Recent Advances in High Cycle Fatigue

High cycle fatigue (HCF) has been the cause of a number of turbine engine failures over the last several years within the U.S. Air Force. Extensive investigations have determined that the root cause when related to material properties, is the accumulation of damage prior to and during service which, in turn, reduces the capability of the material in terms of the fatigue limit strength. Numerous studies have been conducted over the past several years to address the determination of fatigue limits. In this paper, in addition to developing approaches for predicting fatigue limits under various mean stresses and biaxial stress states, methods are presented for accounting for service-induced damage. One of the types of damage involves low cycle fatigue (LCF) which, by itself, should not cause failure but may reduce the HCF capability of the material. Data on the accumulation of damage under LCF or HCF overload transients, and their effects on the HCF strength of a titanium alloy are used to illustrate the inability of linear damage summation laws. A Kitagawa diagram with a short crack correction is found to be useful in assessing the fatigue strength of both smooth and notched specimens.