This paper is devoted to an unresolved model for the simulation of air invasion in immersed granular flows without interface reconstruction between the liquid and the gas. Experiments of air invading a granular bed immersed in ethanol were achieved in a Hele-Shaw cell to observe the gas invasion paths and to calibrate the numerical multiscale model. The grains movements are computed at a fine scale using the non-smooth contact dynamics method, a time-stepping method considering impenetrable grains. The fluid flow is modelled by equations averaged using the volume fraction of fluid and computed at a coarse scale with the finite element method. A phase indicator function is used to dissociate the gas and the liquid constituting the fluid and to compute the density and viscosity of the fluid at each position. It is moved using a convection equation at each time step. The fluid, solid and phase indicator function computations are validated on simple cases before being used to reproduce experiments of air invasion in immersed granular flows. The experiments are supported by simulations in two dimensions to refine the study and the understanding of the invasion dynamics at short times.

In this study, a general frictional cohesive zone model (FCZM) dedicated to quasi-brittle fracture is proposed to describe the mechanical response of an interface under combined traction or compression and shear loadings. Under combined traction and shear loadings, mixed-mode I + II cohesive zone model, as proposed by Camanho et al. (2003), is used to express the mixed-mode response of the interface and the dependence to the loading path consistent to the one expected in quasi-brittle fracture. Under combined compression and shear loadings, the novelty lies in the proposed coupling between Mode II cohesive behavior and frictional behavior based on the damage level leading to a progressive rising of the frictional stress associated with the softening part of the cohesive behavior of the interface. FCZM thus describes a smooth transition from a cohesive zone to a pure frictional contact zone. Applied to the masonry context, this general FCZM can be fully characterized through two fracture tests carried out on small masonry assemblages. Finally, FCZM is implemented in LMGC90 discrete element code and used to simulate the experimental response of an unilateral cyclic shear test carried out on a triplet of limestone blocks assembled by two mortar joints.

Multibody models are often coupled with other domains in order to enlarge the scope of computer-based analysis. In particular, modeling multibody systems (MBSs) in interaction with granular media is of great interest for industrial process such as railway track maintenance, handling of aggregates, etc. This paper presents a strong coupling methodology for unifying a multibody formalism using relative coordinates and a discrete element method based on non-smooth contact dynamics (NSCD). Both tree-like and closed-loop MBSs are considered. For the latter, the coordinate partitioning techniques is applied in the NSCD framework. The proposed approach is applied on the slider-crank mechanism benchmark. Results are in very good agreement with results obtained with other techniques from the literature. Finally, a multibody model of a tamping machine is coupled to a discrete element model of railway ballast in order to analyse efficiency of track maintenance. This application demonstrates that the dynamics of the machine must be taken into account so as to estimate the performance of the maintenance process correctly.

The present work focuses on the impact of the chemical degradations of cementitious materials such as Delayed Ettringite Formation (DEF) on the overall material properties. DEF is an endogenous pathology due to the crystallization of ettringite within voids and cracks. The crystallization pressure in the porous cement paste induces swelling and cracking by differential expansion. The study aims to characterize the evolution of effective material properties (diffusion coefficient, apparent tenacity) with respect to DEF. A non-linear chemo-mechanical modeling is proposed where the entire diffusion-precipitation-pressurization-crack process is solved in a staggered approach. The diffusion-precipitation mechanism is translated by a rough chemical model. The resulting local volume fraction of ettringite is estimated by a finer micro-mechanical-based model using the effective elastic properties of the cement paste. The crack initiation and propagation is estimated with a dedicated cohesive zone model including pressure effect. Some applications of the model are presented.

Ce travail présente une modélisation chimio-mécanique du béton reposant sur le couplage entre le transport réactif et la réponse mécanique du système afin d'évaluer l'évolution des paramètres apparents du béton. Ce modèle repose sur une représentation d'un milieu poreux fissuré, dont la fissura-tion est traitée par un modèle de Zone Cohésives Frottantes. Il est étudié en particulier les réactions de gonflements dont les pores et les fissures sont mis sous pression. Une application à la Réaction Sulfatique Interne (RSI) est présentée mettant en avant l'impact de la granulométrie sur le gonflement du béton.

In the context of lifetime extension of nuclear power plants, the french Institut de Radioprotection et deSûreté Nucléaire (IRSN) conducts researches to predict the aging of cementitious materials. The presentwork is focusing on the impact at the mesoscale of the chemical sulfate attacks on the overall materialproperties. The sulfate attacks are characterized by the ettringite precipitation that induces great pressurein the porous cement paste. The effect of the local swelling of cement paste on concrete, conducts somecracking by differential deformation. These cracks become preferred loci for ions diffusion and furtherettringite precipitations. These strongly coupled phenomena suggest a non-linear chemo-mechanicalmodelling where diffusion, precipitation, pressurization and crack process should be simultaneouslyinvolved:1. Species diffusion: diffusion is modeled by Fick’s law in a cracked porous medium [1] [2];2. Chemical reactions: knowing species concentrations in solution, different types of reactions suchas aqueous, solid and sorption reactions [3] are modelled in order to estimate the amount of ettringitevolume;3. Mechanical: knowing the created solid volume, a local pressure is calculated in the poro-mechanicalmodel and in the cracks. The crack initiation, propagation and pressurization are based on a dedicatedCohesive Zone Model [4].The presented applications will focus on the impact of material properties, such as matrix composition,on strain kinetics. The impact of the clinker composition is estimated by a dedicated hydration computingwithin an analytical homogenization framework.REFERENCES[1] J.M. Segura and I. Carol. On zero-thickness interface elements for diffusion problems. InternationalJournal for Numerical and Analytical Methods in Geomechanics, 28:947 – 962, 2004.[2] L. Bichet. Taking into account the transport mechanisms in the fracture of heterogeneous materials:application to the nuclear power plant aging. PhD thesis, Univ. Montpellier II, 2017.[3] C. de Dieuleveult, J. Erhel, and M. Kern. A global strategy for solving reactive transport equations.Journal of Computational Physics, 228:6395 – 6410, 2009.[4] F. Perales, S. Bourgeois, A. Chrysochoos, and Y. Monerie. Two field multibody method for periodichomogenization in fracture mechanics of non linear heterogeneous materials. Engineering FractureMechanics, 75:3378 – 3398, 2008.

In the context of the lifetime extension of nuclear power plants, the french Institut de Radioprotectionet de Sûreté Nuclaire (IRSN) conducts researches to predict the ageing of cementitious materials. Thepresent work is focusing on the impact of the chemical degradations such as Delayed Ettringite Formation(DEF) on the overall material properties at the aggregates scale. DEF is an endogenous pathologyrelated to the cement composition and to the early age chemical reactions where thermal conditionsinduced dissolution of primal ettringite and later moisture environment can lead to ettringite reprecipitation.The crystallization pressure in the porous cement paste thus conducts both to its swelling andsome cracking by differential strains. These cracks are preferential location for ions diffusion and furtherettringite reprecipitations. These strongly coupled phenomena suggest a non-linear chemo-mechanicalmodelling where the diffusion, precipitation, pressurization and crack process should be solved in acoupled approach. In an modelling, for each time step, one calculates:Species diffusion: to describe the environmental impact on the concrete. Diffusion is modeled by Fick’slaw in a porous medium. This model follows the framework of [1] and it takes into account the resistiveimpact of the crack on the normal flow;Chemical reactions: knowing species concentrations in solution, one models different types of reactionssuch as aqueous, solid and sorption reactions [2]. Thus, the amount of ettringite volume is evaluated;Mechanical: knowing the solid volume, a local pressure is calculated in the poro-mechanical model andin the cracks. The crack initiation, propagation and pressurization are based on a dedicated CohesiveZone Model [3].The presented applications will focus on the impact of material properties, such as volume fraction ofinclusion and matrix composition, on strain kinetics.REFERENCES[1] L. Bichet. Taking into account the transport mechanisms in the fracture of heterogeneous materials:application to the nuclear power plant aging. PhD thesis, Universit´e de Montpellier II, 2017.[2] C. de Dieuleveult, J. Erhel, and M. Kern. A global strategy for solving reactive transport equations.Journal of Computational Physics, pages 228 : 6395 – 6410, 2009.[3] F. Perales, S. Bourgeois, A. Chrysochoos, and Y. Monerie. Two field multibody method for periodichomogenization in fracture mechanics of non linear heterogeneous materials. Engineering FractureMechanics, pages 75 : 3378 – 3398, 2008.