Phononic crystals can be designed to have bandgaps---ranges of frequencies whose propagation through the material is prevented. They are therefore attractive for vibration isolation applications in different industries, where unwanted vibrations reduce performance. Yet, important steps are still to be made for the integration of phononic crystals into engineering practice. For instance, methods for large scale production are still in development. Furthermore, it is essential that design methods are established to enable the design of phononic crystals that meet all of the, often conflicting, requirements for practical applications. This thesis focuses on the latter challenge by proposing a computational design method for phononic crystals based on the combination of an advanced finite element method and level set-based topology optimization.@en

Reducing greenhouse emissions can be achieved by better insulating buildings. However, wall thickness increases significantly if conventional insulation materials are used. Conventional vacuum insulation panels (VIPs) offer a solution to this problem. VIPs have potential, but can not compete with conventional insulation materials due to their high pricing. A new concept vacuum insulation panel is introduced by the start-up company IQ-Bizz. To what extent this concept panel is a feasible alternative to conventional VIPs is the main research question of this thesis. The concept panel consists of two honeycomb plates, an intermediate foil and an MF2 panel envelope. The panel's geometry is chosen such that heat transfer through the panel is minimal and failure of the panel does not occur. Finite element models, verified by analytical calculations, have been used for evaluating the concept panel's structural and thermal performance. A thermal performance (R-value) of 6.23 m^2K/W is evaluated for a one-centimetre thick square panel with sides of one metre. Conventional VIPs of equal size have an R-value of 2.5 m^2K/W. The global warming potential of the concept is roughly 9 kg compared to 15 kg for conventional VIPs. A conventional VIP's price of roughly 21 euros is compared to the material costs of the concept panel of 8 euros. All performance parameters of the concept panel are higher compared to conventional VIPs. However, the concept panel's thermal performance is expected to be lower, and its environmental impact and price higher. Further research is required for a better evaluation of these performance parameters. Nevertheless, this thesis shows that the vacuum insulation panel concept has promising potential.

Due to the growing quest for renewable energy, wind turbines grow in size. This means that higher demands are posed on the transporting structures, also called tower grillages, which brings its challenges for their design. The fact that the structures considered in this thesis are placed on a ship makes the design even more complicated. Therefore, a procedure is developed in this thesis that is able to optimize a tower grillage, while keeping the underlying ship intact. This tool is based on a combination of size and shape optimization.

The optimization is performed with the dual annealing algorithm. The objective is mass reduction, with the stress in the grillage, and stress and buckling in the ship as constraints. These constraints are taken into account by a penalty function. The design variables are the angles at which the radial supports attach and the thicknesses of the radial supports and the other elements of the grillage. The main question that is answered in this thesis is how this optimization procedure can help in the design of a tower grillage, where the underlying ship structure is implemented as a boundary condition.

First, a single layout is optimized with the ship modelled as a rigid boundary condition. Next, a model of a section of the ship is made and used as the boundary condition on the tower grillage. A comparison between these models showed that the main differences in stress between the two optimization results
are seen in the largest parts of the grillage: the sides, the flanges and the can. The maximum stress in the latter two elements is larger, which also results in larger thicknesses in the optimization with the ship. This affects the average mass in the ship, which is increased with 5.7% when compared to the grillage on the rigid constraint.
After that, a full grillage is optimized. A first attempt demonstrated that the stress in the ship could only be decreased a little with the initial settings of the optimization. That required a slightly different model, to make sure that the stress in the ship remains acceptable. With that model, a mass decrease
of 32 tons could be obtained, which is a decrease of 9.1 % compared to the original design by Vuyk Engineering.

From the different optimization steps became clear that the main factors influencing the optimal distribution of radial supports are the lengths of the supports, and the location of the outer brackets. The compliance of the ship changes the stresses in the grillage, and therefore the optimization result. The
stress in the ship can be reduced only to some extent, so the effect of a stress violation in the initial design is that a new bracket design needs to be made. The results show that the stress is governing for the current optimization, and buckling is not.
The conclusion is that the current procedure can help in the design by providing a global image of lighter layouts which avoid stress and buckling constraint violations in the ship. However, the limitation of the research is that the result is only a global image. It is therefore considered not worth the effort and
time to apply the current method in an engineering environment. It does however show the potential of this method, so future enhancements can make the procedure suitable for engineering.

Multi-stable beam-type metastructures exhibiting snap-through behavior have been extensively studied in recent years , as their stable states can be maintained without the need of external power supply. By arranging a series of beams exhibiting bi-stability, multi-stable metastructures can be constructed. However, current designs of multi-stable metastructures are limited in terms of structural kinematics and the associated functionalities are not fully explored. This thesis aims to present design strategies that can facilitate new kinematic behavior and functionalities for multi-stable metastructures (i.e., energy absorption and shape reconfiguration). Specifically, we investigate the additional rotational degrees of freedom by incorporating rotational compliance in both 2D and 3D designs. In doing so, multi-stable metastructures are capable of realizing both translational and rotational motion, facilitating their applicability in soft robotics and deployable structures. Moreover, the energy dissipation of multi-stable metastructures are studied, where we have proposed design strategies that can enhance the energy absorption without using more materials. In addition, it is demonstrated that multi-stable metastructures can also be designed to realize shape reconfiguration of a morphing surface, where the stability requirement and accessible configurations have been presented. Such multi-stable metastructures exhibiting translational and rotational degrees of freedoms hold great potential for developing reconfigurable structures and energy absorbers.@en

In conventional aircrafts, lift control is achieved by using flap systems. It would be beneficial if flap systems could be replaced by a variable-camber morphing wing. It has been shown that variable-camber morphing wings can significantly improve the aerodynamic performance of the aircraft due to the smoothness of the surface, making it possible to fly more efficiently, reduce fuel consumption and reduce the impact on the environment. However, the design of such a variable-camber morphing wing is challenging due to the conflicting requirements of the structure. The wing should be flexible so it can morph, stiff so it can withstand aerodynamic pressures and light weight to reduce fuel consumption. The aim of this work is to provide a method for the density-based topology optimization of compliant morphing structures. The method includes a novel formulation for the objective function which compares the deformed shape of the structure with a desired deformed shape by using a dot product. This method is applied to obtain an optimized design of a compliant variable-camber morphing wing. The obtained design was converted to a prototype by 3D printing and an experiment was performed to assess if the deformed shapes of the prototype were similar to the ones predicted by the analysis in the topology optimization. The experiment showed that for small deformations the output shape matched the predicted output shape. For larger deflections, there was a slight difference. However, the obtained shapes were still quadratic-like and so it is expected that for larger deformations the designed trailing edge will still have superior aerodynamic performance than conventional flap systems.

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.

Bistable vibration energy harvesters are an interesting alternative to their linear counterparts. They allow for large amplitude oscillations between their stable equilibria, from which much energy can be generated. However, the stable equilibria are separated by a potential energy barrier that has to be overcome. Therefore, we cannot guarantee these oscillations, and the performance advantage diminishes. As a solution to this, a novel method of stiffness compensation in compliant bistable mechanisms is explored to lower the potential barrier. This method makes use of interaction between the buckling modes. Whereas this phenomenon is most undesired in structures due to their increasing proneness to catastrophic failure, we cleverly use it to our advantage. During the deflection required for the large amplitude oscillations, a transition between these buckling modes occurs, causing the increase in potential energy. By bringing the corresponding buckling loads closer together, the transition is eased and the potential barrier is lowered. An analytical framework was set up as a fundamental test of this method. Using a discrete analytical model of a bistable buckled four-bar linkage with torsion springs, it was shown that the potential barrier can be flattened upon matching the first two critical buckling loads, resulting in static balancing. This was achieved by making two torsion springs three times stiffer with respect to the other two springs. To put theory into practice, three compliant mechanisms were designed using the ratio between the first two buckling loads. Their force-deflection characteristics were experimentally determined and it was shown that the stiffness may be tuned according to the ratio between the buckling loads. Furthermore, it was shown that in designs having the first two buckling loads equal to each other, near zero stiffness is achieved. Hence, this method is proven a successful addition to the arsenal of methods in stiffness compensation and static balancing of compliant mechanisms.

In this thesis a proof-of-principle demonstration is developed that uses the heat flux between a probe and a sample as a proxy for their separation. The proposed architecture uses a probe that consists of a bilayer cantilever with an attached sphere at its free end. The deflection of the cantilever that is caused by the heat input is measured using the optical beam deflection method. To eliminate temperature dependent effects the temperatures of the probe and the sample are kept constant. Moreover, a total internal reflection microscopy is included to provide an independent measurement of the separation between the probe and the sample. This architecture allows the measurement of the heat flux as a function of only the separation.
An equation is derived that relates the output signal of the instrument directly to the heat flux that is absorbed by the probe. It couples the top-level design parameters to the system output and is used to study and design the separate elements. In addition to the design of the instrument, the research contributes a detailed study of the influences of the microsphere and the microcantilever on the heat flux measurement.

@en

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

Stiffened shells and plates are widely used in engineering, but their performance is highly influenced by the arrangement, or layout, of stiffeners on the base shell or plate and the geometric features, or topology, of these stiffeners. Moreover, structures with modules are beneficial, since it allows for increased quality control and more accessible mass production. The aim of this work is to develop a method that simultaneously optimizes the topology of the modular stiffeners and their layout on a base shell or plate. This is accomplished by introducing a fixed number of module stiffeners which are subject to density based topology optimization and a mapping of these modules to a ground structure of stiffeners. To illustrate potential applications, several stiffened plates and shell examples are presented. After optimization, these examples were converted to three-dimensional physical structures using additive manufacturing. All examples demonstrated that the proposed method is able to generate clear topologies for any number of modules and a distinct layout on the base.

In this thesis, a methodology is developed that allows for both reliable and computational efficient robustness analysis of aircraft component distortion. This research applies to dis- tortion caused by reductive manufacturing processes that are used to obtain monolithic components from pre-stressed stock material. With the developed methodology, orienta- tions of any component within rolled plate stock material can be found where distortion is most robust. Part distortion is defined as a deviation in shape of an aircraft component from original intent as a result of the component’s reductive manufacturing process. As extreme precision is required in aircraft component assembly, the distortion phenomenon is highly undesired. The developed methodology in this thesis contributes to AIRBUS’ objectives to minimize part distortion related issues. In this thesis, the fundamentals of part distortion are studied. It is found that aircraft com- ponents distort as a result of residual stresses that are present in stock material from which the components are manufactured. As residual stress in rolled plate is subjected to substan- tial variation, part distortion is stochastic in nature. Positions of the components in rolled plate are searched for where distortion is most robust. The robustness of distortion refers to the insensitivity of distortion to uncertainty in residual stress. A mathematical stochastic representation of residual stress in rolled plate is developed showing high coherence with experimental measurement data provided by AIRBUS. For elementary geometries, the rela- tionship between distortion robustness and residual stress is derived analytically. The developed method for predicting distortion robustness is more than one hundred times more efficient in terms of computation cost compared to state-of-the-art methods and al- lows for reliable robustness predictions. In the developed method, three-dimensional po- sitioning of a component in rolled plate can be simulated where state-of-the-art distortion modeling tools usually stick to one dimension. The developed methodology is put to the test in a case study concerning an aircraft stiffener component. The case study emphasizes the significance of robustness predictions; distor- tion dispersion is found to be relatively large compared to the distortion magnitude and significant correlation is found between the component’s orientation in rolled plate and the level of robustness. Positions of components in rolled plate can be found where distortion is extremely robust. Moreover, a relationship is found between the component’s degree of symmetry and the level of robustness.

An active fluid heat exchanger can be controlled effectively using Peltier elements to condition the temperature of the fluid flowing through the heat exchanger. The thermal resistance of the heat exchanger can be reduced to increase the speed of controlling the fluid's temperature. Topology optimization is used in this study to find the geometry of a heat exchanger with reduced thermal resistance.
The design of a heat exchanger using topology optimization requires the coupling of the fluid flow equations and the energy equation in a finite element model with a continuous design variable. The existing optimization models perform well when the goal of the optimization problem is to minimize viscous dissipation. A weighted sum multi-objective function is however necessary to optimize the thermal performance of a design, and the correct choice of weights to meet design specifications is difficult to arrive at.
The drawback in the existing model is that the conductivity distribution is defined as a function of the design variable of the optimization problem. This results in infeasible designs when the goal of the optimization problem is to minimize only thermal resistance, and this is demonstrated with several numerical examples along with a motivation for a new formulation.
A new formulation for conductivity distribution is proposed in this thesis. The new formulation defines the conductivity distribution in terms of the velocity field in the design domain. The new formulation is capable of significantly reducing the thermal resistance of the heat exchanger, and this is demonstrated with a numerical example. Finally, a 3d design case is implemented, the results of the optimization routine are post-processed and the performance of the baseline design from ASML is compared with the topology optimized design.

Origami has become a source of inspiration in engineering for structural design and the creation of mechanical metamaterials. In the often used analytic geometric and energy methods, it is difficult to incorporate deformation of the faces in non-rigid origami. However, deformation of the faces can play a significant role in their mechanical behaviour. This thesis aims to investigate and improve the design of non-rigid origami structures by using the Finite Element Method (FEM), which is well suited to model all deformations of these structures.
In collaboration with the Bertoldi Group at Harvard University, FEM is applied on the new bistable non-
rigid origami structure called the Star bellow. The suitable building blocks of the FEM approach, are
finite rotation shell elements to model the faces of the structure, torsional spring elements to model the crease lines, and a Newton-Raphson arc-length control solution procedure. In addition to the numerical analysis, experimental work is performed. A parametric study, FEM simulations, and physical load tests of prototypes, are performed to understand the mechanical properties of the Star bellow structure and to improve its bistable behaviour. In addition, the Star bellow structure was considered as a unit cell and tessellated into a 1D-array to create a Star bellow metamaterial. Simulations and physical load tests were performed to investigate the influence of the tessellation on the bistable properties of the metamaterial.
A design improvement was successfully proposed with a decrease of the minimum force value of 0.11
[N]. The tessellated Star bellow metamaterial into a 1D-array is feasible and keeps its bistable proper-
ties, although the bistability is reduced compared to a non-tessellated, single Star bellow structure.
The application of FEM to study the Star bellow structure enhances the design process of the Star
bellow. It has been shown that a parametric study using FEM is a way to gain understanding of the
structure’s mechanical properties. Furthermore, a design change of the Star bellow was proposed
which improved its bistable properties. It is also found that the results of the FEM simulations, for the
force values, did not match the results of the physical load tests for various reasons. To improve this,
another load testing setup and a more advanced implementation of the torsional spring elements to
model the crease lines is advised.

So far, structural topology optimization has been mainly focused on linear problems. Much less attention has been paid to geometrically nonlinear problems, although it has a lot of interesting applications. The objective of this work is to implement a method to solve topology optimization problems under finite displacements and rotations. The main focus is on designing stiff structures, but also an outlook is presented on the design of structures that follow a prescribed equilibrium path.

The Newton-Raphson incremental-iterative procedure is implemented to solve nonlinear problems. Arc-length control is used to be able to overcome limit points and to obtain faster convergence. It is shown by means of numerical experiments that this method leads to correct results.

Since nonlinear analysis, especially in combination with topology optimization, is computationally expensive, an attempt is made to develop a new reduction method. This method uses load dependent Ritz vectors as a reduced basis and was originally used in linear dynamic problems. However, it is demonstrated that this method will in general not lead to accurate results in nonlinear static problems. This is due to the fact that deformation modes that are not excited by the external force, cannot be described by the basis.

The topology optimization process is implemented without reduction method. An adjoint formulation is derived to obtain sensitivities in a computationally efficient way. By means of several examples of end-compliance minimization, it is demonstrated that in most cases, especially for shell structures, this method leads to better performing designs than topology optimization based on linear analysis.

The stringent and conflicting requirements of optomechanical instruments and the ever-increasing need for higher resolution and quality imagery demands a tightly integrated design approach. The aim of this study is to drive the thermomechanical design of multiple components of an optomechanical instruments by the system’s optical performance. This work addresses the combination of structural-thermal-optical performance analysis and multicomponent topology optimization while taking into account both component and system level constraints. A 2D two-mirror example demonstrates that the proposed approach is able to improve the system’s spot size error by 95% compared to uncoupled system optimization to below the system's diffraction limit.

When drilling for oil or gas, wells are typically constructed using a conventional or telescopic well design. After a section of the well has been drilled, a steel casing (or liner) is inserted into the hole preventing the collapse of the hole. The next well section has a smaller diameter as it’s casing must pass through the previously installed casing. This process leads to a stepwise reduction of the well diameter which, especially in deep or complicated wells, restricts the maximum production rate of oil and gas.

Mono-diameter technology utilizes casings which are plastically deformed after they have been placed in the well. This plastic deformation is done by pulling a conically shaped tool, known as the cone, through the newly installed casing. The plastic deformation increases the diameter of the new casing to (nearly) the same as that of the previous casing, therefore the well diameter no longer decreases with every new section.

The expansion requires a force in the order of 1000-2500[kN]. The work done by this force leads to high local heat generation due to friction and plastic deformation. This heat, poses a risk for the expansion process as the lubricant (RPSLF) decomposes above 250[C]. Decomposition of the lubricant leads to an increase of the friction, inducing more heat release and further decomposition of the lubricant. The expansion force may then exceed the pulling capacity of the drill rig or strength of the drill string, leaving the cone stuck in the well.

Previous research provides the distribution of pressure on the contact surface of the cone, the strain distribution inside the liner and the temperature dependent friction coefficient of the lubricant, by combining these the frictional heat is calculated. Heat generation from plastic deformation is a complex phenomenon
especially at low strain (-rates). The Taylor-Quinney coefficient defines the ratio between the work done on a material and the heat generated. A strain-dependent model for the Taylor-Quinney coefficient, developed by Zehnder, is used to determine the heat release from plastic deformation.

A numerical heat transfer model is built in Matlab, based on the finite volume method. The model uses a mesh based on skewed quadrilateral cells. Numerical stability of the model is verified for the cell size, time step and time integration method. Movement of the cone and the plastic deformation of the casing are modeled by taking a reference frame fixed relative to the motion of the cone and adding a ”convective” term inside the liner. Heat transfer phenomena at all boundaries are investigated using thermal resistance models.

A series of experiments is executed to verify the numerical model. The
experiments consist of full-scale expansion test using a cone equipped with 30 thermocouples placed just below the contact surface. Additionally also the temperature of the liner, expansion speed and expansion force are measured and recorded.

Comparison of the experimental and numerical results leads to the observation that the pressure profile determined in previous work is likely valid at the first onset of expansion. Over time, the forming of a lubricant film however leads to the re-distribution of roughly 20% of the force from the rear of the cone to the front. When the pressure profile is corrected for this effect; the temperatures inside the cone, temperature of the liner, expansion force and overall heat balance show a good match between the numerical and experimental results with an error of around 5%.

From the heat balance based on the experimental results, it follows that between 54% - 60% of the work done on the system is converted into heat. As all work done in the form of friction is converted into heat the remaining energy must be lost in the work done to plastically deform the casing, this provides indirect evidence for the validity of the Zehndermodel.

The validated numerical model is used to simulate a reference case field expansion, the maximum contact temperature here is 164.4[C]. This value is well below the lubricant decomposition temperature of 250[C], which leads to the conclusion that the process is safe and the expansion speed could even be increased from a thermal point of view.

This thesis studies a controllability approach for general resonant compliant systems. These systems exploit resonance to obtain a specific dynamic response at relatively low actuation power. This type of systems is often lightweight, is scalable and minimizes frictional losses through the use of compliant hinges. Some insect-inspired Flapping Wing Micro Air Vehicle (FWMAV) designs are based on such a resonant compliant system. These designs are carefully tuned such that a particular resonance response of the system corresponds to the desired wing flapping motion. The system’s resonance response depends strongly on its structural properties (i.e., mass, damping, stiffness and their spatial distribution). Resonance response modifications can, thus, be controlled using carefully selected structural property changes. This thesis contributes to the controllability of resonant compliant systems in general and the compliant FWMAV design in particular. Although there is yet no physical, controllable FWMAV design with integrated active components, the current research strongly indicates the applicability of structural property changes to reach controllability of this type of systems. Effective control requires an integrated approach that considers simultaneously the structure, the wings, the kinematics, the methods to induce property changes and the desired controllability@en

Thermal error reduction is of major importance to the performance of any high-tech machine. Topology optimization can be a tool that is able to design geometries that reduce the thermal error, beyond human creativity. In this research, topology optimization is used to reduce the thermal error on a transient-thermal-mechanical problem. The thermal error of the specific problem was reduced by 30%. Multiphyiscal topology optimization, as applied to the problem, comes with many challenges, under which is the manufacturability of its final design. A new penalization method is proposed, that ensures that the final design contains no intermediate densities, which are common to multiphysical topology optimization, even with SIMP or RAMP penalization. The new penalization method aims at creating intermediate material that is adverse to the optimization problem, causing it to avoid intermediate material. On a test case it was shown capable of creating a well performing black and white design, where grey penalization failed to maintain the performance of the design. Furthermore, an interpretation of the adjoint in the sensitivity analysis is given.