Morphing aircrafts can change their shape according to different flying environment and conditions, which makes their aerodynamic efficiency much better than traditional aircrafts. In order to achieve multi-dimensional deformation, mechanical metamaterials that exhibit designable morphing capability have been widely studied. Particularly, structures with coupled tension‒tsist characteristics are necessary in case that attacking angle should be changed to adjust the aerodynamic load distribution on the wing surface. Therefore, this study proposes a novel metamaterial structure that can exhibit coupled tension‒twist deformation, which significantly increases the twisting angle of a cross section under axial loading. The methodology of this study can provide valuable guideline for the future design of morphing aircrafts.

Models of the metamaterial cell structure were built using beam elements. Two types of beam structures with different coupled tension‒twist properties were designed by cell stacking. The stiffness and coupled tension‒shear deformation of the cells were studied by finite element analysis (FEA). After the cells were stacked, the coupled tension‒shear deformation of the cells transformed into coupled tension‒twist deformation of the beam structures. The deformation capabilities of the beams and related parameters were then investigated. Finally, samples of different lengths of two types of beams were prepared by selective laser sintering (SLS) of PA12 material for experimental verification. Samples were loaded by hanging weights on the free end, and the other end was fixed by an industrial bench vice. The twist angle was measured indirectly using a laser sensor.

The beams were designed with the ability to exhibit coupled tension‒twist deformation with a twist angle higher than 15°. Results show that the twist angle of the four cells combination cantilever beam is significantly greater than that of the two cells combination cantilever beam. Under a tensile load of 46.69 N, the twist angles of the aforementioned beams are 0.667° and 0.479°, respectively, with the results being consistent with the FEA. In addition, weights of the four and two cells combination cantilever beams are 319.94 and 311.32 g, respectively. This means that with 2.77% greater weight, the value of the coupled tension‒twist parameter increases by 42.97%. The twist angle for the cantilever beams is shown to increase linearly with the number of stacked cells, which enables a larger twisting angle if needed.

In this study, a novel mechanical metastructure with coupled tension‒twist deformation capability is proposed. The metastructures can transform the coupled tension‒shear deformation of unit cells into coupled tension‒twist deformation of beams by cell stacking, which can significantly improve section twist angle under axial loads. The proposed designing method is verified by finite element modeling and experimental testing of beam samples.