Static behavior of a disbonded stringer in a co-infused stiffened panel

Structural Health Monitoring deals mainly with sensorized structures where sensors can be secondary bonded on metallic or composite structural elements. Aerospace structural design must account for Damage Tolerance (DT) of structures. To accomplish the airworthiness, a flawed structure is required to stand the design load without any growth and, eventually, repaired. For metallic materials, the damage tolerance approaches are well-established and rely on the evaluation (theoretically and experimentally) of crack propagation velocity. For composite structures the damage-tolerance design is more challenging as the failures that may occur are of different type, most of the times hidden inside the structure and can grow up to a critical size before the conventional inspection techniques detect them. Within the DT approach one of the showstoppers for the full implementation of adhesive bonds in composites (i.e. stringer-skin connections for stiffened plates) are the airworthiness certification requirements for composite aircraft structures as presented within the FAA Advisory Circular 20-107B. In that document the general methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result in catastrophic loss of the airplane, are prescribed. Among the suggested methods, the only one really permitting to achieve the optimal bonding efficiency without the addiction of disbond stoppers (i.e. rivets), is a “repeatable and reliable non-destructive inspection techniques ensuring the strength of each joint”. That assumption implies the implementation of a reliable SHM system capable of monitoring the extent of an eventual disbond until it reaches a critical dimension at limit load. This paper will present the preliminary results of a research activity where the authors apply static loads to a stiffened plate made of a skin and a bonded stringer (co-infused) where a disbond “starter” has been included during manufacturing. The plate has been sensorized with a strain gauge system to detect the disbonding evolution during load application, in order to verify the effectiveness within a DT approach.