6,332 publications from this institution
Knowledge of risk and protective factors for adolescent tobacco use will lead to the development of improved intervention strategies to reduce/prevent tobacco use. Theory and empirical findings demonstrate the multivariate complexity of the etiology of tobacco use. Sociocultural, social/interpersonal, and intrapersonal factors act through mediated chains of ultimate, distal, and proximal influences. Some influences moderate the effects of others. Once tobacco is used, feedback mechanisms modify prior causes that in turn alter subsequent tobacco use behavior. Most theories and cross-sectional, prospective, and causal process studies have contained major limitations: (a) most addressed only small portions of the total picture; (b) most mediational studies did not test for interactions and most moderation studies are based on limited theory (if any); and (c) most theories do not discuss how the causal processes might be different for males and females or for different ethnic groups (special cases of moderation). Furthermore, few studies focused on more distal or ultimate influences or examined multi-stream patterns, and few theories or causal process studies have specified or tested feedback loops. Determining psychosocial risk factors and how they influence tobacco use faces several major challenges, including discovering complex mediating processes, moderating variables, and overcoming limitations of surveys and theory. We offer six recommendations to advance transdisciplinary tobacco-prevention research: (a) base future studies on strong theory and aim to test one or more theories or theoretically derived hypotheses; (b) collect four or more waves of data and adopt dynamic strategies of prediction and analysis, including interactions, indirect effects, feedback loops, and transitions from one level of tobacco use to another; (c) provide evidence of generalizability to sub-populations within the study sample, such as by gender, ethnic group, and socioeconomic status; (d) use high-quality measures and multiple methodologies, including non-panel longitudinal studies, intensive interview, ethnography, experimental intervention, and small exploratory studies as well as further prospective studies; (e) include variables from multiple streams of influence to investigate interrelationships among cultural, social, and intrapersonal factors; and (f) collect data from multiple nested units (e.g., children within families, within schools, within neighborhoods) and employ multi-level analysis methods to investigate interrelationships among ultimate, distal, and proximal variables.
Aspects of postbuckling behavior are investigated for structures undergoing plastic deformation. The structures singled out are characterized by a highly imperfection-sensitive behavior where buckling takes place in the elastic range. A simple model study is carried out and is followed by an analysis of the plastic buckling of a complete spherical shell under external pressure. In both instances, the bifurcation behavior and subsequent deformation of the perfect structure as well as the influence which geometric imperfections have on buckling are studied.
Background: Amyloid plaques composed of the fibrillar form of the amyloid-β protein (Aβ) are the defining neuropathological feature of Alzheimer's disease (AD). A detailed understanding of the time course of amyloid formation could define steps in disease progression and provide targets for therapeutic intervention. Amyloid fibrils, indistinguishable from those derived from an AD brain, can be produced in vitro using a seeded polymerization mechanism. In its simplest form, this mechanism involves a cooperative transition from monomeric Aβ to the amyloid fibril without the buildup of intermediates. Recently, however, a transient species, the Aβ amyloid protofibril, has been identified. Here, we report studies of Aβ amyloid protofibril and its seeded transition into amyloid fibrils using atomic force microscopy. Results: Seeding of the protofibril-to-fibril transition was observed. Preformed fibrils, but not protofibrils, effectively seeded this transition. The assembly state of Aβ influenced the rate of seeded growth, indicating that protofibrils are fibril assembly precursors. The handedness of the helical surface morphology of fibrils depended on the chirality of Aβ. Finally, branched and partially wound fibrils were observed. Conclusions: The temporal evolution of morphologies suggests that the protofibril-to-fibril transition is nucleation-dependent and that protofibril winding is involved in that transition. Fibril unwinding and branching may be essential for the post-nucleation growth process. The protofibrillar assembly intermediate is a potential target for AD therapeutics aimed at inhibiting amyloid formation and AD diagnostics aimed at detecting presymptomatic disease.
The mechanics of plasticity-induced roughening of initially planar polycrystalline metal films upon thermal cycling is presented. The emphasis is on films of FCC metals having grain size comparable to their thickness. In the model, the elastic substrate imposes a biaxial in-plane mismatch strain on the film. Calculations demonstrate how surface undulations develop with a length scale governed by the grain size, when plastic slip initiates above a critical strain. Under monotonic straining, the undulation amplitude grows in proportion to the imposed strain with extent dependent on the slip anisotropy. Upon cyclic plastic straining, three growth stages have been identified with the possibility of substantial roughness development. To cite this article: G. Parry et al., C. R. Mecanique 336 (2008).
Experiments are performed on micron-scale single-crystal prototypical structural elements experiencing combined torsion and bending to gather data on their load-carrying capacity in the range of size and strain relevant to micron-scale structures for which little data are available. The observed strengthening dependence on size for the structural elements is in general accord with trends inferred from prior tests such as indentation and pure torsion. In addition, the experiments systematically reveal the strengthening size-dependence of structural elements whose surface has been passivated by a very thin Cr coating, an effect shown to have substantial strengthening potential. A state-of-the-art strain gradient plasticity theory is used to analyze the structural elements over the entire range of size and loading. While the computed trends replicate the experimental trends with reasonable fidelity, the predictive exercise, which is representative of those that will be required in micron-scale structural analysis, brings to light constitutive and computational issues which will have to be addressed before micron-scale plasticity theory can serve as effectively at the micron scale as conventional plasticity does at larger scales.
