BRITTLE-DUCTILE TRANSITION IN POROUS ROCKS BY CAP MODEL

A pressure-dependent cap model that is capable of describing the britt le-ductile transition in porous rocks is proposed. The model consists of a hardening Drucker-Prager yield cone with a nonassociative how rul e, an ellipsoidal cap with an associative flow rule, and a failure or limiting surface. This surface designates the stress states at the inc eption of failure below brittle-ductile transition, or the onset of id eally plastic response above brittle-ductile transition. The appropria tely combined structures of the associative and nonassociative flow ru les eliminate the difficulties associated with a singular corner regio n at the intersection of the cap and failure surface. A cross hardenin g between the cap and cone is discussed. The model is capable of a goo d qualitative agreement with experimental data, as demonstrated by the results of the triaxial compression tests under various amounts of th e confining pressure. A stress-strain response with inelastic volume d ilatancy at lower confinements, and an increased ductility and hardeni ng rates at higher confinements, are well predicted.