Morphing Skins Research Articles

A novel cellular structure with center-symmetric cell walls for morphing applications

Cellular structures are potential candidates for the supporting framework of flexible morphing skins. Unlike traditional zero Poisson’s ratio (ZPR) cellular structures with axisymmetric cell walls, this paper proposes a novel cellular structure with center-symmetric cell walls. The in-plane mechanical properties of this novel structure are explored through theoretical analysis and substantiated by both finite element simulations and experimental tests. Compared to the classic accordion honeycomb structure, the elastic modulus in the x-direction of the novel structure is reduced by an average of 58%, the strain amplification rate is increased by 122%, and the in-plane stiffness anisotropy is improved by 200%. These findings suggest that the proposed structure offers far superior stiffness anisotropy and large deformation potential, making it more suitable for morphing applications than the traditional ZPR cellular structures.

In-plane mechanical properties of a novel cellular structure for morphing applications

Cellular structures provide outstanding mechanical characteristics such as low density, zero Poisson’s ratio, and out-of-plane bearing capacity, displaying their extensive application prospects as a supporting framework for flexible morphing skins. Unlike traditional cellular structures that produce two-dimensional deformation under in-plane loading, this work proposes a novel cellular structure that generates local three-dimensional deformation activated by in-plane tension. The designed topology evolves from accordion honeycomb by tilting an angle of the inclined cell walls. The equivalent in-plane tensile modulus, shear modulus, and local out-of-plane deformation are theoretically deduced considering three-dimensional internal forces existing in the inclined cell walls. The finite element simulations are established, and the corresponding experimental tests are carried out to validate the rationality of the analyzed results. Then the effects on mechanical performance versus geometric parameters of the unit cell are investigated, and the theoretical and simulated results illustrate good agreement generally.

Manufacturing and characterisation of 3D printed thermoplastic morphing skins

Morphing aircraft are one promising solution to reducing the aviation industry’s environmental impact because they can make aircraft more aerodynamically efficient, thereby reducing greenhouse gas emissions. The design of morphing skins is inherently challenging due to the competing design constraints of needing to be stiff out-of-plane to resist pressure loads while being compliant in-plane to minimise actuation energy. Previous work by these authors introduced the novel concept of Geometrically Anisotropic Thermoplastic Rubber (GATOR) morphing skins that take advantage of multi-material 3D printing and structural scaling laws to allow for better compromises between these competing design constraints. In this work, multi material Fused Filament Fabrication methods were used to 3D print the proposed GATOR sandwich panels from two different stiffnesses of thermoplastic polyurethane to demonstrate the manufacturing process and experimentally quantify the performance of some simple embodiments of the GATOR concept. The panels were mechanically tested to determine their flexural and extensional stiffnesses. Results show that the panels can be extended to a stretch ratio of λ = 1.6 with a linear response at low stretch ratios. The results from the flexural experiments show that the flexible face sheets are capable of withstanding some compression before buckling, which results in a linear behaviour at small forces. These results show the promise of the GATOR skin concept and motivate future development of designs that further exploit the underlying principles of the concept.

Elastomer-based skins for morphing aircraft applications: Effect of biaxial strain rates and prestretch

There is an emerging trend in the morphing aircraft research where two or more morphing degrees of freedom are used on a wing which leads to the concept of polymorphing. The skin of the morphing wing must be flexible in the morphing direction but stiff in other directions to withstand the aerodynamic loads and maintain the airfoil shape. Polymorphing changes the loadings profile (from uniaxial to biaxial) and increases the complexity of designing suitable morphing skins. Furthermore, elastomeric materials used on morphing wings are usually prestretched to prevent wrinkling and to increase their out-of-plane stiffness. This paper focuses on elastomeric morphing skins and it studies the effect of biaxial strain rates and prestretch ratios on important mechanical properties such as stiffness, hysteresis losses (%), and stress relaxations (%) from an experimental perspective. Three polymeric materials are considered: Latex, Oppo, and Ecoflex. This study provides a mechanical comparative understanding of the three polymers used in the morphing wing under biaxial loading (two morphing degrees of freedom).

The cooperative effect of short-cut carbon fiber and carbon black on the performance of electric-driven morphing skins

Shape memory polymers (SMPs) with variable stiffness properties become the most promising candidate materials for morphing skins. Due to the influence of external environment, electric-driven composite skin is becoming the trend of development. In this paper, rubber toughed hydro-epoxy morphing skins filled with conductive carbon black (CB) and short-cut carbon fiber (SCF) were prepared. The total mass fraction of CB and SCF was fixed at 2.6 wt %. By adjusting the content ratio of CB and SCF (CB/SCF), the in-plane mechanical property, fracture morphology, thermal-mechanical behavior, electronic resistivity, and out-of-plane shape memory behavior of five types of skin were investigated. These results showed that the addition of CB and SCF had a synergistic effect on skin performance. When CB/SCF was 5:5, the composite skin had the highest tensile strength and the lowest glass transition temperature (Tg). In the thermo-active shape memory test, the composite skin presented good shape fixity rate (Rf) and shape recovery rate (Rr). Due to the bridging effect of SCFs on CB particles, the composite skin with CB/SCF ratio of 5:5 has the minimum resistivity and can return to its initial shape within 29 s at 60 V.

An electric‐driven variable stiffness morphing skin: Preparation and properties

AbstractWith variable stiffness properties, shape memory polymers (SMPs) become the most promising materials of morphing skins. In this article, based on hydro‐epoxy resin/carboxyl‐terminated butadiene acrylonitrile/maleic anhydride matrix, composite morphing skins are prepared by doping conductive carbon black (CB). By adjusting the content of CB, in‐plane and out‐of‐plane properties of skins can be adjusted. Because of the in‐plane tensile stress acting on the skin during flight, tensile test is required. The results indicate that in‐plane tensile strength of skin decreases as the CB content increases. In electrical resistivity test, the electronic resistivity of skin decreases sharply at a low‐percolation threshold (1.9 wt%) and then maintains a low and steady level. As for out‐of‐plane properties, thermo‐ and electro‐active shape memory test are studied. All of the selected typical composite skins have 100% thermo‐driven shape recovery rate. Some skins have 100% thermo‐driven shape fixity rate and fast electro‐active shape recovery speed. When the CB content is 2.6 or 3 wt%, the skin can recover its out‐of‐plane deformed shape completely at 150 V. Benefit from easy control and remote drive, the composite SMP skins are proved to be a promising candidate material for morphing skins.

Structural design and analysis of an anisotropic, bi-axially morphing skin concept

Morphing skins as structural component in shape adaptive wings are still in their early development phase, as they need to combine contradicting requirements, such as extreme anisotropic mechanical behaviour, low structural thickness and air-tightness. Various morphing skin approaches have been designed for confined problems such as camber morphing and low load scenarios. However, to expand the applicability of morphing wings, a morphing skin with full in-plane deformability and an out-of-plane stiffness suitable for manned aircraft is necessary. In this work, a novel, elastomer free layered morphing skin is designed, manufactured, applied to a camber morphing transition region for small aircraft and analysed. The layered morphing skin is based on stacked, stiff platelets contributing to the out-of-plane stiffness, while compliant ligaments connecting the platelets provide in-plane compliance. Therefore, the layered morphing skin shows extreme orthotropy and can independently deform in both in-plane directions with an initial modulus of 198 kPa. Deformation analysis of the layered morphing skin on the camber morphing transition region confirms the bi-axial deformability and shows strains in span and chord up to 10% and 16%, respectively. Conducted pressure tests indicate an out-of-plane stiffness high enough for small aircraft, despite the demonstrator being manufactured from a polymer. The layered morphing skin concept is a promising base for bi-directionally deformable morphing skins.

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.

Shape memory textile composites with multi-mode actuations for soft morphing skins

Natural creatures morph their soft skins to adapt to the surrounding environments. Such unique morphing could be of great utility when creating bio-inspired soft robots. Here, we design shape memory textiles that may be used to create soft morphing shells woven using various kinds of fibers, including shape memory alloy wires. We explore the effects of fiber configurations, textile orientations, and lamination numbers in terms of actuation behaviors (deflections and blocking forces). Finally, soft morphing shells prepared from multiple laminates of shape memory textiles were created to allow three-dimensional surface morphing using two-dimensional laminated composites.

Investigation on the mechanical properties of topologically optimized cellular structures for sandwiched morphing skins

The mechanical performances of sandwiched morphing skin used for morphing aircraft are principally determined by the cellular substructure undertaking morphing stress. In this paper, in order to guide the design of sandwiched morphing skins utilized in different morphing applications, several optimal topologies of cellular-based structures are calculated, and the mechanical properties of them are evaluated.Initially, the topology optimization technique is utilized to minimize stiffness in the direction of deformation. This is followed by establishing 3D models through post-processing. Then the experiments and finite element analyses are performed to evaluate the designs and compare them with conventional regular honeycomb structure as well as zero Poisson’s ratio structure. Among them, a novel load transmitting block is designed for transferring shear load using axial movement. The simulated and experimental results show the superiority of optimized topologies. Also, the consistency of them guarantees the reliability of finite element model. Based on it, this research investigates the mechanical performances of periodic cellular structures and the influence depending on the number of cells. The results of the effective transverse modulus, the effective shear modulus, the out-of-plane displacement and the twist angle give support for the development of sandwiched morphing skins in real applications.

Design of cellular based structures in sandwiched morphing skin via topology optimization

Sandwiched morphing skin, composed of cellular based structure and flexible face-sheets, is one of the most promising concepts which can be used for morphing aircraft. Apparently, cellular based structure, with ability to endure aerodynamic pressure and morphing capability, is the most critical component of sandwiched morphing skin. This paper presents a design process for optimal topologies of cellular based structures for two-dimensional morphing skins, and this design procedure is effortless to be adjusted to be applied under different sets of boundary and loading conditions. The topology optimization problem formulation is established based on the simplified isotropic material with penalization (SIMP) interpolation method coupled with the method of moving asymptotes (MMA), meanwhile, Heaviside filter is adopted for reducing the occurrence of transition elements. After several iterations, various optimized topologies of unit cell are calculated corresponding to different morphing applications such as span morphing, sweep morphing and two-dimensional morphing. The mechanical properties of cellular topologies are investigated by comparing with conventional regular honeycomb structure and zero Poisson’s ratio structure via simulation after establishing 3D models of topologies. Results indicate that the design method using topology optimization technique is entirely feasible and optimal topologies of cellular based structure appear to provide superior performance.

Shape optimisation of composite corrugated morphing skins

Abstract One of the challenging parts of the morphing wing is the anisotropic skin, which must be flexible enough to allow the wing to change its shape and at the same time being stiff enough to withstand the aerodynamic loads. Composite corrugated skins have exceedingly anisotropic behaviour as they are stiff along the corrugation direction but flexible in transverse direction. Hence, elastomeric coated composite corrugated panels have been proposed as a candidate for application in morphing wings. This paper presents the shape optimisation of the corrugation with respect to better performance of the morphing skin and manufacturing constraints. The shape of the skin is optimised by minimising the in-plane stiffness and weight of the skin and maximising its flexural out-of-plane stiffness. The objective functions were obtained from homogenised model that depends on geometric and mechanical properties of the coated corrugated panel by means of finite element method for thin beams. A few methods of optimisation were considered: aggregated and genetic algorithm methods as representative of two major categories of multi-objective solving methods. A number of different approaches are proposed in order to solve the problem, such as corrugated skin with and without elastomer coating. The advantages of the new optimised shape of the corrugated skin over the typical shapes are discussed.

Effect of the Backward-Facing Step Location on the Aerodynamics of a Morphing Wing

Over the last decade, aircraft morphing technology has drawn a lot of attention in the aerospace community, because it is likely to improve the aerodynamic performance and the versatility of aircraft at different flight regimes. With the fast paced advancements in this field, a parallel stream of research is studying different materials and designs to develop reliable morphing skins. A promising candidate for a viable morphing skin is the sliding skin, where two or more rigid surfaces remain in contact and slide against each other during morphing. The overlapping between each two panels create a backward-facing step on the airfoil surface which has a critical effect on the aerodynamics of the wing. This paper presents a numerical study of the effect of employing a backward-facing step on the suction side of a National Advisory Committee for Aeronautics (NACA) 2412 airfoil at a high Reynolds number of 5.9 × 106. The effects of the step location on the lift coefficient, drag coefficient and critical angle of attack are studied to find a favorable location for the step along the chord-wise direction. Results showed that employing a step on the suction side of the NACA 2412 airfoil can adversely affect the aforementioned aerodynamic properties. A drop of 21.1% in value of the lift coefficient and an increase of 120.8% in the drag coefficient were observed in case of a step located at 25% of the chord length. However, these effects are mitigated by shifting the step location towards the trailing edge. Introducing a step on the airfoil caused the airfoil’s thickness to change, which in turn has affected the transition point of the viscous boundary layer from laminar to turbulent. The location of the step, prior or post the transition point, has a noteworthy effect on the pressure and shear stress distribution, and consequently on the values of the lift and drag coefficients.

ERRATUM to ‘Design and Experimental Characterisation of a Morphing Wing with Enhanced Corrugated Skin’

‘Design and Experimental Characterisation of a Morphing Wing with Enhanced Corrugated Skin’ by F Previtali, AF Arrieta and P Ermanni published in Journal of Intelligent Material Systems and Structures 2016; 27(2): 278–292. DOI: https://doi.org/10.1177/1045389X15595296.Owing to errors made at SAGE, the following reference in the above article is incorrect:Previtali F, Arrieta AF and Ermanni P (in press) Investigation of the requirements for passive morphing skins. Smart Materials and Structures.This reference should be replaced with:Previtali F (2015) Morphing wing based on compliant elements. Diss. ETH No. 22705, Department of Mechanical and Process Engineering, ETH Zurich.SAGE apologises to the author and to the readers.

Multi-objective optimization for the geometry of trapezoidal corrugated morphing skins

Morphing concepts have great importance for the design of future aircraft as they provide the opportunity for the aircraft to adapt their shape in flight so as to always match the optimal configuration. This enables the aircraft to have a better performance, such as reducing fuel consumption, toxic emissions and noise pollution or increasing the maneuverability of the aircraft. However the requirements of morphing aircraft are conflicting from the structural perspective. For instance the design of a morphing skin is a key issue since it must be stiff to withstand the aerodynamic loads, but flexible to enable the large shape changes. Corrugated sheets have remarkable anisotropic characteristics. As a candidate skin for a morphing wing, they are stiff to withstand the aerodynamic loads and flexible to enable the morphing deformations. This work presents novel insights into the multi-objective optimization of a trapezoidal corrugated core with elastomer coating. The geometric parameters of the coated composite corrugated panels are optimized to minimize the in-plane stiffness and the weight of the skin and to maximize the flexural out-of-plane stiffness of the skin. These objective functions were calculated by use of an equivalent finite element code. The gradient-based aggregate method is selected to solve the optimization problem and is validated by comparing to the GA multi-objective optimization technique. The trend of the optimized objectives and parameters are discussed in detail; for example the optimum corrugation often has the maximum corrugation height. The obtained results provide important insights into the design of morphing corrugated skins.

Investigation of the optimal elastic and weight properties of passive morphing skins for camber-morphing applications

The aerodynamic performance of wing structures is directly related to their external geometry. The idea of seamless shape adaptation of the wing geometry (or morphing) has emerged to provide the capability of operating optimally in a wide range of conditions. Of particular importance to realize the potential of morphing is the ability of the wing skin to conform to the different geometrical contours. Several concepts for morphing skins have been presented to address this design challenge, each presenting peculiar strengths and weaknesses depending on the chosen combination of material and structural arrangement. This paper investigates the generic structural properties of a passive morphing skin design to allow for optimal shape adaptation through cambering. The properties of the morphing skin are included among the design variables to identify their optimal value; multi-objective optimizations are used to obtain parametric results. The results indicate the need for a high anisotropy, both between membrane and bending properties and between the skin’s principal directions. The impact of the skin weight on the wing design is also shown.

Double-walled corrugated structure for bending-stiff anisotropic morphing skins

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.

Lumina: A Responsive Luminous Material for Architectural Skins Design

This research explores the potential for developing responsive composite materials with sensing, kinetic and luminous capacity for application in the design of responsive architectural morphing skins. We integrate sensing devices and building skin as one 'integrated' entity, eliminating the need to embed discrete components in a vulnerable system. This investigation develops and explores the properties and performance of a new material, Lumina for application as a lightweight, flexible and economical luminous architectural skin that responds to proximity and lighting stimuli. The design exploration uses silicone rubber, glow pigments, embedded physical computational and shape change material. It is controlled using parametric design processes.

Optimal Cellular Core Topologies for One-Dimensional Morphing Aircraft Structures

Unlike a conventional aircraft’s wing, a morphing aircraft’s wing could undergo large deformations in order to fly efficiently. This requires a wing’s skin meeting conflicting requirements such as large deformation capability to allow morphing of the underlying structure and high flexural stiffness to maintain the airfoil shape. In this paper, the design of composite skins with a cellular core is considered for the particular case of one-dimensional morphing. Cellular core topologies are calculated using a multi-objective genetic algorithm coupled with a local search optimizer. Morphological filtering is used to remove small features in the topology. As a multi-objective problem, no single solution emerges as the clear best solution because of conflicting objectives. However, the solutions found help guide the design of cellular-based morphing skins. A design is selected from the set of solutions obtained, and a cellular core under combined in-plane morphing and out-of-plane loading is examined with respect to the local stresses, the energy of deformation and the core’s out-of-plane deformation to validate the approach used.

Designing Architectural Morphing Skins with Elastic Modular Systems

This paper discusses the issues of designing architectural skins that can be physically morphed to adapt to changing needs. To achieve this architectural vision, designers have focused on developing mechanical joints, components, and systems for actuation and kinetic transformation. However, the unexplored approach of using lightweight elastic form-changing materials provides an opportunity for designing responsive architectural skins and skeletons with fewer mechanical operations. This research aims to develop elastic modular systems that can be applied as a second skin or brise-soleil to existing buildings. The use of the second skin has the potential to allow existing buildings to perform better in various climatic conditions and to provide a visually compelling skin. This approach is evaluated through three design experiments with prototypes, namely Tent, Curtain and Blind, to serve two fundamental purposes: Comfort and Communication. These experimental prototypes explore the use of digital and physical computation embedded in form-changing materials to design architectural morphing skins that manipulate sunlight and act as responsive shading devices.