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The present paper overviews the recent progress on HE obtained atHYDROGENIUS. The influence of hydrogen and strong test frequency on fatigue crackpath is discussed with a particular attention. The mechanism of change in fatigue crackpath depending on test frequency is explained by the coupled effect of hydrogen inducedlocalized plasticity at crack tip and test frequency.The test frequency of the fatigue test was switched from f = 2 Hz to f = 0.02 Hz andthe crack growth behaviour was observed by the replica method. These two step fatiguetests were repeated and the variation of the crack growth behaviour by switching thetest pattern from f = 2 Hz to f = 0.02 Hz was investigated.Particularly important phenomena are the localization of fatigue slip bands and alsostrong frequency effects on fatigue crack growth rates. For example, with a decrease infrequency of fatigue loading down to the level of 0.02 Hz, the fatigue crack growth rateof a Cr-Mo steel was accelerated by 10 - 30 times. The same phenomenon also occurredeven in austenitic stainless steels at the frequency of the level of 0.001 Hz. Striationmorphology was also influenced by hydrogen.The crack path of the hydrogen-uncharged specimen was monotonic and showed noparticular variation even after switching the test frequency from f = 2 Hz to 0.02 Hz andalso 0.02 Hz to 2 Hz. The monotonic moderate curving of the crack path was caused bythe growth of plastic zone size due to increase in the crack length, i.e. the stressintensity factor range. Namely, the plane stress condition is gradually satisfied and thecrack extension by shear mode ahead of crack tip becomes dominant near specimensurface. On the other hand, the crack of hydrogen-charged specimen grew in theinclined direction under f = 2 Hz, though the crack grew straight under f = 0.02 Hz.