Mohamad Jrad

Granular media failure along triaxial proportional strain paths

This paper revisits a famous phenomenon sometimes observed in granular soils: liquefaction. Liquefaction occurs when all stress components vanish. According to the loading conditions, and the initial void ratio of the soil specimen, it can be shown that a convenient stress quantity passes through a peak, and then decreases. It is usually thought that the deviatoric stress peak also corresponds to a failure state, since a sudden collapse of the specimen is expected from this point according to the loading control adopted. After a brief review of the theoretical background describing the occurrence of such phenomena (liquefaction and failure), the results of laboratory tests run along a triaxial proportional strain loading path are presented. Then, the main conclusions drawn are discussed and confirmed from numerical tests using a discrete element method. In both approaches, it is shown that the choice of the stress response variable is fundamental, in order to properly detect such a failure.

Experimental analyses of the behaviour of saturated granular materials during axisymmetric proportional strain paths

Instability of granular materials is usually studied using undrained triaxial compression tests or constant shear drained tests. These tests are usually performed on fully undrained or fully drained samples respectively. In this paper, experimental investigations of the behaviour of loose saturated sand under proportional strain paths performed by imposing a partial drainage condition are presented. Partially drained conditions observed in situ are due to potential pore pressure variations. Different drainage condition changes can lead to different stress–strain responses, and it is shown that the strength mobilisation for expansive drainage conditions is lower than what is usually obtained for undrained conditions. The stability analyses based on experimental results have shown that the second order work criterion, rewritten using the relevant control parameters, is a good tool to investigate the onset of instability. L’étude des instabilités dans les milieux granulaires est souvent menée en utilisant des résultats d’essais triaxiaux de compression non-drainés ou des essais à contrainte déviatoire constante. Ces essais sont réalisés respectivement en condition totalement non-drainée ou totalement drainée. Nous présentons dans cet article une étude expérimentale réalisée sur sable d’Hostun lâche saturé sous chargement proportionnel en déformation en imposant un drainage partiel. Ce drainage partiel est dû à une augmentation de pression interstitielle et reproduit des conditions in situ. Ces différentes conditions de drainage conduisent à des réponses contraintes-déformations différentes et l’on montre que la résistance mobilisée dans le cas d’un essai à dilatance imposée est inférieure à celle classiquement obtenue durant les essais non-drainés. L’analyse de stabilité réalisée en utilisant les résultats expérimentaux montre clairement que le critère du travail de second ordre, réécrit en utilisant les variables de contrôles adéquates, est un indicateur pertinent pour l’étude du déclenchement de l’instabilité.

Experimental and numerical analyses of failure in very loose sands

ABSTRACT It is now recognized that some collapses can occur before the Mohr-Coulomb plastic limit criterion is reached, and our recent results highlight that these collapses occur under a diffuse mode of failure. We present and discuss experimental data on constant shear drained tests. A non-associated elastoplastic multimechanism model is briefly presented and numerical results are compared with our data obtained constant shear drained tests. These simulations are in agreement with experiments. The second order work is computed and the loss of stability, corresponding to the vanishing of the second order work, is very close to the experimental collapse point.

Phase Transformation States of Loose and Dense Granular Materials under Proportional Strain Loading

AbstractDepending on the rate of external loading, the permeability, and the boundary conditions, different drainage conditions are expected during the loading of sands. These drainage conditions are thus neither fully drained nor fully undrained as usually assumed for saturated soils, which implies simultaneous changes in pore volume and in pore water pressure. For a given material, the transition between contractive behavior (increase of the pore water pressure) and dilative behavior (decrease of the pore water pressure) defines the phase transformation state. It is found that the position of this phase transformation state, which is known to be dependent on the relative density, depends on the rate of dilatancy (e˙v/e˙1) imposed on the sample. Constraining a loose sample to dilate leads to an unstable state earlier than observed during undrained tests. Even dense samples, which do not exhibit a nonlocalized unstable state during undrained tests, are prone to collapse if the rate to dilatancy is high en...

Instability in Loose Sand: Experimental Results and Numerical Simulations with a Microstructural Model

Under certain loading conditions, loose sand can develop instability at a shear stress level much lower than the critical state failure line. To analyze these types of problems, we have adopted the micromechanics model developed by Chang and Hicher for modelling granular material behaviour. The stress-strain relationship for a granular assembly is determined by integrating the behaviour of the inter-particle contacts in all orientations. The constitutive model is applied to simulate undrained triaxial, constant-q and proportional strain tests on loose Hostun sand. Experimental results are used to evaluate how well the model can capture the modes of instability at the assembly level. The notion of control variables is discussed according to these different loading conditions.

Analytical and Finite Element Approaches for the Drilling Modelling

Perform drill point design is one of the major problems for the drill manufacturers. To enhance drill performance they have to elaborate prototypes and carry out many tests to progress step by step to an optimised geometry. Model and simulate drilling operations is a very interesting way to obtain useful information for the drill manufacturing process. In this work, a geometrical and thermomechanical analytical model and a finite element approach of drilling were used. While the first gives very quickly some global information, the second gives more details but after a long calculation time. It is shown that the two approaches are complementary and that they may be used with advantage for the drill design.

Dilatancy Phenomenon Study in Remolded Clays – A Micro-Macro Investigation

The aim of this study is to analyze the influence of stress path on the dilatancy behavior of clays. Triaxial tests were conducted on saturated, remolded clay specimens. During triaxial loading, two different stress paths were considered to bring the stress level to a given point in the (\( p^{\prime } - q \)) plane. After triaxial testing, scanning electron microscopy (SEM) observations and X-ray microtomography (XR-μCT) scans were performed on subsamples. The obtained images were processed using different software and thus the geometric characteristics of pores and cracks were identified. The microstructural characteristics were linked to the dilatancy phenomenon of specimens. The results indicate that dilatancy is influenced by stress path and may be attributed to the evolution of pore geometry and the presence of local open cracks in highly over-consolidated (OC) clays.

Geometrically Nonlinear Field Fracture Mechanics and Crack Nucleation, Application to Strain Localization Fields in Al-Cu-Li Aerospace Alloys

The displacement discontinuity arising between crack surfaces is assigned to smooth densities of crystal defects referred to as disconnections, through the incompatibility of the distortion tensor. In a dual way, the disconnections are defined as line defects terminating surfaces where the displacement encounters a discontinuity. A conservation statement for the crack opening displacement provides a framework for disconnection dynamics in the form of transport laws. A similar methodology applied to the discontinuity of the plastic displacement due to dislocations results in the concurrent involvement of dislocation densities in the analysis. Non-linearity of the geometrical setting is assumed for defining the elastic distortion incompatibility in the presence of both dislocations and disconnections, as well as for their transport. Crack nucleation in the presence of thermally-activated fluctuations of the atomic order is shown to derive from this nonlinearity in elastic brittle materials, without any algorithmic rule or ad hoc material parameter. Digital image correlation techniques applied to the analysis of tensile tests on ductile Al-Cu-Li samples further demonstrate the ability of the disconnection density concept to capture crack nucleation and relate strain localization bands to consistent disconnection fields and to the eventual occurrence of complex and combined crack modes in these alloys.