A highly deformable morphing skin unit cell employing snapping of CFRP tape springs
Creators: name: Kölbl, Michael
Publisher: name: ETH Zurich
Year: 2019

description: Structural skins for aeronautical applications must withstand high loads, inhospitable environments and provide superior mechanical properties (i.e. high local out-of-plane stiffness) in combination with a smooth and closed aerodynamic surface. Lightweight alloys and fibre reinforced plastics meet these requirements in standard wings, however as sheet material they cannot undergo elastic deformations large enough to enable morphing.

Current morphing skin approaches often utilise lattice metastructures filled with relatively dense elastic polymers with inferior mechanical properties (compared to composites and alloys) for creating a smooth, closed and compliant cover. However, the local out-of-plane stiffness of such structures is low and prone to local buckling. Therefore an unfolding skin unit (USU) was designed that avoids elastomeric fillers or sliding mechanisms, yet provides large deformability.

The novel structural design makes use of the elastic buckling of tape spring (TS) ligaments which act as hinges. These hinges support stiff platelets that only undergo rigid body motions during the in-plane deformation of the USU, meaning that common high performance alloys or composites can be used for the platelets.

The USU's kinematics is based on two discrete morphing states, namely folded and extended. While folded, one stiff auxiliary platelet is stored below two precisely lined up, in-plane movable platelets and connected to the latter via the elastically buckled TS ligaments. During the extension process, the top platelets diverge from each other and the buckled ligaments unfold due to their stored strain energy. Once completed, the auxiliary platelet fills the gap between the top platelets, creating a smooth and closed surface that is supported by the straightened TS ligament hinges.

The TS ligaments themselves were mouldlessly manufactured, employing an asymmetric [02|90] layup of thin ply carbon prepregs (NTPT Thinpreg 513, M40J fibre) with 40gsm per ply. These specifications and a curing temperature of 120°C lead to an induced transverse curvature radius of 34mm at room temperature due to thermal residual stresses. Other layups with the same material were investigated, too, but showed less induced curvature.

In a first preliminary experimental study with an oversized USU, the folding response of the USU and the out-of-plane strength of the auxiliary platelet were investigated with four different geometries for the tape spring hinges. The results were benchmarked against an USU that used transversely flat [0|90|0] sheets made from NTPT Thinpreg as hinges for the auxiliary platelet. The preliminary experiments showed that the tape spring ligament hinges lowered the actuation force for folding the USU, while simultaneously improving the out-of-plane strength of the auxiliary platelet by a factor of three, compared to the sheet ligaments.

An ideal morphing skin would exhibit zero in-plane stiffness, significant in-plane deformability and extremely high out-of-plane stiffness. Similarly, the presented USU shows a high out-of-plane to in-plane stiffness ratio which makes it suitable candidate for adaptive skin applications. Due to the fact that only stiff, additively manufactured platelets and tape spring hinges made from CFRP are used, the use of elastomeric polymers becomes obsolete. Consequently problems, such as local buckling and poor mechanical performance are avoided. Furthermore, an additional benefit of the USU is the smooth and closed surface, achieved in the two final morphing states.

The development of the USU is still in its preliminary stage and further work is necessary. This includes the miniaturisation and a detailed finite element analysis of the USU, as well as a refined experimental setup.