A huge enhancement of fatigue crack growth rates was observed on a 15-5PH martensitic stainless steel under gaseous hydrogen. The observation of fracture surfaces showed a change in fracture mechanisms depending on hydrogen pressure and loading condition, namely the stress intensity factor amplitude and the loading frequency. The present study was undertaken in order to analyze the effect of gaseous hydrogen on the out-of-plane displacement at the crack tip plasticity by means of out-of-plane displacement measurements. An interferometric microscope was used to obtain high resolution measurements of out-of-plane displacement. These measurements showed an increase in out-of-plane displacement in presence of gaseous hydrogen compared to an inert or slightly aggressive environment, depending on the hydrogen pressure and the maximum stress intensity factor applied during static or cyclic loading. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.