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931.
Dimensional analysis and scaling in mechanical mixing for fabrication of metal matrix nanocomposites
S. García-Rodríguez N. Alba-Baena N.M. Rudolph J. Wellekoetter X.C. Li T.A. Osswald 《Journal of Manufacturing Processes》2012,14(3):388-392
For a successful enhancement of mechanical properties of metal matrix nanocomposites, a homogeneous nanoparticle dispersion and distribution in the solidified metal is required. Mechanical mixing can be used for initial break-up of agglomerates, and its study can be simplified with dimensional analysis. Using this technique, mixing time and vortex height were assessed while varying fluid properties, impeller angle, and angular speed. Three relevant dimensionless numbers were recognized: the Reynolds (Re), Froude and Galilei (Ga) numbers. Based on blade and impeller shaft angles, a modified Froude number (Fr*) was defined. These parameters were calculated experimentally, varying angular speed from 200 to 1000 rpm for three different impeller angles: 0°, 15° and 30°. This procedure was performed with three fluids: water, and two aqueous glycerin solutions (25% and 50% by volume). Digital images were taken and processed to measure vortex height. Mixing time was measured for water at 0° impeller angle, angular speed ranging from 200 to 1200 rpm. Results showed an optimal dimensionless mixing time with respect to Re. A linear relationship was found between dimensionless vortex height and Fr*. The first had a second order polynomial relationship with the product ReFr*, regardless of impeller angle. This relationship, together with the Ga, specific for each fluid, allows scaling the results to other fluids such as molten pure aluminum. This study allows experimenting in simpler systems that involve transparent fluids, room temperature and low cost, to then elaborate a prediction of vortex height in fluids where measurements are difficult and costly, such as molten metals. 相似文献
932.
Simon?BrinerEmail author Michael?Hartmann Robert?Finger Bernard?Lehmann 《Mitigation and Adaptation Strategies for Global Change》2012,17(4):337-355
We assessed the economic suitability of 4 greenhouse gas (GHG) mitigation options and one GHG offset option for an improvement
of the GHG balance of a representative Swiss suckler cow farm housing 35 Livestock units and cultivating 25 ha grassland.
GHG emissions per kilogram meat in the economic optimum differ between the production systems and range from 18 to 21.9 kg
CO2-eq./kg meat. Only GHG offset by agroforestry systems showed the potential to significantly reduce these emissions. Depending
on the production system agroforestry systems could reduce net GHG emissions by 66% to 7.3 kg CO2-eq./kg meat in the most intensive system and by 100% in the most extensive system. In this calculation a carbon sequestration
rate of 8 t CO2/ha/year was assumed. The potential of a combination of the addition of lipids to the diet, a cover of the slurry tank and
the application of nitrification inhibitors only had the potential to reduce GHG emissions by 12% thereby marginal abatement
costs are increasing much faster than for agroforestry systems. A reduction of the GHG emissions to 7.5 kg CO2-eq./kg meat—possible with agroforestry only—raised costs between 0.03 CHF/kg meat and 0.38 CHF/kg meat depending on the production
system and the state of the system before the reduction. If GHG emissions were reduced maximally average costs ranged between
0.37 CHF/kg meat, if agroforestry had the potential to reduce net GHG emissions to 0 kg CO2-eq., to 1.17 CHF/kg meat if also other options had to be applied. 相似文献
933.
The ball-end milling process is widely used for generating three-dimensional sculptured surfaces with definite curvature. In such cases, variation of surface properties along the machined surface curvatures is not well understood. Therefore, this paper reports the effect of machining parameters on the quality of surface obtained in a single-pass of a ball-end milling cutter with varying chip cross-sectional area. This situation is analogous to generation of free form cavities, pockets, and round fillets on mould surfaces. The machined surfaces show formation of distinct bands as a function of instantaneous machining parameters along the periphery of cutting tool edge, chip compression and instantaneous shear angle. A distinct variation is also observed in the measured values of surface roughness and micro-hardness in these regions. The maximum surface roughness is observed near the tool tip region on the machined surface. The minimum surface roughness is obtained in the stable cutting zone and it increases towards the periphery of the cutter. Similar segmentation was observed on the deformed chips, which could be correlated with the width of bands on the machined surfaces. The sub-surface quality analysis in terms of micro-hardness helped define machining affected zone (MAZ). The parametric effects on the machining induced shear and residual stresses have also been evaluated. 相似文献
934.
Modeling and interpretation of fiber orientation-based failure mechanisms in machining of carbon fiber-reinforced polymer composites 总被引:1,自引:0,他引:1
Kevin A. Calzada Shiv G. Kapoor Richard E. DeVor Johnson Samuel Anil K. Srivastava 《Journal of Manufacturing Processes》2012,14(2):141-149
The development and implementation of a microstructure-based finite element model for the machining of carbon fiber-reinforced polymer composites is presented. A new approach to interfacial modeling is introduced where the material interface is modeled using continuum elements, allowing failure to take place in either tension or compression. The model is capable of describing the fiber failure mode occurring throughout the chip formation process. Characteristic fiber length in the chips, and machining forces for microstructures with fibers orientated at 0°, 45°, 90°, and 135° are examined. For model validation purposes, the model-based machining performance predictions are compared to the machining responses from a set of orthogonal machining experiments. A parametric study is presented that identifies a robust tool geometry, which minimizes the effects of fiber orientation and size on the machining forces. 相似文献
935.
Numair Ahmed Andrew Carlson John A. Rogers Placid M. Ferreira 《Journal of Manufacturing Processes》2012,14(2):90-97
This paper demonstrates the use of a flexible instrumented stamp to enable automated micro transfer printing as a route to large-area, deterministic assembly of microstructured device components or ‘inks’. The ability to instrument the stamp, a critical component for retrieval and placement of a micro device, to detect contact and monitor localized forces during critical events in the printing process not only allows for the development of a robust manufacturing process, but also for a unique vantage point from which to study fundamental issues and phenomena associated with adhesion and delamination of thin films from a variety of substrate materials. This paper presents basic design analysis on the requirements of the cantilevers for compatibility with a typical transfer printing environment. Off-the-shelf thin film strain gages are integrated with a thin elastomeric post as a preliminary prototype and the feasibility of transfer printing with it is demonstrated. Further, the set-up is calibrated to produce force signals for event detection and in situ diagnosis of the process. 相似文献
936.
Does safety climate moderate the influence of staffing adequacy and work conditions on nurse injuries? 总被引:1,自引:0,他引:1
Mark BA Hughes LC Belyea M Chang Y Hofmann D Jones CB Bacon CT 《Journal of Safety Research》2007,38(4):431-446
PROBLEM: Hospital nurses have one of the highest work-related injury rates in the United States. Yet, approaches to improving employee safety have generally focused on attempts to modify individual behavior through enforced compliance with safety rules and mandatory participation in safety training. We examined a theoretical model that investigated the impact on nurse injuries (back injuries and needlesticks) of critical structural variables (staffing adequacy, work engagement, and work conditions) and further tested whether safety climate moderated these effects. METHOD: A longitudinal, non-experimental, organizational study, conducted in 281 medical-surgical units in 143 general acute care hospitals in the United States. RESULTS: Work engagement and work conditions were positively related to safety climate, but not directly to nurse back injuries or needlesticks. Safety climate moderated the relationship between work engagement and needlesticks, while safety climate moderated the effect of work conditions on both needlesticks and back injuries, although in unexpected ways. DISCUSSION AND IMPACT ON INDUSTRY: Our findings suggest that positive work engagement and work conditions contribute to enhanced safety climate and can reduce nurse injuries. 相似文献
937.
Population dynamics and regulation in the cave salamander <Emphasis Type="Italic">Speleomantes strinatii</Emphasis> 总被引:1,自引:0,他引:1
Salvidio S 《Die Naturwissenschaften》2007,94(5):396-400
Time series analysis has been used to evaluate the mechanisms regulating population dynamics of mammals and insects, but has
been rarely applied to amphibian populations. In this study, the influence of endogenous (density-dependent) and exogenous
(density-independent) factors regulating population dynamics of the terrestrial plethodontid salamander Speleomantes strinatii was analysed by means of time series and multiple regression analyses. During the period 1993–2005, S. strinatii population abundance, estimated by a standardised temporary removal method, displayed relatively low fluctuations, and the
autocorrelation function (ACF) analysis showed that the time series had a noncyclic structure. The partial rate correlation
function (PRCF) indicated that a strong first-order negative feedback dominated the endogenous dynamics. Stepwise multiple
regression analysis showed that the only climatic factor influencing population growth rate was the minimum winter temperature.
Thus, at least during the study period, endogenous, density-dependent negative feedback was the main factor affecting the
growth rate of the salamander population, whereas stochastic environmental variables, such as temperature and rainfall, seemed
to play a minor role in regulation. These results stress the importance of considering both exogenous and endogenous factors
when analysing amphibian long-term population dynamics. 相似文献
938.
939.
Further evidence of nuclear reactions in the Pd/D lattice: emission of charged particles 总被引:1,自引:1,他引:0
Almost two decades ago, Fleischmann and Pons reported excess enthalpy generation in the negatively polarized Pd/D-D2O system, which they attributed to nuclear reactions. In the months and years that followed, other manifestations of nuclear
activities in this system were observed, viz. tritium and helium production and transmutation of elements. In this report,
we present additional evidence, namely, the emission of highly energetic charged particles emitted from the Pd/D electrode
when this system is placed in either an external electrostatic or magnetostatic field. The density of tracks registered by
a CR-39 detector was found to be of a magnitude that provides undisputable evidence of their nuclear origin. The experiments
were reproducible. A model based upon electron capture is proposed to explain the reaction products observed in the Pd/D-D2O system.
相似文献
Pamela A. Mosier-BossEmail: |
940.