Radical innovations for all aircraft systems and subsystems are needed for realizing future carbon-neutral aircraft, with hybrid-electric aircraft due to be delivered after 2035, initially in the regional aircraft segment of the industry. Electrical energy storage is one key element here, demanding safe, energy-dense, lightweight technologies. Combining load-bearing with energy storage capabilities to create multifunctional structural batteries is a promising way to minimize the detrimental impact of battery weight on the aircraft. However, despite the various concepts developed in recent years, their viability has been demonstrated mostly at the material or coupon level, leaving many open questions concerning their applicability to structural elements of a relevant size for implementation into the airframe. This review aims at providing an overview of recent approaches for structural batteries, assessing their multifunctional performance, and identifying gaps in technology development toward their introduction for commercial aeronautic applications. The main areas where substantial progress needs to be achieved are materials, for better energy storage capabilities; structural integration and aircraft design, for optimizing the mechanical-electrical performance and lifetime; aeronautically compatible manufacturing techniques; and the testing and monitoring of multifunctional structures. Finally, structural batteries will introduce novel aspects to the certification framework.

This detailed study proposes a structural health monitoring system which enables the identification, localisation, and correct measurement analysis, in relation to the damage and debonding induced by low energy impacts within aircraft composite wing panels. The said system has been envisaged as an offline system which aims to be considered as a valid alternative method in relation to the current first two maintenance approach levels: visual inspection, which is to be followed if necessary by ultrasonic scanning techniques. The architecture includes two different technologies which act at different frequency ranges: high-frequency sensors/actuators piezoceramics and low-frequency distributed fiber optic sensors. Experimental and numerical results on small stiffened panels are illustrated in this study, where technological verification and validation have been assessed within a laboratory-controlled environment. In addition, the potential benefit by utilising such techniques within the design of the aircraft composite structures has also been illustrated; in comparison with the current aircraft composite structures, a higher weight saving and better performing structures is foreseen.

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