Since a lot of bridges in the Netherlands have degraded over time, they require strengthening or (partial) replacement. Previously, the possibility to apply 3D printed fibre reinforced polymers (FRP) for strengthening of bridge decks was investigated by Arup. In this study was concluded that the field of movable bridges with timber decks has the highest potential. The timber deck, laid on steel stringers, should be replaced by an arch-shaped 3D printed FRP element, which will be placed in between the stringers. The print/material partner from the previous study retired and therefore, new research had to be done with material from another supplier, printed by another partner. This research focuses on the configuration and the properties of the new material, a recyclable thermoplastic glycol-modified polyethylene terephthalate (PETG) composite with glass fibres, and the optimization of the preliminary design.

The starting point of this research project was to investigate what the best material configuration was in terms of amount of fibres and way of printing. A fibre volume fraction (Vf) of 30% (GF30%) and 45% (GF45%) was considered. Is it possible to achieve more strength and stiffness by having a higher Vf without the material being too brittle or reducing the bond capacity of the layers? Since the fibres are orientated in the direction of printing, tensile tests in the principal direction will be performed to investigate the effect of the Vf.

With 3D printing, the element will be printed layer by layer. During the printing process, the material is melted, extruded, and cools down to harden. The longer it takes to print one layer, the more the material can cool down before the next layer is printed on top, the larger the temperature difference between the two layers will be. This difference in temperature determines the bond strength of the layers. Therefore, several layer times (80, 100, and 120 seconds) are considered to investigate what the maximum layer time should be before the bond strength decreases too much. This will be examined by tensile testing perpendicular to the print direction.

The test results showed that the structural performance of the GF45% material has a better structural performance than GF30% without being more brittle or having a significant lower strength in transverse direction. The GF45% material had a strength of 102.1 MPa and a stiffness of 22,040 N/mm2 in principal direction compared to 71.3 MPa and 13,920 N/mm2 respectively for GF30%. The two materials, printed with 80 seconds layer time, had a comparable strength in transverse direction with 20.4 MPa (GF30%) and 19.7 MPa (GF45%). The increased layer time was tested with the GF30% material. When the layer time was increased to 100 and 120 seconds, the strength decreased: 8.7 MPa for 100s and 7.7 MPa for 120s. Thus, the optimal configuration for the material is a 45% Vf and a maximum layer time of 80 seconds.

Having the optimal configuration determined, the mechanical properties of this material configuration should be established to be able to make a model for the design of the bridge deck element. These properties will be used in the finite element analysis (FEA) of the final design. The mechanical properties of the material are derived from tensile tests in longitudinal and transverse direction, compression tests in both directions, and shear tests. The investigated shear strength is the shear strength between the print layers, called the interbead shear strength (IBSS).

With these mechanical properties known, the design of the bridge deck component was optimized. The arch shape of the preliminary design is kept. The infill, design of print path and thickness of the top plate, side plates and arch will be varied. In this way, three distinctive design variants were made. From the analysis, it was derived that: the print path should have the same start and end point to enable symmetric stacking of the layers; the most extensive infill of the three variants performs the best from a structural point of view; and the thicknesses of the top plate, the arch and the side plates should be 2t, 3t, and 1t – 2t respectively. The thickness t is the thickness of a print layer, which is the width of the bead (6.0 millimetres). Since local eccentricities, due to transition points from single to double or double to triple layers, introduces unfavourable bending and thus, an increase in local stresses, local eccentricities should be avoided in the design of the print path. Lastly, the shape of the arch was varied. The circular shaped arch is preferred over a parabolic shaped arch because of better printability, although the structural design checks were comparable.

The results from the tests proved that a bridge deck element according to the final design is suitable for strengthening application. The two tested components showed consistent behaviour and were able to carry a wheel load. Moreover, a crack initiation and propagation failure mode occurred. This meant that the force remains at a certain load level above the required minimum equivalent wheel load without having collapsing failure. So, decks of movable bridges can be strengthened by the designed component with the 45% Vf and 80 s layer time configuration.

During the component tests, the failure mechanisms occurred was cracking at the intersection between the stiffeners and the arch. For follow-up, one could investigate a way to improve the connectivity between two stiffeners coming together at the arch, for example by increasing the overlap. Another issue for further research is the fatigue performance of the material and this component since this research is based on static analysis. Besides the properties and design aspect, assembly of the component should be investigated. Inverted T-girders with an additional plate on top could be an alternative to the I-girders as stringers, like in the preliminary design.

In Additive Manufacturing (AM), typically a trade-off exists between part quality and build time. Part orientation with respect to the print direction may significantly influence both. In this thesis, the consequences of part orientation on support volume requirements are studied. Build time, material consumption, and post-processing efforts are influenced by the amount and configuration of required supports. Using triangular surface meshes, the support requirement for a given part orientation is calculated for each triangle facet individually and summed. Gradient descent methods are used to optimize part orientation for minimum support volume. To enable implementation of gradient descent optimization, focus is placed on obtaining derivative information of the support volume on a per-facet basis. The resulting support volume function contains discontinuities, for which smooth
approximation strategies are implemented. This approach is first applied to convex shapes, with promising results. For non-convex shapes however, non-local information is required. A novel method for indicating the presence of on-part supports is presented. All possible candidates for support on part are computed for each facet before the start of the optimization process. The resulting connectivity set is an inherent property of the shape and only requires calculation once. The new method is tested using numerical experiments, which indicate that gradient-based optimization of the smooth volume
function outperforms the population-based approaches commonly used in the literature. Moreover, the presented work provides a framework for optimizing total part cost in which other metrics are easily appended.

Additive Manufacturing (AM) plays a crucial role in the revolution towards Industry 4.0, by enabling the direct translation of digital 3D models into physical objects while reducing process steps and minimizing human intervention. While conventional AM machines are generally limited to three-axis movement, multi-axis AM equipment extends the manufacturing flexibility by enabling the fabrication of freeform layers, thus creating new opportunities to improve part quality, though at the cost of increased complexity in process planning. Wire Arc Additive Manufacturing (WAAM) is a multi-axis technique for producing large metal components, with potential applications in maritime, aerospace and civil infrastructure. However, this potential is hindered by factors such as deformation during fabrication, which compromises part precision and can lead to process failure. Recently, a computational approach has been developed to reduce distortion by optimizing the fabrication sequence. While promising, the optimized sequence is characterized by large variations in layer thickness, rendering them non-manufacturable. This research proposes numerical methods to evaluate and restrict layer thickness in fabrication sequence optimization, ensuring uniform thickness within each layer and a consistent average thickness across the entire sequence. A 3D computational framework has been developed, integrating latest advancements in sequence optimization. To address the intensive computation in 3D, this framework features a parallel implementation using the PETSc library. This framework enables the numerical assessment of the method’s performance and provides a foundation for future experimental validation.

The primary objective of this research is to develop an efficient protocol which can be used to 3D print multi-material microfluidic devices with a high resolution. During this research, the fabrication of multi-material microfluidic valves is discussed as a showcase to verify the multi-material protocol, using a single affordable printer and multiple resin vats. In contrast to single material microfluidic fabrication methods, complex geometries can be created by the use of combinations of stiff and flexible materials in a single 3D print. This protocol aims to streamline the fabrication process while ensuring precise feature reproduction and robust mechanical properties in multi-material 3D printed microfluidic parts.

In this study, the effects of UV light exposure on feature accuracy and mechanical performance is systematically investigated. It is observed that for the rigid material, Anycubic High Clear, sample sizes increase and void features shrink when the exposure to UV light increases. For the soft material, Liqcreate Elastomer-X, shrinkage rates after swelling due to IPA absorption are compared under different conditions, revealing that shrinkage occurs more rapidly with a heat source than at ambient temperature.

Mechanical properties are further evaluated through tensile testing of four sets of printed dogbones, showing that extended UV exposure enhances mechanical properties such as the Young's modulus, ultimate tensile strength and strength at break. Elastomeric materials assessed in this study demonstrate an optimal measurement accuracy within a strain range of 10% to 50%. The influence of print orientation is assessed for the hard material. This experiment is executed for layer thicknesses from 10 µm to 200 µm across horizontal, vertical, and diagonal orientations, with vertically printed samples being closest to the intended dimensions.

A comprehensive multi-material 3D printing protocol based on the existing "print-pause-print" technique and utilizing the software UVTools is presented. Finally, a microfluidic Quake valve is designed and optimized for 3D printing, its performance is analyzed through finite element (FEM) simulation and analytical calculations.

The results of this study offer valuable insights into the optimization of multi-material 3D printing for microfluidic applications, highlighting several critical parameters that affect feature resolution and mechanical performance. The proposed protocol and findings serve as a foundation for future advancements in the fabrication of complex microfluidic devices.

Qualification of the mechanical properties of Wire Arc Additive Manufacturing (WAAM) products is an essential step for a successful introduction of this process in industry. To realise this, the relationship between processing conditions, the thermal history and the resulting mechanical properties needs to be established. Hardness is an easy-to-check property that correlates well with such tensile properties. This thesis describes the prediction of the relationship between thermal cycling during deposition of a WAAM produced multilayer block and the resulting hardness. A 3D transient heat model was built to investigate the effect of the thermal behaviour. The weld process was simulated using temperature boundary conditions instead of a direct heat source to calculate the thermal field. The material deposition was simulated using the element birth-death method. The calculated thermal data was used for hardness estimation based on the t8/5 time, i.e. the time to cool from 800 to 500 °C, to determine the hardness distribution throughout the deposited blocks. Temperature was measured during the experiments with thermocouples to validate the thermal model, while hardness was measured to verify the hardness results. The thermal boundary condition based model was found to be in good agreement with experimental results. The modelled results show a correct prediction of the decrease in hardness with increased layer height.

Additive manufacturing (AM) is quickly becoming one of the more popular methods to manufacture components made of Ti-6Al-4V in the aerospace and automobile industry due to its flexibility in producing complex geometries and reducing tooling costs. As the world of additive manufacturing is still relatively young, there is no globally accepted method for the certification of AM parts, allowing freedom to develop procedures to utilize the advantages of AM. To greatly reduce material wastage, the use of smaller specimens has been explored to characterize the mechanical properties of the material.

This study aims to analyze the influence of test specimen size on the near threshold fatigue properties of Ti-6Al-4V manufactured by conventional processes and Laser powder bed fusion (LPBF) . Two different specimen sizes were utilized, the standard size and the sub-sized specimens. The effect of two heat treatments, stress relief (SR) and annealing (AN) on the LPBF material on the fatigue properties was also studied. In addition, the effect of anisotropy on the fatigue threshold value was also touched upon in this study. A fatigue test setup for the sub-sized samples was designed and a constant Kmax test was employed to characterize the threshold properties. The direct current potential drop (DCPD) method was employed to measure the crack length during the test.

The fatigue threshold of the SR microstructure did not show any effect due to sample size. This was attributed to the partly intergranular crack propagation observed playing a prominent role in the fatigue threshold. The conventional and annealed samples witnessed a drop in threshold values of about 0.5 MPa√m when sub-sized samples were used. It was observed that the sub-sized samples had a greater crack depth at threshold leading to increased constraint at the crack tip and hence smaller degree of crack closure explaining the drop in threshold values.

The current study has thus established successfully the feasibility of using sub-sized specimens to characterize the near threshold fatigue properties of conventionally and additively manufactured Ti-6Al-4V.

In this work, the possibilities to approach various nonlinear moment-angle characteristics with a kinematically indeterminate rigid body balancer with torsion springs are examined. These torsion springs are mounted on the axes that intersect the rigid bodies. The rigid body balancer is coupled to an inverted pendulum. Although the kinematic indeterminate nature of the system enables the balancer to rotate non-proportionally along with the pendulum, the kinematics should correspond with the equilibrium configurations of the system. The required system parameters as spring stiffnesses and element lengths are obtained by optimization with a genetic algorithm. In addition to the standard optimization case, the effects of prestressed springs with contact release, nonlinear springs, optimizable initial configuration and an extra segment on the approximations are studied as well. Moreover, an extra objective function that concerns the distribution of energy among the springs is introduced. Eventually, the results that are obtained by the proposed method are verified with an experimental setup that contains a prototype of the system. The experimental results show agreement with the model with 93.47% work reduction. The corresponding model reduces the required work with more than 99%, which is higher than found in the state of the art.

Functionally graded materials (FGM) are a class of materials in which the chemical composition or microstructure varies as a function of position, offering unique material properties. In this study, a functionally graded structure was manufactured using Gas Tungsten Arc Welding and a double wire feed device. The wire feed rate was changed step by step of the austenitic AISI 316L and ferritic AISI 430L stainless steel wire, creating a chemically graded duplex stainless steel structure. The structure, approximately 30 mm in height, was investigated using Optical Microscopy, X-Ray Diffraction, X-Ray Fluorescence and Energy Dispersive X-Ray Spectroscopy. The transverse section is composed of large elongated grains. The chemical analysis revealed a relatively smooth change in Nickel and Molybdenum composition over the section, due to remelting of previously deposited layers. The graded material showed a gradual transition in phase fractions from mainly austenite and some ferrite to mainly ferrite and some austenite, to fully ferric structure. There were no brittle phases detected in the structure.

Present stroke rehabilitation devices for the arm are often difficult to use by the patient himself and cannot be used at home. A compliant shell mechanism could overcome the shortcomings of the current available devices. By the use of monolithic shell mechanisms a simple to use device can be designed, which can be made wearable. It would make it easier for stroke rehabilitation patients to do repetitive rehabilitation exercises at home. The focus lies on balancing the gravity during the lifting of the upper-arm. To achieve that a negative stiffness in the compliant shell mechanism is necessary.
Negative stiffness arises when tape springs are bent and buckle for a short range of motion. Tape springs are thin-walled beams with a curved cross section. The short range of the negative stiffness limits the use for static balancing over a longer range of motion. In this project an analysis is presented on how the range of the negative stiffness can be increased by changing the geometry. The addition of longitudinal curvature to the tape spring results in a more gradual negative stiffness for a longer range of motion. It is shown why the addition of longitudinal curvature results in a more gradual and longer range of negative stiffness during the bending of shell.

The outstanding mechanical properties of graphene have made it a suitable candidate for awide range of sensor and actuator applications in modern technology. However, before the full potential of future applications can be achieved, a proper characterisation of the fundamental properties of graphene is crucial. The aim of this project was to contribute to the understanding of the mechanics of graphene membranes in presence of surface imperfections. To this end two configurations are investigated: ribbons and cantilevers, respectively. Wrinkled graphene nanoribbons are used to investigate the mechanical behaviour during the transition from the wrinkled state to the flat state. A molecular dynamics model has been developed of a single layer graphene ribbon to describe both the formation of wrinkles as well as the transition from the wrinkled state to the flat state. Also, a continuum model was developed to investigate the formation of wrinkles in graphene nanoribbons. Different constitutive laws have been investigated to describe the mechanical response of wrinkled membranes during the transition from the wrinkled state to the flat state. It was concluded that an exponential version of Hooke’s law fails to describe this transition correctly. The transition is however well described by the first order compressible Ogden’s law. Ogden’s law provided further insights into different mechanical properties of the wrinkled layer. Ogden’s law predicted that wrinkledmembranes exhibit a negative Poisson’s ratio at small strains, which is in agreement with previous research. Also, Ogden’s law predicted a decreasing shearmodulus and an increasing Poisson’s ratio after flattening of the membrane. Single layer graphene cantilevers show great potential, however, due to the difficult manufacturing of these fragile structures they remain virtually unstudied. Herein, a molecular dynamics model has been developed to investigate if nanocantilevers could be stabilised by implying a curvature. We found that, depending on the aspect ratio of the membrane and the applied rate of curvature, single layer graphene cantilevers could be (partly) stabilised by implying a curvature. In conclusion, with this research we provided new insights for designing and investigating the next generation of graphene nanoelectromechanical devices.

Adolescent Idiopathic Scoliosis (AIS) is a condition of the spine, often characterized by a three- dimensional spinal deformity. Treatment usually involves interventions like exercise, bracing, or if necessary, surgery. Often bracing is prescribed to stop curve progression so that surgery can be avoided. Traditional scoliosis braces are usually rigid devices, which displace the spine to the desired corrective position. With efficacies over 90%, these braces can be quite effective when worn enough. Unfortunately, the activities of daily living (ADL) for patients are reduced drastically when wearing a brace, and as a consequence compliance towards the braces is low. Since AIS develops in around 3% of all adolescents, of which approximately 10% has progressive curves that require treatment of some sort, the need for effective, comfortable bracing is high. To increase the ADL of patients, the current focus has been shifted towards designing a compliant scoliosis brace that can provide needed corrective forces and allow motion.
This thesis focusses on the evaluation of spinal motions through the design of a motion capture experiment. The goal of this work is to provide general knowledge about these spinal motions for clinicians, researchers and mechanism designers, such that they can make use of the provided analysis for the design of a new, compliant scoliosis brace. Parts of this analysis are implemented in a brace design quantification strategy, which can be used to facilitate such a brace design project. The key contribution of this master thesis is the characterization of spinal motions for specific vertebrae, to provide substantial kinematic data for the design of a compliant scoliosis brace.

The field of nanotechnology has been quickly growing over the last few decades and many different functional Nano Electro Mechanical Systems(NEMS) can now be made.
To further increase the possibilities and functionality of NEMS, new materials have to be characterized. Of the new materials which become available at the nanoscale, graphene is one of the most promising. This is mainly due to the combination of its extraordinary strength, electric and thermal conductivity, and low weight.
In order to be able to use graphene's full potential in future applications, the elastic properties, such as the Young's modulus and the bending rigidity, have to be known.

These elastic properties have been obtained following different approaches. However, the obtained values are scattered. This scattering has a few reasons.
Firstly, experimental research into graphene is difficult, due to the small scale, big influence of the environment, and the difficulty of fabricating well defined graphene membranes.
Secondly, graphene is a purely two-dimensional material. Therefore, continuum theory is not easily applied. The bending rigidity for example is not related to thickness as it is in a continuum plate.
Finally, graphene exhibits strong mode coupling and is always vibrating due to Brownian motion, the motion which results from the stochastic excitation due to temperature. Parameters extracted from static measures may thus not match the reality or experiments. In conclusion, the elastic properties have to be obtained from the dynamic response, following a multi-modal approach.

As graphene, only one atom thick, is close to the atomic scale, Molecular Dynamics simulations are used to investigate its behavior. To extract parameters, these simulations are compared to an analytical continuum model. In this way, the advantages of continuum mechanics and Molecular Dynamics simulations are combined with a dynamic, multi-modal approach to extract the bending rigidity and Young's modulus of graphene.

In the continuum model, the equations of motion of a circular graphene plate and membrane are derived from a Lagrangian approach. The governing equations are discretized using admissible functions that satisfy boundary conditions. Furthermore, the geometric nonlinearity is included, as graphene is so thin, and is thus easily driven into the nonlinear regime.

The basic principle of Molecular Dynamics simulations is to solve Newton's equation of motion for every single atom of a system. The force acting on the atoms is described by a potential field. The equations of motion are then integrated over time.

The mode coupling in graphene is shown to be so strong that the energy in all modes is equal after some time. Therefore, the only steady state attainable is the state in which the energy in all modes is equal, which corresponds to the Brownian motion. The eigenfrequencies are obtained from the time response of the atoms in the graphene membrane, excited by Brownian motion. The obtained eigenfrequencies are compared to the values obtained from the continuum model. From this comparison the bending rigidity of graphene is extracted. This is done following an optimization approach, which minimizes the difference between the eigenfrequencies obtained from the continuum and the Molecular Dynamics model. Including multiple modes is shown to be a necessity for reaching convergence.
Furthermore, the Young's modulus is obtained. This is done by comparing the geometric nonlinear behavior of graphene obtained in Molecular Dynamics with a continuum prediction of this behavior.

The bending rigidity and the Young's modulus thus have been obtained, independently, from the dynamic response of graphene obtained in Molecular Dynamics.