High Cycle Fatigue of turbo machinery blades is a significant design problem because one of the turbine stages may operates very close to the resonant condition and lead to fatigue failures. In order to assess the crack initiation life of a turbine blade, it is essential to correlate vibration to fatigue. Often a crack initiates from the material imperfections under the combination of steady stresses and fluctuating stresses in high cycle fatigue phenomena. This work models a turbine blade as an untwisted, non tapered cantilever beam with asymmetric cross section. The natural frequencies of the turbine blade were determined by using modal analysis in ANSYS. Nozzle excitation frequencies and forces were determined from the analysis of flow path field between stator and rotor blades. The critical condition at which natural frequencies are coincident with nozzle excitation frequencies were spotted from the Campbell diagram. Steady stresses and dynamic stresses were calculated in ANSYS using excitation forces corresponding to the resonance condition. The stress results obtained were compared with the analytical approach. The true stresses in the vicinity of the defect were calculated by Neuber’s rule with dynamic stresses as input. Local strain around defect was calculated through the formulae given by Martin et al. Crack initiation life was predicted by solving strain life equation.
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- Prediction of the Crack Initiation Life of Turbine Blade
Sharadchandra D. Jog
- Springer Netherlands
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