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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]

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 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]

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]

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]

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]

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]

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]

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]

For the design of smart composite structures, the placement of piezoelectric patch transducers is a very important issue. For sensing and passive vibration damping applications, the electric charge and the generalized electromechanical coupling coefficient are important parameters. This article presents an approximative but efficient method for the calculation of the generalized coupling coefficient using finite element simulation results. A smart composite turbomachine blade serves as a test structure for the new method. The influence of different laminate integration layers, positions, rotation angles, patch size variations and performance benefits of increased allowable strain is tested with the new procedure. The efficient nature of the method allows for parametric placement studies or even exhaustive search to find the global positioning optimum for the structure under investigation. The proposed method is compared to state-of-the-art procedures for the calculation of the generalized electromechanical coupling factor. The 1:5 scale models of full carbon fabric/epoxy blades were manufactured with structurally integrated piezo modules. Two different piezo placement positions inside the blade were investigated experimentally. The coupling coefficient was determined and compared to the calculated values. [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]

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]