinct and substantive actor-related approach, recognised that leadership is one ingredient amongst other factors, including important structural opportunities and constraints. It considers three potential ambiguities/tensions within Kelly’s conceptualisation of leadership related to the social construction of workers’ interests, spontaneity of workers’ action and the ‘leader/follower’ interplay. The review also identifies two important limitations, related to the union member/bureaucracy dynamic and the role of left-wing political leadership, and concludes by signalling different forms of leadership relationships on which further refinement and development would be fruitful