From 3D Models to 3D Prints: an Overview of the Processing Pipeline

Due to the wide diffusion of 3D printing technologies, geometric algorithms for Additive Manufacturing are being invented at an impressive speed. Each single step, in particular along the Process Planning pipeline, can now count on dozens of methods that prepare the 3D model for fabrication, while analysing and optimizing geometry and machine instructions for various objectives. This report provides a classification of this huge state of the art, and elicits the relation between each single algorithm and a list of desirable objectives during Process Planning. The objectives themselves are listed and discussed, along with possible needs for tradeoffs. Additive Manufacturing technologies are broadly categorized to explicitly relate classes of devices and supported features. Finally, this report offers an analysis of the state of the art while discussing open and challenging problems from both an academic and an industrial perspective.Comment: European Union (EU); Horizon 2020; H2020-FoF-2015; RIA - Research and Innovation action; Grant agreement N. 68044

Challenges and Status on Design and Computation for Emerging Additive Manufacturing Technologies

The revolution of additive manufacturing (AM) has led to many opportunities in fabricating complex and novel products. The increase of printable materials and the emergence of novel fabrication processes continuously expand the possibility of engineering systems in which product components are no longer limited to be single material, single scale, or single function. In fact, a paradigm shift is taking place in industry from geometry-centered usage to supporting functional demands. Consequently, engineers are expected to resolve a wide range of complex and difficult problems related to functional design. Although a higher degree of design freedom beyond geometry has been enabled by AM, there are only very few computational design approaches in this new AM-enabled domain to design objects with tailored properties and functions. The objectives of this review paper are to provide an overview of recent additive manufacturing developments and current computer-aided design methodologies that can be applied to multimaterial, multiscale, multiform, and multifunctional AM technologies. The difficulties encountered in the computational design approaches are summarized and the future development needs are emphasized. In the paper, some present applications and future trends related to additive manufacturing technologies are also discussed

Efficient contouring of functionally represented objects for additive manufacturing

Functionally (implicitly) defined 3D objects allow us to quite easily model parts with complex topology such as lattices and organic-like structures with a high level of flexibility. Previous works in this area are based on the direct generation of CNC programs for the 3D printing of these objects and are backed by the growing support for this input format from hardware manufacturers. Efficient contouring of functionally defined models, however, is not an easy task. In this paper, we develop an algorithm for contour extraction of implicitly defined objects for direct additive manufacturing (AM). By comparing various adaptive and exhaustive (non-adaptive) methods of the function representation contouring for AM (FRepCAM), we make a set of recommendations for its usage depending on the specific resolution of the printer. In particular, we use a novel criterion based on affine arithmetic to maintain efficiency while preserving the robustness of the contouring process. The techniques mentioned were evaluated for algebraic and non-algebraic solids and heterogeneous models under a resolution that is comparable with that of current AM technology. The results show that the chosen adaptation criteria allow us to efficiently obtain a contour for complex models and generally outperform those of traditional algorithms based on exhaustive enumeration, especially for high-resolution contouring. In addition, the results present proof of the printability of implicitly defined objects with different 3D printing techniques

SOLID-SHELL FINITE ELEMENT MODELS FOR EXPLICIT SIMULATIONS OF CRACK PROPAGATION IN THIN STRUCTURES

Crack propagation in thin shell structures due to cutting is conveniently simulated using explicit finite element approaches, in view of the high nonlinearity of the problem. Solidshell elements are usually preferred for the discretization in the presence of complex material behavior and degradation phenomena such as delamination, since they allow for a correct representation of the thickness geometry. However, in solid-shell elements the small thickness leads to a very high maximum eigenfrequency, which imply very small stable time-steps. A new selective mass scaling technique is proposed to increase the time-step size without affecting accuracy. New ”directional” cohesive interface elements are used in conjunction with selective mass scaling to account for the interaction with a sharp blade in cutting processes of thin ductile shells

Topology optimization of cables, cloaks, and embedded lattices

Materials play a critical role in the behavior and functionality of natural and engineered systems. For example, the use of cast-iron and steel led to dramatically increased bridge spans per material volume with the move from compression-dominant arch bridges to tensile-capable truss, suspension, and cable-stayed bridges; materials underlie many of the major technological advancements in the auto and aerospace industries that have made cars and airplanes increasingly light, strong, and damage tolerant; and the great diversity of biological materials and bio-composites enable complex biological and mechanical functions in nature. Topology optimization is a computational design method that simultaneously enhances efficiency and design freedom of engineered parts, but is often limited to a single, solid, isotropic, linear-elastic material. To understand how the material space can be tailored to enhance design freedom and/or promote desired mechanical behavior, several topology optimization problems are explored in this dissertation in which the space of available materials is either relaxed or restricted. Specifically, in a discrete topology optimization setting defined by 1D (truss) elements, tension-only systems are targeted by restricting the material space to that of a tension-only material and tailoring a formulation to handle the associated nonlinear mechanics. The discrete setting is then enhanced to handle 2D (beam) elements in pursuit of cloaking devices that hide the effect of a hole or defect on the elastostatic response of lattice systems. In this case the material space is relaxed to allow for a continuous range of stiffness and the objective is formulated as a weighted least squares function in which the physically-motivated weights promote global stiffness matching between the cloaked and undisturbed systems. Continuous 2D and 3D structures are also explored in a density-based topology optimization setting in which the material space is relaxed to accommodate an arbitrary number of candidate materials in a general continuum mechanics framework that can handle material anisotropy. The theoretical and physical relevance of such framework is highlighted via a continuous embedding scheme that enables manufacturing in the relaxed (or restricted) design space of lattice-based microstructural-materials. Implications of varying the material design space on the mechanics, mathematics, and computations needed for topology optimization are discussed in detail.Ph.D

Multiscale optimization of non-conventional composite structures for improved mechanical response

Nowadays, due to governmental requirements to control climate change, there is a great inter- est on the part of the automotive and aerospace industry to design structures as light as possible, without jeopardize their performance, thus increasing their efficiency. Multi-material design is a way to achieve this goal, as will be shown in this work In this work, multi-material design is considered with the goal of improving the structure’s stiffness, strength, and non-linear behaviour when it yields. Firstly, a microstructural topology optimization is carried out seeking for multi-material microstructures with increased stiffness and strength compared to equivalent single-material microstructures. Afterwards, this study is further extended to perform multi-scale topology optimization, where a concurrent optimization of ma- terial and structure is done. Ultimately, the non-linear behaviour of hybrid fibre reinforced com- posites is optimized in order to introduce a so-called “pseudo-ductility”. Two different optimization problems are formulated and solved here. One compliance mini- mization with mass constraint problem and another stress-based problem where the maximal von Mises stress is locally minimized in the unit-cell. The multi-material design is investigated here using two different approaches. On one hand, the two solids coexist being bonded together across sharp interfaces. On the other hand, a functionally graded material is obtained as an extensive smooth variation of material properties on account of varying composition’s volume fractions of both solids throughout the design domain. The compliance-based optimization results show that multi-material microstructures can be stiffer compared to single-material ones for the same mass requirement. Regarding the stress-based problem, lower stress peaks are obtained in bi-material design solutions and, specially, in the case of graded material solutions. As regards multi-scale topology optimization, the results show that a multi-material structure can be stiffer than its single-material counterpart, which is in accordance with the microstructural study performed earlier. Hybrid composites can achieve the so-called “pseudo-ductile” behaviour mimicking the well- known elastic-plastic behaviour. To understand under what circumstances such behaviour is ob- tained, optimization problems are formulated and solved here. Two different types of optimiza- tion problems are considered. Firstly, one finds out the optimal properties of fibres to hybridize and get the pseudo-ductile behaviour. Once an optimal hybridization is found, another optimiza- tion problem is solved in order to understand the influence of the fibre dispersion on the composite response. The optimal results obtained show hybrid composites having a considerable pseudo- ductile behaviour.Atualmente, devido às imposições governamentais para controlar as alterações climáticas, existe um grande interesse por parte da indústria automóvel e aeroespacial para o projeto de es- truturas o mais leves possíveis, sem se comprometer o seu desempenho, aumentando assim a sua eficiência. O projeto multimaterial de estruturas é um dos caminhos para se alcançar este objetivo, conforme será mostrado neste trabalho. Neste trabalho, considera-se o projeto multimaterial de estruturas com o objetivo de se melho- rar a rigidez, resistência, e comportamento não linear após cedência. Primeiro, é feita uma otimi- zação de topologia ao nível da microestrutura procurando-se microestruturas multimateriais com maior rigidez e resistência quando comparadas com microestruturas de material único equivalen- tes. Depois, este estudo é explorado também no contexto de otimização topológica multi-escala, onde é realizada uma otimização concorrente do material e estrutura. Por fim, o comportamento não linear de compósitos híbridos reforçados por fibra é otimizado com vista à introdução de um efeito de “pseudo-ductilidade”. São formulados e resolvidos aqui dois problemas diferentes de otimização. Um problema de minimização de compliance (flexibilidade) sujeito a um constrangimento de massa e outro pro- blema baseado na tensão, onde a tensão máxima de von Mises é localmente minimizada na célula unitária. O projeto multi-material é investigado aqui utilizando duas diferentes abordagens. Numa das abordagens, os dois sólidos coexistem na sua forma discreta originando-se interfaces com uma variação abrupta de propriedades. Na outra abordagem, obtém-se um material de gradiente funcional onde existe uma suave variação das propriedades obtida variando pontualmente a fração volúmica dos sólidos ao longo de todo o domínio de projeto. Os resultados da otimização baseada na compliance mostraram que microestruturas multimateriais podem ser mais rígidas quando comparadas com as de material único para o mesmo requisito de massa. Relativamente ao pro- blema baseado na tensão, são obtidos picos de tensão mais baixos nas soluções constituídas por duas fases discretas de material e, sobretudo, nas soluções de material de gradiente funcional. No que que diz respeito à otimização topológica multi-escala, os resultados mostraram que uma estrutura multimaterial pode ser mais rígida que uma estrutura de material único equivalente, o que está de acordo com o estudo realizado anteriormente ao nível da microestrutura. Os compósitos híbridos conseguem alcançar um comportamento designado de “pseudo-dúc- til”, imitando o conhecido comportamento elasto-plástico. Para melhor se compreender sob que circunstâncias tal comportamento é obtido, são formulados e resolvidos problemas de otimização. São assim considerados dois tipos diferentes de problemas de otimização. Primeiramente, desco- brem-se quais as propriedades ótimas das fibras a hibridizar, obtendo-se o comportamento pseudo-dúctil. Assim que hibridização ótima tenha sido descoberta, outro problema de otimização é resolvido de modo a perceber-se a influência da dispersão das fibras na resposta do compósito. Os resultados ótimos obtidos mostram compósitos híbridos tendo um comportamento pseudo- dúctil considerável

Efficient fast Fourier transform-based solvers for computing the thermomechanical behavior of applied materials

The mechanical behavior of many applied materials arises from their microstructure. Thus, to aid the design, development and industrialization of new materials, robust computational homogenization methods are indispensable. The present thesis is devoted to investigating and developing FFT-based micromechanics solvers for efficiently computing the (thermo)mechanical response of nonlinear composite materials with complex microstructures