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PUL-AERO: High Quality Curved Aerospace Composites

Profiles like stringers, frames and beams are important structural elements of an aircraft, and profiles of a variety of shapes and complexity are estimated to represent about a third of an airframe by weight. The substitution of these, today mainly aluminium components, by CFRP parts offers a weight saving potential of more than 20%.

This weight reduction can only be achieved if suitable production methods for complex CFRP components become available, which can reduce the manufacturing costs by a significant factor compared to prepreg (autoclave) technology. Otherwise the desired increase of CFRP parts in an airframe will fail due to the high production costs.

Pultrusion is widely used for production of linear, non-aerospace grade composite parts, where its benefits include the possibility for continuous production, the repeatability of the process and the low production costs (calculated as cost per meter of composite produced). However, the drawbacks of the pultrusion process for the aerospace industry are; the difficulty in producing curved parts, and the difficulty in handling aerospace grade epoxy resins, which require careful temperature control and slow curing (resulting in slow pultrusion line speed).

The PUL-AERO project set out to resolve the technical problems of pultrusion when it comes to the production of curved aerospace composites parts and take advantage of the benefits of the process.

The objective of the PUL-AERO project was to design and build a pultrusion line for the production of composite stringers for the aerospace industry.

Work programme 

The PUL-AERO project has developed a two-stage pultrusion line for the production of straight and curved composite profiles, complete with resin injection/recirculation technology, process monitoring/QA and in-line NDT inspection. The process offers the benefits of continuous production, repeatability, low production costs and the potential flexibility to manufacture stringers with different curvatures. This gives the process the potential to provide a cost-effective solution whist meeting stringent aerospace quality standards.   

The PUL-AERO pultrusion line has a number of special features:

Separation of the process into two stages

In stage 1, linear heated dies partially cure the resin in order to obtain dimensional stability. Then, in stage 2, a post-forming die introduces any required curvature and fully cures the resin. This design feature offers the potential to create stringers with a range of different curvatures by simply replacing the post-former die, rather than having to reset the whole pultrusion process. Sections may also be removed after stage 1 and post-formed off-line, enabling the production of joggled pultrusions.

A resin injection/recirculation system

Resin injection ensures the correct amount of resin is introduced for complete wetting of the fibres and considerably reduces wastage over a resin bath approach. Sensors in the collector tray monitor the resin temperature and viscosity and only recirculate the resin if specific condition criteria are met, improving the consistency and quality of the pultruded product.

An integrated process monitoring and QA system

The sensors collect information from the line and provide the user with information about the line operation. The output of the system is a complete signature of the process, which can accompany the produced part. This is seen as very attractive by aerospace companies in order to fulfil certification standards.

In-line NDT system

Real time inspection of parts allows any inconsistencies in the manufacturing process to be identified immediately, leading to a reduced scrap rate and early remedial actions. It prevents parts being shipped around the world for inspection only to be rejected, saving both time and money. 

Fig 1. A schematic diagram of the PUL-AERO pultrusion process
Fig 1. A schematic diagram of the PUL-AERO pultrusion process

TWI was involved in:

  • Modelling of the thermoset resin properties in order to design the pultrusion process based on values of materials properties 
  • Three-point bend testing of the T-section composite stringers with digital image correlation and acoustic emission monitoring (Figure 2). This allowed the relative contributions of shear and bending to be established, in order to determine whether the stringers met the performance criteria 
  • Cost analysis of the PUL-AERO pultrusion process against state-of-the-art alternative manufacturing processes for aerospace stringers and identification of market potential. This ensured that the innovative fabrication route was commercially viable

The PUL-AERO project has developed and built an advanced pultrusion line for the manufacture of aerospace-grade stringers that are cheaper and lighter than the competition.

A commercial pultrusion line (including the equipment and technology developed in the PUL-AERO project) is housed in an aerospace dedicated wing at Exel Composite’s manufacturing facility in Runcorn, UK. The consortium is working with the relevant bodies to achieve the aerospace qualification standards. Serial production is expected to begin in 2018.

It is expected that there is also a market for the PUL-AERO technology within the automotive, energy, construction, marine and sport/leisure industries.

References and credits

This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 605613.

Project partners:
Advances & Innovation in Science and Engineering Co, Greece
Cranfield University, UK
Israel Aerospace Industries Ltd, Israel
Isojet Equipment Sarl, France
TWI Ltd, UK
Exel Composites UK, UK

For more information, please email contactus@twi.co.uk

Fig 2. Three-point bend testing of a PUL-AERO stringer
Fig 2. Three-point bend testing of a PUL-AERO stringer
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