An investigation into the frequency dependence upon the fatigue crack growth rate conducted by a novel fatigue testing method with in-situ hydrogen-charging

Abstract

The most widely used testing methods for evaluating the hydrogen embrittlement resistance are classified into the following two types: (1) testing in high-pressure hydrogen gas and (2) testing in air using hydrogen pre-charged specimens. Testing in hydrogen gas is technically difficult and expensive, because high-pressure gas equipments composed of such as a high-pressure vessel and piping system has to be introduced. On the other hand, in the case of pre-charging method, the outgassing of hydrogen from the specimen occurs during the test. Therefore, hydrogen embrittlement could not be evaluated accurately by the pre-charging method in long-term tests such as a fatigue test. In a previous study, to evaluate hydrogen embrittlement effectively, a novel experimental method was developed, in which four-point bending fatigue test system was performed with continuous circulation of a hydrogen-charging solution into a pipe specimen. This new testing system using hydrogen-charging solution enables an easy yet reasonable evaluation of hydrogen embrittlement for a long-term fatigue test. In this study, the frequency effect on the crack growth rate due to hydrogen was investigated by this new testing method. Fatigue crack growth tests at a test frequency ranging from 1 to 0.0002 Hz demonstrated that the fatigue crack growth rate was faster in the presence of hydrogen than in the absence of hydrogen at all test frequencies. Further, the increase in crack growth rate became more pronounced with decrease in test frequency. This frequency dependence upon crack growth rate was discussed by considering the difference between the effects of external hydrogen and internal hydrogen.