Microgravity Experiments on Granular Materials

Abstract

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.

The role of gravity or pressure and contact stiffness in granular rheology

“…These results can be significant for planetary studies regarding the shear strength of the granular material on extraterrestrial bodies such as Moon or Mars. Significant amount of experimental work using parabolic flights have shown the increase in shear strength of the material with decreasing gravity [18][19][20][21][22][23] without proper explanation. We propose that the anisotropy, i.e., the rearrangement in the contact network, is the key mechanism controlling this dependence if no new g-dependent effects or forces are involved.…”

“…However, particles used in these cases were extremely rigid. Few studies were performed systematically investigating the flow behavior in the low pressure/gravity regime [18][19][20][21][22][23]. Figures 3(a) and (b) show shear stress-pressure curves for different values of normal stiffness k n and external gravity g, respectively.…”

“…However, very little is known regarding the same in the presence of a broader range of normal stress. Shear tests performed on parabolic flights have shown an increase in the effective friction at low confinement [18][19][20][21][22][23].…”

“…Different symbols represent different values of κ as given in the legend of figure 6. The solid line represents the corresponding fit to(19). Local data are shown for γ γ…”