Recently, as a part of the development of new and renewable energy sources, the use of hydrogen, especially hydrogen fuel cell vehicles (FCV) and hydrogen stations, has been actively expanded. Various studies have been conducted on the behavior of structural materials in the production, storage, and transport of hydrogen under high-pressure hydrogen or liquid hydrogen environments. As the application field of hydrogen energy is expanding, development of technology for preventing accidents such as gas leakage and explosion due to damage caused by hydrogen environment embrittlement (HEE) of the materials is an urgent task.
From this point of view, the small punch (SP) test method using small size (subsize or miniaturized) specimens can be applied as a simple measurement method for evaluating a gas hydrogen embrittlement behavior of materials in that it is easy to vary the testing temperature, and to know the ductile-brittle transition behavior. In this study, the hydrogen embrittlement characteristics were evaluated qualitatively and quantitatively by applying the in-situ SP test method at room temperature and low-temperatures for the structural materials for hydrogen energy uses. In order to quantitatively evaluate the sensitivity of hydrogen embrittlement through the SP test results, an influencing parameter of relative reduction of thickness (RRT) at failed parts was introduced at the fracture portion and its application reliability was examined in comparison with the relative reduction of area (RRA) which was obtained by the SSRT using the conventional uniaxial tensile test.
On the other hand, in order to understand the behavior of the energy facility materials under an extreme environment like impact loading, it is necessary to investigate the strain hardening behavior and the failure mechanism depending on the strain-rate applied. The deformation behavior under impact loading interesting in such energy facility materials is generally known to be in an intermediate strain-rate range. However, it was not easy to obtain useful data on the deformation and fracture behavior of materials in the intermediate strain-rate range using conventional test equipment. This is because most of the measured load signals contain significant oscillations and are solved by a numerical smoothing process of the data. In this study, in order to solve this problem, a drop-bar type impact test apparatus capable of tensile test at the intermediate strain-rate was newly constructed. Using this constructed impact tester, impact tests were carried out at room temperature and cryogenic temperature (77 K) for steels for energy uses. The effects of strain-rate and test temperature on the strength and elongation were examined and compared with quasi-static test results. When a drop-weight type impact test system having a long output bar constructed was used, the dynamic load signal obtained at intermediate strain-rate range could exclude the influence of the reflected wave interference and the oscillations due to the inertia effect, therefore material characteristic values could be obviously defined. Using the constructed impact test apparatus, impact tensile properties of three kinds of structural metals of 3% Mn steel, 18% Mn steel and 9% Ni steel were evaluated at RT and 77 K.
As results, this study has developed simple test methods using small size specimens and evaluated the properties of metallic materials for energy uses under typical extreme environmental conditions. It is suggested that the developed methods can be applied to the evaluation and screening of materials for energy equipment under actual usage environment.