Scale effects for strength, ductility, and toughness in "brittle” materials

Decreasing scales effectively increase nearly all important mechanical properties of at least some "brittle” materials below 100 nm. With an emphasis on silicon nanopillars, nanowires, and nanospheres, it is shown that strength, ductility, and toughness all increase roughly with the inverse radius of the appropriate dimension. This is shown experimentally as well as on a mechanistic basis using a proposed dislocation shielding model. Theoretically, this collects a reasonable array of semiconductors and ceramics onto the same field using fundamental physical parameters. This gives proportionality between fracture toughness and the other mechanical properties. Additionally, this leads to a fundamental concept of work per unit fracture area, which predicts the critical event for brittle fracture. In semibrittle materials such as silicon, this can occur at room temperature when the scale is sufficiently small. When the local stress associated with dislocation nucleation increases to that sufficient to break bonds, an instability occurs resulting in fractur

Geometric origin of mechanical properties of granular materials

Some remarkable generic properties, related to isostaticity and potential energy minimization, of equilibrium configurations of assemblies of rigid, frictionless grains are studied. Isostaticity -the uniqueness of the forces, once the list of contacts is known- is established in a quite general context, and the important distinction between isostatic problems under given external loads and isostatic (rigid) structures is presented. Complete rigidity is only guaranteed, on stability grounds, in the case of spherical cohesionless grains. Otherwise, the network of contacts might deform elastically in response to load increments, even though grains are rigid. This sets an uuper bound on the contact coordination number. The approximation of small displacements (ASD) allows to draw analogies with other model systems studied in statistical mechanics, such as minimum paths on a lattice. It also entails the uniqueness of the equilibrium state (the list of contacts itself is geometrically determined) for cohesionless grains, and thus the absence of plastic dissipation. Plasticity and hysteresis are due to the lack of such uniqueness and may stem, apart from intergranular friction, from small, but finite, rearrangements, in which the system jumps between two distinct potential energy minima, or from bounded tensile contact forces. The response to load increments is discussed. On the basis of past numerical studies, we argue that, if the ASD is valid, the macroscopic displacement field is the solution to an elliptic boundary value problem (akin to the Stokes problem).Comment: RevTex, 40 pages, 26 figures. Close to published paper. Misprints and minor errors correcte

Illness in Children after International Travel: Analysis from the GeoSentinel Surveillance Network

OBJECTIVE: By using a large, multicenter database, we investigated the characteristics and morbidities of 1591 children returning from 218 global destinations and presenting for care in 19 countries. METHODS: Data reported to the GeoSentinel Surveillance Network between January 1997 and November 2007 were analyzed, to assess demographic features, travel characteristics, and clinical diagnoses of ill pediatric travelers. Data were compared between children and adults and among 3 pediatric age groups (0–5 years, 6–11 years, and 12–17 years). RESULTS: Children were predominantly tourist travelers returning from Asia, sub-Saharan Africa, or Latin America. Compared with adults, children disproportionately presented within 7 days after return, required hospitalization, lacked pretravel health advice, and had traveled for the purpose of visiting friends and relatives. Diarrhea (28%), dermatologic conditions (25%), systemic febrile illnesses (23%), and respiratory disorders (11%) accounted for the majority of diagnoses reported for children. No fatalities were reported. Diarrhea occurred disproportionately among children after exposure to the Middle East/North Africa, dermatologic conditions after exposure to Latin America, systemic febrile illnesses after exposure to sub-Saharan Africa or Asia, and respiratory disorders after exposure to Europe or North America. The proportionate morbidity rates of travelassociated diseases differedamongthe pediatric age groups and between children and adults. CONCLUSIONS: The health care utilization patterns before and after travel and the profiles of travel-associated health problems differed between children and adults. Health professionals providing pretravel advice need to consider destination- and age-specific susceptibility to travel-related morbidities and develop prevention strategies accordingly. Pediatrics 2010;125:e1072–e108

Maltreatment of Strongyloides Infection: Case Series and Worldwide Physicians-in-Training Survey

10.1016/j.amjmed.2006.05.072American Journal of Medicine1206545.e1-545.e8AJME