Design and analysis of RTS steered-fibre composites

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 shearing (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.