Fused Deposition Modelling (FDM) is a common 3D printing technology based on the extrusion of thermoplastic filaments. While it was initially used only for prototyping, it is nowadays shifting towards manufacturing of mechanical components. 4D printing is a very novel technology to produce smart materials and structures through 3D printing of shape memory materials. Due to the specific process of FDM, the material obtains a characteristic mesostructure, which can be controlled through the print process. It is well known that mechanical properties like strength and toughness of the printed material significantly differ from those of the filament material and that they depend on the mesostructure. However, a real understanding of the material behaviour and the governing phenomena is still missing. The common modelling approach is to consider it as a composite laminate. In this proposal, I show that such models cannot capture the complex behaviour of FDM materials beyond the linear elastic regime. I argue that it can only be understood by considering nonlinear effects at the mesostructure, which needs to be interpreted as a 3D structure of bonded fibres rather than an anisotropic solid. Based on these observations, I will develop a new theoretical and computational framework, where representative volume elements of the mesostructure are modelled as an arrangement of beams with adhesive bonding and are linked to the macroscale through a multiscale approach. To make such computations feasible, it will be necessary to adopt modelling simplifications and a major challenge will be to find the right level of simplification that still can capture the relevant effects. It will also require fundamental development of novel high-order/low-cost numerical methods. The results of the successful project will be a clear understanding of the mechanics of FDM materials as well as tools for the design, analysis, and optimization of FDM structural components.

3D-printing is an innovative technique to manufacture three-dimensional objects with complex shape. Processed materials are metals, ceramics or polymers depending on the working principle of the printer. During the point- or layer-wise printing the material experiences a transition from a liquid to a solid and a temperature change accompanied by changes in the thermomechanical and caloric material behavior. The process leads to gradients in the material properties and to residual stresses which influence the mechanical behavior and the shape of the structure in desired or undesired manners. Since the processed materials are inelastic these effects depend on time and temperature and the parameters of the printing and post-treatment process. Due to the lack of understanding, missing constitutive models and simulation tools, such problems are usually faced by costly trial-and-error methods. To keep the costs in view, this project is focused to 3D-printing of filler-modified polymers which are exothermally curing under UV radiation. Our main objectives are to understand, to model, to simulate and to optimize the printing and post-printer processes of filler-modified polymer structures. Therefore, the experiments start with investigations of the UV-induced curing of filler-modified and unfilled polymers: calorimetric, rheometric and volumetric experiments under UV- and temperature-control are planned. Printed and post-treated tensile bars of composites are analysed in dependence on the process-parameters like temperature, UV-intensity, layer thickness or time-scales. Using the data of the unfilled polymer, a degree of cure-dependent model of thermoviscoelasticity will be developed. It describes curing-induced changes in the material properties, is fitted to the experimental data and implemented into a finite element code. A differential equation with UV intensity-dependent parameters is developed to describe the evolution of the degree of cure. If the glass transition temperature of the fully cured polymer is above the curing temperature, diffusion control is taken into account. The distribution and the geometry of the filler particles in printed samples are studied by electron microscopy and the influence of the filler to the material behavior of the composites by mechanical testing. Merging this information, a reference volume element is created whose homogenized behavior is computed under further assumptions with the finite element implementation of the thermoviscoelastic model for the matrix and compared with experiments. Lastly, the simulation chain is applied to simulate the printing process and the post treatment of lattice structures. At different times during the post treatment, the measured and simulated geometries and residual stresses are compared. If the validation is successful, the simulation chain provides optimal process parameters minimizing residual stresses and keeping the printed shape stable and within admissible tolerances.