Morphing Structures Research Articles

3D printing of robotic soft actuators with programmable bioinspired architectures

Soft actuation allows robots to interact safely with humans, other machines, and their surroundings. Full exploitation of the potential of soft actuators has, however, been hindered by the lack of simple manufacturing routes to generate multimaterial parts with intricate shapes and architectures. Here, we report a 3D printing platform for the seamless digital fabrication of pneumatic silicone actuators exhibiting programmable bioinspired architectures and motions. The actuators comprise an elastomeric body whose surface is decorated with reinforcing stripes at a well-defined lead angle. Similar to the fibrous architectures found in muscular hydrostats, the lead angle can be altered to achieve elongation, contraction, or twisting motions. Using a quantitative model based on lamination theory, we establish design principles for the digital fabrication of silicone-based soft actuators whose functional response is programmed within the material's properties and architecture. Exploring such programmability enables 3D printing of a broad range of soft morphing structures.

Compression twist deformation of novel tetrachiral architected cylindrical tube inspired by towel gourd tendrils

Inspired by the coupling geometrical relations of left-hand (LH) and right-hand (RH) climbing towel gourd tendrils within single branch, innovative chiral architected cylindrical tube is proposed, which can convert axial compression loads into angular rotation deformation. Firstly, two series of tetrachiral cylindrical tube samples with rotation disk on the mirror plane (middle cross-section) are designed, where the numbers of unit cells and tetrachiral nodes radius are different. Secondly, the cylindrical tetrachiral tubes are fabricated with Selective Laser Sintering (SLS) nylon sintering techniques, and in-situ compression tests are performed for studying the relation between axial compression strain and rotation angle of the spinning disk. It is found that the proposed innovative tetrachiral cylindrical tube is able to generate lateral rotation, and linear relation between rotation angle and compression force can be harvested. With the progress of micro- and nano-manufacturing techniques, the proposed tetrachiral cylindrical tube with compression-twist deformation mechanism demonstrates robust mechanical performances for future industrial applications, such as: shape memory morphing structures in aerospace engineering, smart actuators and propellers, smart flexible microelectronics and biomechanical devices.

Low energy actuation technique of bistable composites for aircraft morphing

Morphing structures for lightweight and energy-efficient aircraft mobile surfaces have been investigated for several years. This paper presents a novel lightweight, passive and low-energy morphing surface concept based on the “lever effect” of a bistable composite plate that can be integrated in aircraft moving surfaces. By using appropriate boundary conditions, it is demonstrated that the magnitude of the activation force on the bistable composite can be tailored to match the differential pressure on the aircraft's airfoil. As a consequence, the bistable laminate can be used as a passive morphing surface. Both numerical simulations and experimental testing are used to prove this concept on a NACA 2412 airfoil structure. The results show that, by choosing proper configuration of constraints, lay-up and aspect ratio of the bistable composite, it is possible to tailor and activate the snap-through mechanism in a passive manner. The proposed concept would save significant weight when compared to an active morphing concept.

Dynamics and control of twisting bi-stable structures

Compliance-based morphing structures have the potential to offer large shape adaptation, high stiffness and low weight, while reducing complexity, friction, and scalability problems of mechanism based systems. A promising class of structure that enables these characteristics are multi-stable structures given their ability to exhibit large deflections and rotations without the expensive need for continuous actuation, with the latter only required intermittently. Furthermore, multi-stable structures exhibit inherently fast response due to the snap-through instability governing changes between stable states, enabling rapid configuration switching between the discrete number of programmed shapes of the structure. In this paper, the design and utilisation of the inherent nonlinear dynamics of bi-stable twisting I-beam structures for actuation with low strain piezoelectric materials is presented. The I-beam structure consists of three compliant components assembled into a monolithic single element, free of moving parts, and showing large deflections between two stable states. Finite element analysis is utilised to uncover the distribution of strain across the width of the flange, guiding the choice of positioning for piezoelectric actuators. In addition, the actuation authority is maximised by calculating the generalised coupling coefficient for different positions of the piezoelectric actuators. The results obtained are employed to tailor and test I-beam designs exhibiting desired large deflection between stable states, while still enabling the activation of snap-through with the low strain piezoelectric actuators. To this end, the dynamic response of the I-beams to piezoelectric excitation is investigated, revealing that resonant excitations are insufficient to dynamically trigger snap-through. A novel bang–bang control strategy, which exploits the nonlinear dynamics of the structure successfully triggers both single and constant snap-through between the stable states of the bi-stable twisting I-beam structures. The obtained optimal piezoelectric actuator positioning is not necessarily intuitive and when used with the proposed dynamic actuation strategy serve as a blueprint for the actuation of such multi-stable compliant structures to produce fast and large deflections with highly embeddable actuators. This class of structures has potential applications in aerospace systems and soft/compliant robotics.

Compliant structures based on stiffness asymmetry

ABSTRACTOne of the key problems in the development of morphing aircraft is the morphing structure, which should be able to carry loads and change its geometry simultaneously. This paper investigates a compliant structure, which has the potential to change the dihedral angle of morphing wing-tip devices. The compliant structure is able to induce deformation by unsymmetrical stiffness allocation and carry aerodynamic loads if the total stiffness of the structure is sufficient.The concept has been introduced by building a simplified model of the structure and deriving the analytical equations. However, a properly designed stiffness asymmetry, which is optimised, can help to achieve the same deformation with a reduced actuation force.In this paper, round corrugated panels are used in the compliant structure and the stiffness asymmetry is introduced by changing the geometry of the corrugation panel. A new equivalent model of the round corrugated panel is developed, which takes the axial and bending coupling of the corrugated panel into account. The stiffness matrix of the corrugated panel is obtained using the equivalent model, and then the deflections of the compliant structure can be calculated. The results are compared to those from detailed finite element models built in the commercial software Abaqus. Samples with different geometries were manufactured for experimental tests.After verifying the equivalent model, optimisation is performed to find the optimum geometries of the compliant structures. The actuation force of a single compliant structure is first optimised, and then the optimisation is performed for a compliant structure consisting of multiple units. A case study is used to show the performance improvement obtained.

Multidisciplinary challenge in the design of a MWCNTs-based polymer smart structure

Smart structures represent of course a current challenge for the application on the aircrafts. Dealing for example with morphing and variable-shape structures, the skin needs to face extremely high strains, while withstanding the operational loads in order to ensure a smooth profile to the complete system. In this context, the authors led the design and manufacturing of carbon fiber-reinforced composite panels, including different filler infusions based on multi-walled carbon nanotubes (MWCNTs) technology. The vibro-acoustic tests have been carried out on an innovative formulation for the characterization of the damping properties related to such micro-handling treatments. The percentage of nano-filler has been chosen so as to be close to the percolation threshold of the material (about 5wt%). As a result of synergic collaboration between Smart Structures Lab of University of Naples “Federico II” and University of Salerno within H2020-MASTRO (Intelligent bulk MAterials for Smart TRanspOrt industries) research project, a multi-functional composite concept has been idealized and developed. The main purpose is to develop intelligent bulk materials for the transport field based on the novel concepts like self-sensing, self-deicing, self-curing, self-healing and selfprotection methodologies to enhance consumer safety, component life-span and performance while reducing maintenance and manufacturing costs. The functionality of the developed components will be demonstrated under relevant conditions at prototype level with special attention to the aerospace structures.

Evaluation of morphing effectiveness of twist morphing structure achieving arbitrary twist angle distribution

Evaluation of morphing effectiveness of twist morphing structure achieving arbitrary twist angle distribution

Thermal modeling and design of the anisogrid morphing structure for a modular optical telescope concept

To meet the requirements for the next generation of optical space telescopes, a paradigm shift is required from current structures that are static, heavy, and stiff toward innovative structures that are adaptive, lightweight, versatile, and intelligent. A morphing or adaptive structure, the thermally actuated anisogrid morphing boom, can be used to meet the design requirements by making the primary structure actively adapt to the on-orbit environment. The adaptive anisogrid structure is actuated through the intelligent application of thermal gradients. This active primary structure improves structural and thermal stability performance, reduces mass, and enables mission architectures. This effort expands on the author’s previous work by incorporating the impact of thermal coupling and demonstrating an updated architecture. This paper introduces a thermally isolated version of the thermal morphing anisogrid structure to enable control of the thermal losses between active members. To evaluate the isolation design, the stiffness and thermal conductivity of these isolating interfaces is addressed. This paper determines that the applied morphing error remains below 5% across all stiffnesses if the joint thermal conductivity is below 0.2 W/(mK). This paper investigates the performance of the thermal morphing system under a variety of structural and thermal isolation interface properties and determines the linear operational regime.

Viscoelastic bistable behaviour of antisymmetric laminated composite shells with time-temperature dependent properties

Due to the bistable characteristics, antisymmetric laminated ([+α/−α]n) composite bistable shells have been used as novel morphing structures in many engineering fields. Previous theoretical analyses were mainly based on the hypothesis that the composite shell is elastic, which leads to unexpected calculation and prediction errors. In this paper, the fiber reinforcements are assumed to be elastic while the matrix is treated as a viscoelastic material. The resulting viscoelastic material properties of the composite shell are obtained through the experimental test and numerical modeling. Based on the classical lamination theory, together with the principle of minimum potential energy and Maxwell viscoelasticity model, a theoretical model is developed to predict the bistable behaviour of those composite shells with viscoelastic material properties. Subsequently, the influences of applied temperature and relaxation time on the second stable configuration of bistable composite shells are analytically investigated. The results are then compared with those obtained from the experiments and numerical modeling. Comprehensive results show that the principal curvature of the shell's second stable state increases as the applied temperature and relaxation time increase. In contrast, the twisting curvature of the second stable shape generally decreases with the relaxation time increasing but increases with the applied temperature rising.

Shape memory composite cellular plan-forms for shape and area morphing

In this paper we demonstrate a two-dimensional deformation mechanism with shape memory alloy (SMA) wire reinforced composite unit cell based plan-form structure which can produce various combinatorial effects of shape change and area change. Active deformation is realized using thermal SMA wire thermoelectric actuators. Compliant topological constructs are arrived at by consider passive and active load path decoupling and sub-optimal shape change and area change. Out-of-plane bending and buckling of the cellular membrane is restricted with the help of first auxiliary rib type stiffeners and later with monolithic elastic thermoplastic fabricated by laser machining. Systematic incorporation of various small-scale deformation features lead to large improvement in actuation efficiency. Structural compliance without and with the actuators are analyzed based on finite element simulation and experimental validation. A super-cell plan-form is characterized which shows interesting possibilities in structural morphing applications. The proposed multi-cell structures have potential applications membrane like structural morphing which can be integrated with transverse load carrying members.

Swelling-induced wrinkling in layered gel beams.

Gels are widely employed in smart mechanical devices and biomedical applications. Swelling-induced bending actuation can be obtained by means of a simple bilayer gel beam. We show that this system can also exhibit wrinkling patterns of potential interest for structural morphing and sensing. We study swelling-induced wrinkling at the extrados of a bilayer gel beam with the softer layer on top. The bent configuration at finite strain is recovered first and, starting from it, a linear perturbation analysis is performed. We delimit the zone corresponding to wrinkling modes in a parameter plane encompassing a mechanical stiffness ratio and a geometric top layer to total height ratio. Interestingly, we observe that surface instability precedes and envelopes wrinkling modes of finite wavelength. Finally, we discuss the effect of changes in stiffness and of the Flory-Huggins parameters χ on the size of the wrinkling domain.

Highly orthotropic panels structural stability, Farrar and Bloch waves theory

PurposeThe purpose of this paper is to present the studies to develop structural highly orthotropic panels. These types of panels provide the capabilities required for structural morphing. They are highly deformable in one given direction; and in the perpendicular direction, one must be compliant to sustain the internal loads per certification stiffness and strength requirements.Design/methodology/approachNeither classic orthotropic panels nor regular cellular solids Bloch wave theories are strictly applicable on their study. A combination of computational method, MATLAB-coded, to analyze stability and classic structural beam theory is studied. Then, non-linear finite element method models are developed for an aerospace control surface application; their results are compared with reported Bloch wave sequences on periodic cellular solid panels.FindingsThe stability along stiffer direction is a requirement to obtain a continuous deformation and plasticization sequence of the cell rows in the perpendicular direction. A sample panel is sized and 3D modeled, and then produced using additive layer manufacturing process to demonstrate the initial stages of a validation and verification campaign.Originality/valueThis paper provides a new method to mechanical characterize highly orthotropic panels.

P.1.k.002 - Risk factors for suicide in the late-life depression: age and sex-related differences

Morphing capability is absolutely vital for aerospace vehicle to gain predominant functions of aerodynamics, mobility and flight control while piercing and re-entering the atmosphere. However, the challenge for existing aerospace vehicle remains to change its structure of nose cone agilely. This paper carries out a lot of observational experiments on honeybee's abdomen which enhances the flight characteristics of honeybee by adjusting its biomorphic shape. A morphing structure is adopted from honeybee's abdomen to improve both the axial scalability and bending properties of aerospace vehicle, which can lead to the super-maneuver flight performance. Combined with the methods of optimum design and topology, a new bionic morphing structure is proposed and applied to the design of morphing nose cone of aerospace vehicle. Furthermore, simulations are conducted to optimize the structural parameters of morphing nose cone. This concept design of biomimetic nose cone will provide an efficient way for aerospace vehicle to reduce the aerodynamic drag.

Fiber optic shape sensor system for a morphing wing trailing edge

The objective of this work is to present a conceptual design and the modelling of a distributed sensor system based on fiber optic devices (Fiber Bragg Grating, FBG), aimed at measuring span-wise and chord-wise variations of an adaptive (morphing) trailing edge. The network is made of two different integrated solutions for revealing deformations of the reference morphing structure. Strains are confined to typical values along the span (length) but they are expected to overcome standard ranges along the chord (width), up to almost 10%. In this case, suitable architectures may introduce proper modulations to keep the measured deformation low while preserving the information content. In the current paper, the designed monitoring system combines the use of a span-wise fiber reinforced patch with a chord-wise sliding beam. The two elements make up a closed grid, allowing the reconstruction of the complete deformed shape under the acceptable assumption that the transformation refers to regular geometry variations. Herein, the design logic and some integration issues are reported. Preliminary experimental test results are finally presented.

Multi-parameter actuation of a neutrally stable shell: a flexible gear-less motor.

We have designed and tested experimentally a morphing structure consisting of a neutrally stable thin cylindrical shell driven by a multi-parameter piezoelectric actuation. The shell is obtained by plastically deforming an initially flat copper disc, so as to induce large isotropic and almost uniform inelastic curvatures. Following the plastic deformation, in a perfectly isotropic system, the shell is theoretically neutrally stable, having a continuous set of stable cylindrical shapes corresponding to the rotation of the axis of maximal curvature. Small imperfections render the actual structure bistable, giving preferred orientations. A three-parameter piezoelectric actuation, exerted through micro-fibre-composite actuators, allows us to add a small perturbation to the plastic inelastic curvature and to control the direction of maximal curvature. This actuation law is designed through a geometrical analogy based on a fully nonlinear inextensible uniform-curvature shell model. We report on the fabrication, identification and experimental testing of a prototype and demonstrate the effectiveness of the piezoelectric actuators in controlling its shape. The resulting motion is an apparent rotation of the shell, controlled by the voltages as in a 'gear-less motor', which is, in reality, a precession of the axis of principal curvature.

Design and mechanical testing of a variable stiffness morphing trailing edge flap

Morphing structures that are both lightweight and conformal to the aerofoil are currently being considered as promising candidates for the next generation of aircraft high-lift systems. Utilizing spatially variable stiffness materials in morphing structures leads to a possible reduction in the actuation energy requirement and also enables geometric control over the deformed shape of the morphing structure, resulting in enhanced aerodynamic and aeroacoustic performance. In this study, a design optimization methodology has been developed to identify the required material stiffness variations of a morphing structure for target optimal deformed shapes. In the optimization scheme, a layer-wise sandwich beam model is used to predict the structural behaviour of the flap with a specific material stiffness variation. Two-dimensional fluid/structure static aeroelastic interaction analysis is performed in the design optimization. Finite element analysis and mechanical tests were also carried out for a chosen optimization result to study the actuation requirements and the capability of control over the deformed shape of the morphing trailing edge. Numerical and experimental results confirm the feasibility of the proposed optimization methodology for identifying the required stiffness variation in the core and also ways of using rapid prototyped honeycomb core to realize the honeycomb core stiffness variations are discussed.

Morphing structures and fatigue: The case of an unmanned aerial vehicle wing leading edge

AbstractWith the emergence of novel aircraft structural concepts, which make use of large‐scale shape deflections to achieve improved flight performance across significantly different flight regimes and missions, unusual crack paths and mixed‐mode crack propagation situations may take place. Consequently, morphing concepts require a full understanding of the materials' behaviour in primary or secondary structures to reliably withstand unusual and demanding operating conditions.A case study of a morphing leading edge developed for the wings of an unmanned aerial vehicle is presented in this paper. Leading edges are subjected to damage originated by bird strike or other events, and those damages can compromise the structural integrity and stability of the flight. Due to the deflections during the morphing structure actuation, cracks will propagate eventually until critical sizes. Possible crack propagation scenarios are presented, considering the service operation of the morphing leading edge.

Thermal morphing anisogrid smart space structures: Part 1. Introduction, modeling, and performance of the novel smart structural application

To meet the requirements for the next generation of space missions, a paradigm shift is required from current structures that are static, heavy, and stiff to innovative structures that are adaptive, lightweight, versatile, and intelligent. The largest benefit provided by this new structural concept is in the ability to deliver high precision position stability. The conventional high precision structural design uses two decoupled systems to achieve positional stability. First, a high mass structure delivers the effectively infinite stiffness and thermal stability so that no deformations occur under all operational loading conditions. Second, to meet the morphing and on-orbit positional requirements, supplementary mechanisms provide the nano, micro, and macro displacement control required. This paper proposes the use of a novel morphing structure, the thermally actuated anisogrid morphing boom, to meet the design requirements through actively morphing the primary structure in order to adapt to the on-orbit environment and meet both requirements in a consolidated structure. The proposed concept achieves the morphing control through the use of thermal strain to actuate the individual helical members in the anisogrid structure. Properly controlling the temperatures of multiple helical members can introduce six degree of freedom morphing control. This system couples the use of low coefficient of thermal expansion materials with precise thermal control to provide the high precision morphing capability. This concept has the potential to provide substantial mass reductions relative to current methods and meet the high precision displacement requirements of spacecraft systems. This paper will detail the concept itself, demonstrate the modeling procedure, and investigate the design space to quantify the potential of the thermally morphing anisogrid smart structure.

Study on Material Strength of Underwater Deployable Structures

Underwater deployable structures are known as Ocean Morphing Structures, which can be compacted and transported by underwater robots then morph into the final form upon arrival. The underwater deployable structures are made of waterproof fabrics to provide a lightweight, compact solution for the underwater application. This paper focuses on the fabric materials strength used for the ocean morphing structures’ fabrication. The material of Polyethylene Terephthalate (PET) film and the glued seams are tested in dry and wet conditions. This work provides the experimental study of the physical strength for the effect of fabrication direction, fabric thickness and application environment.

Design and Analysis of Biomimetic Nose Cone for Morphing of Aerospace Vehicle

Morphing capability is absolutely vital for aerospace vehicle to gain predominant functions of aerodynamics, mobility and flight control while piercing and re-entering the atmosphere. However, the challenge for existing aerospace vehicle remains to change its structure of nose cone agilely. This paper carries out a lot of observational experiments on honeybee's abdomen which enhances the flight characteristics of honeybee by adjusting its biomorphic shape. A morphing structure is adopted from honeybee's abdomen to improve both the axial scalability and bending properties of aerospace vehicle, which can lead to the super-maneuver flight performance. Combined with the methods of optimum design and topology, a new bionic morphing structure is proposed and applied to the design of morphing nose cone of aerospace vehicle. Furthermore, simulations are conducted to optimize the structural parameters of morphing nose cone. This concept design of biomimetic nose cone will provide an efficient way for aerospace vehicle to reduce the aerodynamic drag.