6,332 publications from this institution
Constraints on diffusive creep cavity growth along grain boundary facets are studied for the limiting case when all facets oriented approximately normal to an applied tensile load are uniformly cavitated. This situation represents the opposite limiting case to when cavitated facets are well-separated and do not interact with each other. The analysis is done for a 3-D periodic polycrystalline model of grains in the shape of the Wigner-Seitz cells of a f.c.c. lattice. The grains have freely-sliding boundaries and deform in a nonlinear viscous manner in response to applied stress. Expressions for the cavity growth rate and the strain and time to rupture are compared with results of prior work in which cavitated facets are well-separated, and this gives a good understanding of the ranges of stress and temperature over which cavity growth is constrained and rupture lifetime is increased. The time to rupture, which is taken here to mean cavity coalescence on the damaged facets, is seen to depend strongly on the proximity of cavitated facets, at least when cavity growth is constrained. However, the strain to rupture is observed to lack this strong dependence although for constrained conditions, the cavitation process contributes substantially to the total strain when cavitated facets are closely-spaced. When cavitated facets are well-separated, the polycrystal is seen to achieve a relatively constant strain rate. By comparison, the strain rate is seen to vary substantially with time when cavitated facets are closely-spaced. The time and strain to rupture as well as strain rate versus time curves are calculated as functions of applied load and temperature for nickel as a representative f.c.c. metal.
We observed an overall significant association between younger age at menopause and higher risk of CHD among women who experienced natural menopause and never used hormone therapy. This increased risk was observed among current smokers but not among never smokers. The apparent elevated risk of CHD with decreased age at natural menopause among smokers might reflect residual confounding by smoking.
Several approaches have been proposed to model binary outcomes that arise from longitudinal studies. Most of the approaches can be grouped into two classes: the population-averaged and subject-specific approaches. The generalized estimating equations (GEE) method is commonly used to estimate population-averaged effects, while random-effects logistic models can be used to estimate subject-specific effects. However, it is not clear to many epidemiologists how these two methods relate to one another or how these methods relate to more traditional stratified analysis and standard logistic models. The authors address these issues in the context of a longitudinal smoking prevention trial, the Midwestern Prevention Project. In particular, the authors compare results from stratified analysis, standard logistic models, conditional logistic models, the GEE models, and random-effects models by analyzing a binary outcome from two and seven repeated measurements, respectively. In the comparison, the authors focus on the interpretation of both time-varying and time-invariant covariates under different models. Implications of these methods for epidemiologic research are discussed.
The rolling process is widely used in the metal forming industry and has been so for many years. However, the process has attracted renewed interest as it recently has been adapted to very small scales where conventional plasticity theory cannot accurately predict the material response. It is well-established that gradient effects play a role at the micron scale, and the objective of this study is to demonstrate how strain gradient hardening affects the rolling process. Specifically, the paper addresses how the applied roll torque, roll forces, and the contact conditions are modified by strain gradient plasticity. Metals are known to be stronger when large strain gradients appear over a few microns; hence, the forces involved in the rolling process are expected to increase relatively at these smaller scales. In the present numerical analysis, a steady-state modeling technique that enables convergence without dealing with the transient response period is employed. This allows for a comprehensive parameter study. Coulomb friction, including a stick–slip condition, is used as a first approximation. It is found that length scale effects increase both the forces applied to the roll, the roll torque, and thus the power input to the process. The contact traction is also affected, particularly for sheet thicknesses on the order of 10 μm and below. The influences of the length parameter and the friction coefficient are emphasized, and the results are presented for multiple sheet reductions and roll sizes.
