• You, Chao
• Reed, Philippa
• Soady, K. A.
• Achintha, Mithila

Abstract

In service, turbine components are subjected to<br/>low-cycle fatigue (LCF) during start-up and shutdown operations, especially at the fir tree root<br/>blade-disc connection which has a complex<br/>geometry and corresponding high stress<br/>concentration. Shot peening generates<br/>compressive residual stress and strain hardening<br/>which can improve fatigue life [1]. However,<br/>prediction of the fatigue life of shot-peened<br/>components under LCF is challanging due to<br/>difficulties associated with predicting residual<br/>stress relaxation, especially in regions of high<br/>stress concentration. The current study aims to<br/>develop a validated 3-D eigenstrain-based<br/>modelling tool to model residual stress relaxation<br/>under LCF in shot-peened notch geometries. The<br/>residual stress and strain hardening profiles caused<br/>by shot peening have been first evaluated by<br/>experiments and then incorporated into the finite<br/>element (FE) model separately.<br/>The material under investigation is FV448 - a<br/>ferritic heat resistant steel representative of those<br/>used for steam turbine blades. An industrially<br/>applied shot peening treatment (intensity: 13A,<br/>coverage: 200%) for steam turbine blades has been<br/>applied to U-notched samples (Kt = 1.58)<br/>representative of the real fir tree geometry. The<br/>LCF behaviour of the shot-peened sample has been<br/>evaluated by three-point bend tests with a load<br/>ratio R = 0.1 [2].<br/>Residual stress variation with depth at the notch<br/>root of shot-peened samples was measured before<br/>and after fatigue load cycles, using an X-ray<br/>diffraction (XRD) device and an incremental layer<br/>removal approach achieved by electropolishing. In<br/>addition, an EBSD-based approach [3] has been<br/>used to measure the plastic strain caused by shot<br/>peening, which was then used to determine the<br/>local strain hardening levels in peened samples.<br/>In the FE model, a combined isotropic-kinematic<br/>hardening material model has been applied,<br/>considering both the monotonic and cyclic<br/>mechanical properties of FV448 which have been<br/>determined experimentally [2]. The residual stress<br/>distribution in peened samples was simulated as<br/>an elastic response of the whole component to the<br/>predicted misfit strain (i.e. eigenstrain) caused by<br/>shot peening [4]. In order to incorporate the<br/>effects of strain hardening into the FE model,<br/>varying local yield stresses were defined at<br/>different depths within the surface layer affected<br/>by shot peening. Residual stress relaxation after 1<br/>cycle and 50% life (about 15000 cycles) was then<br/>simulated by applying a similar load as in the real<br/>experiment to the FE model; the applied nominal<br/>strain range ∆ɛ in the loading direction was 0.68%.<br/>The results show that the full residual stress<br/>distribution has been accurately modelled in the<br/>peened notched sample. The modelling results of<br/>residual stress relaxation after cyclic loading (∆ɛ =<br/>0.68%) match well with experimental data; a 20%<br/>relaxation was observed after the first cycle but<br/>with no further relaxation during subsequent<br/>fatigue cycles.<br/>This study suggests that the hybrid eigenstrain/FE<br/>approach is particularly effective in modelling<br/>residual stresses in shot-peened components with<br/>notch geometry. This approach is helpful in<br/>evaluating the benefit of shot peening by<br/>effectively predicting residual stress relaxation<br/>after fatigue loading

Topics

• impedance spectroscopy
• surface
• polymer
• x-ray diffraction
• experiment
• steel
• fatigue
• electron backscatter diffraction
• isotropic