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 fins do not contain muscles, yet they can change the shape of their fins with high precision and speed while producing large hydrodynamic 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 acrylic (PMMA) outer beams (‘hemitrichs’) connected with rubber ligaments which are 3–4 orders of magnitude more compliant. Combinations of experiments and models of these synthetic rays show strong nonlinear geometrical effects: the ligaments are ‘mechanically invisible’ at small deformations, but they delay buckling and improve the stability of the ray at large deformations. We use the models and experiments to explore designs with variable ligament densities, and we generate design guidelines for optimum morphing shape (captured using the first moment of curvature), that capture the 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.

Editorial: Composite materials in biological and bioinspired systems: Part II, Biological and bioinspired composites

“…The following study by Das et al . [ 8 ] introduces a synthetic morphing beam inspired by natural fish fins, comprising acrylic outer beams connected by highly compliant rubber ligaments. Through experiments and modelling, it is demonstrated that the ligaments exhibit nonlinear geometrical effects, enhancing the stability of the beam at large deformations while remaining mechanically invisible at small deformations, providing insights for optimizing morphing shape and stiffness in bioinspired structures for diverse applications in aerospace, biomedicine, and robotics.…”