A highly anisotropic morphing skin unit cell with variable stiffness ligaments

Michael Kölbl,Maria Sakovsky,Paolo Ermanni

doi:10.1016/j.compstruct.2020.112801

Abstract

Despite major advances in morphing wing technology, morphing skins as a structural part of an adaptive aerospace system are still in their early development phase due to heavily contradicting requirements, such as highly anisotropic mechanical behaviour, air-tightness and lightness. Usually, airtightness in structural morphing skins is achieved with elastomeric covers which show poor mechanical performance and high weight. A novel design for an elastomer-free morphing skin unit cell is introduced and analysed in this work. A foldable unit cell is manufactured fully from lightweight engineering materials, based on hinge-like carbon fibre reinforced polymer ligaments. The latter reversibly fold a supported mid-section in order to generate large in-plane displacements with low actuation forces, while preserving a smooth surface in both states. The geometric parameters of the unit and the ligament design itself determine the mechanical response of the system. Within the design space of the unit cell, extreme global strains up to 100% and highly anisotropic mechanical behaviour is achieved, where resistance against aerodynamic loads exceeds the in-plane actuation force by a factor of 3.64. When used periodically, the novel unit cell is a promising base for a functional morphing skin system involving large displacements.

Highlights

Despite major advances in morphing wing technology, morphing skins as a structural part of an adaptive aerospace system are still in their early development phase due to heavily contradicting requirements, such as highly anisotropic mechanical behaviour, air‐tightness and lightness
Airtightness in structural morphing skins is achieved with elastomeric covers which show poor mechanical performance and high weight
In-plane folding of unit cell The experimentally observed folding process and the simulated change of transverse ligament curvature, Δκ, of two unit cells with α 1⁄4 70 and inwards and outwards facing tape spring ligaments are shown in Figs. 8 and 9, respectively

Summary

Introduction

Despite major advances in morphing wing technology, morphing skins as a structural part of an adaptive aerospace system are still in their early development phase due to heavily contradicting requirements, such as highly anisotropic mechanical behaviour, air‐tightness and lightness. A foldable unit cell is manufactured fully from lightweight engineering materials, based on hinge‐like carbon fibre reinforced polymer ligaments The latter reversibly fold a supported mid‐section in order to generate large in‐plane displacements with low actuation forces, while preserving a smooth surface in both states. As an integral part of the structure, they have to provide superior mechanical properties such as high out‐of‐plane (OOP) and shear stiffness in combination with a smooth and closed aerodynamic surface. Conventional wings achieve this with high performance composites or aluminium alloys, which cannot undergo elastic in‐plane (IP) deformations of the magnitude required for adaptive wings [2]. Corrugated structures, a second approach, exhibit a distinct orthotropic behaviour, which allows a high compliance transverse to the

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