Fatigue crack growth properties of hydrogen-charged ductile cast ironThursday (29.09.2016) 09:45 - 10:00 Part of:
The recent launch of the first commercial fuel cell vehicles is one reason for the increased interest in the influence of hydrogen on the strength of low-cost materials. Ductile cast iron, for example, is a potential candidate material for hydrogen gas pipes due to its wide usage for city gas pipelines. While there are already some investigations on the influence of hydrogen on the tensile properties of ductile cast iron reported, there is only little knowledge of its influence on the fatigue crack growth behaviour.
In the present work, fatigue crack growth tests on ductile cast iron were performed under different conditions. Fatigue cracks were initiated at small drilled holes and the propagation rates were measured at constant stress amplitude up to a crack length of approximately three millimetres. To investigate the influence of the hydrogen amount charged inside the material, test specimens were charged using the soaking method for different periods, and the hydrogen content was measured after fatigue testing. It was found, that the content of hydrogen inside the material increases approximately linearly with charging time, even after a period of 1000 hours. Interestingly, there is only a small influence on the fatigue crack growth properties when the specimens were charged for 100 hours or longer. However, a significant increase of the crack propagation rates was observed for specimens after 24 hours or charging compared to non-charged specimens. Furthermore, the influence of testing frequency was studied. A significant increase of the crack propagation rate was found at a frequency of 0.5 Hz compared to 5 Hz. While this effect was more pronounced after hydrogen charging, it was also observed for non-charged specimens. Additionally, fractographic investigations were performed after fatigue testing in order to identify changes in the fracture mode and the crack shape. In conclusion, a significant influence of hydrogen loading on the fatigue crack growth behaviour of ductile cast iron was found. In order to assess the viability of this material for the desired application, further research is necessary.