TWI is supporting one of our Industrial Members, Carten Controls, to better qualify expected materials performance in this relatively new market by performing fracture mechanics tests in high pressure pure hydrogen in accordance with ISO 11114 Part 4.
This standard method is to ensure the compatibility of cylinder and valve materials used for gas transportation from hydrogen embrittlement from the gas contents. The standard was developed in response to catastrophic failure of cylinders containing hydrogen gas or hydrogen enriched gases, due to the embrittlement of the containment steels. It also provides a method to approve newer steels, with higher tensile strength but reduced impurities, such as sulphur and phosphorous, to be demonstrated for their suitability for hydrogen storage service, alongside more conventional lower grade steels.
Test Method B in the standard is a fracture mechanics test (Method A is a disc test), and the test method involves incrementally loading a 26mm wide pre-cracked compact tension specimen in steps, with a twenty minute hold duration at each step to determine whether any crack propagation can be detected from the direct current potential drop (DCPD) monitoring on the specimen. The tests are done at room temperature within a high pressure gas environment containing the relevant embrittling gas, in this case high purity hydrogen. The largest applied load (in terms of stress intensity factor, K, for the specimen) before crack extension occurs is defined at the threshold value, KIH, for the specimen. If two repeat specimens confirm values of KIH above the target value (equal to 60/950 multiplied by the UTS) then the material is qualified to ISO 11114-4 Method B.