Investigation of manufacturing effects by strength assessment, NDI and guided waves based SHM in composite plates reinforced with bonded stringers

摘要

Sensorized structures aimed at Structural Health Monitoring implementation in aircraft components are among the mostpromising approaches for a next future evolution of the current maintenance procedures (and consequently for operativecosts reduction) but also for relevant modifications of design rules and manufacturing processes. Adhesive bondsrepresent an efficient manufacturing process for composite structural parts due to the possibility to co-bond or co-curetogether thin plates and stiffeners leading to weight saving, although they have not been massively employed yet. One ofthe showstoppers for the full implementation of adhesive bonds in composites are the airworthiness certificationrequirements for composite aircraft structures as presented within the FAA Advisory Circular 20-107B. In that documentthe general methods for substantiating the limit load capacity of any bonded stiffener, the failure of which would result incatastrophic loss of the airplane, are prescribed. Among the suggested methods, the only one really permitting to achievethe optimal bonding efficiency without the addiction of disbond stoppers (i.e. rivets), is a “repeatable and reliable nondestructiveinspection techniques ensuring the strength of each joint”. This paper presents experimental activities that theauthors have carried out to characterize two stiffened plates that are “nominally” equal, but obtained by differentmanufacturing processes; the two plates have been statically characterized performing strength tests, inspected bytraditional NDI (ultrasonic C-Scan) and implementing a guided waves based SHM system designed for stringerdebonding detection and characterization. A critical analysis of the preliminary experimental results as well as acomparison with the expected nominal structural performances will be presented. Afterward a comparison of the threeapproaches adopted for structural health status characterization before and after stiffener disbonding will be presentedlooking at the possible implementation of a SHM system committed at satisfying the certification requirements of theAC 20-107B.