Extension of an energy-based life prediction method to low and combined cycle fatigue regimes

An energy-based fatigue life prediction method has been developed to accurately predict lifetimes of coupon specimens in excess of 10⁵ cycles. The method has been shown to agree with empirically determined room temperature high-cycle fatigue data for both Al 6061-T6 and Ti-6Al-4V in uniaxial, bending, and shear at various stress ratios (R). As with any life prediction method, using a testing scheme to accurately predict fatigue performance from a reduced data set greatly reduces test time and material costs. For gas turbine engine components, this can account for a large portion of development costs, making the use of reduced order models very attractive. The stress state of these components can be difficult to characterize and simulate, as they are subjected to both low-cycle fatigue and high-cycle fatigue from both mechanical and vibrational loading. Mechanical loading is generally within the low-cycle fatigue regime and attributed to throttle excursions of various flight maneuvers or engine start-up/shut-down cycles over the course of a component’s lifetime, typically less than 10⁵ cycles. Vibrational loading causes high-cycle fatigue, sometimes 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, as combined cycle fatigue, is necessary to develop robust design techniques for gas turbine engine and turbomachinery in general. This study focuses on extending a previously developed energy-based fatigue life prediction method to account for both low-cycle fatigue and combined cycle fatigue of Al 6061-T6511 cylindrical test specimens subjected to various stress ratios, mean stresses, and high-cycle fatigue–low-cycle fatigue interaction.