This paper presents a multiscale hybrid approach for predicting damage and failure of laminated composite structures based on the thermo-mechanical properties (stress/strain behaviour and strength) of the unidirectional plies. This kind of approach is thus predictive for different stacking sequences. The approach introduces viscosity of the matrix in order to obtain an accurate description of the mesoscopic behaviour, especially the non-linearity under shear loading. The failure criterion used is based on physical principles and introduces micromechanical aspects (such as the effect of the local debonding on the non-linear failure behaviour) at the mesoscopic scale. The main improvements, over those proposed in the second world-wide failure exercise, are related to (1) the evolution and effects of the mesoscopic cracks and (2) the coupling between those cracks and delamination (inter-ply damage). This approach has been implemented in an implicit finite element code in order to predict the strength of composite structures, exhibiting different levels of complexity (unnotched plates, open-hole plates) and subjected to complex loadings (membrane or bending loadings). All the 13 Test Cases of the third world-wide failure exercise have been solved.

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.

Electrification of aircraft propulsion is one key enabler to cut emissions from aviation and meeting the EU Green Deal target of carbon neutral air travel by 2050. While batteries enable highest energy efficiency, their rather low energy density will remain the bottle neck, even with maturing Li-ion technology. Multifunctional electrical energy storage, equivalently referred to as structural batteries capable of storing electrical energy while bearing mechanical loads, could overcome this energy density limit of conventional, monofunctional batteries, as they offer integration of energy storage at near-to-zero weight penalty. So far none of the many structural battery concepts investigated over the last decades has shown multifunctional efficiency adequate for aeronautic applications, and several gaps in research, technology development and in airworthiness certification have never been tackled. This paper presents the progress in two EU-funded research projects, SOLIFLY and MATISSE, targeting structural battery technology for aerospace applications and its potential for aircraft electrification. Structural battery electrochemistry based on energy dense active material, and cell and structural integration concepts have been developed and demonstrated, allowing projections that improved technology could double the effective system-level energy density of monofunctional batteries, significantly reducing the battery weight penalty, the main barrier to the introduction of batteries for larger aircraft.