Fatigue behavior of steels for critical members and durability models with stress ratio growth

The use of high-strength steels allows you to reduce the weight of the object, increase reliability and safety. Authors offer a new interpretation of critical members based on risk analysis and the concept of safety. Their performance determines the level of losses during the operation of mechanical systems. Critical members made of high-strength steels can be subjected to cyclic loading with a high stress coefficient. Known disadvantage of high-strength steels is their vulnerability to most impact factors, including cyclic loading with high cycle asymmetry. Cylindrical samples of heat-treated steels 09Cr16Ni4Nb and 13Cr15Ni4NМо3 were tested for tensile fatigue. Test results were presented in the form of a durability equation. Smith charts in relative stresses were obtained by simple transformations from a durability equation. Obtained charts showed a somewhat anomalous concave shape instead of an expected convex. This indicates a loss of sensitivity to mean stresses at high cycle asymmetry. Authors explain this phenomenon from the point of view of combining the concepts of fatigue and fracture mechanics. Model of a durability equation is proposed based on the concept of combining fracture mechanics and fatigue methodology. Its application in practice is a priori and requires assigning only one parameter in the form of the slope of the S-N curve.