This study investigates the fatigue characteristics of the high-quality IQ variant of case-hardened 18CrNiMo7-6 gear steel, widely used in aerospace applications, particularly in high-load components such as gears in Ultra High Bypass Ratio (UHBR) engine systems. A major challenge in these gearboxes is the extensive cyclic loading of components, which is not fully addressed by current standards.

The research explores the Very High Cycle Fatigue (VHCF) behavior up to one billion load cycles, providing essential data for material model development. Carburization parameters, including different carbon contents and hardness levels, are varied to assess their impact on fatigue performance. Additionally, the effect of the stress ratio on fatigue life is evaluated. Failure mechanisms are investigated through detailed examination of fracture surfaces using scanning electron microscopy (SEM), analyzing the distribution, size, and composition of inclusions, which are critical for crack initiation and propagation under cyclic loading.

In addition to VHCF analysis, crack propagation behavior is examined. Cyclic fatigue tests on compact tension (CT) specimens have been used to investigate how carburization, and hence different carbon content, affects sensitivity to crack growth. The fracture toughness and the threshold values for crack growth were also determined taking into account different carbon contents. This study provides insights into the factors influencing fatigue performance and crack propagation, which are crucial for improving performance in aerospace applications.

In addition to these findings, an innovative calculation method for probabilistic fatigue life modeling was implemented as part of the project. The fatigue life modeling approach used in this work is based on the concept of the weakest link. An innovation is the generation of the input parameter local hardness, which is determined by tests on samples that represent defined microstructural states of case hardening. This novel aspect allows the local hardness to be omitted from the variable derivation if the test statistics are sufficiently good. In addition, the model calculates component lifetimes so that in a reversed procedure specific lifetime calculations based on given loads are possible. This probabilistic approach can handle any failure probabilities, providing local distributions. Additionally, it makes objective statements about component durability impacts when the carburization or carbon depth profile is altered, given the corresponding residual stress depth profile.