Nicholas C. Costes

Assessment of Localized Deformations in Sand Using X-Ray Computed Tomography

The internal fabric and localized deformation patterns of triaxial sand specimens were investigated using computed tomography (CT). Three displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses (0.05, 0.52, and 1.30 kPa) in a microgravity environment aboard the Space Shuttle during the NASA STS-79 mission. CT scanning was performed on these flight specimens, as well as on an uncompressed specimen and a specimen tested in a terrestrial laboratory at 1.30 kPa effective confining stress. CT demonstrated high accuracy in detecting specimen inhomogeneity and localization patterns. Formation of deformation patters is dependent on the effective confining stress and gravity. Multiple symmetrical radial shear bands were observed in the specimens tested in a microgravity environment. In the axial direction, two major conical surfaces were developed. Nonsymmetrical spatial deformation was observed in the 1-G specimen. Analysis tools were developed to quantify the spatial density change. Void ratio variation within and outside the shear bands is calculated and discussed.

Microgravity experiments on granular materials

Gravity induced stresses in terrestrial laboratory specimens comprising of relatively weak and soft granular materials, are often of the same magnitude as the external tractions that are applied during investigations of constitutive behavior especially at low intergranular stress levels. The presence of heterogeneous strain and stress fields within such a specimen makes it difficult if not impossible to obtain objective and unambiguous constitutive properties and to devise relevant constitutive equations. To fill this technological gap, microgravity experiments are now planned which will be performed during future flights of the Space Shuttle, in conjunction with ground-based tests. This paper deals with analytical and experimental issues related to constitutive modeling of granular materials.

Constitutive and stability behavior of soils in microgravity environment

The constitutive behavior of soils such as strength, stiffness, and localization of deformations are to a large extent derived from interparticle friction transmitted 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-Coulomb strength envelope, which is the criterion most frequently used, is unclear both with respect to interparticle friction and cohesion. Because of the impossibility of eliminating gravitational body forces on earth, the weight of soil grains develops interparticle compressive stresses which mask true soil constitutive behavior even in the smallest samples of models. Therefore the microgravity environment induced by near-earth orbits of spacecraft provides unique experimental opportunities for testing theories related to the mechanical behavior of soils. Such materials may include cohesionless soils, silt, clay, industrial powders, crushed coal, etc. Th...

Computed tomography investigation of microgravity-tested sand samples

Computed Tomography (CT) is being used to investigate the complex internal structure of axisymmetric (triaxial) sand specimens. A series of triaxial experiments was conducted on dry Ottawa sand specimens at very low effective confining stresses in a microgravity environment aboard the Space Shuttle during two missions. Post-flight analysis includes studying the internal fabric and failure patterns using CT. In addition ground-tested specimens subjected to different compression levels are scanned to investigate the evolution of instability patterns, quantify void ratio variation, and provide a direct comparison with microgravity specimens. For an upcoming Shuttle mission, trial specimens are scanned to investigate an experimental reforming method for flight and evaluate techniques for reconstituting specimens. The CT technique demonstrates good ability to detect specimen inhomogeneities and localization patterns, and quantify void ratio variation within sand specimens.

Issues on Geomechanics

The microgravity environment induced by near-earth orbits of spacecraft, along with the hard-vacuum, of space through the use of a wake shield, provide unique experimental opportunities for testing theories related to the mechanical behavior of terrestrial materials that are essentially granular without any physical or chemical cementation between the grains. Such materials may include cohesion less soils, industrial powders, pharmaceutical products, industrial or agricultural grains, crushed coal, etc. This paper will focus on such issues and will include a microgravity experiment currently planned to study the behavior of dry and saturated sands at very low effective stress levels. This experiment is scheduled to be performed during a Space Shuttle-MIR mission in 1996.