# Optical Sensing Technologies for Cryo-Tank Composite Structural Element Analysis and Maintenance
## 2 Industrial Requirements  
The most immediate application of health monitoring would be during flight, and the information provided could also be vital in post-flight analyses to detect anomalies and provide corrections to the design in order to improve reliability, which is the most important parameter, along with cost.  
After manufacturing, cryo-tank components are usually required to undergo acceptance pressure tests to simulate the flight environment with appropriate margins and reduce the probability of a structural failure in flight. Such tests usually start with a quasi-static pressurization, and then, if success is involved, large facilities are needed to implement the final functionality test, simulating fully operative conditions.  
Thus, monitoring during a test would allow the test engineers to decide to stop the test when an item is revealed to be critical. In the end, no propagation or initiation of defects is allowed during such tests by the relevant standards. To monitor its status, the case is subjected to non-destructive inspections NDIs before and after the test. Such controls are very time-consuming and costly. For this reason, finding signs of defect evolution using the instrumentation involved in the test would have a strong impact on the optimization of the overall production and verification processes. In particular, the LH2 storage concept demonstrator being developed at H2 ELIOS [1] (Figure 1) consists of the following components:  
* A composite tank, consisting of domes and a middle skirt;
* A metal joint system;
* An encapsulating-type geometry.  
The test article taken into consideration for the preliminary SHM implementation and functionality verification is representative of the tank's cylinder material. Typical skirt-cylinder joints are indeed considered as a critical region for flaws onset, and the highest strain variations, during pressurization, are actually expected to solicit the middle section of the cylinder.  
The performance requirements for the measurement system specialized for this application are as follows:  
* Easy and fast application for compliance with industrial applications;
* Robustness during process operations and testing;
* Negligible losses due to hydrogen darkening;
* Atex cables designed for use in hazardous environments where explosive atmospheres may be present;
* Distributed measures to detect local variations in strain;
* A sample rate that can capture variations in the strain trend;
* Handling of a large amount of data, reducing cable routing complexity;
* The possibility of embedding the sensor inside the composite material before curing.  
Figure 1: LH2 storage concept demonstrator being developed at H2 ELIOS (patented by ACITURR) [1].  
The latter requirement is a desirable one: this capability would indeed allow for the monitoring of the tank throughout the production cycle, delivery, and flight. In particular, it would come in handy to intercept anomalies when they occur or to obtain a large amount of direct data from the structure if local failures occur. For this feature to be available, the sensing elements shall not jeopardize the structural performance. Moreover, they shall withstand the very low temperature reached during gaseous H2 filling, and compensation for thermal effects is possible.