In various engineering fields it is required to design material layouts with improved performance or tai- lored properties. Hereby, the structural integrity should not be compromised, such that material failure is avoided. Topology optimization enables the systematic design of structures presenting enhanced performance with respect to criteria, such as mass, volume, stiffness, or eigenfrequencies. In the majority of research works in topology optimization, material failure is addressed by constraining or minimizing the stress state in simplified analyses, in which the material is modeled as isotropic linear elastic. However, brittle and quasi-brittle materials, such as concrete and rock, progressively degrade on a micro-scale, before material rupture occurs macroscopically. Using continuum damage mechan- ics, the state of material degradation may be modeled through one or multiple internal variables that are thermodynamically irreversible. However, modeling damage leads to a computationally intensive, non-linear and path-dependent optimization problem. The limited previous research suggests, though, that accounting for material degradation throughout the design phase can significantly improve the per- formance and reliability of the resulting designs. In this work, a level set-based topology optimization approach is proposed to generate steel- reinforced concrete layouts with enhanced damage resistance. Additionally, a thorough comparison of addressing material failure in a linear stress-based and a non-linear damage-based TO approach is conducted. The geometry of the designs is represented implicitly through a level set function. The level set and state fields are discretized using the extended finite element method, which alleviates the need to remesh in each optimization iteration. Assuming isotropic damage, strain softening degradation in the concrete material is modeled through a single scalar damage variable. The steel as well as the un- damaged concrete are modeled as isotropic linear elastic materials. To avoid instability and localization of deformation, non-locality is introduced through a gradient-enhanced damage model. Boundary and interface conditions are imposed weakly using Nitsche’s method. Numerical instabilities due to small material subdomains are mitigated through face-oriented ghost stabilization. The optimization prob- lems are solved through mathematical programming using the globally convergent method of moving asymptotes. Accounting for the irreversibility of damage, the required shape-sensitivities are evaluated semi-analytically through an adjoint approach. The proposed optimization approach is applied to two-dimensional numerical problems from the literature. The results show that resorting to a simplified linear analysis is insufficient to generate struc- turally reliable designs of brittle material. When modeling the non-linear damaged material behavior throughout the optimization process, though, resulting layouts show an enhanced load-bearing capacity and resistance to damage. It is further demonstrated that the maximum damage level in steel-reinforced concrete layouts can be controlled, through an optimized distribution of reinforcing steel material.

Negative linear compressibility (NLC) describes the relative increase or decrease in a material's linear dimension when subjected to an increase or decrease in external pressure, or a decrease or increase in internal pressure, respectively. This is a rare material property found in only a few naturally existing materials. These materials are not only limited by their availability but also by the range, strength, and stability of NLC behavior. This limitation can be addressed through the design of NLC metamaterials, which are engineered materials composed of repeating architectures or material layouts on the microscopic scale, known as base or unit cells. These cells define the macroscopic properties of the material. In this case, negative linear compressibility.

While there are different methods for designing NLC metamaterials, none of them involve the use of topology optimization (TO), which serves as a powerful tool for designing optimized metamaterial structures. Materials can be designed for different parameters such as base material and pressure applied, while also considering different constraints that may be application-specific.

This study aims to create isotropic NLC metamaterials by designing NLC metamaterial unit cells using a systematic design methodology. We achieve this goal using a density-based TO approach, incorporating different constraints and selecting appropriate parameters to obtain different metamaterial unit cells that exhibit NLC behavior in both two and three dimensions.

The resulting 2D designs exhibited an NLC value of -2.370 %/bar and -3.367 %/bar. While the 3D designs exhibited an NLC value of -2.212 %/bar and -3.534 %/bar. The obtained NLC value and the efficacy of the design method are validated through numerical analysis and experimental testing, with the experimental design showing a maximum deviation of 26.099% from the NLC value obtained through TO. Finally, comparative and parameter studies helped better elucidate the advantages, limitations, and areas for improvement of the methodology used to obtain these designs.

This thesis report contains two papers. The first paper is a literature study on mechanisms, smart actuated materials and controllable joints. In this paper, several smart materials are identified and it is discussed how they can be embedded in different mechanisms. From this paper, it is concluded that fast-response smart material twisting actuators do currently not exist. The second paper studies the design and development of such an actuator. The study presents a design and manufacturing methodology of a planer actuator that generates out-of-plane rotation. It first investigates different out-of-plane deformation modes and how they can be used to achieve the desired motion. Based on an analytical model on the shape morphing of piezoelectric macro fibre composites, a methodology is developed to generate out-of-plane twisting deformation. This concept is used to design a carbon black electrode pattern, which can be spray-deposited on a kapton substrate. This electrode is subsequently used to selectively actuate parts of a P(VDF-TrFE- CTFE) polymer layer. The selective stimulation of the layer results in the desired twisting deformation. This study demonstrates how flat designs can be laminated in a planar additive manufacturing process to induce complex 3D motion. The spray-deposition process was capable of manufacturing bimorph actuators with a 1.6mm resolution. The resulting actuators have a length of 41mm and a width of 10 to 20mm and a thickness of 120 to 139micron. The experiments are used to characterize the effect of the design parameters such as actuator width and thickness on the magnitude of the deformation. As theorized in the analytical model, the thinner more slender samples show the largest rotation which is measured to rotate up to 3.38 degrees. From the experiments it is also found that thinner samples show dielectric breakdown at much lower voltages, around 100V, compared to thicker samples from the same design that performed up to 480V. The study also investigates the quality of the deformation of the actuators, i.e. pure twisting or a combination of twisting and bending deformation. It is verified that uneven layers or asymmetric actuators show significant unimorph bending behaviour, with displacements up to 1.5mm while rotating 3.38 degrees. To the author's best knowledge, this study presents the first working prototypes of fast-response smart material twisting actuators.

Given the growing number of environmental and societal concerns we confront today, the idea of sustainability has gained importance. At the same time, new strategies for improving the performance of structures and systems have been developed due to developments in engineering and computational design. This research aims to generate a sustainable design using topology optimization by focusing on design for disassembly. One advantage of design for disassembly is that when a product can be disassembled, the parts can be reused, repaired, recycled, and remanufacture. This facilitates other aspects of product sustainability, such as the product's life cycle and end-of-life. A structure is divided into two parts and attached by a connection point, this connection point is called the connector. Due to sustainability, the connection method needs to be a non-destructive method, which in this case is the bolts. Next to the connector, two voids are required to insert, tighten and remove the bolts. Therefore, in this research, a structure is optimized using topology optimization and simultaneously optimizing the position of cut lines and connectors. The approach taken uses level set functions to model the cut of the structure, as well as the connectors and the voids. Then, they are converted into a density field using a smoothed Heaviside function. A Solid Isotropic Material with Penalization (SIMP) motivated method is used to join all the different density fields into an equation for the interpolated elasticity modulus. The optimization aims to minimize compliance with volume and no-overlap constraints. The non-overlap constraint is applied to the connectors.
The structure and the position of the cut line and the connectors are optimized using the Method of Moving Asymptotes (MMA) method. A gradient based sensitivity analysis is used in the MMA. Afterwards, the influence of the cut line, the connector and the voids are observed individually. After optimizing the parts individually, the full optimization was performed, where the structure, the cut lines and the connectors with the voids were optimized. Furthermore, a parameter study was done to observe their influence on the final layout. The optimizer's behaviour was observed by looking at the optimization results and the parameter study. For example, how the optimizer tends to stack some connectors together to create a member of the structure or the influence of the voids.

With the approach presented, the main idea of optimizing a structure using topology optimization and simultaneously dividing it and optimizing the connector's position is obtained. However, the optimization has some limitations, as some assumptions and design considerations are not accurate, further research is needed to get accurate results.

The main problem in density-based two-fluid optimization is the fact that the two fluids often mix in optimal designs. Therefore, state-of-the-art two-fluid heat exchanger optimization includes non-mixing constraints. However, the current non-mixing constraints can only impose a constant wall-thickness between the two fluids. Depending on the optimization problem, the forming of a wall with a variable thickness is advantageous for the heat transfer in the heat exchanger. A non-mixing constraint that can only impose a constant wall-thickness cannot further improve the heat transfer objective in such an optimization problem, since a variable wall-thickness cannot be generated. In these optimization problems, a non-mixing constraint that allows a variable wall-thickness to be formed can generate heat exchangers with a higher efficiency. In the proposed optimization method, a non-mixing constraint is provided which guarantees a pre-defined minimum wall-thickness separating the two fluids, and also allows the optimizer to locally increase the wallthickness above the minimum wall-thickness. The advantage of the proposed non-mixing constraint is that
the minimum wall-thickness can be set based on a manufacturing limit, while the optimization algorithm can vary the wall-thickness based on advantageous heat transfer.
In this report a method is proposed for the design of a two-fluid heat exchanger with density-based topology optimization. The density-based topology optimization method has two design variables to distinguish between the two fluids and solid material. The first design variable distinguishes between the fluid and solid material regions and the second design variables distinguishes between the two fluids. The non-mixing constraint relies on a two design variable method combined with a two-step filtering and projection method to generate a variable wall-thickness. The two-step filtering and projection method is applied to the second design variable to guarantee a solid material region with the minimum wall-thickness; the non-mixing region. The first design variable can be used to generate additional solid material regions that are combined with the non-mixing region to form a variable wall-thickness.
The research question is : Can a non-mixing constraint for density-based two-fluid topology optimization be created that guarantees a minimum wall-thickness and also allows for a wall-thickness larger than the minimum implemented within a finite element computational framework?
The optimization problem formulation used in this project is a heat transfer objective that is maximized with two pressure drop constraints, one for each fluid channel. The proposed non-mixing constraint is applied to a variety of 2D optimization problems where different design domains, heat exchanger configurations, materials and parameter settings are used to investigate the influence on the optimization behaviour. The results show that the non-mixing constraint guarantees a pre-defined minimum wall-thickness and also allows for a wall-thickness larger than the minimum wall-thickness. In addition, a method to determine a suitable parameter continuation scheme fine-tuned based on the parameter settings is provided. The proposed optimization method allows for two-fluid heat exchangers with identical and different fluids. A variety of material parameters is used to show the effect on the material interpolation and the optimization behaviour. Depending on the initial design and design domain, the optimization algorithm generates an optimal design with a constant wall-thickness or variable wall-thickness. Finally, to illustrate the possibilities with the proposed non-mixing constraint two 3D optimization problems are computed and one of the optimal designs is post-processed and manufactured.

Adequate mechanical ventilation serves as the first and most important life-saving appliance during a tunnel fire. The most severe threat during a tunnel fire is the smoke. Most victims get incapacitated by the smoke, after which they decease from intoxication and/or suffocation. By means of mechanical ventilation, smoke-free escape routes via the central egress corridor are created. Two stages are considered in mechanical ventilation system design. The first stage involves the identification and positioning of jet fans to create a longitudinal ventilation system in the tunnel. The dangerous undesired reverse flow of smoke in the tunnel, back-layering, can be prevented by achieving a minimum critical air velocity. The second stage is a positive pressure establishment in the central egress corridor with respect to the tunnel. This is essential to prevent smoke flow from the tunnel into the central egress corridor through open escape doors. The tunnel ventilation performance is highly influenced by the position and heat release rate of the fire. Given the uncertainty concerning the fire, it is imperative that longitudinal ventilation is designed to consistently meet the back-layering constraint across all possible fire scenarios. For enhanced longitudinal ventilation reliability, jet fan placement at the tunnel entrance is preferable, while positive pressure ventilation benefits from a scattered jet fan layout.
A design approach based on the fundamentals of topology optimization is used to construct a systematic design method. To streamline the design of ventilation within the central egress corridor, optimizing longitudinal tunnel ventilation design while minimizing pressure downstream of the fire in the tunnel is advantageous. To prevent energy dissipation, a secondary objective specified as a penalization objective was introduced to promote the required distance between jet fans. This objective determines the placement of jet fans by considering the positioning of nearby jet fans within a specified distance, influenced by a penalization exponent. To address the fire related uncertainty, a scenario-based approach is applied. This method has the capacity to accommodate multiple fire scenarios simultaneously, where the designer can select the desired quantity.

Introduction: Atherosclerosis is characterised by the buildup of plaque within the arterial wall and it is often the underlying cause effect of deaths related to cardiovascular diseases. Thin cap fibroatheromas are plaques with a high risk of causing clinical events due to rupture and espousal of thrombogenic components to the bloodstream. The rupture of the plaque is not yet fully understood and for this reason, tissue engineered plaques were created in a previous study to assess the rupture of the plaques based on the displacement field registered with Digital Image Correlation during a uniaxial tensile experiment. The knowledge of the material properties of the tissue-engineered fibrous cap structures makes it possible to link deformations to external loads and contributes to the understanding of plaque rupture. The study aims to create a pipeline for local mechanical property characterisation of tissue engineered fibrous plaque structures.
Methods: In this novel method inverse Finite Element Method (iFEM) was combined with the Differential Evolution machine learning algorithm to assess global and local mechanical properties of tissue engineered fibrous plaque structures. The method required three main steps. Step one was the implementation of the uniaxial tensile test into a computational model using ABAQUS version 2016 Finite Element Method (FEM) software. To couple loads and deformations the hyperelastic reduced polynomial function of second order was implemented in the FEM. The characterisation of the c10 [kPa] and c_20 [kPa] parameters in the model is the main focus of this study. After the creation of the FEM, the computed displacement field and the previously registered DIC displacement field were implemented into the iFEM pipeline. Preliminary to the experimental data study, the pipeline was tested on a synthetically generated displacement field, in order to investigate the expected accuracy of the method. In step two the global mechanical properties of the fibrous plaque structures were investigated, using the assumption of homogeneous material property distribution in the samples. The resulting material properties after the global estimation served as an initial guess for the local estimation procedure. In step three the local material properties were investigated by creating sections with independently variable material properties, thus introducing heterogeneous distribution of material properties within the samples.
Results: The global mechanical property assessment was carried out successfully and the resulting material properties are within the range of previously reported stiffness values of plaques with a similar composition. Local mechanical properties were characterised using up to twelve independently variable material parameters to investigate the heterogeneous mechanical behaviour of the constructs.
Conclusion: During this project a new method was established to assess the local mechanical properties of tissue engineered fibrous cap structures. The pipeline shows high potential to be useful when investigating plaque rupture in a controlled environment using tissue engineered constructs. The knowledge of local material properties in combination with local deformations is a great addition to the understanding of plaque rupture.

High performance machines rely on fast moving parts and generally avoid resonance for improved accuracy. To improve the dynamic properties of these high performance machines, their parts are optimized via a lengthy iterative process. Topology optimization for vibrations problems could shorten this time consuming design process and provide a more optimal design compared to the manual iteration process.

In the field of topology optimization for vibration problems there are various methods to solve a given problem. The two most commonly used methods in recent research are the density approach and the level-set approach. The characteristics of the density and level-set approach are well understood in context of topology optimization for vibration problems, however a direct comparison between these two methods has not yet been conducted. Several crucial aspects of topology optimization for vibration problems will be investigated, such as localized eigenmodes, mode multiplicity, grey areas and efficiency for practical applications. Additionally, the applicability of these aspects will be tested in the academic and industrial field to determine their values when applied in industry. This thesis provides an extensive study of various design cases in which the density and level-set topology optimization approaches are compared on their ability to solve vibration problems. These design cases are based on frequently used design cases in literature which are generally seen as benchmark problems.

For this thesis it is opted to have as many similarities between the density and level-set approach as possible, to ensure a fair comparison between the two methods. To accomplish this, the level-set approach uses a density based mapping in combination with material parameter sensitivities and the method of moving asymptotes (MMA) to update the design variables. Furthermore, the level-set function is parameterized with compactly supported radial basis functions (CSRBF). This leaves the difference that the density approach uses element densities as design variables, whereas the level-set approach uses expansion coefficients as design variables.

The design cases indicate that the density approach is versatile as it is able to solve a wide variety of problems. Additionally, there are less parameters, which makes this method easier and faster to work within an industrial setting. Furthermore, the method produces well-performing designs even with more difficult tasks, such as a coarse mesh.
Although occasionally localized eigenmodes occurred whilst using this method, they do not seem to interfere with the final result. Thus, the density approach is less time consuming to setup and needs less tuning of the method specific parameters.
On the other hand, results from the design cases also indicated that the level-set approach is able to produce designs with an improved objective function at the cost of possibly more tuning of method specific parameters. Furthermore, the level-set approach is able to solve all the design cases without the occurrence of localized eigenmodes. Although the level-set approach is less optimal for coarse meshes, it outperforms the density approach at more refined mesh sizes. Additionally, it features a crisp geometry description by the zero level-set contour. Thus, the level-set approach is able to produce more optimal designs without the occurrence of local eigenmodes at the cost of more complexity and possibly more tuning of the method specific parameters.

To conclude, both approaches have unique properties to be able to solve vibration problems. The density approach is more applicable as a standard approach in an industrial setting due to it being more robust and the method is less time consuming. However, the level-set approach should be opted for more complex vibration problems due to the crisp geometry definition of complex geometric features and its ability to outperform the density approach.
A practical application has been solved with an optimization run, where the use of a set of predefined parameters that solved the benchmark cases has been used. Additionally, an optimization run where all parameters are optimized for the specific design case was performed to see the ultimate performance. The level-set approach was able to outperform the density approach in the predefined parameter case, whereas the ultimate performance case gave usable results for both methods. The differences came down to a more improved objective function for the density approach, or a more simplistic and lighter design for the level-set approach.