The proposal aimed to extend the capabilities of a composite laminate design tool D2B (Designed to Build) to demonstrate a flexible load carrying structural skin for morphing wings. A unique composite skin construction that allowed the skin stiffnesses to vary spatially so as to provide the most flexible skin that can be morphed with minimum energy requirements, while aerodynamic load carrying capability is maintained. The software tool was used to produce laminate designs that fully take into account coupled bending and in-plane stiffnesses so that both load paths achieve proper load transmission from the points of actuation loads on the skin to the fixed points, while the coupled tailored bending stiffness distribution ensured achieving prescribed deformations. The desired stiffness distribution was achieved by spatially varying the fibre orientation of the individual layers of the laminate by adopting a steered fibre construction, and by selectively terminating certain layers of the laminate to create a blended laminate thickness variation.
The integrated tools were used in an optimisation formulation with the objective of achieving a user defined deformed shape with constraints on strength, stiffness, fabrication limitations, actuation forces, while accounting for large deformations and aerodynamic loads. The optimisation formulation was also be able to calculate the energy requirement to achieve the deformed shape. The optimal fibre path distribution of the individual layers was designed in a two-step design formulation. The first step produced theoretically optimal stiffness distribution of the skin in terms of stiffness matrices, while satisfying strength and a limited number of manufacturing constraints. In the second step, fibre paths of the individual layers were computed so as to achieve theoretically optimised stiffness distribution. The optimal design was fabricated using a state-of-the-art fibre-placement machine.
Morphing skins are skins which undergo large deformations and change from one state to the other mainly to optimally adapt the underlying structure to the real time flight requirements. Design of such skins, e.g. in the leading or trailing edge of an aircraft wing, is contradictory in the sense that the skin has to be flexible enough to be able to deform to the aerodynamically optimal configuration with the minimum required actuation energy and on the other hand has to be stiff enough to withstand aerodynamic loads. Combination of contradicting requirements puts fibre steered and/or variable thickness laminates as promising candidates. The objective of MOSKIN project was to establish the framework to design such laminates from conceptual phase to laminates which can be manufactured. To facilitate reaching the main goal of the project, which was the development of stiffness tailoring software tool for variable stiffness laminates, the requirement for aerodynamic analysis was eliminated by pre-selecting a target shape to which the initial configuration has to deform. Part of the development was to create an in-house structural FEM code required to perform the analysis, evaluate the deformations and the corresponding sensitivities, which are used as inputs of the stiffness optimiser. The stiffness optimiser outputs the updated properties to the FEM and the loop continues until convergence.
The tailoring tool was developed based on a multi-step approach which separates the structural performance and manufacturing aspects and uses different algorithms, which can best address each issue, in different steps. In the structural performance optimisation step, instead of fibre angles, the laminate stiffness was used as the design variable to eliminate the complexities such as nonconvex design space and large number of design variables. Also, instead of computationally expensive finite element analysis, convex approximations of structural performance are used to evaluate the objective and constraint functions in each optimisation loop. These approximations are built based on the sensitivities of objective and constraints and are updated after each optimisation loop by finite element analysis of the laminate with updated stiffness properties. In the fibre angle/path retrieval step, distance between the approximated performance or the laminate stiffness of the optimised stiffness laminate and the desired manufacturable laminate was used as the objective. The manufacturing constraints include the maximum steering curvature, balanced-symmetric configuration and insertion of ±45-degree layers as the outer layers of the laminate.
In the beginning, the demonstration article was selected to be a real size leading edge. CoDeT received the geometry and loading of the initial and target configurations from the ITD partners, however, the actuation system and thickness of the laminate were not delivered to CoDeT due to confidentiality issues. Although the developed tool was verified using the design case of a variable thickness straight fibre panel from a previous research, after a lot of trial and error in selecting the actuation loads and laminate thickness, CoDeT was not able to design the leading edge skin such that it can deform to the defined target shape precisely enough. Since trial and error cannot continue forever and due to the limited time and budget, the demonstration article of choice was changed to be a flat panel, the initial and target shape and loading was provided by the topic manager. The initial design trials were not successful and the reason was diagnosed to be the selected laminate thickness which did not allow the bending stiffness of the variable stiffness panel to be within the required range to deform to the specified target shape, whatever the fibre angles are. Later, by knowing the bending stiffness of the isotropic panel, the deformation of which under a load case was set as the target shape, the thickness of the laminate was selected such that a reasonable range of bending stiffness is resulted and hence the variable stiffness panel could always deform to the target shape under any other load cases.
The designed panel was manufactured with an Automated Fibre Placement (AFP) machine and tested using actuators and sandbag weights to resemble dead aerodynamic loads. The measured deflections from the test are very close to the FE analysis results and the discrepancy could be assigned to the manufacturing of the panel.