Optimization of Surface Enhancement Methods to Improve the Fatigue Life of Turbine Engine Components

The fatigue life of turbine engine components is substantially extended by surface enhancement (SE) using shot peening, laser shocking, etc., which produces a layer of compressive residual stress. Loss of the compressive layer, which relaxes with time at elevated engine temperatures, reduces the resistance to high cycle fatigue failure, shortens component life and limits engine performance. Retention of surface compression will improve engine robustness and maintenance economy. The rate of residual stress relaxation depends upon the degree to which the surface is cold worked. The cold work produced by different SE methods can be controlled, allowing current and new SE methods to be developed for better retention of surface compression. A model expressed in terms of properties measurable by x-ray diffraction will be developed to predict the relaxation of compressive residual stress produced by shot, laser shock peening (LSP), and gravity peening in Inconel 718 and Ti-6A1-4V at engine temperatures. Improved retention of component life extending compression produced by low cold work SE, and the feasibility of modifying SE process parameters to improve retention of compression in Ni and Ti alloys will be demonstrated in Phase I. New, and improved surface enhancement methods will be developed in Phase II.