Lattice materials have been studied extensively for their advantageous mechanical properties like high stiffness-to-weight and strength-to-weight ratios. These properties make them suitable for a multitude of applications like load-bearing structures, energy absorption and impact attenuation. Amongst several planar microarchitectures, the Kagome topology is found to be optimal for effective mechanical properties like macroscopic elastic modulus. However, it has been shown that lattice defects such as nodal misalignments are detrimental to the macroscopic stiffness of the Kagome lattice. From a realistic standpoint, any manufactured lattice is expected to inevitably have these defects, which naturally makes the favorable stiffness properties of the Kagome inaccessible.

This work takes a step towards designing planar lattices with defect insensitive macroscopic stiffness. A novel design strategy is proposed for generating stiff, lightweight and robust lattice materials. The Kagome lattice is chosen as base architecture for the developed designs due to its excellent stiffness-to-weight property and extreme sensitivity towards defects. The generated designs involve appending the Kagome bulk with two unique and carefully designed microarchitectures. The first is a lattice developed using combinatorial design strategies, which demonstrates properties like perturbation-dependent switching between mechanism-states and load-bearing states. The second is a polarised lattice that enables localisation of mechanisms at specific sites of the lattice. This arrangement forms a "composite" lattice which when appropriately designed demonstrates robustness against manufacturing defects.

Development of such an architecture is carried out through an iterative design and analysis process which is based on reformulated matrix methods. A mass-spring model is used in contrast to regular pin-jointed models and a dynamical matrix method is developed as an alternative to traditional matrix analysis. Suitable designs formed out of this iterative process are verified for their stiffness properties using Finite Element models. Robustness against nodal defects is observed in well-designed lattice models. Satisfactory performance of such designs indicates the efficacy of the adopted design strategy which could potentially be applied to any lattice microarchitecture other than Kagome for robustness.

Wire arc additive manufacturing (WAAM) is attributed to higher material deposition rates and medium to large build volumes. Integration of multi-axis (more than three) material deposition kinematics in WAAM can bring a paradigm shift in the metal additive manufacturing industry. The material build-up mechanism employing directed energy deposition results in localized melting and solidification of feeding wire making the fabricated structure inherently prone to deposition defects such as dimensional distortion, residual stresses, solidification cracking, and porosity. Optimization of welding process parameters can improve cracking and porosity behavior. However, the problem of distortion is inevitable, and current mitigation strategies rely on post-processing or symmetric material deposition; while the former decreases the effectiveness of WAAM and increases costs, the latter could only be implemented for specific parts. Tackling this problem with the help of computational design tools is a recent approach, and Fabrication Sequence Optimization dictating material deposition is numerically predicted to limit the extent of the distortion effectively compared to conventional planar horizontal deposition; however, this still lacks experimental validation. This research presents a series of fabrication experiments using Al5356 Aluminum-magnesium alloy wire in a 6-axis robotic WAAM setup in two parts: (1) Process parameters identification for optimal bead characteristics for Al5356 wire, (2) Fabrication of single-bead multi-layered thin-walled shell structures using planar and optimized material deposition. A digital image correlation-based non-contact in-situ distortion measurement setup is constructed to analyze strain development during fabrication process and 3D scanning of fabricated structures is performed to estimate dimensional deviation. The experimental findings revealed an improvement in geometrical formation and a qualitative indication of distortion minimization with an optimized fabrication sequence.

In this thesis a new efficient model has been developed in the thermal analysis of Laser Powder Bed Fusion, an Additive Manufacturing (or: 3D-printing) method that is used to manufacture metal products. During this process, a laser is used as a heat source to melt metal powder grains to each other, that are solidified when cooled down. In Laser Powder Bed Fusion high temperatures changes arise at the laser spot, that are in a scale smaller than the scale of the manufactured product. This requires a very fine spatial discretization (or: mesh) if the thermal analysis is done purely numerical. A fine mesh leads to high computational cost. Various methods have been previously developed to reduce this costs, like the semi-analytic method. The new model in this thesis is ased on a the previously developed thermal model that uses the semi-analytic method....

Laser Powder Bed Fusion (LPBF) is a metal additive manufacturing process in which a three-dimensional object is obtained by selectively melting and fusing a metallic powder with a heat source, such as a laser beam, in successive thin layers. This process allows to create lightweight parts with complex geometry. However, parts created by LPBF processes may present poor surface quality and can be prone to high residual stresses and deformations that arise during manufacturing. To be able to understand the process and investigate the relation between the thermal history of the part and the deformations that arise, researchers have proposed different thermal models in literature. Most of these models are purely numerical methods in which the temperature history is predicted by implementing finite element and finite difference schemes. However, a finer discretization is required to capture the steep temperature gradients that arise at the vicinity of the laser spot, which results in computational expensive models.
In the Precision and Microsystems Engineering (PME) Department at TU Delft, a semi-analytical approach was proposed to study the thermal history of LPBF processes. This method combines an analytical solution that captures the high spatial gradients of the laser beam and a numerical solution that corrects the analytical solution and enforces the boundary conditions. When the laser beam is near the boundary, the analytical solution is corrected first with the method of images, which dramatically reduces the computational cost when obtaining the solution. However, this approach is only valid for convex polyhedrons. If the domain is non-convex, the method of images cannot be applied. Therefore, the high spatial gradients have to be dealt with finer discretization, and the numerical correction's computational cost becomes prohibitive.
In this thesis, a thermal model is developed for LPBF processes using the semi-analytical approach with a modification in the method of images for concave polyhedrons. In this case, a correction to the method of images, denoted as anisotropic mirror sources, is introduced. This method allows to apply image sources to concave geometries. With this modification, the numerical correction in the semi-analytical method can be implemented with a coarse discretization with high computational efficiency.
Predictions of the proposed thermal model are compared to the predictions of a semi-analytical method without using the method of images. This comparison allows us to validate the proposed thermal model's computational efficiency as it can obtain the thermal history of concave polyhedrons in a computationally inexpensive manner. More importantly, the thermal model proposed demonstrates the possibility of using the method of images in concave polyhedrons.

Additive manufacturing (AM) has revolutionized part manufacturing, offering unprecedented design freedom and the ability to fabricate intricate structures. Computational design methods like topology optimization (TO) effectively capitalize on AM's design freedom. However, manufacturability constraints must be considered to ensure reliable manufacturing. A significant constraint is local overheating, prevalent in metal additive manufacturing, leading to defects, part distortion, and diminished mechanical properties. While overhangs are commonly associated with overheating, they are not the only geometric features prone to overheating. This thesis addresses the overheating issue in topology optimization by a novel geometric approach. Specifically, we propose to estimate the local conductivity around each element by evaluating the material distribution in its vicinity. This is then used to generate a pseudo temperature field (i.e., hotspot map) to assess overheating risks. Formulated as a constraint in topology optimization, our approach creates optimized structural layouts free of local overheating during additive manufacturing. Additionally, the approach is implemented in space-time topology optimization, limiting overheating risks by simultaneous optimization of structural layout and fabrication sequence. Compared with existing overheating prevention methods, the geometry-based constraint demonstrates significant computational advantages. Transient thermal AM simulations were conducted on the final designs obtained using the physics-based method from the literature and the novel geometry-based overheating prevention method. The numerical results have shown that the proposed geometric approach is efficient and effective in controlling local overheating in topology optimization for metal additive manufacturing.

In recent studies, selection criteria for lattice materials suitable for active shape-morphing have been composed. These criteria have directly resulted in the proposal of a lattice that is able to compete with the Kagome lattice, which in the literature is identified as a lattice material with optimal in-plane stiffness and shape-morphing capabilities. The selection of suitable lattice materials is mostly related to the properties of the lattice before shape-morphing. Naturally the question 'What is the effect of morphing on the macroscopic properties of the lattice material?' arises.

This research investigates the relationship between kinematic properties of the pin-jointed Kagome truss and elastic macroscopic properties of the welded-jointed Kagome lattice during deformation. The kinematic properties of the Kagome truss are explored at all possible collapse configurations. The same collapse configurations of the pin-jointed truss are then used to determine the elastic response of the welded-jointed twisted lattice with the use of the commercial finite element program ABAQUS.

The results show that only the pin-jointed initial configuration of the Kagome truss can support all macroscopic loads without inducing a collapse of the truss, no other pin-jointed configuration shares this property. Indicating different elastic responses of the initial welded-jointed lattice and the twisted lattices. The results of the finite element analysis support this finding, as it can be shown that the type of deformation of the initial configuration of a welded-jointed lattice differs from the deformation of any other twisted configuration. A stark difference between Young's modulus is observed between the initial configuration and the twisted configurations. The deformation induced by single member actuation creates a weak layer in the lattice. Therefore the attractive properties of the Kagome lattice are not preserved during shape-morphing.

Fused filament fabrication (FFF) is the most used additive manufacturing technique that uses a heated nozzle to melt a polymer and a feeder to extrude it on a buildplate. The dependencies of temperature, shear-rate, viscosity and pressure of the melt create complex dynamics within the nozzle, which causes inconsistent extrusion. To improve extrusion control, a better understanding of the dynamics within the nozzle is required. The greatest knowledge gap comes from a lack of experimental data on the pressure inside the nozzle, due to the challenging environment for sensors. This study presents a novel way of monitoring the pressure inside the nozzle of an FFF 3D printer. A pin that is in direct contact with the melt transfers the force applied by the melt through a hole in the nozzle to an externally mounted load cell. The set-up has proven to provide reliable, repeatable pressure data in steady-state, static extrusion. The experimental data on different nozzle geometries and materials, with different flows and temperatures, has been compared to theoretical pressure calculations to identify non-linearities that influence the pressure such as entrance effects, temperature non-uniformity and viscoelastic behaviour of the melt. The proposed design can be used to gain more knowledge on the extrusion process to further develop extrusion control.

Fused deposition modeling (FDM) is one of the fastest-growing additive manufacturing processes in recent years. Nevertheless, the mechanical properties cannot be matched to those of traditional manufacturing methods such as injection molding.The research presented in this report aims to demonstrate the limitations of fusion deposition models and proposes automated fiber placement (AFP) as a way to mitigate them by improving transversal tensile strength and elastic modulus.
The background and main characteristics of the stated processes, as well as the state of the art, are discussed in this paper. It is explained how both methods were integrated for a production line built specifically for the execution of this research. The key parameters are listed, along with their impact on the end part.
Mechanical parameters such as tape-to-print bonding strength, tape-to-tape bonding strength, molding unidirectional fabric tensile strength, and 3D printed part tensile strength were all compared in the experiments. The major challenges to be addressed are surface roughness, print-to-tape contact area, and heat transfer when taping, according to the results. Yet, there is still a large potential for these methods to achieve better performance.

As a type of powder-bed-based Additive Manufacturing (AM), Selective Laser Melting (SLM) is widely used for building metallic lattice structures. However, during SLM process, geometrical imperfections and defects, such as strut over-sizing or under-sizing, typically exist in components due to overheating, which strongly influence their mechanical response. Therefore, it is common that the as-built lattice has discrepancies from the designed lattice structure in mechanical properties. The main aim of this study is to simulate the transient temperature field of lattices during SLM process to predict the possible manufacturing issues for the improvement of product quality. Based on a technique, the moving grid method, growing struts of a lattice whilst it being built by SLM can be regarded as moving domain problems, and thermal process of a building lattice can be simulated numerically. The primary contribution of this study is to evaluate the effect of process parameters, including laser power, energy density and material deposition rate, on thermal evolution of a lattice during SLM. Another contribution is related to the comparison of transient temperature field between different lattice architectures. Analyzing the influences of various factors is important to recognize the thermal evolution of a building lattice. The results reported in this study offer a reference for quality improvement of produced parts, to realize that both geometry and mechanical properties of the as-built lattices as close as possible to their as-designed counterparts.

Magnets for High Energy Physics applications built to date are generally superconducting magnets, which operate at cryogenic temperatures. The reliability and safety of the applications are entirely dependent on good design which in turn rely heavily on predictable materials performance. Where at these low temperatures the fracture toughness is of importance to be known (alongside mechanical properties such yield and tensile strength). At CERN at the materials and engineering department (EN-MME-MA) a testing facility is being commissioned for the measurement of mechanical at cryogenic temperatures. A tensile test facility has been realised, yet no such set-up is available for fracture toughness measurement. The aim of this thesis was to develop a test set-up for the measurement of the fracture toughness in order to realise a universal cryogenic testing system, which can be used for both tensile tests and fracture toughness test at low temperatures. A design for a set-up is proposed for the measurement of the fracture toughness which can be employed within the current cryostat. The design of the tooling and cryostat have been extensively verified using numerical methods taking into account thermal effects (such as conduction and contraction) and the varying material properties at these low temperatures. A modified C(T) specimen is proposed for more robust and reliable set-up. For this modified specimen it is shown with numerical methods that the modification are expected to have a negligible impact on the fracture toughness measurements. Tests have been performed using the proposed design with four specimen fabricated from two different materials (SS316L and Ti6Al4V), at both room temperature and at 4 K (using liquid helium). The set-up is shown to provide sufficient data for the characterisation of the fracture toughness for both Linear Elastic Fracture Mechanics tests as well as Elastic Plastic Fracture mechanics tests.

Wire Arc Additive Manufacturing (WAAM) is a manufacturing technique with the ability to produce large metal parts with relatively complex geometrical shapes. One obstacle limiting full exploitation of WAAM in the industry is uncertainty on the mechanical properties of manufactured parts. Mechanical properties are strongly influenced by the microstructure of the material which is a product of the thermal history of the material. Thermal history of parts manufactured with WAAM is complex which leads to uncertainty on mechanical properties and microstructure. Therefore a methodology to predict thermal history, microstructure and hardness of metal-based additive manufactured parts is presented, validated and applied in this study. The methodology is applied to high strength low alloy s690 steel parts produced with WAAM. First a part level thermal process model is utilised to predict the thermal history. Subsequently, a model is developed to predict the microstructure by modelling microstructure phase transformations. Based on thermal history and microstructure phase fractions, Vickers hardness is predicted. Predictions are validated with experimental measurements and observations obtained from a collaborating master thesis. Part temperatures of the thermal model agreed well with measurements. Although the predicted microstructure phase fractions show a lower ferrite content than experimentally observed, the trend of the microstructure phase fractions along the part height is the same in the prediction and the experiment. The predicted hardness is 49 HV higher than the measured hardness. However, hardness predictions and measurements show the same trend along the part height. The methodology to predict the thermal history, microstructure and hardness was applied on a study in which the height of a wall was varied and on a study on a geometry in which two walls cross each other at the center. For both studies the martensite content decreases over the part height, whilst the bainite content increases over the part height. This is caused by the decrease of cooling rate over the part height. Hardness also decreases over the part height. For both the change in microstructure phases and the hardness over the part height, the largest change was observed at the bottom of the part. For the study with the crossing geometry, no significant difference was found for the microstructure and hardness between the center and side of the crossing.

This thesis a method is described to include manufacturing constraints of the additive manufacturing process into topology optimization. Topology optimization often results in complex structures, which can only be produced by additive manufacturing. Although additive manufacturing has much fewer design restrictions compared to conventional manufacturing methods, it has its own limitations. The most prominent limitation is the overhang constraint, which causes the need for support structures. In this thesis front propagation is utilized to crudely mimic the printing process. Making use of existing numerical methods for front propagation, a numerically efficient algorithm is produced that can identify parts of the structure that do not adhere to the overhang constraint, during the topology optimization process. Since the sensitivities of the algorithm are available, it can be used in the gradient based topology optimization to ensure printability of the final part. Due to the continuous description of the constraint, it can be applied to unstructured meshes and for variable overhang angles. Furthermore, it is shown that the constraint can be parallelized which is demonstrated on large scale 3D problems.@en

The urge for more sustainable and cost-efficient repair methods for damaged parts is growing every day. On the other hand, laser metal deposition (LMD) is gaining momentum. This flexible technique, where material is added in a layer-by-layer fashion with a minimal heat input offers solutions for a wide range of production and repair purposes. Especially restoration or replacement of large and complex high-performance parts with long production times could profit from these developments. Industrial gears are certainly such type of parts. Occasionally, unforeseen tooth breakage of gears occurs, often leading to the replacement of the parts. No reliable repair techniques have emerged yet, due to the desired material properties and the uneven distributed hardness seen in carburized gears. This study was initiated to examine the feasibility of LMD for the repair of industrial gears with the goal to reduce both material waste and down time while retaining or exceeding the required mechanical performance. The initial part of the research is focused on finding the best suitable material and deposition parameters to ensure the desired mechanical properties of the core and a good bonding of the structure to the gear. Multiple deposition parameters are varied followed by destructive and non-destructive testing. It was found that the mechanical properties of deposited structures Inconel 718 and its bonding to a carburized substrate can be adjusted by accurately selecting the deposition parameters. Large differences in outcome were obtained by changing the deposition direction between each layer and tuning of the shielding gas flow rate. In further investigation the effects on the mechanical behavior of a deposited tooth structure were studied by altering the thermal cycle during and after deposition. The tests are performed on a specially developed static load testing setup. The results show a negative correlation between the interpass temperature and both the yield and ultimate tensile strength. This effect is attributed to a lower cooling rate during solidification at higher interpass temperatures. Ultimately, the results of a deposited tooth with hard flanks and built on a case hardened base were compared to a case hardened gear tooth that was manufactured according to the regular production method. It is found that the deposited structure exhibits significantly lower yield strength. Nonetheless, the superior ductility of the newly deposited tooth structure in the presented work is considered as a promising method for future gear repair.

Trendelenburg gait is an abnormal gait that occurs when hip abduction muscles are weakened. These weakened muscles cannot deliver enough force to prevent the pelvis from drooping when standing on one leg. This results in an abnormal gait where the upper body sways from left to right to keep the center of gravity above the standing foot. Besides standing out from other people when walking, this causes problems at the hip joint, knees, and ankles. This thesis presents a conceptual design of a compliant hip orthosis to prevent Trendelenburg gait. The design has a high adduction over flexion-extension stiffness ratio, meaning that the upper body is supported while walking is still possible. Also, the design exerts minimal shear forces on the human body, increasing comfort, and does not protrude from the body, making it aesthetically more appealing.

Powder bed fusion (PBF) is a widely used metal additive manufacturing method where a part is built with almost no limitation on its geometrical layout. However, parts built by PBF typically suffer from distortions and residual stresses which depend on the PBF processing conditions and the geometrical layout of the part. In order to investigate this relation, a new computational tool is presented. The unique property of our methodology is to keep the total number of degrees of freedom constant by utilising a moving grid to represent a growing part with time. Thermal transients are calculated and are coupled to a mechanical boundary value problem to evaluate the evolution of temperature history and residual stresses. The significant gain in the computational efficiency of the moving grid technique is demonstrated in comparison to the state of the art models available in the literature.

Discrete Dislocation Dynamics concerns the analysis of microplasticity in which the dislocations, a line defect in the crystallographic periodicity, are treated as separate moving entities inside an elastic continuum. This analysis at the mesoscale can give valuable insights in the nature of dislocations and the behaviour of macroscopic plasticity. Although an analytical solution for dislocations exist, special measures are to be taken for the solution of boundary value problems. In this thesis a Discrete Dislocation Dynamic model in a two-dimensional plane-strain formulation for edge dislocations is proposed using DE-FEM, a finite element formulation enriched with discontinuities to model dislocations independent of the mesh. Although the modelling of a stationary field with dislocations is straightforward, the dynamic solution becomes much more complicated. It requires the accurate stress retrieval of the numerical stresses close to an artificial singularity and various attempts, such as eliminating the singularity by subtracting the analytical solution or incorporating a singular core enrichment, were proven to be ineffective. Recommendations are given for future attempts, but also an alternative numerical superposition approach is explained. Its results are comparable to existing methods in terms of accuracy, but the computation time is not optimal for the current implementation.

Externally triggered lengthening or shortening (actuation) of one or more members in a lattice material can be used to achieve macroscopic shape changes. The three main attributes looked for in 2D lattice materials to be deemed suitable for actuation are in-plane isotropy, high specific elastic properties and limited energy requirement for actuation. However, no infallible topological criteria have yet been discovered that determine which micro-architectures meet these requirements. The Kagome micro-architecture yields the best-performing lattice material for actuation known to date.

This thesis contributes to the search for micro-architectures that perform similar to Kagome lattice material, or even outperform it. Four novel micro-architectures are introduced. Each of the new micro-architectures is verified to constitute a stiff (i.e. stretching-dominated) isotropic lattice material. Still, not all of them result in optimal elastic moduli. One of the proposed designs does perform identically to the Kagome structure in terms of elastic moduli. Moreover, it requires less energy for actuation in the range of relative densities of interest. On the other hand, its attenuation distance is shorter; actuation deformations damp out within a shorter distance than in Kagome lattice material.