Size-Scaling of Tensile Failure Stress in a Hot-Pressed Silicon Carbide

Quasi-static Weibull strength-size scaling of hot-pressed silicon carbide is described. Two surface conditions (uniaxial ground and uniaxial ground followed by grit blasting) were explored. Strength test coupons sampled effective areas from the very small (4 × 10⁻³ mm²) to the very large (4 × 10⁴ mm²). Equibiaxial flexure and Hertzian ring crack initiation were used for the strength tests, and characteristic strengths for several different specimen geometries were analyzed as a function of effective area. Characteristic strength was found to substantially increase with decreased effective area for both surface conditions. Weibull moduli of 9.4- and 11.7 well-represented strength-size scaling for the two ground conditions between an effective area range of 10⁻¹ and 4 × 10⁴ mm². Machining damage was observed to be the dominant flaw type over this range. However, for effective areas <10⁻¹ mm², the characteristic strength increased rapidly for both ground surface conditions as the effective area decreased, and one or more of the inherent assumptions behind the classical Weibull strength-size scaling were in violation in this range. The selections of a ceramic strength to account for ballistically induced tile deflection and expanding cavity modeling are considered in context with the measured strength-size scaling. The observed size-scaling is briefly discussed with reference to dynamic strength.     

Using Hertzian Indentation to Understand the Strength and Ballistic Resistance of Silicon Carbide

This article presents an initial evaluation of the usefulness of spherical or Hertzian indentation for the determination and/or validation of constitutive models and for a potential link to ballistic resistance (interface defeat). Recent advancements in producing more confident elevated stress levels in Hertzian indentation make the usefulness of this test much more feasible. There are very few experimental techniques that can produce the high stresses, strains, and pressures produced in Hertzian indentation tests. Additionally, they are also relatively simple, repeatable, timely, and inexpensive. There is still the disadvantage that the stresses, strains, and pressures are not explicit outputs of the experiments requiring the constitutive response to be inferred by performing computations and comparing the computed force–deflection results to the experiment. Preliminary results indicate that Hertzian indentation can be used to help define the constitutive response but it is still uncertain if it can be used to rank a ceramic's ability to produce interface defeat.     

中欧美拉伸锚固长度比较研究

对中欧美规范中拉伸锚固长度的定义和计算公式进行了比较,并选取不同工况,重点考虑了钢筋间距和直径、混凝土强度和保护层厚度的影响,用规范公式对梁和板中的锚固长度进行了计算分析。通过对比发现,在几种主要因素影响下,Eurocode 2锚固长度曲线变化趋势和极限锚固长度曲线变化趋势整体上比较接近。当考虑钢筋间距和直径的影响时,在梁中和板中,GB 50010—2002的变化趋势和Eurocode 2比较相似,值也比较接近。在考虑混凝土强度和保护层厚度时,GB 50010—2002精细方法计算值在影响因素取值较大时,对于板来说偏保守,建议取个适当的折减系数。ACI318-05多数因素影响下均偏保守,建议考虑相应因素的影响时取合适的折减系数。     

Vapor Annealing as a Post-Processing Technique to Control Carrier Concentrations of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Thin Films

This article demonstrates that carrier concentrations in bismuth telluride films can be controlled through annealing in controlled vapor pressures of tellurium. For the bismuth telluride source with a small excess of tellurium, all the films reached a steady state carrier concentration of 4 × 10¹⁹ carriers/cm³ with Seebeck coefficients of −170 μV K⁻¹. For temperatures below 300°C and for film thicknesses of 0.4 μm or less, the rate-limiting step in reaching a steady state for the carrier concentration appeared to be the mass transport of tellurium through the gas phase. At higher temperatures, with the resulting higher pressures of tellurium or for thicker films, it was expected that mass transport through the solid would become rate limiting. The mobility also changed with annealing, but at a rate different from that of the carrier concentration, perhaps as a consequence of the non-equilibrium concentration of defects trapped in the films studied by the low temperature synthesis approach.     

Perceptual-based quality assessment for audio–visual services: A survey

Accurate measurement of the perceived quality of audio–visual services at the end-user is becoming a crucial issue in digital applications due to the growing demand for compression and transmission of audio–visual services over communication networks. Content providers strive to offer the best quality of experience for customers linked to their different quality of service (QoS) solutions. Therefore, developing accurate, perceptual-based quality metrics is a key requirement in multimedia services. In this paper, we survey state-of-the-art signal-driven perceptual audio and video quality assessment methods independently, and investigate relevant issues in developing joint audio–visual quality metrics. Experiments with respect to subjective quality results have been conducted for analyzing and comparing the performance of the quality metrics. We consider emerging trends in audio–visual quality assessment, and propose feasible solutions for future work in perceptual-based audio–visual quality metrics.     

Dietary exposure to copper in the European Union and its assessment for EU regulatory risk assessment

EU legislation requires a multimedia exposure assessment for substances supplied within the EU. Dietary intake is the main source of exposure for the majority of the population hence an essential component of the human risk assessment. This paper describes the available data for dietary copper and its use in estimating daily intake including variability and determinants of exposure. Typical and reasonable worst-case estimates are derived for the general population from the available peer reviewed literature. Intakes from drinking water are found to exhibit more variability than those from food. Therefore, different exposure scenarios are derived to reflect the range of acute and chronic exposures that may occur. Estimates of typical copper intakes for the EU population are in the range 0.8-1.8 mg/day. Typical copper intakes of men are higher than those of women while the intake among the general adult population is higher than that of the elderly. Intakes of both men and women are generally close to the WHO normative requirements but may be somewhat lower in specific locations where background levels of copper are unusually low. Alcoholic beverages represent minor contribution daily copper intakes. Intakes for children are rather variable ranging broadly from 0.7 to 1.5 mg/day and are somewhat age and sex dependent. Greater uncertainty applies to the assessment of local exposure incorporating food produced on land directly impacted by contemporary copper industry emissions. Specifically, the extent to which soil is enriched in copper in these conditions is unclear. However, effective homeostatic control mechanisms in plants limit uptake and transfer to the human food chain. A best estimate of 0.25 mg/day in addition to regional exposure was derived. Drinking water is estimated to contribute only marginally to total copper intake in most cases. Higher intakes may occur in areas of poor water quality and/or corroded distribution systems. Such elevated exposures appear unusual but their frequency is unknown.     

Improvement of predicted fine and total particulate matter (PM) composition by applying several different chemical scenarios: a winter 2005 case study

A new method using several different chemical scenarios is developed to predict chemical composition of fine (PM2.5) and total (PM10) aerosol. This method improves the accuracy of predicted PM concentrations. The Mesoscale Model version 5 (MM5) and a 3-dimensional Eulerian chemical model (CAMx4.2) are used to predict PM2.5 and PM10 concentrations using gridded input emissions (from the "Total" group) over a 48-72 h time period for Christchurch (New Zealand) for winter 2005. The aerosol concentrations are obtained for four different chemical compositions (chemical scenarios) of the input aerosol emissions. PM2.5 chemical compositions are based on previous Christchurch winter studies and from observations in other countries with similar winter pollution problems, and used in CAMx4.2 to model seven winter 2005 heavy pollution episodes. The error between observed and modelled PM2.5 concentrations is based on predictions of fine aerosol that are derived from linear regression with PM10. It is used to find the minimum difference between modelled and observed PM2.5 for an observation site located in the Christchurch residential area. Combination of the chemical scenarios with analysis of the minimum error is used to create a new complex chemical scenario. The new complex scenario is used to re-calculate all pollution episodes to obtain new values of PM with minimum error compared with observed aerosol concentrations. Mean Absolute Error of the calculated PM2.5 (for all pollution episodes) decreased from 21-24 microg m(-3) to 14-16 microg m(-3) compared with observations. The chemical composition of the modelled PM2.5 is also discussed.     

Density effects on activated sludge zone settling velocities

A recently developed method to manipulate biomass density based on the addition of synthetic microspheres was used to isolate density effects on zone (hindered) settling velocities in samples from full-scale activated sludge plants over a range biomass densities and concentrations. Settling velocities increased by as much as a factor of five as density was increased by microsphere addition. The effects of density on the Vesilind sedimentation model parameters (V=V(o)e(-kX) , where V is the settling velocity, X is the biomass concentration, and V(o) and k are constants) were evaluated. The parameter V(o) was positively correlated with density for values greater than approximately 1.02 g/mL, while k values were approximately constant in this range. New models were developed for settling velocity as a function of both density and biomass concentration, and these may be useful for future incorporation with clarifier models to improve predictions of system performance, particularly when biomass characteristics known to affect density are variable, such as polyphosphate and nonvolatile suspended solids content.     

Filament content threshold for activated sludge bulking: artifact or reality?

An often-noted feature of activated sludge settleability is the existence of a threshold content of filamentous bacteria, below which filaments have a relatively small effect on settleability, and above which settleability is much more sensitive to filament content. This concept has historically been based on an apparent inflection point in settleability/filament content curves. However, this was demonstrated to be an artifact of data presentation on semi-logarithmic scales. Analyses of several published data sets indicated that there was little or no evidence of a threshold effect when data were presented on linear scales, particularly around the 150 mL/g sludge volume index (SVI) "threshold". Rather, there appears to be a generally continuous (and often nearly linear) relationship between filament content and SVI, suggesting that the threshold effect, as commonly perceived, is not accurate. A modified empirical model of filament content and settleability was also presented.