Sensors 2023, 23, 455. https://doi.org/10.3390/s23010455 www.mdpi.com/journal/sensors Article Preliminary Results of a Structural Health Monitoring System Application for Real-Time Debonding Detection on a Full-Scale Composite Spar Monica Ciminello 1, *, Bogdan Sikorski 1 , Bernardino Galasso 1 , Lorenzo Pellone 1 , Umberto Mercurio 1 , Antonio Concilio 1 , Gianvito Apuleo 2 , Aniello Cozzolino 2 , Iddo Kressel 3 , Shay Shoham 3 and Moshe Tur 4 1 Adaptive Structures Division, The Italian Aerospace Research Centre (CIRA), 81043 Capua, Italy 2 Research Division, Piaggio Aerospace Industries, 81043 Capua, Italy 3 Advanced Structural Technologies, Engineering Center, Israel Aerospace Industries (IAI), Ben Gurion International Airport, Tel Aviv 70100, Israel 4 School of Electrical Engineering, Tel-Aviv University (TAU), Tel Aviv 70100, Israel * Correspondence: m.ciminello@cira.it Abstract: The present paper reports the outcomes of activities concerning a real-time SHM system for debonding flaw detection based on ground testing of an aircraft structural component as a basis for condition-based maintenance. In this application, a damage detection method unrelated to struc- tural or load models is investigated. In the reported application, the system is applied for real-time detection of two flaws, kissing bond type, artificially deployed over a full-scale composite spar un- der the action of external bending loads. The proposed algorithm, local high-edge onset (LHEO), detects damage as an edge onset in both the space and time domains, correlating current strain levels to next strain levels within a sliding inner product proportional to the sensor step and the acquisition time interval, respectively. Real-time implementation can run on a consumer-grade com- puter. The SHM algorithm was written in Matlab and compiled as a Python module, then called from a multiprocess wrapper code with separate operations for data reception and data elaboration. The proposed SHM system is made of FBG arrays, an interrogator, an in-house SHM code, an orig- inal decoding software (SW) for real-time implementation of multiple SHM algorithms and a con- tinuous interface with an external operator. Keywords: structural health monitoring; real-time processing; composite structures; sensors; damage characterization; smart devices 1. Introduction In recent years, in order to ensure safe operation of UAVs, especially over populated areas, the industry has moved towards establishing airworthiness requirements, such as STANAG 4671 [1], moving away from commercial manned aircraft airworthiness regula- tions. An attractive option for maintenance of structural integrity is the use of structural health monitoring (SHM) systems. The aim of SHM is the autonomous structural airwor- thy assessment of individual vehicles, alerting for maintenance actions only as needed. It is expected that when fully developed, SHM will qualify as one of the ‘repeatable and reliable non-destructive inspection techniques’ mentioned in the aforementioned regula- tions. This general case has a specific problem when it comes to bonded structures. Bond- ing can generate significant advantages for the production of composites, which can be reduced by the applicable directives. STANAG 4671 states the criteria that shall be fol- lowed in designing a bonded structure in the USAR.573(a)(5) based on the same criteria present in the EASA and FAA commercial manned regulations. The cited standard states Citation: Ciminello, M.; Sikorski, B.; Galasso, B.; Pellone, L.; Mercurio, U.; Concilio, A.; Apuleo, G.; Cozzolino, A.; Kressel, I.; Shoham, S.; et al. Preliminary Results of a Structural Health Monitoring System Application for Real-Time Debonding Detection on a Full-Scale Composite Spar. Sensors 2023, 23, 455. https://doi.org/10.3390/ s23010455 Academic Editor: Vittorio Passaro Received: 14 November 2022 Revised: 22 December 2022 Accepted: 29 December 2022 Published: 1 January 2023 Copyright: © 2023 by the author. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).