Explosion in a LH2 vessel

An explosion occurred during tensile tests of metals in a liquid hydrogen vessel. efforts were made to prevent air from entering by using a flexible top skirt designed to allow for movement of the tensile machine. A purge was also applied at each testing cycle.
[Ordin, NASA (1974)]

The INITIATING CAUSE was the formation of an air-hydrogen explosive mixture inside the liquid hydrogen apparatus, and the following ignition .

The most probable cause was air which had condensed in the LH2 vessels system due to leaks. This air was most likely solidified and collected in the bottom of the vessel. Purging after each test did not warm the vessel sufficiently to remove the air. Fracturing of the specimen probably provided sufficient energy for ignition.

ROOT CAUSE could be then identified in an inadequate procedure applied at the end of the test and aiming to purge completely the testing vessel. Moreover, it should be possible to identify the presence of frozen ai inside the vessel by simple visual inspection. The fact that air could leak into the vessel speaks for a shortcoming of the design.

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It is not completely clear when exactly the explosion took place.