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.

The use of composite materials in civil aviation is today certainly widespread, but sometimes this not implies that the final product is obtained exploiting the full potential of this kind of materials. The complexity of the composite material systems, the difficult repeatability of the products and their inspection, the high sensitivity to damages and defects, more severe certification constraints, often can discourage the use of these materials, especially for small aircrafts. New design approaches are necessary to overcome the current design methods applied by the aircraft manufactures, which often generate oversized and expensive structures respect to the expected ones. On the base of some results and experiences accrued, and in progress, in some projects coordinated by CIRA and funded by Italian Aerospace Research Programme, this work summarises advanced and new design approaches aimed at reducing the conservativisms of the current design. New protocols and paradigms are finalized to an improved and less time consuming building block approach, by means of the use of numerical procedures and more effective tests and tools for the material characterisation, also considering the use of structural health monitoring systems. Sensors/actuators, applied in the last decades for maintenance purpose, are now also under investigation to improve the design methods. In this framework, the use of general-purpose software, in-house developed codes and dedicated testing approaches, have become part of an improved new building block and structural sizing approach, for an efficient design of the next generation composite structures.

This paper presents a cross-correlation function-based method applied to a spatially shifted differential strain readout vectors using distributed sensors under backscattering random noise and impact excitations. Structural damage is generated by low/medium energy impact on two aeronautical 24-ply CFRP (carbon fiber reinforced plastic) stiffened panels. Two different drop impact locations, two different sensor layouts and two different post-impact solicitations are provided for a skin-stringer debonding detection and length estimation. The differential signal with respect to an arbitrarily selected grounding is used. Then the effects of noise filtering are evaluated post-processing the differential signal by cross-correlating two strain vectors having one sensor gauge position lag. A Rayleigh backscattering sensing technology, with 5 mm of spatial resolution, is used to log the strain map. The results show a good coherence with respect to the NDI (nondestructive inspection) performed by ultrasonic C-scan (an ultrasonic imaging system) flaw detector.

This work resumes the results achieved until today in the European project AirGreen 2 of Clean Sky 2 programme, deriving from the application of two different dry preforming processes for the manufacturing of a composite outer wing box of the next generation turboprop aircrafts. Liquid Resin Infusion and Out of Autoclave techniques, by Hand-Layup and Automated Fiber Placement, are considered. The optimisation and validation of the manufacturing processes have been done according to key performance indexes: weight and cost reduction, lower energy consumption, high productivity and minimal reworking time, less intensive labour, minimal scrap and less waste of materials. The work has been performed through manufacturing tests and optimisation of the process parameters, implementation of several bagging techniques, numerical simulations of the infusion process and material characterization tests in operative conditions, from coupons level up to details and elements level (flat stiffened panels). Pro and cons, suggestions and technical considerations useful for the next step of the project (final manufacturing of large parts and components) are assessed.

The object of this work is to show the potentiality of the application of SHM systems and PFA methodology, from the beginning of the design phase of wing box composite stiffened panels, for obtaining lighter structures respect to the current ones. The common industrial approach to satisfy the certification requirements (EASA AMC 20-29) for composite structures, based mainly on the application of high conservative knockdown factors to the material strength properties and/or performing extensive test campaigns, can lead to oversized structures and to an increase in costs and timing. The use of information coming from SHM systems and of the PFA methodology could exploit the full potential of damaged composite materials, in favour of a greater weight reduction. By detecting the damages thanks to SHM systems, the structure could be designed with higher design allowables (more reliable detection of BVID) improving the static strength for a reduced damage size detection. Two aircraft wing box composite stiffened panels have been preliminary designed under static compressive load, one panel at the wing root and the other one at the wing tip, according to the traditional industrial design approach. Then, they have been re-designed releasing some of the current conservative criteria, because they were considered resolved by SHM systems: no BVID knockdown factor, no notch material design allowables (only bonded joints and bonded repair are considered) have been applied. The new design has shown the potential weight reduction achievable, the design parameters and panel subparts to which the panel weight is more sensitive. The results of these analyses provide fundamental requirements for the SHM system definition in terms of “where to monitor and why”. Successively, in order to exploit the actual residual strength of impact damaged panels, PFA has been performed also on the stiffened panels considering a discrete damage model against the traditional design approach; the latter, based on the first ply failure design criteria and assuming the structure uniformly damaged by using reduced design allowable obtained at coupon level. The results show that a significant weight reduction is potentially achievable by using PFA, and the potentiality of this methodology as a valid design tool to investigate a new design philosophy based on the evaluation of the residual strength of panels with discrete damages.