POST-DOC 24 mois , comportement mécanique de fibres de lin et carbone

Estimation of anisotropic mechanical parameters of flax and carbon fibres by instrumented nanoindentation testing

Univ. Bretagne Sud, Lorient, France

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Mission (scientific project) The innovative manufacturing of these materials, for example by fibre placement methods, the durability of structures (fatigue, shaping) and the replacement of mineral fibres (glass, carbon) with fibre from agricultural bioresources (flax) are very much in line with the scientific orientations of UBL’s Industry Department.

Mission Recherche
The challenge of this project is to feed a multi-scale approach to predict fatigue life. A PhD thesis (with GSeaDesign, design company based in Lorient) was completed in 2017 on the proposal of a multi-scale approach to predict fatigue life by homogenizing the properties of the constituents (fiber, matrix) and by localizing the loadings (mechanical, thermal …). A second PhD in the same collaborative framework has just begun on the continuity of this topic. One of the keys to these multi-scale methods is the knowledge of the behaviour (mechanical, for example) of the constituents. However, they are highly anisotropic in the case of carbon fibres or flax fibres. Moreover, the small size of these objects (a few micrometres in diameter) makes it almost impossible to determine the 9 (orthotropic case) or 5 (transverse isotropy) of the elasticity constants. This is not to mention that in the case of flax fibers the behaviour is strongly nonlinear (viscoelasticity (see Keryvin et al., 2015)). The objective is to extract the mechanical properties of these constituents by instrumented nanoindentation tests. The project leader has a great deal of experience with this type of experimental techniques and the extraction of properties by inverse analysis. (V. Keryvin et al., 2017), using experimental equipment and software used at the IRDL. Nevertheless, specific difficulties, and not least, remain for these materials. We read the main locks here. 1. Regarding the elasticity properties, for an isotropic material, a single test theoretically allows access to the plane strain modulus. For an anisotropic material, the difficulty is greatly increased since the indentation modulus measured depends on the 5 (or 9) elasticity parameters of the material. Analytical solutions that are exact but complex (Vlassak et al., 2003) or approximate and simple but limited (Delafargue and Ulm, 2004) have nonetheless been proposed. We will start from these existing solutions. 2. Carbon fibers, unlike glass fibers, besides the fact that they are very anisotropic (contrast of 40 between longitudinal and transverse moduli) are in addition themselves heterogeneous. Crystalline planes may have a tendency to buckle, for example (Barsoum et al., 2004, Gross et al., 2013). The type of indenter vis-à-vis the orientation of these plans, as well as the level of loading will have to be adapted. 3. Indenting on a fiber surrounded by matrix and other fibers does not allow the fiber to be considered as isolated. Taking into account an isolated fiber or in a fiber environment will have to be taken into account for example by a specific numerical approach (L. Charleux et al., 2015). Methodologically, the project should proceed according to the following elements: • Experimental work – Selection of carbon and flax fibers from unidirectional folds from other laboratory studies – Cutting of samples at different angles with respect to the direction of the folds of the unidirectional – Small-scale indentation (~ 100 nm depth) of the different samples with different indenters (cone, pyramid, sphere) – – imprint imaging by atomic force microscopy • Modeling work – Implementation of direct analytical models in scripts (eg Python) – Creation of optimization scripts using analytical models allowing via experimental data to extract the mechanical properties of the fiber • Simulation work

Mission Recherche
– Creation of numerical models (finite elements) representative of indentations carried out taking into account § the angle between the axis of the fiber and the indenter § type of indenter (cone, pyramid, sphere) § of the matrix – Use of inverse methods integrating the previous numerical models It should be noted that some works may be done in relation with colleagues: • DR. Jean-Pierre Guin (IPR, U. Rennes 1) for a complementary experimental part in nano-indentation and atomic force microscopy • Dr. L. Charleux (SYMME, U. Savoie-Mont-Blanc) for a complementary numerical part • Dr. O. Arnouldt (LMGC, U. Montpellier II) for a complementary analytical modelling