A hybrid modeling method of fatigue crack growth for gas turbine blades under combined high and low cycle fatigue loadings

Abstract

Fatigue crack is one of the major faults of the gas turbine blades. Previous modeling studies on fatigue crack growth of blades mainly focus on a single numerical or analytical approach considering different sizes of cracks under various fatigue loadings. However, the actual crack propagation process is time-varying. The iteration and update of the model are necessary to evaluate the fatigue life of blades precisely. This paper proposes a hybrid modeling method to study blade crack growth under combined high and low cycle fatigue (CCF) loadings. The method emphasizes the combination and interaction between the finite element (FE) numerical simulation and the analytical calculation based on the fracture mechanics model. The current crack propagation length of the blade FE model is calculated from the crack growth rate obtained by the stress intensity factor (SIF) range according to Paris law. Then, a new SIF range is resolved from the FE model with the updated crack length. The proposed method is verified by the first-stage compressor blade from an in-service gas turbine. Results show that the crack growth is faster using the proposed hybrid modeling method than the traditional method. The update of the SIF range under CCF loadings cannot be ignored when predicting the fatigue life of the blade. Also, a sensitivity analysis is carried out. Suggestions are given on how to set the crack extension length at each stage during modeling, especially when the blade approaches failure.