Caratterizzazione e ottimizzazione delle proprietà meccaniche di compositi realizzati tramite Continuous Fibre Fused Filament Fabrication

This PhD Thesis deals with the field of additively manufactured composite materials. The key characteristics of this technology, herein referred to as Continuous Fibre Fused Filament Fabrication (C4F), will be introduced in Chapter 1, demonstrating that its potential lies in the combination of the advantages of additive manufacturing with the superior mechanical properties of continuous fibre composites. Through topology and fibrepath optimization algorithms, it is possible to produce highly efficient lightweight structures, offering excellent mechanical performance with very low weight. Additionally, at least in principle, this manufacturing approach leverages the remarkable attributes of “tool-less” production and “lot-size independence”. However, as will be shown in Chapter 2, the technology is not yet fully mature, with several manufacturing issues still to be improved, and the as-printed quality still characterized by non-negligible defects. The deposited fibres are often not well aligned along the designed paths, especially for small curvature radii and, even more detrimental, there are numerous and large voids that significantly reduce the bonding between the individual extrudates, here named beads. The material characterization presented here has been done on the higher-quality material obtained through a thermal post-processing. Generally speaking, it could be argued that this microstructural and mechanical characterization differs from the existing literature due to its greater focus on the bead-based architecture and how it influences the mechanical behaviour, particularly failure modes. These experimental outcomes will then be considered to “inform” the establishment of an analytical model, specifically conceived and developed for these materials. Such model not only accurately predicts the elastic properties, but also allows to estimate the bead stresses. In doing so, our model offers some advantages over the state of the art for 3D-printed composites: it accounts for the large-scale heterogeneity resulting from bead-based deposition; and fits within a multi-scale approach, which is often more effective in describing damage mechanisms, especially under fatigue conditions. Throughout Chapter 3, the mathematical formulation of the model will be presented, followed by its implementation and experimental validation for a unidirectional material. In the following Chapter 4, the analytical model will be incorporated into a framework for 3D-printed components, whose localized and curvilinear fibre reinforcement – as resulting from fibrepath optimization – introduces a level of complexity that is not trivial to simulate. More specifically, our model will be used to assign elastic properties to finite mesh elements, and then to calculate the meso-scale stresses from the simulation’s results. After describing the steps of this modelling approach, it will be implemented for some case studies, featuring an increasing level of complexity. This modelling strategy can be considered an useful achievement, as it may assist the design of topologically optimized parts, fully exploiting the anisotropic performance of C4F composites.