Morphing Structures Research Articles

Three-dimensional printing of functionally graded liquid crystal elastomer.

As a promising actuating material, liquid crystal elastomer (LCE) has been intensively explored in building diverse active structures and devices. Recently, direct ink writing technique has been developed to print LCE structures with various geometries and actuation behaviors. Despite the advancement in printing LCE, it remains challenging to print three-dimensional (3D) LCE structures with graded properties. Here, we report a facile method to tailor both the actuation behavior and mechanical properties of printed LCE filaments by varying printing parameters. On the basis of the comprehensive processing-structure-property relationship, we propose a simple strategy to print functionally graded LCEs, which greatly increases the design space for creating active morphing structures. We further demonstrate mitigation of stress concentration near the interface between an actuatable LCE tube and a rigid glass plate through gradient printing. The strategy developed here will facilitate potential applications of LCEs in different fields.

Potassium-insertion in polyacrylonitrile-based carbon fibres for multifunctional energy storage, morphing, and strain-sensing

By increasing the number of functions a structure performs it is possible to save weight and volume on a systems level. Ion-insertion in carbon fibres (CFs) is a way to create multifunctional structures for energy storage, morphing, and strain-sensing. Previous studies have focussed on lithium- and sodium-insertion to create multifunctionality. However, with a larger ionic radius and a chemistry more amenable to insertion in polyacrylonitrile (PAN)-based CFs, potassium-insertion is a promising way forward. Here, a study is conducted to examine potassium-insertion in intermediate modulus PAN-based CFs for multifunctionality. Electrochemical cycling shows a maximum reversible capacity of 170 mAh/g, with ex-situ mechanical testing showing a small impact on the CFs’ mechanical properties post-cycling. Operando measurements show a maximum reversible CF expansion during potassium-insertion of 0.24%, and analytical modelling illustrates that such strains can generate significant deformations in a morphing structure. A voltage-strain coupling of 0.26 V/unit strain is also found. Results are compared with previous work on lithium- and sodium-insertion, with the conclusion that potassium-insertion is more promising than sodium-insertion, but less so than lithium-insertion for multifunctional structures.

Passive load alleviation on wind turbine blades from aeroelastically driven selectively compliant morphing

AbstractLoad alleviation control is highly desirable to reduce penalties associated with the added structural mass required to withstand rare load scenarios. This is particularly true for wind turbine designs incorporating long‐span blades. Implementation of compliance‐based morphing structures to modify the lift distribution passively has the potential to mitigate the impact of rare, but integrally threatening, loads on wind turbine blades while limiting the addition of actuation and sensing systems. We present a novel passive load alleviation concept based on a morphing flap exhibiting selective compliance from an embedded bistable element. A multifidelity, aeroelastic tool is used to study the shape adaptability of a morphing flap indicating that passive changes from high lift generation to load alleviation configurations can be achieved by exploiting the energy of the flow. This mechanism offers a method to reduce catastrophic peak loads potentially, thus offering the possibility to lower the overall structural weight of wind turbine blades.

Reversible actuation for self-folding modular machines using liquid crystal elastomer

Robot and mechanism designs inspired by the art of Origami have the potential to generate compact, deployable, lightweight morphing structures, as seen in nature, for potential applications in search-and-rescue, aerospace systems and medical devices. To generate the folding of these origami-inspired designs, previous work has demonstrated several actuation methods (e.g. pneumatics, electrical motors, artificial muscles). However, it is challenging to obtain actuation for self-folding machines that is patternable, reversible, and made with a scalable manufacturing process. In this work, we use liquid crystal elastomer (LCE), as an artificial muscle to obtain tendon-driven actuation with a layer-by-layer process, to generate reversible self-folding modules using a Sarrus linkage mechanism. The Sarrus mechanism enables biaxial folding with a single unidirectional actuation, and allows pop-up designs of origami-inspired patterns such as a crane and a lily. In this paper, we demonstrate the design, fabrication, and reversible self-folding actuation of lightweight modules as well as distributed actuation of a crawler composed of the Sarrus mechanism modules. We predict the reversible fold angles given the contraction of the LCE actuation layer, and demonstrate that one single module is capable of lifting and holding 13 times and 38 times its weight, respectively. Additionally, we demonstrate traveling wave gaits in the modular crawler by sequentially actuating the Sarrus modules to achieve worm and caterpillar inspired locomotion, and investigate how this locomotion can be improved with directional friction pads. Finally, we show how a simplified model can be used to simulate the locomotion of this crawler, and compare the experimental and simulated locomotion.

Compliant morphing structures from twisted bulk metallic glass ribbons

In this work, we investigate the use of pre-twisted metallic ribbons as building blocks for shape-changing structures. We manufacture these elements by twisting initially flat ribbons about their (lengthwise) centroidal axis into a helicoidal geometry, then thermoforming them to make this configuration a stress-free reference state. The helicoidal shape allows the ribbons to have preferred bending directions that vary throughout their length. These bending directions serve as compliant joints and enable several deployed and stowed configurations that are unachievable without pre-twist, provided that compaction does not induce material failure. We fabricate these ribbons using a bulk metallic glass (BMG), for its exceptional elasticity and thermoforming attributes. Combining numerical simulations, an analytical model based on a geometrically nonlinear plate theory and torsional experiments, we analyze the finite-twisting mechanics of various ribbon geometries. We find that, in ribbons with undulated edges, the twisting deformations can be better localized onto desired regions prior to thermoforming. Finally, we join multiple ribbons to create deployable systems with complex morphing attributes enabled by the intrinsic chirality of our twisted structural elements. Our work proposes a framework for creating fully metallic, yet compliant structures that may find application as elements for space structures and compliant robots.

Regulating the mechanical behavior of metamaterial microlattices by tactical structure modification

Microlattice structures possess properties imparted by the architected microstructure of their constituents (unit cells) and micro/nanofeatures rather than their bulk material properties. Post-processing techniques have revealed intriguing length scale effects, encompassing enhanced behavior or shape morphing of various structures. Although monolithic structures consisting of single unit cells have been successfully fabricated by advanced additive manufacturing techniques, such as multiphoton lithography, innovations towards the tactical architecture of localized failure and design-inherent controllable collapse have been fairly limited. Using as a point of reference the octet truss structure, we designed substitutional unit cells and slip-plane defect-like features inspired by crystalline materials. Through finite element analysis, we investigated how the directional effective stiffness of a material can be augmented and altered in predetermined orientations. Fabricating these structures by multiphoton laser lithography and testing them by in situ scanning electron microscopy-nanoindentation we discovered a remarkable enhancement of the structural integrity, stiffness, and strain energy density, emulating the corollaries of hardening bulk materials. In addition, we observed that these structures demonstrate localized plastic deformation and collapse, distinguishing their mechanical behavior from the conventional layer-by-layer collapse reported in previous studies. The nanofeatures detected in fractographies obtained by helium ion microscopy provided further insight into the collapse mechanisms of the structures. The present design methodology of strategically placed structural features yields architected microstructures with microlattice geometries that can be used to enhance and control the mechanical performance of metamaterial structures.

A highly anisotropic morphing skin unit cell with variable stiffness ligaments

Despite major advances in morphing wing technology, morphing skins as a structural part of an adaptive aerospace system are still in their early development phase due to heavily contradicting requirements, such as highly anisotropic mechanical behaviour, air-tightness and lightness. Usually, airtightness in structural morphing skins is achieved with elastomeric covers which show poor mechanical performance and high weight. A novel design for an elastomer-free morphing skin unit cell is introduced and analysed in this work. A foldable unit cell is manufactured fully from lightweight engineering materials, based on hinge-like carbon fibre reinforced polymer ligaments. The latter reversibly fold a supported mid-section in order to generate large in-plane displacements with low actuation forces, while preserving a smooth surface in both states. The geometric parameters of the unit and the ligament design itself determine the mechanical response of the system. Within the design space of the unit cell, extreme global strains up to 100% and highly anisotropic mechanical behaviour is achieved, where resistance against aerodynamic loads exceeds the in-plane actuation force by a factor of 3.64. When used periodically, the novel unit cell is a promising base for a functional morphing skin system involving large displacements.

Parametric optimization for morphing structures design: application to morphing wings adapting to changing flight conditions

Morphing structures can allow significant improvements in performance by optimally changing shape across varying conditions. A critical barrier to the design of morphing structures is the challenge of determining how optimal shape changes as a function of the many operating conditions that affect optimality. Traditional engineering optimization techniques are able to determine an optimal shape only for one condition or an aggregation over operating conditions (i.e., optimizing average performance). Parametric optimization is an alternative approach that can solve a family of related optimization problems simultaneously. Herein the authors analyze the design of a structurally consistent camber morphing wing for light aircraft applications using parametric optimization techniques. The approach combines rigorous consideration of structural constraints via Class/Shape Transformation (CST) equations and use of a C1-continuous analytical representation of wing outer mold line geometry with the Predictive Parametric Pareto Genetic Algorithm (P3GA), an algorithm for nonlinear multi-parametric optimization. The system is tuned to maximize lift-to-drag ratio, a key metric for aircraft flight range. Kriging-based interpolation is applied to P3GA output to obtain an optimal solution map determining optimal shape variable values as a function of flight conditions (airspeed, angle of attack, and altitude). Solutions obtained from iterated use of traditional optimization techniques are utilized to benchmark the more novel and efficient parametric optimization accuracy. Results show that parametric optimization is useful for optimizing morphing structures across a range of operating conditions.

Self-sensing-based deflection control of carbon fibre-reinforced polymer (CFRP)-based shape memory alloy hybrid composite beams

The fabrication of carbon fibre-reinforced polymer (CFRP)-based shape memory alloy hybrid composite (SMAHC) beams is highly challenging because conductive carbon fibres alter the electrical characteristics of the beams when driven by an electrical current. Here, we propose and fabricate a CFRP-based SMAHC beam with embedded SMA actuators using the electrical insulation methods. We firstly implemented a self-sensing-based deflection control of the CFRP-based SMAHC beam for unit-step and sinusoidal wave targets using its resistance as a feedback signal for estimating self-sensed deflection. The results reveal that the deflection was well controlled; however, the responses were slightly delayed. Therefore, in our future studies, we will concentrate on overcoming this response delay. Nevertheless, we found that the proposed SMAHC beam is highly feasible and can form the basis for future self-sensing-based controllable CFRP composite morphing structures.

Laser pyrolysis for controlled morphing and chemical modification on 3D microlattices

3D microlattices fabricated through multi-photon lithography have been widely utilized for photonics, bio-scaffolds and architected metamaterials. Post-processing techniques have accentuated the reduction of the overall size to the nanoscale and the modification of the material properties of these structures. However, there are no processing techniques that can locally modify the morphology and the properties of microscale polymeric structures. In this study, our objective is to demonstrate localized nanoscale morphing and material modification of polymer 3D lattice structures through continuous wave laser induced selective pyrolysis, leading to local carbonization. Our approach of focused laser heating on single beam members precipitates thickness reduction from 500 nm to 250 nm. Local size reduction can further embrace collective mechanical deformation and form new complex geometries. Through spatially-variant laser heating on the periodic structures, curved and gradient three-dimensional shapes can be fabricated. Finite element analysis explained the resulted changes and predicted the enhanced mechanical performance. Ultimately, in situ scanning electron microscopy of the microindentation process proves that the modified structure demonstrates a substantially improved mechanical performance through the combination of high strength and ductility of the ceramic and polymer counterparts. Our findings and main results demonstrate the significance of the laser as a vital tool to ameliorate the geometrical complexity and material heterogeneity for enhanced mechanical performance.

Experimental study of multi-stable morphing structures actuated by pneumatic actuation

In this paper, we proposed a new pneumatic actuation method inspired from a soft robot, which could drive the multi-stable carbon fiber-reinforced polymer (CFRP) structure transition among five stable configurations under air pressure. Multi-stable structure was manufactured by the carbon fiber prepreg laid in asymmetrical and anti-symmetric layup methods and stacked at the joint during the preparation process according to the designed scheme. In experiments, multi-stable structures with two different fibers layups were prepared to study the snap-through behaviors. The snap-through process of two kinds of multi-stable structures was investigated by experiment. The parameters used to characterize efficiency of the pneumatic actuator actuating the multi-stable structures, including air pressure and snap time, were measured. The influence of the pneumatic actuator on the curvature of multi-stable structures was discussed by comparing the shape characteristic of multi-stable structures with or without pneumatic actuation, and it showed pneumatic actuator has little effect to the structure. The results demonstrated that the proposed pneumatic actuation could be used to drive multi-stable morphing structure flexibly. The combination of multi-stable laminates develops the function of multiple point bending and shape retention of pneumatic actuation.

Structural morphing in a symmetry-mismatched viral vertex

Large biological structures are assembled from smaller, often symmetric, sub-structures. However, asymmetry among sub-structures is fundamentally important for biological function. An extreme form of asymmetry, a 12-fold-symmetric dodecameric portal complex inserted into a 5-fold-symmetric capsid vertex, is found in numerous icosahedral viruses, including tailed bacteriophages, herpesviruses, and archaeal viruses. This vertex is critical for driving capsid assembly, DNA packaging, tail attachment, and genome ejection. Here, we report the near-atomic in situ structure of the symmetry-mismatched portal vertex from bacteriophage T4. Remarkably, the local structure of portal morphs to compensate for symmetry-mismatch, forming similar interactions in different capsid environments while maintaining strict symmetry in the rest of the structure. This creates a unique and unusually dynamic symmetry-mismatched vertex that is central to building an infectious virion.

Cell Cycle Progression Determines Wing Morph in the Polyphenic Insect Nilaparvata lugens.

SummaryWing polyphenism is a phenomenon in which one genotype can produce two or more distinct wing phenotypes adapted to the particular environment. What remains unknown is how wing pad development is controlled downstream of endocrine signals such as insulin and JNK pathways. We show that genes important in cellular proliferation, cytokinesis, and cell cycle progression are necessary for growth and development of long wings. Wing pad cellular development of the long-winged morph was characterized by a highly structured epithelial layer with microvilli-like structures. Cells of adult short wing pads are largely in the G2/M phase of the cell cycle, whereas those of long wings are largely in G1. Our study is the first to report the comparative developmental and cellular morphology and structure of the wing morphs and to undertake a comprehensive evaluation of the cell cycle genes necessary for wing development of this unique, adaptive life history strategy.

Control Authority of a Camber Morphing Flying Wing

Flying wings achieve their roll, pitch, and yaw controllability exclusively through spanwise variations of lift and drag due to the lack of an empennage. Instead of using conventional control surfaces, which increase the aerodynamic drag due to gaps and discontinuities, a morphing structure can be employed. This work considers a previously developed camber morphing flying wing, which uses 10 electromechanical actuators to achieve spanwise-varying trailing edge deflections, resulting in variations of the lift and drag distributions and generating moments around the roll, pitch, and yaw axes. To investigate the control authority of the aircraft across all flight maneuvers, a series of multi-objective aerostructural optimizations is carried out. The performance for the individual maneuvers, as well as the interaction of simultaneous roll, pitch, and yaw moments and the resulting control authority, is assessed. Because the same actuators are used for all maneuvers, a high degree of interaction has been found between the achieved rolling and yawing moments. By minimizing the additional drag with the morphing actuation, the undesired effect of adverse yaw is minimized. The results describe a possible control strategy for flying wing aircraft with optimal drag performance.

Nonlinear dynamics of a conical dielectric elastomer oscillator with switchable mono to bi-stability

Dielectric elastomer actuators (DEAs) are an emerging type of soft actuator that show many advantages including large actuation strains, high energy density and high theoretical efficiency. Due to the inherent elasticity, such actuators can also be used as soft oscillators and, when at resonance, the dielectric elastomer oscillators (DEOs) can exhibit a peak oscillation amplitude and power output with an improved energy efficiency in comparison to non-resonant behaviour. However, most existing DEOs have a fixed pre-defined morphology and demonstrate a single stable equilibrium, which limits their versatility. In this work, a conical DEO is proposed which may exhibit either monostability (i.e. one stable equilibrium point) or bistability (two equilibria). The system demonstrates a transition between two regimes using a voltage control. Such a feature allows the DEO system to have multiple oscillation modes with different equilibrium points and the transition between equilibria is controlled by an effective control strategy proposed in this work. A mathematical model based on the Euler-Lagrange method is developed to investigate the stability of this system and its complex nonlinear dynamic response in unforced and parametrically forced cases. This design has potential in more advanced and versatile DEO applications such as active vibrational controllers/ shakers, active morphing structures, smart energy harvesting and highly programmable robotic locomotion.

Composite wing box deformed-shape reconstruction based on measured strains: Optimization and comparison of existing approaches

The reconstruction of the displacement field of a structure (shape sensing) has become crucial for the Structural Health Monitoring of aerospace structures and for the progress of the recently developing morphing structures. As a consequence, shape sensing techniques based on discrete surface strains measurements have seen a consistent expansion in the last few years. In this paper, the three main shape sensing methods, the Modal Method, the Ko's displacements theory and the inverse Finite Element Method, are presented. The most recent and also novel improvements are discussed and added to the methods' formulations. Then, the three methods are numerically applied to a complex aerospace structure such as that of a composite wing box experiencing bending and twisting deformations. For the first time, a detailed investigation on the optimal strain sensors configuration is performed for all the three techniques simultaneously. Finally, the methods' performances, in terms of accuracy of the reconstruction and of number of required sensors, are compared. The three methods show different characteristics that make them suitable for different applications, depending on the level of accuracy and the number of strain information required. The iFEM is proven to be the more accurate but the more demanding in terms of required sensors; the Ko's displacement theory is capable of giving a rough estimation of the displacement field, but requires a small amount of sensors; the Modal Method represents a trade-off between the other two in terms of accuracy and number of sensors required.

On the design of three-dimensional mechanical metamaterials using load flow visualization

Mechanical metamaterials have generated considerable interest due to their unique attributes such as light weight, high strength, enhanced energy absorption, high impact, and fracture resistance. However, most of the design methodologies in literature are predominantly computational and provide limited user-insight (especially for three-dimensional designs). This paper presents an alternative two-phase design methodology for the conceptual synthesis of three-dimensional tunable mechanical metamaterials. In the first phase, we obtain a kinematically feasible conceptual topology using a building block-based design approach. In the second phase, the conceptual topology is refined using shape/size optimization to meet specific elastic properties and conform to manufacturing requirements. The building block-based approach uses a unique visualization formulation that represents the deformation behavior as ‘load flow’ in the constituent members. An immersive virtual reality tool is developed to mitigate the visual difficulty in the design of three-dimensional microstructures. Furthermore, the load flow formulation is used to propose a qualitative classification scheme for negative Poisson’s ratio elastic metamaterials based on relative values of shear moduli and bi-axial elastic stiffnesses. The efficacy of the framework is illustrated through the design of microstructures with negative Poisson’s ratios in spatial directions and the ability to achieve isotropic microstructures from a qualitative understanding of the conceptual designs. Such a framework can lead to the design of synthetic materials that have potential applications in multifunctional structures, shape morphing structures, and soft robotics.

Global patterns of reproductive and cytotype diversity in an invasive clonal plant

The reproductive systems of invasive species play a key role in determining their geographical distributions. Oxalis pes-caprae is a clonal, polyploid, heterostylous, plant native to South Africa, but now invasive in all major Mediterranean climatic regions. Here, we extend earlier surveys in the Western Mediterranean basin of floral morph ratios, reproductive traits and ploidy level to include populations from South Africa and introduced regions of Australia, California and Chile. We sampled a total of 104 populations, 33 in South Africa, 29 in Australia, 27 in California and 15 in Chile and collected data on floral morph representation (trimorphic, dimorphic, monomorphic), flower size, fruit set, and ploidy level using flow cytometry. There were significant differences among regions in floral morph structure of populations, reproductive traits and ploidy level. Populations in South Africa were exclusively tristylous and largely tetraploid (4x); Australian populations were mostly pentaploid (5x, 65.5%), comprised exclusively of the short-styled morph, with the remaining populations either dimorphic or trimorphic. Populations in California and Chile were comprised exclusively of the 5x short-styled morph. Fruit set varied dramatically among populations with no fruit produced in 5x populations. Our study demonstrates striking geographical variation among regions in reproductive systems ranging from a mixture of sexual and clonal reproduction in the native range to exclusively clonal propagation in some introduced regions. This variation is likely to have important consequences for local adaptation and should be considered in future management decisions of invasive populations.

Design and analysis of a configuration-based lengthwise morphing structure

The ability of changing aircraft wing shape or geometry has drawn great attention of researchers in the past decades. This paper proposes a new approach to design a lengthwise morphing structure for an airfoil morphing wing. The structure is composed of n planar mechanism units. Each unit is designed to have 3 degrees-of-freedom (DOFs) that can be fully controlled by two actuators using lockable joints. This structure is reconfigurable through dual mode switching by locking specific joints. The end-effector of the mechanism can reach any desired position and the morphing wing can change to any desired shape. When locking three or four actuators, the structure turns into a statically determinate or indeterminate truss structure to withstand required loads. Force analysis for the structure during morphing motions is conducted based on statics, and an optimization method is used to determine the appropriate number of actuators and their placements. Prototypes with or without skins are fabricated to verify the feasibility of the design.

Tapered elasticæ as a route for axisymmetric morphing structures

Transforming flat two-dimensional (2D) sheets into three-dimensional (3D) structures by combining carefully made cuts with applied edge-loads has emerged as an exciting manufacturing paradigm in a range of applications from mechanical metamaterials to flexible electronics. In Kirigami, patterns of cuts are introduced that allow solid faces to rotate about each other, deforming in three dimensions whilst remaining planar. In other scenarios, however, the solid elements bend in one direction. In this paper, we model such bending deformations using the formulation of an elastic strip whose thickness and width are tapered (the 'tapered elastica'). We show how this framework can be exploited to design the tapering patterns required to create planar sheets that morph into desired axisymmetric 3D shapes under a combination of horizontal and vertical edge-loads. We exhibit this technique by recreating miniature structures with positive, negative, and variable apparent Gaussian curvatures. With sheets of constant thickness, the resulting morphed shapes may leave gaps between the deformed elements. However, by tapering the thickness of the sheet too, these gaps can be closed, creating tessellated three-dimensional structures. Our theoretical approaches are verified by both numerical simulations and physical experiments.