Application of Lightning Strike Protection on Thermoplastic Structures by Automated Fiber Placement

Kurzfassung

Lightning strikes can affect the airline operations, cause costly downtimes and service operations. Similar to thermoset composite parts, for carbon fibre reinforced thermoplastic (CFRTP) parts a layer of lightning strike protection (LSP) also has to be applied on the outer surface to protect the airplane structure. The automated integration of this material onto highly double curved surfaces like the rear-end of a fuselage causes multiple challenges for the production and had been investigated here. Automated Fiber Placement (AFP), where several tows can be laid up simultaneously on a surface, seems to be an appropriate technology for this application. Because of the narrow tow width of 6.35 mm, steering of the material may be realized, which in some cases is necessary for a layup with parallel courses and a constant distance between each other. Furthermore this technology is already in use to manufacture the structure itself and therefore no other cost intensive acquirements of machines are needed. In serial production of large scale thermoset aircraft parts, these LSP layers are applied either manually by workers or automatically using Automated Tape Laying (ATL) Technology. Airbus, for example, uses ATL for deposition of these layers in their A350 wing [1]. For thermoplastic LSP, a method for automated application of layers into a composite structure is described in [2]. In their project called Arches Box TP, STELIA Aerospace showed the automated layup onto a thermoplastic structure with AFP [3, 4]. However, further detailed information like material configurations, process parameters, appropriate layup design or electrical properties of lightning strike protected CFRTP parts produced with AFP could not be found. Due to this lack of process understanding, the automated layup of LSP on a CFRTP structure using AFP technology had been analysed here. Because there was no market-ready AFP material available, an appropriate material configuration had to be determined first. This final material, consisted out of raw materials like perforated (PCF) or expanded (ECF) copper foils, thermoplastic films and reinforcements, had to fulfil the requirements for an AFP process. With the manufactured LSP material automated layup trials with AFP and Xenon heating flash lamp had been realized. Focus had been the handling of the material in general. Appropriate process parameters like heating power, layup speed or compaction force had been identified with regard to the layup capability and quality. Using LSP tows with a width of 6.35 mm automatically leads to the question of the layup design and which one is most appropriate in terms of lightning strike protection. Therefore possible designs like crossed tows, longitudinal overlap or mitre joint had been identified. These designs had been investigated regarding contact quality and electrical properties. The fundamental trials presented in this paper demonstrate the feasibility of the integration of a LSP layer using AFP. Further on, a hybrid LSP material that fulfils the requirements to an AFP process and its processing window is presented. Test specimens with appropriate AFP designs under manufacturing aspects and different LSP material combinations had been produced and the electrical properties had been investigated. All results will be presented in the final paper.