Jean-Francois Metayer, Donald Suntrup III, Charles Radin, Harry Swinney and Matthias Schroeter
Shearing of frictional sphere packings
(841K, pdf)

ABSTRACT.  We measure shear stress in packings of glass spheres by pulling a thin 
metal plate vertically through a bed of volume fractions phi, which is 
set (before the plate is pulled) to be in the range 0.575 to 0.628. 
The yield stress increases exponentially with phi with a change in slope at phi approximately 0.594. 
An analysis of the shear stress fluctuations provides a measure of the shear modulus divided by shear zone width; 
this measure also exhibits distinct regimes above and below phi approximately 0.594. 
The yield stress and the rescaled shear modulus are velocity independent over 4 decades.