Aeromechanical modelling of rotating fan blades to investigate high-cycle and low-cycle fatigue interaction

摘要

Gas turbine engine components are subject to both low-cycle fatigue (LCF) and high-cycle fatigue (HCF) loads. To improve engine reliability, durability and maintenance, it is necessary to understand the interaction of LCF and HCF in these components, which can adversely affect the overall life of the engine while they are occurring simultaneously during a flight cycle. The LCF loads result from the aircraft flight profile and are typically high stress, nominally rotational and aerodynamic loads. HCF loads are a consequence of high frequency vibrations, such as the fluctuating loads on blades as they rotate through the wakes from the upstream inlet guide vanes (IGVs). This thesis simulates a fully-coupled aeromechanical fluid-structure interaction (FSI) analysis in conjunction with a fracture mechanics analysis to predict the effect of representative fluctuating loads on the fatigue life of blisk fan blades. This was achieved by comparing an isolated rotor to a rotor in the presence of upstream IGVs. Two rotor blade geometries were considered in order to assess the effect of changing geometric parameters such as chord length and airfoil thickness on the subsequent vibrational response, subjected to an identical form of HCF aerodynamic forcing from IGV wakes. A fracture mechanics analysis was used to combine the HCF loading spectrum with an LCF loading spectrum from a simplified engine flight cycle in order to determine the extent of the fatigue life reduction due to the interaction of the HCF and LCF loads occurring simultaneously. The major contribution to the field of computational blade aeroelasticity is determined to be the reduced fatigue life of the blades predicted by a combined loading of HCF and LCF cycles from a crack growth analysis, as compared to the effect of the individual HCF and LCF loadings. In addition, the HCF aerodynamic forcing from the IGVs excites a higher natural frequency of vibration of the rotor blade, which is shown to have a detrimental effect on the fatigue life. The results indicate that fluid-structure interaction, blade-row interaction and HCF/LCF interaction are an important consideration when predicting blade life at the design stage of the engine. The implications of this research can influence future experimental studies that aim to generate meaningful fatigue data, which will assist in the management of safe operation of gas turbines.