# Optical Sensing Technologies for Cryo-Tank Composite Structural Element Analysis and Maintenance
## 1 Introduction  
The H2 ELIOS project [1] is an ambitious initiative to develop an innovative hydrogen storage solution for the aviation sector [2]. The main idea is to reduce emissions and minimize the environmental impact of the aircraft by developing a lightweight [3] and cost-effective solution for storing liquid hydrogen, utilizing sustainable polymer-based materials for the tank structure [3]. In the development of composite cryo-tank technology, the Boeing Company has been building cryo-tanks focused on composite material since 2010. Key design features included one-piece wall construction to minimize overall weight (eliminating the need for a bellyband joint), 3-dimensionally (3D) reinforced y-joint material to alleviate stress concentrations at the tank-to-skirt interface, and a purgeable fluted core skirt to carry high axial launch loads [4]. Gloyer-Taylor Laboratories Inc. (Tullahoma, TN, USA) [5], an aerospace engineering research and development company, has announced breakthrough results in its ultra-lightweight cryogenic tanks made from graphite fiber composites and other materials. According to GLT, its cryogenic tanks have demonstrated a huge 75 percent mass reduction when compared to \"state-of-the-art aerospace cryo-tanks (metal or composite).\"  
The AIMEN Technology Centre has achieved a milestone in the field of industrial innovation with the 3D manufacture of an innovative cryogenic tank demonstrator, designed for future applications in the storage of liquid hydrogen (LH\({}_{2}\)) in aircraft [6]. LH2 is a key fuel used for space launchers but is also being developed by Airbus and other aircraft OEMs for zero-emission aviation. When transferring LH2 from a delivery or storage tank to vehicle tanks, NASA [7] and other entities have reported high losses due to boil-off of the cryogenic liquid to vapor and other issues that still need to be addressed. The EU, indeed, recognizes hydrogen as a clean fuel with the potential to reduce emissions in the aviation sector, but current technologies limit the use of hydrogen-powered aircraft due to challenges in terms of storage safety, weight, and cost-efficiency. Airworthiness regulation [8] must be formulated to support the adoption of hydrogen. Although its gravimetric energy density is 2.5 times [9] greater than that of kerosene, its volumetric energy density is 4 times lower, leading to larger tanks and posing significant limitations for cost-effectiveness on medium to long-range flights. The new generation of cryo-tank storage systems must comply with specific analysis [10]. Integral tanks, which are part of the aircraft's main structure, seem promising, but existing designs show limitations in the integration with the airframe and insulation capabilities. To address these issues, the study in [11] proposes the analysis of an integral tank concept featuring a double wall architecture with vacuum insulation and outward-facing structural reinforcements connecting to the fuselage skin. Preliminary results point to fuel containment efficiencies consistent with previous designs, with buckling stability identified as the critical design criterion. For the new generation of cryogenic composite vessels, additional standards and optimized on-board inspection techniques need to be implemented more than ever. Due to their complex geometry, sometimes characterized by encapsulated shell assemblies and peculiar material properties, internal, and therefore hidden, defects such as flaws, delamination or joint debonding may occur. As a result, conventional inspections and maintenance procedures can be very expensive and difficult. For many reasons, among the other extreme pressure and temperature levels, the tank system must be verified both prior to flight and throughout its service life. Currently, tomography is one of the most used techniques for this kind of analysis. It guarantees a complete and detailed survey but is expensive and time consuming [12].