All three MGM test cells were processed as planned during the mission. Data include three video images of each cell taken every second, plus nearly 15 megabytes of pressure measurements.
After return to Earth, each specimen was impregnated with an epoxy to stabilize the sand column so it could be handled. A silhouette of each cell was obtained every 5° (72 edge profiles in a full rotation) and converted into measurements of the cell diameter vs. preflight diameter.
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Next, the cells were examined by computer tomography (CT scan) at Los Alamos National Laboratory. A 1024x1024-pixel image was made every 1 mm along the length of the specimen for up to 120 images per column. From this, three-dimensional images were produced to reveal internal details. The last examination involved sawing the columns into disks to allow detailed examination under an optical microscope.
The CT scans show features unlike those seen in ground-based tests. Cross-sections have areas of generally uniform density outside of shear zones. Cross-sections at right angles to the axis of compression show lower and higher density areas seeming to separate into radial streams, tied together toward the center of the specimen, and at right angles to the outer surface. In vertical sections, a shear cone and shear plane are visible.
The data processing and analysis effort thus far has been very successful and shows exciting results giving researchers unique data on the mechanics of granular materials. One finding is that gravity appears to hinder load oscillation phenomena which are much more distinct in the flight data than in ground testing. New peak strength and friction angle information show promise of proving a long-term hypothesis in the geotechnical community.
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Author: Dave Dooling
Curator: Linda Porter
NASA Official: Gregory S.
Wilson
