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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]

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]

In order to increase the comfort of vehicle drivers, automotive panel structures are normally damped on their surfaces. Common surface damping treatments like free layer damping or constrained layer damping have the drawback of heavy weight and temperature as well as frequency dependent damping performance. New alternatives with simple structure, high robustness and light weight are sought. In this contribution, three different passive and active damping approaches were investigated: interface damping (ID), active constrained layer damping (ACLD) and particle damping (PD). They were surveyed under same boundary conditions and their performances were compared in terms of weight. The results show that ID strongly decouples the vibration from the source to the panel and provides steady performance over the whole frequency range. ACLD is a lightweight treatment with high damping capability for special mode shapes. PD is an effective simple damper providing excellent damping performance. [ABSTRACT FROM PUBLISHER]

In a previous work, the authors have presented a comprehensive procedure for the direct characterization of single piezoelectric ceramic fibers in terms of butterfly and polarization loops and blocking force. The ability to investigate single fibers is relevant for optimizing their manufacturing processes, for quality control purposes, and for modeling the response of components and structures. In this study the novel testing procedure is used to characterize commercially available fibers distributed by Advanced Cerametrics Inc., CeraNova Corp., and Smart Material Corp., respectively, and to compare their performance with fibers developed at Empa. Their porosity, grain size and phase composition were investigated to correlate the ferroelectric properties with the microstructure. Fibers supplied by Smart Material Corp. exhibited the best ferroelectric performance, in particular the highest saturation and remnant polarization, the lowest coercive field and the highest P-E loop squareness. The said properties result from a low porosity, a sufficiently large grain size and a phase composition near the morphotropic phase boundary. After removing a surface layer dominated by a rhombohedral phase, Empa fibers developed maximum average free-strains 15% larger than any commercially available fiber. Better control of the sintering atmosphere thus promises to be the key to very high performance fibers. [ABSTRACT FROM AUTHOR]

The article presents a study that investigates the electric conductivity and the mechanical properties of electrochemically synthesized polypyrrole/p-toluene sulfonic acid (PPY/pTSA) as a function of its processing parameters. It examines the influence of polymerization current density and counter ion concentration on the electric conductivity of thick polymer film. Mechanical testing of samples showed that the monomer concentration has the greatest influence on the elongation at break.

This article presents a successful extension of passive R-L shunt damping to piezoelectric ceramic elements working in direct 3-3 mode and a performance comparison to elements working in indirect 3-1 mode. A new circuit topology is implemented to synthesize the very large inductances required by the low inherent piezoelectric device capacitance at relatively low frequencies. This allows for efficient tuning of the R-L circuit to the structure resonance frequency to be damped. The vibration suppression performance of monolithic piezoelectric ceramic actuators and active fiber composites is compared in this study. For this purpose, different actuators are bonded on aluminum cantilever plates. An integrated FE model is implemented for the prediction of structure resonance frequencies, optimum values for electric components, and the resulting vibration suppression performance. The passive structure, bonded active patch, and shunted electrical network are analyzed within the same FE model. Active fiber composite patches working in the direct 3-3 mode show equivalent specific damping performance compared to conventional monolithic 3-1 actuated patches. Issues related to the sensitivity of R-L shunts to variations in environmental and operational conditions are discussed in this study. In short, monolithic actuators operating on the 3-1 piezoelectric effect seem to be the best for use in R-L shunting. [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]