High-Cycle Fatigue in a Hydraulic Turbine Runner

A hydraulic turbine is an equipment where hot-formed blades are welded to band and crown by double-fillet welds. Decades of operating experience have shown that fatigue cracks develop in hydraulic turbine runners where both stress concentrations and material defects can be observed, as often occurs in the welded zones of such runners. In the present paper, a welded joint between the blade and the band or crown of a Francis turbine runner is considered, and the failure mechanism due to high-cycle fatigue loading produced by operational starts and stops is analysed. Such a welded joint can be idealised as a T-joint with a circular-shaped transition zone between blade and band (or crown), subjected to cyclic bending induced by the water action. A semielliptical surface crack is assumed to exist in the above transition zone, and the crack propagation is numerically examined by using the stress-intensity factor values obtained from finite element analyses. Experimental fatigue testing results are employed to substantiate these numerical estimations.