Time history interfacial conditions in multiscale computations of lattice oscillations

We make a systematic study on the time history interfacial conditions in multiscale computations for crystalline solids in one space dimension. The exact interfacial condition is derived rigorously. For a class of approximate time history kernel functions, the error is estimated. In particular, a cut-off of the exact kernel function is a special case. Moreover, an effective numerical algorithm is proposed for the convolution.     

Dyadic Green's function of multilayer cylindrical closed and sector-structures for waveguide, microstrip-antenna, and network analysis

A clear and systematic method to derive the spectral- and space-domain dyadic Green's function of arbitrary cylindrical multilayer and multiconductor structures is proposed. The derivation is either done for a circumferentially closed or a cylindrical sector structure, which is bounded by electric or magnetic walls in an azimuthal direction. The solution for the dyadic Green's function in the spectral domain is obtained via an equivalent circuit. Relations between the spectral and space domains for the dyadic Green's functions are derived using eigensolution expansions. Finally, the dyadic Green's function is applied to the problem of finding the propagation constants of the two-layer dielectric rod.     

A comparative study of mentoring among men and women in managerial, professional, and technical positions.

Linkages between a global measure of mentoring experiences, gender, and 4 outcome variables were investigated. Also, the moderating effects of gender were examined to determine whether mentoring is differentially associated with career outcomes for men and women. Business school graduates (147 women and 173 men) provided information about their backgrounds, companies, positions, mentoring practices, compensation, and compensation satisfaction. Individuals experiencing extensive mentoring relationships reported receiving more promotions, had higher incomes, and were more satisfied with their pay and benefits than individuals experiencing less extensive mentoring relationships. There were no gender differences with regard to the frequency of mentoring activities, and gender did not moderate mentoring–outcome relationships. The results are discussed within the context of a $7,990 income difference between men and women. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Constructs and assessment center dimensions: Some troubling empirical findings.

Maintains that exercise design alone serves as weak evidence for construct validity due to the complexity of the typical assessment center rating and evaluation process. Evidence that assessment ratings accurately reflect the complex traits they purport to measure has not been provided to date. To explore this issue, the interrelationships among dimensional ratings between and within exercises in 3 assessment centers were examined. 559 candidates for upper-level management from a multinational firm, a civil service commission, and a retailer were assessed on managerial skills (including communication and interpersonal skills). Postexercise ratings of assessment dimensions were correlated and factor analyzed. The resulting factor pattern for all 3 organizations represented exercises rather than dimensions. In 2 of the organizations, the mean correlation among across-exercise ratings of individual dimensions was near zero. These findings suggest that assessment ratings do not measure the intended constructs. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The “compleat” engineer

There is a rising tide of opinion holding that manpower needs are not so much numbers as they are for an engineer better prepared in every way for the age in which he will live. The report on which this article is based was started in 1952 and published in 1955; recent events have only added urgency to the opinions and ideas expressed.     

A Material Model for the Cyclic Behavior of Nitinol

The uniaxial behavior of Nitinol in different forms and at different temperatures has been well documented in the literature. Mathematical models for the three-dimensional behavior of this class of materials, covering superelasticity, plasticity, and shape memory effects have been previously developed. Phenomenological models embedded in FEA analysis are part of common practice today in the development of devices made out of Nitinol. In vivo loading of medical devices has cyclic characteristics. There have been some indications in the literature that cyclic loading of Nitinol modifies substantially its behavior. A consortium of several stent manufacturers, Safe Technology and Dassault Systèmes Simulia Corp., dedicated to the development of fatigue laws suitable for life prediction of Nitinol devices, has conducted an extensive experimental study of the modifications in uniaxial behavior of both Nitinol wire and tubing due to cyclic loading. The Abaqus Nitinol material model has been extended to capture some of the phenomena observed and is described in this article. Namely, a preload beyond 6% strain alters the transformation plateaus; if the cyclic load amplitude is large enough, permanent deformations (residual martensite) are observed; the lower plateau increases; and the upper plateau changes. The modifications to the upper plateau are very interesting in the sense that it appears broken: its start stress gets lowered creating a new plateau up to the highest level of cyclic strain, followed by resuming the original plateau until full transformation. Since quite often the geometry of a device at the point at which it is subjected to cyclic loading is very much dependent on the manufacturing, deployment, and preloading sequence, it is important that analyses be conducted with the original material behavior up to that point, and then with the cyclic behavior thereafter.     

Accelerated Fatigue Testing of Stent-Like Diamond Specimens

In this study, we investigated the fatigue behavior of stent-like diamond specimens with particular attention paid to the nature of the test specimen, the constitutive model for the finite element analyses and the displacement condition. A newly designed test rig did enhance the investigation and results with respect to the simulation of the expected in vivo displacement conditions. The excellent performance of the new test method presented within our study provides a good basis for future tests without risk of compromised results due to differing characteristics between test specimens and finished stents, inappropriate displacement conditions or constitutive material model and provides a high reliability and applicability of the results to actual stents.     

Effects of Cyclic Loading on the Uniaxial Behavior of Nitinol

The widespread development and use of implants made from NiTi is accompanied by the publication of many NiTi material characterization studies. These publications have increased significantly the knowledge about the mechanical properties of NiTi. However, this knowledge also increased the complexity of the numerical simulation of NiTi implants or devices. This study is focused on the uniaxial behavior of NiTi tubing due to cyclic loading and had the goal to deliver both precise and application-oriented results. Single aspects of this study have already been published (Wagner in Ein Beitrag zur strukturellen und funktionalen Ermüdung von Drähten und Federn aus NiTi-Formgedaechtnislegierungen, Ph.D. Thesis, 2005; Eucken and Duerig in Acta Metall 37:2245-2252, 1989; Yawny et al. in Z Metallkd 96:608-618, 2005); however, there is no publication known that shows all the single effects combined in a ??duty cycle case.?? It was of particular importance to summarize the main effects of pre-strain and subsequent small or large strain amplitudes on the material properties. The phenomena observed were captured in an extended Abaqus^® Nitinol material model, presented by Rebelo et al. (A Material Model for the Cyclic Behavior of Nitinol, SMST Extended Abstracts 2010). The cyclic tensile tests were performed using a video extensometer to obtain accurate strain measurement on small electro-polished dog-bone specimen that were incorporated into a stent framework so that standard manufacturing methods could be used for the fabrication. This study indicates that a prestrain beyond 6% strain alters the transformation plateaus and if the cyclic displacement amplitude is large enough, additional permanent deformations are observed, the lower plateau and most notably the upper plateau change. The changes to the upper plateau are very interesting in the sense that an additional stress plateau develops: its ??start stress?? is lowered thereby creating a new plateau up to the highest level of cyclic strain, followed by resuming the original plateau until full transformation. This study was conducted in the course of the work of a consortium of several stent manufacturers, SAFE Technology Limited and Dassault Systèmes Simulia Corp., dedicated to the development of fatigue laws suitable for life prediction of Nitinol devices.