Incorporation of Residual Stresses in the Fatigue Performance Design of Ti-6AI-4V

摘要

The high cycle fatigue (HCF) performance of turbine engine components has long been improved by the introduction of a surface layer of compressive residual stress, usually by shot peening. However, credit has not been taken for the improved fatigue performance in component design; rather shot peening is used primarily as an additional safe guard against fatigue failure. Recently, laser shock processing (LSP) and low plasticity burnishing (LPB) have been shown to provide spectacular fatigue and damage tolerance improvement by introducing deep or through thickness compression in fatigue critical areas. These new processes have been introduced primarily to improve an existing inadequate design, and credit for the fatigue benefits is not taken in the initial design. This paper describes a design methodology and testing protocol to take credit for beneficial residual stresses in component design to achieve a required or optimal fatigue performance. A protocol has been developed for designing a residual stress distribution using surface treatments to achieve a targeted HCF performance. The protocol is applied to a 1st stage Ti-6Al-4V compressor blade to provide the optimal leading edge damage tolerance. The use of finite element modeling (FEM), linear elastic fracture mechanics, and x-ray diffraction (XRD) mapping of the residual stress field to develop an LPB generated residual stress distribution is described. A novel adaptation of the traditional Haigh diagram to estimate the compressive residual stress magnitude for optimal fatigue performance is introduced. Fatigue results on both blade-edge feature samples and fretting damaged samples with various kf are compared with analytical predictions provided by the design methodology.