Starting from the original formulation of the Granta-Gravel and Cam-Clay constitutive models, the framework of elasto-plasticity has been successfully applied to describe the geomaterials behaviour. Thanks to the modifications proposed by different authors, several aspects of the granular materials behaviour were progressively taken into account, such as the non associated flow rule and the effects of induced anisotropy. Recently, a series of triaxial laboratory tests performed by di Prisco and Imposimato (1996) has put in evidence an aspect which is disregarded in elasto-plasticity: the time dependency of the mechanical behaviour of loose sand specimens. The same authors proposed to interpret the observed behaviour within the framework of visco-plasticity. In this paper, the non reversible behaviour of an idealised granular material composed of a 2D stack of disks is analysed by performing a series of Distinct Element numerical simulations (PFC-2D code). Starting from different stress levels (from isotropic to close to failure) previously recorded during a biaxial compression simulation, a series of finite stress probes is applied to the numerical specimen. All the probes have the same (small) magnitude, but they differ in direction in the principal stress plane. For each probe, the stress increments are applied to the boundary plates in an unique finite step, and the corresponding strain increments are recorded during the following time period. The obtained results show that the transient state is reached passing through two phases: a first one, where mainly elastic strains take place as the stress increment propagates within the specimen followed by a second one where the measured strains (non reversible) increase under constant stress; The duration of both phases considerably increases as the initial stress state approaches to failure. These results, which confirm the adequacy of a visco-plastic flow rule, are put in relation to the specimen structure progressive rearrangement caused by the applied stress-increment.

Micromechanical investigation of the visco-plastic behaviour of granular materials

CALVETTI, FRANCESCO
1999-01-01

Abstract

Starting from the original formulation of the Granta-Gravel and Cam-Clay constitutive models, the framework of elasto-plasticity has been successfully applied to describe the geomaterials behaviour. Thanks to the modifications proposed by different authors, several aspects of the granular materials behaviour were progressively taken into account, such as the non associated flow rule and the effects of induced anisotropy. Recently, a series of triaxial laboratory tests performed by di Prisco and Imposimato (1996) has put in evidence an aspect which is disregarded in elasto-plasticity: the time dependency of the mechanical behaviour of loose sand specimens. The same authors proposed to interpret the observed behaviour within the framework of visco-plasticity. In this paper, the non reversible behaviour of an idealised granular material composed of a 2D stack of disks is analysed by performing a series of Distinct Element numerical simulations (PFC-2D code). Starting from different stress levels (from isotropic to close to failure) previously recorded during a biaxial compression simulation, a series of finite stress probes is applied to the numerical specimen. All the probes have the same (small) magnitude, but they differ in direction in the principal stress plane. For each probe, the stress increments are applied to the boundary plates in an unique finite step, and the corresponding strain increments are recorded during the following time period. The obtained results show that the transient state is reached passing through two phases: a first one, where mainly elastic strains take place as the stress increment propagates within the specimen followed by a second one where the measured strains (non reversible) increase under constant stress; The duration of both phases considerably increases as the initial stress state approaches to failure. These results, which confirm the adequacy of a visco-plastic flow rule, are put in relation to the specimen structure progressive rearrangement caused by the applied stress-increment.
1999
NUMOG VII
9058090957
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/665761
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