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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...

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...

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.

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.

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%.

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

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.

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.

Scientific Reports, 9 (1), ISSN:2045-2322

Although good progress was made by two international benchmark exercises on in-plane permeability, existing methods have not yet been standardized. This paper presents the results of a third benchmark exercise using in-plane permeability measurement, based on systems applying the radial unsaturated injection method. 19 participants using 20 systems characterized a non-crimp and a woven fabric at three different fiber volume contents, using a commercially available silicone oil as impregnating fluid. They followed a detailed characterization procedure and also completed a questionnaire on their set-up and analysis methods. Excluding outliers (2 of 20), the average coefficient of variation (c v ) between the participant´s results was 32% and 44% (non-crimp and woven fabric), while the average c v for individual participants was 8% and 12%, respectively. This indicates statistically significant variations between the measurement systems. Cavity deformation was identified as a major influence, besides fluid pressure/viscosity measurement, textile variations, and data analysis. Fil: May, D.. Institut Für Verbundwerkstoffe Gmbh; Alemania Fil: Aktas, A.. National Physical Laboratory; Reino Unido Fil: Advani, S.G.. University Of Delaware; Estados Unidos Fil: Berg, D. C.. Technische Universität Clausthal; Alemania Fil: Endruweit, A.. University of Nottingham; Estados Unidos. Science and Technology Facilities Council of Nottingham. Rutherford Appleton Laboratory; Reino Unido Fil: Fauster, E.. Montanuniversitat Leoben; Austria Fil: Lomov, S. V.. Katholikie Universiteit Leuven; Bélgica Fil: Long, A.. University of Nottingham; Estados Unidos. Science and Technology Facilities Council of Nottingham. Rutherford Appleton Laboratory; Reino Unido Fil: Mitschang, P.. Institut Für Verbundwerkstoffe Gmbh; Alemania Fil: Abaimov, S.. Skolkovo Institute Of Science And Technology; Rusia Fil: Abliz, D.. Technische Universität Clausthal; Alemania Fil: Akhatov, I.. Skolkovo Institute Of Science And Technology; Rusia Fil: Ali, M. A.. Khalifa University Of Science And Technology; Emiratos Arabes Unidos Fil: Allen, T. D.. University of Auckland; Nueva Zelanda Fil: Bickerton, S.. University of Auckland; Nueva Zelanda Fil: Bodaghi, M.. Institute Of Science And Innovation In Mechanical And Industrial; Portugal Fil: Caglar, B.. Koç Üniversitesi; Turquía Fil: Caglar, H.. Koç Üniversitesi; Turquía Fil: Chiminelli, A.. Instituto Tecnologico de Aragon; España Fil: Correia, N.. Institute Of Science And Innovation In Mechanical And Industrial; Portugal Fil: Cosson, B.. Imt Lille Douai; Francia Fil: Danzi, M.. Eth Zürich; Suiza Fil: Dittmann, J.. Universität Stuttgart; Alemania Fil: Ermanni, P.. Eth Zürich; Suiza Fil: Francucci, Gaston Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina Fil: George, A.. University Brigham Young; Estados Unidos Fil: Grishaev, V.. Skolkovo Institute Of Science And Technology; Rusia Fil: Hancioglu, M.. Koç Üniversitesi; Turquía Fil: Kabachi, M. A.. Eth Zürich; Suiza Fil: Kind, K.. Universitat Technical Zu Munich; Alemania Fil: Deléglise Lagardère, M.. Imt Lille Douai; Francia Fil: Laspalas, M.. Instituto Tecnologico de Aragon; España Fil: Lebedev, O. V.. Skolkovo Institute Of Science And Technology; Rusia Fil: Lizaranzu, M.. Instituto Tecnologico de Aragon; España Fil: Liotier, P. J.. Université de Lyon; Francia Fil: Middendorf, P.. Universität Stuttgart; Alemania Fil: Morán, J.. Universidad Nacional de Mar del Plata; Argentina Fil: Park, C. H.. Imt Lille Douai; Francia Fil: Pipes, R. B.. Purdue University; Estados Unidos Fil: Pucci, M. F.. Université de Montpellier; Austria Fil: Raynal, J.. Groupe Institut de Soudure; Francia Fil: Rodriguez, E. S.. Universidad Nacional de Mar del Plata; Argentina Fil: Schledjewski, R.. Montanuniversitat Leoben; Austria Fil: Schubnel, R.. Groupe Institut de Soudure; Francia Fil: Sharp, N.. Purdue University; Estados Unidos Fil: Sims, G.. National Physical Laboratory; Reino Unido Fil: Sozer, E. M.. Koç Üniversitesi; Turquía Fil: Sousa, P.. Katholikie Universiteit Leuven; Bélgica Fil: Thomas, J.. Khalifa University Of Science And Technology; Emiratos Arabes Unidos Fil: Umer, R.. Khalifa University Of Science And Technology; Emiratos Arabes Unidos Fil: Wijaya, W.. University of Auckland; Nueva Zelanda Fil: Willenbacher, B.. Institut Für Verbundwerkstoffe Gmbh; Alemania Fil: Yong, A.. National Physical Laboratory; Reino Unido Fil: Zaremba, S.. Universitat Technical Zu Munich; Alemania Fil: Ziegmann, G.. Technische Universität Clausthal; Alemania

This paper presents a study of the mechanical response of hexachiral honeycombs with transversally curved ligaments under planar uniaxial tensile, compressive and shear loads. The impact of the chiral cell design parameters on the resulting macroscopic behaviour is assessed utilising finite elements calculations. It is shown that the presence of ligament curvature permits to attain mechanical responses which are not achievable through conventional chiral honeycomb designs. In addition, the resulting responses exhibit, for all considered load cases, significant tunability through the investigated geometrical design parameters. Two chiral lattices with identical geometries, only differing in their ligament curvature, were manufactured and experimentally tested to validate the finite elements predictions. A connection and assembly strategy is presented and utilised, offering a fast and robust approach to build larger finite lattice structures through 3D printed single basic cells. The hexachiral lattices were tested in tension, compression and in-plane shear, showing good agreement with the numerical predictions.

This paper presents the proof-of-concept for a 4D printed active compliant hinge with a selectively variable stiffness for the deployment and reorientation of satellite appendages. We use 4D printing to create an active compliant hinge capable of bending to a given angular position, holding the position without consuming energy and reorienting itself multiple times in a slow and controlled manner without using rigid mechanisms and, therefore, requiring no lubrication. The deployment and the reorientation of the hinge are achieved by exploiting thermally induced stiffness modulation of one of the constituting materials and two antagonistic shape memory alloy actuators. The hinge is specifically designed for the case study of a 6U CubeSat with two orientable solar panels. In this work, we first explain the working principle of the hinge and propose three different actuation strategies to increase the energy collection of the considered CubeSat. Second, we describe the specific functional and geometric requirements of the hinge, the resulting design and the fabricated functional prototype. The latter is tested in a standard laboratory environment to measure the range of motion, the energy consumption and the actuation time. Finally, the feasibility of the three proposed actuation strategies is evaluated considering the corresponding net increase in collected energy. The results show that the hinge is compatible with the stowing requirements and capable of achieving maximum angular positions larger than 90° in both directions and holding any intermediate position with an accuracy of less than 3°. The three actuation strategies considered lead, in a standard laboratory environment, to an increase in energy generation between 54% and 72%., Smart Materials and Structures, 30 (8), ISSN:1361-665X, ISSN:0964-1726

Corrugated laminates have highly anisotropic stiffness properties. Therefore, they are interesting candidates for flexible skins. The geometrically non-linear stiffness response of corrugated sheets is crucial for applications with large deformations. The present paper analyzes the governing mechanisms that drive the non-linear tensile behavior and suggests a model to analyze corrugated structures. The mechanical response mainly depends on the geometry such as amplitude and thickness of the corrugation and the material properties. The proposed model calculates the mechanical response based on a simplified model consisting of rods and discrete torsional springs. It is verified by comparison with non-linear structural analysis with FEM and experimentally validated by using samples manufactured by 3D printing. The paper also presents a parametric study investigating the influence of the geometry on the non-linear behavior and we identify the limits of linear approximation of the structural response of corrugated laminates.

The aerodynamic and structural performance of a morphing wing concept, based on fully compliant structures and actuated by closed-loop controlled solid state piezoelectric actuators, is investigated numerically and experimentally. The morphing wings are designed for a 1.75-m-span unmanned aerial vehicle operating at up to 30 m/s, following lightweight aeronautical construction principles. The goal of providing roll controllability exclusively through morphing is achieved with a concurrent aerostructural optimization, considering static and dynamic aeroelastic effects. The aeroelastic response of the wings is experimentally assessed through wind tunnel tests, performed at different speeds, angles of attack, and actuation levels. The test campaign confirms the ability to achieve lift and rolling moment variations while maintaining a high aerodynamic efficiency, and the results closely match the numerical predictions. An 8-min flight test is performed by replacing the unmanned aerial vehicle wings with the morphing system, demonstrating the capabilities of the concept in its operational environment. This experimental assessment confirms the performance of the design, its robustness, and the possibility of implementing the morphing concept and the required high-voltage control system in small unmanned aerial vehicles. Ultimately, the results show the possibility of replacing the conventional ailerons of similarly sized airplanes with the proposed solution, achieving significant efficiency improvements while guaranteeing controllability.

In axially loaded corrugated laminates consisting of more than one layer interlaminar stresses occur due to the curvature which potentially cause delamination of the laminate. Therefore, it is crucial to know how geometry and lay-up influence the interlaminar stress and hence the risk of delamination. In this paper, a parameter study is presented that studies the influence of corrugation amplitude and different lay-ups on interlaminar stresses. We considered two geometries, one consists of circular sections and the other is a sinusoidal shape. From the parameter study we can derive favorable configurations to minimize normalized interlaminar stresses. A numerical model is used to calculate the stress distribution in the cross-section of the corrugation. It considers a generalized plane strain state and uses a unit-cell approach. The model is validated with experiments in a tensile test using digital image correlation (DIC).

Despite major advances in morphing wing technology, morphing skins as a structural part of an adaptive aerospace system are still in their early development phase due to heavily contradicting requirements, such as highly anisotropic mechanical behaviour, air-tightness and lightness. Usually, airtightness in structural morphing skins is achieved with elastomeric covers which show poor mechanical performance and high weight. A novel design for an elastomer-free morphing skin unit cell is introduced and analysed in this work. A foldable unit cell is manufactured fully from lightweight engineering materials, based on hinge-like carbon fibre reinforced polymer ligaments. The latter reversibly fold a supported mid-section in order to generate large in-plane displacements with low actuation forces, while preserving a smooth surface in both states. The geometric parameters of the unit and the ligament design itself determine the mechanical response of the system. Within the design space of the unit cell, extreme global strains up to 100% and highly anisotropic mechanical behaviour is achieved, where resistance against aerodynamic loads exceeds the in-plane actuation force by a factor of 3.64. When used periodically, the novel unit cell is a promising base for a functional morphing skin system involving large displacements., Composite Structures, 254, ISSN:0263-8223, ISSN:1879-1085

Mechanical metamaterials undergoing extreme deformations span an ever-increasing design space of mechanical performance. However, achieving selective deformability in load-carrying metamaterials remains unexplored. Anisotropic thin fiber-reinforced composite shells, which are soft in bending and stiff axially, present an attractive option for addressing this challenge but are difficult to realize in practice due to fabrication complexity. In this work, an integrated fabrication technique enabling single-step curing of complex composite mechanical metamaterials is proposed. By using 3D-printed tooling and silicone spacers, composite assemblies can be cured in an autoclave without the need for post-cure bonding of individual shells. The technique reduces manufacturing times and eliminates adhesive bonds, which add mass to the structure and can be points of failure. The proposed technique is demonstrated on a modified rotating square auxetic metamaterial geometry, with fabricated prototypes withstanding up to 60% global tensile strains elastically. The composite anisotropy is, moreover, used to control the deformation mechanism in the metamaterial, thereby delaying failure of the structure and allowing tunability of elastic properties. This work sets the stage for the use of composites as a means of expanding the design space achieved by mechanical metamaterials for shape adaptation in lightweight, load-carrying applications., Composite Structures, 246, ISSN:0263-8223, ISSN:1879-1085

Multifunctional materials play a vital role in attaining adaptability, autonomy and many such benefits with no added material cost and weight. However, even today multifunctional structures entail more than one material to obtain combined sensing and actuation, resulting in incompatibility during fabrication as well as service. In this study, a single material that changes its shape with temperature and simultaneously measures the extent of this deformation is designed, making it a viable solution to remotely monitor actuators or change the orientations of sensors, for example in pipelines, space, or the human body. To realize this combination of properties, a composite made from shape-memory polymers – PLA and PEA, and piezoelectric barium titanate nanoparticles is developed and investigated. We explore the possibilities to tune our actuation temperatures from 100 °C down to body temperatures, and present a robust sensor capable of withstanding temperatures ranging from 23 °C to 100 °C, and to over 5000 operation cycles. We use direct-write 3D printing to form different shapes from this multifunctional material. This shape-memory composite has a recovery rate of ∼98% and the sensor has a linear response in the force range of 0.1 – 1 N. Our material with presented improvements towards 4D printing offers applications in electroactive scaffolds, artificial organs for surgical training as well as adaptive electronics.

A heterogeneous system with enhanced dielectric and tribological properties is developed for an electrostatic tuneable friction damper. Such system consists of a carbon fibre reinforced polymer stiffening element which is reversibly laminated onto a host structure by means of electrostatic fields. Damping is achieved when the maximum shear at the interface between stiffening element and structure exceeds the shear strength of the electrostatically laminated interface. The permittivity of the composite material, consisting of barium titanate particles in an epoxy matrix, follows the logarithmic mixing rule formulated by Lichtenecker. In order to exploit the beneficial effect of the high-permittivity filler material, a particle concentration of at least 30 % by volume is necessary. To improve the reduced wear resistance caused by the addition of such filler material, ceramic platelets are introduced and magnetically aligned. This measure reduces the wear volume by more than 70 % without affecting significantly its coefficient of friction. By increasing permittivity and the static friction coefficient, the required force to achieve damping can be realised at lower electric fields compared to commercially available polymer films.