## 2 Hydraulic Tests of Type 4 Containers, Motivation for Research and Goals  
In the case of type 4 containers, the dominating failure mode is directly related to the liner. Despite its low price, if a liner is faulty, it eliminates a significantly more expensive product from use, i.e., the whole container. Some liner defects result from errors in the manufacturing technology, limitations related to the used material or design, and exploitation errors related to the whole container. On the other hand, a liner defect may contribute to the occurrence of phenomena, such as blistering, buckling, aging, or increased hydrogen permeability, later when in contact with hydrogen. Every new type 4 container construction first undergoes hydraulic tests: static and fatigue tests. The cost of a necessary series of tests is high; however, it is known, from the many years of research by the authors of this paper, that in the hydraulic tests of a finished container, especially a new one, a negative test result is usually obtained due to a faulty liner. Below are some selected examples of liner defects detected as a result of the authors' own research on type 4 hydrogen high-pressure containers. Defects were usually related to:  
* Air bubbles and other inclusions in the liner material;
* Non-homogeneity of liner geometry and wall thickness;
* Residual stress in the polymer after the liner production process;
* Cracks in the liner material resulting from cyclic loads (Figure 3);
* Cracks in the liner material resulting from low temperatures, below \(-40\)\({}^{\circ}\)C (Figure 4);
* Defects in the area of the liner/boss joint;
* Fatigue cracks in the area of a metal boss (Figure 5). In the case of liner defect, the composite comprising the load-bearing surface of a container usually remains intact; however, fatigue cracks occur in the liner, usually located near the dome or at the joint between the dome and the cylindrical part. There are also defects in the area of the connection between a boss and the liner material or, less frequently, boss defects resulting from fatigue cracks in the material. At the development stage of new container construction, defects may be caused by both technological errors in the production of the liner itself or an incorrect design of the load-bearing overwrap. The non-uniform stress distribution in the composite layer of the construction may result in a local concentration of stress in the whole construction, which, in turn, can initiate cracks directly in the liner. In a previous study, unoptimized stress distribution in the geometry of a composite structure was observed and, as a consequence, the liner cracked during an ambient-temperature pressure cycle test in the 20-875-bar pressure range (Figure 6). Figure 5: View of a damaged boss after fatigue testing. (**a**) Crack in the metallic boss and (**b**) crack in the inner side of the liner’s dome. Figure 6: A 5-degree segment of the vessel model (ANSYS V15) presented normal stress (fiber orientation direction) in the composite material; (**a**) hotspot located in the dome area was the reason for liner damage and medium leakage [30]. (**b**) View of the leaking hydrogen vessel during the hydraulic test due to the cracks in the PE liner in the outlet dome area (water drops on the dome surface leaking through the pores in the composite shell). Errors related to incorrect container design, both in the area of the boss or liner and a composite reinforcement, can be verified at the stage of introductory/qualification research (numerical and/or hydraulic tests of finished containers) and should not cause exploitation problems in certified containers. Approval for use in the case of containers means that they have successfully completed a whole cycle of tests at the type-approval stage; this is why, provided that the production procedures are repeatable, defects related to an incorrect construction design should not occur during normal exploitation.