International standards and codes dedicated to design of pressure vessels are still unable to competitively ensure safe design and fitness for service of steel vessels for high pressure gaseous hydrogen. Emptying and shallow pressure cycles subject the material to hydrogen enhanced fatigue. A pre-normative project, MATHRYCE under the EU joint research program focused in this subject through material and component testing, analytical work, review of design methodologies and international collaboration.
An easy to implement, safe and economically competitive vessel design methodology is targeted. Steps towards this goal were taken by deepening our understanding on hydrogen enhanced fatigue in different kinds of laboratory specimens and real vessels designed for hydrogen service at maximum 45 MPa pressure. This included cyclic pressure testing of artificially notched vessels both in hydrogen and inert environment.
The effect of hydrogen pressure, frequency and mechanical loading parameters (ΔK, Sa) on fatigue crack initiation and propagation was analyzed. Attention was paid on the definition of “initiation” and influence of hydrogen on the relative parts of initiation and propagation on the fatigue life of a component. A good correlation between results with various test types was found. Particularly promising was the match between the measured — and estimated — crack growth rates in laboratory specimens and vessels. This supports our proposal for a safe design procedure based on crack growth and defect tolerant approach. Recommendations for implementation in a new international standard, on how to properly address hydrogen enhanced fatigue based on laboratory tests, were given and will be summarized in this presentation.
Our results indicate that crack initiation from inclusions or other small microstructural features is not necessarily affected by hydrogen to a similar extent as crack growth, but when initiated, the remaining life may be short due to fast growth. This is challenging for design and inspection rules to allow economically competitive construction of hydrogen equipment without compromising safety.
Fatigue Crack Initiation and Propagation Behavior of a Nickel Alloy ; A Study of Structural Integrity Assessment Method Based on Polycrystalline FE Analysis