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Composite joints for improved mechanical and electrical performance

Periodic Reporting for period 2 - C-JOINTS (Composite joints for improved mechanical and electrical performance)

Reporting period: 2018-07-01 to 2019-09-30

The C-JOINTS project aims to introduce new joining concepts, enhance existing joining technologies and propose an innovative way for lightning strike protection (LSP) that removes the needs for insertion of metallic meshes in the composite (hence simplifying the composite manufacturing process).

The versatile nature of the C-JOINTS project, reflected by the main aim of optimising a wide range of joints found in aircraft, is in-line with the scope of Activity Line B: High Versatility and Cost Efficiency of the Airframe ITD that is oriented on “More global aero structural optimisations… can lead to further focused on drag, weight and manufacturing processes”.

Finally, the project will be consistent with the overall strategic objectives of the CSJU Joint Technical Programme and Airframe ITD, in which key directions of progress among others are:
• Developing structures with optimised usage of volume and minimized weight, cost and environmental impact.
• Ensuring the environmental and production efficiency.
Initial screening work on laser riveting, thermal spraying and alternative manufacturing methodologies generated useful information to further the project goals. Laser riveting of composites to metal was too technically immature for this project as the work found significant issues around thermal degradation of the composite because of the laser processing. The thermal spray trials showed a variable catch efficiency across the surface of the sample panels. The trials show that surface characteristics of either the fibre or the resin matrix have a significant influence on the ability of the material to accept the coating. From the work to date, the fibres exposed by the surface preparation of the composite catch more metal than the resin matrix, in reverse of what is expected.

In order to enable some of the joining methods proposed in the DoA it has been necessary to trial alternative composite manufacturing methods. The existing wet lay-up process is not compatible with the metallic stitching in particular and so trials were conducted to assess the existing materials for their suitability for other composite manufacturing methods. In light of the needs of the Topic Manager, these trials have focussed on methods available to them and without significant equipment purchases. It was conducted with the existing materials to minimise any qualification or certification issues with the existing aircraft. The Project has successfully managed to use the existing materials in a low temperature (approximately 40°C) resin infusion process to produce test parts of suitable quality. This is preferred manufacturing method as it allows the combination of different joining methods whilst also reducing the part count.

Novel joints were designed and manufactured incorporating tufting and thermal spray technologies for enhanced lightning strike protection. The tufting solutions down selected copper and aluminium threads as the most promising materials. The tufting process was developed in order to identify the best methods, thread thicknesses and alloy grades. The thermal spraying activities compared copper and aluminium coatings and characterized their properties: e.g. electric resistance (sheet resistance) as a function of the number of passes (correlated to a gram per square meter value), corrosion resistance. The best results were summarized in the deliverables, methodologies were produced and future designs and tufting/thermal spray innovative solutions were identified.

The project was overall successful and is paving the way towards lighter composite-based aeroplanes with efficient lightning strike protections.
The new innovative joints and production methodologies, including electro-magnetic compatibility (EMC) treatments (e.g. tufting and metal spray) were developed and demonstrated equivalent to enhanced performance than the reference joints taken from an existing aircraft design.

An example of this is through the use of thermally sprayed metallic coatings to replace the copper mesh as lightning strike protection material within the composite component. The benefits of this include reduced manufacturing complexity, easier maintenance of the protection and lighter components through the reduced metal content for equivalent performance.

The use of metallic tufting through composite components also demonstrated a lower-weight alternative to embedded copper straps for transferring lightning currents through composite materials as part of the grounding path. It should also be used to provide a grounding path through the composite that does not include the fastener itself, reducing the risk of damage to the fastener.
Tufting Process
thermal spray (Al) on demonstrator
lightning strike specimens with no structural damage
Copper-tufted composite fabric
lightning strike specimens with no composite damage