Evaluation of hydrogen concentration in high strength steel by using finite element method

Abstract

In order to compute hydrogen concentration distribution under mechanical loading, a stress-hydrogen diffusion or distribution coupling computing method had been developed based on Oriani’s theory. In this theory, it is assumed that hydrogen concentration between lattice sites and trapping sites are locally balanced, and a number of trapping sites are described as a function of equivalent plastic strain. In this study, this method was implemented to commercial finite element (FE) analysis software, Abaqus, with its user subroutine UMAT and UMATHT. This method was applied to analyze concentration of hydrogen near a blunting crack tip in pure iron. It is shown that the results of hydrogen distribution were nearly identical compared to the Sofronis’s study. Then, apparent diffusion coefficient and a relationship between the number of trapping sites and equivalent plastic strain were determined by using an electrochemical technique for high strength steel. The apparent diffusion coefficient was stable under elastic deformation; on the other hand it was drastically decreased as plastic strain increased. The material parameters were applied the blunting crack problem and the results were compared to that of pure iron. It is shown that hydrogen concentration of lattice site of high strength steel was more than three times as high as that of pure iron.

Keywords

• hydrogen embrittlement
• finite element method
• hydrogen-stress coupling problem
• high strength steel
• crack
• diffusion