Volume 2, Issue 8 (September 2005)
A Design Methodology to Take Credit for Residual Stresses in Fatigue Limited Designs
High cycle fatigue (HCF) performance of turbine engine components has been known for decades to benefit from compressive surface residual stresses produced by shot peening. Recently laser shocking 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. However, the lack of a comprehensive design method that defines the depth and magnitude of compression required to achieve a design fatigue life has prevented surface enhancement from being used for more than a safeguard against HCF damage initiation. The present paper describes a design methodology and testing protocol to allow credit to be taken for the beneficial compression introduced by surface enhancement in component design to achieve a required or optimal fatigue performance.
A detailed design method has been developed that relates the required fatigue life, the mean and alternating applied stresses, and the damage in terms of Kf to the residual stress at the fatigue initiation site required for the targeted HCF performance. The method is applied to feature specimens designed to simulate the fatigue conditions in the trailing edge of a first stage low pressure Ti-6-4 compressor vane to provide the optimal trailing edge damage tolerance. A novel adaptation of the traditional Haigh diagram to estimate the compressive residual stress magnitude for optimal fatigue performance is introduced. Fatigue results on blade-edge feature samples are compared with analytical predictions provided by the design methodology.