Journal of Intelligent Material Systems and Structures

Broad-scale area change of a non-porous surface while maintaining resistance to aerodynamic loading was demonstrated through the development of a passive elastomeric matrix composite morphing skin. The combined system includes an elastomer-fiber-composite surface layer that is supported by a flexible honeycomb structure, each of which exhibit a near-zero in-plane Poisson’s ratio. A number of elastomers, composite arrangements, and substructure configurations were evaluated and characterization testing led to the selection of the most appropriate components for prototype development. The complete prototype morphing skin demonstrated 100% uniaxial extension accompanied by a 100% increase in surface area. Results from out-of-plane pressure loading showed that out-of-plane deflection of less than 0.1 in. (2.5 mm) can be maintained at various levels of area change under pressures of up to 200 psf (9.58 kPa). Applications to wing span morphing UAVs are also discussed.

This study presents and examines the concept of flexible skins for morphing aircraft applications comprising of a cellular honeycomb core covered by a compliant face-sheet. The overall properties of the flexible skins are then largely governed by the characteristics of the cellular honeycomb core, which are in turn dependent on the cell parameters. The results of this study showed that the cellular cores could easily undergo global strains over 10 times greater than the virgin material of which they were built. The in-plane stiffness of the cellular cores is generally several orders of magnitude lower than the virgin material. Using cores that are thicker than isotropic sheet skins, the out-of-plane stiffness can be many times greater than the sheet skin for comparable mass (due to porosity of the cellular core). In general, honeycomb cores with positive cell angles (as opposed to auxetic cores) produce a higher out-of-plane stiffness. For cellular cores made from high-strain capable materials and undergoing large strains, geometric and material non-linearities need to be considered. When the cores are stretched along the principle axes they geometrically stiffen, thereby reducing the maximum global strains achievable. When material softening is considered, the forces required to deform the cellular core to large global strains are reduced.

Cellular honeycomb cores with overlying flexible face sheets have been proposed for use as flex skins for morphing aircraft. The cellular cores, which provide underlying support to the face sheets for carrying aerodynamic loads, must have low in-plane stiffness and high in-plane strain capability. For one-dimensional morphing applications such as span-, chord-, or camber-change, restraining the Poisson’s contraction (or bulging) that a conventional cellular honeycomb core would otherwise experience in the non-morphing direction results in a substantial increase in the effective modulus in the morphing direction. To overcome this problem, this article develops zero Poisson’s ratio hybrid and accordion cellular honeycombs. Cellular Material Theory is extended, and analytical solutions for the mechanical properties and global strains of the hybrid and accordion cellular honeycombs are developed. The analytical results show excellent agreement with ANSYS finite element results. Comparing the properties shows that the hybrid and accordion honeycombs proposed have generally similar in-plane axial stiffness and strain capabilities to conventional honeycombs when the latter are unrestrained in the non-morphing direction. However, with the zero Poisson’s ratio of the hybrid and accordion honeycombs, it is observed that the axial stiffness in the morphing direction will not increase when the skins are restrained in the non-morphing direction. The zero Poisson’s ratio of the accordion and hybrid cellular honeycombs is not helpful from an out-of-plane load carrying ability standpoint. However, the out-of-plane load carrying ability of the accordion honeycombs can be superior to those of conventional honeycombs if the ‘continuous fibers’ are sufficiently thick, leading to a very large modulus in the non-morphing direction. The effective out-of-plane stiffness of hybrid cellular honeycombs, on the other hand, is poorer than conventional cellular honeycombs.

This article focuses on flexskins comprising of a cellular substructure and pretensioned facesheet for shear morphing applications. The unit cell of the substructure is a strand with some strain-relief feature, and it supports a segment of pretensioned facesheet. The function of the strain-relief feature is to reduce the peak strains in the strand and consequently the actuation work during shear morphing. Central hexagonal cells, elliptical cells, and half elliptical cells in the strand provided the sought strain relief at the edges of the strand, but the sharp corners and the proximity of these central features to those from adjacent strands during shear morphing led to a tendency for the facesheet to wrinkle. Alleviating wrinkling by increasing facesheet pretension increases the morphing actuation work requirement. Facesheet wrinkling can also be reduced by increasing strand separation, but this too requires higher prestrain in the facesheet to limit out-of-plane displacement under aerodynamic loads. The best solution was found to be the use of Gaussian- or Cosine-shaped curved strands which reduced morphing actuation force requirements by around 30% compared to straight strands used on a previous demonstration aircraft, and the peak strain levels to about 1.2% (down from 3.3% for straight strands), while avoiding facesheet wrinkling. Going from a unit cell to a finite strip accounting for boundary effects it was observed that the curved strands near the rigid boundaries of the skin panel come very close to the boundaries at high morphing angles, promoting wrinkling in the facesheet. A gradient reduction in the amplitude of the curved section along the length of a strip of flexskin approaching the problematic boundary alleviates this situation. Other approaches examined prior to the adoption of the smooth curved strands, such as selective bonding of facesheet to the strands, varying the strand thickness, or offsetting the central strain relieving feature between successive strands, were unable to eliminate facesheet wrinkling at shear morphing angles.

Morphing aircraft wings require flexible skins that undergo large strains, have low in-plane stiffness, and high out-of-plane stiffness to carry aerodynamic loads. For some morphing applications deformation and low stiffness in the flexskin is required in one direction. In these cases, a flexible matrix composite (FMC) skin is proposed as a possible solution. A FMC comprises of stiff fibers embedded in a soft, high-strain capable matrix material. The matrix-dominated direction is aligned with the morphing direction. This allows the skin to undergo large strain at low energy cost. However, the high-stiffness in the fiber-dominated direction allows application of pretension along this direction, without rupture, and is critical for the membrane skin to carry out-of-plane pressure loads without excessive deformation. An analysis for a FMC skin panel is developed and validated against experiment. The analysis is used to conduct design studies. Comparison of the FMC skin to a matrix-only skin illustrates the importance of the fiber’s stiffness in tolerating pretension and limiting out-of-plane deformation under load. The other dominant parameter that limits out-of-plane deformation is panel size, with smaller lengths in the non-morphing direction proving beneficial. In general, fiber and matrix modulus has limited effect on out-of-plane deformation of flexskin panels.

Reconfigurable and morphing structures may provide a range of new functionalities such as optimization over broad operational conditions and multi-mission capability. This article introduces a new generic approach to achieving large strains in materials with high elastic moduli (5-30 GPa). The work centers on creating variable stiffness composite materials which exhibit a controllable change in elastic modulus (bending or axial) and large reversible strains (5-15%). We have performed a simulation study to better understand the implication of various geometric design parameters on the elastic and deformation behavior. Using this information, a series of prototype materials were prepared using a commercial shape memory polymer, and measurements on these materials indicate a controllable change in stiffness as a function of temperature with large reversible strain accommodation. We have fabricated and tested several design variations of laminar morphing materials which exhibit structural stiffness values of 8-12 GPa, changes in modulus of 15-77x and large reversible axial of 2-10%. Results indicate that significant controllable changes in stiffness are possible. Further, agreement between simulations and prototype material properties indicate that simulations may be used an effective screening tool to specify micromechanical design variations for specific application requirements.