# Optical Sensing Technologies for Cryo-Tank Composite Structural Element Analysis and Maintenance
## 1 Introduction
There is therefore an industrial demand for the implementation of different, integrated and flight-ready systems. From a safety and maintenance perspective, an integrated monitoring system working during operational life appears to be a useful tool [13]. In this sense, the standard regulations require sensors able to work in harsh conditions [14]: ATEX-certified sensors, preventing embrittlement, exhibiting EM immunity, limiting their impact on weight, reducing the thermal bridge and hydrogen darkening effects. Currently, many areas foresee a significant growth of fiber optic sensor applications, from medical to civil, aerospace, and industrial applications [15]. This kind of monitoring architecture is highly desirable for non-destructive assessment for many reasons. Among others, one recalls the capability to resist under chemically aggressive environments, the high integrability, and the characteristic of hosting many sensitive elements on a single fiber. Bragg grating arrays (FBGs) and long-period gratings (LPGs) have been developed during the last decades and allow very accurate sensing. For the most demanding applications, a quasi-continuous sensing technology is available based on Rayleigh backscattering, enabling the monitoring of tens of meters with millimeter-resolution. This kind of system is generally used for monitoring large civil infrastructures such as bridges, railways, and so on [16]. In this paper, different optical monitoring technologies are analyzed, with the perspective of developing a robust and integrated approach for SHM applications on composite cryo-tanks, particularly during acceptance static pressure tests. Specifically regarding pressurized tank items, some studies in the literature report the implementation of monitoring systems by using fiber optic sensors. Strain sensors are used to monitor distributed strain in composite overwrapped pressure vessels (COPVs) throughout the manufacturing process until qualification testing [17]. Sensible elements are embedded along circumferential patterns, at various locations along the structural thickness. Filament-wound cylinders are investigated with multiple layers of carbon fiber, integrated with glass fiber composite on the outer surface for extra durability. Each cylinder is filled with water and pressurized to complete the process. In [18; 19], cryogenic y-joint tests validated the performance of distributed optical strain sensing at liquid helium temperatures (around 130 \({}^{\circ}\)K). SHM experiments are conducted to explore the durability and survivability of the fiber optic sensor system and related surface bonding techniques through multiple liquid helium temperature excursions and mechanical load cycles. The results on a reduced-scale tank showed that internally bonded strain gauges often failed, while the use of fiber optics sensors seemed a viable method to mitigate the evidenced problems, especially in harsh or extreme environmental conditions. In [20], the issue is again faced. Indeed, strict process control is required during bonding operations. The latest non-destructive joint inspection techniques cannot quantify the strength of the bond and only indicate the presence of disbonds or delaminations. Expensive and time-consuming proof-load testing of the joints is required to demonstrate structural performance. This work focuses on experimentally evaluating joint-health monitoring with piezoelectric sensors exposed to repeated loadings until failure. In order to verify the functionality of fiber optic sensors at cryogenic conditions, several studies have been conducted. In [21], the authors described the experimental characterization of thermal responses of four commercially available optical fiber samples with different polymeric coatings in the temperature range from 5 K to 300 K: two with acrylate coatings of different thickness, one with a polyimide coating and one with a polyether-ether-ketone (PEEK) coating.