Multiscale thermo-mechanical modeling of semi-crystalline polymers : application to additive manufacturing by selective laser sintering
(2022)
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- Semi-crystalline polymers exhibit performances that are highly dependent on their micro-structure as induced by the thermo-mechanical processes they are subjected to. In this thesis we developed a multiscale modeling and simulation framework able to predict the thermo-mechanical response of semi-crystalline polymers including crystallization and porosity evolution, with an application to additive manufacturing by selective laser sintering. Firstly, an enhanced phase field model was developed for the numerical simulation of crystallization in semi-crystalline polymers. The model is based on coupling the heat equation with the Allen-Cahn equation, which is derived from the Gibbs-Thomson solid-liquid interface equation. Starting from the nucleation of spherulites, existing phase field models can simulate their evolution in a surrounding liquid and separate the amorphous and crystalline phases. However, the predictions of the morphological characteristics of the spherulites remain qualitative only. Moreover, the predicted spherulite evolution as a function of crystallization temperature is not consistent with experimental results. In our work, existing phase field models were enhanced in order to obtain experimentally consistent results. We used spherulite growth, crystal morphology, and crystallinity degree in spherulite, as measures to determine the model accuracy. The model is numerically implemented using the finite difference method so that 2D and 3D simulation results are presented and compared to experimental data, illustrating the quantitative adequacy of the predictions with experimental evidence. Secondly, full-field micromechanical simulations were conducted on the micro-structures generated by the enhanced phase field model in order to predict the effective mechanical properties. Care is taken to obtain Representative Volume Elements (RVEs) by computing the number of subcells in each spherulite and the number of spherulite nucleations in each RVE. An FFT solver was used to determine the thermo-mechanical properties for a range of crystallinity ratios. Validation against experimental data and comparison with simpler models was done. Thirdly, we developed the constitutive laws enabling the prediction of the thermo-viscoelastic-viscoplastic behavior of semi-crystalline polymers. The crystalline lamellae follow an orthotropic elasto-viscoplastic model based on the activation of slip systems, while the amorphous phase is viscoelastic. Some material parameters were identified directly while others were reverse engineered from experimental measurements. Finally, a mesoscale model was proposed for the density evolution of an initially low density polymer grain powder which becomes compacted after sintering. The approach is based on a generalized self-consistent model, the analogy between linear elasticity and Newtonian fluid and the mass conservation equation. All the models have been implemented in a research version of the Digimat software.
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Bahloul, A. (2022). Multiscale thermo-mechanical modeling of semi-crystalline polymers : application to additive manufacturing by selective laser sintering. https://hdl.handle.net/2078.5/102935