A design approach for components in ultralong fatigue life with step loading

An overview of critical variables that affect fatigue failure with respect to steel components in ultralong life regimes is presented. The key role of hydrogen trapped by non-metallic inclusions in the ultralong life fatigue failure process is documented. The role of non-metallic inclusions on ultralong fatigue life is shown in the master curve of ODA (Optically Dark Area surrounding a non-metallic inclusion at fracture origin) growth. The master curve of ODA growth shows the correlation of the size of the ODA with the size of the non-metallic inclusion as it corresponds to fatigue life. The ability to predict the presence of non-metallic inclusions in steels with extreme value methods is incorporated with the master curve of ODA growth to determine the maximum threshold stress for ultralong fatigue life using the image parameter model. Most machine components experience variable loads in service. A design approach is introduced for calculating the effects of different loading levels for ultralong fatigue life.