Effects of shot peening on short crack growth rate and resulting low cycle fatigue behaviour in low pressure turbine blade material

Abstract

The effect of shot peening on subsequent low cycle fatigue behaviour of a representative low pressure (LP) steam turbine blade material has been investigated in bend test samples. An analysis of the short fatigue crack growth behaviour has been conducted. For samples with no stress concentration feature shot peening was found to have a more evident beneficial effect at lower strain levels than at higher strain levels whereas for samples with a stress concentration feature, the beneficial effect was retained even at higher strain levels.Pre-existing cracks were observed on the shot peened surface, which started to grow at 10~25% of fatigue life in the low cycle fatigue regime. The crack propagation rate was slower than that observed in the ground sample suggesting that the shot peening process delayed crack propagation. This improvement in fatigue life has been attributed to the significant slowing of small cracks while growing through surface regions of significant compressive residual stresses and local work hardening developed by the shot peening process. Once cracks in the notch root have penetrated this region in the depth direction, faster crack growth rates, similar to those observed in the ground case, were seen.

3-D analysis of fatigue crack behaviour in a shot peened steam turbine blade material

“…A full description of the fatigue sample and experiment details have been provided in previous publications [31,[34][35][36] and the relevant details are reported here only briefly. The material in this study is a steam turbine blade material, FV448, a ferritic heat resistant steel with a tempered martensitic microstructure.…”

“…The present study is part of a larger project detailed in [31,[34][35][36], focussed on understanding the effects of shot peening on fatigue crack initiation and propagation behaviour in steam turbine blade materials in the LCF regime. Both mechanical serial sectioning and X-ray tomography (using a lab source) were used to study in detail the 3D fatigue crack morphology under these shot peened conditions, and this paper focuses on these investigations.…”

Stress Relaxation in Shot-Peened Geometric Features Subjected to Fatigue: Experiments and Modelling

“…In previous experiments [3], four-point bend fatigue loading was carried out with load ratio R p = 0.1 and peak load P max = 16794 N, using a sinusoidal waveform of frequency of 20Hz. In the FE simulation, the same load values were applied, but a triangular waveform was chosen.…”

“…It should, however, be appreciated that the increased surface roughness caused by shot peening can actually reduce crack initiation time as a result of the presence of micro stress concentrators [1]. However, it has been determined that the beneficial effects of the surface compressive residual stress and strain hardening usually dominate any detrimental surface roughness effects and the overall effect of shot peening is to increase fatigue life [1,3,4]. Shot peening is particularly attractive for application at geometric stress concentrations.…”

“…Turbine blades of conventional power plants are mostly manufactured from ferritic heat resistant steels [1,2,3], and are susceptible to damage and crack formation in regions of contact exposed to both centrifugal loading and oscillatory vibrations. One particular location where high stress concentrations are present and which is therefore susceptible to fatigue failure is the fir-tree blade root.…”