Stiff morphing composite beams inspired from fish fins

Abstract

Morphing materials are typically either very compliant to achieve large shape changes, or very stiff but with small shape changes that require large actuation forces. Interestingly fish fins overcome these limitations: Fish do not contain muscles, yet they can change the shape of their fins with high precision and speed while producing large hydrodynamics forces without collapsing. Here we present a “stiff” morphing beam inspired from the individual rays in natural fish fins. These synthetic rays are made of PMMA outer beams (“hemitrichs”) connected with rubber ligaments which are 3-4 orders of magnitude more compliant. Combinations of experiments and models show strong nonlinear geometrical effects in these synthetic rays: The ligaments are “mechanically invisible” at small deformations, but they delay buckling and improve the stability of the ray at large deformations. We used the models and experiments to explore designs with variable ligament densities, and we generated design guidelines for optimum morphing shape (captured using the first moment of curvature), and that capture trade-offs between morphing compliance (ease of morphing the structure) and flexural stiffness. The design guidelines proposed here can help the development of stiff morphing bioinspired structures for a variety of applications in aerospace, biomedicine, or robotics.