As the microstructures of engineering materials become increasingly complex, they give rise to advanced fracture processes. Despite independent efforts, consistently accounting for fracture anisotropy and mode-mixity in phase field modeling remains challenging. Motivated by the anisotropic, mixed-mode fracture processes observed in composites, the proposed methodology introduces a direction- and mode-dependent fracture energy to model crack resistance in a physically consistent manner. Contrary to methods commonly seen in the literature that incorporate anisotropic crack resistance through the crack surface density, in this work, the fracture energy is defined by a function that takes the crack orientation and mode-mixity as arguments. The model was applied to several numerical examples, ranging from isotropic mode-independent conditions to anisotropic mixed-mode conditions. In all cases, the results show good agreement with reference solutions and available experimental trends. ... Read more

The composite additive manufacturing industry keeps growing every year, alongside the need for more lightweight, problem-specific structural components in EV, wind, space, sports, and other industries. This need has led to a great deal of research on the optimisation of composites, as they offer far more lightweight design possibilities and are more problem-direction-specific, which makes them more efficient. This, however, comes with some difficulties that are addressed in this thesis, namely the manufacturability of such designs.

The thesis combines the SIMP method for topology optimisation with an ANDFO approach to fibre orientation optimisation methods. The SIMP method provides a well-defined variable parameter that can be used to retrieve a manufacturable design without grey areas when implemented with a power scale, as in this thesis. The ANDFO, on the other hand, uses normal distribution weight functions to guide the fibre orientation towards a discrete value without requiring multiple design parameters per layer.

The model is initially compared for its manufacturability efficiency against other optimisation methods, such as level-set or Heaviside methods, as well as against lamination parameters and other methods for laminate design.

The model, which works with a mix of heuristic and concurrent combination methods, is shown to have verified results when tested against existing literature and demonstrates some improvement in both manufacturability and compliance levels. The randomness test, however, still shows that even with the use of a GCMMA solver, the non-convexity of the problem causes the randomised initialisation point to yield different results. This effect is mainly observed in the fibre orientations, as they are more susceptible to such variations. ... Read more

Launch vehicles and payload adapters are mainly composed of thin-walled cylindrical shells and conical shells, where conical shells are necessary when transitioning from a larger to a small diameter. These thin-walled shell structures are constructed with metallic or composite materials. Mass is a critical factor in the design and operation of launch vehicles, so it is desirable to save as much mass as possible on primary (core stage, interstage) and secondary structures (payload adapters). This objective can be achieved by maximizing the radius-to-thickness ratio in conical and cylindrical shell structures. However, structures with high radius-to-thickness ratios are usually more susceptible to buckling failure. Traditionally, conical and cylindrical shells are designed and analyzed as independent structures. State-of-the-art manufacturing and numerical methods also allow designers to consider novel shapes and joints to save mass and increase volume, for example, combining the conical and cylindrical shells in a single integrated structure. By combining these two sections with a seamless toroidal transition, designers can remove the heavy, stiff interface ring that often connects the two independent structures, which potentially saves mass. An example of this type of integrated structure is the NASA Universal Stage Adapter.

Demonstrating the ability to successfully predict the buckling behavior of integrated conical-cylindrical shells is a critical step in the development of buckling design guidelines for this class of structures. Although there are numerous papers documenting the test and analysis correlation of conical and cylindrical shells separately, there is a limited number of research papers specifically related to the test and analysis of an integrated conical-cylindrical shell under axial compression. Furthermore, a modeling methodology has yet to be proven to predict the buckling behavior of these integrated shell structures.

To address this need, a buckling analysis methodology was developed which successfully predicted the buckling behavior of a composite cylindrical shell with a nontraditional composite layup. The methodology was further applied to an integrated conical-cylindrical composite shell. The finite element model included as-built geometric imperfections and thickness variations, and a geometrically nonlinear analysis was used to predict the buckling behavior for both the cylindrical and the conical-cylindrical shells. A composite conical-cylindrical shell was designed, built, and tested until buckling. The observed buckling behavior was in good agreement with the predicted behavior. Since the test specimen buckled elastically, it could be reused for further testing. The specimen was modified with additional composite plies that were added to the transition region. The same finite element modeling approach was also used to successfully predict the buckling of the composite conical-cylindrical shell with the modified design. This additional test provided further validation of the modeling methodology.

After validation, the modeling methodology was used to investigate whether the current buckling design methodology for conical and cylindrical shells can be applied to integrated conical-cylindrical shells. This begins with comparing the buckling response of conical and cylindrical shells, and how they compare with the buckling response of an integrated conical-cylindrical shell using an eigenvalue analysis (buckling equation) and a geometrically nonlinear analysis (implicit quasi-static analysis).

The buckling behavior and imperfection sensitivity of the conical-cylindrical shell was used to assess
the traditional buckling design methodology. It was determined that the traditionally
recommended knockdown factors may not be conservative for conical-cylindrical shells
in some cases. It was also discovered that the effects of geometrical nonlinearity may be
more influential than imperfections for conical-cylindrical shells, which is contrary to the
case of the individual components.

To help quantify the relative importance of various design parameters, a Polynomial Chaos Expansion was employed to express the critical buckling load of a conical-cylindrical
shell as a function of the shell thicknesses, cone angles, transition geometry and axial stiffness. Polynomial Chaos Expansion was also used to highlight differences in the predicted buckling loads obtained from a buckling eigenvalue analysis and a geometrically nonlinear implicit dynamics analysis. Isotropic and composite shells were considered separately. Due to its capacity to successfully predict the buckling load of a conical-cylindrical shell, the Polynomial Chaos Expansion of the buckling load may be a useful design tool during launch vehicle sizing studies, as it may limit the number of finite element analyses required, particularly in the early design stages.

This research aimed to present the fundamental buckling behavior of conical-cylindrical shells through numerical and experimental means. This led to the conclusion that the traditional buckling design approach for cone and cylinders is not appropriate for conical-cylindrical shells. Additionally, it may be more mass efficient to design a conical-cylindrical shell that has a lower buckling capability because it is less sensitive to imperfections. The recommendations provided are based on experimentally validated data and observations, which provides credibility to the conclusions and recommendations. ... Read more

This thesis investigates the constitutive modeling of GrCop-42 Low Cycle Fatigue (LCF) behavior and its application to the combustion chamber of a liquid rocket engine developed by Pangea Propulsion. The main objective is to develop a predictive framework for fatigue life estimation under severe thermomechanical loads, which induce cyclic viscoplasticity and progressive damage, ultimately leading to the prediction of the chamber failure mode known as the Doghouse effect. The Chaboche model was adopted as the baseline for cyclic plasticity and extended to account for viscoplastic and damage effects. Viscous behavior was described using
the Norton–Bailey law. Damage was accounted for with two different models: as a reduction in cross-sectional area, affecting the material’s mechanical response only in tension, and as a reduction in stiffness, influencing the response under both tensile and compressive loading. In both cases, damage is represented as a second-order tensor, making the resulting damage state anisotropic. Additionally, three damage evolution laws — linear, multilinear, and nonlinear (exponential), with and without isotropic softening — were evaluated and compared
with experimental observations. Due to the lack of available LCF data for GrCop-42, a dedicated test campaign was designed by Pangea Propulsion and performed by an external laboratory to provide reference data for model calibration and validation. The implementation began with the development of a fitting tool based on the residual sum of squares method, enabling the extraction of Chaboche parameters from the experimental
dataset. This tool was first implemented in MATLAB and used to simulate the stress–strain response of a LCF test, which was then compared and verified against data available in the literature. Building on this step, the Chaboche plasticity model was implemented in an Abaqus User Material (UMAT) subroutine, where the LCF test conditions were replicated on a dogbone specimen and verified against published results. The UMAT was subsequently extended to include viscous effects and verified using the same methodology. Finally, the damage model was incorporated into the UMAT and verified through both analytical computations and comparisons with literature. With all tools successfully verified, the validation phase was conducted against the experimental data obtained from the test campaign. The results reveal that at elevated temperatures, GrCop-42 exhibits a three-stage fatigue response: initial isotropic softening, stabilization, and final softening leading to failure. This behavior was best captured using a nonlinear (exponential) damage evolution law combined with isotropic
softening and kinematic hardening, with the damage modeled as a reduction in stiffness.. The application of the non-linear model to the combustion chamber demonstrated that the UMAT successfully predicted mid-channel crack formation and the Doghouse effect, with damage accumulation occurring primarily during shut-off phases. Specifically, two firing scenarios—10 seconds and 300 seconds—were analyzed, highlighting that once thermal steady state is achieved, viscous effects become negligible for the mission durations studied. In conclusion, this thesis provides a validated modeling approach for predicting low-cycle fatigue-driven failure in GrCop-42 and proves its applicability in the framework of the combustion chamber. ... Read more

This thesis was conducted within Aerospace Propulsion Products, a company specialized in rocket engine igniters and Parachute Deployment Devices (PDDs) for space applications. The objective of this research was to develop a high-fidelity numerical model in Python capable of accurately predicting the performance of a PDD system. Given input parameters such as component geometry, propellant load, material properties, and environmental conditions, the model serves as a predictive tool to assist engineers in designing and optimizing PDDs for future planetary exploration missions. To achieve this, an extensive study of parachute ejection mechanisms was first conducted, analysing the physical principles governing gas generator combustion, gas flow dynamics, structural interactions, and aerodynamic forces. A numerical model was then formulated using fundamental principles of combustion, ballistics, physics, and thermodynamics, incorporating detailed representations of the gas generator, plenum, and parachute pack. Once developed, the model was calibrated using experimental data and systematically verified and validated against deployment tests to assess its accuracy in predicting key performance metrics such as pressure profiles, reaction forces, and ejection velocity. The validated model was then subjected to a sensitivity analysis to identify the most influential parameters affecting system performance. The results demonstrated that parameters such as the discharge coefficient, chamber volumes, plenum dimensions, and propellant loading significantly impact the pressure buildup, deployment timing, and ejection dynamics. Additionally, variations observed in experimental repeatability highlighted the inherent uncertainties in physical testing, further emphasizing the value of numerical modelling in PDD development. By providing a robust and accurate simulation framework, this research contributes to the advancement of next-generation PDD systems. The model developed in this study offers engineers a powerful tool for optimizing PDD designs, ensuring reliable deployment performance across various mission scenarios. Future work will focus on further refining the model by incorporating additional experimental data and extending its applicability to a wider range of deployment conditions. ... Read more

Hyper-Velocity Impacts (HVI) from micrometeoroids and orbital debris pose a significant threat to satellites in low Earth orbit due to the higher density of sources and the resulting increased impact frequency. Understanding the stress field and dynamic behavior around impact points is critical for satellite design, structural integrity, and platform stability assessment. A coupled Finite Element and Smoothed Particle Hydrodynamics (SPH) methodology implemented in LS- DYNA explicit software is used to simulate HVI effects. Without compromising the reliability of the results and ensuring their continuity at the methodological interface, the aim is to take advantage of the strengths of both simulation methods. Although previous studies have used SPH-FEM coupling to model Hyper-Velocity Impacts, the focus of this thesis is on characterizing the stress field surrounding the impact zone. It has been observed that a portion of the stress wave is reflected at the interface between the two numerical methods within the plate. This reflection is not caused by a physical obstacle and is therefore numerical and artificial. A comparative analysis of stress signals collected near this numerical modeling discontinuity demonstrated that the implementation of a SPH-FEM hybrid elements interface exhibited superior performance in mitigating this effect in comparison with a tied type contact. Indeed, stress waves can smoothly move from the impacted region towards the external domain of the structure, exhibiting only minor internal reflection at the interface between the two numerical methodologies. Furthermore, the impact of the SPH lattice on stress wave propagation was explored. It was found that the default modeling approach had a detrimental effect on uniform stress propagation in the plate, as it introduced preferential directions of propagation. This issue was addressed by implementing a custom SPH lattice that ensures the isotropic properties of the material selected for modeling the plate. The propagation of impact-induced effects is ensured to be independent of the direction of study. Initial calibration and validation were conducted on a single flat plate system, followed by an extension to a full Whipple shield simulation. With regard to the latter, not only stress data, but also the HVI-induced vibration field within the plates was studied. This was achieved by collecting the out-of-plane velocity signal at variable distances from the impact site. Nevertheless, further studies are necessary for further refinement and validation. ... Read more

This thesis project focuses on the evaluation of the Extended Finite Element Model (XFEM) for modelling skin-stringer separation in thermoplastic composite stiffened panels. The study uses test results from a reference study in the literature on a PEKK-FC carbon composite panel with three stringers joined to the skin using a short-fibre reinforced butt joint. The modelling strategy was developed using Double Cantilever Beam (DCB) specimens. The XFEM-based panel models were then developed using two different damage initiation criteria: Quadratic Stress Criterion (QUADS) and Maximum Principal Stress Criterion (MAXPS). These models were compared with the Virtual Crack Closure Technique (VCCT) model from the reference study. The project also addresses the challenges posed by the crack tip impingement problem in the XFEM-based model with MAXPS and tests different strategies to overcome it. A proof of concept is provided for using the UDMGINI subroutine to address these challenges, setting the stage for future studies and algorithm development. ... Read more

Experimental and numerical characterisation of a novel filler material was carried out, followed by an investigation of the elastoplastic behaviour in a butt joint. The study aims at understanding the behaviour of the novel material when subjected to loads as expected in the butt joint.

Skin-stiffened structures are commonly used in aerospace applications, because of the excellent strength-to-weight ratios. For this, it is important to understand the behaviour of all the materials which go into the structure. The filler material under consideration is a novel material developed for the application of a butt-joint section.

The current study is made in two phases. First, the filler material in characterised for plasticity experimentally, followed by a numerical characterisation stage, wherein the tensile tests are replicated using Abaqus. After this, short butt-joint sections were taken and subject to 3-point bending tests. FEM simulations were carried out for the 3-point bending test in order to study the state of stresses and strains occurring on the filler section.

In a nutshell, a novel filler material was first characterised experimentally as well as numerically, with special emphasis on a real-world application.
... Read more

In the context of damage tolerance for aeronautical structures, substantial research has focused on simulating skin-stiffener separation in stiffened composite panels. This separation is marked by unstable crack growth at the skin-stiffener interface, which can lead to structural collapse in the post-buckled regime. Recent post-buckling tests on thermoplastic butt-joint single stiffener panels indicate the development of delamination within the skin before crack propagation occurs at the skin-stiffener interface. This delamination is likely triggered by the crack extension process prior to the buckling tests, where the skin was subjected to out-of-plane loads away from the stiffener, promoting the extension of the artificial crack at the interface. It has been hypothesized that this delamination results from crack migration from the skin-stiffener interface into the ply interfaces within the skin. Crack migration, which involves complex interactions between delamination and matrix cracks, is crucial for improving numerical models. For accurate prediction, these models must capture both matrix crack and delamination interactions.

Cohesive zone models, in conjunction with the eXtended Finite Element Method (XFEM) and cohesive elements (CE), have been employed in literature to model the interaction between matrix cracks and delamination. While previous approaches often enrich cohesive elements using user subroutines, this thesis aims to leverage ABAQUS's built-in methods to model crack migration. A series of migration test simulations were conducted to evaluate the combined XFEM and CE approach. The LaRC05 failure criterion in ABAQUS was applied to initiate inclined matrix cracks within the plies, while delamination was modeled with standard 8-node linear cohesive elements. A three-dimensional mesoscale model of the test specimen was developed to simulate the migration test. The LaRC05 criterion successfully captured the orientation changes in matrix cracks due to changes in shear stress, consistent with experimental results. However, the predicted migration distance was 2-3 times greater than observed experimentally. A parametric study revealed that lower matrix strength and fracture energy facilitated migration, although increasing these parameters did not result in a consistent delay in migration, with discrepancies arising at higher values. Despite this, the methodology demonstrated the ability to predict crack migration tendencies and is considered suitable for structural-level applications.

Simulations of the butt-joint thermoplastic skin-stiffener panel under bending were also performed using a global-local modeling approach. The 19-ply skin was meshed with shell elements, while the local model explicitly represented the outer two plies (45/-45) with solid elements and the remaining 17 plies with shell elements. Three modeling approaches were explored: (i) damage only at the skin-stiffener interface, (ii) damage at both the skin-stiffener and ply interfaces (global-local model), and (iii) matrix cracks combined with delamination at both interfaces (global-local model). The first two approaches predicted mode I crack extension at the skin-stiffener interface, with no interlaminar damage in the second approach. However, the third approach using XFEM-CE predicted significant matrix cracking in the outer ply beneath the filler material, which further initiated delamination at the 45/-45 interface. This method successfully predicted delamination migration in the stiffened panel, demonstrating its capability to capture complex damage interactions at the structural level. ... Read more

A new multiscale fracture mechanics methodology is being developed at TU Delft for composite materials. This methodology aims to unlock the use of hierarchical multiscale frameworks to link sub-ply fracture processes to laminate failure. In this thesis, this methodology is extended to 3D, both in terms of its analytical formulation and the numerical implementation of the pre-processing segment. Suitable conditions for three-dimensional periodicity were developed considering cases that do not appear in the two-dimensional case. Algorithms for three-dimensional crack identification were laid out. A series of representative loading cases were studied, including uniaxial and biaxial stretch, for an unidirectional ply and for a [0,90,0] laminate. Only a small number of analyses were carried out, due to the high computational cost compared to the two-dimensional case. A roadmap for further implementation is laid out, including a discussion of computational challenges for the implementation of three-dimensional multiscale fracture. ... Read more

Mitigation strategies which eliminate existing space debris, such as with Active Space Debris Removal (ASDR) missions, are now more important than ever regarding the ever-growing space debris population problem. One of the considered ASDR approaches uses a net as a capturing strategy. The benefits of such strategy are to allow for a large capturing distance, high compatibility for different space debris sizes and reduced accuracy requirements.

A key requirement of any ASDR missions is that during capture, no new space debris is to be generated during the process. However, when simulating net capturing in the literature, the potential to break of vulnerable structures, like antennas or solar panels is often neglected. Such elements may show an enhanced risk of failure, especially if these are already damaged, potentially contributing to even more space debris.

A discrete Multi-Spring-Damper net model was used to simulate the 20 m/s-frontal impact of a 30 m x 30 m net onto an ESA Envisat mock-up. The Envisat was modelled as a two rigid-body system with a Single-Degree-of-Freedom hinge connection. A sequential modelling strategy was implemented, which de-coupled all the necessary dynamic and structural models. More than two large sub-structures (the Ka-band antenna dish and solar array) were found to have a high likelihood of breaking, leading to the recommendation of several design mitigation strategies using two types of sensitivity analysis. With secondary space debris being generated, net capturing is found to be riskier than originally assumed throughout the literature. ... Read more

To survive on the surface of a near atmosphere-less Mars, humans must be able to provide thermal energy to buildings to sustain temperature changes from -73°C to 20°C. The goal of the thesis is to create a passively-working thermal battery using materials currently available to humans; it must be the sole heat source to the Martian buildings. Metal-organic framework materials adsorb CO2 in a reversible exothermic chemical reaction, thereby providing heat to the buildings.
 
A model was made in ABAQUS to simulate the adsorption mechanism; two parallel coupled simulations were run in a staggered approach, heat conduction and mass diffusion, to simulate chemical reaction.
 
The model was then used, with the chosen materials, to simulate several geometry permutations of Martian buildings, the common denominator being the square area of the liveable space and walls. The material was able to provide a significant portion of the required heat and saved up to 97% power otherwise demanded from conventional energy sources. ... Read more

Topology optimization (TO) has proven to be a capable design methodology for the realization of lightweight solutions that maximize structural stiffness or other design objectives. Due to its capability to be adapted to suit a wide range of objectives and constraints, both structural and non-structural, TO has been widely applied in all industries, including aerospace. The application of TO in secondary structures offers the scope for further weight savings and, therefore, this thesis investigates the employment of TO on a galley structure provided by Safran-Cabins. The galley structure is an essential equipment for the functioning of passenger aircrafts and hence, provides an ideal product, which once optimized could offer widespread weight saving.

The next generation of commercial aircraft require solutions that are lightweight and yet economic to manufacture. TO solutions are renowned for their complex architectures and frequent employment in conjunction with additive manufacturing. However, for products such as galleys, which are required to be manufactured in large quantities, complex manufacturing strategies are not economically feasible. Therefore, this thesis proposes a modularization strategy that can be used to augment the monolithic TO solution into an assembly of simpler and identical modules. The modular design strategy has proven to be a capable strategy to enhance manufacturability as well as reduce costs over the board and offers a unique opportunity to leverage the capabilities of TO for employment on a wider range of products.

To achieve the aforementioned goal, an image moment-based modularization strategy is investigated which treats the TO solutions as digital images and identifies positions of simple bar/beam modules within the image based on matching of the image moments. Through a detailed investigation, a fragment constrained bar matching strategy is developed. It is demonstrated that the proposed matching strategy is capable of identifying positions of bar/beam modules within TO solutions from the literature. Additionally, a post-processing strategy is developed to augment the obtained modules and their positions into simplified and assembled frame structures analogous to the underlying TO solution. The developed framework is employed on the topologically optimized galley, and its practical capabilities and limitations are identified, thus providing a foundational work to be further refined.
... Read more

The demand for efficient lightweight structures grows rapidly in the aerospace sector. Topology optimization, introduced in the late 80s, is a method capable of producing such structures and has been mainly studied for isotropic materials. On the other hand, the performance of composite structures, which are already widely employed in the aerospace industry, can be now improved due to the latest advances in manufacturing. Automated Fiber Placement has paved the way for further exploiting the capabilities of composites and led to the introduction of an optimization method, namely three-step variable stiffness design, that alters the fiber orientation within each ply, leading to a variable stiffness laminate. In this thesis, a staggered optimization method that simultaneously improves the material and the structural performance of balanced and symmetric laminated composite structures under planar loading is developed by coupling the two aforementioned methods. A finite element code is developed based on the equations for a static and linearly elastic problem. For the topology optimization problem, the nodal density values are used as design parameters and the Solid Isotropic Material with Penalization approach is chosen, along with a density filtering. The first step of the variable stiffness design method is implemented, using the nodal lamination parameters as design variables. The coupling of the two optimization techniques is performed through a staggered optimization, where both techniques are implemented successively, at each iteration. A gradient-based scheme is used for both topology optimization and variable stiffness design and the steepest descent method is implemented for the design update. The sensitivity analysis is performed based on the continuous adjoint method. Three different examples are demonstrated in this thesis; the case of a flat composite plate, a lug and an aircraft chair bracket. The whole optimization procedure, including pre- and post-processing, is carried out using an algorithm developed in MATLAB. The meshes are externally created and imported in the code. Convergence studies and a comparison with indicative examples from the literature are performed in order to verify the obtained results. A parametric analysis of the plate studies the effects of the different topology optimization parameters. Finally, comparative analyses are executed for the three aforementioned designs investigating both the structural and the computational efficiency of the results obtained with each of the three optimization methods, i.e. topology optimization, variable stiffness design and staggered optimization. ... Read more

Wind turbines play an increasingly imporant role in the energy production of our time. In order to optimize the performance of wind turbine blades, this thesis work aims at assessing the possibility of using panel methods for gradient based optimization of the aerodynamics of wind turbine blades. Specifically, the method employed has used Dirichlet boundary condition, a fixed wake for optimization and a free wake model for validation. The panel method developed has been validated against the MIRAS software and CFD results. The results of the optimization are compared against the Glauert optimum blade. The blade is parameterized using NACA profiles and the twist and chord are used as design
variables. Two optimizations have been performed: an unconstrained optimization, which has shown to take advantage of limitations of the panel method model; a second optimization is performed applying a thrust constraint and with tighter bounds on the design variables, which is capable of achieving realistic results. The main conclusion is that realistic blade designs can be achieved using a fixed wake panel method for aerodynamic optimization, although ultimately the performance of these designs should be assessed using
higher fidelity models. ... Read more

The launch vehicle industry is undergoing a major shift as new commercial organizations are staking claim to large market shares with bold technologies. To stay competitive in this new era, launch vehicle companies must develop new technologies to reduce launch costs while serving the growing demand for space accessibility. A possible solution is to use additive manufacturing in combination with topology optimization to relieve manufacturing and assembly burdens in addition to reducing launch mass. Topology optimization is a mathematical method that optimizes the distribution of material in a design domain for a given combination of loads, boundary conditions and constraints with the objective of maximizing mechanical performance. Now an increasingly popular design tool in the aerospace and automotive industries, topology optimization is used to redesign structures to reduce mass while increasing stiffness. To effectively apply this design tool to launcher structures, the optimization process must take into account multiple load cases to represent the large number of load cases such structures are subjected to, throughout the various flight phases. In this thesis, a design methodology was established for topology optimization of launcher structures under multiple load cases by redesigning a launcher structure produced by Airbus Defence and Space Netherlands. The thesis used two analytical models; a simple cantilever beam model to set baseline expectations of design methodologies which were then verified on the launcher structure demonstrator model. To achieve the objective, a suitable multiple load case objective function was identified for the design problem at hand by testing different objective functions on both analytical models. In the process, efficient ways to accommodate the large number of load cases in the objective were also studied through which promising methods to reduce the number of load cases were identified. The final optimized design was compared to the original launcher structure which showed that the optimized design exhibited organic design features, higher stiffness with the same mass. This thesis provided insight into multiple load case topology optimization which is expected to accelerate its adoption in the launch vehicle industry and pave a new path for more efficient space travel. ... Read more

Structural members carrying dominantly compressive forces are present in many types of structure. These members are referred to as columns and are frequently present in for example lifting appliances and offshore structures. Whether or not a column consists of longitudinally welded subsections is relevant for its mechanical performance in compression. Steel columns consisting of longitudinally welded members contain residual stresses caused by the non uniform longitudinal expansion and shrinkage during the welding process. The distribution and magnitude of these residual stresses are dependent on the dimensions of the heat affected zone (HAZ), and varies for instance as function of the welding procedure, and whether or not post weld measures are taken to diminish these residual stresses. In this thesis the width of the region in a column's cross section where tensile residual stress is present is referred to as the HAZ width. Using practical experiments an estimate is made of realistic HAZ widths in column structures using _nite element analyses. These column structures where welded and kept free of restraints during this process, so that a resulting curvature is developed upon cooling down to ambient temperature. The established value of the width of the tension zone is on the order of the thickness of the column's cross section. Based on existing norms a trapezoidal distribution of residual stress having a width of the earlier found value is applied on a _nite element model of a single square column, both for pinned and clamped boundary conditions. Mesh independence of the obtained results are verified by convergence studies. The compressive load capacity of the considered column is accected in the intermediate range of slenderness ratios, and shows a maximum reduction of approximately 21%. The effect of welding residual stresses is also investigated on a scaled model inspired by an existing design of a tower crane. A crane section is modelled by four vertical columns connected by multiple side bars. These side bars reduce the effective slenderness ratio of the columns loaded in compression, and therefore a less severe effect of the residual stresses on the collapse load is found. ... Read more

Mines and improvised explosive devices are the cause of many mortalities of vehicle occupants. Both experimental and numerical research in this field aims to improve the safety of military vehicles. TNO has advanced Finite Element (FE) models to simulate such events. The numerical research is done to better understand mine blasts. Another aim is to reduce experimental costs for Defence Material Organization (DMO). For full vehicle simulations the empirical Westine model is the basis of the in-house TNO mine blast model. The model consists of a triangular pressure pulse consistent with the Westine impulse which is calibrated using the a test rig developed by TNO and DMO. Experiments are compared with numerical results using the TNO mine blast model and the jump height, representative for total impulse transferred, shows scattered results for different vehicle tests. A possible cause can be accuracy of the current mine blast model. This research will study a new technique to validate mine blast models. When this new technique results in improvements in validation of mine blast models it can be used to study and improve the current TNO model. The aim of the new validation method is to calculate the interaction pressure of an plate excited by a mine blast loading. The obtained pressure can be compared with existing mine blast models for validation of the mine blast model. The methodology is based on solving the inverse problem which requires transient Digital Image Correlation (DIC) measurements of the deforming plate for the duration of the mine blast loading. Such measurements are done with the state of the art test setup from TNO and DMO. Several methods to solve the inverse problem will be studied. The proposed best way to solve the problem at hand is by solving the corresponding optimization problem using an iterative gradient of descent algorithm. The main difficulty in this approach will be the calculation of the gradient of the objective function. To do this the corresponding transient adjoint problem has to be solved. First an algorithm to solve the inverse problem is implemented for a linear Kirchhoff Love plate model to study the behaviour of the inverse problem for a relatively simple test case. For this algorithm the transient adjoint problem is derived. The forward and adjoint problem are numerically solved. The implementation is verified using the method of manufactured solutions and a convergence study. The algorithm is verified using three different bench mark test pressures. It was found that without any exception the displacement of the algorithm converged with great accuracy to the applied displacement. The corresponding pressure converged good for smooth pressure distributions. Non smooth pressure distributions representative for mine blast interaction pressure did not converge. This shows the non-uniqueness of the solution of the inverse problem. It was realized after these tests that the forward problem acts as a low pass filter for time and spatial oscillations of the pressure distribution. This implies that the inverse problem in non-unique and that noise in the displacement data will be amplified in the obtained pressure from the inverse solution. The total force and radial position as function of time, obtained after integration over the spatial domain, for a localized load are accurately captured back. These parameters could be useful to validate a mine blast model such that research was continued for a continuum model. A non-linear elastic material model is proposed to model the deforming plate assuming monotonically increas- ing strain. This model is used and calibrated against the Johnson-Cook plasticity model. One plate simulation excited by the TNO mine blast model verifies that the non-linear elastic and Johnson-Cook material model result in almost the same displacement. The adjoint problem for a general continuum with elastic material model is derived. The same optimization algorithm is implemented for the continuum model using the calibrated non-linear elastic material model. The forward and adjoint problem are solved using the author’s transient FEM implementation which is verified using commercial software. From the benchmark tests it was verified that the displacement converged quite well however less compared to the linear problem studied earlier. The corresponding pressures behaved similar. It was concluded that the total force and centroid position of a localized load are not always in agreement for the benchmark and solved pressure. This research shows that the limitations of the inverse problem shed light on the limitations in the validation methods employed by many researchers in the field. ... Read more

Aiming to enhance the performance of the industrial design process, structural optimization techniques have been proposed as an alternative to traditional design and optimization techniques.
They own the potential of achieving an optimal distribution of the material through the design domain, thus reducing waste loss and weight, and increasing the ability to carry the loads and the overall efficiency of the design.
During this thesis work it has been developed a 3D Fluid-Structure Interaction model for beam-like structures, such as wind turbine blades. The cross-section geometry of the beam can be optimized using structural optimization techniques, such as size, shape or topology optimization.
The proposed 3D beam is partially based on the formulation of the classical beam element for slender beams (Euler-Bernoulli), including Saint-Venant torsional effects for isotropic materials, and with the addition of the terms related with the coupling between axial and torsion, and bending and torsion contributions, which may arise when using non-linear materials.
The stiffness information of the beam is interpolated from its cross-section geometries and materials, which can vary along the beam length.
The cross-section geometries are defined on a XFEM mesh. The fluid and solid domains are specified using a Level Set Function. This provides a smooth geometry and crisp representation of the solid/fluid interface without the necessity of re-meshing, as in the case of classical FEM. A 2D fluid simulation based on Incompressible Navier Stokes flow at low Reynolds number is carried around each cross-section, in order to obtain the aerodynamic loading over its contour. This aerodynamic loading serves as an input for the beam model, to compute deformation of the beam. This deformation is mapped onto the cross-sections, obtaining the updated displacements and rotations of the geometry. With the updated geometry the fluid field is altered and it needs to be updated as well, forming a non-linear iterative process that loops until a converged structure is obtained.
The 3D FSI model is solved on a monolithic Newton-Raphson solver that treats all the equations involved at once. The Jacobian terms derived for the monolithic solving scheme that has been developed for the forward analysis allow a straightforward computation of the sensitivities using adjoint method. This sensitivity analysis makes possible the optimization of the geometry of the cross-sections based on certain criteria and constraints. ... Read more