The Clean Sky 2 D-Joints (Design of innovative composite hybrid joints with electromagnetic compatibility) project ran from May 2020 until December 2022. TWI coordinated the initiative with support from Cranfield University and Brunel University's Composites Centre. The topic manager was Evektor of the Czech Republic.
The Challenge of Lightning Protection and Composites
Modern composite materials have many advantages when used for aircraft, notably for weight reduction when compared to old-fashioned, all-metal aircraft construction.
However, the use of composite materials created a new challenge with regards to lightning strikes, since they cannot channel the electrical charge from a lightning strike.
All-metal aircraft naturally channel electricity from a lightning strike to its trailing edges, where it is safely discharged, whereas technologies need to be introduced to composites that form external parts or structures on an aircraft to achieve the same outcome.
Of course, it is important to be able to discharge electricity from a lightning strike in order to prevent safety concerns and damage to the aircraft.
Lightning Strike Protection Technologies for Composites
The C-Joints project, a forerunner to the D-Joints project, successfully developed two technologies to provide a solution to the problem of discharging electricity from composite aircraft parts.
TWI’s thermal spraying was applied as one solution, which involves spraying metal particles onto composites to create metallic connections that can carry energy between composite panels as well as between metal and composite panels.
The other solution developed by C-Joints was ‘tufting,’ an industrial stitching process that uses robotics to sew a thin copper thread through a composite panel to enable electrical conductivity.
Testing
The D-Joints project was created to test and validate the effectiveness of the two solutions developed by C-Joints. Once tested, the results could be incorporated into a ‘sizing tool,’ which is a software tool for engineers that helps specify the technical parameters, material characteristics and joint size required when designing aircraft with composite parts.
A nose section demonstrator was tested by being subjected to simulated lightning strike conditions, with the results showing that both thermal spray and tufting offered effective protection for aerostructures against lightning, including indirect effects such as the electromagnetic fields that lightning generates.
Radiated field ground tests were used for the assessments and the project finding informed the creation of the sizing tool.
Speaking on the results, Sonia de la Cierva, Project Officer at Clean Aviation, said, “The tufting and thermal spraying technologies were tested and validated for electromagnetic compatibility testing at demonstrator level, on the nose section of the Evektor EV-55 aircraft."
Benefits
The sizing tool helps engineers when designing specialised joints to be embedded in aircraft panels and structures, saving time in the design process through easy access to relevant data through a convenient user interface.
According to TWI, the tool saves weight and costs by ensuring the exact amount of protective material is used, which also reduces fuel consumption and lowers emissions.
Clean Aviation's de la Cierva said: “A further deliverable, encapsulating the compiled test outcomes and assessment of the environmental benefits is currently in progress," adding, "This will take the form of a summary document that classifies the benefits of the investigated technologies with evaluations of their cost and weight reduction."
