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Background: In vitro assessment is mandatory for artificial heart valve development. This study aims to investigate the effects of pulse duplicator features on valve responsiveness, conduct a sensitivity analysis across valve prosthesis types, and contribute on the development of versatile pulse duplicator systems able to perform reliable prosthetic aortic valve assessment under physiologic hemodynamic conditions. Methods: A reference pulse duplicator was established based on literature. Further optimization process led to new designs that underwent a parametric study, also involving different aortic valve prostheses. These designs were evaluated on criteria such as mean pressure differential and pulse pressure (assessed from high‐fidelity pressure measurements), valve opening and closing behavior, flow, and regurgitation. Finally, the resulting optimized setup was tested under five different hemodynamic settings simulating a range of physiologic and pathologic conditions. Results: The results show that both, pulse duplicator design and valve type significantly influence aortic and ventricular pressure, flow, and valve kinematic response. The optimal design comprised key features such as a compliance chamber and restrictor for diastolic pressure maintenance and narrow pulse pressure. Additionally, an atrial reservoir was included to prevent atrial–aortic interference, and a bioprosthetic valve was used in mitral position to avoid delayed valve closing effects. Conclusion: This study showed that individual pulse duplicator features can have a significant effect on valve's responsiveness. The optimized versatile pulse duplicator replicated physiologic and pathologic aortic valve hemodynamic conditions, serving as a reliable characterization tool for assessing and optimizing aortic valve performance. [ABSTRACT FROM AUTHOR]

Current transcathether heart valves rely on metal mesh stents, resulting in discontinuous stent‐leaflet interfaces that introduce stress concentrations, reducing valve lifetime. This work aims to investigate non‐conventional methods to create a fully polymeric transcatheter heart valve, exhibiting a quasi‐continuous interface with hemocompatible leaflets with high durability potential. Polyetheretherketone (PEEK) is of particular interest as a cardiovascular material due to hemocompatibility and mechanical resilience, highly relevant for catheter delivered valves. For increased reproducibility and design freedom, an autoclave process was used to manufacture thin‐ply PEEK composite stents. We demonstrated that a suitable membrane material during manufacturing is essential to evenly distribute pressure around the stent. Meanwhile, a modified vacuum forming process was used to simultaneously form and attach PEEK leaflets to the stent using a heated mold. This simple and robust method enables rapid manufacturing of an integral PEEK‐based valve design, resulting in improved stent‐leaflet bonding, demonstrating an alternative to conventional dip‐coating. The customized vacuum forming causes a controlled annealing effect in semicrystalline materials such as PEEK. Processing PEEK leaflets at higher mold temperatures results in higher crystallinity, elastic modulus, and bond strength. These processes enable greater design flexibility and promote composite materials for use in heart valve devices. Highlights: Developed manufacturing processes for PEEK‐based heart valve implants.Proof of concept for thermoforming of semi‐crystalline PEEK leaflets.Demonstrated effect of processing temperature on valve mechanical properties.Autoclave processing and vacuum forming enable new heart valve designs. [ABSTRACT FROM AUTHOR]

Multi‐stable elements are commonly employed to design reconfigurable and adaptive structures, because they enable large and reversible shape changes in response to changing loads, while simultaneously allowing self‐locking capabilities. However, existing multi‐stable structures have properties that depend on their initial design and cannot be tailored post‐fabrication. Here, a novel design approach is presented that combines multi‐stable structures with two‐way shape memory polymers. By leveraging both the one‐way and two‐way shape memory effect under bi‐axial strain conditions, the structures can re‐program their 3D shape, bear loads, and self‐actuate. Results demonstrate that the structures' shape and stiffness can be tuned post‐fabrication at the user's need and the multi‐stability can be suppressed or activated on command. The control of multi‐stability prevents undesired snapping of the structures and enables higher load‐bearing capability, compared to conventional multi‐stable systems. The proposed approach offers the possibility to augment the functionality of existing multi‐stable concepts, showing potential for the realization of highly adaptable mechanical structures that can reversibly switch between being mono and multi‐stable and that can undergo shape changes in response to a change in temperature. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Shape transformation offers the possibility of realizing devices whose 3D shape can be altered to adapt to different environments. Many applications would profit from reversible and actively controllable shape transformation together with a self‐locking capability. Solutions that combine such properties are rare. Here, a novel class of meta‐structures that can tackle this challenge is presented thanks to multi‐stability. Results demonstrate that the multi‐stability of the meta‐structure is strictly tied to the use of highly anisotropic materials. The design rules that enable large‐shape transformation, programmability, and self‐locking are derived, and it is proven that the shapes can be actively controlled and harnessed to realize inchworm‐inspired locomotion by strategically actuating the meta‐structure. This study provides routes toward novel shape adaptive lightweight structures where a metamaterial‐inspired assembly of anisotropic components leads to an unforeseen combination of properties, with potential applications in reconfigurable space structures, building facades, antennas, lenses, and soft robots. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we report on the wave transmission characteristics of a hybrid one dimensional (1D) medium. The hybrid characteristic is the result of the coupling between a 1D mechanical waveguide in the form of an elastic beam, supporting the propagation of transverse waves and a discrete electrical transmission line, consisting of a series of inductors connected to ground through capacitors. The capacitors correspond to a periodic array of piezoelectric patches that are bonded to the beam and that couple the two waveguides. The coupling leads to a hybrid medium that is characterized by a coincidence condition for the frequency/wavenumber value corresponding to the intersection of the branches of the two waveguides. In the frequency range centered at coincidence, the hybrid medium features strong attenuation of wave motion as a result of the energy transfer towards the electrical transmission line. This energy transfer, and the ensuing attenuation of wave motion, is alike the one obtained through internal resonating units of the kind commonly used in metamaterials. However, the distinct shape of the dispersion curves suggests how this energy transfer is not the result of a resonance and is therefore fundamentally different. This paper presents the numerical investigation of the wave propagation in the considered media, it illustrates experimental evidence of wave transmission characteristics and compares the performance of the considered configuration with that of internal resonating metamaterials. In addition, the ability to conveniently tune the dispersion properties of the electrical transmission line is exploited to adapt the periodicity of the domain and to investigate diatomic periodic configurations that are characterized by a richer dispersion spectrum and broader bandwidth of wave attenuation at coincidence. The medium consisting of mechanical, piezoelectric, and analog electronic elements can be easily interfaced to digital devices to offer a novel approach to smart materials. [ABSTRACT FROM AUTHOR]

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%. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 V f = 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

One of the outstanding challenges in phononic crystals and acoustic metamaterials development is the ability to tune their performance without requiring structural modifications. We report on the experimental demonstration of a tunable acoustic waveguide implemented within a two-dimensional phononic plate. The waveguide is equipped with a periodic array of piezoelectric transducers which are shunted through passive inductive circuits. The resonance characteristics of the shunts lead to strong attenuation and to negative group velocities at frequencies defined by the circuits' inductance. The proposed waveguide illustrates the concept of a controllable acoustic logic port or of an acoustic metamaterial with tunable dispersion characteristics. [ABSTRACT FROM AUTHOR]

Fiber patch placement (FPP) is a manufacturing technique for discrete variable stiffness composites. In the FPP approach, a structural component is assembled from a multitude of discrete fiber patches. However, due to the discontinuous fibers at patch edges, complex stress distributions occur. To date, a holistic FPP design framework that combines a tailored patch placement method with a dedicated mechanical model for the analysis of patched laminates does not exist. This article introduces a novel approach for the design of fiber patched laminates. It is based on the sequential placement of patches on a finite element shell mesh, using a critical element and angle selection routine in order to optimally locate and orientate fiber patches. They are added to the 3D mesh by employing a highly efficient kinematic draping algorithm. Strength-critical regions of the resulting fiber patched laminates are identified by state-of-the-art finite element analysis and extracted to a shear-lag–based mechanical submodel dedicated to the detailed analysis of patched laminates. The patch placement routine terminates once all design optimization criteria are met. The efficiency of applying optimized patch reinforcements on a continuous fiber-reinforced base laminate is demonstrated using the example of an individualized biomedical component. The work at hand presents the first patched laminate design framework combining a patch placement strategy coupled with a dedicated mechanical model. As a consequence, a substantial progress in the design of patch laminated structures is achieved. [ABSTRACT FROM AUTHOR]

Combining additive manufacturing (AM) with carbon fiber reinforced polymer patched composites unlocks potentials in the design of individualized, lightweight biomedical structures. Arising design opportunities are geometrical individualization of structures using the design freedom of AM and the patient-individual design of the load-bearing components employing carbon fiber patch placement. To date, however, full exploitation of these opportunities is a complex recurring task, which requires a high amount of knowledge and engineering effort for design, optimization, and manufacturing. The goal of this study is to make this complexity manageable by introducing a suitable manufacturing strategy for individualized lightweight structures and by developing a digitized end-to-end design process chain, which provides a high degree of task automation. The approach to achieve full individualization uses a parametric model of the structure which is adapted to patients' 3D scans. Moreover, patient data is used to define individual load cases and perform structural optimization. The potentials of the approach are demonstrated on an exoskeleton hip structure. A significant reduction of weight compared to a standard design suggests that the design and manufacturing chain is promising for the realization of individualized high-performance structures. [ABSTRACT FROM AUTHOR]

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

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

In liquid composite molding (LCM) processes such as resin transfer molding (RTM), particle distribution can be problematic as the particle fillers can be filtered by the reinforcement fibers during the resin infusion process. In this paper, the filtration of alumina and silica nanoparticles in the production of aramid fiber epoxy composites is characterized. The laminates are produced by in‐plane RTM and the effects of selected process variables on the laminate particle distribution are investigated. The objective is to evaluate the assumption that nanoparticles due to their small physical size inherently do not filter in resin infusion processes. The nanosilica particles are found to effectively not filter, while the nanoalumina particles are much more sensitive to filtration as they formed micro‐scale agglomerates as small as a few microns in size prior to injection. The filtration behavior follows a simple theoretical model for micro‐scale particle filtration, already existing in the literature. For the filtration sensitive particles, it was found that the filtration is influenced by the preform fiber volume content. Other common process variables such as resin viscosity, particle concentration in the injected resin, and saturated resin flow time (resin overflow volume) are found to be filtration independent and do not change the filtration behavior. POLYM. ENG. SCI., 59:22–34, 2019. © 2018 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

Tailless swept wing airplanes rely on variations of the spanwise lift distribution to achieve controllability in all axes. As every flight condition requires different control moments, the conventional discrete control surfaces will be practically continuously deflected, leading to drag penalties. Shape adaptation base on chordwise morphing can achieve continuous deformations of the wing profile, leading to local lift variations with minimum drag penalties. As the shape is varied continuously along the wingspan, the lift distribution can be tailored to each flight condition. Tailless aircraft appear therefore as prime candidates for morphing, as the attainable benefits are potentially significant. This work presents a methodology to determine the optimal planform, profile shape, and morphing structure for a tailless aircraft. The employed morphing concept is based on a distributed compliance structure, actuated by piezoelectric elements. The multidisciplinary optimization considers the static and dynamic aeroelastic behavior of the structure and aims to maximize the aerodynamic efficiency of the plane while guaranteeing its controllability by means of morphing. The potential of the resulting wing design is fully exploited by means of a second optimization process, which identifies the actuation configuration resulting in the highest aerodynamic efficiency for a wide variety of control moments. [ABSTRACT FROM AUTHOR]

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

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

This letter presents a cellular solid characterized by extreme tailorability of its mechanical response over a large range of macroscopic strains. The proposed periodic structure is based on a hexagonal chiral lattice topology, modified by introducing transverse curvature in the ligaments, key to the resulting unconventional behavior. The resulting topology allows to exploit different levels of structural hierarchy for tailoring the mechanical behavior of the continuous medium. The capability of the initially curved ligaments to change shape under global deformations is exploited to alter the microstructural motions and - as a result - the macroscopic response. We explore the effect of the additional geometrical design parameter, namely the transverse ligament curvature, on the strain-dependent stiffness of the chiral lattice, evaluating its interplay with the conventional attributes defining the chiral topology and assessing the attainable envelope of mechanical responses. Purposely choosing the parameters permits to tailor the behavior of the chiral lattice, enabling extreme variations in stiffness. The possibility to attain effective negative and zero-stiffness regimes over large compressive strains further exemplifies the potential of this design. These unique characteristics can be used to attain specific wave propagation properties, augment structural damping, control vibrations and noise, and tune the deformation of compliant structures. [ABSTRACT FROM AUTHOR]

This paper reports on the control of longitudinal wave propagation, in the kHz frequency range, using local and interconnected LC (inductance-capacitance) shunts distributed periodically along a rod. The LC shunts are connected to piezoelectric inserts and tuned to engender narrow or broadband pass-bands in the forbidden band frequency range. The Bragg-scattering bandgaps are the result of the periodic mechanical mismatch between PMMA (polymethyl-methacrylate) of the rod and PZT (lead-zirconate-titanate). The narrow pass-bands correspond to the local configuration, where an equivalence between the mechanical impedance of the PMMA and PZT occurs around the shunt resonance frequency. Conversely, the interconnected shunts give a way to an electrical medium through which energy can propagate parallel to its mechanical counterpart, leading to broad pass-bands. This paper presents analytical models for calculating the dispersion and displacements of the 1D medium with interconnected LC shunts. An analytical formulation is also introduced to expediently identify the location of bandgaps and pass-bands in the medium comprised of local LC shunts. Moreover, analytical investigations are carried out to elucidate different physical phenomena giving rise to these pass-bands. The findings are experimentally validated using a finite periodic rod. The ability to tune the dispersion properties of the medium to control the width or depth of the bandgap, by utilizing local or interconnected shunts, offers a new and powerful application for piezoelectric shunts. [ABSTRACT FROM AUTHOR]

In this paper, a reliable and reproducible experimental procedure for the study of the through-thickness flow induced by the compaction of a saturated porous media is presented. Experimental fluid pressure data are exploited in the validation of a fully coupled fluid-mechanical model and the verification of the related material parameters. The experimental results show overall good agreement with the numerical solution, for all three configurations tested. In addition, up-scaling rules have been identified, which relate the consolidation time with the fluid viscosity and the number of layers. [ABSTRACT FROM AUTHOR]

This paper reports on the wave transmission characteristics of two configurations of a hybrid one dimensional (1D) medium. The hybrid characteristic is the result of the coupling between a 1D mechanical waveguide in the form of an elastic beam and an electrical network. The first configuration investigated is based on an LC high-pass network, while the second configuration is based on an LC band-pass network. For both networks, the capacitors are represented by a periodic array of piezoelectric elements that are bonded to the beam coupling, the mechanical and electrical domains, and thus the two waveguides. The coupling is characterized by a coincidence in frequency/ wavenumber corresponding to the intersection of the dispersion curves. At this coincidence frequency, the hybrid medium features attenuation of wave motion as a result of the energy transfer to the electrical network. This energy exchange is depicted in the dispersion curves by eigenvalue crossing, a particular case of eigenvalue veering. This paper presents the numerical investigation of the wave propagation in the considered media along with experimental evidence of the wave transmission characteristics. The ability to conveniently tune the dispersion properties of the electrical network by varying the inductance is exploited to adapt the periodicity of the domain, i.e., monoatomic and diatomic unit cell configurations. [ABSTRACT FROM AUTHOR]

The quality of composite parts manufactured by resin transfer molding is sensitive to material and process variations during the preform impregnation. In order to improve the process robustness in two-dimensional injection cases, this work proposes a fast method for tracking and controlling the resin flow through the preform, using only a small number of pressure sensors embedded in the mold. The approach combines pressure signals and flow modeling in a quick algorithm that returns on-line estimations of the flow front profiles. Virtual and real injection tests demonstrated the accuracy of the methodology and provided information for designing the sensing system. Moreover, the investigation proved the feasibility of a simple and effective procedure of flow rate control. Based on a computer-controlled pressure regulator acting on the inlet gate, a feedback mechanism was implemented in the system and allowed keeping the flow front velocity within a target range of values. Such a control procedure may be used to limit the formation of voids and enhance the final part quality. [ABSTRACT FROM AUTHOR]

Distributed compliance systems with integrated variable stiffness elements show great promise for reconciling the conflicting requirements of morphing. The distinct structural properties of each equilibrium configuration allow bi-stable laminates to provide stiffness variability in a purely elastic, energy-efficient manner. This article presents a novel morphing concept based on a distributed arrangement of embeddable variable stiffness bi-stable composites inside a 500 mm chord NACA 0012 profile (where ‘NACA’ is the National Advisory Committee for Aeronautics). The structural response of the aerofoil is assessed numerically and experimentally, with a particular focus on the global stiffness modification potential via the snap-through of the component laminates. Extending the validated finite element models to include a weak static aeroelastic coupling permits evaluation of the aerodynamic adequacy of the final, passively morphed shapes. This concurrent aero-structural methodology is finally employed to develop an improved design. The results allow for assessing the feasibility and potential of the innovative morphing approach exploiting selective compliance provided by the stiffness variability of the integrated bi-stable elements. [ABSTRACT FROM AUTHOR]

Textile permeability is a generally anisotropic material property, which characterizes the ease of establishing a resin flow through the fibrous reinforcement in Liquid composite molding (LCM) processes. Unidirectional injection experiments are commonly performed to determine in-plane permeability. Effective permeability values have to be measured along three different textile directions to calculate the full in-plane permeability tensor. This article presents a strategy to reduce the number of the required unidirectional experiments to two or even one by considering the angle that the flow front forms with the measurement direction. The relationship between this flow front angle and the permeability tensor elements was derived theoretically and verified by both simulations and experiments with various textile reinforcements. In addition, two methods were investigated to measure the flow front angle and the effective permeability during the experiments: a standard approach based on visual observations and a new method that relies on three pressure sensors, applicable also in the case of nontransparent tooling. The results show that: (I) the two methods provide consistent measurements and are substantially equivalent; (II) the strategy devised to characterize permeability by measuring the flow front angle is effective and accurate; (III) the proposed procedure allows reducing considerably the time and the material samples required for permeability characterization by unidirectional experiments. POLYM. COMPOS., 37:2037-2052, 2016. © 2015 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

In this study, nanoalumina (Al2O3) and nanosilica (SiO2) particles are evaluated as tougheners for a high glass-transition temperature (Tg) epoxy system in correlation with liquid composite molding (LCM) processability. The aim of this paper is to directly compare the effectiveness of nanoalumina and nanosilica of the same nominal particle size as epoxy tougheners on the same neat resin system. The epoxy resin system used in this study was Dow D.E.R. 330 amine cured epoxy with a Tg of 150℃. Both particle types are observed to be Tg neutral and increase fracture toughness of the base epoxy system. Between the two particle types, nanoalumina is found to be more effective than nanosilica in terms of achievable fracture toughness at a given particle loading. As resin viscosity increases with particle addition, the addition of fewer particles with the use of nanoalumina is also beneficial to LCM processing where a lower viscosity is preferable. [ABSTRACT FROM AUTHOR]

In this article, a compliant morphing wing featuring an innovative load-carrying, highly anisotropic, doubly corrugated morphing skin is introduced. A multi-disciplinary design methodology is used to optimally generate the compliant structure with the aim of maximising the produced rolling moment, while minimising mass and drag. The design tool considers the three-dimensional, aeroelastic behaviour and structural constraints. In particular, a parametric metamodel is used to identify the best morphing skin design. The results show that the wing can achieve high levels of control authority and has a lower or equivalent weight compared to conventional wings. A wing demonstrator is manufactured and its aeroelastic performance is tested. The measurements of the displacement field show an appreciable deformation without shape discontinuities. Low-speed wind tunnel tests indicate that the designed wing can produce roll moments that are sufficient for replacing conventional ailerons. Moreover, the obtained changes in shape have a negligible effect on the zero-lift drag, thus demonstrating the aerodynamic efficiency of profile changes achieved through morphing. An effective solution for covering the used corrugation while allowing for shape changes is also introduced and tested. [ABSTRACT FROM AUTHOR]

Morphing airfoils promise advances in performance and efficiency when compared to conventional designs. However, the conflict of requirements between compliance and stiffness, which is characteristic of shape-adaptable structures, often leads to compromise-driven solutions. This article presents the concept of a variable-camber airfoil with adjustable stiffness. Smart elements in the airfoil skin permit to adapt the structural rigidity to the system’s operational states, resulting in more efficient morphing and potentially lighter designs. It is shown by a numerical study that with respect to a reference configuration, the actuation energy is reduced by up to 97% and the structural mass can be lowered by up to two thirds. Furthermore, experiments on a scaled airfoil structure with a variable-stiffness skin based on electro-bonded laminates demonstrate effectivity and integrability of the proposed structural concept, which permit changes in cambering stiffness by a factor of at least 70. [ABSTRACT FROM AUTHOR]

The knowledge of the through thickness permeability is important for the design of modern liquid composite molding processes. In through thickness permeability measurements, the textile undergoes flow-induced compaction, changing the distribution of fiber volume content. As a consequence, results of permeability measurements show a dependency on the applied injection pressure, further denoted as apparent permeability. In the study presented in this paper, saturated permeability measurements were conducted for different fiber volume contents, varying the injection pressure. At the lowest fiber volume content used, the apparent permeability strongly varied with pressure. With increasing fiber volume content the dependency on injection pressure decreased, but it was not negligible. We used a simulation model for coupling of flow and fiber deformation in liquid composite molding in order to predict the apparent permeability in saturated through thickness permeability measurements. The input data for the simulation model were determined by measurements of saturated permeability, using a very low injection pressure, and textile compaction properties. With the proposed methodology, a good agreement of simulation and experimental results was achieved. [ABSTRACT FROM PUBLISHER]

Morphing promises to enhance the performance of wing-like structures by allowing for operating optimally in a wide range of flying conditions. Yet, a key unresolved problem is the realization of a skin capable of concurrently carrying bending and shear loads, as well as allowing for significant levels of in-plane stretching. In this article, a novel concept exhibiting these desirable characteristics is introduced by means of a double walled structure hereafter called double corrugation. Numerical results show that the double corrugation is capable of offering a high bending stiffness while achieving, with low applied forces, a 20% in-plane stretching. The numerically obtained results are validated with experimental tests, showing the feasibility of the concept. Furthermore, the structural characteristics of the double corrugation are optimized for different material constructions. Nonlinear optimization results concurrently considering strength, bending stiffness, axial compliance and weight show the capabilities of this concept to potentially address the conflicting requirements of morphing skins. [ABSTRACT FROM AUTHOR]

Mechanical metamaterials offer unique possibilities to tune their mechanical response by adjusting their geometry, without the complexity that the thermodynamics and kinetics of materials synthesis otherwise impose. In this work, the tuning of the quasi‐static and wave propagation properties of micro‐lattice structures are explored using numerical methods. The ability to independently modify the elastic moduli and the dispersion properties of the material by appropriately placing micro‐inertia elements is demonstrated. The numerical methods used for this investigation are also presented. [ABSTRACT FROM AUTHOR]

This work reports on the adoption of electric fields to tune the mechanical behaviour of structural elements. A mechanical characterization procedure, consisting of double lap joint and 3-point bending tests, is conducted on copper-polyimide laminates while applying electric fields of varying intensity. Field dependence and, thus, adaptability of shear strength and bending stiffness are shown as a function of the overlapping length and interfaces number, respectively. Further, the impact of remaining charges is investigated in both testing configurations. The findings herein lay the foundation for the implementation of electro-adaptive components in structural applications. [ABSTRACT FROM AUTHOR]

A novel semi-passive morphing airfoil concept based on variable bending–twist coupling induced by adaptive shear center location and torsional stiffness is presented. Numerical parametric studies and upscaling show that the concept relying on smart materials permits effective twist control while offering the potential of being lightweight and energy efficient. By means of an experimental characterization of an adaptive beam and a scaled adaptive wing structure, effectiveness and producibility of the structural concept are demonstrated. [ABSTRACT FROM AUTHOR]

Shunted piezoelectric transducers can be used to dissipate vibration energy of a host structure. The synchronized switch damping on inductance is a shunting technique characterized by a nearly rectangular shape of the resulting voltage on the transducer being in antiphase with the structure’s velocity. As for these systems, previous studies have reported the strong relationship between the dissipated energy and the slope of the voltage signal occurring during the switch. This implies that any electrical losses have to be minimized in order to increase the slope of the voltage signal and, thus, the damping performance. The rate of change of the voltage represents a critical issue for autonomous shunts, where the switch can be inefficient because the power for switching the circuit is not supplied by an external source but is supplied by the vibrating structure itself. In this study, a new technique for improving the damping performance of autonomous synchronized switch damping on inductance is proposed based on controlling the switch with a square wave signal that reduces its electrical losses. An experimental validation of the proposed shunting technique is carried out in order to assess the performance in both the cases of a single-tone and multimodal responses of the structure. [ABSTRACT FROM PUBLISHER]

Shunted piezoelectric elements have been studied for several years as promising devices for vibration damping. Different shunting techniques have been developed to deal with the vibration energy in the appropriate way. The energy dissipated by all these techniques, expressed in terms of loss factor or damping ratio, mainly depends on two different contributions: the electromechanical coupling and the shunt design. Therefore, an accurate prediction of the damping is based on a reliable identification of the generalized coupling coefficient that completely describes the electromechanical coupling. In this study, a robust method for the measurement of this coefficient is proposed, where the influence of the inherent damping of the structure is also considered. This method is based on the analysis of the dynamic response of the structure when the piezoelectric patch is connected to a resonant shunt. The proposed method is applied to different sample structures, and the measured generalized coupling coefficients are used for predicting the values of damping attainable with different shunting techniques (such as the resonant shunt or the synchronized switching damping). Vibration tests are then carried out on the same shunted structures, and the analytical prediction of the damping is compared with the experimental results. [ABSTRACT FROM AUTHOR]

Semi-active structural elements based on variable stiffness represent a promising approach to the solution of the conflict of requirements between load-carrying capability and shape adaptivity in morphing lightweight structures. In the present work, a structural concept with adaptive bending–twist coupling aiming at a broad adjustment range of coupling stiffness while maintaining high flexural rigidity is investigated by analysis, simulation and experiment. [ABSTRACT FROM AUTHOR]