25,374 research outputs found
Expandable coating cocoon leak detection system
Development of system and materials for detecting leaks in cocoon protective coatings are discussed. Method of applying materials for leak determination is presented. Pressurization of system following application of materials will cause formation of bubble if leak exists
Trends in Black-White Test-Score Differentials
Until the 1970s, there were few signs of change in the historic difference of one standard deviation between average ability or achievement test scores of blacks and whites in the United States. From about 1970 to the mid- to late 1980s, there was a substantial convergence of the average achievement test scores of black and white youth; however, from the mid- to late 1980s to 1992, test scores began to diverge again. Although we place the greatest weight on data from the National Assessment of Educational Progress (NAEP), the convergence also appeared in other test series. Herrnstein and Murray's highly visible work, The Bell Curve, stands almost alone in minimizing the importance of the convergent trend. We also find a longer-term trend of convergence between the verbal abilities of blacks and whites in data from the General Social Survey (GSS), which covers adult cohorts born since 1909.
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Image Transformations and Printing of Plaster Layers in Spiral Growth Manufacturing
Spiral growth manufacturing (SGM) is a high speed rapid manufacturing technique in
which objects are built up, layer by layer, by simultaneously depositing, levelling and selectively
consolidating thin powder layers onto a rotating build platform. The size and position of the
jetted droplets are mapped by the position and greyscale level of pixels within an 8 bit greyscale
bitmap image. This paper reports on the development of software in which mathematical
algorithms apply geometric transformations to images in preparation for printing onto a rotating
substrate. In support of this work, dimensional accuracy measurements of printed images and
methods to correct radial print density variations are reported. The accuracy of printed images
were found to be within ±0.2mm of their predicted size. The experimental work is briefly
extended to the direct printing of plaster layers, formed by mixing two reactive ink solutions.Mechanical Engineerin
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Further Developments in Process Mapping and Modelling in Direct Metal Selective Laser Melting
This paper advances previous reported work on the mapping and modelling of single
tracks and layers produced in powder beds of tool steel and stainless steel powders by a CO2
laser. For single tracks it reports on predicted and simulated track masses. It validates the
simulations, including the use of absorption close to 1.0 when cylindrical tracks are formed. It
also reports on melt pool temperature calculations and estimated melt pool dimensions which are
used, in conjunction with bed physical properties, to explain why the single tracks form as either
continuous with a crescent shape cross-section, continuous with an elliptical section,
discontinuously irregularly broken, discontinuously balled or only partially melted as scan
speeds and laser powers change. It then extends its scope, experimentally, to consider effects of
scan spacing on single layer formation.Mechanical Engineerin
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Spiral Growth Manufacturing (SGM) – A Continuous Additive Manufacturing Technology for Processing Metal Powder by Selective Laser Melting
Spiral growth manufacturing is a new innovative powder based rapid manufacturing
technique. The innovation exists in the methodology in which powder layers are deposited.
Unlike other pre-placed powder systems, the deposited layers move relative to the location at
which they are processed. This is made possible by a rotating build drum into which powder is
deposited, in spiralled layers, from a stationary hopper. With this configuration powder can be
continuously deposited and levelled and simultaneously processed, eliminating delays in the
build cycle. Stainless steel and cobalt-chrome powder is selectively melted using a 100W flash
lamp pumped Nd:YAG laser. This paper reports on factors affecting build rate and on build
strategies for creating a number of axis-symmetric thin and thick walled cylinders. Experimental
results suggest that build rate for thin walled structures bonded to a substrate will ultimately be
governed by tangential movements of the powder particles when frictional forces are not
sufficient to accelerate the particles along a curved path, provided that enough laser power is
available for melting. Even melt pool balling, which is evident when melting one layer at high
speeds, diminishes in multiple layer builds due to re-melting and infilling.Mechanical Engineerin
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