Mechanics of Granular Materials (MGM)

Objectives

An assessment of the influence of solid-solid and solid-fluid interactions on the mechanical behavior of granular materials under various drainage conditions, when such materials are subjected to very low confining pressures/effective stresses can be performed in the absence of a gravitational potential.

Shuttle-Mir Missions
STS-79, STS-89

Approach
A set of nine very low effective confining stress level experiments, were conducted in the microgravity environment of the Space Shuttle (STS-79, September, 1996 and STS-89, January, 1998). The test specimens consisted of subangular quartz sand with average grain size of 0.22 mm. The sand was placed into a natural rubber latex membrane having a thickness of 0.30 mm. The ends of the specimens were sealed by the tungsten carbide end platens. In each test, axial compression unloading and reloading cycles were completed at regular intervals. Axial load, axial displacement, confining pressure, ambient temperature, and acceleration data were recorded. Video cameras were used to record the behavior of the specimens. A grid was printed on the elastic membrane that facilitated the tracking of the compression motion. The test specimens were impregnated with epoxy, following the tests, for postflight x-ray computed tomography.

Results
In the absence of cohesion, the friction angles observed were in the range of 75 (± 1) to 70 (± 1) degrees at the lowest confining stress level to 56 degrees at the highest level. The dilatancy angles for all experiments were in the range of 30 degrees. The deformations were relatively uniform in all cases right up to the maximum displacements. No shear bands or other forms of localized deformation were visible at the surfaces of the specimens. The overall behavior of the specimens was brittle-ductile with significant material instabilities only at the 0.05 kPa level, resulting in strain-softening. More ductile behavior was seen at the 0.52 and 1.30 kPa levels. X-ray computed tomography studies show relatively uniform internal structure with radial-turbine screw fan-like patterns appearing almost periodically, where the fans constitute zones of dilation. The specimens'overall nominal stress-strain response curves display periodic patterns of minor instability, with stress variations resembling stick-slips that were of a magnitude 5-10% of the overall stress levels. These periodic material instabilities appear to be independent of confining stresses in the ranges that were studied. All six experiments showed significant initial stiffnesses, which were comparable to the regularly spaced observed unloading-reloading stiffness responses. It appears that there is little or no coupling between the unloading-reloading stiffness modulus behavior, which is predominantly elastic, and the confining stress level, or the amount of inelastic deformations induced. Ground-based experiments were compared to the flight data, and it was shown that the in-space experiments showed consistently higher friction and dilatancy angles at the lower confining stresses. Analysis of the flight experiments and the terrestrial tests show that there is internal consistency in the material properties except for the very high dilatancy and friction angles, and elastic moduli seen in the microgravity experiments. These findings have large impact on engineering and science involving the mechanics of granular materials, such as Earthquake engineering, geotechnical engineering, coastal and ocean engineering, sediment transport, geophysics, storage and handling of bulk (granular) solids, etc.

Earth Benefits
The advantage of studying the mechanics of granular materials in microgravity is the low stress environment which cannot be duplicated in ground-based laboratories. Predicting the behavior of saturated loose sands during earthquakes or wave loading depends on the understanding of interactions between granular materials and the interstitial water that surrounds the grains; during these the unstable states gravity is a secondary influence. Microgravity, provides the ideal setting for studying the behavior of cohesionless materials in the absence of body forces. These studies have potential application for the design engineering of buildings in unstable geological settings subject to erosion, earthquakes, and water saturation.

Publications
Runesson, K., Larsson, R., and Sture, S. Localization in hyperelastic-plastic porous solids subjected to undrained conditions, Int. J. Solids and Structures, Pergamon Press, Vol. 35, No. 31-32, pp. 4239-4255, 1998.

Sture, S., Costes, N.C., Batiste, S.N., Lankton, M.R., Alshibli, K.A., Jeremic, B., Swanson, R.A., and Frank, M., Mechanics of granular materials at low effective stresses, ASCE, J. of Aerospace Engineering, Vol. 11, No. 2, July, pp. 67-72, 1998.

Jeremic, B., and Sture, S., Tensor objects in finite element programming, Int. J. Numerical Methods in Engineering, Vol. 41, 113-126, 1998.

Jeremic, B., Runesson, K., and Sture, S., Large deformation elasto-plastic analysis of geomaterials: from experiment to numerical predictions, Proc. Int. Conf. on Comp. Meth. and Advances in Geomech., Wuhan, PR China, November, 1997, Vol. 2, 678-692.

Alshibli, K.A., Swanson, R.A., Costes, N.C., Sture, S., Batiste, S.N., and Lankton, M.R., Mechanics of granular materials under very low effective stresses in a microgravity environment, Proc. Am. Geophysical Union, Boston, Massachusetts, April, T21B-8, 1998.

Swanson, R.A., Alshibli, K.A., Frank, M., Costes, N.C., Sture, S., Batiste, S.N., Lankton, M.R., and Jeremic, B., Mechanics of granular materials in microgravity at low effective stresses, Proc. Am. Geophysical Union, Boston, Masaachusetts, April, T21B-7, 1998.

Alshibli, K.A., Sture, S., and Costes, N.C., Effect of inclusions on plane strain behavior of sand, ASCE, Proc. 12th Engineering Mechanics Conference, Univ. of California, San Diego, May, CD-ROM, 1998.

Costes, N.C., and Sture, S., A mobility concept for Martian exploration, Proc. ASCE, 5th Int. Conference on Engineering, Construction and Operations in Space, Vol. 1, 489-494, 1998.

Jeremic, B., Runesson, K., and Sture, S., Finite deformation hyperelasto-plasticity of geomaterials: Numerical integration algorithms, Proc. 13th U.S. Natl. Congress of Applied Mechanics, Univ. of Florida, MA9, June, 1998.

Sture, S., Modeling of granular materials at low stresses, Proc. 13th U.S. Natl. Congress of Applied Mechanics, Univ. of Florida, MB9, June, 1998.

Sture, S., and Costes, N.C., Mechanics of granular materials at very low effective stresses, Proc. 13th U.S. National Congress of Applied Mechanics, University of Florida, gainesville, MB9, June, 1998.

Sture, S., "Experiments and Analysis of Granular Materials aboard the Space Shuttle", Invited Lecture, Colorado School of Mines, Department of Engineering, Golden, Colorado, March 31, 1998.

Sture, S., "Mechanics of Granular Materials under Low Effective Stresses and Static Conditions", Invited Lecture, Cornell University, Department of Theoretical and Applied Mechanics, April 15, 1998.

Sture, S., "Microgravity and Low Stress Experiments on Granular Materials aboard the Space Shuttle," Moran Lecture in Engineering, University of Notre Dame, Indiana, April 30, 1998.

Sture, S., "Mechanics of Granular Materials at Low Effective Stresses", Fourth Microgravity Fluid Physics and Transport Phenomena Conference, Cleveland, Ohio, August 14, 1998.

Sture, S., "Constitutive Issues in Soil Liquefaction, Int. Workshop on the Physics and Mechanics of Soil Liquefaction", NSF and Johns Hopkins University, Baltimore, Maryland, September 10, 1998.

(Lectures and proceedings papers related to NASA/JSC organized symposia in connection with 180-day results reports, 360-day reports, and NASA microgravity results symposia (organized by John Uri et al., at JSC, and held at NASA Ames Research Center, April 1-3, 1998, etc. are not included. It is assumed that these have found their way into your database separately.)

Batiste, S.N., Lankton, M.R., and Sture, S., "Mechanics of Granular Materials: STS-89/Mir8 experiments," Second Phase 1 Research Program Results Symposium, NASA, Ames Research Center, April 1, 1998. Report

Sture, S., Costes, N.C., Batiste, S.N. Lankton, M.R., Alshibli, K.A., and Swanson, R., 180-day Report: Mechanics of Granular Materials Experiment Research Results, Report to NASA, Johnson Space Center, June, 1998.

Principal Investigators
Stein Sture, Ph.D.
University of Colorado at Boulder

Co-Investigators
Dr. Nicholas C. Costes
Dr. Khalid AlShibli
Dr. Roy A. Swanson
Mr. Mark Lankton
Ms. Susan Batiste
Mr. Tae-Hyung Kim
Ms. Maria P. Tchonkova

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