  Features of rheological laws applied to solid-like granular materials are
recalled and confronted to microscopic approaches via discrete numerical
simulations. We give examples of model systems with very similar equilibrium
stress transport properties -- the much-studied force chains and force
distribution -- but qualitatively different strain responses to stress
increments. Results on the stability of elastoplastic contact networks lead to
the definition of two different rheological regimes, according to whether a
macroscopic fragility property (propensity to rearrange under arbitrary small
stress increments in the thermodynamic limit) applies. Possible consequences
are discussed.
