Mechanics of Cohesive-frictional Materials最新文献

The accurate prediction of the behaviour of geostructures is based on the strong coupling between the pore fluid and the solid skeleton. If the relative acceleration of the fluid phase relative to the skeleton is neglected, the equations describing the problem can be written in terms of skeleton displacements (or velocities) and pore pressures.

This mixed problem is similar to others found in solid and fluid dynamics. In the limit case of zero permeability and incompressibility of the fluid phase, the restrictions on the shape functions used to approximate displacements and pressures imposed by Babuska–Brezzi conditions or the Zienkiewicz–Taylor patch test hold.

As a consequence, it is not possible to use directly elements with the same order of interpolation for the field variables.

This paper proposes a generalization of the fractional-step method introduced by Chorin for fluid dynamics problems, which allows to circumvent BB restrictions in the incompressibility limit, thus making it possible to use elements with the same order of interpolation. Copyright © 2000 John Wiley & Sons, Ltd.

In the present paper we analyse the conditions for band-shaped localization to occur in the hydro- mechanically coupled problem. The governing continuum equations are based on the porous media theory applied to an elastic–plastic solid (skeleton) at small deformations. In the localization analysis, the concept of regularized discontinuity is extensively used at the application to the conservation laws of momentum and mass. As a result, we obtain a coupled localization condition, where the situation of partly drained conditions is discussed and compared to the two extreme cases of fully drained and undrained situations. A simple numerical experiment for a 1D problem is carried out using an interface FE-formulation. Copyright © 2000 John Wiley & Sons, Ltd.

The degradation process of natural stones is reproduced by performing low-cycle-number, uniaxial compression fatigue tests on three different qualities of marble. A typical feature of fatigue tests is that repetition of loading cycles produces a progressive accumulation of permanent strain in the specimen, rather than any significant decay in the material's elastic moduli. This permanent deformation has been recognized as both due to the opening of micro-cracks as well as due to shear-like mechanisms. The relative contribution of each of these mechanisms to the overall behaviour of the materials depends heavily upon the orientation of the rift plane of the specimen, that is, whether the specimen has been cut with its axis orthogonal to the marble rift plane or parallel to it. In the first case, shear-like deformations are the most evident signs of the failure mechanism, while in the second, the marked increases in volume due to micro-cracking are evident early. Scanning electron microscope studies performed, evidence that there are two different phases in the marble's behaviour: the first one is characterized by calcite grain decohesion, that is, the grains remain integral, but detach along their interfaces upon repeated load cycles (grain de-cohesion); while in the second stage, there are evident signs of transgranular micro-cracking. These two distinct phases are shown to correlate with different stages in the material's macroscopic response to cyclic loading. Copyright © 2000 John Wiley & Sons, Ltd.

This paper presents an experimental investigation and numerical modelling of the behaviour of a porous chalk by taking into account effects of saturating fluid. Two representative fluids, oil and water are considered. The laboratory testing programme includes two topics. First, conventional hydrostatic and triaxial compression tests are carried out on samples which are saturated with water and oil, respectively. The sensitivity of chalk behaviour to saturating fluid is clearly shown. Secondly, specific water injection tests are conducted in which water is injected into chalk samples initially saturated by oil, under different hydrostatic and deviatoric stress states. Additional plastic deformation induced by water flooding is determined. In the second part of paper, an elastoplastic model with two yield surfaces is proposed. The water-induced plastic deformation is described by introducing an additional plastic mechanism related to the behaviour jump between two material states of chalk. An overall good correlation is obtained between numerical simulations and experimental data not only for conventional tests, but also for water injection tests. Copyright © 2000 John Wiley & Sons, Ltd.

This work is directed toward the formulation and analysis of the modified Cam-clay critical state model within the framework of isotropic multiplicative finite strains. A suitable energy function, in which the shear modulus is made to depend on the mean pressure, is chosen allowing the hyperelastic response to be energy conserving. As a result of the use of Eulerian logarithmic stretches as strain measures in conjunction with an exponential approximation of the plastic flow rule, the small strain integration algorithms, and the corresponding consistent tangent operators, automatically extend to the finite strain regime. Numerical simulations are provided to demonstrate the stability and good performance of the proposed formulation. Copyright © 2000 John Wiley & Sons, Ltd.

Discrete simulations by the distinct element method are performed for modelling an ensiled granular material. Starting from the material microstructure on a microscopic scale, two procedures are considered in order to simulate the mechanical behaviour of the material within a silo on a megascopic scale. On the one hand, a direct discrete simulation from the microscopic scale to the megascopic scale is carried out. On the other hand, a two-step simulation procedure is performed: the determination of the macroscopic behaviour from the microscopic scale, followed by a continuum analysis from the macroscopic scale to the megascopic scale. The macroscopic parameters are derived from discrete simulations of usual rheological tests. An experimental validation of the distinct element method is presented by simulating a biaxial compression test. The macroscopic parameters of the ensiled material are then identified and introduced into a finite element analysis with an elastic–plastic constitutive model using the Mohr–Coulomb criterion. The results of the two above procedures are compared and in this way the relevance of the discrete simulation is demonstrated. Copyright © 2000 John Wiley & Sons, Ltd.

In this paper catastrophic landslides on a discrete failure plane are considered, which are explained by a mechanism of frictional heat generation that gives rise to high pore-water pressures generation inside the shear band. The problem is governed by a set of coupled diffusion–generation equations for the pore pressure and the temperature inside the shear band at the base of the landslide. The thermo-poro-mechanical coupling leads to frictional softening due to effective stress reduction and explains the extremely low apparent friction angles for rapid slides documented in the literature. The analysis suggests also that the shear band thickness must scale with the geometric dimensions of the slide. Copyright © 2000 John Wiley & Sons, Ltd.

Many concrete structures may be submitted to high rate dynamic loadings (impacts, explosions, etc.). So, it is necessary to know the behaviour of this material in order to predict the response of the structure. Under dynamic loading an increase of apparent compressive strength is observed. This may be due to a rheological effect or to a structural effect. Dynamic compression tests are available using the split Hopkinson pressure bars. Forces and velocities are obtained on each face of the specimen. A simulation of these tests is carried out using two different models. The first, based on plasticity theory, is a non-associated model with a failure surface of the Ottosen type. This model is independent of the strain rate. The second model is an elasto-viscoplastic model with a damage law. It only takes into account the main features of the behaviour, the sliding phenomena, the damage and the softening. The comparison of experimental results and simulations leads us to consider the structural effect as an inertial confinement responsible for the apparent increase in strength. Copyright © 2000 John Wiley & Sons, Ltd.