Surface Toughening - Ein kleiner Streifen mit großem Einfluss auf die Verbindungsfestigkeit von geklebten Primärstrukturen im Projekt HAP

Abstract

In the HAP (High Altitude Platform) project, the German Aerospace Center (DLR) is developing a solar-powered long-range aircraft, various payloads for Earth observation missions, the ground control station and operating concept. The mission conditions and objectives pose major challenges to the aircraft structure, especially with respect to lightweight design.

From the point of manufacturing, the 27m long wing has to be manufactured in separate 1.2m long sub segments. The load carrying structure builds a round tube spar. In order to achieve the required degree of lightweight design, adhesively bonded joints are provided for the primary structure made of fiber composite material. The extremely thin structures with simultaneous highly rigid laminate orientations induce failure-triggering stress concentrations in the bonded scarf joint, which can lead to total failure of the structure during months of continuous operation.

With the help of local Surface Toughening [1], the stress concentrations in the adhesive layer can be significantly reduced as FEA simulations show. Complementing the simulations, the concept is validated at coupon level and then transferred to the semi-automated production of the round tube spars using the winding process.

[1] M.J. Schollerer, et al., J. Adhes., 95:5-7, p.495-514, (2019).