The effect of hydrogen on the shear localization and associated crack nucle
ation has been investigated by means of a three point bending test of hydro
gen-charged steels. The ductile crack growth resistance in terms of the slo
pe of R-curve was lowered under the presence of hydrogen, the decrease bein
g more pronounced in the steel with more abundant slip constraint phases al
ong grain boundaries. Enlargement of size and reduction in depth/width rati
o of primary dimples, occasionally associated with quasi-cleavage, were obs
erved on the fracture surface of the hydrogen-charged steels. By means of a
FEM calculation, the increase of the nucleation void volume fraction local
ized at the crack tip with strain localization as well was shown to take pl
ace in the hydrogen-charged steel in consistent with enhanced shear instabl
ity. It was discussed that the evolution of vacancy-type defects, rather th
an void nucleation at second phase particles. in the course of plastic stra
ining was enhanced under the presence of hydrogen, reducing the ductile cra
ck growth resistance.