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 paper identifies the desirable attributes of a flexible skin of a morphing wing. The study is conducted using airfoil camber morphing as an example. The ideal flex-skin would be highly anisotropic, having a low in-plane axial stiffness but a high out-of-plane flexural stiffness. Reduced skin axial stiffness allows morphing at low actuation cost. However, for some substructure and actuation designs, a lower limit on the skin's in-plane axial stiffness may be required to prevent unacceptable global camber deformation under aerodynamic loads. High flexural stiffness prevents local deformation of skin sections between supports due to aerodynamic pressure loads, and avoids buckling of skin sections under compression as the airfoil cambers under actuation force. For the camber morphing application the strain levels in the flex-skin are not expected to exceed around 2%. If the axial stiffness of the flex-skin is reduced significantly, it may be necessary to consider aerodynamic stiffness (negligible vis-à-vis structural stiffness for classical airfoils) to accurately calculate deformation under loading. The approach followed in the study can be used to identify specifications for the skin and then reverse engineer and design highly anisotropic composite skins that meet the specifications.

An experimental investigation was conducted to determine the pseudoelastic hysteresis damping characteristics of Ni-Ti Shape Memory Alloy (SMA) wires. The comprehensive study examines the effects of cycling, oscillation frequency, strain amplitude, temperature, and static strain offset on the pseudoelastic stress-strain hysteresis of SMA wires under axial loading. Experimental data are obtained for complete austenite-martensite transformation hysteresis as well as partial transformation hysteresis. The results indicate that as the frequency of excitation increases, the reverse phase transformation from detwinned martensite to austenite commences at higher stress levels, and the area of the hysteresis loop decreases. This results in a rapid initial decrease in energy dissipation, but approaches a stable level by about 10 Hz. The energy dissipation at 10 Hz was found to be about 50% of that observed at very low frequencies. The energy dissipated at higher temperatures was found to be up to 40% lower than that observed at 90'F. The energy dissipated per cycle was found to be larger when lower values of static strain offset were used.

The pseudoelastic stress/strain hysteresis behavior observed in nickel-titanium (Ni-Ti) shape memory alloys (SMAs) above the austenite finish temperature can be exploited to provide passive structural damping in a variety of applications. The present study characterizes the damping behavior of Ni-Ti SMAs using the complex modulus approach, commonly used in structural dynamics for the characterization of damping materials. Results indicate that as excitation frequency increases, the loss modulus (a measure of the damping) undergoes a rapid initial decrease. The value of loss modulus (and available damping) at 6-10 Hz is approximately 50% of that at low frequencies, but does not show significant reduction thereafter. As the cyclic strain amplitude increases, the storage modulus (a measure of the stiffness) initially undergoes a rapid decrease, implying that the material softens with increasing motion amplitude. As the static strain offset increases, the loss modulus decreases, and the storage modulus increases. The loss modulus decreases at temperatures above , while the storage modulus shows a significant increase above associated with the SMA operating outside its ideal pseudoelastic temperature window. The experimental stress/strain hysteresis loops and the idealized loops based on complex modulus characterization compare very well for small cyclic strain amplitudes, but may differ for higher amplitudes. However, the energy dissipation estimates from the idealized and experimental hysteresis loops compare very well over the entire range of cyclic strain amplitudes, indicating that complex modulus characterization is well suited for estimating the damping capability of SMAs.