Periodic Reporting for period 2 - FDM^2 (Structural multiscale modelling of extrusion-based 3D and 4D printed materials)
Okres sprawozdawczy: 2021-12-01 do 2023-05-31
Fused Deposition Modeling (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 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 behavior and the governing phenomena is still missing. A common modeling approach is to consider it as a composite laminate. In this project, we show that such models cannot capture the complex behavior of FDM materials beyond the linear elastic regime. We 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 fibers rather than an anisotropic solid. Based on these observations, we are developing a new theoretical and computational framework, where representative volume elements of the mesostructure are modeled as an arrangement of beams with adhesive bonding and are linked to the macroscale through a multiscale approach. To make such computations feasible, it is necessary to adopt modeling simplifications and a major challenge is to find the right level of simplification that still can capture the relevant effects. It requires 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-printed structural components, with the goal of leveraging this technology as a reliable and cost-effective manufacturing technology for industrial applications.
(1) We have performed extensive experimental investigations, mostly mechanical testing of test specimen printed with different layup orientation and different process parameters (extrusion temperature etc.), but also temperature measurements during the extrusion process and the subsequent cooling. The purpose of these experiments is (a) to obtain a deeper understanding of the specific behavior of FDM materials and the underlying mechanisms, (b) to retrieve the material parameters necessary for calibrating the simulation models, and (c) validating the simulation models. Dedicated studies were performed for optimizing the test procedures and specimen design, since no standards are available for testing FDM materials.
(2) We have built nonlinear finite element models to represent the mechanics of the material at the mesoscale for different layup orientations. Different modeling approaches with different levels of simplification were pursued and are compared to each other in terms of accuracy and computational efficiency.
(3) Simulations of the extrusion process have been performed to predict important properties like bond strength between filaments and residual stresses within the printed material. Also here, different approaches with different levels of simplification have been considered: a) ALE-CFD simulations of the viscoelastic melt, including all stages of the extrusion process, b) heat transfer simulations with element activation to model the deposition. For the latter, a multi-level adaptive mesh coarsening approach was developed, enabling efficient simulations also for larger objects.
(4) Work in (2)+(3) was made with established commercial software. In parallel, advanced numerical methods, in particular isogeometric analysis and isogeometric collocation methods, were developed as basis for performing simulations as in (2)+(3) in an in-house code in the second halve of the project, in order to improve the numerical efficiency of the simulations and to overcome limitations currently posed by the available software and methods.
(2) We have built nonlinear finite element models to represent the mechanics of the material at the mesoscale for different layup orientations. Different modeling approaches with different levels of simplification were pursued and are compared to each other in terms of accuracy and computational efficiency.
(3) Simulations of the extrusion process have been performed to predict important properties like bond strength between filaments and residual stresses within the printed material. Also here, different approaches with different levels of simplification have been considered: a) ALE-CFD simulations of the viscoelastic melt, including all stages of the extrusion process, b) heat transfer simulations with element activation to model the deposition. For the latter, a multi-level adaptive mesh coarsening approach was developed, enabling efficient simulations also for larger objects.
(4) Work in (2)+(3) was made with established commercial software. In parallel, advanced numerical methods, in particular isogeometric analysis and isogeometric collocation methods, were developed as basis for performing simulations as in (2)+(3) in an in-house code in the second halve of the project, in order to improve the numerical efficiency of the simulations and to overcome limitations currently posed by the available software and methods.
State of the art for performing structural analysis of FDM-printed materials and structures is to use classical lamination theory, where each layer is modeled as an anisotropic continuum. The experimental investigations of this project have shown that such modeling approaches cannot describe the material behavior beyond the linear elastic regime. We have shown that nonlinear structural effects at the mesoscale govern the nonlinear material behavior at the macroscale, e.g. the printed material may behave either brittle or ductile depending on the layup orientation. We have developed a novel modeling and simulation approach, which captures the relevant nonlinear effects on the mesoscale and the complex material behavior at the macroscale as observed experimentally on simple test specimens. The final goal of the project is to have a simulation framework which allows for accurate prediction of the mechanical behavior until failure for complex printed structures as a function of the various controllable print parameters. This method shall then be used to optimize the print process parameters individually for each print to obtain optimized structural performance as well as to produce materials with specifically designed mechanical behavior. The potential impact is that FDM shifts from a technology of hobby 3D printing to a reliable, flexible, and cost-efficient technology for manufacturing structural components as well as designed materials.