Exploring the bending behavior of 3D star-shaped auxetic metamaterials for morphing airfoil applications

This study comprehensively investigates the bending behavior of 3D star-shaped auxetic (3D-SAU) metamaterials, which exhibits a distinctive negative Poisson's ratio (NPR) effect along three orthogonal spatial directions. Parametric analysis reveals that the out-of-plane NPR effect is significantly enhanced by increasing the hexagon side length ( l ), whereas reduced by increasing the offset angle ( α ) or interlayer spacing ( h ). Results extracted from experimental and numerical three-point bending tests indicate that, auxetic (3D-SAU and 3D-RE) metamaterials exhibit 3.5 times to 10.7 times higher bending compliance than the traditional BCC one, due to their distinct deformation modes under bending loads. Notably, the architecture of the 3D-SAU unit cell, featuring reversely staggered star-shaped frames, facilitates a unique deformation mechanism dominated by rod rotation, thereby ensuring the metamaterials superior structural compliance under bending loads. Furthermore, the potential application is explored by integrating 3D-SAU metamaterials into morphing airfoils actuated by shape memory alloy (SMA) skins. A functionally-graded design for the unit cell’s α , has been introduced to enable controllable and localized deformation of the airfoil profile. Through a combined experimental and numerical approach, it is found that the 3D-SAU metamaterials not only achieves exceptional deformability but also effectively reduces stress concentration through its tailored compliance. This work establishes a novel design framework for high-performance morphing airfoils that achieves a synergy between large, controllable shape adaptation with inherent impact-resistance for advanced aerospace applications.