  The question when and how dense granular materials start to flow under
stress, despite many industrial and geophysical applications, remains largely
unresolved. We develop and test a simple equation for the onset of quasi-static
flows of granular materials which is based on the frictional aging of the
granular packing. The result is a non-monotonic stress-strain relation which -
akin to classical friction - is independent of the shear rate. This relation
suffices to understand the below-threshold deformations of aging granular
media, and its solid-to-liquid transition. Our results also elucidate the
(flow) history dependence of the mechanical properties, and the sensitivity to
initial preparation of granular media.
