Abstract
Morphing structures are commonplace in nature and very often these structures are not only able to change shape, but also to dynamically alter parameters, for example, stiffness. It is this combination of adaptability of form and of parameters that enables birds to fly, fish to swim, and mammals to run.This research focuses on the development of a morphing structure capable of switching stiffness under different loading conditions. The objective is to investigate the feasibility of a novel morphing trailing edge based on the interactions of an internal bioinspired structure and vacuum-packed particles. The bioinspired structure resembles the fish skeleton which has several isolated cells along the airfoil camber, filled with granular media, and fluidic channels are built throughout the cells to remove the air and compress the grains by vacuum pressure (vacuum-packed particles).
Vacuum-packed particle structures can actively adjust the stiffness of granular assemblies via the process of granular jamming, which enables granular materials to behave as either liquid or solid depending on how tightly the grains are compressed together. The ability to control stiffness in this manner offers many possibilities for the design of morphing and smart structures by enabling them to soften while morphing (vacuum off) and then stiffen (vacuum on) once the desired shape has been achieved to withstand external loads.
Three different granular materials were analyzed using four-point bending testing. In addition, digital image correlation strain mapping was used to measure the full-field axial and transverse strains in a ground coffee beam. The results of the tests revealed a non-linear softening stiffness region and a reduction in beam thickness, indicating that the grains split apart because they cannot withstand the increase in tensile stress.
Initial testing of the new morphing trailing edge with vacuum-packed particles proved the viability of the concept through successful cantilever testing (tip deflections) under various vacuum pressure levels. This prototype showed the ability of vacuum-packed particles to switch the stiffness of the morphing trailing edge with an increase of stiffness between 50 to 300% by adjusting the vacuum level in a range from 20 kPa to 99 kPa. A non-linear finite element method based on a plasticity model was developed in parallel to predict the quasistatic behaviour.
Research into developing an analytical model of the granular structure incorporating a non-linear material stress model based on the Mohr-Coulomb failure envelope was developed to obtain faster and more accurate solutions than the non-linear finite element analysis, showing a relative error of less than 10% compared with experimental data. The model was then integrated with a composite structural analysis to predict the deformation of the new morphing trailing edge concept. The numerical solutions are consistent with the experimental results.
The novelty of this research is the development of a beam model that is able to capture the variation of stiffness for vacuum-packed particles (VPP) cells. This new model allows the capturing of the flexural stiffness of numerous cell layouts that can vary the vacuum pressure level and can be pressurised or unpressurised to control and adapt the deformation of the overall structure. The main findings of this model are that it was experimentally validated with three distinct granular materials and was able to capture the non-linear behaviour under large strains. Numerous cell layouts, including the FishBAC structure, have been validated for small-strain analysis, the majority of which is primarily linear behaviour. In addition, the beam model was coupled with a panel-based aerodynamic solver to create a Fluid-Structure interaction (FSI) simulation. The FSI simulation allowed the development of a whole new passive control scheme. The control passive scheme relies on the increase in wing speed to change the trailing edge deflection of the structural-cell layout which can regulate and adapt the shape by adjusting the vacuum pressure or closing the ports to pressurised or unpressurised cells. In comparison to a standard trailing edge, it was discovered that the novel morphing structure has an improved lift-to-drag ratio (more than a 44% increase) and can be adapted depending on the speed of the wind.
This study established the fundamental concepts that enable vacuum-packed particles to be used as a variable stiffness mechanism for morphing structures. The implications of this variable stiffness mechanism will provide potential solutions to the trade-offs that most of the morphing aerostructures designs face, encouraging further research.
| Date of Award | 27 Sept 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Steve G Burrow (Supervisor) & Ben K S Woods (Supervisor) |