P. Dumont

Microstructure and deformation micromechanisms of concentrated fiber bundle suspensions: An analysis combining x-ray microtomography and pull-out tests

The non-Newtonian rheology of concentrated fiber suspensions, such as short fiber reinforced polymer composites during their processing, depends on both the microstructure of their fibrous network and the deformation micromechanisms arising at fiber–fiber contacts. In this work, these two aspects are investigated using model concentrated fiber suspensions made up of short glass fiber bundles impregnated in a transparent polymer. For that purpose, multiresolution x-ray microtomography was used to analyze the fibrous microstructures, showing that the studied suspensions exhibit a planar fiber orientation with fairly straight fiber bundles, the connectivity of which can be modeled by the geometrical statistical tube model. Besides, bundle–bundle contact forces together with the interaction between the bundles and the suspending fluid are analyzed by using pull-out experiments. These tests allow the influence of the pull-out velocity, the confining stress and the volume fraction of fiber bundles on the pull-o...

Cellulose crystals plastify by localized shear

Significance While most attention has so far been devoted to the tensile properties of crystalline cellulose, the main elementary building block of plants, we show here using atomistic simulations that their shear is also an important mode of deformation, occurring at stress levels lower than tension with much larger ductility. We also demonstrate how crystalline defects like dislocations drastically facilitate plasticity. This analysis can be used as a basis for the micromechanical modeling of cellulose microfibrils that are currently considered as promising eco-friendly alternatives to synthetic fibers for structural materials. Cellulose microfibrils are the principal structural building blocks of wood and plants. Their crystalline domains provide outstanding mechanical properties. Cellulose microfibrils have thus a remarkable potential as eco-friendly fibrous reinforcements for structural engineered materials. However, the elastoplastic properties of cellulose crystals remain poorly understood. Here, we use atomistic simulations to determine the plastic shear resistance of cellulose crystals and analyze the underpinning atomic deformation mechanisms. In particular, we demonstrate how the complex and adaptable atomic structure of crystalline cellulose controls its anisotropic elastoplastic behavior. For perfect crystals, we show that shear occurs through localized bands along with noticeable dilatancy. Depending on the shear direction, not only noncovalent interactions between cellulose chains but also local deformations, translations, and rotations of the cellulose macromolecules contribute to the response of the crystal. We also reveal the marked effect of crystalline defects like dislocations, which decrease both the yield strength and the dilatancy, in a way analogous to that of metallic crystals.

Three-dimensional visualization and quantification of the fracture mechanisms in sparse fibre networks using multiscale X-ray microtomography

The structural changes that are induced by the initiation and the propagation of a crack in a low-density paper (LDP) were studied using single edge-notched fracture tests that were imaged under an optical microscope or in laboratory or synchrotron X-ray microtomographs. The two-dimensional optical images were used to analyse the links between the mesoscale structural variations of LDP and the crack path. Medium-resolution X-ray three-dimensional images were used to analyse the variations in the thickness and local porosity of samples as well as their displacement field that were induced by the LDP fracture. High-resolution three-dimensional images showed that these mesostructural variations were accompanied by complex fibre and bond deformation mechanisms that were, for the first time, in situ imaged. These mechanisms occurred in the fracture process zone that developed ahead of the crack tip before the crack path became distinct and visible. They were at the origin of the aforementioned thickness variations that developed more particularly along the crack path. They eventually led to fibre–fibre bond detachment phenomena and crack propagation through the fibrous network. These results can be used to enhance the current structural and mechanical models for the prediction of the fracture behaviour of papers.