Fatigue Damage Mechanism and Stress Relaxation in Shot Peened and Polished Nickel Base Materials

Abstract

Aircraft turbine engine disks operate at high cyclic loads. Low cycle fatigue, by definition involves localized plastic deformation, typically in areas with geometrical stress concentrations and results in multiple cracking. It is common engineering practice to introduce compressive residual stress by processes such as shot peening on the surface of these highly loaded parts to improve fatigue strength. The fatigue strength is improved because of the compressive residual stress offsets some of the operating tensile loads. The combined stresses, when they accumulate cycles above yield will gradually cause the surface compressive residual stress to be reduced until the surface becomes residually stressed in tension which will lead to crack initiation and crack propagation. This work shows the different damage mechanisms found in shot peened and polished specimens after fatigue testing using rotating beam.