A Multi-Phase-Field Anisotropic Damage-Plasticity Model for Crystalline Rocks

Many engineering applications, such as geological disposal of nuclear waste, require reliable predictions on the thermo-hydro-mechanical responses of porous media exposed to extreme environments. This presentation will discuss the relevant modeling techniques designed specifically for such environmental conditions. In particular, we will provide an overview of the coupling method of crystal plasticity and multi-phase-field model designed to replicate the thermal- and rate-dependent damage-plasticity of crystalline rock. Special emphasis is placed on capturing the intrinsic anisotropy of salt grain in 3D with respect to damage behavior and plastic flow by incorporating the crystallographic information of salt

Thermo-hydro-mechanical simulation of a full-scale steel-lined micro-tunnel excavated in the callovooxfordian claystone

The paper presents an interpretation of the full-scale ALC1604 in situ heating test carried out in Callovo-Oxfordian claystone (COx) in the Meuse/Haute-Marne underground research laboratory (MHM URL). The MHM URL is a site-specific facility planned to study radioactive waste disposal in the COx. The thermo-hydro-mechanical (THM) behaviour of the host rock is significant for the design of the underground radioactive waste disposal facility and for its long-term safety. When subjected to thermal loading, the Callovo-Oxfordian claystone of low permeability (~10-20-10-21 m2) exhibits a strong pore pressure response that significantly affects the hydraulic and mechanical behaviour of the material. The observations gathered in the in situ test have provided an opportunity to examine the integrated thermo-hydromechanical (THM) response of this sedimentary clay. Coupled THM numerical analyses have been carried out to provide a structured framework for interpretation, and to enhance understanding of THM behaviour of COx. Numerical analyses have been based on a coupled theoretical formulation that incorporates a constitutive law specially developed for this type of material. The law includes a number of features that are relevant for a satisfactory description of the hydromechanical behaviour. By performing the numerical analysis, it has been possible to incorporate anisotropy of material parameters and of in situ stresses. The performance and analysis of the in situ tests have significantly enhanced the understanding of a complex THM problem and have proved the capability of the numerical formulation to provide adequate predictive capacity

Multiphysical failure processes in concrete: a consistent multiscale homogenization procedure

Durability and strength capabilities of concrete materials are vastly affected by the combined action of temperature and mechanical loading, which give rise to multiphysical failure processes. Such a phenomenon involves complex cracking, degradation and transport mechanisms on different scale lengths of concrete mixtures which, in turn, depend on the particular properties of the different constituents. Thus, the macroscopic observation of relevant concrete mechanical features such as strength, ductility and durability are the result of several different properties, processes and mechanisms which are not only coupled but moreover, depend on multiple scales. Particularly, regarding the pore pressure and thermal actions, most of the degradation processes in concrete are controlled by the heterogeneities of the microscopic scale. In the case of the mechanical actions both the micro and mesoscales play a relevant role. In this context, multiphysical failure processes in cementitious material-based mixtures like concrete can only and fully be understood and accurately described when considering its multiscale and multiconstituent features. In the realm of the theoretical and computational solid mechanics many relevant proposals were made to model the complex and coupled thermo-hydromechanical response behavior of concrete. Most of them are related to macroscopic formulations which account for the different mechanisms and transport phenomena through empirical, dissipative, poromechanical theories. Moreover, although relevant progress was made regarding the formulation of multiscale theories and approaches, none of the existing proposals deal with multiphysical failure processes in concrete. It should be said in this sense that, among the different multiscale approaches for material modeling proposed so far, those based on computational homogenization methods have demonstrated to be the most effective ones due to the involved versatility and accuracy. In this work a thermodynamically consistent semi-concurrent multiscale approach is formulated for modeling the thermo-poro-plastic failure behavior of concrete materials. A discrete approach is considered to represent the RVE material response. After formulating the fundamental equations describing the proposed homogenizations of the thermodynamical variables, the constitutive models for both the skeleton and porous phases are described. Then, numerical analyses are presented to demonstrate the predictive capabilities of the proposed thermodynamically consistent multiscale homogenization procedure for thermo-mechanical failure processes in concrete mixtures

Retention and permeability properties of damaged porous rocks

International audienceThe objective of this research work is to model the influence of deformation and damage on the permeability and retention properties of cracked porous media. This is achieved thanks to the introduction of microscale information into a macroscopic damage model. To this end, the Pore Size Distribution (PSD) of the material is coupled to the mechanical behaviour of the rock. Changes to this distribution due to deformation and damage are modelled and then used to capture induced changes to the retention and permeability properties of partially saturated materials. Rock microstructure is characterized by the size distributions of natural pores and cracks, which are used to update intrinsic permeability with Hagen-Poiseuille flow equation and Darcy's law. The void space occupied by water is computed by integrating the pore size distributions of natural pores and cracks up to the capillary pore radius (r(sat)). Laplace equation is used to relate r(sat) to the capillary pressure. The paper explains how to update PSD parameters with the macroscopic variables (such as deformation and damage), and then how to update permeability and retention properties with the PSD parameters. Conventional triaxial compression tests are simulated under controlled capillary pressure and under controlled water content. The proposed model captures well the intrinsic permeability decrease associated to the elastic compression of the natural pores, followed by the permeability jump due to crack opening. The modeling framework can be adapted to any rock constitutive model, including thermo-hydro-chemo-mechanical couplings. Applications may be found in energy production, ore exploitation and waste management

Correlation between Sub-Tg relaxation processes and mechanical behavior for different hydrothermal ageing conditions in epoxy assemblies

The aim of this study is to understand aging phenomena by monitoring physical parameters after real and simulated aging experiments. This study focuses on aluminum-epoxy assemblies, which are commonly used on spacecraft structures. Different samples are submitted to simulated aging tests. Influence of temperature and moisture is analyzed. Evolution with aging is characterized at two different scales. The macroscopic behavior of the assemblies is studied by single lap shear test. A decrease in the shear rupture stress is observed with increasing temperature and relative humidity. It is demonstrated that temperature has more important influence. The molecular behavior in the adhesive joint is studied by dynamic dielectric spectroscopy measurements. This experiment gives access to molecular mobility in the adhesive. Dipolar entities are identified as evolving with aging conditions. The temperature is more effective than moisture at this scale. An interpretation of the molecular mobility before and after aging shows that water is an important parameter of this study. A link between mechanical and molecular behavior with hydrothermal aging is found. The decrease of mechanical properties occurs while failures become interfacial. In the same time, the interactions between hydroxyether and water increase. The evolution of the macroscopic behavior of the bonded assemblies is due to this combination observed at different scales

Impact of temperature on the pullout of reinforcing geotextiles from unsaturated silt

This study investigates the thermal soil-geosynthetic interaction mechanisms of reinforcing geotextiles confined in compacted silt that may be encountered when using mechanically-stabilized earth (MSE) walls as geothermal heat sinks. A thermo-mechanical geosynthetic pullout device was used that incorporates standard components for geosynthetic pullout or creep testing but also heating elements at the top and bottom of the soil box to apply boundary temperatures and dielectric sensors embedded in the soil layer to monitor distributions in temperature and volumetric water content. Two test series were performed: the first involves monotonic pullout of woven polypropylene geotextiles after reaching steady-state conditions under different boundary temperatures without a seating load, and the second involves monotonic pullout of woven polyethylene-terephthalate geotextiles after reaching steady-state conditions under different boundary temperatures with a seating pullout load. The results indicate that the pullout resistance of both geotextiles decreased with increasing temperature. Although heating led to drying of the unsaturated silt layers as expected, measurements from the second test series indicate accumulation of water at the silt-geotextile interface. An effective stress analysis considering thermal softening of soils indicates that the increase in effective saturation at the silt-geotextile interface was the cause of the decrease in pullout resistance with heating

Finite element modeling of thermo-hydromechanically (THM) coupled problems in frozen ground engineering: state-of-the-art

Fully coupled Thermo-Hydro-Mechanical (THM) modeling has been widely studied in various areas of geomechanics, owing to the multiphase nature of geomaterials. Several researches have dealt with THM coupled modeling of geomaterials in high temperature regimes, but a limited work is available for geomaterials in low temperature regimes. A review and summary of existing work in the literature on THM coupled modeling of frozen soils is presented here. THM coupled modeling in general and its applications are pointed out. The basic governing equations of a coupled THM model in general form, namely mass, momentum and energy balance equations, are discussed. A review of fully coupled models is made and the numerical aspects of THM modeling are briefly discussed. A mechanical constitutive model makes up an important component of a fully coupled THM model and a brief review of existing constitutive models for frozen soils is presented. The models reviewed range from elastoplastic models to viscoplastic or creep and damage coupled models. Some models that consider different approaches from the plasticity framework are briefly reviewed. The state-of-the-art is summarized by pointing out the main aspects of THM coupled modeling and directions for future work