W. Hassan, Rolls-Royce Corporation, Indianapolis, IN; D. Ryan, Honeywell Engines, Phoenix, AZ; B. Abu-Nabah, University of Cincinnati, Cincinnati, OH; P. B. Nagy, University of Cincinniti, Cincinnati, OH
Near surface residual stresses directly influence the fatigue life of critical engine rotating components. Depending on sign and magnitude a near surface residual stress gradient can either inhibit or accelerate fatigue initiation and crack propagation. Shot peening is a common surface treatment that can enhance machining residual stresses by increasing the magnitude and depth of compression. Shot peening is intended to create a uniform, consistent, and reliable sub-surface compressive residual stress layer. Recently, it has been demonstrated that, in contrast with most other materials, shot-peened nickel-base super alloys exhibit an apparent increase in eddy current conductivity at increasing inspection frequencies, which can be exploited for nondestructive residual stress assessment of subsurface residual stresses. Honeywell Aerospace is particularly interested in DP718 Ni-base superalloy as it is the main Ni alloy used in the manufacturing of critical rotating components in its engines. We will present the results of the process used to build sets of flat and curved samples with graduated residual stress profiles. The process, which utilizes design of experiment (DOE) approach, targets producing samples with four different stress profiles namely, as shot peened, 25% reduction in peak stress from the as shot peened case, 50% reduction in peak stress from the as shot peened case, and finally 75% reduction in peak stress from the as shot peened case. Thermal relaxation of the stress profiles is used to achieve these different levels. The model that relates the temperature and time parameters to the peak compressive stress level will also be presented and discussed. X-ray diffraction measurements will be used to quantify the generated profiles and the results will be compared to the residual stress profiles obtained using eddy current measurements at high frequencies (up to 80 MHz).
