Study On Fatigue Crack Propagation Characteristics Of Wind Turbine Pitch Bearings

Wind turbine pitch bearings,as vital components of wind power generators,are subjected to complex loading conditions such as alternating loads and impact loads.Under these conditions,fatigue crack propagation in pitch bearings is prone to occur,leading to failure and damage,consequently affecting the overall service life of the wind power generator.This paper focuses on the theoretical analysis and calculation of the contact load distribution in pitch bearings.By combining fatigue crack propagation experiments with finite element simulation analysis,the research investigates the mechanism of fatigue crack growth in pitch bearings.The main contents are as follows:1.A static mechanical overall model of a 3.4 MW pitch bearing was established,and theoretical calculations of the contact load were performed to obtain the maximum contact load under extreme operating conditions.Local submodels were created for the"bolt hole in the variable pitch bearing"and the"rolling element/rolling track".2.Standard CT specimens of 42CrMo were prepared,and fracture toughness tests were conducted to determine the fracture toughness of the material.Fatigue crack growth tests were performed under different loads and stress ratios,and the relationship between the crack growth rate da/dN and the stress intensity factor rangeΔK was obtained.The applicability range of three commonly used crack growth models,namely Paris,Walker,and Forman,was analyzed under different stress ratio conditions..3.A three-dimensional finite element model of the CT specimen was established using Abaqus.The distribution pattern of the stress intensity factor at the crack front was obtained,and a comparison analysis was conducted between the simulation results and the theoretical calculations of the stress intensity factor to validate the accuracy of the simulation method.Numerical simulations of fatigue crack growth were performed using ANSYS-Workbench.The fatigue crack growth process of the CT specimen was simulated,and the results were compared with experimental data to verify the accuracy of the numerical simulations.4.Finite element analysis was conducted to investigate the fatigue crack growth characteristics of the bolt hole submodel in the variable pitch bearing.For the submodel with a bolt hole,initial cracks of different length-to-depth ratios(a/b)were introduced at the bolt hole,and crack propagation calculations were performed.The results indicated that as the length-to-depth ratio(a/b)increased,the crack became more prone to initiation.Additionally,by varying the initial crack angle,it was observed that the crack was most likely to initiate when the crack plane was aligned with the axial direction of the bolt hole.Fatigue crack growth life calculations were carried out for this submodel.5.Fatigue crack characteristics analysis was conducted on the rolling element/rolling track submodel.By varying the initial crack distance from the contact center of the rolling element,it was found that the crack was more prone to propagation at a distance of 2mm.The predominant crack types observed were a combination of Mode II and Mode III cracks.By controlling the angle between the crack plane and the rolling track surface,it was observed that as the angle increased,the crack plane experienced an increase in shear stress perpendicular to the propagation direction.When the angle was 120°,the Mode III stress intensity factor,_Ⅲ,at the crack front reached its maximum value.Fatigue crack growth life calculations were performed for this submodel..