Metallic materials, e.g. used for pipelines, have to show a high corrosion fatigue durability against gaseous hydrogen in context of embrittlement. This work shows the influence of gaseous hydrogen on the fatigue behaviour of the pipeline steel X56 (API 5L, 1.8724) in the low cycle fatigue regime (LCF). Therefore, strain-controlled fatigue tests are conducted with internal and external hydrogen pressurisation and as a reference in ambient air using both traditional specimen in an autoclave as well as hollow specimen.

The evaluation of the test results focuses on the determination of the stress-strain behaviour and the cyclic softening and hardening behaviour. Based on this evaluation the investigated steel shows a significant reduction in lifetime at high strain amplitudes under gaseous hydrogen compared to the reference investigations in air. Compared to the full specimens, the hollow specimens achieved higher lifetimes at comparable local stresses as well as in air and under hydrogen environment. The evaluation of the stress-strain behaviour in form of the cyclic stress-strain curves and the cyclic deformation curves showed for both specimen geometries no significant difference in stress amplitude, but the lifetime reducing effect of the hydrogen is pronounced. The fatigue life reducing influence of the hydrogen can only be observed for lifetimes up to 1∙10⁶ cycles. From this point the strain life curves of air and hydrogen merge into one another. In order to evaluate the different lifetimes determined for both specimen geometries, considerations are made regarding the influence of size effect.