Adaptive stiffness in lattice metastructures through tensile-buckling inspired topology morphing

This paper explores the use of simultaneous tensile buckling of unit cells to induce a transformation in lattice topology. Under tension, unit cells undergo passive transformation from a rectangle-like to a triangle-/pentagon-like topology, with an associated change in the effective stiffness properties. This behaviour is investigated through finite element analysis and experiments, with analytical results providing insights into the observed behaviour. The analysis identifies (i) that the initial unit cell topology (rectangular) is dominated by membrane effects, (ii) the transformation phase is associated with negative stiffness, and (iii) once formed, the new topology (triangular/pentagonal) exhibits increased stiffness in both compression and tension. Finite element analysis confirms that the unit cell behaviour is also preserved in lattices. Under tension, the lattice undergoes a seven-fold increase in stiffness as it transitions from its initial to the new topology, with a regime of negative stiffness during this transformation accounting for approximately 82% of its total elastic deformation. This new approach to elastically tailor the nonlinear response of (meta-)materials/structures has the potential to contribute to the development of novel tensile energy absorbers

Morphing composite cylindrical lattices: modelling and applications

Morphing composites have the potential to significantly reduce the mass and volume of deployable space components, expanding the applicability of smaller spacecraft to interplanetary missions and beyond. In this thesis, the analytical modelling of the morphing composite cylindrical lattice is further developed from a one-dimensional model by including the transverse curvatures, membrane strains and thermal residual effects, to improve the applicability of the morphing lattice to deployable systems. To this end, the analytical model is utilised in the development of a deployable space boom and a new deployable toroidal lattice. The morphing composite lattice is a structure comprising helical strips of carbon fibre composite, bound to a cylindrical shape. By tailoring the manufacturing parameters of the composite strips, the lattice is capable of morphing from a compact stowed shape to an extended deployed shape and any configuration in-between. In this work, the modelling of the lattice is enhanced by including the transverse curvature and membrane strains in the calculation of the stability landscape. The new formulation is capable of modelling lattice strips of arbitrary width that use symmetrical laminates. Further improvements include the addition of thermal strains and curvatures that develop during the cool down after curing at an elevated temperature. These thermal components have the potential to significantly alter the stability landscape of the lattice. This model was subsequently used to develop a thermally actuating lattice that is stowed at room temperature and self-deploys at 120°C. The accuracy of these models is verified through comparison with both finite element modelling and experimental testing. The developed model is then used to design two lattices for use in a deployable space boom. This boom weighs only 0.4kg and morphs from a compact 1U (1000 cm3 ) form factor, to an extended length of 2m. The lattices of the boom are tested in deployment force and bending experiments, which correlate well with numerical models. Finally, a new type of morphing lattice has also been developed that deploys along a toroidal path, rather than a straight one. It achieves its unique deployment path by the tailoring the pitch of the lattice fasteners. A numerical model of the curved lattice is developed to predict the stability landscape, which is verified by comparison with experimental testing

A variable-topology morphing composite cylindrical lattice

Morphing composite structures are of significant interest due to the fact that they exhibit superior mass‐to‐ stiffness ratios and a large degree of tailorability in comparison to traditional materials and structures. One such morphing composite structure is the multistable composite cylindrical lattice. Current work introduces a novel variable‐topology morphing mechanism to it through the use of both permanent magnets and electro-magnets. By replacing a set of mechanical fasteners from the central intersection of the lattice strips with a bespoke variable‐topology mechanism introduces a controllable and replicable semi‐autonomous means for topology morphing. The variable‐topology mechanism allows the structure to transition from being a linear deployment actuator to one that deploys along a curved path, without need for external mechanical input. The behaviour of both the variable‐topology mechanism and the topology‐changing cylindrical lattice are thoroughly characterised through a combination of mechanical and virtual tests

Morphing lattice boom for space applications

Structures used in space applications demand the highest levels of stiffness for their mass whilst also performing in a hostile environment. To partly address these requirements and so as to also pack efficiently for stowage during launch we propose a new type of compact telescopic morphing lattice space boom. This boom stows within a 1U CubeSat volume and is lightweight being only 0.4 kg. The boom has a total length of 2 m in its deployed state which is 20 times its stowed height. The device comprises two multi-stable cylindrical composite lattices that are joined telescopically. These lattices nest inside one another in the stowed configuration, with the objective of improving packaging efficiency. Notably, prestress and lamina orientation are used to smoothly change shape from being compact when stowed to being extended when deployed. The lattices in the boom have been designed to maximise deployment force and to be self-deploying by tuning manufacturing parameters. As a result, only a small, lightweight mechanism is required to regulate deployment speed of the lattice boom. By reversing its direction, this mechanism can be used to retract the lattice boom to its stowed configuration, thereby enabling two-way reconfigurability

Toroidal deployment of morphing cylindrical lattices

Morphing composites are relatively new types of structure that change their shape due to inherent combinations of anisotropy, pre-stress and curvature. They have found use in deployable spacecraft structures, including booms and solar generators. However, little work has been completed on the utilisation of morphing composites in the design of curved space structures, e.g. parabolic dish antennae. This study focuses on the development and testing of a morphing composite cylindrical lattice that deploys along a toroidal (curved) path, which has potential to be used in future deployable antennae. This structure achieves its curved deployment through the tailoring of lattice fastener pitch, where the fasteners are more closely spaced on the inside of the curve and further apart on the outside. A numerical model of this new structure is developed and compared with a straight cylindrical lattice of similar configuration, to determine the effect that curving a lattice has on the stability landscape. The numerical model shows that the curving lattice produces a larger longitudinal curvature during deployment and smaller transverse and twisting curvatures, than the equivalent straight lattice. It also produces a marginally smaller force and extends to a slightly shorter deployed state. This numerical model is verified by comparison with experimental testing, which shows good agreement in key areas of the stability landscape

Reconfigurable helical lattices via topological morphing

Composite materials can enhance morphing and deployable structure capability due to their high degree of tailor ability and their favourable stiffness- and strength-to-weight ratios. One such structure, the bistable helical lattice, is augmented in current work. To date this type of structure, shows promise in aerospace systems which require linear actuation. Herein, morphing capabilities are enhanced by removing traditional mechanical fasteners at the joints, and replacing them with magnets which allow detachment and re-attachment in a controlled, purposeful way. Within a helical lattice structure, joint detachment creates new functionality by allowing a new topology to be formed which is used to convert a linear actuator to one that is curved and then back again, when the joints are reattached. The required force to actuate the topological change is characterised through the use of both finite element analysis and experimental testing. The structural response is observed through the manufacture and testing of a demonstrator which replaces the traditional joints with a series of magnets in order to capture this variable topology behaviour

Thermal stresses in composite cylindrical lattices

Deployable spacecraft technology should be both lightweight and compact for storage while also being rigid and expansive once deployed. A new type of structure that can meet both of these requirements is the morphing cylindrical lattice. This multi-stable structure can morph from a compact stowed state, to a long and slender deployed beam. It comprises narrow strips of carbon fibre composite material, making it particularly suitable for deployable booms, solar arrays and antennae. While existing modelling techniques focus on predicting the stability of lattices using symmetrical laminates, current work extends upon state-of-the-art by including the effects of thermal strains and curvatures that arise in non-symmetrical laminates when cured at elevated temperatures. As non-symmetrical laminates cool during post-cure, thermal stresses increasingly develop due to the variation of in-plane thermal expansion coefficient through the thickness. The model developed in this work, includes thermal stress effects, allowing for the design of thermally actuating lattices. This model is verified through comparison with finite element analysis and experimental data, both of which show excellent agreement

Static test of a thermoplastic composite wingbox under shear and bending moment

The proof of an aircraft’s structural integrity and safety is typically provided by analysis and supported by structural test evidence. The experimental test of a wingbox, which is the main structural component of a wing, can provide useful data on assessment of its structural performance in an actual airplane for the design objectives. To this end, a testing fixture was designed and manufactured to introduce a prescribed shear force and bending moment at one end of a variable stiffness thermoplastic composite wingbox and react the load at the other end. We report experimental results and compare them with detailed finite element data