MGM fact sheet: operations
Mechanics of Granular Materials
At the start of MGM activities in space, the crew opens the lockers and removes the test cells from their storage positions. One test cell is installed on the test/observation pad and the others are stored until needed. The crew connects the PGSC and the video system, and connects electrical lines for power and data. They also insert new memory cards for the start of experiments with each new test cell. The crew activates each experiment sequence via the PGSC, but the experiments will be automatic once started.
Using the PGSC, the crew commands the stepper motor to drive the platen against the specimen at a speed of 35 mm/hr (1.4 in/hr). This compresses the specimen by 37.5 mm by the end of the experiment. During the relief phase of each cycle, the specimen may expand by up to 9 mm as the individual grains shift and realign. The latex sleeve will move with the sand so the grid pattern reveals changes to the cameras.
The test will proceed in five discrete cycles, consisting of a compression and a rebound. The specimen is expected to increase in volume and decrease in density. This means the specimen actually gets bigger and looser as it is compressed, the same phenomenon often seen processes involving granular materials.
|Click on the picture at left to get a 100K, 9-frame movie showing how one MGM specimen was compressed during STS-79. The speed of the movie is misleading; the complete sequence takes about an hour.|
Six MGM test cells will be flown on STS-89. On STS-79, air filled the gaps between sand grains. On STS-89, the sand will be less dense (65% vs. 85%). The experiments will repeat the STS-79 sequence both for calibration against the STS-79 specimens and for scientific data. Then the other three test cells will load their specimens with small-displacement compression/extension cycles. These conditions match those of an earthquake.
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