Digital Image Correlation Measurement of Localized Deformation in Carbon Steel in the Presence of Hydrogen

Although there have been many reports related to localized plastic deformation in the presence of hydrogen, this process has not been clearly elucidated to date. In our previous report, we conducted tensile tests on hydrogen-charged carbon steel S25C. Using the digital image correlation method, we were able to measure the time evolution of the equivalent strain distribution and the equivalent strain rate distribution up to the crack initiation around the notch bottom. We then revealed that the hydrogen induced an increase in the equivalent strain rate during the deformation process and a decrease in the equivalent strain when the time the crack started to grow. In this study, we performed similar measurements on carbon steels S15C and S55C and evaluated how the deformation progressed until the crack started to grow depending on the carbon content. The results showed a decrease in the local equivalent strain when the time the crack started to grow in all of the investigated materials, but we could not identify any clear trend ;with respect to a carbon-content dependence. However, we observed that smaller carbon contents (i.e., higher ferrite volume fractions) resulted in greater significance of the localization of the deformation due to the hydrogen and in a greater rate of increase in the local strain rate. In addition, on the basis of the equivalent strain distribution, we revealed that the deformation tended to localize in the ferrite phase.