Fatigue life reduction of steels in pressurized hydrogen as a consequence of changes in short crack propagation mechanisms

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Material damage resulting from hydrogen is still not clearly understood despite many research activities. On the one hand, the hydrogen-induced damage processes are influenced by many factors which are specific to material, loading and environment. On the other hand, the main mechanisms of hydrogen embrittlement (HELP HEDE, AIDE, HESIV as well as pressure theory) can occur in a combined manner, so that different embrittlement mechanisms are dominating in different stages of material damage evolution. Moreover, hydrogen embrittlement in the case of cyclic loading is still lacking fundamental investigations.Within the framework of the proposed research project, the mechanisms of hydrogen-induced material damage of different types of steel in the low and high cycle fatigue regime will be characterized and represented in a microstructure-based model usable for simulation calculations. For practical reasons, two steels from different application areas were selected with different microstructures and susceptibility to hydrogen absorption, i.e., the metastable austenitic steel X2CrNi19-11 and the soft-martensitic steel X3CrNiMo13-4. An extensive database for these steels is available, describing the effect of different parameters on the mechanical properties under hydrogen conditions. Fatigue tests in vacuum will serve as reference for the test results obtained in pressurized hydrogen and helium. Detailed metallographic and fractographic microstructural investigations are intended to show the effect of hydrogen on the material damage process (change in the dislocation arrangement, short crack growth and phase transformation). Thus, the damage evolution will be monitored regarding initiation site and frequency of occurrence of damage throughout the major part of fatigue life starting from the formation of microstructurally short cracks until the transition to long crack propagation.The experimentally characterized hydrogen-induced damage mechanisms will be transferred into a physically-based simulation model. By including the identified damage mechanisms into an already existing short crack model, which is based on the boundary element method, the effect of hydrogen on the microstructural damage evolution will be described systematically. In this way, the modeling contributes to the development of an improved understanding of material damage under the influence of hydrogen. The experimental results and the development of the simulation model will both provide a sound basis for a better material utilization in components being exposed to hydrogen and will strongly contribute to an unerring material adjustment towards hydrogen-resistant microstructures.