Fused Filament Fabrication (FFF) is a common 3D printing technology, where a thermoplastic filament is extruded through a heated nozzle and deposited layer by layer on a build platform. Accordingly, the material can be considered as an assembly of fibers, which are bonded to each other through a diffusion welding process. This process results in a very specific mesostructure of the material, which is determined by several printing parameters like layer height, layer orientation, road width, air gap, etc., and this geometric layout of the mesostructure has a major impact on the mechanical properties of the printed material. The relation between mesostructural layout and resulting material stiffness and strength has been investigated in several experimental studies [1]. Moreover, there exist several publications focusing on modelling and predicting these material properties, which are mostly based on classical lamination theory. In this contribution, we investigate the relation between mesostructural properties and resulting macroscopic properties, with a special focus on material toughness. Experimental results show that these relations are more complex than the ones observed for stiffness and strength, e.g., depending on the of layer orientations the resulting material can be either brittle or highly ductile [2]. Moreover, we find that existing models cannot predict such a behaviour. Based on the experimental findings, we build numerical models which describe the complex nonlinear macroscopic material behaviour by geometrically resolving the mesostructure while resorting to simple, linear elastic, constitutive models for the filament material itself. Numerical examples with different layups show the versatility and efficiency of the approach.