Double-walled vessels are recognized as an effective storage solution for liquid hydrogen (LH2) due to their superior thermal insulation. Similar to a thermos flask, the vacuum space between the inner cryogenic vessel and the outer vacuum shell prevents convective heat transfer, which would otherwise result in the rapid boil-off of the stored LH2. A critical component of this technology is the inner support structure which supports the inner vessel within the outer vacuum shell. It must be robust enough to handle operational loads and potential crash events specific to aviation applications, prevent excessive thermo-mechanical stresses caused by the contracting inner vessel, and minimize heat transfer through conduction. Despite its importance, there is a noticeable lack of comprehensive designs and research addressing this component, particularly in the context of aviation.

The primary objective of this thesis is to develop a design methodology for creating effective inner support structures for double-walled vessel technology. The research is grounded in a practical context through a collaboration with AeroDelft, a student team at TU Delft currently retrofitting a Sling 4 aircraft for hydrogen-powered electric flight. As part of the next milestone in their Project Phoenix, AeroDelft aims to store 6 kg of LH2 in a double-walled vessel to meet the mission's requirements.

To begin, functional, operational, and constraint requirements were established for the inner support structure. A baseline design for the inner vessel and outer shell was then developed. Subsequently, four distinct inner support structure concepts were introduced, each with its own design intents and key considerations. These concepts were subjected to various analyses, including modal, thermo-mechanical, crash loads, and heat leakage, to assess their viability and ability to meet essential functional requirements. To determine the most performative design(s), the concepts were evaluated and compared based on the following performance metrics: gravimetric efficiency, heat leakage, safety, and feasibility of manufacturing and assembly.

The analyses demonstrated that all four design concepts meet the essential functional requirements, providing a solid foundation for further development or potential prototyping. While the focus was on a vessel designed for a small aircraft, the methodology and design insights are applicable to larger systems. Hence, this research contributes to advancing cryogenic storage solutions for hydrogen-powered aviation.

This thesis explores the potential of Type IV conformable compressed hydrogen pressure vessels to enhance the efficiency and performance of Urban Air Mobility (UAM) vehicles, specifically focusing on the Varuna EVTOL. The motivation for this research stems from the pressing need for sustainable and environmentally friendly propulsion systems in the aerospace industry. Hydrogen, with its high energy density and zero carbon emissions, presents a promising solution, but its storage poses significant challenges, particularly in weight-sensitive applications like UAM.

Growing concerns about the environmental impact of aviation have sparked interest in hydrogen aircraft as a greener alternative. Hydrogen can be used to power existing turbofan engines or electrical motors via a fuel cell, eliminating carbon emissions not only during flight, but also during production, provided renewable energy sources are used. However, adopting hydrogen as fuel introduces technological challenges, particularly with regard to on-board storage. Integral tanks, which are part of the aircraft's main structure, seem promising but existing designs show limitations in their integration with the airframe and insulation capabilities.

To address these issues, this study proposes an integral tank concept featuring a double wall architecture with vacuum insulation. The main advantage of this design is the use of an external stiffened wall that can be directly connected to the remaining airframe. In addition, having stiffeners on the outside ensures the required space for systems routing and addresses concerns with the crash worthiness of the structure. A parametric method, coupled with finite element analysis is developed to size the external load bearing wall, enabling quick analysis and mass estimations of different tank configurations. The method consists of a sizing optimization with the objective of minimizing the structural mass under constraints on the strength, buckling stability and fatigue behaviour.

The feasibility of the concept is then evaluated on an aft tank for a short/medium range aircraft in configurations with and without a forward tank. Preliminary results under this realistic scenario point to fuel containment efficiencies of up to 0.71, which are consistent with existing designs. Moreover, buckling stability is identified as the critical design criterion, highlighting the importance of using a stiffened shell design. These findings show the viability of the proposed concept from a structural standpoint and provide the basis for further research. The optimum solution at an aircraft level can be obtained by integrating the developed framework into a multidisciplinary aircraft design tool.

Over the last decades, wind turbine blades have continuously grown in size to harvest further power from the wind. Longer blades have an increased structural mass and thus suffer from increased gravitational and inertial loads. Current developments in the field of wind turbine blade design strive for lightweight, high-bending stiffness, yet cost-effective structures. An approach to drive down mass is through the combined optimisation of aerodynamic and structural properties of a blade, which is commonly referred to as aeroelastic tailoring. Current structural optimisation strategies of wind turbine blades could benefit from existing aerospace structural tailoring approaches, such as straight-fibre variable stiffness design. Straight-fibre variable stiffness, also referred to as multi-patch laminate blending, consists in partitioning a structure into constant laminate regions that are locally optimised. Those regions are then blended to bring back continuity, thus ensuring structural integrity and manufacturability. Locally optimising regions allows the stiffness to be tailored to the specific needs of each region and allows the definition of load paths within the laminate. Laminate blending enables further structural tailoring through variable stiffness composite design while preferring the use of conventional laminate patches over more costly continuous fibre angle variation.

The present research aims to investigate the structural performance potential for straight-fibre variable stiffness laminates in wind turbine blade design. This research objective was tackled in two phases. First, a design methodology is proposed to couple a wind turbine aeroelastic optimisation framework with a laminate blending design algorithm. Second, the proposed design framework was applied to a blade section to evaluate the achievable structural performance when straight-fibre variable stiffness laminates are introduced in large-scale structures. The design framework for straight-fibre variable stiffness design of a blade section first optimises the lamination parameter distribution for each laminate region of the design. Then a conversion of the lamination parameters design to a stacking sequence is performed, followed by the enforcement of laminate blending constraints.

To quantify the performance of the blending approach, the DTU 10MW reference turbine blade was selected as the baseline structure. Specifically, a section of this blade was assessed, taken at the maximum chord location. At this site, the largest trailing-edge panels are present, and structural requirements such as buckling are critical. The objective of the laminate blending optimisation was extended to maximise the buckling performance of the considered section while matching a tip deflection requirement. When refining the number of laminate regions present on the trailing-edge panels, improvements in buckling performance up to 68% are achieved, with a limited increase in tip deflection of 4.8%. Overall, the methodology presented in this research highlights the potential improvements achieved with straight-fibre variable stiffness laminates when applied to a blade section. Further research is recommended in refining the modelling of a blade structure, namely to define sandwich materials and shear webs. Even though a methodology to evaluate the aeroelastic response of a full blade with variable stiffness structures is presented, this methodology was not assessed. The application of this approach to a full blade design could highlight the tailoring potential of stiffness variation and load redistribution enabled through laminate blending for aeroelastic applications.

Fibre-reinforced laminated composites are constructed layer-by-layer, allowing their material properties to be easily tailored relative to their isotropic counterparts (metallic alloys). Moreover, a composite structure with variable stiffness (VS) can be made by spatially tailoring stiffness across a laminate. This permits better load distribution within a structure, efficiently using a given material, and allow for lighter construction. Typically, the design of VS structures follows a bi-level procedure: first, the structure’s stiffness distribution is optimised using lamination parameters (LPs), and second, the stacking sequences (SS) of differently oriented fibres are designed to achieve the desired stiffness distribution. However, the designs envisioned using LPs are only sometimes exactly matched by the designed SS. The shortcomings faced during this conversion are called the ’Inverse Problem’. Upon reviewing the existing techniques in literature to handle the inverse problem, it was understood that converting LPs into SS is challenging without incurring substantial computational costs or imposing significant restrictions on allowable fibre orientations in the design. Such restrictions tend to under-utilise the directional properties of the fibres. Thus, there was a need to explore and develop better stiffness design methods for laminated composites by bridging the gap between LPs and SS in a computationally efficient way.

In response to this challenge, a hierarchical design framework was proposed to handle the Inverse Problem. Diverging from conventional methods that directly design the SS and try to match a given set of LPs, this framework divides the problem into two distinct stages. Initially, the focus is to use the In-Plane LPs and determine the number of layers in each orientation within a laminate, also called the Fibre Angle Distribution (FAD). Subsequently, the FAD serves as an interim solution, and the SS can be designed by using the Out-of-Plane LPs along with it. This problem partitioning enhances computational efficiency and offers potential benefits in solving the Inverse Problem more effectively. Given the time constraints inherent to a master thesis, the primary undertaking of this study was to efficiently design the FAD while accommodating a wide range of possible fibre orientations. To address this, a novel method using the Fast-Fourier Transform (FFT) was developed to facilitate FAD design with ply angle multiples of 15° (or [∆15° ] = [0, ±15, ±30, ±45, ±60, ±75, 90]). Moreover, empirical guidelines for SS design, such as the Symmetry and Balancing rule, were incorporated into the design step.

The primary contribution of this report is introducing an FFT-based method to design FADs, a novel addition to the existing body of research. Upon extensive testing, it was shown that this implementation could convert LPs into multiple unique FAD solutions in less than 0.3 seconds on a regular office laptop, outperforming other methods in literature by at least ten times. Furthermore, the implementation was also used to demonstrate the benefits of designing laminated composites using [∆15°] over the conventional [∆45°] orientations (or [0, ±45, 90]). In light of the positive results, it is pointed out that the In-Plane LPs (and consequently the In-Plane stiffness) are known to be sensitive only to the FAD and not their SS. As such, the readers of this thesis are presented with a very computationally efficient approach for designing laminated composites for In-Plane Stiffness...

During this research, a new approach to predict impact damage was evaluated using the PAL-V Liberty’s propeller blade as the test subject. The purpose of this research was to provide a simple method for impact damage prediction which will support the certification activities of the PAL-V propeller.
This method involves conducting a threat assessment, a Probability of Detection analysis, executing ASTM D7136 impact and quasi-static tests on coupon samples and conducting quasi-static tests on the propeller blade itself. The results from these tests were used to create a two-mass model which can predict the contact force during a propeller impact event. This model was validated using impact test on the propeller blade.
The study conducted a series of ASTM D7136 impact tests and quasi-static tests on coupons, revealing that the coupons exhibited greater load-bearing capacity under impact loading compared to quasi-static loading. These tests also showed that the coupons have an increased stiffness when subjected to impact loading, requiring a multiplication factor of 1.47 to align their stiffness with quasi-static conditions.
The research also involved determining boundary conditions for propeller tests, which led to the development of a custom clamping mechanism. During the quasi-static tests, the differences in stiffness between coupon and propeller tests were identified. These coefficients served as the basis for a predictive
model involving a two-mass, spring and damper system, with parameters derived from coupon tests and the propeller quasi-static tests. After performing the validation impact tests on the propeller blade, it was determined that this model successfully predicted contact forces within 13% of actual test values.
The research demonstrated the potential to predict impact energy requirements for barely visible impact damage, without the use of FEA. The model’s accuracy, while it can be improved, is sufficient to predict impact events effectively, offering a simpler alternative to complex FEA models.

The automotive industry has been in an increasing drive toward finding more sustainable transportation alternatives to abandon the era of Internal Combustion Engines (ICE). This was manifested in the large market share of Battery Electric Vehicles (BEVs) that have been established in recent years. As BEVs are not an efficient option in long-haul transportation, Fuel-Cell Electric Vehicles (FCEVs) have presented themselves as a prominent choice. To maximize the attainable driving range, gaseous hydrogen is compressed to 70 MPa per recent regulations. As a result, hydrogen storage pressure vessels need to provide high structural integrity, whilst being as lightweight as possible. This has led to the introduction of Carbon Fiber Reinforced Polymer (CFRP), where type IV Composite Pressure Vessels (CPVs) with CFRP overwrap and plastic liner are gaining popularity. The mass implementation of FCEVs is however directly dependent on the cost and reliability of CPVs. Increasing the reliability levels inherently lead to minimizing the materials used inducing cost reduction possibilities, and presenting FCEVs as a more feasible solution. To attain such high levels of reliability, a thorough understanding of the manufacturing process of CPVs is required. This research project aims at contributing to an increased level of understanding of CPVs by investigating the influence manufacturing parameters have on the final product quality. This study is done in collaboration with Plastic Omnium, which allowed for the possibility of obtaining experimental data for validation purposes. The approach followed in this research project is by developing an analytical model that is capable of representing the manufacturing process of CPVs. The objective is to be able to link the manufacturing parameters to the vessel’s quality for a given vessel configuration. The analytical model takes the vessel configuration as an input, alongside the definition of the considered manufactured parameters, winding tension force and internal liner pressure, and outputs quantitative predictions of changes in dimensions, compaction and burst performance. The model was found on the basis of adopting a common analysis method for composite materials, Classical Laminate Theory (CLT) and adapting it to be capable of representing the manufacturing process. A major challenge in developing such a model is dealing with uncured composite materials, where different assumptions were imposed for the model to be physically sound. Hypotheses were generated using the analytical model for eight different vessel configurations, where either the vessel configuration was changed or the magnitude of manufacturing parameters. The proposed experimental campaign consists of microscope inspection and burst testing of 22 sub-scale vessels in total, where the obtained data were used to prove or disprove the hypotheses. The analytical model and experimental results showcased the significant influence varying tension force and internal pressure have on the end-product quality where at different parameter settings, differences up to 4 % in Fiber Volume Fraction (FVF), 4.5 % in porosity content and 8 % in burst performance were observed with respect to a baseline configuration. Changes in vessel configuration, by varying stacking sequence grouping or liner diameter, led to notable differences in compaction and burst performance. The work presented in this report highlights the significance manufacturing parameters have on the scatter and reliability of CPVs and showcases the possibility of improving mechanical performance for a fixed design. Additionally, a framework is proposed in this work to help in defining the optimum process parameters for different vessel configurations. This work serves as a basis for future investigations into further optimizing the process parameters definition as it has been shown that there is still room for further improvements.

The development of novel materials has been of great interest to the aerospace industry in the pursuit of efficiency and cost reduction. Fibre-steered variable stiffness laminates have promising characteristics due to the ability to better tailor designs for expected loading, as the fibre orientation can be varied within a layer. This thesis was created to develop a fibre placement planning algorithm for fibre-steered layers on moderately curved open shells for a given fibre angle distribution.

The fibre-steered layers are to be produced by Automated Fibre Placement (AFP) without any course overlaps, which means that several manufacturing defects have to be considered. Gaps between courses are inevitable due to course shifting in combination with the prescribed fibre paths, and tow cuts run the risk of tow straightening due to loss of steering control of the AFP roller head. Furthermore, steering limits have to be observed to prevent tow buckling.

The thesis objective was approached by dividing it in three parts. Firstly, the geometry of the mould and the course paths was defined parametrically in the form of NURBS surfaces and curves. These have been widely used in Computer Aided Modelling (CAM) and have extensive support in terms of existing algorithms and documented work. Additionally, performing most operations in the parametric space allows for greater accuracy compared to general finite element methods.

Secondly, the fibre angle distribution was converted into a fibre layer. This was initiated by generating initial course centre lines, which were then fit onto the parametric surface. Representations of tow courses were made by geodesically offsetting the course centre lines in the binormal direction. A curve-curve intersection algorithm was then used to accurately detect tow overlaps, which were handled according to the tow placement strategy. Two tow placement strategies have been developed, one for general use and one to specifically prevent tow straightening. In addition, curvature of the course centre line was registered to check for tow buckling, and the minimum cut length was enforced by evaluating the length of tow segments.

Lastly, fitness criteria for fibre-steered layers were defined. Layers were evaluated for gap percentage, fibre angle error and number of cuts. These can be used to objectively grade layers against each other, or to assess the effects of fibre steering against conventional laminates.

Reducing the emissions of ultra-fine particles in the vicinity of airports to minimise detrimental health effects for near-airport residents and airport personnel has been the main objective of the project. This report details the realisation of this goal which was done by designing a hybrid aircraft with notable improvements in the efficiency and emission characteristics. The aircraft in question is named the ’Low Emission Alternative Fuel’ aircraft, or ’LEAF’. Design of essential components and systems is presented along with evaluation of aspects needed for LEAF aircraft to enter the market by 2035.

This project aims at developing a model to predict the damage initiation and propagation in a cylindrical filament-wound cord-rubber structure under internal pressurization using non-linear FEA, and is conducted in cooperation with TANIQ BV, Netherlands.
Cord-reinforced rubber composites are used in several safety-critical industries such as oil and gas and civil plumbing. Despite their widespread use, limited research is available that focuses on the single-cycle damage phenomenon that may occur in events such as over-pressurization.

The aim of this project is to close this knowledge gap by developing a theory that accounts for the key damage modes present in CRC structures. This involves experimental studies for material characterization and identification of relevant damage modes, the creation of a novel fibre overlap model that accurately replicates the meso-level filament-wound structure, and translation of the experimentally verified damage modes into functional damage initiation and propagation laws using a global-local FEA model. Verification of the created damage model is done experimentally on samples manufactured and tested at TANIQ, with differences between model predictions and experimental burst pressures being $\approx 6.5\%$. This is a marked improvement over Taniq's current FEA model, which overpredicts the solution by $\approx 27\%$.

The successful implementation of this model would help industries like TANIQ build efficient, strong, and lightweight rubber composite parts for various industries, thus adapting aerospace design principles to the development of more commonplace apparatus.

The recent drive towards carbon-neutral transportation has led the automotive industry to the development of fuel cell electric vehicles (FCEVs). These are especially convenient in the trucking industry, where the required long ranges and the slow recharging capabilities of battery electric vehicles (BEVs) make this alternative not suitable at this time. A crucial component of any fuel cell system is the hydrogen storage solution. This is most often a type IV composite pressure vessel (CPV), where gaseous hydrogen is compressed and stored at 70 MPa. CPVs are sophisticated structural components and the full understanding of their mechanical behaviour is yet to be achieved. This study presents a modelling framework for the prediction of the mechanical response of CPVs during pressurization. The framework begins with an analytical adjustment of the vessel geometry and material composition to match the properties of the filament-wound tank. Then, the analysis is solved numerically. The model takes into account damage progression to simulate the vessel’s deformational behavior and estimate the burst pressure. The modelling strategy is highly replicable, it reduces significantly the computational time of the analysis with respect to the previous benchmark, and it is able to predict burst with reasonable accuracy both in the cylinder and in the dome region of the vessel. The numerical results are correlated to the experimental data collected during burst testing at cellcentric GmbH in Stuttgart, Germany.

Straight fiber variable stiffness laminates use multiple overlapping patches to tailor the fiber orientations in particular regions of the laminate. A novel, automated framework for the design of manufacturable laminate has been developed using different stages. In the first and second stage an optimal stiffness distribution is determined using lamination parameters and converted to stacking sequences on a per region basis. In the third and fourth stages cellular automata and network theory are used to introduce cohesion between regions and comply with current manufacturing standards. Plates in uniaxial compression and cylinders in bending show an increase in buckling performance of 42% and 30% respectively. Such improvements do allow weight savings between 7-11% for these types of structures. When the additional cost of manufacturing and the cost-to-weight ratio for airlines are considered, these laminates already have the potential to significantly reduce the cost of air travel.

Due to an ever-growing demand for sustainable transportation, there is an increased need for the development of hydrogen as an energy carrier. Based on mass, hydrogen contains about three times as much energy as conventional gasoline. However, the volumetric energy density, which is the amount of energy per volume unity, is much lower for hydrogen. Therefore, it needs to be stored either at cryogenic temperatures or at hyperbaric pressure. The most common nominal working pressures (NWP) for hydrogen are either 350 or 700 bar. Because of these excessive pressures, the composite pressure vessels (CPVs) require to become thick-walled. This makes the out-of-plane stress components significant which requires calculation models that consider these effects.

This thesis evaluates, whether the use of tow-preg material, could reduce the amount of material necessary to attain a predefined burst pressure. Currently, the production of CPVs is mainly executed using wet winding. The use of this process results in varying material properties due to differences in fibre volume fraction. Also, the friction coefficient is minimal because the resin has a low tackiness. This friction is required for the production of winding paths, which deviate from the traditional geodesic angle. Alternatively, tow-preg material introduces constant material properties and possibly an elevated friction coefficient. This friction coefficient provides an opportunity to vary the winding angle through the thickness, by which the material strength can be made more uniform in every lamina. However, the challenge during the design and production of CPVs is that several effects are present which alter the mechanical response such as varying stiffness on the dome-cylinder interface, non-geodesic winding angle on the dome and resin-rich areas. This thesis will attempt to describe the mechanical behaviour of the CPV, by including several phenomena in numerical models and to minimise the material usage.

The combination of a relatively recent emissions scandal and the ever growing incentive to provide sustainable transportation solutions, led to the fact that the transition to electrically propelled land-based vehicles is more prominent than ever. All prominent European car-makers announced diverse fully and partially electric product portfolios for 2021 and beyond with strong indication that the era of the Internal Combustion Engine (ICE) is slowly fading into history. While Battery Electric Vehicles (BEVs) are already well on their way in establishing a strong market share in the personal vehicle domain, the field of long haul transport is left to search for solutions elsewhere as battery technology does not present an efficient solution at this time. Fuel-Cell Electric Vehicles (FCEVs) pose a solid performance base to close the gap that batteries cannot
and provide sustainable, zero emissions solutions, for all heavy transport sectors - automotive, aviation and maritime. Within fuel cell systems, a component whose criticality is often overlooked or even understated is the hydrogen storage tank. Current state of the art uses type IV composite pressure vessels (CPVs) with a plastic liner and Carbon Fiber Reinforced Plastics (CFRP) composite overwrap to ensure structural integrity. Recognizing the complexity of a CPV is crucial in order to allow for the development of robust and worthwhile
design processes that could yield a significant decrease in material use, making FCEVs more cost effective and an attractive long-term solution. This thesis aims to provide some detail on the behavior of CPVs and their depth through the development of an automated numerical analysis framework and the execution of a focused experimental study that examines the impact of laminate stacking sequence as well as manufacturability factors on overall CPV behavior and, in particular the cylinder-dome transition of the vessels. The details and experimental correlation of the developed numerical analysis framework are shown and discussed. The framework heavily utilizes the outputs of an industry standard filament winding software, Composicad, which is used as a basis that is then further refined in order to generate a satisfactory geometric description of the vessel and its laminate composition. Major emphasis is placed on the discussion of processes made for the geometry correction and attention is placed on particular factors that were also tested in the experimental study - hoop ply drop-off and stacking sequence. The framework is correlated to experimental results and laminate configurations from previous and current experimental results to showcase its versatility and potential for future optimization studies given the high level of automation involved. Two Finite Element (FE) models are investigated within the scope of the developed framework - a solid and shell element model. The performance and predictive ability for each are presented and discussed. The experimental study presents data on the impact of stacking sequence effects and hoop ply drop-off
on CPV behavior - in particular, the cylinder-dome transition. The influence of both parameters are discussed and contextualized with previous studies in order to identify relevant trends. Novel results were obtained for the relationship between hoop layer placement and burst performance. The results further indicate a delicate interaction between vessel stacking sequence, burst strength and manufacturing robustness. The results of this study highlight important factors for future numerical studies of CPVs and showcase the development of an analysis approach with low computational cost while retaining acceptable accuracy. Furthermore, the experimental study helps put focus on future areas in need for further investigations in order to identify design-critical performance trends.

Weight reduction in vehicles offers many advantages. Especially in aircraft, one of the advantages of weight reduction less need for power which is translated into less fuel consumption and lower emissions. Composite materials are now a popular construction material in new passenger aircraft, making them lighter by taking advantage of their high specific properties. However, composite materials can offer even greater specific properties and as a result lighter structures, by utilizing composite laminate blending. Laminate blending is the local optimization of laminates while taking into account the continuity, structural integrity and manufacturability guidelines. The main project started by developing an algorithm that blends laminates with a large number of sections and then a blended laminate with 25 sections was manufactured using the algorithm mentioned above. The manufactured laminate showed improved stiffened driven buckling performance and out of plane displacement imperfections due to thermal stresses in the autoclave caused by the stiffness mismatch in the ply drop areas of the laminate.

This thesis project has to do with the investigation of the mechanical behavior in blended laminates under buckling loading, by building FEM models that accurately simulate the stresses and out of plane displacement as well as the thermal effects taking place during the cooling down in the autoclave. A global-local modelling technique is used to model the stresses, where the local ply drop section FEM models are simulated by 3D elements and the global laminate model by 2D elements. As far as the out of plane displacement is concerned, an algorithm that calculates the stiffnesses of the ply drop sections in order to transfer them back into the global model is built, that works for any blended laminate. Finally, a 2D thermal FEM model is built that simulates the imperfections of a blended laminate due the thermal stresses in the autoclave caused by the stiffness mismatch in the ply drop areas of the laminate. All in all, the results show that the stress and out of plane displacement results of the FEM models built in this thesis project can in many cases be substantially different from simple FEM models that do not take into account the ply drop sections. Furthermore, the thermal FEM model showed that imperfections due to thermal stresses can be accurately simulated.

Type IV composite pressure vessels (CPVs) are used commercially for the gaseous storage of hydrogen in fuel cell electric vehicles (FCEVs). However, their economic implementation requires material optimization and a reliable prediction of the vessel strength. In this regard, their burst when loaded under internal pressure is impacted by the combined effect of the stacking sequence design and the variability of mechanical properties resulting from the manufacturing process. This work shows a framework for analysis that accounts for some of these manufacturing-induced characteristics in the mechanical response, namely the relation between the vessel stacking sequence, its final geometry, and the material properties. Furthermore, vessel burst pressures are alternatively estimated from the failure criteria evaluation in constitutively elastic analyses and the modeling of damage progression. Predictions are reasonably accurate when the collapse occurs in the cylinder (+2.2 %), although a more considerable discrepancy exists with experimental results when vessels fail in the dome transition region (+12.3 %).

Fuel cell electric vehicles can provide the possibility to meet CO₂ emission reduction targets, but these vehicles require cost effective energy storage solutions. The current most mature technology uses compressed hydrogen, stored in composite pressure vessels (CPVs) manufactured by filament winding. Structural optimization of CPVs is important to meet both safety and cost requirements. This research aims to contribute to future CPV optimization strategies by investigating the effects of stacking sequence on the burst pressure and strain response. Two different winding angles were considered, related to a helical type winding and circumferential type winding, which were varied in sequence in terms of positioning and grouping. The manufacturing parameters were kept constant for all considered stacking sequences and were applied to a sub-scale pressure vessel geometry. Detailed experimental characterization revealed differences in burst pressure and strain measured with full-field digital image correlation with predefined deformation parameters. Analytical and finite element methods were used to capture related mechanical effects. The strain and burst pressure results from both approaches were correlated to conclude that the order of a sequence affects the structural performance, causing differences in CPV burst pressure of at most 49%. The results show that specific sequence choices can lead to higher burst pressures.

To achieve CO2 emissions reductions, the automotive industries are moving towards more sustainable solutions. One of those are fuel cell electric vehicles (FCEVs) which rely on the chemical reaction of hydrogen to produce electricity. The hydrogen is stored in a gaseous and compressed form in composite pressure vessels (CPVs) which must be subjected to numerous tests for the certification. The end-of-line (EOL) test is a pressurization up to 105 MPa, compulsory for each tank before in-service life. This test could be used to quality assure the component and to verify the state of the CPV. Knowledge regarding damage formation and progression during a CPV pressurization must be obtained for the purpose. The goal of the research is to study which damage mechanisms take place and how their characteristics change when analyzing different layups. This is done pressurizing vessels with different stacking sequences in a specially designed testing chamber. During the test, optic and acoustic data are recorded. The acoustic emissions of the CPVs are detected using 120 sound pressure sensors and processed using a delay-and-sum beamforming algorithm. The optic data are analyzed with digital image correlation (DIC). The results show that only one damage mechanism takes place in the investigated pressure range (5-105 MPa). This has been identified as interfiber-fracture (IFF), happening both in the hoops and helical layers. Computer tomography scans have been used to validate the results. It is discovered that the grouping and the positioning of the hoop and helical layers has a significant influence on the acoustic behavior of the vessel. All the specimens of a layup follow a specific acoustic pattern: they have similar characteristics that can be used to associate a specimen to an analyzed stacking sequence. It has been shown that DIC has limited suitability regarding IFF detection in the hoop layers. It can identify IFF only if the hoops are the outermost plies of the laminate. However, it seems promising to predict the formation of other damage mechanisms such as IFF in the superficial helicals and delaminations. The findings contribute to a deeper understanding of the damage mechanisms taking place during an EOL pressurization. They also clarify the influence of the stacking sequence on the damage characteristics.

The use of fiber-reinforced composites in the aerospace industry has increased over the past couple of decades thanks to their high specific properties. Fiber-reinforced composites also enable engineers to tailor the stacking sequence to satisfy the required stiffness or strength requirements. However, locally optimizing the stacking sequence of composite laminates introduces incompatibilities between the locally optimized sections which jeopardize the structural integrity of the panel. This highlights the importance of “laminate blending” which is a term given to problems that involve the local optimization of laminates while taking into account continuity guidelines to ensure manufacturability. In this work, a constant thickness blended demonstrator of 32 layers and 5x5 sections was designed, manufactured and tested for its stiffness and critical buckling load. The results were compared to a constant stiffness conventional laminate of the same geometry and weight. The critical buckling load of the blending demonstrator was 80.9% higher than that of its conventional counterpart and the stiffness was 93.75% higher.