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In addition to the well-known threats of bird and hail strikes, small unmanned aerial vehicles (sUAV) pose a new threat to manned aviation. Determining the severity of collisions between sUAVs and aircraft structures is essential for the safe use and integration of drones in airspace. A generic analytical calculation model needs to be developed to supplement the existing test and simulation data. This paper presents an analytic model for drone collisions with perpendicular and inclined targets. The targets have a rigid or elastic material behavior. The aircraft impact model, which is used for the design of nuclear reactor structures, is transferred and adjusted for sUAV impacts to calculate the impact force. A mass- and a burst load distribution are needed as input parameters. Both distributions are determined for an sUAV design depending on the flight direction. Compared to previous calculations, the new approach is to consider a moving target structure, which produces more realistic results. We compare the calculation results with simulation data from sUAV collisions with a commercial airliner windshield from the literature. The calculations show plausible results and a good agreement with literature data. Subsequently, the influence of the input parameters on the impact force is investigated. We see that spring stiffness, target mass, burst load distribution and damping have minor influence on the overall impact force. The impact velocity, mass distribution and flight orientation on the other hand have a major influence on the impact force. Further tests are needed to validate the impact model.

We present an integrable, sensor inlay for monitoring crack initiation and growth inside bondlines of structural carbon fiber reinforced plastic (CFRP) components. The sensing structures are sandwiched between crack stopping polyvinyliden fluoride (PVDF) and a thin reinforcing polyetherimide (PEI) layer. Good adhesion at all interfaces of the sensor system and to the CFRP material is crucial as weak bonds can counteract the desired crack stopping functionality. At the same time, the chosen reinforcing layer must withstand high strains, safely support the metallic measuring grids and possess outstanding fatigue strength. We show that this robust sensor system, which measures the strain at two successive fronts inside the bondline, allows to recognize cracks in the proximity of the inlay regardless of the mechanical loads. Feasibility is demonstrated by static load tests as well as cyclic long-term fatigue testing with up to 1,000,000 cycles. In addition to pure crack detection, crack distance estimation based on sensor signals is illustrated. The inlay integration process is developed with respect to industrial applicability. Thus, implementation of the proposed system will allow the potential of lightweight CFRP constructions to be better exploited by expanding the possibilities of structural adhesive bonding.

In order to obtain a widespread application of Additive Manufactured (AM) technology in the aircraft industry for fatigue critical parts, a detailed characterization of the Fatigue and Damage Tolerance (FDT) behavior of structural components is required. Metal AM techniques in particular are prone to internal defects inherently present due to the nature of the process, which have a detrimental effect on fatigue properties. In the present work, Ti6Al4V and Inconel 718 coupons with artificially induced defects of different dimensions were produced by the Laser Powder Bed Fusion (LPBF) technique. Fatigue tests were performed, and a different defect sensitiveness was observed between the two materials with Inconel being more defect tolerant compared to Titanium. The environmental role at the crack tip of internal defects was discussed, and based on a purely fracture mechanics approach, a simplified stress-life-defect size model was finally devised. The experimental test results together with the information obtained from the fracture surface analysis of tested samples are used to validate the model predictions. The proposed approach could be adopted to define a critical defect size map to be used for tailored Non-Destructive Testing (NDT) evaluation.

Fully laminar aircraft are one step towards reaching eco-efficient aviation. However, high system complexity and significant manufacturing effort prevent the wide usage of existing laminarisation concepts such as laminar flow control, which are rarely found in commercial aircraft. Hybrid laminar flow control concepts reduce the manufacturing effort significantly at the cost of only achieving partial laminar flow. This paper presents extended hybrid laminar flow control concepts for fully laminar wings, with reduced system complexity. A detailed study of structural and aerodynamic requirements provides the foundation for partial design solutions of active suction structures. The authors derive two concepts for active suction panels from the structural design space. While the first concept relies on state of the art manufacturing techniques, the focus of the second concept is on additive manufacturing technologies. Based on these concepts, it is feasible to design fully laminar wings with structurally integrated active suction systems. The authors propose an aerodynamic test strategy for further developing extended hybrid laminar flow control.

The use of pressure-actuated cellular structures (PACS) is an effective approach for the application of compliant mechanisms. Analogous to the model in nature, the Venus flytrap, they are made of discrete pressure-activated rows and can be deformed with high stiffness at a high deformation rate. In previous work, a new innovative approach in their integral textile-based manufacturing has been demonstrated based on the weaving technique. In this work, the theoretical and experimental work on the further development of PACS from simple single-row to double-row PACS with antagonistic deformation capability is presented. Supported by experimental investigations, the necessary adaptations in the design of the textile preform and the polymer composite design are presented and concretized. Based on the results of pre-simulations of the deformation capacity of the new PACS, their performance was evaluated, the results of which are presented.

Launch-vehicle primary structures like cylindrical shells are increasingly being built as monolithic composite and sandwich composite shells. These imperfection sensitive shells are subjected to axial compression due to the weight of the upper structural elements and tend to buckle under axial compression. In the case of composite shells the buckling load and imperfection sensitivity depend on the laminate stacking sequence. Within this paper multi-objective optimizations for the laminate stacking sequence of composite cylinder under axial compression are performed. The optimization is based on different geometric imperfection types and a brute force approach for three different ply angles . Decision tree-based machine learning is applied to derive general design recommendations which lead to maximum buckling load and a minimum imperfection sensitivity. The design recommendation are based on the relative membrane, bending, in-plane shear and twisting stiffnesses. Several optimal laminate stacking sequences are generated and compared with similar laminate configurations from literature. The results show that the design recommendations of this article lead to high-performance cylinders which outperform comparable composite shells considerably. The results of this article may be the basis for future lightweight design of sandwich and monolithic composite cylinders of modern launch-vehicle primary structures.

Thin-walled shells like cylinders, cones and spheres are primary structures in launch-vehicle systems. When subjected to axial loading, bending or external pressure, these thin-walled shells are prone to buckling. The corresponding critical load heavily depends on deviations from the ideal shell shape. In general, these deviations are defined as geometric imperfections, and although imperfections exhibit comparatively low amplitudes, they can significantly reduce the critical load. Considering the influence of geometric imperfections adequately into the design process of thin-walled shells poses major challenges for structural design. The most common procedure to take into account the influence of imperfections is based on classical buckling loads obtained by a linear analysis which are then corrected by a knockdown factor. The knockdown factor represents a statistical lower-bound with respect to data obtained experimentally for different types of thin-walled shells. This article presents a versatile and simple numerical design approach for buckling of critical shell structures. The new design procedure is based on the reduced stiffness method and leads to significantly improved critical load estimations in comparison to lower-bounds obtained empirically. An analysis example is given which is based on the launch-vehicle stage adapter (LVSA) of NASAs Space Launch-system (SLS).

Unavoidable deformations occur during part production due to the non-isotropic nature of carbon fibre reinforced plastics (CFRP). These deformations often lead to dissatisfaction of tolerances or result in cost and time intensive rework of the tooling. In a cost driven production environment, similar to the automotive industry, it is essential to predict the deformations early on in the part development process in order to compensate toolings accordingly. In future applications, Fast Curing Epoxy Resins (FCER), with curing times of less than 20 min, will play a key role in high rate CFRP-production at low cost. The present paper reports on a comprehensive experimental study on different FCER systems. It includes the thermo-chemical characterization of neat resin samples as well as the quantification of spring-in deformations of L-profiles. Essential part and processing parameters, as the lay-up, the curing temperature, the cooling rate and the fibre volume fraction are varied and their effect on process induced deformations is quantified. Results for FCER system are compared to slower curing systems to assess differences.

In der Automobilindustrie ist die Anwendung von monolithischen Profilstrukturen etabliert. Jedoch bietet der Einsatz von Hybridprofilen, die aus einem faserverstarkten Polymer (FVK) und einer duktilen Metalllegierung bestehen, eine im Vergleich vorteilhafte Kombination aus mechanischen Eigenschaften und resultierendem Bauteilgewicht. Um eine effiziente Fertigung und damit den industriellen Einsatz von Metall-FVK-Hybridprofilen anzustreben, werden in diesem Kapitel zwei intrinsische Fertigungsprozesse vorgestellt. Dabei ermoglichen die Prozesse einerseits die Verarbeitung einer thermoplastischen und andererseits einer duroplastischen Kunststoffmatrix. Neben der Entwicklung und Validierung dieser Produktionsprozesse werden, mit dem Ziel einen bestmoglichen Korrosionsschutz bei einer zugleich hohen Haftfestigkeit zu realisieren, Konzepte zur Gestaltung der Grenzschicht der artverschiedenen Werkstoffe betrachtet. Des Weiteren werden Methoden zur Simulation und zur mechanischen Prufung der gefertigten Hybridprofile aufgezeigt. Zusatzlich soll dabei auch auf die Messung und die gezielte Modifikation von Eigenspannungen, die sich infolge der Verarbeitung der verschiedenen Materialien einstellen, eingegangen werden.

Tori-spherical shells are often used as enclosures of pressure vessels in ocean and civil engineering. When subjected to external pressure, these thin-walled shells are prone to buckling. The corresponding critical buckling pressure heavily depends on deviations from the ideal shell shape, but also the yield strength as well as the knuckle radius. This article summarizes and analyzes all known experimental results for tori-spherical shells under external pressure. A detailed numerical elastic-plastic buckling analysis of a tori-spherical bulkhead is presented including details regarding non-linear material model, finite-element modeling and solver settings. In addition, a wide variety of empirical design and numerical geometric imperfection approaches for tori-spheres are presented, applied and validated. Among the geometric imperfection approaches are realistic measured geometric imperfections, dimple imperfections, flat patch imperfections and reduced stiffness methods. Large scale parametric imperfection amplitude analyzes are performed in order to determine which imperfection concept delivers in general the best fit to experimental results. New design factors for different tori-spherical shell geometry configurations are consequently developed and validated with experimental results. The new design factors lead to significantly improved critical load estimations in comparison to lower-bounds obtained empirically.

Der Einsatz von Leichtbautragstrukturen bietet heutzutage die Moglichkeit eine signifikante Gewichtsreduzierung zu realisieren. Bei der Gestaltung dieser Leichtbautragstrukturen mussen, je nach Anwendungsfall, eine Vielzahl von Anforderungen berucksichtigt werden. Die komplette Substitution eines Werkstoffes ist fur die konsequente Nutzung des Leichtbaupotentials nicht immer zielfuhrend. Eine optimale Gesamtstruktur besteht aus einer hybriden Werkstoffkombination, dem sogenannten Multi-Material-Design. Der Ansatz der Hybridisierung von Strukturkomponenten ruckt somit immer starker in den Vordergrund und kann grundsatzlich nach zwei unterschiedlichen Methoden erfolgen. Zum einen konnen zwei Bauteile aus Faserverbundkunststoff und Metall durch nachgeschaltete Fugeprozesse, wie beispielsweise Nieten, Schrauben oder Kleben, gefugt werden. Nachteil dieses Ansatzes ist neben dem Aufwand fur den Fugeprozess die zusatzliche Masse, durch die das Leichtbaupotential nicht vollkommen ausgeschopft werden kann. Zum anderen besteht die Moglichkeit der Herstellung des Hybridverbunds in einem einstufigen Prozess, wobei die Verbindung der verschiedenen Materialien im Ur- oder Umformprozess ohne einen nachfolgenden Fugeschritt erfolgt. Das entstehende Bauteil dieses einstufigen Prozesses wird als intrinsisches Hybrid bezeichnet. Aufgrund der intrinsischen Hybridisierung entstehen neue Gestaltungsmoglichkeiten und produktionstechnische Vorteile, aber auch Herausforderungen in Bezug auf die Prufung, Simulation und Herstellung. Im Rahmen des Schwerpunktprogramms 1712 der Deutschen Forschungsgemeinschaft wurde hierzu auf den Fachgebieten Produktionstechnik, Mechanik und Werkstoffwissenschaften intensiv Forschungsarbeit geleistet.

Based on the description of the objectives of active shape control, this chapter lays the foundation for modeling integrated continuous actuation by means of simple analytical considerations on beam models. At first isotropic materials are considered. The views are then extended to anisotropic materials on the basis of flat panels made of activatable fibre composites in order to demonstrate the possibilities of using deformation coupling. Finally, an insight into current research for active shape control is given.

Curved structures made of fibre-reinforced plastics tend to show an out-of-plane failure type, when loaded under bending. One method to determine the related tensile strength in thickness direction, is the standardised unfolding test related to ASTM D6415 with L-profile specimens. However, the standard shall be used for unidirectional materials only, which is not the case for multidirectional laminates. Therefore, this publication presents methods to determine the layer-wise stress and strain state as well as occurring failure types of multidirectional laminates. An extension of the Lekhnitski formulae is presented and its results are validated by comparison with high resolution strain measurements by Digital Image Correlation. Based on the analytical solution, a failure analysis using Cuntze’s Failure Mode Concept is conducted. It was revealed, that the failure load and position can be predicted accurately, if a failure type related strength –in-plane or out-of-plane failure– is used. In the presence of [Formula: see text]-layers higher deviations occur between test and prediction and therefore limits the validity of the presented analytical failure analysis.

Metallic grid stiffened cylinders are often used as tank and interstage structures in launch-vehicle systems. When subjected to axial compression, these thin-walled shells are prone to buckling. The corresponding critical buckling load heavily depends on deviations from the ideal shell shape. In general, these deviations are defined as geometric imperfections and they can significantly reduce the critical load. Considering the influence of geometric imperfections into the design process of thin-walled shells poses major challenges for structural design. This article presents an overview regarding the modelling, analysis and design of metallic grid stiffened launch-vehicle cylinders. In comparison to commonly studied unstiffened cylinders, complex shells applied in aerospace engineering can have a completely different structural response to axial compression than their unstiffened counterparts. Advanced shell buckling design conceps like perturbation approaches and energy barrier criteria are applied and validated with experimental data. Finite element models are presented and described in detail. Important aspects like skin and weld buckling as well as the influence of cutouts are analyzed and discussed.

Thin-walled cylindrical shells are primary structures in aerospace, marine and civil engineering. A major loading scenario for these imperfection sensitive shells is axial compression. For this load case, there is a critical disagreement between the theoretical and experimental critical load. This difference is mainly contributed due to shape deviations of the shell middle plane which are commonly described as geometric imperfections. However, some deviations between theoretical and experimental buckling loads are sometimes so severe that other imperfection types are possibly to blame. This article describes experimental and numerical studies on the influence of loading imperfections on the buckling load of thin-walled cylinders. A global loading imperfection was applied by mistake during a buckling test of a thin-walled cylinder which led to a severe buckling load reduction and the corresponding load level was similar to the post-buckling load. Also, a series of experimental studies with localized loading imperfections is described which significantly reduced the buckling load of CFRP cylinders. The experimental results of this article represent an example for some of the very low buckling knockdown factors from early experimental campaigns and give insights in how to avoid these critical imperfections in experimental buckling campaigns.

For the design of thin-walled cylindrical shells under axial compression empirical knockdown factors are applied. These knockdown factors are based on experimental results from the beginning of the 20th century and have been shown to be very conservative for modern shell structures. In order to determine less conservative and physically based knockdown factors for the design of axially loaded shells, different analytical and numerical design approaches have been developed. In this paper common as well as new shell design approaches are presented in detail and evaluated regarding the lower-bound buckling load. Among these design approaches are the EN 1993 1–6, the reduced energy method, linear buckling eigenmode imperfections, perturbation approaches and the new threshold knockdown factors. Important analysis and modeling details of each design approach are described and test examples are given and validated. Advantages and disadvantages of each approach are listed and design recommendations are given. A comparison of deterministic design approaches with modern probabilistic design methods is shown and the range of application of both design philosophies is discussed. Orthogrid stiffened cylinders with weld lands from NASAs Shell Buckling Knockdown Factor Project (SBKF) are modeled, analyzed and lower-bound buckling load calculations for improved knockdown factors are shown.

The stability failure of the axially loaded cylindrical shell is considered as the last unresolved classical stability problem, although it has been investigated for over 100 years. Therefore designers rely on the application of empirical knockdown factors from the 1960s like the NASA SP-8007 for cylindrical shells and the NASA SP-8019 for truncated conical shells which are very conservative for modern shell structures. Perturbation approaches for the design of axially loaded cylindrical and conical shells are presented in this paper. These approaches deliver knockdown factors for a physical based estimation of the lower-bound buckling load and are suitable for research and industrial applications as they are independent from imperfection measurements and easy to implement. The corresponding numerical models are validated by means of high-fidelity buckling experiments and it shows that experimental buckling loads can be calculated very precisely in contrast to the previous methodology. Additionally, new robust knockdown factors are proposed for preliminary shell design which are based on curve fitting of numerical knockdown factors of the perturbation approaches. Thus, it is possible to utilize the load bearing capability of launch-vehicle primary structures up to 40% more effectively, resulting in considerable weight saving potentials for composite shell structures.

A new finite element procedure for thin plates and shells is presented. It combines a geometrically non-linear, rotation-free Kirchhoff–Love formulation with triangular and quadrilateral Bernstein–Bezier elements and C 1 and G 1 inter-element continuity conditions , as well as boundary conditions for clamping and for symmetry. The formulation is free from transverse-shear locking and relies on a high polynomial degree to mitigate membrane locking. Bernstein–Bezier elements are, as opposed to NURBS, suitable for arbitrary triangulations. Unlike with Hermite elements, no stress concentrations occur if the boundary is partially clamped and the formulation can be potentially extended to stiffened plates and shells and to step-wise changes of thickness and material properties . The convergence behaviour is demonstrated and the computational efficiency is compared with that of C°Reissner–Mindlin elements on several numerical examples.

In this paper, a smart cure cycle with an additional cooling step for co-cure bonded fiber metal laminates (FML) is developed to reduce process related thermal residual stresses . An online measuring technique with fiber Bragg grating (FBG) sensors is used to gain a clear understanding of the formation process of residual stresses during cure, as well as to determine the resulting residual stress level. Fiber optic sensors are integrated in carbon fiber reinforced plastic (CFRP) – steel laminates. The recorded processing strain curves enable the characterization of the mechanical interaction of the metal and CFRP-layers. By introducing an additional cooling-step, the moment of the connection between the individual components is being influenced. The conducted experiments show a significant reduction of the resulting stress-free temperature. A variety of temperature cycle modifications, like different cooling temperatures, as well as modified cooling and heating rates, are investigated. The impact of these parameters on the process-induced strains and stresses are detected by the fiber optic strain sensors. The thermal residual stresses are reduced by up to 23% for the specimens in focus, when the developed cure cycles with FBG sensors for a monitored cooling and reheating are used.

For the design of thin-walled spherical shells under external pressure empirical knockdown factors are applied. These knockdown factors are based on experimental results from the beginning of the 20th century and have been shown to be very conservative for modern shell structures. In order to determine less conservative and physically based knockdown factors for the design of spherical shells, different analytical and numerical design approaches have been developed. In this paper common as well as new shell design approaches are presented in detail and evaluated regarding the lower-bound buckling pressure. Among these design approaches are the reduced stiffness method, measured geometric imperfections and perturbation approaches. Important analysis and modeling details of each design approach are described, and test examples are given and validated. Advantages and disadvantages of each approach are listed, and design recommendations are given. Practical shell buckling design examples are demonstrated by means of a tori-spherical bulkhead and a deep-sea spherical pressure hull. In addition, a collection of about 700 experimental knockdown factors for spherical shells under external pressure is given in the Elsevier repository.

Digitization, autonomous driving and lightweight construction are the major future challenges in automotive engineering. This means that more and more complex driver assistance systems, engine control units, infotainment systems, actuators, sensors, etc. must be installed and wired. However, from a lightweight point of view, these cables are additional weight without any structural benefit and only affect the weight balance.

The research described in this paper is dedicated to the use of continuous fibers as reinforcement for additive manufacturing, particularly using Shotcrete. Composites and in particular fiber reinforced polymers (FRP) are increasingly present in concrete reinforcement. Their corrosion resistance, high tensile strength, low weight, and high flexibility offer an interesting alternative to conventional steel reinforcement, especially with respect to their use in Concrete 3D Printing. This paper presents an initial development of a dynamic robot-based manufacturing process for FRP concrete reinforcement as an innovative way to increase shape freedom and efficiency in concrete construction. The focus here is on prefabricated fiber reinforcement, which is concreted in a subsequent additive process to produce load-bearing components. After the presentation of the fabrication concept for the integration of FRP reinforcement and the state of the art, a requirements analysis regarding the mechanical bonding behavior in concrete is carried out. This is followed by a description of the development of a dynamic fiber winding process and its integration into an automated production system for individualized fiber reinforcement. Next, initial tests for the automated application of concrete by means of Shotcrete 3D Printing are carried out. In addition, an outlook describes further technical development steps and provides an outline of advanced manufacturing concepts for additive concrete manufacturing with integrated fiber reinforcement.

Within the collaborative FP7 project PoLaRBEAR a high-performance anisogrid prepreg structure concept has been developed considering high quality assurance demands and the need of a state of art manufacturing process (advanced fiber placement). This paper reports on the design, analysis, manufacturing and test of an anisogrid prepreg panel under uniaxial compression. The anisogrid structures consist of intersecting, integral manufactured stiffeners with different directions (helical, circumferential and/or axial) and a load bearing skin. The stiffeners are manufactured from prepreg layers with 60% fiber volume ratio. An alternating pattern of cut and uncut layers is used within the intersection knots to avoid thickening. In addition, there are special interface layers in the stiffener laminate in order to ensure a proper connection between the grid and the load bearing skin even within the postbuckling range. Optical measurements showed that the panel was burdened by manufacturing induced geometric imperfections (process inducted deformations or spring-in) which caused a slight curvature of the anisogrid panel. Geometrically non-linear analyzes were performed using ABAQUS and B2000++ in order to analyze the pre- and post-buckling response of the panel under axial compression with and without imperfections. Numerical and experimental results are in good agreement if the process induced deformations are considered.

In comparison to bolted joints, structural bonds are the desirable joining method for light-weight composite structures. To achieve a broad implementation of this technology in safety critical structures, the issues of structural bonds due to their complex and often unpredictable failure mechanisms have to be overcome. The proposed multifunctional bondline approach aims at solving this by adding two safety mechanisms to structural bondlines. These are a design feature for limiting damages to a certain size and a structural health monitoring system for damage detection. The key question is whether or not the implementation of both safety features without deteriorating the strength in comparison to a healthy conventional bondline is possible. In previous studies on the hybrid bondline, a design feature for damage limitations in bondlines by means of disbond stopping features was already developed. Thus, the approach to evolve the hybrid bondline to a multifunctional one is followed. A thorough analysis of the shear stress and tensile strain distribution within the hybrid bondline demonstrates the feasibility to access the status of the bondline by monitoring either of these quantities. Moreover, the results indicate that it is sufficient to place sensors within the disbond stopping feature only and not throughout the entire bondline. Based on these findings, the three main working principles of the multifunctional are stated. Finally, two initial concepts for a novel multifunctional disbond arrest feature are derived for testing the fundamental hypothesis that the integration of micro sensors into the disbond stopping feature only enables the crack arrest and the health monitoring functions, while reaching the mechanical strength of a conventional healthy epoxy bondline. This work therefore provides the fundamentals for future investigations in the scope of the multifunctional bondline.

The paper deals with the experimental determination of material properties required as model parameters in Cuntze's invariant-based Failure-Mode-Concept, applied to transversely-isotropic UD materials. For the envisaged 3D strength failure conditions (criteria) of this material, 5 strengths and 2 friction values must be determined. So far, no concept for the determination of the physically necessary internal friction values has been agreed to. In this publication bi-axial ARCAN tests are performed to obtain the friction values from data points on the inter-fiber failure fracture curve. The initiation of inter fiber failure is identified through sudden strain changes, recorded with strain gauges and digital image correlation. Further, the strength values for one specific material (M21/T700GC) are determined. To obtain the necessary shear strength three different shear test setups are investigated: the ±45°-Tension, the IOSIPESCU and the ARCAN test. They are compared considering the smoothness quality of their stress field distribution. The presented methodology enables to determine the two internal friction values.

Based on the experience in the past and the occurrence of in-service damages, the authorities restrict today the application of adhesive bonding of composite structures for aircraft applications. However, certification limitations can be overcome if occurring disbonds within a bond are stopped by implemented design features, so-called disbond stopping features. Consequently, a novel bondline architecture for bonded composite joints is proposed. By implementing a distinct rather ductile thermoplastic phase, a physical barrier for growing disbonds is obtained and thus a fail-safe design, respectively. Moreover, the joint is established by using two different joining technologies, namely adhesive bonding and thermoset composite welding. A sophisticated manufacturing technique is developed for the hybrid bondline concept to achieve a high strength joint. The joint׳s quality is examined by means of several analytical methods like microsections, scanning electron microscopy (SEM), and energy-dispersive X-Ray (EDX) analysis. Additionally, the mechanical performance is evaluated by static Double Cantilever Beam (DCB) and Single Lap Shear (SLS) tests.

Neben der Gewichtsreduktion durch den Einsatz von Faserverbundwerkstoffen kann im Flugzeugbau die Widerstandsreduktion durch die Laminarhaltung der Grenzschicht am Tragflugel einen signifikanten Beitrag leisten. Ein wichtiger Baustein im Zusammenfuhren beider Ansatze ist die multifunktionale Flugelvorderkante in Multimaterialbauweise.

This article contains examples to demonstrate the use of different design concepts for cylindrical shells under axial compression. The examples are based on shells which were manufactured according to electroplating, machining, welding (isotropic cylinders) and prepreg hand layup on a mandrel (composite cylinders). Three of the four shell series are characterized by pure elastic buckling and one shell series buckled in the elastic-plastic region. All relevant data for the numerical analysis are described in the article and summarized in the Elsevier repository of this article (geometry, material, measured imperfection data and Python-ABAQUS scripts). The design concepts are based on the geometric imperfection signatures, probabilistic and deterministic lower-bound methods. The design concepts are representative for the development of design approaches for imperfection sensitive shells from the early 1980 to the late 2010 and are validated with experimental data. Recently developed design lower-bound curves for axially loaded cylinders are presented and compared with currently used design criteria like the Eurocode EN 1993-1-6 and the NASA SP-8007. The results of this article show that the design of imperfection sensitive cylinders has been significantly improved in the last 30 years.

Deep spherical shells are often used as pressure vessels in ocean and aerospace engineering. When subjected to external pressure, these thin-walled shells are prone to buckling. The corresponding critical buckling pressure heavily depends on deviations from the ideal shell shape. In general, these deviations are defined as geometric imperfections, and although imperfections exhibit comparatively low amplitudes, they can significantly reduce the critical load. Considering the influence of geometric imperfections adequately into the design process of thin-walled shells poses major challenges for structural design. The most common procedure to take into account the influence of imperfections is based on the classical buckling pressure obtained by a linear analysis which are then corrected by a knockdown factor. The knockdown factor represents a statistical lower-bound with respect to data obtained experimentally for different types of thin-walled shells. This article presents a versatile and simple numerical design approach for deep spherical shells under external pressure. The new design procedure leads to significantly improved critical load estimations in comparison to lower-bounds obtained empirically. Different design example are given and validated with experimental results.

Thin-walled conical shells are used as adapters between cylindrical shells of different diameters in launch-vehicle systems. Conical shells carry heavy payloads and are consequently subjected to axial compression. The buckling load of these shells is very sensitive to imperfections (geometry, loading conditions) which results in a critical disagreement between theoretical and experimental results for axially loaded conical shells. The design of these stability critical shells is based on classical buckling loads obtained by a linear analysis which are corrected by a single knockdown factor (0.33 - NASA SP-8019) for all cone geometries. This practice is well established among designers and hasn't changed for the past 50 years because the buckling behavior is till today not very well understood. Within this paper an analytical and numerical lower-bound procedure for conical shells under axial compression is proposed. Data of previous experimental testing campaigns are used to validate the new design criteria for different conical shell geometry configurations. The whole design concept is demonstrated by means of the Interstage 1/2 of the Vega launcher and it is concluded that a revision of the current design recommendation for conical shell structures may results in a significant weight reduction potential.

In this paper a physically based and deterministic design procedure for spherical shells under external pressure is introduced. Within the new design concept the membrane energy of a sphere is incrementally reduced by means of perturbation cutouts, until a bending energy dominate state is identified. The threshold between membrane energy state and bending energy state represents a robust plateau for the buckling pressure. A comprehensive numerical investigation was performed in order to study the influence of radius-to-thickness ratio (R/t) as well as the dome height-to-base radius ratio (H/r). The results verify that both geometric properties ratios significantly influence the lower-bound buckling pressure, especially if plastic buckling occurs. Improved shell buckling design factors are given in the form of an simple analytic equation. The corresponding threshold KDFs were validated with a large number of buckling experiments and deliver much higher KDFs than currently used empirical guidelines. Based on the new design criterion lower-bound estimation for the buckling pressure of a tori-spherical bulkhead and the inner dome of the cryogenic upper stage ESC-A from the European space launch-vehicle Ariane 5 are determined.

This paper overviews the efforts that led to new improved knockdown factors for the design of cylindrical shells under axial compression. The corresponding design methods were derived by means of three step procedure involving deterministic methods for the buckling load prediction, modern experimental results and an extensive probabilistic analysis. The new design procedure is demonstrated by means of a full-scale primary launch-vehicle shell and the results show an estimated weight reduction of about 20%. The potentials of the new knockdown factors are shown and open questions regarding the influence of manufacturing specific imperfection signatures on the buckling load are addressed and design implications are derived and discussed. Dynamic load-controlled simulations with imperfection signatures for different manufacturing qualities and processes are performed and the influence of load introduction and mechanical boundary conditions on the buckling load is studied. From the results it is concluded that the commonly used practice of displacement-controlled shell buckling experiments and numerical simulations is sufficient for conservative design of real shell applications.

In this paper, a procedure for designing a lattice fuselage barrel has been developed and it comprises three stages: first, topology optimization of an aircraft fuselage barrel has been performed with respect to weight and structural performance to obtain the conceptual design. The interpretation of the optimal result is given to demonstrate the development of this new lattice airframe concept for the fuselage barrel. Subsequently, parametric optimization of the lattice aircraft fuselage barrel has been carried out using Genetic Algorithms on metamodels generated with Genetic Programming from a 101-point optimal Latin hypercube design of experiments. The optimal design has been achieved in terms of weight savings subject to stability, global stiffness and strain requirements and then was verified by the fine mesh finite element simulation of the lattice fuselage barrel. Finally, a practical design of the composite skin complying with the aircraft industry lay-up rules has been presented. It is concluded that the mixed optimization method, combining topology optimization with the global metamodel-based approach, has allowed to solve the problem with sufficient accuracy as well as provided the designers with a wealth of information on the structural behaviour of the novel anisogrid composite fuselage design.

In the presented study the response of a carbon-glass composite/steel/(elastomer) multi-material structure to low-velocity impact was investigated experimentally. Two different steel layer thicknesses, 0.125 mm and 0.25 mm, as well as the addition of a rubber layer were assessed regarding their influence on internal and external damage, the absorbed energy, force evolution, and deformation. It was found that the doubling of the steel layer thickness stiffens the structure marginally and induces deeper indentations but has no significant influence on the absorbed energy or the damage threshold F D . The additional rubber layer increases the damage threshold load. Below this threshold, the response is dominated by global, elastic deformation decreasing internal and external damage and thereby the amount of absorbed energy. Above F D , large local deformations around the centre of impact lead to a more critical surface damage which dissipate a part of the energy decreasing the extend of delaminations.

Technologien zur Senkung des Treibstoffbedarfs und der CO2-Emissionen stehen im Fokus der Verkehrsflugzeugentwicklung. Die Widerstandesreduktion durch die Laminarhaltung der Grenzschicht am Tragflugel kann hier einen signifikanten Beitrag leisten. Zum Erreichen dieses Ziels ist die Einhaltung hoher Anforderungen an die aerodynamische Oberflache notwendig, was gerade in einer Faserverbundbauweise besondere Herausforderungen mit sich bringt.

In this paper, the deformation of cylindrical composite structures employing piezoelectric actuators is investigated. It specifically addresses radial deformations of a composite cylinder generated by macro fibre composites (MFC). The purpose is to experimentally characterise the shape as well as the magnitude of the composite cylinder’s deformation using an optical full-field 3D measurement system. Besides cylinder expansion and contraction under operating conditions, shell bending appeared to contribute significantly to the overall deformation behaviour. With the data provided the development of an adaptive primary aircraft structure is conceivable.

Within this paper, the structural optimization of stiffened composite panels is considered. The application example is a wing of a long–range aircraft, whereby the flexibility has to be increased due to flight physical demands. To develop the new structural design concept, numerical optimization is used. The structural analysis is carried out utilizing detailed finite element models (DFEM) with linear static and linear eigenvalue simulations. Using this DFEM– and gradient–based optimization environment, designs are analyzed and optimized regarding their mass, considering damage tolerance and stability requirements.

The design of thin-walled structures still relies on empirical guidelines like the NASA SP-8007 for cylinders up till now. This lower-bound guideline does not include important mechanical properties of laminated composite materials, such as the stacking sequence. New design approaches that allow taking full advantage of composite materials are therefore required. Within this paper newly developed innovative design approaches for axially loaded cylindrical shells are pre-sented and validated by means of high-fidelity buckling experiments of unstiffened composite cylinders. The considered composite shells were designed, manufactured and tested at the German Aerospace Center (DLR) in Braunschweig. The results show that the buckling load can be determined very accurately in contrast to the previous methodology.

A currently used guideline for cylinder structures under axial compression is the NASA SP-8007 which is based on empirical data from the 1960s. This guideline provides knock-down factors (KDF) for the lower bound of the buckling load which depend on the cylinder radius-to-thickness ratio but neglect the influence of the cylinder length L. Experimental results indicated an influence of the cylinder length on the buckling load but a clear dependency could not be established because of the insufficient amount of available data. A comprehensive numerical investigation was performed in order to study the influence of length effect on the lower bound of the buckling load. The numerical analysis is based on the single boundary perturbation approach (SBPA) for cylindrical shells. The results verify that there is a significant influence of the cylinder length L on the lower bound of the buckling load. Semi-analytic knock-down factors for the stability failure of axially loaded cylindrical shells were determined which can be used for a simple and fast approximation of the lower bound of the buckling load. The corresponding SBPA thresholds were validated with a number of high fidelity buckling experiments and deliver much higher KDFs than currently used empirical guidelines.

This paper is focused on a method for calculating the optimal drape origin on arbitrary surfaces considering a minimal shear-deformation of the applied fiber-reinforced material. The presented method is based on the mesh from a triangulated surface and therefore not restricted by the assumption of a mathematical differentiable surface description. It is shown how the information of the mesh can be used to calculate the point-wise Gaussian-curvature on the given surface. It is further shown how this curvature information can be used to construct a directed planar graph and how this graph can help calculating the optimal drape sequence in order to minimize the shear-deformation of the applied fabric. The paper is concluded by a demonstration of the developed method for different example surfaces.

Thin-walled conical shells are used as adapters between cylindrical shells of different diameters in launch-vehicle systems or as tailbooms in helicopters. A major loading scenario for conical shells is pure bending. The buckling moment of these shells is very sensitive to imperfections (geometry, loading conditions) which results in a critical disagreement between theoretical and experimental results for conical shells under pure bending. The design of these stability critical shells is based on classical buckling loads obtained by a linear analysis which are corrected by a single knockdown factor (0.41 - NASA SP-8019) for all cone geometries. This practice is well established among designers and hasn't changed for the past 50 years because the buckling behavior is till today not very well understood. Within this paper a reduced stiffness analysis for conical shells under pure bending is performed. Data of previous experimental testing campaigns are used to validate the new design criteria for different conical shell geometry configurations. The results show that the application of the new design recommendation for conical shell structures results in increased knockdown factors for the buckling moment which in turn may lead to a significant weight reduction potential. All ABAQUS-Python scripts and the results generated for this article are deposited in the Elsevier repository.