Quasistatic rheology and the origins of strain

Features of theological 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 theological 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. (C) 2002 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.