MGM fact sheet: introduction
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Mechanics of Granular Materials
Second round of space experiments on STS-89 will help us
understand better the behavior of soils, powders, and other solid particles
under very low confining pressures
Anyone who has ripped open a vacuum packed pouch of coffee has experienced a fundamental aspect of granular mechanics: a singular shift in conditions can drastically change the properties of a bulk material. While the atmosphere presses on the pack, the grains push against one other, locking each other in place, creating a stiff "brick." Once pressures are released, the grain assembly becomes very weak and soft, and moves about freely, almost like a liquid.
During critical, unstable states - like liquefaction of saturated, loose sand during an earthquake - gravity acts as a "follower load" that makes the structure collapse. Even under laboratory conditions, this is too rapid to allow detailed study of intergranular forces and conditions. Further, gravity-induced stresses complicate the analysis.
To understand how granular materials behave under low stresses, NASA has sponsored the Mechanics of Granular Materials (MGM) experiment for flights aboard the U.S. Space Shuttle. In orbit, MGM uses the weightless environment of orbital flight to test soil under very low pressures. The results will further understanding of the behavior of granular materials and help in conceptual and analytical modeling. This will be applied to improving foundations for buildings, managing undeveloped land, and handling of powdered and granular materials in chemical, agricultural, and other industries.
The first flight of MGM, on STS-79 (September 1996) was highly successful. One of the main findings is that the lower the confining pressure on the dense specimens, the higher the friction angle becomes (i.e., the specimens become stiffer). The second flight, scheduled for STS-89 in January 1998, will comprise twice as many experiment runs and expanded test conditions.
MGM traces its origins to studies to help design the wire mesh wheels for the Lunar Rover Vehicles driven by astronauts on the last three Apollo missions in 1971-72. Results from MGM may be applied in the future to advanced rovers for the exploration of Mars in addition to terrestrial needs.
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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack