In order to apply the high strength steel sheet with tensile strength is above 1200MPa, it is necessary to assess the delayed fracture susceptibility caused by hydrogen entering from the environment where steel components are exposed, and also necessary to adopt proper evaluation method of the delayed fracture. In this study, to clarify the influence of various experimental methods on delayed fracture susceptibility of high tensile steel sheets, slow strain rate technique (SSRT), constant load test (CLT) and conventional strain rate technique (CSRT) were demonstrated using commercial high strength steel sheet, and the fracture surfaces were observed. The results show that fracture stress decreased with the increase in the diffusible hydrogen content, and the fracture limit curves of SSRT and CLT were almost the same, whereas that of CSRT shifted to higher stress level. The area fraction of brittle fracture of SSRT, which is the sum of intergranular and quasi-cleavage fracture, increasing with the diffusible hydrogen content, is almost the same as that of CLT, however the area fraction of brittle fracture of CSRT is lower. It is supposed that SSRT and CLT owe to accumulated hydrogen concentration and CSRT owes to accumulated less than that of the others. Thus, both SSRT and CLT are suitable methods to assess the delayed fracture susceptibility; SSRT is superior to the point of testing time, and CLT is superior to the point of reproducing the environments. However, CSRT is suitable to classify materials susceptibility of the delayed fracture immediately on the condition of high diffusible hydrogen content.
The application of high-strength steel sheets in automobiles has been increased to achieve low bodyweight and simultaneously enhance crashworthiness. High-strength steel sheets are susceptible to hydrogen embrittlement and it is essential to evaluate their delayed fracture resistance for appropriate use. Delayed fracture resistance is typically evaluated using the relationship between the amount of diffusible hydrogen and fracture strength obtained from a constant load test and the slow strain rate technique (SSRT). It is difficult to monitor the amount of diffusible hydrogen invading from the environment; the thermal desorption analysis is not a non-destructive analysis to obtain the amount of diffusible hydrogen and diffusible hydrogen is easily desorbed from the specimens. The hydrogen permeation test easily monitors the invasion of diffusible hydrogen. In this study, we evaluated the delayed fracture resistance of high-strength steel sheets using the hydrogen permeability obtained from the hydrogen permeation test. As a result, relationships between hydrogen permeability, mechanical properties obtained from SSRT, and the brittle fracture surface ratio were found to be consistent among various hydrogen invasion conditions, such as under hydrogen charging and corrosive environments. Furthermore, little diffusible hydrogen was detected using the hydrogen permeation test. Thus, delayed fracture resistance obtained from the relationship between hydrogen permeability and its mechanical properties proves the effectiveness of this method.
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.
In order to evaluate hydrogen concentration under mechanical loading, a stress-hydrogen diffusion or distribution coupling computing method was developed based on Oriani's theory. The 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. It is shown that the results of hydrogen distribution were nearly identical compared to the Krom's study, and it is also shown that hydrogen concentration was higher supposing high strength steels.
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