Bidirectional Stiffness Material: A Potential Solution for Passive Shape-Morphing Wings

Abstract

An ornithopter is a flying device created by mimicking the motion of birds or insects. Its wings play significant roles in determining flight performance and stability. For example, butterfly wings are demonstrated to possess significant differences in bending stiffness when considering the bending direction (up or down). Novel concepts are needed to build a passive shape-morphing wing structure that could change shape smoothly during flapping, and thus enable efficient flight. Traditionally, ornithopter wing structures have employed hinges between rigid wing sections or hinges along the leading edge to introduce the desired wing flexibility. Motivated by the scale structure found in butterfly wings, it was hypothesized that a biomimetic scale-type material system would possess the desired bidirectional stiffness response. A material system was manufactured by 3-D printing of polyhedral-shaped particles, assembling such particles in a dense planar array, and joining them with carbon fibers. In particular, the polyhedral shapes considered were tetrahedra, truncated tetrahedra, and Janus-type tetrahedra made of half soft and hard solid. These systems were tested in single-point load bending. For regular tetrahedra, the assemblies possess the same stiffness in both deflection directions ( !! !!#$ = 1 , due to symmetry), while the assemblies of the truncated tetrahedra ( !! !!#$ = 25.2 ) and of the Janus-type tetrahedra ( !! !!#$ = 4.7 ) exhibited significant bending stiffness asymmetries. These results confirm the initial hypothesis that scale-type structures are responsible for the bending stiffness asymmetry in biological wing structures. The results show that the proposed material system can imitate the function of a wing, and that the passive shapemorphing wing constructed using a bidirectional stiffness material presents a potential solution for ornithopters.