Postprocessing and simulation methods for additively manufactured continuous fibre reinforced plastics

A new technology based on fused filament fabrication that allows the manufacturing of continuous fibre reinforced composites has been developed in the recent years. Fibres are being embedded in a plastic that is molten and laid out on two-dimensional layers. These layers are stacked on top of each other, thereby creating a three-dimensional part. However, alignment of the polymer chains during the extrusion causes an insufficient bonding between the layers. Furthermore, the printing process causes a high air-void ratio. These effects lead to a reduced strength and stiffness. To address these drawbacks, this study investigated the influence of a thermal treatment process on the mechanical performance of continuous and chopped fibre reinforced polyamide 6 in the build-up direction. A correlation between the annealing temperature and the mechanical performance was found. The Young’s modulus increased by a factor of three, while the ultimate tensile strength (UTS) increased by 186% for the continuous glass fibre reinforced material. A temperature dependent transition from the gamma to the alpha phase was observed while the crystallinity only slightly changed. For the continuous fibre reinforcement an increase in the air-void ratio was observed. Based on these findings, the influence of a post pressure treatment process on the mechanical performance in build-up direction and perpendicular to fibre direction within the layer was investigated to address the high air-void ratio. Annealing at a pressure of 1 MPa was found to homogenize the material and led to a significant increase of both the tensile strength (55 MPa) and Young’s modulus (5 GPa). Increasing the pressure to 3 MPa only slightly increased the mechanical performance, whereas a further increase to 6 MPa caused no significant changes. While at 1 MPa the air-voids are significantly decreased within the part, 3 MPa were required to decrease the air-void ratio close to the fibre bends. In order to evaluate potential use-cases it is important to understand the materials failure behaviour. Simple failure theories used for homogenous materials, like the von Mises failure criterion, are not able to predict the failure of composite materials. Due to the vastly different properties of the fibres and the matrix the failure process is more complex. To investigate the failure behaviour of the postprocessed material a micromechanical model was developed. Based on sophisticated models for the constituents a unit cell was used for virtual testing. It was found that, despite the low fibre volume content, Puck’s failure criterion was applicable. Consequently, the it was used to simulate the materials potential uses in the front spoiler of the university’s student race car.