FEA-based fatigue assessment methodology for hydraulic turbine runners

Turbine runners experience start-stop cycles and vibration cycles. Cracks initiated from manufacturing defects and propagated by start-stop cycles become critical, when the stress intensity range due to vibration loading exceeds the threshold for fatigue crack growth. Assuming a crack to grow under a constant stress equal to that at the most highly loaded location at the welded joint between the blade and the band or crown of a Francis turbine runner yields a conservative estimate of the life of the runner. A more accurate prediction of fatigue life is obtained by considering the growth of a crack in the real, inhomogeneous stress field. For an idealised T-joint under pure bending, the stress field has been determined by means of finite element analysis. Both a finite and a zero transition radius have been modelled, and the influence of the mesh size on the maximum stress has been investigated. The growth of a semi-elliptic surface crack in the stress field of the T-joint has been analysed using stress intensity factors due to Pommier et al. Theoretical predictions are in acceptable agreement with observations from fatigue testing.