Dapta

Design and analysis of RTS steered-fibre composites

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Box_manufacturing_Q42021
Reduced scaled wing box demonstrator with RTS skins (reproduced with permission from iCOMAT)

Dapta has recently been developing a new software tool for the design and analysis of steered-fibre composites as part of a consortium of companies within the Innovate-UK/ATI-funded MASCoTS project. In this post, we take a brief look at steered-fibre composites and particularly RTS composites. What are they? Why do we want to use them? And how do we design with them? Read on to find out more.  

Composites in Aerospace 

The latest generation aircraft are increasingly made of composite materials, with the A350 and 787 being over 50% composites in weight [1]. The main benefits of composites are the increased stiffness and strength to weight ratios provided by these materials. 

Components in aerospace are mostly made from laminates of thin plies, where the fibres in each ply run in a straight line. However, the plies are only very stiff and strong in the fibre direction. This means that engineers need to design laminates with plies stacked in many different directions to get the desired performance, whilst being as light as possible. Indeed, more structural weight means more fuel burn and fewer passengers or less payload. So minimising the mass of the structure is usually a primary design target in aerospace applications.

What are steered-fibre composites and why are they beneficial? 

With steered-fibre composites (also called variable-stiffness composites), engineers have the option to change the fibre directions within each ply, as well as stacking plies with different fibre orientations through the laminate thickness. 

The use of steered-fibre composites opens-up the design space and has been shown to lead to lighter, more efficient designs in a number of applications, for example, by steering fibres around holes or cut-outs in complex parts [2]. By steering the fibres, the number of stacked plies and their surface area can be optimised for structural efficiency, which leads to lighter designs. The use of fewer plies and the increased control over ply shapes has another benefit: it potentially speeds-up the manufacturing process and reduces material wastage.

The ability to design the local stiffness of composite parts more effectively can also enable the use of composites for other applications, which have been largely un-exploited for commercial aircraft. A recent study estimated that the use of steered fibre composites for aeroelastic tailoring could reduce the fuel burn by 1.5% compared to straight-fibre composites [3]. In this case, the aeroelastic benefits are due to the ability to control the passive wing deflections in flight, reducing aerodynamic drag and improving load distributions when hitting gusts for example.     

The research and development into steered-fibre composite manufacturing technologies and their applications is still very much an active field [4]. 

What is RTS?

Rapid tow sheering (RTS) is a new patented manufacturing method for steered-fibre composites that promises to revolutionise the way that high-performance composites will be designed in the future. It is being developed by the University of Bristol spinout iCOMAT (icomat.co.uk), who state that RTS is the world’s first automated composites manufacturing process that can place carbon fibre tapes along curved paths without generating defects, at rates suitable for high volume manufacturing. 

One of the main benefits of RTS is that the fibres can be steered to a tight radius (50mm), without generating fibre wrinkling, gaps or overlaps, which are defects that can significantly compromise the structural performance. This is a key advantage compared to other steering methods, such as ATL, AFP or TFP (yes – we do love three-letter acronyms!). 

In addition, with RTS the thickness of the ply changes as a function of the steering angle (or more precisely “shearing”-angle) as shown in Figure 1. This means that thickened laminate regions can be introduced purely by locally shearing the fibres, potentially removing the need to add plies near joints or other high stress areas. 

RTS_steering_concept
Fig 1 - RTS ply thickness t changes as a function of the shearing angle Θ

What are the challenges of designing with steered-fibre composites?

Commercial software for Computer Aided Design (CAD) and Finite Element Analysis (FEA) do not currently allow users to easily define composite materials with curved fibre paths. This lack of design and analysis tools has effectively prevented the use of steered fibre composites in industrial applications in recent years. 

A consortium of iCOMAT, Dapta, MSC Software and TWI was formed in 2020, and funding obtained from the UK ATI for a project, MASCoTS, to address this specific challenge. Since then, the project developed RTS hardware, design software, and led to the manufacture and testing of two structural demonstrators. In this project Dapta specifically focussed on the development of multidisciplinary optimisation tools for steered fibre composite wing structures. A reduced scale box demonstrator was designed by Dapta and manufactured by iCOMAT. The structural test of the box was completed at TWI in early 2022.

Our design approach for steered fibre components

 
The RTS design software developed by Dapta in the MASCoTS project defines plies in terms of parametric fibre paths, which are functions of local fibre and machine head angles. These design variables are defined at discrete “control points” on the surface of the component, with intermediary values being interpolated using linear or higher-order variations as shown in Figure 2. The approach can be combined with standard composite design parametrizations, which allow ply drops and stacking sequence variations to occur as required.   
path_control_points
Fig 2 - Sheared fibre path as a function of control point design values

A key benefit of this approach is that the number of design variables (i.e. the number and location of path control points) can be varied as needed, whilst guaranteeing continuous fibre paths. In addition, manufacturing constraints such as the maximum path curvature or shearing angle can be implemented relatively easily. 

The software currently automates the creation and analysis of MSC Nastran FEM models based on a few RTS laminate property inputs. Analysis outputs are also read automatically, which effectively allows us to execute parametric design studies and fully automated design optimisations. Multiple FEM model analyses can be chained or coupled with other analyses, enabling a truly flexible and multidisciplinary design approach to steered fibre composites.

The Dapta design software has the capability to rapidly translate the optimised designs into 3D geometry STEP files of the paths which can be directly imported into CAD / CAM software tools for manufacturing purposes.

References:

[1] Xu, Y.; Zhu, J.; Wu, Z.; Cao, Y.; Zhao, Y.; Zhang, W. A Review on the Design of Laminated Composite Structures: Constant and Variable Stiffness Design and Topology Optimization. Adv. Compos. Hybrid Mater. 2018, 1, pp. 460–477

[2] C.S. Lopes, Z. Gürdal, P.P. Camanho, Tailoring for strength of composite steered-fibre panels with cutouts, Composites Part A: Applied Science and Manufacturing, Volume 41, Issue 12, 2010, Pages 1760-176. 

[3] Timothy R. Brooks, Design Optimization of Flexible Aircraft Wings Using Tow steered Composites, PhD Thesis, University of Michigan 2018

[4] Jeff Sloan, Tow steering, Part 2: The next generation, Compositesworld, 2021,  https://www.compositesworld.com/articles/tow-steering-part-2-the-next-generation

Download a copy of our case studies below. Fill in your details for instant access or get in touch with us to find out more. 

Case Study 1: Optimal compression panel design with RTS composites

 
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