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Polymers have the potential to replace metallic or bioprosthetic heart valve components due to superior durability and inertness while allowing for native tissue-like flexibility. Despite these appealing properties, certain polymers such as polyetheretherketone (PEEK) have issues with hemocompatibility, which have previously been addressed through assorted complex processes. In this paper, we explore the enhancement of PEEK hemocompatibility with polymer crystallinity. Amorphous, semi-crystalline and crystalline PEEK are investigated in addition to a highly crystalline carbon fiber (CF)/PEEK composite material (CFPEEK). The functional group density of the PEEK samples is determined, showing that higher crystallinity results in increased amount of surface carbonyl functional groups. The increase of crystallinity (and negatively charged groups) appears to cause significant reductions in platelet adhesion (33 vs. 1.5% surface coverage), hemolysis (1.55 vs. 0.75%∙cm−2), and thrombin generation rate (4840 vs. 1585 mU/mL/min/cm2). In combination with the hemocompatibility study, mechanical characterization demonstrates that tailoring crystallinity is a simple and effective method to control both hemocompatibility and mechanical performance of PEEK. Furthermore, the results display that CFPEEK composite performed very well in all categories due to its enhanced crystallinity and complete carbon encapsulation, allowing the unique properties of CFPEEK to empower new concepts in cardiovascular device design., Biomaterials Advances, 146, ISSN:2772-9508

Shape memory alloys (SMA) allow for the realization of smart actuators capable of achieving large stresses and large strains but highly demanding in terms of power input. This work presents a solution integrating shape memory polymers (SMP) in a novel type of selectively compliant actuator to reduce the power input of SMAs. The thermally induced variation in stiffness of the SMP is used to achieve large deformations by temporarily increasing the compliance of the actuator and to lock the actuator in a deformed state by restoring the initial stiffness. The behavior of the actuator is simulated taking into account the viscoelastic behavior of the SMP and validated through a comparison with experimental results. The latter show that the proposed actuator can achieve a maximum contraction of 3.0% and hold a contraction of about 1.6% multiple times without constantly powering the SMA. Finally, a reconfigurable sandwich panel is considered as possible application. A distributed actuator network is implemented in the face sheets of the panel and a digital image correlation system is used to prove the capability of the proposed structure of undergoing large deformations, holding a deformed shape without consuming energy, and recovering its initial shape. A further development of this panel might find application as support structure for morphing aerodynamic surfaces or reconfigurable antennas. ISSN:1045-389X ISSN:1530-8138

Shape adaptable structures are desired in many fields of application such as aerospace, soft robotics, and architecture. Multi-stable structures can enable shape adaptation as they possess multiple stable morphologies that can be held without the need of external work. Many concepts to realize multi-stable systems have been proposed in the last decades. However, the vast majority of multi-stable structures exhibit minimal changes in shape or high coupling between stable states, which limits their applicability. Recently, a novel class of multi-stable composite structures has been investigated, showing that a periodic arrangement of square frames combined with pre-stretched membranes enables many stable states together with large shape transformations. To investigate how this new class can be employed in a broader range of applications, this study extends the design space to any N-sided regular polygon frame. The multi-stability is investigated through experiments and finite element (FE) analysis. The polygonal frames and their respective periodic arrangements possess a large number of stable states, with similar behavior to that of the square frames. Moreover, neutral stability is observed for the limit case of a circular polygonal frame. This study proves that the concept is highly flexible in terms of design, thus opening up new possibilities for applications of multi-stable composite structures., Composite Structures, 313, ISSN:0263-8223, ISSN:1879-1085

This work introduces a novel concept of modular, shape-adaptable sandwich panel with a distributed actuation system based on shape memory alloys (SMA). The panel consists of a modular arrangement of rigid cells connected with compliant active joints. Each joint hosts a SMA wire, which can be controlled independently, enabling the panel to achieve multiple shapes and complex curvatures with a single design. A numerical model of the actuators is developed combining the SMA model proposed by Brinson with a finite element model of the compliant joints, and validated against experimental results. Further, a demonstrator of the panel is manufactured and tested implementing four different actuation patterns and measuring the final shapes with a digital image correlation system. The results prove the capability of the proposed concept to achieve both in plane and out-of-plane deformations in the order of millimeters to centimeters, and to reproduce shapes with double curvatures. With the possibility to integrate sensors and additional components inside the core, the proposed shape-adaptable panel can be used to realize smart structures, which might be used for morphing aerodynamic surfaces or reconfigurable space structures., Journal of Intelligent Material Systems and Structures, 33 (11), ISSN:1045-389X, ISSN:1530-8138

Thin-shell carbon fiber composites have great potential for structures that require large recoverable deformations, high stiffness and low weight, as in deployable space structures, biomedical devices and robotics. Despite being astonishingly flexible in fiber direction, such thin shells are highly sensitive to off-axis loading. This relates to the high manufacturing complexity and sensitivity to imperfections, revealing the need for in-depth understanding and enhancement of their transverse response. This paper provides crucial insights into influencing factors of thin-shell composites’ transverse strength using a highly controllable manufacturing technique to create novel thermoplastic thin-ply (35 μm) carbon fiber-PEEK laminas. The effects of fiber type, microstructure and polymer morphology as well as their interactions, are addressed towards a drastic increase in performance. A combination of microstructure tuning and isothermal crystallization can provide thin-shell composites with a more than 150% improved transverse performance compared to the state-of-the-art. The conducted analysis reveals the sensitivity to all related processing conditions and highlights the effect of their accurate control., Composites Part B: Engineering, 261, ISSN:1359-8368, ISSN:1879-1069

Morphing wings are expected to have transformative impact on future transportation and energy systems. To enable analysis and optimization of morphing wings, efficient numerical models are critical...

Composite rods pultruded from commingled yarns consisting of glass fibres and polypropylene, polyamide 12, polycarbonate, polyethylenterephthalate and polyetherimide are assessed for porosity, hydrothermal aging and thermal breakdown behaviour according to IEC 62217 and IEC TR 62039 standards. The results indicate that the quality of the glass fibres, the fibre-matrix interface and the thermomechanical properties of the polymer affect the electrical performance of the composite. Of the materials examined, the glass fibre-polyethylenterephthalate pultruded rods were the most promising in terms of overall performance by successfully passing all tests, however these results also vary with material supplier.

Flying wings achieve their roll, pitch, and yaw controllability exclusively through spanwise variations of lift and drag due to the lack of an empennage. Instead of using conventional control surfa...

Continuous carbon fibre composite additive manufacturing opens up new possibilities for automated, cost-effective manufacturing of highly-loaded structures. This is achieved by the high design freedom of the process, allowing to tailor the fibre placement and by thereby fully exploiting the anisotropy and strength of the composite material. On the other hand, compliant or so-called morphing mechanisms – exploiting the elastic properties of the material to achieve shape changes – show great potential in improving the flight performance of aerospace structures. Such structures exhibit complex internal topologies, making them prohibitively expensive to manufacture with conventional processes. Combining additive manufacturing of composites with the utilization of morphing mechanisms has the potential to concurrently reduce manufacturing cost whilst greatly improving the flight performance of aerospace structures. The applicability of composite additive manufacturing to morphing aerospace structures is discussed in this letter. For the first time, the complete composite primary- and morphing-structure of a fixed-wing drone was additively manufactured. The drone was successfully flight-tested, evaluating the potential of combining these two emerging technologies.

In this study, three novel thermoplastic impregnation processes were analyzed towards automotive applications. The first process is Thermoplastic Compression Resin Transfer Molding in which a glass fiber mat is impregnated in through thickness by a thermoplastic polymer. The second process is a melt-thermoplastic Resin Transfer Moulding (RTM) process in which the glass fibers are impregnated in plane with the help of a spacer. The third process, stamp forming of hybrid bicomponent fibers, coats the fibers individually during the glass fiber production. The coated fibers are used to produce a fabric, which is then further processed by stamp forming. These three processes were compared in a life cycle analysis (LCA) against conventional resin compression resin transfer moulding with either glass or carbon fibers and metal processes with either steel or aluminum that can be new, partly or fully recycled using the case study of the production, life and disposal of a car bonnet. The presented LCA includes the main phases of the process: extraction and preparation of the raw materials, production and preparation of the mold, process, and energy losses. To include the life of the analyzed bonnet, the amount of diesel that is used to drive the weight of the bonnet for 300′000 km is calculated. In this LCA, the disposal of the bonnet is integrated by analyzing the used energy for the recycling and the incineration. The results show the potential of the developed thermoplastic impregnation processes producing automobile parts, as the used energy producing a thermoplastic bonnet is in the same range as the steel production., Journal of Cleaner Production, 330, ISSN:0959-6526

In many applications, structures need to reversibly change between different 3D shapes to adapt to different environmental conditions or operational requirements. To date, the majority of reconfigurable structures require continuous actuation to hold a selected configuration. Multi-stable elements could potentially be used for shape reconfiguration because they can maintain different 3D shapes without needing a continuous power supply. However, when integrating multi-stable elements with other components, boundary conditions are negatively affecting the multi-stability. A new concept that combines flat fiber-reinforced polymer (FRP) composite shells with bi-axially pre-stretched membranes to realize highly multi-stable structures was recently presented by the authors. In this study, the concept of FRP metamaterials is adapted to realize a hinge that possesses five stable angular configurations. Thanks to the low influence of the boundary conditions on the multi-stability and the large shape change that the structure undergoes, the proposed hinge design demonstrates that FRP metamaterials enable the realization of highly reconfigurable structures. The hinge can achieve angular positions larger than 180° in both directions. Two parametric studies show that the angular positions of the hinge can be tailored by changing either the bending stiffness of the FRP composite shells or the pre-stretch of the membrane. A reconfigurable array of stiff panels connected by adaptive hinges is presented, showing potentials for the realization of a solar array that can change its shape to track the sun or be reversibly stowed and deployed.

Airborne wind energy (AWE) is a power production technique aiming to harvest energy from high-altitude winds through tethered aircraft. In the ground-based power generation concept, the aircraft fl...

This study explores the toughening of fiber-reinforced composite laminates to prevent against mode 1 delamination by using a selective placement of nanosilica particles in only the out-of-tow interlaminar regions of the laminate. In place of a conventional homogenous particle distribution throughout the laminate, “selective toughening” through controlled particle deposition is examined with the objective to increase the nanosilica toughening efficiency. Using a laboratory-scale manufacturing route conceptually similar to a combined prepreg and resin-film process, uni-directional carbon fiber composite laminates containing high glass-transition temperature amine-cured Dow D.E.R. 330 epoxy are produced from both particle distribution configurations. Comparisons are made by double cantilever beam testing for mode 1 delamination fracture energy G1C and by examination of the fracture surfaces. The results show that further nanosilica toughening efficiency is possible with local deposition and toughening compared to the conventional homogenous particle distribution throughout the laminate. For the same total nanosilica particle content in the laminate, the delamination toughening effects are maintained or improved when locally toughened in only the out-of-tow interlaminar regions. For mode 1 delamination initiation and propagation, fracture energy increases in the range of 60% over the untoughened laminates are found for the laminates with a local particle distribution. By comparison, those laminates with a conventional homogeneous particle distribution saw increase of 20–35% over the untoughened laminates. The implications of the localized toughening approach are discussed to provide further guidance in optimizing the use of nanosilica particles and particle toughening in general in composite laminates.

This article presents the design, optimization and performance assessment of a novel structure-actuation morphing concept for a flying wing, enabling the flight control for straight flight and around the pitch and roll axes. The applied camber-morphing concept utilizes an optimized selectively compliant internal structure, combined with electromechanical actuators to achieve a trailing edge deflection. These deflections lead to variations of the local and global lift, permitting to control the flight of the aircraft. The aero-structural behaviour of the wing is analysed using a coupled three-dimensional aerodynamic and structural simulation tool. An optimization of the planform, aerodynamic shape, internal structure and actuation parameters is performed to attain a longitudinally stable and aerodynamically efficient flying wing. The drag increment caused by morphing is minimized through the numerical optimization, resulting in high aerodynamic efficiency across a range of flight speeds. The stiffness and morphing capabilities of the manufactured wing are characterized experimentally and are compared with the numerical predictions, and the aerodynamic and aeroelastic behaviour of the wing is investigated through wind tunnel tests. The test results indicate the ability of the flying wing to achieve sufficient variations in lift, roll and pitch to control the flight completely through camber morphing.

Heart failure is a raising cause of mortality. Heart transplantation and ventricular assist device (VAD) support represent the only available lifelines for end stage disease. In the context of donor organ shortage, the future role of VAD as destination therapy is emerging. Yet, major drawbacks are connected to the long-term implantation of current devices. Poor VAD hemocompatibility exposes the patient to life-threatening events, including haemorrhagic syndromes and thrombosis. Here, we introduce a new concept of artificial support, the Hybrid Membrane VAD, as a first-of-its-kind pump prototype enabling physiological blood propulsion through the cyclic actuation of a hyperelastic membrane, enabling the protection from the thrombogenic interaction between blood and the implant materials. The centre of the luminal membrane surface displays a rationally-developed surface topography interfering with flow to support a living endothelium. The precast cell layer survives to a range of dynamically changing pump actuating conditions i.e., actuation frequency from 1 to 4 Hz, stroke volume from 12 to 30 mL, and support duration up to 313 min, which are tested both in vitro and in vivo, ensuring the full retention of tissue integrity and connectivity under challenging conditions. In summary, the presented results constitute a proof of principle for the Hybrid Membrane VAD concept and represent the basis for its future development towards clinical validation., Annals of Biomedical Engineering, 49 (2), ISSN:1573-9686, ISSN:0191-5649, ISSN:0090-6964

Flow-induced fiber-bed compaction has considerable effects on through-thickness permeability due to resulting changes in fiber-volume-fractions. This study introduces a novel method for the simultaneous characterization of compaction and unsaturated out-of-plane permeability of engineering textiles in one single experiment. The measurement technique relies on visually tracking flow-front positions and compaction during through-thickness impregnation using a camera and a rigid tool equipped with a pressure sensor and a transparent window. This strategy allows flow-induced fiber-bed compaction to be considered in unsaturated permeability measurements, providing permeability at different fiber-volume-fractions, whilst also providing dry and wet compaction curves. This new measurement tool also readily verifies any race-tracking that may occur during impregnation, which drastically influences flow behavior. The method is successfully validated through testing of glass fiber NCF and woven fabrics, where permeability results are compared to analytical solutions, while compaction results are compared to those obtained from a universal testing machine. © 2020 Elsevier Ltd., Composites Part A: Applied Science and Manufacturing, 141, ISSN:1359-835X, ISSN:1878-5840

Physical reconfiguration of spacecraft antennas promises to achieve on-demand performance while staying within the power constraints of space systems. Demonstrations thus far, however, have not resulted in structures compatible with the space environment as concepts have been either heavy, had low mechanical vibration frequencies, or could not be realized from space-qualified materials. This study proposes a novel physical reconfiguration concept based on stretchable mechanical metamaterials. The approach is validated through application to a frequency reconfigurable patch antenna. The antenna is realized from thin fiber reinforced polymer composites providing a dielectric support layer for a thin conductive film. Finite element simulations are used to select the metamaterial geometry and FRP layup to meet antenna requirements, in particular to maintain a Poisson's ratio of -1 over a large range of applied strains. Measurements of the prototype demonstrate a 19% frequency drop in response to a unixial tensile strain of 27.6%, comparable to theoretical predictions and competitive with the state-of-the-art. In addition, the antenna prototype is extremely lightweight, with an average density of only 0.15 g/cm3, shows stiff behaviour with a fundamental mechanical vibration frequency of 45.9 Hz, and is manufactured from space-appropriate materials.

Multi-stability in lightweight structures has been the subject of intensive research. This property is advantageous for the realization of shape adaptable structures because it allows, for example, maintaining different configurations of the structure without the presence of a continuous power supply. However, the integration of multi-stable components into complex structures is not widely implemented yet due to high coupling between the stable modes, suppression of stable modes due to the influence of boundary conditions, and complex fabrication techniques. In this work, a novel class of highly multi-stable periodic structures is presented. The investigation is first conducted on a single cell structure that possesses eight stable modes and, second, expanded to grid structures with periodicity. The multi-stability of the unit cell is preserved in the periodic structures and, in fact, the grid possesses further stable configurations not observed in the unit cell. Prototypes are fabricated by combining flat thin fiber-reinforced polymer (FRP) composite frames with bi-axially pre-stretched membranes. Therefore, the proposed approach enables the fabrication of highly multi-stable FRP grids without the need of a mold. Finite element analysis and experimental results show that the multi-stability property depends on the level of anisotropy of the laminate employed. Highly anisotropic laminates strengthen the multi-stability while isotropic ones suppress it. The realization of such highly anisotropic components with additive manufacturing techniques such as 3D-printing has been proved to be feasible, enlarging the range of materials suitable for the proposed concept. The presented technique is expected to be advantageous for the realization of highly reconfigurable, yet foldable, space habitats and antennas.

Pultrusion is a rapid and cost-effective manufacturing technology for continuous fibre reinforced thermoplastic composite profiles. As the cross-sections of pultruded profiles grow to meet increasing performance requirements, manufacturing challenges concerning heat transfer are encountered. In this study, a two-dimensional finite element model was used to simulate the heat transfer and fluid flow physics of the pultrusion process for increasing diameters from Ø 5–Ø 40 mm. To facilitate the experimental validation, a novel batch-wise pultrusion concept is introduced in which the impregnation process is observed in-situ using a transparent die. The pultrusion studies, conducted on glass-fibre/amorphous polyethylene terephthalate (GF/PET) commingled yarns, show that – with proper design – pultrusion is able to deliver consistent, high quality (void content < 2%) profiles up to at least Ø 40 mm., Composites Part B: Engineering, 227, ISSN:1359-8368, ISSN:1879-1069

International audience; This paper reports the results of an international benchmark exercise on the measurement of fibre bed compaction behaviour. The aim was to identify aspects of the test method critical to obtain reliable results and to arrive at a recommended test procedure for fibre bed compaction measurements. A glass fibre 2/2 twill weave and a biaxial (±45°) glass fibre non-crimp fabric (NCF) were tested in dry and wet conditions. All participants used the same testing procedure but were allowed to use the testing frame, the fixture and sample geometry of their choice. The results showed a large scatter in the maximum compaction stress between participants at the given target thickness, with coefficients of variation ranging from 38 % to 58 %. Statistical analysis of data indicated that wetting of the specimen significantly affected the scatter in results for the woven fabric, but not for the NCF. This is related to the fibre mobility in the architectures in both fabrics. As isolating the effect of other test parameters on the results was not possible, no statistically significant effect of other test parameters could be proven. The high sensitivity of the recorded compaction pressure near the minimum specimen thickness to changes in specimen thickness suggests that small uncertainties in thickness can result in large variations in the maximum value of the compaction stress. Hence, it is suspected that the thickness measurement technique used may have an effect on the scatter.

In this work, the impact of high stress gradients, found in bending of thin unidirectional fiber reinforced shells (~0.1 to 1 mm), on compressive micro-buckling failure, was analyzed. Such thin shells show increased resistance to compressive failure under high curvatures, which may even allow tensile fiber damage to drive ultimate failure for very low thickness (e.g., Composites Science and Technology, 216, ISSN:0266-3538, ISSN:1879-1050

ASME 2020 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, ISBN:978-0-7918-8402-7

Material based actuation with metallic fibers, for example shape memory alloys (SMA) is gaining popularity to replace the conventional bulky actuators used for shape morphing in aerospace sectors. However, Joule heating arising from electrical actuation of SMA affects the interfacial bonding between the SMA and the composite matrix and thus reduces the life span of the structure. Insulating the SMA from the composite matrix will tremendously increase the service life of these reconfigurable structures. Three-dimensional (3D) printing of functional elements during the fabrication phase of the composite structures permits the flexibility to form complex shaped reconfigurable lightweight aerospace components. Here, we present a novel technique to embed polymer encapsulated functional elements into structural composites. We use the direct-write (DW) technique to coat SMA with a polymer solution while simultaneously printing them onto carbon fiber prepreg. We develop high performance polymeric inks - polyetherimide and polycarbonate - compatible with the DW technique, the coating, as well as the composite. In addition to SMA, the technique can also be easily extended to embed various kinds of other functional fibers into composites, in any shape or form. Additionally, we also demonstrate the application of this technique to integrate SMA with polymeric structures towards actuators for robotics grippers or surgical tools. Successful demonstrators of our technique, in the form of SMA-CFRP hybrid and SMA-polymeric leaf-spring actuators are presented.

This work presents the design, optimization and testing of a novel lattice-structure based morphing wing. The lattice-structure concept spans a large design space, including the possibility to vary among others material, number, distribution and the orientation of the lattice rods. The used parametrization scheme considers both the positioning of the Carbon Fiber Reinforced Polymer (CFRP) rods and their orientation in spherical coordinates within the wing, thus allowing to appropriately cover the design space, while reducing the set of variables during the design optimization procedure. The morphing deformations are relying on electromechanical servomotors. Design objectives include weight minimization and structural requirements, while achieving sufficient roll control. The local deformation induced by the electromechanical actuators is distributed by an internal skeleton structure across the rear section of the wing. An extensible skin is ensuring a smooth cambering and minimizes the required actuation energy. The in-house developed and validated simulation environment couples the aerodynamic pressure and the structural deformations, to accurately predict the aeroelastic response of the wing to aerodynamic and actuation forces, considering large deformations. In addition to the lattice structure, the aeroelastic optimization also considers the actuation layout, and the layup-thickness of the wing skin. Planform and airfoil shape are fixed to a NACA0012 airfoil with 80 cm span, and 30 cm chord. The structural and morphing behavior was evaluated on a technology demonstrator. The demonstrator provides large continuous shape changes, improving the aerodynamic performance and achieving large deflections and high rolling moment coefficients. This is mainly achieved by exploiting the interaction of the tailored internal structure and the actuation system. Since the deformation is distributed over a large portion of the wing, local stress concentrations are minimized, and actuation forces are reduced. Wing-up bending tests have been carried out, confirming the simulation results, respectively the load-carrying capability of the presented concept. Furthermore, actuation tests resulted in peak-to-peak trailing edge deflections of 31mm, respectively a rolling moment coefficient of 0.062, which are consistent with the simulation results and guarantee sufficient roll control.

In this article, a novel shear stiff, hinge-less, and truss-like structure for morphing skins is presented and investigated. Adaptive skins are an essential component of morphing wings, which enable perimeter length changes with low deformation energy requirements and increase the efficiency of the morphing system. Based on fundamental considerations for morphing skins, a high shear to morphing stiffness ratio is targeted. The underlying deformation energies for compliant structures are evaluated and provide the basis for the developed geometry of the novel structure. The sizing parameters of the structure are optimized for shear stiffness and morphing stiffness by an evolutionary multi-objective optimization algorithm. A representative geometry is manufactured and tested to verify the nonlinear simulations for shear and morphing stiffness. Furthermore, various cover options forming the aerodynamic surface were investigated and compared in terms of out-of-plane deformations and for their interaction with the underlying structure. The chosen concept, comprising of the novel substructure and sliding covers, exhibits superior performance, reaching a shear to morphing stiffness ratio up to 300, while forming a closed aerodynamic surface.

Air-texturisation is a process that adds bulkiness to bundles of fibres. In this study, the permeability and compaction behaviour of air-texturised glass fibre rovings are experimentally characterised and compared to conventional unidirectional rovings. Based on radial impregnation experiments and single-step compaction/decompaction tests, the following main findings are highlighted: Compared to conventional unidirectional-rovings, the normalised permeability of the air-texturised rovings was approximately three times higher along the fibre direction and 40 times higher transverse to the fibre direction. Accordingly, the degree of anisotropy was approximately one magnitude lower. At a compaction pressure of 1 and 5 bar, the air-texturised rovings were compacted to a volume fraction of Vf=0.34 and 0.43, respectively, which was approximately 30% lower than the volume fraction achieved for the conventional unidirectional-rovings. Finally, it was observed that the decompaction of air-texturised rovings exhibits a more distinct elastic response when unloaded., Journal of Composite Materials, 54 (27), ISSN:0021-9983, ISSN:1530-793X

Very thin carbon fiber composite shells can withstand large bending curvatures without failure. The resulting high tensile and compressive strains require accurate modeling of the fiber-dominated non-linear effects to predict the mechanical response. To date, no universal modeling technique can precisely capture the behavior of such structures. In this work, successful representation of composite’s response was achieved by utilizing single fiber tension and compression experimental data, implemented to extend a basal-plane-realignment based non-linear carbon fiber material model. Numerical techniques were adopted to model the bending behavior of unidirectional carbon fiber composites that was recorded in a comprehensive experimental campaign. Observations show that high material non-linearity leads to a non-negligible neutral-axis shift and drastic reduction of bending modulus due to compressive softening. Tensile fiber failure is the driving mechanism in thin shells flexure allowing for elastic compressive strains of up to 3% without micro-buckling. As a result, a remarkable flexibility in thin shells is realized. With increasing thickness, the elastic flexibility is reduced as the failure-driving mode switches to compressive micro-buckling., Composites Science and Technology, 199, ISSN:0266-3538, ISSN:1879-1050

Large design and manufacturing effort for high load carrying composite structures results from anisotropic material behavior, tedious curing or forming conditions as well as high sensitivity to manufacturing defects. Such challenges limit the design freedom and result in large cost and time effort. A novel design approach is proposed to realize load carrying structures based on the utilization of the outstanding flexibility of thin composite shells and the “complexity for free” approach of additive manufacturing. To this purpose, highly integrated structures are created by folding cured and thin composite shells around additively manufactured internal core topologies. The developed structures do not require complex molding approaches, while maintaining a high degree of manufacturing quality. A multidisciplinary design optimization is used to fully exploit the design freedom and the load carrying capabilities of the structure. Following the design concept, a UAV wing structure that carries more than 100 times its own weight is developed, optimized and tested to validate the design approach and demonstrate load carrying ability and manufacturing quality., Composite Structures, 250, ISSN:0263-8223, ISSN:1879-1085

Morphing skins as structural component in shape adaptive wings are still in their early development phase, as they need to combine contradicting requirements, such as extreme anisotropic mechanical behaviour, low structural thickness and air-tightness. Various morphing skin approaches have been designed for confined problems such as camber morphing and low load scenarios. However, to expand the applicability of morphing wings, a morphing skin with full in-plane deformability and an out-of-plane stiffness suitable for manned aircraft is necessary. In this work, a novel, elastomer free layered morphing skin is designed, manufactured, applied to a camber morphing transition region for small aircraft and analysed. The layered morphing skin is based on stacked, stiff platelets contributing to the out-of-plane stiffness, while compliant ligaments connecting the platelets provide in-plane compliance. Therefore, the layered morphing skin shows extreme orthotropy and can independently deform in both in-plane directions with an initial modulus of 198 kPa. Deformation analysis of the layered morphing skin on the camber morphing transition region confirms the bi-axial deformability and shows strains in span and chord up to 10% and 16%, respectively. Conducted pressure tests indicate an out-of-plane stiffness high enough for small aircraft, despite the demonstrator being manufactured from a polymer. The layered morphing skin concept is a promising base for bi-directionally deformable morphing skins., Aerospace Science and Technology, 120, ISSN:1270-9638

Fiber patch placement (FPP) is a manufacturing technique for variable stiffness composites. In the FPP approach, a structural component is assembled from a multitude of discrete fiber patches, thus allowing for an easy tailoring of the layup to the local load state. However, due to the discontinuous fibers at the patch edges, complex stress distributions occur in patched laminates. To date, an efficient method for the analysis of patched laminates on macro-scale does not exist. This article introduces a 2D planar shear-lag model (SLM) based on thin-plate mechanics and a simplified interlaminar shear stress formulation. It is shown how the governing partial differential equation system is assembled and how boundary conditions are set. The model is solved numerically using the Finite Element Method. For verification the 2D SLM is compared to a full 3D linear-elastic solution. It can be shown that the SLM allows for accurate prediction of stress fields disturbed by interrupted fibers with considerably improved numerical efficiency. An application example shows that the SLM resolves the effects of discontinuities significantly better than a state-of-the-art shell element modeling approach. As a consequence, a substantial progress in the design of patch laminated structures is achieved.

The design of morphing structures requires attaining concurrently light-weight, load-carrying and shape adaptable systems that minimize complexity, actuation requirements and part count. Exploiting the external loads to produce local stiffness variations induced by elastic instabilities offers the potential to activate shape deformations purely passively. Following this approach, we present the structural response study of wings capable of attaining compliant buckling-induced twisting deformations. The structure is designed such that a specific level of aerodynamic load triggers the elastic instability in a shear web part of the wing box. The buckling-induced variation in effective shear stiffness, resulting from the development of a stable, reversible diagonal tension field, leads to a twisting deformation. The structural behaviour is characterized by two drastically different mechanical responses, delimited by the onset of elastic instability. The influence of the buckling component design on the attainable change in spanwise angle of attack is investigated for the specific case of a finite composite wing structure under aerodynamic loads. The type of composite material, its thickness, and the fibre orientation are the considered design parameters. The buckling-induced variation in twist angle, and the corresponding aerodynamic pressure redistribution on the wing, is exploited for achieving load alleviation, reducing the global lift coefficient and decreasing the wing root bending moment.

Tailless airplanes with swept wings rely on variations of the spanwise lift distribution to provide controllability in roll, pitch and yaw. Conventionally, this is achieved utilizing multiple control surfaces, such as elevons, on the wing trailing edge. As every flight condition requires different control moments (e.g. to provide pitching moment equilibrium), these surfaces are practically permanently displaced. Due to their nature, causing discontinuities, corners and gaps, they bear aerodynamic penalties, mostly in terms of shape drag. Shape adaptation, by means of chordwise morphing, has the potential of varying the lift of a wing section by deforming its profile in a way that minimizes the resulting drag. Furthermore, as the shape can be varied differently along the wingspan, the lift distribution can be tailored to each specific flight condition. For this reason, tailless aircraft appear as a prime choice to apply morphing techniques, as the attainable benefits are potentially significant. In this work, we present a methodology to determine the optimal planform, profile shape, and morphing structure for a tailless aircraft. The employed morphing concept is based on a distributed compliance structure, actuated by Macro Fiber Composite (MFC) piezoelectric elements. The multidisciplinary optimization is performed considering the static and dynamic aeroelastic behavior of the resulting structure. The goal is the maximization of the aerodynamic efficiency while guaranteeing the controllability of the plane, by means of morphing, in a set of flight conditions.

Continuous lattice fabrication (CLF) was recently introduced as a new additive manufacturing (AM) technology capable of printing continuous fibre-reinforced thermoplastic composites along desired trajectories in three-dimensional space. In a systematic attempt to maximize the mechanical properties of the printed extrudate by minimizing the residual void content, this study investigates the thermal deconsolidation behaviour observed in pultruded unidirectional fibre-reinforced thermoplastic composite material when it is reheated above its melting point and exposed to ambient pressure. Fibre decompaction, generally accepted to be the primary cause for deconsolidation in fibre-reinforced thermoplastics, was investigated to assess the influence of cyclic softening of the fibrous media on the residual void content of the extruded material. The magnitude and rate of fibre decompaction were observed to decrease with the number of consolidation-deconsolidation cycles to which the material was subjected. A model was developed to predict the degree of deconsolidation in the CLF process as a function of temperature, processing speed, and processing history. Based on the deconsolidation behaviour observed, a multi-stage pultrusion module was designed that exploits cyclic softening and was demonstrated to reduce the residual void content of the printed extrudate by over 80%.

Numerical optimization is an indispensable part of the design process of laminated composite structures. Several optimality criteria-based algorithms exist which rely on a sequential resizing and scaling approach. This paper presents a novel design algorithm applicable for stiffness and eigenfrequency optimization of composite structures with concurrent consideration of resizing and scaling operations. A method is introduced that allows for an efficient consideration of nonlinear constraints. This is done by determining stable concurrent scaling parameters from first-order constraint change ratio estimations. Optimization is carried out using optimality criteria in three independent steps, namely with respect to fiber angles, ply thickness ratios, and total laminate thickness. Sensitivity analyses are performed analytically at low computational costs. Numerical examples demonstrate the efficiency and fast convergence of the method. Compared to established algorithms, the number of required function evaluations is reduced significantly.

The description of the fiber bed compaction behavior is essential for the simulation of many composite manufacturing processes. In this study, we propose a material model describing the time-dependent compaction behavior of a dry fiber bed, valid for different fiber volume fractions and strain-rates. The approach is based on a linear generalized Maxwell model, in which stiffness parameters depend on strains, and relaxation times depend on strain-rates. The model parameters and functions are derived following an empirical approach, performing a curve fitting on multiple steps compaction experiments. The material considered is a carbon fiber woven fabric. The model response shows good agreement with the experimental results, proving the validity of the approach. The proposed material model uses only one set of equations to represent both the compaction and relaxation phases and can be easily implemented in numerical simulation tools.

Thin fiber reinforced polymer (FRP) composites are widely implemented in adaptive and morphing structures. However, realization of the necessary complex 3‐dimensional FRP structures requires the use of expensive molds thereby limiting the design space and flexibility. Using the elastic strain energy of pre‐stretched membranes holds potential for addressing this challenge. In this work, a novel manufacturing technique for fabricating 3‐dimensional FRP structures moldlessly is presented where pre‐stretched membranes are used to drive out‐of‐plane buckling instabilities of FRP composite shells. To explore the potential of this approach, a simple square frame design is investigated. An analytical model based on high deformation beam buckling theory is developed for understanding the parameters driving the out‐of‐plane behavior of these structures. Experimental and finite element results are used for model validation and reveal excellent agreement, with errors less than 10% over a large portion of the design space. Analytical and finite element models demonstrate that the out‐of‐plane deformation can be tailored by varying the structure’s geometric and material parameters. A new design space for FRP composite laminates is characterized, enabling highly flexible design. The manufacturing and modeling techniques can be extended to other geometries for the realization and analysis of arbitrarily complex surfaces., Composite Structures, 268, ISSN:0263-8223, ISSN:1879-1085

This paper introduces continuous lattice fabrication (CLF) – a novel additive manufacturing (AM) technique invented for fiber-reinforced thermoplastic composites – and demonstrates its ability to exploit anisotropic material properties in digitally fabricated structures. In contrast to the layer-by-layer approaches employed in most AM processes, CLF enables the directed orientation of the fibers in all spatial coordinates, that is in the x- , y- , and z- directions. Based on a serial pultrusion and extrusion approach, CLF consolidates commingled yarns in situ and allows for the continuous deposition of high fiber volume fraction (>50%) materials along a programmable trajectory without the use of molds or sacrificial layers by exploiting the high viscosities of fiber-filled polymer melts. The capacity of CLF to produce high-performance structural components is demonstrated in the fabrication of an ultra-lightweight load-bearing lattice structure with outstanding stiffness-to-density and strength-to-density performance (compression modulus of 13.23 MPa and compressive strength of 0.20 MPa at a core density of 9 mg/cm 3 ). This digital fabrication method enables new approaches in load-tailored design, including the possibility to build freeform structures, which have previously been overlooked due to difficulties and limitations in modern fiber composite manufacturing capabilities.

Hybrid bicomponent fibres – materials in which reinforcement fibres are individually sheathed in thermoplastic polymer – are proposed as a novel class of preform materials for thermoplastic composites. We assert that by reducing the scale of hybridization between the reinforcement fibres and the matrix polymer to the level of the fibre, a thermoplastic intermediate material with both high drapeability and short consolidation times can be developed. The manufacture of hybrid bicomponent fibres is demonstrated in a scalable, coating process in which glass fibres are combined with several thermoplastic polymer matrix systems in dimensions and proportions suitable for use in thermoplastic composite structures. This novel class of thermoplastic composite preforms is expected to expedite the high volume production of geometrically complex thermoplastic composite parts.

The in situ consolidation of commingled yarns during filament winding is demonstrated on an aramid fibre-reinforced polyamide 6 material. This article is a systematic experimental investigation of the filament winding processing parameters, namely, the heat gun temperature, line speed, fibre tension, compaction force and preheater temperature. Optimizing the processing parameters in this filament winding process produced a fully consolidated material with a void content of ∼0.25% which is comparable to the material quality achieved by means of compression moulding using the same intermediate materials.

This paper presents a detailed investigation about the geometric non-linear stiffness behavior of corrugated laminates in six different load cases. The considered tensile, bending, and shear load cases allow the modeling with a unit-cell approach assuming a generalized plane strain state. The torsional load case is more complex. There the mechanical response depends on the number of unit-cells and the width of the samples in case of geometric non-linearities. We first identify the load cases that show a non-linear stiffness response under large deformation. Then the non-linear behavior is analyzed in detail using numerical simulation and we aim for mechanical explanations to describe the non-linear behavior. The FE simulations of the torsional load case are validated using experiments with 3D printed samples.