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The advances in additive manufacturing removed most of the limitations that were once stopping designers when it comes to the manufacturability of the design. It allowed designers to produce parts with high geometric complexity such as cellular structures. These structures are known for their high strength relative to their low mass, good energy absorption, and high thermal and acoustic insulation compared to their relative solid counter-parts. Lattice structures, a type of cellular structures, have received considerable attention due to their properties when producing light-weight with high strength parts. The design of these structures can pose a challenge to designers due to the sheer number of variables that are present. Traditional optimization approaches become an infeasible approach for designing them, which motivated researchers to search for other alternative approaches. In this research, a new method is proposed by utilizing the material density information obtained from the topology optimization of continuum structures. The efficacy of the developed method will be compared to existing methods, such as the Size Matching and Scaling (SMS) method that combines solid-body analysis and a predefined unit-cell library. The proposed method shows good potential in structures that are subjected to multiple loading conditions compared to SMS, which would be advantageous in creating reliable structures. In order to demonstrate the applicability of the proposed method to practical engineering applications, the design problem of a commercial elevator sling will be considered.

Lattice structures are widely used in nowadays industries in combination with additive manufacturing technology to obtain components with a limited weight and tuneable mechanical properties. However, industries still find challenging a complete implementation of these metamaterials in the product development due to the complexity given by an accurate prediction of the mechanical and fatigue properties. To overcome this limitation, analytical and numerical techniques are developed, to help designers to achieve the desired performances. Finite Element simulations are a common tool utilized in this sense, where solid models can provide accurate results. Nevertheless, the implementation of this technique requires high computational costs, often not compatible with an iterative design process where versions of the component are constantly updated considering the feedback provided by actors having different backgrounds and product interactions. Accurate and computationally efficient simulations strategies are thus required. The proposed thesis investigates three possible simulations ideas able to describe the mechanical properties of the lattice-based components. Two main properties are studied: the lattice structure elastic behaviour, which is important to determine the in-service behaviour of the designed component and the fatigue resistance, which defines the component service duration. Homogenization technique is the first numerical method analysed and it is pivoted on the idea of substituting the intricate lattice geometries with a solid fictitious material displaying the same elastic properties. In this framework, a case study is analysed, where the design process of a total hip replacement prosthetic device is developed. The workflow starts with a preliminary experimental campaign on lattice specimens with the aim of determining the printing quality, the mechanical properties, and the biological characteristics. In this phase, a verification of the homogenization predictions is performed. On this base, the best specimens’ configurations are selected to design and manufacture the prosthetic device. The second simulation technique leverages on the observation of the onedimensional nature of the strut-based lattice structures. Lattice structures’ behaviour can thus be simulated through the usage of truss and beam elements, depending on the stretching or bending dominated nature of the lattice topologies. Based on this observation, two different paths are followed, the first one aiming to improve the fatigue life of lattice components by acting of their orientation in the printing chamber. It is known that printing orientation influences the surface quality of the components and, in lattice struts this effect can be directly linked to a variation in the fatigue life. An optimization algorithm is thus developed, aiming to optimize the fatigue resistance of the manufactured components. Following this idea, a control and an optimized lattice batch are printed and an improvement in the fatigue resistance is found, even if not as large as expected by the simulations. Improvements in the predictions can be observed if the as-build geometry of the struts is considered. The second path is devoted to the computation of the corrective coefficients able to properly describe the elastic properties of bending dominated lattice structures. One-dimensional simulations are normally too severe for bending dominated lattice topologies, and a compensation has to be provided to match the elastic properties calculated trough computational efficient beam models and lattice ones. To address this problem, an optimization routine is developed, where the compensation factors are computed comparing the elastic properties of the beam models and a homogenised solid model taken as reference. A benchmark testing between the beam model, - built with the so computed compensation coefficients - a homogenised, and a solid model is developed. Compensated beam models are found to be able to improve the predictions of lattice structures elastic properties if compared to the homogenization techniques, showing a comparable computational time. Nevertheless, a reduced accuracy is found in presence of dense lattice structures, where the hypothesis of one-dimensional is weaker. The third analysed simulation method aims to obtain a precise fatigue life estimation at the expense of computational time. Starting from an as-build geometry reconstructed trough CT-scan analysis, a finite element simulation built with solid elements is performed. To reduce the computational cost, an innovative finite element theory is adopted, the Finite Cell Method. A two-step simulation is performed, and thanks to the usage of the average strain energy density method, the fatigue life estimation can be obtained. An excellent agreement is found; however, a complete validation is required for this method before its safe implementation in the design process.

Using Additive manufacturing it is possible to manufacture complicated components, that cannot be manufactured using conventional methods. The typical example is the lattice structure. Fabrication of these structures is complicated, and it is different from the fabrication of bulk parts. Using numerical simulation which can reflect process parameters it is possible to analyze the thermal behaviour of vertical and inclined struts fabrication. Results show that the diameter of struts influences weld track width. This influence is caused by preheating the powder material by previous scanning paths. The final geometry of inclined struts is made in more scanning layers. In this work influence of the start and endpoint of trajectory is described.

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Carbon fiber reinforced polymer (CFRP) composites are known for their high stiffness-to-weight and high strength-to-weight ratios and hence are of great interest in several engineering fields such as aerospace, automotive and defense. However, despite their light weight, high stiffness and high strength, their application in these fields is limited due to their poor energy dissipation and vibration damping capabilities. This thesis presents a two-phase microlattice design to overcome this problem. To realize this design, a novel tape casting integrated multi-material stereolithography system is developed and mechanical properties of samples fabricated using this system are evaluated. The design incorporating a stiff phase (CFRP) and a high loss phase, exhibiting high stiffness as well as high damping, is studied via analytical and experimental approaches. To investigate its damping performance, mechanical properties at small-strain and large-strain regimes are measured through dynamic material analysis (DMA) and quasi-static cyclic compression tests respectively. It is seen that both intrinsic (small-strain) and structural (large-strain) damping in terms of a figure of merit (FOM), E1/3tanδ/ρ, can be enhanced by a small addition of a high loss phase in Reuss configuration. Moreover, it is seen that structural damping is improved at low relative densities due to the presence of elastic buckling during deformation. For design usefulness, tunability maps, displaying FOM in terms of design parameters, are developed by curve fitting of experimental measurements. The microlattice design is also evaluated quantitatively by comparing it with existing families of materials in a stiffness-loss map, which shows that the design is as stiff as commercial CFRP composites and as dissipative as elastomers. / Master of Science / Carbon fiber reinforced polymer (CFRP) composites are known for their lightweight, high stiffness and high strength and hence are of great interest in several engineering fields such as aerospace, automotive and defense. However, despite these advantages, their application in these fields is limited due to their poor energy dissipation and vibration damping capabilities. This thesis presents a novel cellular lattice design to overcome this problem. Recent growth in stereolithography (SLA) has enabled the fabrication of complex structures with high resolution. Using this capability of SLA additive manufacturing, a cellular design is developed to improve both the stiffness and damping performance of CFRP composites while reducing weight. Experiments are conducted to determine the stiffness and damping properties and small and large deformations. It is seen that the stiffness and damping properties can be increased through a two-material hybrid design, comprising of a high stiffness phase and a high damping phase, arranged in a specific pattern. The microlattice design is evaluated quantitatively by comparing it with the existing families of materials using an Ashby chart. The design shows a two order-of-magnitude increase in the stiffness-damping performance when compared to commercially available CFRP.

This dissertation presents three studies addressing various modeling and computational aspects of lattice structures. The first study is concerned with characterization of the threshold behavior for very slow (subcritical) crack growth. First, it is shown that this behavior requires the presence of a healing mechanism. Then thermodynamic analysis of brittle fracture specimens near the threshold developed by Rice (1978) is extended to specimens undergoing microstructural changes. This extension gives rise to a generalization of the threshold concept that mirrors the way the resistance R-curve generalizes the fracture toughness. In the absence of experimental data, the resistance curve near the threshold is constructed using a lattice model that includes healing and rupture mechanisms. The second study is concerned with transmission of various boundary conditions through irregular lattices. The boundary conditions are parameterized using trigonometric Fourier series, and it is shown that, under certain conditions, transmission through irregular lattices can be well approximated by that through classical continuum. It is determined that such transmission must involve the wavelength of at least 12 lattice spacings; for smaller wavelength classical continuum approximations become increasingly inaccurate. Also it is shown that this restriction is much more severe than that associated with identifying the minimum size for representative volume elements. The third study is concerned with extending the use of boundary algebraic equations to problems involving irregular rather than regular lattices. Such an extension would be indispensable for solving multiscale problems defined on irregular lattices, as boundary algebraic equations provide seamless bridging between discrete and continuum models. It is shown that, in contrast to regular lattices, boundary algebraic equations for irregular lattices require a statistical rather than deterministic treatment. Furthermore, boundary algebraic equations for irregular lattices contain certain terms that require the same amount of computational effort as the original problem. / text

Advances in additive manufacturing technologies have brought a new paradigm shift to both design and manufacturing. There is a much bigger design space in which designers can achieve a level of complexity and customizability, which are infeasible using traditional manufacturing processes. One application of this technology is for fabrication of meso-scale lattice structures (MSLS). These types of structures are designed to have material where it is needed for specific applications. They are suitable for any weight-critical applications, particularly in industries where both low weight and high strength are desired. MSLS can easily have hundreds to thousands of individual strut, where the diameter of each strut can be treated as a design variable. As a result, the design process poses a computational challenge. Since the computational complexity of the design problem often scales exponentially with the number of design variables, topological optimization that requires multi-variable optimization algorithm is infeasible for large-scale problems. In previous research, a new method was presented for efficiently optimizing MSLS by utilizing a heuristic that reduces the multivariable optimization problem to a problem of only two variables. The method is called the Size Matching and Scaling (SMS) method, which combines solid-body analysis and predefined unit-cell library to generate the topology of the structure. However, the method lacks a systematic methodology to generate the initial ground geometry for the design process, which limits the previous implementations of the SMS method to only simple, axis-aligned structures. In this research, an augmented SMS method is presented. The augmented method includes the integration of free-mesh approach in generating the initial ground geometry. The software that embodies that ground geometry generation process is integrated to commercial CAD system that allows designer to set lattice size parameters through graphical user interface. In this thesis, the augmented method and the unit-cell library are applied to various design examples. The augmented SMS method can be applied effectively in the design of conformal lattice structure with highly optimized stiffness and volume for complex surface. Conformal lattice structures are those conformed to the shape of a part's surface and that can used to stiffen or strengthen a complex and curved surface. This design approach removes the need for a rigorous topology optimization, which is a main bottleneck in designing MSLS.

Il est maintenant possible de fabriquer des structures treillis métalliques facilement avec la fabrication additive. Les structures en treillis peuvent être utilisées pour produire des pièces de faible masse et de haute résistance. Il n’existe pas de méthode de conception pour les structures treillis. Cette thèse se concentre sur les méthodes de conception des structures treillis et la manipulation dans le CAO et FAO pour faciliter l'intégration des structures treillis dans les produits. La thèse a abordé les questions de recherche suivantes:• Pourquoi les structures treillis sont-elles si peu utilisées dans la conception?• Quelles sont les informations nécessaires pour aider les concepteurs à concevoir des pièces contenant des structures treillis?• Comment les structures treillis peuvent-elles être créées rapidement et facilement dans le CAO?Les principales contributions sont les suivantes:• Une évaluation des outils CAO actuels dans la conception de structures en treillis en termes d'interface homme machine, de formats de fichiers CAO et de FAO pour la fabrication d'additive a été effectuée. Les résultats montrent que les outils de CAO et les formats de fichier CAO actuels ont des performances insuffisantes dans le contexte de la conception pour la fabrication d'additive. Les outils de CAO actuels créent et représentent actuellement des structures en treillis utilisant les surfaces limites des volumes. Cela contribue ainsi à la grande taille des fichiers, à une consommation élevée de mémoire vivre, ainsi des opérations fastidieuses pour les modélisations.• Une nouvelle stratégie de conception de structures treillis. Cette méthode sert de guide aux concepteurs pour l'intégration des structures en treillis dans les pièces fabriquées par fabrication additive en utilisant le matériau équivalent. Les concepteurs auront à leur disposition les informations nécessaires pour choisir les types et la densité des structure treillis à utiliser.• Une méthodologie pour calculer les propriétés matériau équivalent. Ces matériaux équivalents remplacent le besoin de créer des structures treillis dans le CAO et de les calculer par éléments-finis. Cela permettra d'économiser du temps dans la création de modèles CAO 3D et les calculs éléments finis.• Les principales caractéristiques géométriques des structures treillis ont été déterminées. Un modèle squelettique a été présenté pour définir les structures treillis à partir de points, de lignes, de sections et de joints au lieu des surfaces et des volumes. Une méthode est présentée pour visualiser et découper les structures treillis à partir du modèle squelette. / It is now possible to manufacture metallic lattice structures easily with additive manufacturing. Lattice structures can be used to produce high strength low mass parts. However, it does not exist a method to design lattice structures for additive manufacturing. This PhD focuses on lattice structure design methods and manipulation in CAD, CAE and CAM tools to facilitate the wide use of lattice structures in products. The thesis addressed the following research questions:• Why are lattice structures so little used in part designs?• What are the information necessary to help designers to design parts containing lattice structures?• How can lattice structures be created quickly and easily in CAD?The main contributions are:• An evaluation of current CAD tools in terms of human machine interface, CAD file formats, CAE and CAM to design lattice structures was conducted. The results show that current CAD tools and CAD file formats have insufficient performance in the context of design for additive manufacturing. Current CAD tools create and represent lattice structures using surfaces and volumes. This contributes to large file sizes, high RAM consumption, as well as time-consuming creations and operations.• A new lattice structure design strategy. This method serves as a guideline for designers to integrate lattice structures in additive manufactured parts using the concept of equivalent material. Designers will be able to choose lattice structure patterns and densities.• A methodology to create equivalent materials is presented. It is solid and does not contain any struts, thus has few surfaces only. With this equivalent material, it will be easier and quicker to conduct FEA due to the small number of surfaces involved. The characteristics of different lattice structure patterns and densities were determined, which are the relative Young’s modulus and relative strength in function of the relative density. This methodology can be applied to all lattice structures.• The main lattice structure geometrical characteristics were determined. A skeleton model was presented to define lattice structures with points, lines, sections and joints instead of surfaces and volumes. A method is presented to visualise in CAD and slice lattice structures in CAM from the skeleton model.