Disbond-Stopping Concepts for Bonded Composite Joints

Bonded composite joints have many advantages in comparison to conventional fasteners. However, adhesive bonding of primary aircraft structures is still a certification issue. One promising way to achieve airworthiness of bonded CFRP joints in accordance with FAA and EASA requirements is the establishment of disbond-constraining design features. Consequently, two novel design features are developed. On the one hand, small pin elements are used as through-thickness reinforcements that reduce stresses in the vicinity of the crack tip and thereby stop crack growth. Preliminary studies showed that small diameter elements are beneficial as disturbance and weakening of adherent parts is reduced and the overall bearing capacity is increased. In contrast to conventional chicken rivets, the elements are not supposed to transfer full operational load. On the other hand, a combination of two different adhesives is explored in a novel concept called hybrid bondline. A thermoplastic phase as second adhesive constrains cracks by posing a distinct physical barrier through a sharp change of materials. Furthermore, the material allows thermoplastic welding instead of adhesive bonding by use of a sophisticated manufacturing technology. Enhanced manufacturing concepts are developed for both features and their mechanical performance is evaluated by means of static and fatigue tests