Capturing brittle-to-ductile transitions in porous rocks via meso-scale informed constitutive modelling

Localised failure under low confinement and diffuse deformation with subsequent localisation under high confining pressure have been widely observed in porous rocks, both in nature and experiments. In this study, a novel mechanism-based constitutive framework, termed the two-scale approach, is established to describe the mechanical responses of porous rocks from a mesoscopic perspective. The proposed model considers a volume element with two distinguished components: an embedded meso-scale localisation band and the surrounding inelastic bulk, whose responses are governed, respectively, by a cohesive–frictional model and an elastoplastic continuum model. The interactions between these two components enable the model to effectively describe both localised and diffuse failure modes under a wide range of stress states, especially representing the complex localisation behaviour within inelastic bulk, which is beyond the capability of all existing constitutive models. In particular, both brittle and ductile responses, as well as their transitions, can be depicted at the meso-scale. Validation against two sets of triaxial compression tests on different sandstones demonstrates its robustness in capturing complex failure mechanisms under varied loading conditions. Furthermore, the model successfully accounts for key features such as the size effect and Lode angle dependence, without requiring additional fitting parameters, which underscores its versatility.