Creep at low stresses: An evaluation of diffusion creep and Harper-Dorn creep as viable creep mechanisms

High-temperature creep experiments often reveal a transition at very low stresses to a region where the stress exponent is reduced to a value lying typically in the range of ∼1 to 2. This region is generally associated with the occurrence of a new creep mechanism, such as grain-boundary sliding, diffusion creep, and/or Harper-Dorn creep. Several recent reports have suggested that diffusion creep and Harper-Dorn creep may not be viable creep mechanisms. This article examines these two processes and demonstrates that there is good evidence supporting the occurrence of both creep mechanisms under at least some experimental conditions. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Predictive validity of six neurodiagnostic tests: A decision theory analysis.

Investigated different aspects of predictive validity for EEG, skull X ray, brain scan, arteriogram, pneumoencephalogram and the Block Rotation Test. The validation sample was based on cases referred to a Neuropsychology Service from 1964-1968. The criterion distribution, brain disease (B) and no brain disease (F), and the test signs, positive (+) and negative -, were both dichotomized for analysis. The overall hit rate, percentage of valid positives, and strength of predictive association were extremely low for several of the tests. However, the predictive utility reversed appreciably for some of the tests when base-rate information (inverse probabilities) and types-of-error risk (cost efficiency) were computed. Implications for diagnosis in neurology and psychology are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Transient behavior of diffusion-induced creep and creep rupture

Steady state solutions to three types of diffusion problems: creep, grain boundary sliding and intergranular crack growth, have been published in the literature. This paper considers, in detail, the events which occur between the time when the external stress is applied and the time when the steady state is eventually reached. The time constant of the transient has been calculated. It is shown how the grain boundary tractions change with time from the initial “elastic” configuration (when sliding has been elastically accommodated) to the steady state “diffusional” configuration (when the sliding rate is diffusionally accommodated). This requires infusion of excess grain boundary dislocations; the distribution of these dislocations is calculated. The results are applied to problems of diffusional creep, grain boundary sliding and intergranular crack growth.     

Transient behavior of diffusion-induced creep and creep rupture

Steady state solutions to three types of diffusion problems: creep, grain boundary sliding and intergranular crack growth, have been published in the literature. This paper considers, in detail, the events which occur between the time when the external stress is applied and the time when the steady state is eventually reached. The time constant of the transient has been calculated. It is shown how the grain boundary tractions change with time from the initial “elastic” configuration (when sliding has been elastically accommodated) to the steady state “diffusional” configuration (when the sliding rate is diffusionally accommodated). This requires infusion of excess grain boundary dislocations; the distribution of these dislocations is calculated. The results are applied to problems of diffusional creep, grain boundary sliding and intergranular crack growth.     

The tertiary creep and necking of creep damaging solids

Tertiary creep and necking of bars composed of creep damaging material are investigated subject to the condition of constant applied load. The creep-damage constitutive model employed in the study is based on the concept of constrained cavity growth which characterizes the phenomenon of grain boundary cavitation in polycrystalline metals under creep conditions. For a perfect bar, tertiary creep arises from the interaction of cavitation with the geometrical effect of uniform thinning of the area cross section. Two coupled differential equations governing the evolution of the axial strain and area cross section are obtained and solved for the case of linear strain dependent nucleation. Necking is analyzed by considering the deformation of an initially imperfect bar modelled as a series of disk elements. The governing equations are integrated in these elements subject to the approximation that stresses and strains remain uniform throughout the deformation. Results indicate that the mechanism of cavitation can significantly affect the creep response of both the perfect and imperfect bars.     

Improved creep strength and creep ductility of type 347 austenitic stainless steel through the self-healing effect of boron for creep cavitation

Composition of type 347 austenitic stainless steel was modified with the addition of boron and cerium. An improvement of creep strength coupled with creep ductility of the steel was observed with boron and cerium additions. The observation of enhanced precipitation of carbonitrides in boron-containing steel over that of boron-free steel may in part contribute to the increase in creep strength. Both grain boundary sliding and nucleation and growth of intergranular creep cavities were found to be suppressed in steel-containing boron. This results in an increase in creep strength and creep ductility. Auger electron spectroscopic analysis of the chemistry of creep cavity surfaces (exposed by breaking the creep-exposed steel specimen at liquid nitrogen temperature under impact loading) revealed the segregation of elemental boron on the creep cavity surface. Boron segregation, on the creep cavity surface in the absence of sulfur contamination, suppressed the cavity growth and provided the steel with a self-healing effect for creep cavitation. Cerium additions enabled boron to segregate on the cavity surface by effectively removing the traces of free sulfur in the matrix by the formation of ceriumoxysulfide (Ce₂O₂S).     

Executive function and theory of mind: Predictive relations from ages 2 to 4.

Despite robust associations between children's theory of mind (ToM) and executive function (EF) skills, longitudinal studies examining this association remain scarce. In a socially diverse sample of 122 children (seen at ages 2, 3, and 4), this study examined (a) developmental stability of associations between ToM, EF, verbal ability, and social disadvantage; (b) continuity and change in ToM and EF; and (c) predictive relations between ToM and EF. Verbal ability and social disadvantage independently predicted changes in EF (but not ToM). Task scores improved with age and showed stable individual differences. The authors examined predictive relations between ToM and EF using partial correlations (controlling for age and verbal ability) and hierarchical regressions (that also controlled for social disadvantage and initial ToM and EF). The findings provide only partial support for the view that ToM is a prerequisite for EF but stronger support for the proposal that EF facilitates children's performance on ToM tasks. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Assessment of intrinsic creep resistance evolution based on the results of constant load creep tests

In the present study a method to assess the intrinsic creep resistance evolution by using data of constant load creep tests is proposed. The investigation has been performed on the austenitic steels X6 CrNi 18 11 and X8 CrNiMoNb 16 16. To develop the constitutive equation describing the intrinsic creep resistance evolution a simple structural mechanical model has been used. The applied model is based on a modification of the Levy-Mises equation for plasticity to consider creep time effects. The proposed model has been verified experimentally. The experimental creep resistance evolution curves have been derived with the aid of strain transient dip test performed in transient, steady state and accelerated creep stage.     

高职辅导员必需的基本能力

随着高等教育从精英化到大众化的转变，大学生人数的急剧增加，学生管理工作也有了新的变化。如何在新的形势下解决高等职业院校学生管理工作中的问题，已成为了一个亟待解决的课题。拥有一支高素质的辅导员队伍，加强辅导员必备的基本能力的培养，是解决这个问题的根本。     

Enhancement of the creep resistance of metals

It is shown that the presence of an unstable surface layer has a very marked effect on the creep behavior of metals. By eliminating this surface layer the creep rate can, for example, be reduced by a factor of 38 for 321 steel tested at 1400° F at 60 pet of the proportional limit. By eliminating the surface layer, activation energy for creep was increased by 13, 21, and 49 kcal/mole for Haynes 188, titanium (6Al-4V), and 321 steel, respectively. This change indicates that two mechanisms control the creep process:     

Primary recrystallization during the creep of alumina

Abstract

Polycrystalline alumina prepared by cold isostatic compaction and sintering has been deformed in compression in the temperature range 1600 – 1700°C and at a stress of 3000 p.s.i. The strain-time curves indicated that under these conditions recrystallization may be initiated at strains as low as 1/4-1/2%. Metallographic examination of the deformed specimens revealed directly the initial stages of recrystallization. Preferential nucleation occurred at, or near, grain boundaries and the tips of microcracks. A mechanism involving the polygonization of dislocations in highly deformed regions of the crystals is proposed as a likely nucleation process.

Résumé

De l'alumine polycristalline préparée par compaction isostatique à froid et frittage a été déformée en compression sous une contrainte de 3000 psi dans un intervalle de température de 1600° à 1700°c. Les courbes déformation-temps indiquent que dans ces conditions la recristallisation peut débuter à des déformations aussi faibles que 1/4-1/2 %. Un examen métallographiq ue des échantillons déformés révèle directement les premiers stades de la recristallisation. Une germination préférentielle se produit aux (ou près des) joints de grains ainsi qu'à la pointe des microfissures. Comme interprétation possible de cette germination, les auteurs proposent un mécanisme faisant appel à la polygonisation des dislocations dans les régions des grains fortement déformées.     

On the short time creep rupture

Short time tensile creep rupture tests were made on 7 mil lamp filament wire and on 7 mil undoped tungsten wire in the temperature range 2500 to 3100 K. Steady-state creep rate was described by an expression having the form: ε =ADσ/E)^m whereA is a constant,D is the self-diffusion coefficient, σ is the creep stress,E is the elastic modulus, andm is 5 for undoped tungsten and 25 for lamp grade wire. Activation energy for time to failure of lamp wire was found to vary from 150 to 185 kcal in the range 2500 to 3100 K. Failure mode in this range was slip at high stresses and intergranular brittle fracture at low stresses. The very high creep-strength observed for lamp wire was attributed to a combination of resistance to diffusional creep at low stresses and resistance to dislocation creep at high stresses. The dispersoid structure of lamp wire was suggested as the origin of strength in both cases. Formerly manager, Research and Development, Refractory Metals Business Section     

The structure of personality capabilities.

Wallace (1966) proposed that personality be construed as a set of abilities. Rather than assessing typical performance, as in trait ratings, he recommended assessing an individual's ability to perform social behaviors. We have elaborated this notion by distinguishing between personality abilities and capabilities. In this article we focus on the capability, that is, the ease with which an individual can display a certain category of social responses. A capability X is assessed with self-reports of (a) likelihood of performing X when perceived to be required, (b) perceived difficulty in performing X, (c) anxiety in performing X, and (d) tendency to avoid performing X. In Study 1 we examined the relations among six measures of 16 interpersonal behaviors in the context of the interpersonal circumplex. The four capability-related measures were shown to be measuring something distinct from the two trait measures. Unlike trait measures, which showed a circular structure in two dimensions, capability measures exhibited a positive manifold structure (i.e., no negative intercorrelations). The first two orthogonal factors were interpreted as Hostility and Nurturance, which are normally bipolar opposites on trait measures. The only dimension to remain bipolar was introversion–extraversion. In Study 2, the nomological network of the capability measures was shown to be consistent with the theoretical construct. For example, high self-esteem and interpersonal control were associated with almost all of the interpersonal capabilities. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Optimizing recording capabilities of the Utah Intracortical Electrode Array

The Utah Intracortical Electrode Array is a unique silicon-based monolithic structure designed for use as a multichannel interface to the central nervous system. In this paper, we describe a series of acute experiments designed to determine the neural recording capabilities of this electrode array and the dependence of the signal-to-noise ratio (SNR) of the recordings on the electrode surface area (length of metallized tip). We found that both separable unit and multiunit cluster responses could be recorded. Additionally, high SNR recordings could be achieved for some electrodes (with electrode tip lengths of 30-220 microns), while recordings with signals substantially greater than the noise could be made from most of the electrodes provided that the proper electrode surface area was used. The demonstrated recording capabilities of the Utah Intracortical Electrode Array and its unique three-dimensional structure should form the basis for innovative physiological investigations into the functional organization of the cortex as well as for long term neuroprosthesis development.     

A comparison between creep of laminated aluminum and the transverse creep behavior of a unidirectional boron-aluminum composite

The transverse creep behavior of a unidirectional 30 vol pct boron/1145-0 aluminum composite material was investigated over the temperature range 573 to 773 K. The creep curve of the composite exhibited primary, steady state, and tertiary stages of creep, as did the unreinforced laminated matrix; however, the primary stage of creep was consid-erably less pronounced in the composite than in the matrix. The minimum (or steady state) creep rate of the composite was less than that of the laminated matrix alone below a transition stress Σ = 1.74 × 10⁻⁴ E whereE is Young's Modulus of aluminum. Above this transition stress, the minimum creep rate of the composite exceeds that of the unreinforced matrix; further, the strain to failure of the composite generally decreased when the applied stress was above the transition stress. The temperature dependence of the minimum creep rate was the same for both the composite and the laminated matrix. Failure in the composite was initiated by debonding of the filament-matrix interface and it is suggested that debonding of the interface contributes in an additive way to the creep of the composite and to a greater extent at high stresses, leading to the transition stress observed. Formerly graduate student