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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. [ABSTRACT FROM AUTHOR]

Abstract: Two new concepts for supporting CAD of repetitively stiffened structures are presented. The two concepts are called D-operator and K-operator, respectively. They have been developed to support preliminary aircraft design and are therefore illustrated by using typical aircraft structures. However, the methods could be applied to any repetitive CAD structure. Cooperation with the software supplier made it possible to implement the K-operator professionally. Sample structures generated with this concept will be illustrated and the results of a CAD based optimization using dynamic objects will be discussed. An outlook will give some ideas for further development and improvements of the method. [Copyright &y& Elsevier]

The axial flow behaviour of unidirectional discontinuous aligned fibre reinforced thermoplastic composites made from commingled yarns is investigated experimentally and modelled employing a micromechanical approach. One fibre that interacts with its immediate neighbourhood is considered and its motion as a result of an externally applied tensile force is simulated. Stress relaxation according to the rheological Herschel–Bulkley model is assumed to describe the flow curve of the micromodel. Based on the measured fibre length distribution and an assumed interaction length distribution the flow curve of a multi-fibre material is modelled as the statistically weighted sum of all considered micromodels each featuring one arbitrary combination of fibre length and interaction length. The modelled flow curves agree well with respective experimentally determined flow behaviour for all considered reinforcement fibre fractions, fibre length distributions, and test temperatures. However, the suggested model contains two independent scaling factors, determination of which requires knowledge of an experimental reference curve. The necessity of scaling is attributed to imperfections in terms of fibre alignment and inhomogeneous blending of the commingled yarn. [Copyright &y& Elsevier]

Resin transfer moulding is a fibre impregnation process which is widely used for manufacturing of lightweight fibre reinforced plastic components for automotive and aircraft industry. For economic manufacturing of larger series of production lower cycle times must be achieved, thus injection strategies which allow a rapid infusion of the resin and impregnation of the fibres must be applied. For process optimisation numerical methods are increasingly applied to reduce time to market, cost and risk. Since temperature has an impact on flow and curing of the injected resin, heat transfer mechanisms between the tool and the uncured composite part is of major importance to achieve reliable numerical results. When fluid flows through stationary fibre beds, hydrodynamic dispersion plays an important role in heat transfer and is found to be a function of Peclet number. To understand the hydrodynamic dispersion phenomena, a test rig was designed to measure heat transfer at steady state conditions. The transverse effective diffusivity was investigated, whereas the influence of cavity thickness, porosity and resin flow velocity on effective thermal diffusivity was evaluated. Based on experimental data a model for heat transfer was developed. Because resin flows through a porous media, thermal diffusion is expected to be a combination of conduction and convection. Therefore the model for transverse diffusion for non-crimped fabrics was found to be a function of the thermal properties of the constituent materials, the fibre volume fraction, the degree of filling and the Peclet number. The model is suitable for all process stages, such as preheating, form filling and curing. [Copyright &y& Elsevier]

Semi-active damping of a clamped plate using PZT with purely resistive circuit or resistive state-switched circuit is studied in this paper. The clamped plate bonded with PZT is modelled using Hamilton’s principle and Galerkin’s method. The optimal shunt resistance for the purely shunt resistive circuit together with the optimal placement of the PZT is investigated. For a resistive-state-switched circuit, Clark’s control logic is employed to optimize the shunt resistance. Numerical simulations for these two systems are also presented. Furthermore, a negative capacitance is added to enhance the damping performance. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• A single hybrid material that acts both as a sensor and an actuator. • Shape memory behavior of polymer blends exploited for actuation. • Piezoelectric properties of barium titanate nanoparticles utilized for sensing. 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. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

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%. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Thin-ply UD CF-thermoplastics can outperform baseline thermoset's transverse strength by ∼ 150%. • Low ply thickness reveals microstructure & crystallinity interactions on transverse strength. • Microstructure tuning on thin CF-thermoplastic shells can enhance highly deformable structures. [ABSTRACT FROM AUTHOR]

The challenge of accomplishing efficient morphing entails providing a vastly anisotropic internal architecture. As a possible solution, selective compliance can be imparted on a larger structural system by embedding its interior with elements exhibiting variable stiffness properties. Multi-stable laminates, which feature markedly different characteristics corresponding to each equilibrium configuration, become promising variable stiffness components. This paper explores the design space of two classes of bi-stable composites specially designed to provide stiffness variability in distributed compliance systems. A further requirement of embeddability is imposed, denoting the ability to constrain two opposite edges of the element without bi-stability loss. Given these two central aspects, the spatial distribution of the piece-wise lamination regions and the overall aspect ratio of the configurations are varied using experimentally validated finite element analysis. Hence characteristic trends are identified, expected to greatly facilitate the design process of shape-adaptable selectively compliant systems based on integrated bi-stable components for stiffness variability. [ABSTRACT FROM AUTHOR]

Composite tape springs are curved shells with the ability to assume two energetically stable states. By carefully selecting layup and mold during manufacturing, these structures can store significant strain energy, making them ideal for deployable systems. Yet, their potential as reversible reconfigurable structures, particularly as adaptive hinges, remains largely unexplored. In structural applications, both stable states of these hinges must exhibit high stiffness to support loads while retaining the capacity for reversible reconfiguration. This paper introduces kinematics-based stacking concepts aimed at enhancing the stiffness of bistable hinges in both states, allowing for specific fold angles without compromising compliance. These designs leverage snap-through kinematics, resulting in unique architectures with customizable stiffness and stability. Additionally, the paper presents a novel energy-based semi-analytical approach for analyzing the stacking concepts. This approach involves constrained exploration of the energy landscape to assess the design space in terms of stiffness and energy barriers. Through comprehensive parametric studies, the research demonstrates significantly increased stiffness and reduced peak stresses compared to single tape spring designs. This research highlights the potential of employing tape springs as reconfigurable load-bearing structures in diverse applications, including precise adjustments of low-mass payloads like solar panels or antennas on space structures. • Kinematically compliant stacking of bistable tape springs is introduced to create high-stiffness bistable hinges without compromising bistability. • New design tools using energy landscape exploration techniques have been developed to generate low-order descriptions of tape spring behavior. • Superposition of these reduced order descriptions correlated by constraints shown as a novel means of designing mechanisms consisting of stacks of bistable elements. • A multistable reconfigurable boom developed for a debris collection satellite using these novel bistable hinges. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanotube papers (CNT papers, also referred to as “buckypapers”) prepared from chemically functionalized single-walled CNTs are being investigated for their mechanical tensile properties. While the Young’s moduli are unaffected by the functionalization with diazonium salts of aniline or aromatic mono- and bis-amides tensile strengths of CNT papers are found to increase with a growing degree of functionalization, and more pronounced with a growing number of amide groups capable of hydrogen bonding. The importance of hydrogen bonding becomes evident after its inhibition through N-methylation of the amide groups, resulting in a distinct reduction of strength values. Scanning electron micrography indicates that a high degree of functionalization or a high number of amide group results in the formation of domains with aligned CNTs. [Copyright &y& Elsevier]

Abstract: Carbon nanotube mats (buckypapers) were prepared from three commercial grades of single-walled carbon nanotubes (SWCNTs) and by two processing variants (i.e. filtration of centrifuged and uncentrifuged dispersions). Material properties such as Young’s modulus, tensile strength, electrical conductivity, electrical capacitance, specific surface area and morphology were investigated and put in relation to the in-plane actuation performance in an aqueous electrolyte (1M NaCl). A dynamic mechanical analyzer was adapted for actuation strain measurements on the samples under various tensile prestress levels. High SWCNT purity and dispersibility were found to be crucial for preparing dense and strong cohesive mats with good actuation performance. [Copyright &y& Elsevier]

Non-consolidated woven fabrics of carbon fibre and poly(laurolactam) fibre commingled yarns were subjected to stamp forming. The temperature profile through the thickness of four ply fabric stacks was experimentally assessed by continuous temperature recording during both heating and moulding. Cooling of the laminate during the combined moulding/consolidation/cooling process step was described by a combined heat transfer and consolidation model. The thermal equation of energy conservation including the heat generation term was solved by means of the method of finite differences, also accounting for variable material parameters such as laminate density and thermal conductivity due to progressing consolidation. The suggested model delivers temperature profiles that excellently agree with experimental data. Moreover, this combined heat transfer and consolidation model is capable of predicting the void content of a laminate made from commingled yarn at any time during stamp forming. Simulation of void content evolution at various processing parameters and yarn architectures demonstrates the practical use of the suggested model and allows for determining processing boundary conditions and requirements of yarn characteristics for the successful use of stamp forming for unconsolidated commingled yarns. [Copyright &y& Elsevier]

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. <0.5 mm). The main scope of this work is to analyze this increased resistance to compressive failure and propose a robust modeling scheme. The mechanical failure response was captured by a shell-buckling experimental campaign. The origins of the increased compressive failure resistance were initially attributed to the reduction of shear stresses acting on the most susceptible domain of a representative wavy fiber. This effect was effectively described by an analytically derived, stress-gradient-dependent parameter. The hypothesis for the establishment of this parameter was corroborated by a numerical micromechanical model adopting the embedded cell approach. This model also revealed important micromechanical interactions which were incorporated by simple stress and strain factors. The derived failure prediction scheme was further extended to include the non-negligible, non-linear elastic material behavior of carbon fibers by means of a numerical algorithm. The validity of the failure prediction model was demonstrated by the successful comparison with results acquired from the shell-buckling experiments on a unidirectional carbon-fiber reinforced epoxy system. To this end, the validity of the initial hypothesis of stress-gradient-dependence on compressive failure was corroborated. Major effect on the overall behavior modeling has carbon fiber's material non-linearity, as well as micromechanical interactions. [Display omitted] [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Fiber patch placement (FPP) is a manufacturing technique for variable-stiffness composites structures. Using FPP, a component is assembled from a multitude of discrete fiber patches. Thus, FPP allows tailoring a composite layup to local load states. However, complex stress distributions occur due to discontinuous fibers at patch edges. This paper investigates the failure of unidirectional patched laminates and assesses the influence of governing design parameters on their strength by nonlinear FE analyses. It is found that, depending on their configuration, fiber breakage, yield-slip, or interface fracture are the leading causes of failure. Moreover, the study reveals that the outer layers of patched laminates are much more susceptible to failure than the inner layers. Based on these findings, three distinct patch overlapping patterns are derived, with each of them having their benefits at different locations in the design space. Lastly, strength optimization measures are investigated, aiming at a load reduction of the outer layers. Patched laminate configurations are found which, for intermediate patch length and thickness, feature up to 70% tensile strength retention compared to continuous laminates from the same material. The findings of this work give valuable guidance for the design and analysis of patched laminates. [ABSTRACT FROM AUTHOR]

Large loads caused by fluid–structure interaction leading to fatigue failure and added robustness of wing-like structures constitute important design challenges to be addressed. A reduction in the penalties associated to the added structural mass required to withstand rare load scenarios by means of load alleviation control is highly desirable, particularly for efficient light-weight engineering systems, such as aircraft and wind turbine blades. Implementation of morphing for modifying the lift distribution to mitigate the impact of rare, but integrity threatening, loads on wing-like structures offers a potential solution for such challenges. In this paper, a passive load alleviation aerofoil concept featuring variable stiffness multi-stable elements is presented. The adaptability in the structural response of the aerofoil when subjected to aerodynamic forces allows for passively changing from a high lift generation shape, to a load alleviation configuration exploiting the energy of the flow. Passive implementations to achieve load alleviation through morphing result in lighter and simpler designs in comparison to actively actuated solutions. [ABSTRACT FROM AUTHOR]

Abstract: The possibility to achieve shape adaptation provides structural systems with the potential to adjust for optimal operation in a wide range of conditions. The formidable challenges posed by shape adaptation, and particularly in morphing applications, can be potentially addressed through the development of distributed compliance systems featuring highly directional structural properties. These characteristics can be further enhanced by embedding in such systems elements featuring variable stiffness. In this paper, a novel type of embeddable variable stiffness elements exploiting thermally-induced multi-stability in unsymmetrically laminated composites is presented. A tailored lay-up exhibiting spatially distirbuted stacking sequences is implemented to achieve multi-stability even when restricting two opposing edges. The difference between the structural responses leading exhibited by the multiple stable shapes of the designed composites are numerically investigated. The restoring force for each stable state is examined yielding a significant variability in the stiffness. Experimental specimens are manufactured and tested showing good agreement with the numerical results, validating the proposed variable stiffness implementation. [Copyright &y& Elsevier]

Abstract: Morphing, understood as the ability to undergo pronounced shape adaptations to optimally respond to a diversity of operational conditions, has been singled out as a future direction in the pursuit of maximised efficiency of lightweight structures. Whereas a certain degree of adaptivity can be accomplished conventionally by means of mechanical systems, compliance allowing for substantial reversible deformability exhibits far more potential as a morphing strategy. A promising solution to the inherent contradiction between high stiffness and reversible deformation capacity posed by morphing is offered by introducing variable stiffness components. This notion indicates the provision of a controllable range of deformation resistance levels in place of fixed properties, as required by real-time shape adaptation dictated by maximum efficiency under changing external conditions. With special emphasis on the morphing context, the current review aims to identify the main tendencies, undertaking a systematic classification of existing approaches involving stiffness variability. Four broad categories in which variable stiffness has been applied to morphing are therefore distinguished and detailed: material engineering, active mechanical design, semi-active techniques and elastic structural behaviour. Adopting a wide perspective, the study highlights key capabilities, limitations and challenges. The need for attention directed to the variable stiffness strategy is recognised and the significance of intensive research activities in a highly integrated and multidisciplinary environment emphasised if higher maturity stages of the concepts are to be reached. Finally, the potential of emerging directions of semi-active design involving electro-bonded laminates and multi-stable structures is brought into focus. [Copyright &y& Elsevier]

Abstract: Bi-stable composites have been considered for morphing applications thanks to their ability to hold two statically stable shapes with no energy consumption. In this paper, the modelling of the dynamic response of cantilevered wing-shaped bi-stable composites is presented. To this end, an analytical model approximating the dynamic response about each statically stable shape of wing-shaped bi-stable composites is derived. Theoretical modal properties are obtained to attain or stabilise a desired configuration following a previously introduced resonant control strategy. The resonant control technique is evaluated for a wing-shaped bi-stable composite subject to aerodynamic loads. Wind tunnel experiments are conducted on a wing-shaped specimen showing the ability of the control strategy to stabilise or attain a desired stable shape under aerodynamic loads. [Copyright &y& Elsevier]

Abstract: Aircraft manufacturers are facing several challenges in the pre-design of aircraft structures. This early stage of the aircraft design has a very multi-disciplinary character. Different competence centres need input data, which is at this point in time to a large extent undefined. Therefore, a large variety of specialised tools is used in order to estimate and predict the required data. If these tools are not compatible, interface problems are the consequence. A permanent improvement of the applied processes with regard to the informal value as well as the applicability remains a continuous challenge. The objective of a collaboration project between Airbus Germany GmbH, the DLR Braunschweig, and the ETH Zurich is to find new methods and approaches to improve accuracy, efficiency, and flexibility of data prediction for primary aircraft structures. The use of modern CAE systems together with the integration of finite element methods into the early pre-design process is a very promising approach [F. Bianconi, P. Conti, N. Senin, D.R. Wallace, CAE systems and distributed design environments, in: XII ADM International Conference, Italy, 5–7 September, 2001 ; M. Pellicciari, G. Barbanti, A.O. Andrisano, Functional requirements for a modern CAD system, in: XII ADM International Conference, Italy, 5–7 September, 2001 ; T. Richter, H. Mechler, D. Schmitt, Integrated parametric aircraft design, in: ICAS 2002 Congress, Institute of Aeronautical Engineering, TU Munich]. The modular and knowledge-based architecture of modern CAE systems allows to represent complex assemblies like aircraft structures by parametric associative and very dynamic models. Design knowledge can be integrated into the modelling [M. Mäntylä, S. Finger, T. Tomiyama, Knowledge Intensive CAD, vol. 2, Chapman & Hall, 1997 ] and different characteristics or individuals of the same structure can be mapped through parameters. This document presents concepts, which allow to design comprehensive digital models of novel aircraft structures whereas the level of the modelling detail shall be variegated flexibly [D.E. Whitney, R. Mantripragada, J.D. Adams, S.J. Rhee, Designing assemblies, Res. Engrg. Design 11 (1999) 229–253 ; P. Aspettati, S. Barone, A. Curcio, M. Picone, Parametric and feature-based assembly in motorcycle design: from preliminary development to detail definition, in: XII ADM International Conference, Italy, 5–7 September, 2001]. The strongly parameterised structures allow calculating and assessing different individuals of a given structure in a very efficient and automated way. This makes parametric associative structures very suitable for optimisation. After structural optimisation tasks have successfully been performed with parametric models [U.M. Fasel, O. König, M. Wintermantel, N. Zehnder, P. Ermanni, DynOPS – an approach to parameter optimization with arbitrary simulation software, Centre of Structure Technologies, ETH Zurich; O. König, R. Puisa, M. Wintermantel, P. Ermanni, CAD-entity based evolutionary design optimization, Centre of Structure Technologies, ETH Zurich, and VGTU, Faculty of Mechanics, Vilnius, Lithuania; U.M. Fasel, O. König, M. Wintermantel, P. Ermanni, Using evolutionary methods with a heterogeneous genotype representation for design optimization of a tubular steel trellis motorbike-frame, Centre of Structure Technologies, ETH Zurich], multi-disciplinary optimisations are gaining importance, since they have the potential to find global optima instead of the discipline-dependent optimal configurations and solutions. [Copyright &y& Elsevier]

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 Molding (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 molding 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. • Comparison of the energy demand of three new thermoplastic impregnation processes. • High environmental potential of thermoplastic composites compared to metal parts. • Comparison of the environmental impact of virgin, partly and fully recycled metals. • Novel thermoplastic impregnation processes are relevant for the automotive sector. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Pultrusion of solid thermoplastic composite rods from commingled yarns with diameters up to Ø 40 mm. • Multi-physics FE model simulating the temperature distribution and evolution during pultrusion. • In-situ observation of novel batch-pultrusion process via a transparent die. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]