Fatigue lifing approaches for shot peened turbine components

Life assessment is of great importance to component repair and replacement scheduling of turbine systems which experience cyclic start-up and shut-down operations during their service lives. As vital parts where a severe stress concentration exists, the fir tree blade-disc interfaces are typically shot-peened. Their fatigue resistance is considered to be improved subsequently due to the surface compressive residual stress (CRS) field and strain hardening resulting from shot peening. However, current life assessment models are relatively conservative, merely considering shot peening as an additional safety factor rather than taking account of the benefits derived from the process. The objective of this research is to develop a life assessment method considering the effects of shot peening in the component lifing protocols. Successful development of this approach will achieve a more cost effective scheduling of repair or replacement of the assets while ensuring sufficient safety margins. In this study, a low pressure steam turbine material, FV448, has been selected. The shot peening induced residual stress profiles as well as their evolution during fatigue loading were measured using the X-ray diffraction (XRD) technique. In the modelling work, a 3D finite element (FE) modelling approach has been developed to predict the residual stress relaxation behaviour during fatigue loading. Both the CRS and the strain hardening fields arising from shot peening have been reconstructed in the FE model as pre-defined conditions: The reconstruction of residual stresses was realised by the inverse eigenstrain approach. In this study, the application of this approach has been extended from a flat surface to a notched geometry. The strain hardening field was reconstructed by modifying the material parameters at different depths based on the shot peening induced plastic strain distribution, which was evaluated utilising an approach based on previous measurement of electron backscatter diffraction (EBSD) local misorientations. By allowing for both beneficial effects of shot peening, the simulated quasi-static residual stress relaxation occurring during the first cycle correlated well with corresponding experimental data. The retention of the CRS field in the notched sample during fatigue loading has been highlighted. The application of total life approaches in predicting the low-cycle fatigue (LCF) life of the shot-peened specimens has been investigated. The Smith-Watson-Topper (SWT) and the Fatemi-Socie (FS) critical plane fatigue criteria have been selected in the present study. The developed FE models incorporating shot peening effects have been used to generate the stress and strain data required by the SWT and FS criteria. A good agreement between experiments and predictions was obtained using this FE-based approach. In addition, the FE analysis shows that the degree of the shot peening benefit in improving fatigue life can be reasonably related to the degree of the reduction in the mean stress level within the shot peening affected layer. The application of a critical distance method considering the stress and strain hardening gradients near the shotpeened surface has been found to effectively increase the accuracy of the life prediction. Damage tolerant approaches have also been employed to assess the fatigue life of the shot-peened notched specimens by predicting the short crack growth behaviour through the shot peening affected layer. This analysis was carried out using both 2D and 3D FE models containing a crack emanating from the notch root. The FE models have been upgraded from the models used to study the residual stress relaxation behaviour. The crack driving force has been appropriately characterised using both linear-elastic and elasto-plastic fracture mechanics (LEFM and EPFM), allowing for the effects of shot peening. An accurate quantification of the retardation of the short crack growth behaviour resulting from shot peening has been subsequently realised. Additionally, the associated crack shape evolution has also been predicted using the developed FE model, which explains the experimentally observed significant differences in crack shape evolution between varying surface conditions. The importance of taking the crack shape effects into account when evaluating the short crack growth behaviour has been emphasised. Overall, a FE tool has been developed in this study which has been demonstrated to be effective in analysing the benefits of shot peening in improving fatigue life. It also helps unveil the mechanism behind this life improvement, which contributes to the development of a robust and convenient lifing method that can be applied to shot-peened components and can be used to guide shot peening optimisation.