Energy-Based Fatigue Life Prediction for Combined Low Cycle and High Cycle Fatigue

Gas turbine engine components are subjected to both low and high cycle fatigue as a result of mechanical and vibrational loading. Mechanical loading is generally within the low cycle fatigue regime and attributed to throttle up/throttle down cycles of various flight maneuvers or engine start-up/shut-down cycles over the course of a component’s lifetime. Vibrational loading causes high cycle fatigue of a multiaxial stress state, and is attributed to various forced and free vibration sources manifested as high order bending or torsion modes. Understanding the interaction of these two fatigue regimes is necessary to develop robust design techniques for gas turbine engines and turbomachinery in general. Furthermore, applying a method to accurately predict fatigue performance from a reduced data set can greatly reduce time and material costs. This study investigates commonly used fatigue life prediction models and techniques in their ability to accurately model fatigue lives of Al 6061-T651 cylindrical test specimens subjected to various stress ratios, mean stresses, and high cycle/low cycle interaction. Comparisons between these models are made and modifications are proposed than can account for these complex loading effects where appropriate.