Effect of hydrogen on fatigue crack growth of metals

The present paper shows several important phenomena obtained by investigations of the effect of hydrogen on fatigue crack growth behaviour, including the measurement of the hydrogen content in various materials such as low-carbon, Cr–Mo and stainless steels. Particularly important phenomena are the localization of fatigue slip bands, strain-induced martensite in Types 304, 316 and even 316L, and also strong frequency effects on fatigue crack growth rates. For example, with a decrease in frequency of fatigue loading down to the level of 0.2 Hz, the fatigue crack growth rate of a Cr–Mo steel is accelerated by 10–30 times. The same phenomenon also occurs even in austenitic stainless steels at the frequency of the level of 0.001 Hz. Striation morphology is also influenced by hydrogen. It has been revealed by re-analysing the results of the authors’ separately published reports that this basic hydrogen embrittlement mechanism is essentially the same throughout all the materials, i.e. low-carbon, Cr–Mo and stainless steels. Thus, the coupled effects of hydrogen content, hydrogen diffusion coefficient (for BCC or FCC), load frequency, localization of fatigue slip bands and strain-induced martensite must be always considered in fatigue test and analysis of hydrogen embrittlement.