MECHANICS OF GRANULAR MATERIALS
Overview
The third flight for MGM is scheduled on STS-107 in 2001. While the investigation is similar to the previous two flights, there are some exciting new changes. Rather than testing with air in the specimen, the sand will be saturated with water. This will allow us to achieve several important objectives. First, we will be able to verify our previous experiments, as tests on dry and saturated materials should yield the same results. Questions regarding cohesion due to possible partial saturation on the dry specimens will be answered. Second, we are implementing a new "reforming" procedure. Three specimens will be launched at 85% relative density. After compression, a specimen will be reformed: the top platen will be raised up to the original position, and extra pore water will be moved into the specimen, until a specimen reaches approximately -10% relative density. This will allow the particles to move around and erase the internal structure of shear bands, etc. Surplus water will then be removed from the specimen, and the particles will form a solid structure, at approximately 10% relative density. The loose specimens will then be tested in three different configurations. Normally consolidated and overconsolidated specimens will be tested under drained conditions to quantify the effect of pre-loading, which is required for stability during launch. Undrained testing will also be performed, to simulate earthquake liquefaction. Using only three test cells, eight tests will be performed, with a ninth sample being brought back down uncompressed to assess the reformation technique in microgravity.
Reformation Technique
Below is a link to a picture of a specimen reformed in the terrestrial environment (Terrestrial Reformation Figure). The specimen was first compressed (dry) on the flight-like bench unit to 25% axial strain, then reinstated to the original size. In order to reform the specimen, the specimen was saturated, and the internal (pore water) pressure brought slightly above the external (water jacket) pressure. This freed the grains of sand and allowed them to move in relation to each other. The specimen reformed very easily to the state shown below. A small amount of jarring of the test cell was used to remove one wrinkle from the middle of the specimen. While a successful reformation, the influence of gravity is clearly evident: rather than a cylindrical specimen, it is rather conical. It is expected that reformation will improve in the microgravity environment, where particles will move around easier, allowing better reformation. Also, the conical appearance of the end product should be eliminated, without the directional preference of gravity.
After proof of concept on specimens as discussed above, work began to apply this method to flight hardware. The resulting method is broken into two distinct steps. In the first step, two actions take place. One, the compression motor is moved back to its pre-test configuration, which raises the top loading platen such that the specimen height is again 150 mm. Two, pore water is moved into the specimen to increase the volume such that the relative density is -10%. At this point, the system is powered off, allowing the operator to agitate the specimen to begin free movement of the particles. Ground tests show that some agitation is helpful in fully breaking up the internal structure. Click on the link below (Flight-Like Reformation Scans) for more information. In flight, the specimen will set in this arrangement for between 4 and 24 hours. The second step takes place after the system is powered on again. The surplus water is removed from the specimen. The system is commanded to extract a volume of water specified by the user. The effective pressure is monitored, and when the internal pressure begins to drop (external pressure is controlled during this procedure) the process is completed. This occurs through the given amount of water being extracted, or the the operator cancelling the process. The operator can perform this step in small pieces, by extracting a small volume at a time.
Terrestrial Reformation Figure
Flight-Like Reformation Scans
Hardware
Minor hardware changes were needed for performing experiments with saturated soil, and accomodating both drained and undrained experiments. The sample side of the accumulator will be filled with water, and the fine pressure sensors were altered to be vented, in order to maintain the 2 psig backpressure in the sample. One sensor measures confining pressure, the other backpressure. During testing, each sensor is monitored and used to control the confining and backpressure, respectively. Overall, the equipment functions in much the same manner as in MGM-I and MGM-II.
Flight Software
The software was updated to run in the Windows environment. Changes were also made to operate the equipment for saturated tests (water in both the water jacket and specimen accumulators) in both the drained and undrained mode, as well as additional functions, such as reforming the specimens.
Ground System Software
New to MGM-III, ground telemetry and uplink commanding will be implemented. State of Health messages, including science data, will be downlinked once per second. This allows the science and engineering teams to monitor the experiments as the progress, and can help in recognising and diagnosing off-nominal situations. Uplink commanding will also be available. Because there are various valves and connectors in the experiment, a crewperson is still needed to help run the experiment. However, portions of the experiment can be run from the ground, saving crew time. The reforming processes are part of the experiments to be run from the ground; this will allow the scientists to have greater involvement in any small adjustments to be made with this new procedure. (The only forseen adjustments are small tweaks in the volume of water to introduce or extract.)
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