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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.

Residual stresses in fiber metal laminates (FML) inevitably develop during the manufacturing process. The main contributor to these stresses is the difference in the coefficients of thermal expansion (CTE) between fibers and metal in combination with high process temperatures. To quantify these stresses, the use of specimens with an asymmetric layup is an easily adaptable method. The curvature that develops after the manufacturing of flat laminates with an asymmetrical layer stack is a measure of the level of residual stresses evolving during cure. However, the accuracy of the curvature evaluation is highly dependent on specimen design and other influencing parameters. This leads to deviations when compared to other methods for residual stress quantification as can be seen from the literature. Therefore, in this work a large set of FML specimens is comprehensively investigated to identify relevant influencing parameters and derive conclusions about specimen design and evaluation techniques. For certain layups and process parameters, there is a good correlation between the curvature and the stress-free temperature, which is further covered by analytical solutions for bimetals. This correlation is the basis to transfer curvature into a stress-free temperature that can consequently be used for the quantification of residual stress levels in more complex FMLs. The transfer is validated by in situ strain measurements during cure using a strain gauge technique. Based on the results, the application of asymmetric specimens for residual stress characterization in more complex laminates is presented in the form of a workflow. The work shows the basic considerations and procedures necessary to use asymmetric specimens for residual stress quantification in FML. Furthermore, the results obtained can also be transferred to other composite materials.

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.

Composite structures in aeroplanes are often thin-walled and lightweight, resulting in significant compliance, which presents a handling and assembly challenge due to the associated part deformations. In order to counteract these deformations, the parts are held in their specified geometry using stiff and correspondingly heavy fixtures or jigs. Mobile industrial robots are very versatile and widely used in industrial volume production, but they are limited in their payload capacity. High-rate production of large aerospace modules requires highly automated flexible assembly processes. The approach presented in this paper is to combine mobile units with lightweight assembly jigs that have the capability of deformation compensation. The subject of the study is a high-rate assembly process for flap modules using an Autonomous Industrial Mobile Manipulator (AIMM) and a lightweight end effector. The end effector has a shape compensation function, implemented by an integrated Stewart platform, which enables the compensation of manufacturing tolerances as well as gravity effects. The compensation function is used in a closed loop and counteracts shape deviations by appropriate fixture shape adjustments. The paper reports on the conceptual design of the assembly scenario, the design of the end effector, its realization and the successful experimental demonstration at 1:1 scale.

The article reports on the influence of annealing PVDF in an autoclave process on the PVDF phase composition. DSC, FTIR and XRD measurements serve to observe the phase changes in an already stretched, polarised and β-phase rich film. Annealing was conducted between 90 and 185 ∘C to cover a broad range of curing processes in an autoclave. The β-phase is found to be stable up to near the melting range at 170 ∘C. At 175 ∘C, the non-piezoelectric α-phase dominates and the piezoelectric γ- and γ′-phases appear. The γ-phase grows at elevated temperatures and replaces the β-phase. This observation stresses the importance of developing new methods to reactivate the polarisation after annealing, in particular for the integration of PVDF as a sensor in laminated structures, such as CFRP.

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.

Lamination parameter optimisation is a highly efficient type of composite optimisation. An equally efficient transformation of lamination parameters into stacking sequences is not yet available. This paper presents a general method for rapidly transforming lamination parameters into continuous stacking sequences. Systematic studies of the relationship between lamination parameters and stacking sequences provide a broad understanding of the transformation problem. An important finding is that multiple stacking sequences share the same lamination parameter set. The transformation is therefore not unique and has to account for multiple layup solutions. The layup retrieval algorithm uses primitive optimisation techniques to search for all optima in the layup space. Ply angles and layer numbers are hereby not restricted. In two representative examples, the authors show the algorithm’s capabilities of finding all stacking sequence solutions of a twelve layer laminate and of finding multiple stacking sequence solutions for arbitrary layer numbers. This makes the algorithm applicable for stacking sequence retrieval, the last step in lamination parameter optimisation.

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.

High aspect ratio aircraft have a significantly reduced induced drag, but have only limited installation space for control surfaces near the wingtip. This paper describes a multidisciplinary design methodology for a morphing aileron that is based on pressure-actuated cellular structures (PACS). The focus of this work is on the transient dynamic system behavior of the multi-functional aileron. Decisive design aspects are the actuation speed, the resistance against external loads, and constraints preparing for a future wind tunnel test. The structural stiffness under varying aerodynamic loads is examined while using a reduced-order truss model and a high-fidelity finite element analysis. The simulations of the internal flow investigate the transient pressurization process that limits the dynamic actuator response. The authors present a reduced-order model based on the Pseudo Bond Graph methodology enabling time-efficient flow simulation and compare the results to computational fluid dynamic simulations. The findings of this work demonstrate high structural resistance against external forces and the feasibility of high actuation speeds over the entire operating envelope. Future research will incorporate the fluid–structure interaction and the assessment of load alleviation capability.

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.

In the presented investigation composites consisting of carbon and glass fiber reinforced plastics in combination with steel and elastomer layers are subjected to experimental drop-weight impact tests and compression after impact tests. The objective is to study the low-velocity impact behaviour, damage resistance and residual strength of composite laminates dependent on position and proportion of additional layers. In continuation of earlier studies two additional laminate configurations containing thin steel layers and two elastomer layers are tested. Impact damage and damage tolerance are determined respecting the aspect of differing bending stiffnesses and structure densities. It is found that concerning the elastomer addition the damage area, which mainly depends on the elastomer layer position, is an essential influencing factor on the residual strength. The steel addition leads to wide delaminations but high residual strengths.

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.

For the design of composite structures with circular cut-outs analytical, experimental and numerical methods are available. This paper investigates numerous exact analytical methods introduced in literature as an example of the Airbus Industries Test Method for determination of open hole tensile strength, conforming to AITM 1-0007. Experimental testing accompanied by digital image correlation of unidirectional 0° and 90° laminates as well as quasi-isotropic CFRP laminates with width-to-diameter (w/d) ratios of 3 and 5 is conducted. In addition, 3D finite element models of the test specimens are created for detailed analysis on lamina level. A preliminary study on numerous analytical methods introduced in literature shows that application of any of those methods results in one of two possible stress distributions. A qualitative comparison of 2D strain fields on laminate level reveals the high prediction quality of the analytical solutions. Furthermore, a detailed stress analysis on lamina level by comparison to a validated FE model is conducted. The results show that exact analytical solutions exhibit a satisfactory stress prediction quality for w/d ratios as low as 3 and thus can be recommended for preliminary design purposes.

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.

For detailed stress distribution analysis of bondlines, non-linear finite element analy- sis (FEA) is necessary. Depending on the load case in relation to shear and tension/ compression adhesives show a different behaviour of the yield point [ 1 ], which is e.g. included in the Mahnken and Schlimmer [ 2 ] model. State of the art for biaxial tested adhesive material-characteristics is the use of bonded tubular butt joints under variable torsion and tension loads. Important for the quality of the determined material values is the alignment of both tubes. The quality is significantly improved, if both tubes are aligned perfectly coaxial. Also, the bondline has to be free of voids. In previous work [ 3 ], Wölper investigated the effects of coaxial and angle deviations for the results of material characteristics using FEA. A slight deviation has a strong negative impact to the results. Particularly for thin film-adhesives with elevated curing temperatures, the change of viscosity of the adhesive and the thermal expansion of the tubes must be considered. Previous investigations regarding the manufacturing of the specimens showed shortfalls in joining and curing them. Due to voids, geometric deviations or poorly-bonded tubes, no reliable results were achieved yet. Therefore, a new assembly- device is developed and tested. The results show well joined tubes without a signifi- cant angle deviation and with an average of 40 μm in coaxial deviation. The thickness of the bondline can be adjusted and is constant over the whole diameter. The new joining-device enables the testing of tubular butt joints to determine biaxial material values of thin higher-temperature-cured film-adhesives. The device is patented to DE 102017114538.9.

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.

Process-induced distortion of composite structures often leads to a violation of tolerances, making the assembly of components difficult and expensive. It therefore can inhibit a cost-effective mass production of high-performance composite structures. Process-induced distortion is often introduced by curved regions of a part due to spring-in. Main drivers are chemical shrinkage of the resin and thermal expansion of both fiber and resin during cooling after demolding. Both contribute to residual strains and consequently lead to distortion of the manufactured part. The spring-in phenomenon has been already addressed in many studies. However, variations in manufacturing and specimen properties inhibit a detailed comparison of the results. Hence, it is difficult to isolate major influencing parameters. Here we show spring-in results of specimens that were manufactured using the very same experimental setup and laminate configuration but different resin and fiber types. It is therefore possible to identify the interaction of the curing temperature and the maximum achievable glass transition temperature of the individual resins as a major influencing factor. Furthermore, it is shown that the properties of the investigated resins do not differ largely in terms of thermal expansion and chemical shrinkage. Moreover, the latter was measured using two different techniques to enable a comparison. Numerical spring-in prediction revealed good accuracy throughout the investigated specimen configurations. Limitations found are the influence of the sewing of fiber textiles and the sensitivity of the model to gradual changes of the layup. Moreover, different homogenization techniques are compared with regard to spring-in prediction accuracy.

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.