On the modelling of flexible graphite
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Flexible graphite (FG) is obtained by compaction of exfoliated graphite in the form of sheets having a thickness of 0.2 to 3 mm and density ρ from 0.7 to 1.9g/cm3. Sigraflex® is a commercial type of FG with ρ = 1 g/cm3 employed as beam dumping material in the Target Dump External (TDE) cores of the Large Hadron Collider (LHC) located in Geneva, Switzerland. The performances of the TDE components are assessed by comprehensive FE simulations whose accuracy depends on the material parameters assigned to each component. In such context, this thesis aims to find the model and the necessary parameters for the mechanical simulation of the Sigraflex® core in the TDE. At first, a review of Sigraflex® and FG properties available in literature will be presented: this will include the description of the microstructure, the collection of the mechanical and thermal properties, and their critical comparison with other well-known types of graphite. Then, an extensive experimental campaign will be reported. For the first time, the application of a Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) technique to FG for the quantitative investigation of the pores’ sizes and shapes and the micro-sheets’ thickness and arrangement, will be described. Moreover, the experimental setup and the mechanical properties obtained by in-plane tension, out-of-plane compression and nanoindentation tests will be outlined and thoroughly discussed. In particular, nanoindentation revealed to be an easy-to-use method to measure the orthotropic elastic properties of FG. Finally, although the focus will be mainly on the mechanical properties, a thermomechanical testing campaign will be presented, too. Based on experimental observations, the behavior of FG will be assimilated to other well-known materials such as crushable foams and crumpled materials. A 1D analytical model will be proposed to decouple the deformation contributions from the graphite-like and crumpled-like behaviors, and the extension of the nonlinear stress-strain behavior to 3D cases will be discussed. A practical solution will be to use a material model found in Ansys LS-Dyna library i.e., MAT_142 transversely crushable foam. Its suitability was investigated by means of 3D FE simulations of nanoindentation and the numerical force-displacement curves will be shown in comparison with the experimental curves. Although not exhaustive, the results are promising and can be considered as a reference point for future works on FG characterization.