Conference Agenda

Geomembranes are materials used as impermeable barriers to guarantee structure waterproofing and the conservation of water. The annual growth of the global geomembrane is anticipated to continue with an increased drought crisis due to climate change.

Even if they are not primarily designed to provide mechanical resistance, geomembranes face challenges from mechanical actions that could potentially degrade their impermeability properties, and so compromise their waterproofing functionality. For example, unprotected geomembrane in contact with a granular layer for drainage may experience tensile forces, that raise the risk of puncturing failure. Furthermore, if improper operating techniques are used during installation, the geomembrane may be susceptible to unexpected mechanical actions, that could result in a premature failure. To the best of our knowledge, no research has been done to interpret the microscopic origin of geomembrane failure.

In this study, a micromechanical model of PVC geomembrane is adopted to explore the elementary mechanisms underlying macroscopic behavior. The model relies on the Discrete Element Method (DEM) to depict PVC geomembrane microstructure at the nanoscale, as a semi-crystalline assembly of polymers embedded in a matrix of plasticizers.

By employing this original multiscale approach, we seek to demonstrate a connection between macroscopic deformation and internal microscopic mechanisms. Through such a detailed analysis at the microstructural level, we show promising results to unravel the intricate elementary processes governing the mechanical response of PVC geomembrane up to failure.

Clays are geo-materials containing extremely fine mineral grains with peculiar hydro-mechanical behavior, the most well-known being the drying shrinkage. Indeed, the nanometric mineral layers can adsorb water in the inter-layer, which induces large deformations. Although the crystalline swelling at the layer scale was identified long ago in the 1950's by XRD, much progress in its fundamental understanding has been made in the last 20 years thanks to the development of molecular simulations. Atomistic modeling offers an unprecedented nanoscale description of the mechanisms of swelling, with quantitative estimates of the mechanical behavior at the scale of a single mineral layer, in particular, the coupling with humidity and the thermo-mechanical response. Yet, the up-scaling from the mineral layer to the clay matrix remains a challenge, in particular, because the mesoscale (nm to µm) is hardly accessible to experimental observation, and the microstructure at this scale can only be inferred indirectly (e.g., from small angle scattering). As an alternative to experiments, granular 'mesoscale' simulations have emerged which aim at taking advantage of the fine understanding obtained at the molecular scale to propose 'coarse-grained' models at larger scales. In this work, we propose a mesoscale model for sodium Wyoming montmorillonite, designed to investigate specifically the anomalous thermo-hydro-mechanical couplings. The model addresses a few key features of the mesostructure: the degree of local anisotropy, the flexibility of the clay minerals, and the hydration states. Investigating the response to mechanical and osmotic loadings, the model exhibits a THM behavior quantitatively consistent with that usually observed experimentally for montmorillonite, although the systems considered are limited to about 100 nm. This suggests that the mesoscale is central in the emergence of the macroscopic mechanical properties of clays.

Every day, wood-pulp-based paper is a relevant engineering material that may exhibit considerable variability in its macroscopic, mechanical properties. To quantitatively decipher the origins of that variability, a multiscale, continuum-micromechanics-based model for the linear elasticity of transversely isotropic paper sheets is presented, which reflects an experiment-based description of the hierarchical, nanoscopic-to-macroscopic scale organization of such sheets and of their constituents. At a length scale of few nanometers, clusters of hemicellulose, lignin, as well as water and extractives deposits, presenting virtually spherical shapes, indiscriminately aggregate to form a polymer blend. Still at a nanometer length scale, collections of cellulose l_α, cellulose I_β, and cellulose II crystals, exhibiting next to elliptical-cylindrical geometries, parallelly precipitate into a contiguous matrix of amorphous cellulose to constitute a cellulose fibril. Further up, at a length scale of few micrometers, bundles of cellulose fibrils, indicating nearly circular-cylindrical configurations, helically accumulate into an adjacent polymer blend matrix to build-up a pulp fiber. Finally, at a length scale of few hundreds of micrometers, constellations of wood pulp fibers, displaying close to elliptical-cylindrical contours; as well as of pores, appearing to spheroidally mirror the transverse shape of said fibers; planarly, arbitrarily combine to materialize as a transversely isotropic paper sheet. Model validation rests on carefully collected datasets, altogether containing over a thousand experimental values, standardly, independently determined across ample regions of space and time, as well as a very, very small number of theoretically predicted values. Based thereon, values for the elasticity of a mean, softwood-based, unbeaten, chemical pulp fiber and of corresponding cellulose fibrils as well as of paper sheets of varying porosity are theoretically (continuum micromechanically) predicted, which agree very well with respective, experimentally determined values. Our investigation enriches fundamental paper research, and at the same time provides a valuable basis for improved paper production, development, and use.