Fatigue Damage Model Of Wind Turbine Blade Under Biaxial Conditions

Full-scale fatigue tests of wind turbine blades are the mandatory means to detect blade design and manufacturing quality.At present,the fatigue tests mainly adopt uniaxial fatigue tests,lead to the low accuracy and time cost.Thus,countries around the world have accelerated the research on the biaxial fatigue test methods.However,the lack of blade damage quantification method for biaxial fatigue tests has led to the difficulty of promoting efficient and accurate biaxial loading tests.Therefore,this thesis proposes a wind turbine blade biaxial fatigue test damage quantification model based on the multi-axial stress state of composite materials to quantify the damage produced by wind turbine blades in fatigue tests and to provide some theoretical support for the development of biaxial full-size structural fatigue tests of wind turbine blades.The main research contents and conclusions of this thesis are as follows:(1)Fatigue cumulative damage theory is explained in detail and a small-sample P-S-N curve modeling of composites is proposed.When quantifying the fatigue damage of blades,it is usually necessary to combine the S-N curve of the material for calculation.Considering the long cycle time and high cost of fatigue test of composite materials,a small sample P-S-N curve fitting method based on Weibull distribution.Firstly,the median S-N curve is obtained by calculating the mean value of the sample;secondly,a random variable function is constructed according to the characteristics of the Weibull distribution,and the median value of the sample is used as the initial value of the iteration to perform the regression optimization;finally,the shape and scale parameters of the Weibull distribution are determined to achieve the fitting of the P-SN curve.(2)The three-dimensional model of the wind turbine blade is established and the dangerous cross-section is obtained.Through the airfoil design software,the original airfoil coordinates are exported,and the three-dimensional spatial coordinates of the blade are obtained through the coordinates transformation formula,and finally,the three-dimensional model of the blade is generated in the model.The blade is imported into the provided 3D model interface,and the material parameters,material layup,mesh division,and working condition loading are set according to the blade requirements,and finally,the stress cloud map of each section of the blade is obtained,and the dangerous section of the blade is determined as the maximum chord length.(3)A damage quantification model under biaxial fatigue test conditions of wind turbine blades is established.Based on the multi-axis fatigue theory of composite materials,a damage quantification model for biaxial fatigue loading test is proposed and combines the simulation data of a 1.5MW wind turbine blade hazardous section to compare and analyze the damage values under uniaxial and biaxial fatigue tests.It is concluded that the accumulated fatigue damage of the biaxial fatigue test is higher than the sum of accumulated fatigue damage of two single-axis fatigue tests under the same number of tests.On this basis,the stress coupling law between the flatwise and edgewise in the biaxial fatigue test is analyzed,which provides some theoretical support for the promotion of the biaxial full-scale fatigue test of wind turbine blades.