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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.

On STS-89, three MGM test cells will be subjected to five cycles of compression and relief (left) and three will be subjected to shorter displacement cycles that simulate motion during an earthquake (right). In the compression/relief tests, the sand particles will rearrange themselves and slightly re-expand the column during relief. Ultimately, each cycle will slowly shorten the column while making it wider. Total displacement is expected to be about 38 mm (1.5 in.). In the short displacement tests (right), the specimen's resistance to compression decreases, even though the displacement remains the same. The specimens will be cycled up to 100 times or until the resistive force is less than 1% that of the previous cycle.

The force and displacement scales of the two graphs are different, with the displacement expected to total about 0.5 mm (0.02 in.) after 5 cycles, and 5 mm (0.2 in.) after 100 cycles. The curves shown are representative of the forces and displacements expected during the tests.

Link to 581x336-pixel, 160K JPG (left) or 250x336-pixel, 64K JPG (right) Illustrator version will be available presently. Credit: NASA/Marshall Space Flight Center.

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.

Spacehab (left) which will carry MGM in the shuttle payload bay,, is prepared for flight on STS-89. Links to 1536x1052-pixel, 736K JPG. Credit: NASA/Kennedy Space Center.

Astronaut Jay Apt installs an MGM test cell, in its water jacket, in a locker in the Spacehab module for experiment runs during STS-79. Operations and equipment for STS-89 will be virtually identical. Links to 1536x1052-pixel, 416K JPG. Credit: NASA/Johnson Space Center.

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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack