An advanced centre box of a vertical tail plane with a side panel from CFRP foam-core sandwich structure

Abstract

Despite its superior specific stiffness, the sandwich design is currently applied in commercial aviation mainly in secondary structures. A major reason for this is its low resistance to a concentrated out-of-plane load, such as impact loading by a foreign object. In this scenario, the resulting damage is also known as foreign object damage and may penetrate further into the sandwich core. An improvement of impact resistance shall be achieved to introduce sandwich into the primary structures. Within the German aeronautics research programme, a primary sandwich structure, i.e. the centre box for a next generation vertical tail plane, was developed. The geometrical characteristics of the sandwich shell of the box, which has a unique lens shaped or lenticular cross section, lead to a noticeable improvement of its impact resistance. The considered sandwich configuration consists of carbon-fibre reinforced plastic skins and a closed cell foam core in between, which allows manufacturing using liquid composite moulding, such as a vacuum assisted infusion process. Embedded core reinforcements and damage arresters, respectively, can be manufactured within the same process step. Furthermore, the developed novel air-coupled ultrasonic scanning can detect the manufacturing flaws contact-freely as well as impact damages and thus enhances the inspection and damage tolerance capability of the sandwich structure. For the proof of structural integrity, a cutout in the critical area of the sandwich panel was defined and modified, such that it has no significant eccentricity and fits into an available shear-compression test facility. Loads are increased stepwise to explore all structural responses including interactions. The results show excellent structural stability with respect to the damage tolerance, even after multiple impact loadings up to 50 J, which agree with previously conducted non-linear structural analysis. A cyclic loading scenario with 8,000 simulated flight cycles in the context of research induces no significant growth of the impact damages and no deteriorating of the structural behaviour, respectively. Finally, the sandwich panel was used to investigate expected thermo-mechanical loadings during the takeoff and landing phases of the reference aircraft.