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THE UNIVERSITY OF CALGARY Eric Snively A ... - Ohio University

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this program. NUAGES typically found one curve per image or two if a medulary<br />

cavity was present in a given region of the metatamal.<br />

4. 30 modeling and finife element mesh generation<br />

To create a 30 model fram image slices stacked along the z-axis, NUAGES<br />

formulates connectivrty between cuwes according to user-supplied commands.<br />

These commands also facilitate mesh creation. For example, the -tri command<br />

generates a tn'angular surface mesh, and tetra creates a tetrahedral volumetric<br />

rnesh. For finite element analysis, a votumetric mesh with tetrahedral elements<br />

was chosen as the reconstructed 30 model.<br />

Other commands entered into NUAGES ensured that the number of<br />

tetrahedra within the mesh was suffciently high for informative stress analysis,<br />

but low enough for efficient proœssing by MARC finite etement software. These<br />

commands were entered as preprocessing options into NUAGES (-popt in the<br />

NUAGES command Iine, followed by the appropnate commands and<br />

arguments).<br />

for example, the command -approx reduces the number of elements by<br />

deleting vertices (points in spacej. if three points form an angle that is less than a<br />

specified nurnber of degrees, one of those points is deleted. The result is a<br />

straight Iine between two vertices; the Iine becomes one side of a larger<br />

tetrahedral element. Large-scale contours are retained whenever three points<br />

subtend an angle greater than the specified amount. Therefore, if a small angle is<br />

chosen, extraneous vertices are deleted, but fidelity to the original shape is

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