The increasing demand for lightweight and cost-effective structures in aerospace and wind energy applications has led to widespread adoption of fibre-reinforced polymers (FRPs). While carbon fibre reinforced polymers (CFRPs) offer excellent stiffness-to-weight ratios, their high cost and poor damage tolerance pose challenges for structural applications such as wind turbine and helicopter rotor blades. Hybrid laminates combining CFRPs and low-cost, higher failure strain (compared to CFRP) Glass fibre reinforced polymers (GFRPs) present a promising alternative, balancing competing objectives of cost and stiffness while providing higher damage tolerance. Most existing optimisation studies of hybrid composite plates treat material selection and ply orientation as decoupled problems, lacking a unified formulation that can capture their interaction holistically. Furthermore, existing approaches prevent the application of gradient-based optimisation via lamination parameters(LP), efficient for stiffness-targeted optimisation, when attempting to unify material assignment and ply orientation within a single design framework. This thesis develops a novel method to enable the use of lamination parameters for hybrid composite laminates by introducing material-dependent dispersion parameters(DP), unifying material selection and ply orientation within a single design formulation. With the material properties of the laminate as a function of volume fraction, the dispersion parameters describe the through-thickness material distribution in a continuous, geometry-independent manner. The combined LP–DP formulation defines a continuous, convex design region amenable to gradient-based optimisation frameworks. The DPs and feasible region are further explored to create a combinatorics-based and deterministic method to constrain the region, creating a relation between LPs and DPs. Further, different optimisation strategies have been discussed to achieve an optimum solution for design problems. A two-level optimisation strategy is roposed, combining a gradient-based optimisation for target parameter identification and a genetic algorithm to retrieve manufacturable, symmetric, and balanced stacking sequences that comply with standard layup rules.

This thesis presents a methodology for constitutive material modeling of flexible targets, with a specific emphasis on finite element analysis. The Johnson-Cook constitutive model is employed in combination with a customized Bao-Wierzbicki damage model, both implemented in Abaqus/Explicit through a user-defined material subroutine (VUMAT). The primary objective is to evaluate the accuracy of the finite element simulations by comparing them to experimental tensile test data. To this end, the dogbone specimen used in an experimental test was reconstructed in Abaqus, and stress-strain as well as reaction force-strain data were extracted from a single finite element located in the necking region. These data were then compared to strain measurements obtained via Digital Image Correlation (DIC) from the physical experiments. The analysis was performed across four mesh densities to assess mesh convergence.

The results indicate a reasonable agreement between the simulated and experimental force-strain and stress-strain responses. Minor discrepancies were observed, largely attributable to uncertainties in the experimental measurements. Overall, the simulations demonstrate the ability of the implemented material and damage models to capture key aspects of the observed mechanical behavior.

This validated simulation framework provides a foundation for future investigations involving high-deformation phenomena. It is particularly relevant for impact analyses, such as bird strike scenarios in aerospace applications.

In this work, a conceptual design was created that integrates battery packs into the wings of a 90-seater electric aircraft. This was done under the challenging requirement of replacing over 35 tonnes of batteries more than once a year. The concept allows the batteries to move spanwise along a rail system and through the rib using a panel structure. The core of this concept is that it avoids the need to remove large skin panels of the wing. From the broader concept, a single component was analyzed in detail using non-linear finite-element models combined with fretting-fatigue analyses and physical testing.

Bird strikes pose a significant threat to aircraft safety, yet accurately predicting their impact behaviour remains challenging due to many uncertainties, such as variations in bird properties, high impact velocities, and limited experimental data. While much research has focused on modelling the bird itself, understanding the structural response of materials, particularly aluminium alloys commonly used in aircraft structures, is just as important. This thesis investigates how different material model definitions in the plasticity regime affect the predicted response of aluminium 2024-T3 under bird strike conditions.
Moreover, it investigates the uncertainties that arise from limited quasi-static testing or usage of inconsistent material properties that can be found in the literature. The work aims to highlight the influence of modelling choices and identify key sources of uncertainty related to material characterisation. Using a surrogate model implementation for two chosen plasticity models, Ramberg-Osgood and Johnson-Cook plasticity models, the uncertainties are being assessed through one-factor-at-a-time sensitivity analysis together with Monte Carlo analysis. The results of this work contribute to improving the reliability of bird strike simulations and provide guidance for future model development.

Inverse design is a concept where one can design a structure via a property-first approach, where the properties of a system act as precursor information to guide the search for a viable design. This concept is classified as one of two: indirect or direct. While the former is no different than a forward-based approach using traditional optimization, a direct inverse design framework attempts to invert the process and directly map the desired properties to a suitable design candidate, which is unconventional. As such, the design process can be positively radicalized, with regards to the required computational resources when designing multiple structures.

In this work, the use of Bayesian machine learning, more specifically Bayesian optimization using Gaussian process regression, is employed to construct a direct inverse design framework using the design of mass-optimized fixed-wing aircraft ribs against buckling as the validation case study in 1, 6, and 10 dimensions. The results are compared to an indirect approach using the same algorithms and demonstrate that barring certain limitations, which are not inherent flaws of the direct approach, the framework designed not only demonstrates sound potential in inverting the forward map, but outperforms the indirect approach when designing multiple structures.

This research equally provides a stepping stone towards future research possibilities in the same field, all culminating in the improvement of the multi-disciplinary design process.

With the progressively more widespread exploitation of post-buckled skin-stiffened structures in the aerospace industry, aimed at making them more weight efficient, assessing the failure of such structures is of utmost importance. Additionally with the trend of moving towards fastener free connections, one of the potential failure modes of skin-stiffened structures is the skin-stiffener separation that leads to a sudden and drastic reduction in the load carrying capacity of the structure. Hence, it is imperative that structures are designed against such failure modes, being crucial to have predictive models for the initiation and propagation of delaminations at the connection interface.

Given this motivation, the present thesis investigates the development of a novel multi-domain semi-analytical cohesive zone approach to predict the initiation and propagation of damage along a two-dimensional interface. In the multi-domain method the main structure being analyzed is decomposed into multiple smaller domains for computational advantage, with each domain having its own set of continuous approximation functions based on the Ritz method. A penalty formulation governs the inter-connection of domains. To extend the multi-domain framework to model damage, the interface connection is modelled as a cohesive zone with the traction-separation law governing its response. The area connection from the multi-domain method is modified to include a damage-dependent penalty stiffness term, whose degradation is controlled by the traction-separation law of the cohesive zone model. Lastly, the energy dissipated to create a crack is included in the formulation. Since the framework is based on the Ritz approach, the appropriate choice of continuous approximations to model damage is investigated, presenting their limitations and the proposed methods to overcome them.

Finally, the results obtained from the multi-domain framework were verified against finite elements for a multitude of cases, which in turn had been validated by experimental data. The results were subsequently followed by discussions and the caveats of the developed framework. Results indicate that the developed multi-domain semi-analytical cohesive zone method predicts the results within 4.3% of finite element results, while taking 75% less time on average.

Finally, having somewhat successfully modelled the initiation and propagation of damage along a two-dimensional interface, this thesis establishes a robust basis for future research involving the use mixed-mode behaviours with multi-linear cohesive laws, whose formulations can just be slotted into the current framework. All of this forms a solid foundation to efficiently model damage initiation in larger structures and, more concretely, skin-stiffener separation.

The established theory of diagonal tension (i.e., post-buckling of skin-stiffened structures under shear load) for curved skin is based on experiments conducted on stiffened cylindrical shells at NACA in the 1940s-50s. However, the experiments were unable to capture all aspects of the phenomenon, leaving knowledge gaps in the understanding of the local distribution of the compressive loads generated on the stiffener, which were filled using the theory of diagonal tension for flat skins. Furthermore, due to the limitations of the time, the secondary bending effects on the stringers were not captured well.

This MSc thesis attempted to fill this gap by performing virtual tests using non-linear finite element modelling, which is enabled by the significant improvements in computational hardware of today. A virtual testing framework is developed based on the stiffened cylinders tested at NACA using the ABAQUS finite element modelling software. The skin and the stringers are modelled using continuum shell elements and the rivets are modelled using point-based fasteners. Furthermore, the generation of the models were parameterised using python scripting with provisions to model imperfections on the cylinders in the shape of the buckling eigenmodes. To validate the modelling framework, FE models of 8 cylinders were created and the results were compared with experimental data. Satisfactory correlation was found between the virtual testing results and the experimental results given the under-reported experimental data and the unknown imperfections in the original test

A custom post-processing method was developed that was able to capture the axial and bending loads that were generated in the stringers, which were used to develop a more detailed understanding of the load-transfer mechanism under diagonal tension. The results were also compared with the semi-empirical design method and the theory of diagonal tension, and significant discrepancies were found pertaining to the local variation of the axial loads and the secondary bending of the stiffeners.

Design methodologies for retrofitting are devised, and the retrofitted fuselage configuration designs were developed and tested for crashworthiness using finite element analysis in this thesis. These retrofits aimed to install hydrogen tanks within the fuselage, contributing to an efficient transition of the aerospace sector towards hydrogen-powered aviation.
Liquid hydrogen is chosen as the fuel source for hydrogen tanks due to its high density compared to other forms of hydrogen storage. However, storing the same amount of energy as traditional fuel still requires four times as much volume. Therefore, installing hydrogen tanks within the fuselage is necessary due to the low volumetric density and the relatively high flammability of liquid hydrogen.

The retrofits were carried out on a typical fuselage, the most common tube and wing aircraft section. The chosen aircraft type was the single-aisle/short-haul aircraft, and the typical section designed for was the F28. The designs aimed to maximize the fuel capacity within the fuselage, ensuring the retrofitted aircraft could achieve the maximum range. The design methodology was based on practical packing solutions commonly used in the logistics sector to transport as much fuel volume as possible within the constraints imposed by the available volume. This approach is especially beneficial for liquefied hydrogen tanks, considering their low volumetric density and the resulting demand for a large volume.

The design methodology is based on packing solutions popularly employed in the transportation and logistics sector to transport large containers of a specific shape in a constrained volume. Packing solutions utilize tessellation, in which a plane of a certain shape is covered or filled by another shape. This method is beneficial for hydrogen tanks since all of them will be of the same shape in a plane, namely, a circle.

Three designs were devised based on two tank orientations. Two designs are in the lateral direction,
while the other is in the longitudinal direction. All three designs use different packing solutions to
design the tank configuration with the fuselage, with the tank supports designed accordingly.
Some tank design choices were influenced by other fuel tanks and liquefied hydrogen tanks related to regulations such as API 620. Five crashworthiness tests were conducted based on criteria outlined in certification standards such as CS-25 or FAR-25. All designs passed the crashworthiness and fuel system criteria, indicating their ability to meet crashworthiness certification. The crashworthiness results for each design were compared and discussed, and observations were noted.

By loading the tanks onto the frames, longitudinal circle packing (LCP) can alter the load path
and facilitate significant friction dissipation. This helps avoid issues with a lighter fuselage, which may not produce enough plastic deformation in the lower fuselage to correspond with the designed crash sequence. On the other hand, LHP lacks this mechanism, leading to extreme acceleration on the tanks. Lateral square packing (LSP) closely resembles the designed loading, follows the crash sequence as intended, and shows the lowest average DRI despite necessitating the highest energy dissipation.

Finally, a trade-off study was conducted to compare the designs based on the retrofitted aircraft’s range, inspectability, crashworthiness, and retrofit cost estimation. LCP was selected as the best design due to its advantage in range and crashworthiness.

The present study focuses on utilizing the Bayesian Optimization Machine Learning algorithm for the weight optimization of a shear web of given size (a x b), material properties, boundary conditions, and loading conditions. The study is carried out in cooperation with GKN Fokker Aerostructures. The main objective of the research is to replace a pristine, unperforated web with a circularly perforated web where the circular cutout is reinforced such that the weight of the web is minimized w.r.t the pristine web while adhering to shear buckling criteria.

The Bayesian Optimization machine learning method is a Parametric Modelling approach which belongs to the search algorithm class whose purpose it is to find the optimum set of input parameters which maximize or minimize a predefined objective function. Bayesian Optimization attempts to model the objective function through a surrogate model. In the present study Gaussian Process-based surrogate models are used. Through the surrogate model the optimization algorithm can search the design space in a probabilistic manner in search of the global objective optimum.

Furthermore, a novel universal Bayesian Optimization convergence criteria was theorized in the present study. The convergence criteria is based on the decrease in change of uncertainty between subsequent evaluated sets of parameters by the optimizer. The performance of both the Bayesian Optimization algorithm and convergence criteria are verified on a three dimensional case study. The verification showed that the results of the optimization are within 3% of an apparent global minimum weight. The convergence criteria verification showed accurate convergence prediction which has as an implication that roughly 40% of the total optimization iterations could have been spared through the use of this criteria.

The demand for more efficient aircraft and new modes of transportation lead to a departure from the traditional tail and wing configuration. In order to operate an aircraft, it needs to be certified. Most Aircraft with a capacity to carry more than 10 passengers have to fulfil a variety of bird strike related certification standards. These standards define an impact velocity to design for, critical locations and tested areas however are chosen based on experience of previous aircraft. This is not possible if the design is radically different, as it is in the case of the Flying-V. The present thesis proposes a methodology to identify and rank all possible bird strike scenarios, based on geometry and flight path. The quantification of impact scenarios may offer a cost-effective way to assess and visualize vulnerabilities, ultimately reducing certification cost and time. Thereby allowing new concepts to be certified. The methodology synthesized in this report relies on decoupled analytical bird strike load models from the late 70s to quantify bird strike intensity. An algorithm has been developed to determine impact scenarios over the area of arbitrary computer aided design (CAD) geometries. Ray tracing allows for the exclusion of areas that can not be hit. The results are not sufficient to determine critical impact locations, as the damage is significantly influenced by the structural response. However, it is possible to quickly generate probable impact scenarios over large areas based on the flight path. Despite the discrepancy between predicted damage and impact intensity, intensity maps can indicate areas of interest for investigation.

This research delves into aircraft crashworthiness, focusing on the innovative Flying-V configuration, aiming to improve safety in unconventional designs. Challenges arise due to the Flying-V's unique V-shaped fuselage, complicating traditional crashworthiness assessments. To address this, the study proposes modelling approaches, particularly for the central part of the fuselage lacking detailed structural information.
Various modelling approaches are explored, building on a finite element model of the Flying-V developed in previous work. Drop tests validate optimal section designs, emphasizing a minimum vertical impact velocity. Spatial variations in Dynamic Response Index (DRI) and Severity Index (SEV) prompt nuanced studies on impact scenarios and potential passenger side loads.
As the analysis progresses, extending the computational domain becomes crucial for reliability. Insights into weight distribution imbalances and challenges with corrective measures emerge from analyses of extended fuselage sections. Spatial fluctuations in DRIs and SEVs underscore the need for a balanced approach between computational efficiency and result realism.
A newly introduced modelling technique leveraging moments of inertia is implemented, yielding realistic results for straightforward scenarios and reducing simulation time significantly. Further analysis explores intricate landing scenarios, highlighting differences between full and reduced models, particularly at elevated pitch angles.
Recognizing the limitations of simplified methodologies, a submodelling technique is proposed for extreme crash scenarios, effectively capturing engine section dynamics with reduced computational time.
While reduced modelling techniques show promise, the study underscores the need for a comprehensive finite element method representation of the Flying-V, recommending successive simulations with a coarse overall mesh followed by submodelling for detailed assessment of critical regions.

This study investigates the development and application of meta-models for crashworthiness assessment of helicopter structures and components. It aims to address the challenges associated with scarcity of data from computationally expensive simulations and experimental drop-tests, and enable the use of surrogates in a crashworthiness optimization framework. Two predictive approaches utilizing Machine Learning techniques are compared to predict and assess the energy absorption of tubular metallic structures for different cross-section configurations. The first approach directly predicts energy absorption, while the second predicts load-displacement curves, from which energy absorption is derived. Results indicate that certain regressors, such as the Transform Target Regressor, the Decision Tree Regressor and the Poisson Regressor, consistently achieve high accuracy in predicting load-displacement curves and energy absorption across the evaluated tubular samples. A low-fidelity model able to provide less accurate but computationally inexpensive information is then introduced. The influence of low-fidelity data is investigated when it serves as additional input alongside high-fidelity data during the training phase of the surrogate model, through a comparative analysis. The research's findings suggest the efficiency of Machine Learning in representing structural behaviour under crushing conditions and highlight the potential for further enhancements through the integration of low-fidelity data, thereby holding promise for extending the methodology to more complex structures.

The thesis aimed to characterize the effects of manufacturing-induced defects, particularly gaps in variable stiffness laminates. The primary focus centered on designing a specimen capable of accurately characterizing gap effects. A refined, element-level specimen design featuring a controlled gauge zone was developed, based on pre-defined requirements to represent common variable stiffness structures accurately. The specimens were manufactured and tested. The experimental work featured Digital Image Correlation, Dye Penetrant Testing, and Optical Microscopy. Experimental observations revealed deformation peaks induced by gaps, along with trends in void formation and ply deformation during curing processes. Furthermore, digital image correlation (DIC) analysis unveiled distinct out-of-plane tendencies associated with gap locations. Discrepancies between finite element method (FEM) results and experimental strain distributions highlighted the need for improved modeling, particularly accounting for ply waviness. The main hypothesis presumes that the effects of gaps are influencing off-axis plies through matrix failures. The hypothesis was not proven true based on insufficient proof.

The need for better fuel efficiency within the growing aviation sector is increasing every day. Meanwhile conventional aircraft are reaching an asymptote regarding fuel efficiency and therefore the focus switches to unconventional aircraft. These aircraft have unique challenges whereof one is the structural connection between engine and aircraft. In this research an automated design methodology is developed which allows for optimization of such a structure. This design methodology uses a parameterised computer-aided design (CAD) connected to Python, which can automatically alter the parameters and make a finite element (FE) model of the design and subsequently run the desired simulations. The
results of the simulations are extracted and post-processed into margins of safety (MoS) using the direct results or (semi-)analytical equations for each individual component. In total five failure modes are assessed, which are Von Mises stress, displacement of engine, crippling, panel buckling and column buckling. The most critical MoS and the relation to the parameters is used in combination with an adaptive damping parameter to optimize each individual part. This design methodology is tested using the Flying-V, which is a novel aircraft configuration in the shape of a V, as a reference aircraft. Therefore, first the requirements, critical loadcases, loads and initial design have been developed based on regulations and previous research regarding the Flying-V and have been used as input for the CAD model and simulations. The design methodology has proven not be perfect as regions with stress concentration cause the minimum MoS to be below zero for all iterations and could not be solved within the time frame of this research. However, the design methodology has proven to be significantly faster compared to a design of experiments used in previous research. Overall it should be noted that there are still many other limitations regarding this method. These limitations are related to the limited amount of failure modes used and the fact that it still requires some kind of Design of Experiment, which is time consuming. The final mass of an optimized engine mounting structure is estimated to be approximately 40% higher than the design methodology used in previous research, which is due to the higher amount of details in the design as well as the significant higher estimated mass for the engine. This means that the current estimated mass for the full engine mounting structure including landing gear and engine is approximately 22150 kg compared to the estimated 17360 kg before.

Furthermore, an assessment of the influence of the developed design on the crashworthiness of the Flying-V has been done. This assessment focuses on one of the four main criteria regarding occupant protection, namely the guarantee of acceptable acceleration and loads sustained by the occupants. This is done by connecting the engine mounting structure to the developed wing-fuselage structure of the Flying-V with a tie connection. In order to compare different cases a new variable has been created, which is based on empirical equations developed by the U.S. Air Force. In addition, a new method based on moments of inertia has been developed to take the rest of the aircraft into account without increasing the size and thus computational effort of the simulation. To validate this method, the wing-fuselage structure has first been extended to fit the structure and results are compared to previous research. This has shown large, unexpected, differences which could not be explained even with thorough investigation. Applying the inertia method to the extended fuselage has shown small but explainable and expected differences, validating that the method has no unforeseen effects on further results. Simulations using this inertia method and the wing-fuselage with full engine mounting structure shows an improved performance in crashworthiness as the engine mount also absorbs some of the energy. Similar simulation without engine shows an even more improved performance, meaning that engine separation is of interest for the Flying-V. Initial further-reaching investigation into engine separation, using a point-mass model, shows that engine separation might be possible without impacting the Flying-V for crashes without any initial yaw, pitch or roll angle.

Bird strike simulations require accurate information about the nonlinear material properties of the bird and structure, dynamic changes in contact between bird and structure, and large displacements and rotations of the structural elements. The soft-body nature of birds allows it to flow fluidly upon impact, making accurate simulation validation difficult. Setting up a model requires evaluating and considering a large number of parameters. To simplify the process of obtaining validated bird strike models, a set of guidelines for setting up the simulations, covering the major parameters that influence the validation of the models, is established. This was done for an EWVT-SPH (Element Weighted Voronoi Tessellation Smoothed Particle Hydrodynamics) bird model by following the building block approach laid out by the aerospace standards of SAE-G28 for both rigid and complaint target structures.

The use of liquid hydrogen LH2 in aviation offers significant potential for reducing CO2 emissions. However, its unique properties, such as rapid dispersion, wide flammability limits, and low ignition energy, introduce explosion safety concerns in confined spaces like the aircraft tail cone, where LH2 tanks are typically placed. Using a numerical approach, this thesis investigates the blast loads and dynamic response of tail cone structures to a confined hydrogen-air detonation, identified as a worst-case explosion scenario.

The LS-DYNA CESE-Chemistry solver, employing an 8-step finite-rate reaction mechanism, was used to model the detonation blast wave. The structural response is coupled via Immersed Boundary Fluid-Structure Interaction (FSI). The detonation model was validated with literature results from a hydrogen-air detonation experiment, while the structural model was validated with blast-loaded circular plates, both showing good agreement.


Results show that the blast wave propagates in the tail cone with multiple reflections, producing 2–3 distinct pressure peaks. While the first pulse is the primary shock-loading in most locations, the second pulse is more significant at the bulkhead center, amplified by shock convergence effects.

The structural response occurs in four distinct phases: initial deformation, flexural wave propagation, secondary pulse loading, and snapback instability. Large deformations and high plasticity were observed at the bulkhead center and edge, identifying them as critical zones for damage. Parametric studies reveal that deflections increase with ignition distance and decrease with bulkhead thickness or radius. A linear relationship was found between a dimensionless impulse parameter and permanent deformations or plastic strains across varying bulkhead configurations.

The insights gained provide a foundation for designing safer hydrogen-powered aircraft and addressing regulatory gaps in explosion safety. Recommendations for future work include extending the model to 3D geometries, incorporating material failure models, and conducting tailored validation experiments.

This thesis investigates the design and analysis of cryogenic liquid hydrogen storage tanks in the context of aircraft retrofit. Hydrogen propulsion offers significant potential for achieving zero emissions in aviation, but its adoption introduces challenges related to safe and efficient storage.
The research focuses on comparing single-wall and double-wall tank architectures, assessing their ability to meet stringent operational, thermal, and structural performance requirements. In the preliminary assessment of a tank design’s viability, two key performance requirements are cruise time and dormancy time. A minimum cruise time of 20 minutes ensures the tank can support basic flight operations, while a dormancy time of 1 day ensures no hydrogen loss occurs if the aircraft remains stationary for an extended period, accounting for potential delays.
The methodology to calculate cruise time involves determining the maximum time the aircraft can remain in the cruise phase based on the inner tank dimensions, fill ratio, and mission profile. The dormancy time is the time required for the tank pressure to reach the venting pressure, at which point hydrogen must be released, and is calculated by implementing a thermodynamic model that simulates the tank's dynamic behavior over time, accounting for heat inflow from the external environment.
The evaluation of the single-wall tank reveals its simplicity and potential cost-effectiveness, but also exposes considerable limitations in terms of thermal insulation for the specific retrofit case study. This design approach is a viable option for larger-scale applications, where a lower surface area-to-volume ratio reduces heat transfer and, consequently, hydrogen boil-off. However, the compact dimensions of the tanks required for aircraft retrofitting present a significant challenge due to the inherently higher surface area-to-volume ratio, which leads to increased thermal losses and prevents the single-wall architecture from meeting the performance requirements imposed by this specific case study.
In contrast, the double-wall tank, equipped with a vacuum layer and multi-layer insulation (MLI), offers improved thermal performance. The heat transfer from the external environment is significantly reduced, allowing to preserve the cryogenic temperature of the hydrogen fuel. However, the added complexity introduces new challenges, particularly regarding the design of the inner vessel support system which must maintain the inner vessel’s position while accommodating thermal displacements and managing structural loads. Assessing the heat leakage budget for the support structure is the final critical step, as it determines the maximum allowable heat inflow through the support system, ensuring the tank meets its dormancy time requirements while allowing for design optimization.
The thesis develops a design methodology for the inner vessel support system, balancing the need for flexibility (to accommodate thermal contraction experienced during the first filling of the tank) with sufficient stiffness (to withstand operational loads, including emergency landing conditions). This approach involves selecting suitable materials and geometries that meet thermal requirements, while accurately determining the support structure’s stiffness properties. Different loading scenarios, such as normal operations and emergency landing conditions, are evaluated to analyze stresses and displacements in both the tank and support system. Adjustments to the design are made if stress or displacement exceed safe limits. The analysis reveals that optimizing the support structure is critical for the double-wall tank’s overall feasibility. While the double-wall design is technically viable and meets the thermal performance requirements, its success depends on further refinement of the support system to minimize heat leakage and ensure structural integrity.
The results of this study suggest that, although single-wall tanks are not suitable for this application, double-wall tanks offer a promising solution for retrofitting aircraft with cryogenic liquid hydrogen storage. Nonetheless, significant challenges remain, particularly in designing efficient support structures that can handle the operational demands without compromising thermal performance. Future work should focus on optimizing the support system design, exploring flexible materials, and considering additional factors such as sloshing loads to further improve tank reliability and performance.
In general, this thesis contributes to the development of a robust methodology for the preliminary design of cryogenic hydrogen storage tanks, providing a foundation for further advancements in hydrogen-powered aviation.

Pushing the envelope of aerospace structures requires the complete exploitation of their potential in terms of load-carrying capacity per unit weight for both economic and ecological reasons: the two most important being the reduction of fuel consumption and greenhouse gas emissions. A key approach involves allowing structures like stiffened panels to function within the post-buckling range domain while in service. To do so, the finite element approach allows a broad design space for researching the post-buckling behaviour of such structures.

Accurately representing post-buckling behaviour in finite element models requires accounting for geometric and loading imperfections. The present study explores their effects on the post-buckling behaviour of a composite L-stiffened panel. A finite element model is created and validated based on an experimental case. This is then further modified to incorporate imperfections. Geometric imperfections are modelled using linear eigenvalue modes, while loading imperfections are introduced via a rigid loading plate making contact at an angle.

The research showed that both first and higher eigenmode combinations for geometric imperfections influence post-buckling behaviour. Their shape and amplitude impact the transition into post-buckling and their ultimate loads. Similar behaviour was also observed for loading imperfections. Additionally, their configuration also showed an offset in axial displacement results. These insights emphasise the need for precise imperfection modelling to promote safer and more efficient post-buckling design of aerospace structures.

The present study, which was carried out in collaboration with GKN Fokker, focuses on incorporating bird strike crashworthiness requirements within a multidisciplinary optimization (MDO) framework. During the preceding three-month internship in the same company, a pivotal contribution to this project was the development of an Abaqus interface for the Multidisciplinary Modeller, MDM, created within the Center of Competence in Design department. MDM is a Python/ParaPy-based automated generator of wings, moveables and flaps, starting from a set of user-specified parameters. The generation of ready-to-run input files thus lays the foundation for the subsequent optimization process, as any changes in materials or geometry can be easily accommodated.

The core objective of the research is to minimize the weight of an aircraft wing while taking into account additional requirements related to the extent of damage caused by bird strikes. Unfortunately, such events occur more frequently than one would be comfortable with, and stringent requirements are set in place to guarantee the safety of the passengers. Among these requirements, the aircraft must be capable of landing safely after such an event, being subject to loads associated with get-home conditions.

As a consequence, two critical constraints are formulated within the optimization framework, addressing the residual strength of the damaged front spar following a bird strike, coupled with a requirement based on a maximum penetration depth. The last constraint has also been included due to the rising popularity of the electric vertical take-off and landing aircraft, which not only fly at low altitudes, thus increasing the risk of bird strike, but may also contain battery packs in the leading edge, for instance, which can pose a significant risk if damaged. To tackle the complexity of this highly-dimensional optimization challenge, a methodology based on Bayesian optimization is proposed, employing surrogate models coupled with a preliminary variable ranking procedure.

The Kriging metamodel is identified as a suitable candidate, thanks to its error prediction capabilities, which are paramount in Bayesian optimization. A variance-based dimensionality reduction method is proposed, which makes use of an initial surrogate to estimate the main and interaction effects of the variables. The quantification of the significance of a variable is expressed as its percentage contribution to the total variance, thus allowing for an intuitive selection of the most important parameters. After the screening procedure is complete, the optimization procedure is carried out in the reduced design space, which uses the constrained expected improvement as an acquisition function. The proposed methodology is then applied on a case study problem, involving a five-bay metallic wing segment subject to the constraints aforementioned, involving 19 design variables representing the thicknesses of various components.

Remarkable weight savings have been achieved, the final result being 40\% lighter than the lightest feasible design among the initial data points. A significant dimensional reduction has also been attained for the maximum depth constraint, which is expected due to the local nature of the impact. Not only did the number of variables greatly decrease from 19 to just 3, but a considerable increase in the accuracy of the corresponding metamodel has also been registered, thanks to an increase in sampling density in the reduced space. However, the variable screening procedure revealed intricate interaction effects with respect to the residual strength of the front spar, emphasizing the nuanced complexity inherent in crashworthiness considerations. Nevertheless, a moderate dimensional reduction has been achieved for this constraint as well, reducing the number of variables to 8, thus proving the efficacy of the proposed variable screening procedure.

In conclusion, the utilization of Kriging models, variable ranking procedures, and Bayesian optimization collectively contributed to the success of achieving remarkable weight savings, proving the efficiency of the proposed methodology. Moreover, it has been shown that the integration of a residual strength requirement is necessary, as many cases were uncovered where no significant penetration occurred, although the application of the considered load case, which is not from critical to an undamaged wing, resulted in high stresses to the front spar of the damaged structure.