Emerging blade technologies are finding it increasingly essential to correlate blade vibrations to blade fatigue in order to asses the residual life of existing blading and for development of newer designs. In this paper an analytical code for Dynamic Stress Analysis and Fatigue Life Prediction of blades is presented. The life prediction algorithm is based on a combination method, which combines the local strain approach to predict the initiation life and fracture mechanics approach to predict the propagation life, to estimate the total fatigue life. The conventional stress based approach involving von Mises theory along with S-N-Mean stress diagram suffer from the drawback that they do not make allowance for the possibility of development of plastic strain zones, especially in cases of low cycle fatigue. In the present paper, strain life concepts are employed to analyse the crack initiation phenomenon. Dynamic and static stresses incurred by the blade form inputs to the life estimation algorithm. The modeling is done for a general tapered, twisted and asymmetric cross section blade mounted on a rotating disc at a stagger angle. Blade damping is nonlinear in nature and a numerical technique is employed for estimation of blade stresses under typical nozzle excitation. Critical cases of resonant conditions of blade operation are considered. Neuber’s rule is applied to the dynamic stresses to obtain the elasto-plastic strains and then the material hysteresis curve is used to iteratively solve for the plastic stress. Static stress effects are accounted for and crack initiation life is estimated by solving the strain life equation. Crack growth formulations are then applied to the initiated crack to analyse the propagation of crack leading to failure. The engineering approximations involved are stated and the algorithm is numerically demonstrated for typical conditions of blade operations.

This content is only available via PDF.