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Sensorised structures aimed at Structural Health Monitoring implementation in aircraft components are among the most promising approaches for a next future evolution of the current maintenance procedures (and consequently for operative costs reduction) but also for relevant modifications of design rules and manufacturing processes. Adhesive bonds represent an efficient manufacturing process for composite structural parts due to the possibility to co-bond or co-cure together thin plates and stiffeners leading to weight saving, although they have not been massively employed yet. One of the showstoppers for the full implementation of adhesive bonds in composites 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”. This paper presents experimental activities that the authors have carried out to characterize two stiffened plates that are “nominally” equal, but obtained by different manufacturing processes; the two plates have been statically characterized performing strength tests, inspected by traditional NDI (ultrasonic C-Scan) and implementing a guided waves based SHM system designed for stringer debonding detection and characterization. A critical analysis of the experimental results as well as a comparison with the expected nominal structural performances will be presented. Afterward a comparison of the three approaches adopted for structural health status characterization before and after stiffener disbonding will be presented looking at the possible implementation of a SHM system committed at satisfying the certification requirements of the AC 20-107B.
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