Transient Hydrogen Diffusion-Elastoplastic Coupling Analysis near a Blunting Crack Tip

Abstract

The effect of hydrogen on the metals is known as hydrogen embrittlement, which affects the structural integrity of a hydrogen energy system. Hydrogen atoms near a crack tip play an important role in the hydrogen embrittlement. In the present paper, we developed a computer program for an transient hydrogen diffusion-elastoplastic coupling analysis by combining an in-house finite element program for hydrogen diffusion analysis with a general purpose finite element computer program for stress analysis. In the hydrogen diffusion equation, we consider both the hydrogen concentration at the normal interstitial lattice sites and that at the trap sites, and also take account of hydrogen diffusion flux due to hydrostatic stress. We use a hypothesis that the hydrogen absorbed in the metal affects the yield stress of the metal. Therefore, the hydrogen diffusion problem is coupled with the elastoplastic stress problem. Using the computer program developed in the present study, we performed the transient hydrogen diffusion-elastoplastic coupling analysis of a cracked plate made of a bcc-metal, and obtained the hydrogen concentration, hydrostatic stress and plastic strain near the crack tip. In the present paper, we discuss the effects of the initial hydrogen concentration, the boundary condition of hydrogen on the free surface and the loading frequency. An important finding obtained from the present study is the fact that the hydrogen concentration near the crack tip depends greatly on the loading frequency. The fact indicates that the fatigue lives of the components in a hydrogen system depend not only on the number of loading cycles but also on the loading time.