This work is focusing on the influence of the specimen geometry for the characterization of the fatigue behavior of AISI 316L and AISI 4140 for hydrogen testing in cryogenic conditions. For this purpose, four different hollow specimen geometries were manufactured. The aim was to enable HCF and VHCF testing under very high testing frequencies at 1000 Hz and cryogenic conditions, i.e. -253 °C and liquid hydrogen, at the same time. For this, the conventional specimen geometry has to be adapted. As a reference, the conventional geometry as well as the adapted geometry without a borehole have been manufactured. Furthermore, the adapted geometry was manufactured with an axial through bore and an axial blind hole.

At first, the tests were performed in a dry state, without additional hydrogen loading or temperature influence, in order to determine the effect of specimen geometry on fatigue behavior. In doing so, two resonance fatigue testing systems were used. The fatigue tests were conducted as multiple amplitude tests (MAT), in which the maximum stress was stepwise increased until failure occurred. The stress ratio was R = -1, i.e. fully-reversed loading. The maximum stress at start was chosen 200 MPa, as the maximum stress was increased by 10 MPa each 100,000 cycles. These tests served as estimation for the fatigue lifetime. Further tests are performed as constant amplitude tests (CAT) in the HCF regime by a testing frequency of approx. 85 Hz, whereas the VHCF regime results in testing frequencies of 1000 Hz. In the case of VHCF testing, the specimen temperature was reduced by pressured air cooling.

The results show a severe influence of the specimen geometry on the fatigue lifetime, resulting in a reduced maximum stress level at failure of up to 24%, comparing the axial through bore and the reference specimen at a testing frequency of 85 Hz. Testing frequencies of 1000 Hz revealed severe specimen heating in case of AISI 316L, resulting in higher maximum strains and thus limited stress controlling at high maximum stress levels.