Applied Loads Optimization For Large Rotor Blade Fatigue Testings

Full-scale fatigue testing is important because it guarantees the safety of blades during their design life.Researchers have studied their equivalent target loads,testing equipments and certification standards to standardize the full-scale fatigue testings.Even though,the current testings cannot accurately apply loads on large blades whose structures are complicated.In order to accurately and efficiently optimize applied loads for large blades,this thesis studies three aspects of the resonance fatigue testing: the characteristic of optimizing applied loads,the approaches to design applied load distributions and the methods to calculate applied loads.The characteristic of optimizing applied loads is studied firstly.By analysing the rules between mass variables and applied loads,optimizing applied loads is pointed to be a multi-variable optimization problem but the rule between every single variable and applied loads is simple.Therefore,particle swarm optimization(PSO)is chosen to be the basic algorithm for optimizing applied loads.Furthermore,based on the practical conditions,the variable and objective function of PSO are modified specifically.Thus,the optimization for applied loads is established.Two results are obtained from the cases,one is that the optimization can efficiently find the best applied loads among thousands of selections.The other is that the edgewise deviations are always large by only mounting additional masses.For accurately designing edgewise applied loads and dual-axial applied loads,the approaches of designing applied load distributions are studied in two aspects.From the aspect of design targets,the distributions of flapwise and edgewise target bending moments are deduced qualitatively by analysing the distributions of cross-section and density.After comparing the distributions between the target and the blade bending moments,the different requirements of designing flapwise and edgewise applied loads are clarified.From the aspect of design approaches,the abilities of mounting additional mass and cutting-off blade in modifying applied load distributions are figured out after studying their effects on structural properties and resonance characteristics.It is found that the ability of an additional mass is proportional to its local displacement,and cutting-off blade can wholly raise bending moment slope and natural frequencies.Based on the researches in this chapter,two results can be obtained.First,mounting additional mass is incapable to design applied loads near blade root,that is the reason why the edgewise deviations are always large by only mounting additional masses.Second,the integration of mounting additional mass and cutting-off blade can help decrease load deviations.Therefore,mounting additional mass and cutting-off blade are combined to optimize applied loads.On the one hand,the length of cutting-off blade is treated as an additional variable in optimization.The cases show that the additional variable can improve both the testing accuracy and testing efficiency to some extent but cannot significantly decrease edgewise deviations.On the other hand,comprehensively considering the different abilities of mounting additional mass and cutting-off blade and the different requirements of designing flapwise and edgewise applied loads,a two-stage fatigue testing method which consists of a dual-axial testing and an edgewise compensation testing is proposed.Compared to the general testing methods,the two-stage method can decrease 50% deviation or larger,but its testing efficiency is between single-axial and dual-axial testings.Besides,the calculation method of applied loads is also studied for efficiently designing applied loads.By physically explaining the geometric characteristics of applied bending moments and ignoring the changes of mode shapes,an efficient geometric expression can be deduced to calculate applied bending moments.Two results are obtained from the cases.First,because mounting additional mass affects resonance characteristics slightly,the geometric expression can accurately calculate applied bending moments of blade which is affected by additional masses only.The result difference between geometric expression and finite element method(FEM)is less than 15%.The corresponding optimization can not only reach the requirement of certification standard but also significantly shorten designing duration.Second,cutting-off blade affects resonance characteristics a bit seriously,the geometric expression and the corresponding optimization are inaccurate enough to calculate and optimize applied bending moments of blade which is affected by both cutting-off blade and mounting additional masses.For improving the accuracy of calculating applied loads,the directions between blade vibration and target loads are compared.It is found that the direction difference leads to varying amplitudes of dual-axial applied loads.To conveniently compare dualaxial applied loads with target loads whose amplitudes are constants,an ”equivalent applied loads” is deduced based on coordinate system transformation and the equivalent damage theory.Then,by rationally using beam and shell models,”equivalent applied loads” is introduced into optimization.The results of cases prove that the ”equivalent applied loads” can accurately predict damage caused by dual-axial coupled loads and the corresponding optimization can avoid insufficient damage.In conclusion,the author of this thesis figures out the characteristic of optimizing applied loads,clarifies the demands of modifying applied load distributions,proposes effective method of designing applied load distributions by scientifically combining mounting additional mass and cutting-off blade,deduces an efficient approach for calculating single-axial applied loads and an accurate approach for calculating dualaxial applied loads.By combining these researches together,the goal that accurately and efficiently designing applied loads on large testing range is achieved.