Design Credit for Compressive Residual Stresses in Turbine Engine Components

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 is not generally taken for the improved fatigue performance in component design. Laser shock processing (LSP) and low plasticity burnishing (LPB) provide impressive fatigue and damage tolerance improvement by introducing deep or through- thickness compression in fatigue critical areas, but have been applied primarily to improve existing, rather than new, designs. This paper describes a design methodology to allow credit to be taken for beneficial residual stresses in component design to achieve a required or optimal fatigue performance. The fatigue design methodology is based on an extension of the traditional Haigh Diagram to include compressive mean stresses. The Smith Watson Topper equation (or other similar equations by Walker or Jasper) is used in combination with Neuber's rule to account for both the stress ratio, R, and stress concentration factors from notches and cracks. The extension of the Haigh Diagram into the compressive mean stress region and the effect of stress concentration factors lead to the identification of a safe range of mean and alternating stresses in which there can be no Mode I crack growth. This in turn is used to determine the minimum and optimum compressive residual stresses needed to mitigate different damage conditions in terms of kf. Case studies are presented illustrating the design approach for Ti-6Al-4V turbine engine compressor blade and vane edges to mitigate FOD and fan blade dovetail surfaces to mitigate fretting damage.