In 2022 Airbus and Boeing combined delivered 1203 commercial aircraft. With an annual predicted growth of 4.3% for the coming 20 years there is an urgent need for end of life solutions that go beyond down-cycling of parts that cannot be reused. Especially carbon fibre reinforced composites are hard to recycle, and attempts to deliver recyclable short fibre reinforced thermoplastic materials see a reduction in specific properties. This is a problem because, the life cycle impact of an aircraft part is predominantly determined by its weight, which drives cumulative CO ₂-emissions over its lifetime. The transition to renewable energy sources by the aviation sector has the potential to change this relationship drastically. Therefore, it is necessary to begin developing methods to account for life cycle impact already at the start of the mechanical design of an aircraft part. This study proposes to apply variable stiffness laminate design to compensate for relatively lower mechanical performance of a recyclable short fibre reinforced composite laminate, with the objective of an overal better life cycle performance of the composite part. This approach is demonstrated using the example of a rectangular plate under uniaxial compression on board of an ATR72. The results show the impact of weight-optimisation on the cumulative CO ₂-emissions for the life span of the aircraft. Two different energy sources are considered: the aircraft powered by conventional jet fuel, and the aircraft powered by hydrogen which has been generated using non-fossil energy sources. The results furthermore clearly show that moving from conventional to renewable energy sources, reduces the impact of part-weight on the accumulated CO ₂-emissions very significantly, bringing recycling considerations more into focus, especially for parts which require regular replacement during the lifespan of the aircraft, for example hydrogen storage tanks.

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This study aims to develop a model to predict the burst pressure of a dry filament wound cord-rubber composite pressure vessel under hydrostatic internal pressurization using a submodelling based global–local FEA model. The model links the global displacements of a rebar-based model to obtain the local deformation state in a single rhomboidal representative volume. Emphasis is placed on capturing the local stress concentrations in the fibers due to the unique filament winding mosaic pattern. Fiber damage is included in the local model using a maximum principle strain criteria. Verification of the created model is done experimentally on industrially manufactured burst-test specimens. Measurements for displacement during the experiments are taken photographically, while the burst pressure is captured using a pressure transducer. The final error between the burst pressure of the samples and the experimental demonstrators is approximately 6.5%, a marked improvement over conventional models with truss and rebar elements as fibers.

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Hydrogen is being investigated as aviation fuel, with the objective to achieve an energy transition for the aviation sector. Effective storage solutions are crucial to mitigate the aerodynamic penalty caused by its low volumetric energy density. The focus of this study is the integration of a cryo-compressed vacuum-insulated storage vessel into the primary structure of aircraft, aiming to enhance structural efficiency. This is achieved by implementing analytical methods to analyse the thermo-mechanical loading of the inner and outer walls of the fuel tank. It is envisioned that the inner wall rather than the outer wall is more suitable to sustain additional loads. However, it is unclear how the cryogenic environment affects the stress state of the composite material.

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Vacuum-insulated, all-composite hydrogen storage vessels are promising for achieving viable gravimetric and volumetric efficiencies in aviation applications. However, stress concentrations arise due to the connections between the composite shells and to the surrounding structure. This study develops analytical models which capture the stress state of composite cylinders subjected to pressure loading and discrete in-plane edge loads. The models allow for easy adjustments in tank geometry, lamination, and load introduction parameters, including the number and width of connection points. This aids in the preliminary structural and thermal analysis of composite hydrogen storage vessels, pushing the implementation of hydrogen as a sustainable fuel for aviation. @en

This Editorial mentioned a paper by Nishimoto et al [1], which was not available to be cited at the time. The paper has now been included in this issue and the reference for it is below. [1] M. Nishimoto, M. T. Kezirian, Safety requirements for Hyperloop transportation systems: Applying NASA human spaceflight safety practices, J. Space Saf. Eng. 10 (4) (2023) 397 – 406, doi: 10.1016/j.jsse.2022.02.009

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Significant improvements in the mechanical performance of a laminated composite structure can be achieved by spatial variation of the stiffness properties within the laminate. Such a variable stiffness (VS) laminate, typically is created by steering the fibers within the laminae, resulting in an in-plane variation of stacking sequence and thus an in-plane variation of mechanical properties of the laminate and subsequent load-redistribution within the laminate. Since the early work on VS laminates [1,2] more than 30 years have passed. In these years, significant improvements in the design, analysis and manufacturing of VS composite laminates have been made. Buckling load improvements in excess of 100% (compared to the best constant stiffness design) are reported for VS laminates [3], and there are numerous examples of case studies on (aerospace) structures [4]. Nevertheless, to the knowledge of the authors, there is still no widespread adoption of VS laminates in the aerospace engineering industry. This raises the question, why VS laminates are not being adopted, and what can be done to change this. Recent work done at the TU Delft has demonstrated that it is possible to achieve variable stiffness laminate designs by means of laminate blending and patching operations, instead of fiber steering. The result is a straight-fiber variable stiffness (SFVS) laminate, see Fig. 1. A computationally efficient design method has been developed for these laminates. Preliminary results show similar improvements in buckling load as have been reported in literature for fiber-steered variable stiffness laminates. The advantage of the proposed design method is that variable stiffness laminates could be achieved by either straight-forward tape-placement machines or pick-and-place operations of pre-cut patches of fabric. In this work the authors have investigated whether this novel approach to variable stiffness laminate design can help the adoption of variable stiffness laminates in aerospace industry. @en

Type IV composite pressure vessels represent the current state-of-the-art for compressed gaseous hydrogen storage in fuel cell electric vehicles. A combination of highly demanding safety regulations and the need for cost competitive solutions make the topic of CPVs particularly challenging. Given the elevated material price of carbon fiber, structural optimality is essential to meet both requirements. Thorough understanding of design parameters and mechanical performance of composite pressure vessels is prerequisite to structural optimality. In this paper we investigate the relation of stacking sequence and circumferential ply drop locations on the mechanical performance of type IV composite pressure vessels subjected with internal pressure. This paper builds on previous studies by the authors, which are enhanced by new numerical and experimental results. An experimental set is used, where for a given layup composition the stacking sequence and the circumferential ply drop locations are varied. The experimental results are complemented by a computationally efficient numerical framework, which is composed by the output of a commercial filament winding software, a self-developed geometry correction algorithm and an automated FE model generation program. The numerical results are compared with outer surface strains obtained by means of three-dimensional digital image correlation, the final burst pressures and the vessel remainders. The achieved burst pressures vary between 152.6 and 188.6MPa, depending on design configuration. For the layup composition in this investigation, the placement of tangentially reinforcing layers (e.g. circumferential and high-angle helical layers) as innermost layers led to overall higher cylinder strengths compared to sequences, where these layers were located as outermost. The retraction of circumferential ply drop locations was found to impact the burst performance differently in dependence of the stacking sequence. For sequences, where circumferential layers were located as outermost, a retraction of ply drop locations by 12mm showed barely any differences in burst pressure (-1.9%). For sequences, where these layers were located as innermost, a severe decrease (-19.1%) was noticed once the ply drop locations were retracted by up to 9mm. The results not only underlined the criticality of both design parameters and their interaction with each other, but also showcased a computationally efficient numerical framework capable of capturing distinct mechanical responses for a variety of layups at least trend-wise.

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The mechanical performance of laminated composite structures can be improved significantly by spatially varying the stiffness properties within the laminate, resulting in a so-called Variable Stiffness (VS) laminate. Typically, the stiffness variation in VS laminates is achieved by steering the fibres within the laminae, resulting in an in-plane variation of stacking sequence and thus an in-plane variation of mechanical properties of the laminate and subsequent load-redistribution within the laminate. The authors use a different approach to design VS composite cylinders, which does not rely on fibre steering. Instead, in-plane variation of the stacking sequence is achieved by stacking patches of unidirectional material or fabric, each of different in-plane dimension. Such a laminate can be designated as straightfibre variable stiffness (SFVS) composite laminate. A method developed at TU Delft to design SFVS laminates is modified to optimize the buckling load of composite cylinders. In this method, the laminate is divided into a discrete number of regions, for each of which the stacking sequence is optimised individually. Stacking sequence continuity is enforced by an algorithm inspired on cellular automata (CA) which relies on the application of a set of simple design rules. The modified method is applied to optimize the buckling load for composite cylinders. The buckling load of the optimised designs is evaluated using commercial finite element software. The effect of the number of design regions into which the cylinder is divided is considered. To conclude an outlook is given on how the modified method can be applied to the design of aerospace structures. @en

The industrialization of fuel cell electric vehicles demands cost efficient storage solutions for hydrogen. While gaseous storage in type IV pressure vessels is currently the most mature technology, further structure optimization needs to be undertaken in order to meet cost requirements. This research investigates the effects of stacking sequence of composite pressure vessels regarding laminate quality, structural deformation and finally burst pressure. Therefore, a known laminate is studied on a subscale vessel geometry with changing stacking sequences. The specimens are pressurized in a specially designed chamber up to burst pressures of 166.11 MPa. Through a multisensor arrangement of stereometric systems, the deformation is tracked up to burst by using 3D digital image correlation. The experimental results show a difference of 67% in burst pressure between the investigated stacking sequences. Experimental cylinder strains and burst pressures are compared to results derived from 3D elasticity theory with implemented first ply failure criterion. Additionally, using X-ray computed tomography and acid digestion tests, insights about the distribution of fiber volume fraction and porosity are provided. For the investigated sequences in this research, the results show the considerable influence of stacking sequence on the laminate quality, the structural deformation and finally the burst pressure of composite pressure vessels. Moreover, it is shown that while the used 3D elasticity approach proved to be a useful tool for the prediction of strains and failure in the cylindrical section, discrepancies between prediction and experiment can arise based on preliminary failure occuring at the cylinder-dome transition. The results therefore emphasize the need for analytical and numerical analysis strategies to consider transition-related effects between cylinder and dome.

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Stacking sequence optimisation can be used to increase the strength or stiffness of a composite laminate, or to reduce its weight subject to a strength or stiffness constraint. Optimisation of larger composite structures consisting of multiple panels may result in stacking sequences of adjacent panels that are incompatible with one another. The act of enforcing stacking sequence continuity to ensure structural integrity and manufacturability of a laminated composite laminate is known as blending. This term was first introduced by Zabinsky (1994). In literature, many methods can be found to implement structural continuity by means of stacking sequence blending in one way or another. The complexity of the problem makes the blending of a structure with a large number of adjacent design regions, and thus stacking sequences, prohibitive. This work introduces a computationally efficient method for stacking sequence blending of composite laminates. The presented method is inspired by cellular automata (CA) and relies on the application of a set of simple rules to solve the blending problem. The presented method is demonstrated using the benchmark 18-panel horseshoe blending problem, Soremekun et al. (2002). Each panel is initialized using a genetic algorithm (GA). The result is fed into the CA-scheme. The obtained results are equal to or better than those reported in the literature and were obtained requiring very little operations. This can be attributed to the increased design space of the presented method compared to literature. The computational efficiency makes the presented method especially interesting for composite structures with a large number of design regions. An outlook on the scalability of the presented method and its limits will be given. Soremekun, G. A., Gürdal, Z., Kassapoglou, C. and Toni, D. (2002), ‘Stacking sequence blending of multiple composite laminates using genetic algorithm’, Composite Structures 56(1), 53–62. Zabinsky, Z. B. (1994), ‘Global optimization for composite structural design’, Monthly technical progress report, under contracts NAS1-18889 (report No. 58) and NAS1-20013, task 2 (report 4). @en

There are many process parameters that are important in automated composite manufacturing, starting with the handling of fabrics in the preforming process. Characterization of fabrics on their preforming qualities – which will be referred to as drapability in this paper – is necessary to tune the manufacturing process. The location where the fabric is held, the amount and direction of tension applied to the fabric, the order in which it is deformed, all play a major role in how much in-plane shear deformation a fabric can undergo. The inclusion of process parameters in the characterization of fabrics on drapability is therefore important. In this paper a method is developed to determine the critical shear angle of a fabric whilst including process parameters. A family of geometric shapes is derived and a testing procedure to determine the critical shear angle of a fabric using these shapes is proposed. It is discussed how the presented process can be used to tune draping and stamp forming processes in automated composite manufacturing.@en

Large classroom sizes are a reality university educators need to contend with, particularly in the first year of a given cohort within a degree programme. Activating and engaging students in these large classroom environments present numerous sets of challenges. These challenges are exacerbated by student learning development needs in the early stages of the degree programme. In their first year, students are still adapting to a new learning environment and are developing new study skills and practices. Early success and failure in courses will shape intrinsic and extrinsic factors that will motivate the student in the remainder of their degree. Thus the perceived challenge of activating large classrooms early in a degree programme goes beyond simple engagement; beneath the core learning objectives of the course are implicit learning objective about developing effective motivation and study skills. This paper examines the efforts to reorganize a first year Mechanics of Materials course taught in the Bachelor of Engineering Programme within the Faculty of Aerospace Engineering at Delft University of Technology University to address this need using a Blended Learning approach.@en

Mechanical properties of fiber-reinforced laminated composite materials are direction-ally dependent. Contemporary laminated composite design aims to make effective use of these directional properties by means of stacking sequence design, selecting the fiber orientation angle of each ply from a predefined set. Automated fiber-placement (AFP) technology can be used to improve the efficacy of composite materials by means of fiber steering. The variation of fiber orientation angles per ply of the laminate yields a variable stiffness (VS) laminate. For optimization purposes it is attractive to design such laminates in terms of lamination parameters (LP), as the number of design variables per point in the structure can be reduced to as little as four dimensionless variables considering balanced symmetric layups, and because many lay-up optimization problems can be made convex by describing them in terms of LPs. VS laminate design in terms of LP requires the obtained LP distribution to be converted into an actual fiber angle design. In a previous study the authors proposed a method to convert VS laminate designs using LPs into fiber angle designs. This method includes a constraint on in-plane curvature, a manufacturing constraint related to AFP. Thickness build-up will occur due to fiber steering. The amount of thickness build-up that results from the obtained fiber angle designs is discussed here as a function of the constraint on fiber curvature. The streamline analogy is used to obtain an estimate for thickness build-up and to determine fiber paths. A square plate loaded in biaxial compression is used to demonstrate the effect of the in-plane curvature constraint on thickness build-up, and several fiber angle designs, thickness distributions and fiber paths are given for this structure.

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