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Morphing Skin with a Tailored Non-conventional Laminate

Final Report Summary - MOSKIN (Morphing Skin with a Tailored Non-conventional Laminate)

Executive Summary:
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 is 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 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.
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, distance between the approximated performance or the laminate stiffness of the optimised stiffness laminate and the desired manufacturable laminate is 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 till ever 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 sand bags 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.

Project Context and Objectives:
Tailoring of composite laminates, meaning designing of the stiffness properties of laminates, can be performed either by assuming the laminate has one constant stiffness property everywhere or different stiffness properties at each spatial location for example by linear variation of fibre angles in each layer. The first scenario can be accomplished by finding the optimum stacking sequence in a laminate with straight fibre plies (Constant Stiffness or CS design). The second scenario has become possible since introduction of Automated Fibre Placement (AFP) machines for manufacturing laminates with steered (curvilinear) fibre path plies (Variable Stiffness or VS design).
Automated fibre placement (AFP) is a recent computer controlled composite manufacturing method which layup the mould or mandrel surface using thermoset, thermoplastic or dry pre-impregnated tows. Each tow comprises a bundle of unidirectional fibres which are typically 1/8, 1/4 or 1/2 inch wide. Usually fibre placement heads are capable of placing up to 32 tows in one pass. Each band of simultaneously placed tows is called a course.
The first objective of MOSKIN is to further develop the software package for optimal design of steered fibre laminates. The design variables of a fibre steered laminate are comprised of the independent stiffness matrices and laminates which are assigned to each node in the FE model. The design approach is based on a multi-step optimisation framework, in which the structural performance and manufacturing aspects are separated and for different aspects different algorithms are used. For structural performance optimisation, stiffness matrices or lamination parameters are used as design variables instead of fibre angles to eliminate the complexities related to non-convex design space and large number of design variables. Also to reduce the computational costs, convex approximations are used instead of FE analysis to evaluate the objective and constraint functions. These approximations are built based on the sensitivities of structural performance measures and are updated after each optimisation loop based on the new stiffness properties of the structure.
The real laminate configuration, including the ply angles and stacking sequence, are retrieved from the optimum stiffness design found in the first step. Therefore, the objective function to be minimised is set to be the distance between the stiffness matrices or the structural performance approximations of the real laminate and the optimum stiffness design. The manufacturing constraints including the maximum steering curvature or outer layers set to be ±45 degree layers for damage tolerance purposes are considered in this step. The tows cannot be curved more than a certain allowable curvature, because extra curvature may lead to wrinkling of the fibres placed inside the curve.
Fibre steered laminates require an extra level of post processing since the obtained laminates from the second step include the nodal fibre angles per layer and not the smooth steered fibre paths. Therefore, the streamline analogy is used to create the smooth fibre paths from the nodal discrete fibre angles meaning that the approach is similar to building the streamlines from the velocity vectors in a potential fluid flow. Adjusting the distance between the course centrelines, decision about the detailed cut/restart locations of the individual fibre-tows and the percentage of allowed overlap (0-100%) at the cut/restart locations are performed as the final post-processing steps.
CoDeT engineering has developed two modules; ALDO (Advanced Laminate Design Optimisation) and OLGA (Optimisation of Laminates using Genetic Algorithms). ALDO is used to find the set or spatial distribution of theoretical in-plane and out of plane stiffness matrices for CS or VS designs. Subsequently, OLGA is used to find the best stacking sequence of layers with straight fibres matching the CS theoretical designs. For the retrieval process of steered fibre laminates, some preliminary codes using a gradient-based optimiser are used which need to be modified by an appropriate initialisation strategy such as GA to increase the robustness and avoid being trapped in local optima. Also a course simulation program is developed which helps in adjusting the distance between the adjacent steered courses to achieve the desired amount of thickness build-ups and/or gaps. This task extends the current capabilities to constructing CATIA files which include the centreline information of steered courses placed on arbitrarily defined 3-D tooling surfaces.
To address analysing the manufacturability of the designs and obtain different process estimates such as time, etc., CoDet’s in-house software will be integrated with the robust software designed by MAGESTIC SYSTEMS [1] to evaluate different process estimates of multiple composite manufacturing processes such as Automated Fiber Placement (AFP) and Automated Tape Laying (ATL).

Project Results:
The multi-step otimisation framework, which is used to design straight and steered fibre laminates, is implemented in ALDO and OLGA software as shown in Figure 3. ALDO is the laminate stiffness optimiser used to design straight and steered fibre laminates and OLGA is the tool to retrieve real single or multi-patch straight fibre laminates from the optimised stiffness design by consideration of manufacturing constraints.
The capabilities of OLGA are enhanced to find the nodal distribution of fibre angles through using Genetic Algorithm and a gradient-based optimiser which works based on approximation techniques. The second level of refinement in OLGA capabilities is embedded in finding the smooth steered fibre paths which minimise the thickness build-ups and spread them as uniform as possible over the laminate. A parameter called average curvature is defined to control the amount of fibre steering when retrieving the spatial distribution of fibre angles from the optimum theoretical distribution of stiffness. Controlling the amount of curvature not only prevents the tows from wrinkling but also limits the amount of gaps and overlaps. Using average curvature rather than local curvature as the constraint for the amount of steering has the benefit of limiting the number of constraints to one constraint per ply. On the other hand, average curvature doesn’t have any control on the local curvatures which are often realised as the measure for manufacturability of fibre paths. However, using the average curvature constraint with the streamline method provides an effective tool for constraining the amount of steering and finding smooth fibre paths. In the second step of the multi-step framework, the structural response and the curvature constraints are approximated in terms of fibre angles and the approximated optimisation problem is solved using dual methods with a gradient-based optimiser.

Figure 3 Multi-step optimisation framework

Potential Impact:
Traditional flight control surfaces such as slats on the wing leading edge and flaps on the wing trailing edge are extended during landing and take-off to provide the required lift at low speed. For example, a higher lift to drag ratio is produced by the air flowing from bottom to the top surface of the wing through the gap between the slats and the wing, however the air flow through the gaps generates a lot of noise.

One of the main challenges of the civil aviation industry is to reduce the environmental impact. Therefore, the morphing leading edge which can deform into different shapes during cruise and non-cruise flight conditions, without the need for slats or gaps, is very interesting for the aviation industry. In addition, seamless control surfaces will contribute into reducing the drag, fuel consumption and CO2 emission through maintaining the laminar air flow.

The same advantages of morphing skins are predictable for winglets which can change their angle during different flight conditions. Traditional winglets are fixed at the cant angle, about 25 degrees from the vertical, and can reduce the aircraft’s fuel consumption by 3 to 5 percent. Boeing and Airbus are investigating movable winglets, changing their angle for different flight conditions, hoping to further reduce the fuel consumption and at the same time reduce the noise that the aircraft makes during landing. The mentioned benefits are not only interesting for military and civil aviation but also can be advantageous in designing the back wings of F1 cars to have the optimum aero-elastic behaviour.

Among different approaches to design morphing skins, fibre steered laminates combine the unique features of tailoring the spatial stiffness of the laminate to meet the required aero-elastic performance, while having the minimum weight and requiring the minimum actuation energy in a uniform thickness laminate. The aforementioned benefits are achievable by optimum exploitation of the anisotropic properties of composites in the optimised fibre steered laminates. The uniform thickness without any ply drops eliminates the design and manufacturing complications due to the stress concentration and non-symmetric laminates.

The developed composite tailoring software can be used by all composite designers in aerospace, automotive, offshore, oil and gas, wind energy industries to design conventional and non-conventional composite structures which have improved structural performance and less weight. The structural performance improvements are not only limited to aero-elastic behavior but include other structural performance measures such as buckling, material failure, stiffness, thermal, vibration and acoustic properties and etc.

A plan for use and dissemination of foreground (including socio-economic impact and target groups for the results of the research) shall be established at the end of the project. It should, where appropriate, be an update of the initial plan in Annex I for use and dissemination of foreground and be consistent with the report on societal implications on the use and dissemination of foreground (section 4.3 – H).
The plan should consist of:

• Section A

The developed composite tailoring software has been introduced to Fokker during a one-day workshop on 29th January 2013 at Papendrecht site. Also the capabilities of the composite tailoring tool are presented during TAPAS II project team meetings to the partners including:
• Airbus
• Fokker Aerostructures
• Ten Cate Advanced Composites
• KVE Composites Group
• Airborne Technology Centre
• Technobis Fibre Technologies
• KE-works
• Delft University of Technology
• University of Twente

A semi-live demonstration of the composite software tool for two case studies was presented on 7th July 2014 at CoDeT office to the team of the fourth work-package of TAPAS II project from Fokker.

A poster presentation will be arranged at "Making aircraft green and affordable" seminar held on 23rd March 2015 in Delft. The presentation will be given to several French and Dutch leading aerospace businesses and knowledge institutes attending the seminar. Effort will be put to create awareness and interest of the developed technology in the Aerospace Industry, through introduction of the composite tailoring software, analytical tools and numerical results together with visualisation of the movies and photos from the manufacturing process and test results and probably demonstration of a physical steered fibre panel.

A webex demonstration of the composite tailoring tool is planned for 18th March 2015 to a large group of engineers of Fokker.

Also it is planned to submit an article to one of the peer-reviewed journals such as Composite Structures, Composites: Part B, etc. to show the effectiveness of fibre steering in designing morphing skins.

• Section B

Considering the valuable knowledge gained in the design, manufacturing and testing of the morphing skin, continuation of the project to design, manufacturing and testing of a full-scale leading edge together with the actuation system and aerodynamic analysis is sought within the framework of CleanSky 2, horizon 2020, or within other opportunities.

The developed composite tailoring software will be equipped with GUI and licence and CoDeT’s marketing team will start commercialising the software.