Harnessing pattern transformation of honeycomb structures for soft actuators design

Periodic elastic perforated plate materials undergo pattern transformation when subjected to uniaxial compressive load exceeding a threshold. However, for some of the conventional regular honeycomb structures, the pattern transformation characterized by local buckling is not the primary deformation pattern. This situation can be changed by tuning the nodes of the honeycomb structure. The pattern transformation is successfully realized in square, rhombic and hexagonal honeycombs (y-direction) by enlarging the nodes, together with the transition of the Poisson's ratio from positive to negative. Additionally, increasing the size of the nodes also significantly enhances the stiffness, where the Young's modulus of the rhombic and hexagonal honeycombs increases by 18.14 times and 19.30 times, respectively. The proposed bi-material honeycomb structure further explicates the conclusion that the pattern transformation is the result of the enhanced stiffness of the nodes. A hypothetical equivalent model is proposed to explain the effect of enlarging the nodes on pattern transformation of honeycomb structures, where larger nodes make the ligaments more susceptible to slight disturbances, leading to new deformation patterns. The pattern transformation behavior is further explored for the design of soft actuator, which is driven by uniaxial compressive load and combined with rigid components to achieve directional movement of the soft robots.