Michael Kölbl

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Kölbl

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Michael

ORCID

0000-0001-8614-5996

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Publications 1 - 8 of 8

Investigation of an adaptive, hinge-less, and highly shear stiff structure for morphing skins
Ott, Valentin; Keidel, Dominic; Kölbl, Michael; et al. (2020)
Journal of Intelligent Material Systems and Structures
A layered, bi-axially morphing skin concept
Kölbl, Michael; Ermanni, Paolo (2021)
Structural design and analysis of an anisotropic, bi-axially morphing skin concept
Kölbl, Michael; Ermanni, Paolo (2022)
Aerospace Science and Technology
Morphing skins as structural component in shape adaptive wings are still in their early development phase, as they need to combine contradicting requirements, such as extreme anisotropic mechanical behaviour, low structural thickness and air-tightness. Various morphing skin approaches have been designed for confined problems such as camber morphing and low load scenarios. However, to expand the applicability of morphing wings, a morphing skin with full in-plane deformability and an out-of-plane stiffness suitable for manned aircraft is necessary. In this work, a novel, elastomer free layered morphing skin is designed, manufactured, applied to a camber morphing transition region for small aircraft and analysed. The layered morphing skin is based on stacked, stiff platelets contributing to the out-of-plane stiffness, while compliant ligaments connecting the platelets provide in-plane compliance. Therefore, the layered morphing skin shows extreme orthotropy and can independently deform in both in-plane directions with an initial modulus of 198 kPa. Deformation analysis of the layered morphing skin on the camber morphing transition region confirms the bi-axial deformability and shows strains in span and chord up to 10% and 16%, respectively. Conducted pressure tests indicate an out-of-plane stiffness high enough for small aircraft, despite the demonstrator being manufactured from a polymer. The layered morphing skin concept is a promising base for bi-directionally deformable morphing skins.
A highly deformable morphing skin unit cell employing snapping of CFRP tape springs
Kölbl, Michael (2019)
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.
A highly anisotropic morphing skin unit cell with variable stiffness ligaments
Kölbl, Michael; Sakovsky, Maria; Ermanni, Paolo (2020)
Composite Structures
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.
A bi-axially zero Poisson's ratio morphing skin system
Kölbl, Michael; Bossart, Dominic; Ermanni, Paolo (2022)
A highly deformable morphing skin unit cell employing snapping of CFRP tape springs
Kölbl, Michael; Schwarzenbach, Fabian; Sakovsky, Maria; et al. (2019)
MECHCOMP 2019, 5th International Conference on Mechanics of Composites, Instituto Superior Técnico, Lisbon, Portugal 1-4 July 2019. Book of Abstracts
Highly Anisotropic Metastructures for Shape Adaptable Aerodynamic Surfaces
Kölbl, Michael (2023)

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