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The University of Southampton

Research project: Life assessment methods for industrial steam and gas turbines

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Crack initiation in steam and gas turbine blade and disc roots can lead to structural failure resulting in loss of revenue for the operator. Validated life assessment models will enable understanding of defect tolerance and crack growth, enabling the scheduling of maintenance work, to prevent such severe outages. The effect of shot-peening surface treatments on improving low cycle fatigue life has been a focus of this work.

Shot-peening results
Shot-peening results

Materials characterisation and mechanical testing of fatigue properties is being used to characterise the behaviour of turbine materials. Computational modelling, including the use of finite element analysis, will allow the generation of validated life assessment models. Research work encompasses both as-designed and repaired blade to disc geometries and will allow for simulations of life-time scatter. Shot peening is a cold-working process in which small spherical shots are used to bombard the surface of components. The procedure induces compressive stress on the surface.

Shot peen overview
Shot peen overview

Nearly all fatigue and stress corrosion failures originate on the surface of a part, but the cracks will not initiate or propagate in a compressively stressed zone, so the compressive stress under the component surface is beneficial in increasing resistance to fatigue failures, including corrosion fatigue. For critical applications, i.e. steam and gas turbine components, an accurate prediction of fatigue lifetime will allow close scheduling of inspection and renewal, meaning the maintenance cycle could be extended, with considerable cost benefits. The detailed behaviour of crack initiation events and the early stages of crack growth are being examined for a number of shot peening conditions and compared with lifing approaches.

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