Session: 01-02-02: Adaptive and Multifunctional Structures

Paper Number: 121418

121418 - Differential Pressure Analysis of 3d Printed Gator Morphing Skins: Novel Experimental Approach 

Morphing aircraft present a potential solution for minimizing the aerospace industry's environmental footprint by decreasing induced drag and, as a result fuel consumption. A primary barrier to their widespread adoption is the competing design requirements of morphing skins. These skins must possess high out-of-plane stiffness to counteract aerodynamic loads, but simultaneously maintain low in-plane stiffness to minimize actuation forces. Moreover, they must provide a smooth aerodynamic surface while remaining a lightweight structure.

Addressing this challenge, a novel concept was introduced by these authors: 3D printed Geometrically Anisotropic ThermOplasic Rubber (GATOR) morphing aircraft skins. It utilizes three main principles, which sets it apart from other methods: 1) the use of multi-material 3D printers, which allow multiple materials to be printed simultaneously, 2) employing multiple formulations of Thermoplastic Polyurethane (TPU) with varying stiffness properties, and 3) exploitation geometric scaling laws and anisotropy to optimize in-plane and out-of-plane mechanical properties. This research evidences a substantial simplification in morphing skin design. Stiff stringers were directly printed onto the skin sheet, creating an impeccable bond between different TPU formulations negating the complex assembly of the aerodynamic skin sheet to the underlying structure. When applied to a 1.0m long FishBAC morphing trailing edge, wind tunnel tests confirmed no decrement in aerodynamic efficiency.

Capitalizing on geometric scaling principles, the GATOR technique printed a zero Poisson's ratio MorphCore directly onto a highly flexible skin sheet using both stiff and soft TPU formulations respectively. The core imparts out-of-plane stiffness, while the skin ensures a smooth aerodynamic surface, collectively producing a lightweight morphing skin in a singular manufacturing process. This tailored printing addresses the conflicting structural prerequisites. As part of this research, a novel static method to assess the differential pressure performance of 3D printed GATOR skins across stretch ratios of λ=1 to λ=1.6 was developed, eliminating the need for complicated, time-consuming and expensive wind tunnel testing. This method fully harnesses the power of additive manufacturing. A single panel was printed, subsequently folded around two end plates and sealed, forming an airtight inflatable chamber. The end plates, attached to a Shimadzu EZ tensile tester for specimen stretching, incorporated both a pressure sensor and supply port. Digital Imaging Correlation (DIC) was employed to record intricate skin sheet deformations, while internal pressure variations were monitored via the pressure sensor. In tandem, a comprehensive high-fidelity FEA model was developed, based on hyperelastic material properties outlined in our previous studies using 3D solid and 2D shell elements to model the core and skin sheet respectively.

These experiments yielded encouraging outcomes. The specimen could be stretched to a ratio of 1.6 while maintaining consistent pressure throughout the extension cycle with only a limited amount of leakage. Notably, the DIC data revealed local out-of-plane deformation of the unsupported skin sheet up to a ratio of λ=1.1. Consequently, overall out-of-plane deformation decreased by 11.2%. However, for stretch ratios exceeding λ=1.1, the out-of-plane deformation increased again to 12.3% due to the enlarged pressurized surface area. The DIC data furthermore showed that core deformation pattern projected onto the skin sheet results in a local strain amplification effect, which as a result can reduce the amount of pre-tension required to reduce the local ballooning effect. The FEA model demonstrated its efficacy in predicting the deformation patterns of the GATOR skin with a very complex core behaviour caused by pre-strained and unstained material properties.

Presenting Author: Rafael Heeb University of Bristol

Presenting Author Biography: Final year PhD student conducting research on 3D printed morphing aircraft skins.