# Optical Sensing Technologies for Cryo-Tank Composite Structural Element Analysis and Maintenance
## 5 Conclusions  
Damage detection is a complex and crucial operation enhancing the level of safety through the early identification of critical flaws before failure or, alternatively, reducing the design penalty factors in composite structures, required by safety and maintenance regulations. Unfortunately, there is a wide range of damage, each exhibiting specific characteristics. Therefore, techniques that can be suitable for one, often are not, or poorly, applicable to others. Even for a single type of defect, different attributes should be considered, and this cannot be achieved with a single technology. With a focus on fiber optic technologies, different systems, despite their specific limitations, can be highlighted for their ability to access some of the traits, distinguishing features to be correlated and merged, in view of an integrated multi-scenario approach. Experiments are conducted on a representative coupon thermoplastic material from the cylinder element, but the reported concepts may be scaled to full-scale systems. From this perspective, the non-destructive methods are not taken into consideration, as they cannot be installed on board or operate during the system's service life, being intended to be used on the ground during scheduled inspections. The SHM system is instead conceived to provide a tool to aid in the real-time detection of structural faults. Three different methodologies are investigated in this work: FBG-based sensing, in both static and dynamic configurations, and distributed sensing for static configurations. The distributed sensing technique provides, at different extents, useful information of the strain map, allowing for a structural overview of the region of interest in both the time and space domains. A visual inspection of the tested coupon confirmed the flaw emergence at 34 cm from the clamped edge (located at 3 cm from the origin), as shown in Figure 15. By comparing the visual inspection with the results of the SHM algorithm (Figure 14), the damage onset is estimated at 1872 mm relative to an origin located at 1544 mm. In addition, the flaw extension can be estimated at 32 mm by fixing its termination at 1904 mm. On the other hand, the FBGs, whether in static or high-dynamic-range configurations, can detect the damage and provide information about its location and evolution in the time domain. Again, the cross-correlation between the two signals allowed the identification of events characterized by a non-negligible energy release, as shown in Figure 16. In the picture, an anomaly in the processed data makes it possible to understand the instant when the carbon fiber enters the fracture process and the propagation begins. Table 1 provides a short summary of the different methodologies under the above assumptions, highlighting their main characteristics. In this way, it is believed to offer a preliminary, yet effective tool for better addressing the issue of damage detection. These considerations are not affected by the herein chosen architectures, since this approach can be easily extended to different methodologies, integrating further capabilities. Figure 15: Detail of the flaw onset on the element and distance from the initial edge. The current activity is part of an ongoing research project that will end with the full-scale functionality test next year. Mapping damage severity or identifying critical region severity is the focus of the final activity. Methodology, M.C. ; software, M.C. ; validation, M.C. ; experimental work, C.C.T. ; sensorization, P.C. ; writing--original draft preparation, M.C. All authors have read and agreed to the published version of the manuscript. The activity herein presented has been carried out within the Project H2 ELOS, \"HydrogEn Lightweight & Innovative tank for zerO-emision aircraft\", funded within the EU research programme and, specifically, the topic HORIZON-JU-CLEAN-AVIATION-2022-01-HPA-03-Large Scale Lightweight Liquid Hydrogen Integral Storage Solutions. Not applicable.