David Grégoire

Mesoscale analysis of failure in quasi-brittle materials: comparison between lattice model and acoustic emission data

Summary The purpose of this paper is to analyse the development and the evolution of the fracture process zone during fracture and damage in quasi‐brittle materials. A model taking into account the material details at the mesoscale is used to describe the failure process at the scale of the heterogeneities. This model is used to compute histograms of the relative distances between damaged points. These numerical results are compared with experimental data, where the damage evolution is monitored using acoustic emissions. Histograms of the relative distances between damage events in the numerical calculations and acoustic events in the experiments exhibit good agreement. It is shown that the mesoscale model provides relevant information from the point of view of both global responses and the local failure process.

Capillary bundle model for the computation of the apparent permeability from pore size distributions

A simplified model based on capillary bundle aimed at computing the intrinsic and the apparent permeabilities of mortar to gas is presented. At the micro-level of capillaries, fluid flow follows the classical Poiseuille’s law combined with Knudsen’s flow. At the macro-level, the equation governing fluid flow is very similar to Klinkenberg’s model, and exhibits a dependence of the apparent permeability to the mean pressure. The input data entering in the analytical model are the pore size distributions directly measured from mercury intrusion porosimetry cut above a threshold diameter in order to avoid the overestimation of the permeability inherent to the bundle model. The model is fitted on experimental data on mortar specimens subjected to damage due to electrical fracturing and a tortuosity factor is identified. Fits on the same material with different states of damage show that the tortuosity decreases as damage progresses.

A novel experimental setup for simultaneous adsorption and induced deformation measurements in microporous materials

A new experimental setup is presented allowing the simultaneous measurement of adsorption isotherms and adsorption-induced deformations. It is composed of a manometric technique coupled with a digital image correlation setup for full-field displacement measurements. The manometric part is validated by comparing adsorption isotherms with those obtained by a gravimetric method. The principles and methods of both adsorption isotherm and induced deformation measurements are presented in detail. As a first application of this new apparatus, the coupling between adsorption and induced deformation is characterised for a microporous media (activated carbon) saturated by pure CO2 (318.15 K, [0-60] bars) and pure CH4 (303.15 K, [0-130] bars). For this very homogeneous porous material, the induced deformation is characteristic of a pure volumetric swelling but the full-field setup may allow the characterisation of the localised pattern of deformation for heterogenous or cracked microporous media.

2D X-FEM Simulation of Dynamic Brittle Crack Propagation

The application of X-FEM technique to the prediction of two dimensional dynamic brittle crack growth is presented in this paper. The method is known to guarantee exact energy conservation in case of crack propagation and it is applied to the simulation of one dynamic crack propagation experiment submitted to a mixed mode loading and showing stop and restart of a crack.

A New Model for Estimating Fluid Transfer Properties of Cementitious Materials

A model capable of predicting transport properties based solely on the pore size distribution of the material and the fluid characteristics has been developed previously by the authors. This model combines a Darcy description of the fluid flow at the macroscale with a Poiseuille/Knudsen flow at the micro-scale. A stochastic approach is then used with regards to pore network generation, in a manner consistent with mercury intrusion porosimetry (MIP). The present paper extends this model to multi-phase flow in an attempt to model partially saturated porous media. Kelvins law defines which pore sizes are invaded with liquid and rules for accounting contributions of the pore network to liquid and vapour flows are introduced. The extended model provides data that are consistent with the Van Genuchten approach to liquid and gas relative permeabilities, compares well with experimental data and provides information of the relative permeability to vapour and liquid water upon damage.

Permeability and relative permeability of mortar undergoing damage: a hierarchical capillary bundle approach

The purpose of this work is to achieve a better understanding of the relationship between mechanical damage, pore size distribution and transport properties of cementitious materials. In the literature, analyses are usually restricted to intrinsic permeability of the material and the evolution of the apparent permeability with respect to the pressure gradient and to the nature of the fluid considered are left aside. A new model capable to provide the apparent permeability of a porous material to gas, directly from the pore size distribution and from the properties of the gas is discussed. Comparisons with experimental data on mortar specimens show that the model can reproduce the intrinsic permeability and its evolution when the material is subjected to mechanical damage, provided the pore size distributions are available. Extension to the transport of different phases (e.g. water and water vapor) is discussed, with a view towards the simulation of nuclear accident in containment vessels. It is shown that small pores that are not affected by damage according to the pore size distribution are of great importance in the evaluation of the relative permeability to liquid and vapor as a function of the saturation. A tentative model is discussed and compared with the existing – standard – approach relying on Van Genuchten relationships.

Correlation during the fracture process analysed with the help of Ripley’s functions

The degradation of quasi-brittle materials encompasses micro-cracks propagation, interaction and coalescence in order to form a macro-crack. These phenomena are located within the Fracture Process Zone (FPZ). This paper aims at providing a further insight in the description of the FPZ evolution with the help of statistical analysis of damage. The statistical analysis relies on the implementation of Ripley’s functions, which have been developed in order to exhibit patterns in image analyses. It is shown how a correlation length may be extracted from the Ripley’s function analysis. Comparisons between experimental and numerical evolutions of extracted correlation lengths are performed.

Analysis of Crack Evolution in Concrete through Combined Acoustic Emission Monitoring and Mesoscale Modelling

In this paper, the fracture process zone (FPZ) is investigated on unnotched and notched beams with different notch depths. Three-point bending tests have been realized on plain concrete under crack mouth opening displacement (CMOD) control. Crack growth is monitored by applying the acoustic emission (AE) technique. The comparison with a numerical model is also realized by using a mesoscopic approach. Such an approach is of particular interest in the analysis of interactions between the cementitious matrix and aggregates. Several AE parameters are examined during the entire loading process, and show that the relative notch depth influences the AE characteristics, the process of crack propagation, and the brittleness of concrete. The numerical load-CMOD curves show that the mesoscopic modelling reproduces well the notch effect and concrete failure. In order to improve our understanding of the FPZ, the width and length of the FPZ are followed based on the AE source locations maps in parallel with the numerical damage fields. An important energy dissipation is observed at the crack initiation in unnotched beams.

Adsorption‐induced instantaneous deformation in double porosity media: modeling and experimental validations

Abstract: Natural and synthetic porous media generally encompass different and distinct porosities: a microporosity where the fluid is trapped as an adsorbed phase and a mesoporosity or a macroporosity required to ensure the transport of fluids to and from the smaller pores. Zeolites, activated carbon, tight rocks, coal rocks, source rocks, cement paste or construction materials are among these materials.

On the Growth, the Arrest and the Restart of a Crack during a Dynamic Brittle Fracture Experiment

Our purpose is to propose a methodology for assessing dynamic crack propagation laws under mixed-mode loading. Dynamic brittle fracture experiments are performed on polymethylmethacrylate (PMMA) in which mode combination changes and crack arrest phases occur. Then, these experiments are numerically reproduced by using the eXtended Finite Element Method (X-FEM) in order to validate the algorithms and the criteria assumed.