The theoretical framework of strain localization is summarized. Two kinds of material are studied: an overconsolidated clay and a sandstone. In the case of the oveiconsolidated clay which is contractant, non-associativity of a plastic flow (Khan's model) is found to be sufficient for the detection of the localization in the hardening regime. But for a strong dilatant material such as Fontainebleau sandstone, the non-associativity is no longer sufficient. Non coaxial flow theory needs to be added to the constitutive law to have bifurcation consistent with data provided by careful experiments.

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