Electrical degradation diagnosis of aerospace carbon fibre composites

Reducing the use of metallic structures on aircraft, by replacement with lighter weight carbon fibre reinforced polymer (CFRP), enables optimisation of aircraft weight. Optimisation of weight for aircraft is critical and this includes the optimisation of the electrically powered propulsion system, where the biggest challenge is electrical power system weight. An example of a near-term aircraft with electrical propulsion is an electrical vertical take-off and land (eVTOL) aircraft, e.g. CityAirbus.
CFRP has excellent mechanical properties, but it has poor conductivity compared to aluminium (~1000 times less). A gap in knowledge on the thresholds for failure of CFRP due to electrical current conduction, and methods to detect these in a non-destructive manner, results in industry standards requiring that CFRP and electrical equipment and cables are kept physically separate. This ensures that in the event of an electrical fault (e.g., due to insulation failure), electrical fault current will not flow through CFRP structures. This leads to volume and weight penalties: additional 30% weight on wiring infrastructure; 10% of power electronic weight, and 30 % of solid state circuit breaker weight is due to metallic casings; 25 % of motor weight is due to metallic end plates and casing. The use of CFRP for these functions in these components will bring weight reductions for the equipment of ~5 - 10 %.
This project will investigate the correlation between the level of Joule heating sustained by a component and the reduction in mechanical properties due to thermal degradation. A major element of the project is the use of NDT methods to assess the level of degradation, and correlate to the level of Joule heating sustained. First, this develops a more efficient route to assessing degradation. This is critical, as there are a wide set of variable parameters which may influence the response of CFRP to Joule heating. Second, this provides a platform for future in-service assessment of CFRP components which have conducted electrical current.

Key Objectives
1. Review types of CFRP used, characteristics of electrical loading, and mechanical thresholds of CFRP in aircraft with electrical propulsion (e.g., EVTOL).
2. Design test matrix for electrical loading of CFRP to lead to different levels of degradation.
3. Experimental capture of datasets for degradation of CFRP due to electrical loading.
4. Assessment of level of degradation using non-destructive test methods, verified by mechanical testing.
5. From degradation thresholds observed, propose design and manufacture modifications to keep degradation below failure thresholds (e.g. layup, electrical bonding, manufacturing method).