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Mechanics of Granular Materials

Abstracts for American Geophysical Union Spring 1998 conference

T21B-07 Mechanics of Granular Materials in Microgravity at Low Effective Stresses. R A Swanson, University of Colorado at Boulder; K A Alshibli, University of Colorado at Boulder; M L Frank, University of Alabama in Huntsville; N C Costes, NASA/Marshall Space Flight Center; S Sture, University of Colorado at Boulder; S N Batiste, University of Colorado at Boulder; M R Lankton, University of Colorado at Boulder; B Jeremic, Clarkson University

The Mechanics of Granular Materials (MGM) project is a recent series of microgravity experiments designed to achieve quantitative understanding of the constitutive properties and deformation behavior of granular materials at very low confining pressure (effective stress) levels. The MGM experiments were conceived and designed to utilize the microgravity environment of low earth orbit and have provided new and unique experimental results on the mechanical behavior of terrestrial granular materials at low stress levels attainable only in the absence of gravitational body forces.

Three displacement-controlled, drained triaxial (axisymmetric) loading experiments were conducted on cohesionless granular soil specimens at very low effective confining stresses in a Spacehab module on the Space Shuttle Atlantis during the STS-79 mission of September 1996. Sand specimens were subjected to quasi-static loading/unloading compression profiles. The cylindrical specimens measured 75 mm in diameter and were 150 mm long, confined by a thin, flexible latex rubber membrane printed with grid patterns to facilitate video camera tracking of specimen motions. The specimens were mounted between smooth tungsten carbide end platens within a test cell apparatus featuring a Lexan water jacket allowing the enclosed specimen to be sealed at a controllable water confining pressure. The average grain size of the subangular Ottawa quartz sand was 0.22 mm, and the specimens' relative densities were 86.5% (+/- 0.8%). The confining stresses of the three experiments were 0.05, 0.52, and 1.30 kPa.

The MGM flight hardware also comprised control and data recording systems housed in a flight locker that also served as the stage for conducting and video recording of experiments. Detailed recording of data during experiments, included axial load and displacement, confining pressure, bulk volume change, and ambient pressure, temperature and acceleration levels. The most significant findings derived from the data are seen in the relation between principal stress difference and nominal axial strain responses that are quite unusual and differ among the three confining pressure cases. Analysis of the flight data shows very high friction angles in the range of 58 deg. to 75 deg. The dilatancy angles were also unusually high, in the range of 12 deg. to 14 deg. We present findings derived from data analysis and discuss implications on the mechanics of granular materials at low confining stresses. Specimens are impregnated with epoxy post-flight to allow handling and x-ray computed tomography (CT scan) examination of internal structure. CT images and derived volume renderings that have revealed unique shear band formation evidence and other never before observed soil fabric features are presented.

Six additional experiments were performed on dry, loose specimens during the reflight of MGM on STS-89 in January 1998 under the same confining pressures. Three experiments were subjected to quasi-static axial loading/unloading and the other three were subjected to cyclic loading. Preliminary results of these experiments will be given in the present paper with details presented in our companion paper.

T21B-08 Mechanics of Granular Materials Under Very Low Effective Stresses in a Microgravity Environment. K A Alshibli, NASA/Marshall Space Flight Center, R A Swanson, University of Colorado at Boulder; M L Frank, University of Alabama in Huntsville; N C Costes, NASA/Marshall Space Flight Center; S Sture, University of Colorado at Boulder; S N Batiste, University of Colorado at Boulder; M R Lankton, University of Colorado at Boulder

Uncemented granular materials such as cohesionless soils, crushed rock and industrial powders exhibit constitutive properties and deformation behavior such as strength, stiffness, plastic hardening and localization of deformations that are to a large extent derived from interparticle friction between solid particles and particle groups. Interparticle forces are highly dependent on gravitational body forces. At very low effective confining pressures, the true nature of the Mohr envelope, which defines the Mohr-Coulomb failure criterion for soils, as well as the relative contributions of non-frictional components to soil's shear strength, cannot be evaluated in terrestrial laboratories. The weight of soil grains produces interparticle compressive stresses that mask the true constitutive behavior in cohesionless granular materials, even in the smallest samples. The Mechanics of Granular Materials (MGM) project is a series of microgravity experiments designed to achieve quantitative understanding of the constitutive properties and deformation behavior of granular materials at very low confining pressure (effective stress) levels. The MGM experiments were conceived and designed to utilize the microgravity environment of low earth orbit and have provided new and unique experimental results for testing theories on the mechanical behavior of terrestrial granular materials at low stress levels realizable in the absence of gravitational body forces.

A total of nine displacement controlled triaxial (axisymmetric) compression tests have been performed in a Spacehab module of the Space Shuttle Orbiter to achieve the MGM science objectives. The first three tests (F1) were performed on dry, medium-dense silica sand specimens during the STS-79 Shuttle-Mir mission in September 1996. All tests were performed successfully under 0.05 kPa, 0.52 kPa, and 1.30 kPa confining pressures, and were subjected to quasi-static axial loading/unloading profiles. Six subsequent tests (F2 and F3) were performed on dry, loose specimens during the STS-89 Shuttle-Mir mission in January 1998 under the same confining pressures. Three F2 experiments were subjected to quasi-static axial loading/unloading whereas the three F3 experiments were subjected to cyclic loading.

Experimental results are reported and interpretation given in terms of granular material constitutive properties. The results are derived from the detailed record of the flight data including axial load, axial displacement, confining pressure, bulk volumetric changes, and ambient temperature and acceleration levels, as well as 360 deg. video camera recording of specimen deformations. The sand specimens are processed post-flight by epoxy impregnation followed by x-ray computed tomography (CT) examination of internal specimen structure that has revealed unique shear band formation and other never before observed soil fabric properties. CT images and volume renderings will be presented and discussed.

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