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Morphing Skin with a tailored Non-Conventional Laminate


Morphing skins are skins that undergo large deformations and change from one state to the other, mainly to optimally adapt the underlying structure to the real time flight requirements. The 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 the MOSKIN project is to establish the framework to design such laminates from conceptual phase to laminates that can be manufactured. To facilitate reaching the main goal of the project, which is the development of stiffness tailoring software tool for variable stiffness laminates, the requirement for aerodynamic analysis is eliminated by pre-selecting a target shape to which the initial configuration has to deform. Part of the development is 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.

As shown in Figure 1, the tailoring tool is 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 is 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, the distance between the approximated performance or the laminate stiffness of the optimised stiffness laminate and the desired manufacturable laminate is used as the objective to be minimised. The manufacturing constraints include the maximum steering curvature, balanced-symmetric configuration and insertion of ±45 degree layers as the outer layers of the laminate.

optimisation loop

Figure1 Multi-step optimisation framework

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, the designed variable stiffness leading edge skins were not able to deform to the defined target shape precisely enough as shown in Figure 2.


Figure2 Deformed QI and VS laminates under two actuation loads (at points 1 and 2) and comparison with the initial and target shapes (actuation loads: Fx(1) = -78 N, Fy(1) = -902 N, Fx(2) = 1571 N, Fy(2) = -716 N)

Due to the limited time and budget, the demonstration article of choice was changed to be a flat panel. The target shape was set to be the deformation of the panel made of an isotropic material and under certain loading conditions. The objective was to design a variable stiffness composite panel which can deform into the target shape under any other loading conditions. The initial design trials were not successful and the reason was diagnosed to be the selected laminate thickness that did not allow the bending stiffness of the variable stiffness panel to be within the required range to deform to the specified target shape, regardless of the stiffness distribution. 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 sand bags weights to resemble dead aerodynamic loads as shown in Figure 3. The measured deflections from the test are very close to the FE analysis results as shown in Figure 4 and the discrepancy could be assigned to the manufacturing strategies and defects, testing apparatus and numerical simulation assumptions.


Figure3 Panel testing under simulated service and actuation loads


a) under dead load b) under dead and service loads

Figure4 Correlation between numerical and experimental results 


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