Research On Stiffness Degradation Model Of Composite Wind Turbine Blades

Wind turbine blade(hereinafter called the blade)is the key part of wind turbines to capture wind energy.In recent years,with the increasing capacity of wind turbines,the size of blade increases consequently,the quality problems of blade are becoming increasingly prominent.The failure probability of blade with flat,lightweight and flexible structure is as high as 40 percent under the continuing action of varying wind loads with random direction,and instantaneous impact of strong gust.Facing the grim situation,how to analysis fatigue performance of composite blade with high precision and high efficiency is very important for improving the fatigue life of blade and ensuring the safe and efficient operation of blade.The aim of this dissertation is to explore the evolution law of fatigue performance of composite blade,stiffness is chosen as a characteristic parameter to characterize the fatigue properties of composites and their components.Based on the progressive research ideas of “material level”,“structural level”,“product level” with blade,the law of stiffness degradation are investigated systematically for the composite laminates of blade,beam substructure of blade and composite blade,respectively,further providing theoretical basis for service life prediction of blade and wind power equipment construction.The main contributions of this study include the following aspects:(1)A stiffness degradation model during the life cycle of composite laminates for blade is proposed.Based on the macro-fatigue damage model of composite,and considering the stages and non-linear characteristics of the fatigue damage process of composite laminates,a stiffness degradation model of composite laminates for blade is established by combining with fatigue damage theory,Paris formula and non-linear mathematical model,and this model is defined as “material-level” stiffness degradation model for blade.Comparing the existing stiffness degradation models,the case study result shows that the proposed stiffness degradation model is more accurate to describe the law of stiffness degradation of composite laminates for blade during the life cycle.(2)A stiffness degradation model of beam substructure for blade(composite I-beam)is established.To study the law of stiffness degradation of beam structure for blade,the concept of blade substructure is introduced.A composite I-beam with blade cap,web and bonding layer is adopted to simulate the beam structure of blade with “I” shaped cross section.Using classical laminate theory,composite damage theory and continuum damage mechanics theory,the static and fatigue characteristics of the composite I-beam are analyzed respectively,and the stiffness degradation model of composite I-beam with adhesive layer is established based on progressive fatigue damage.And this model is defined as “structure level” stiffness degradation model for blade.The results show that the proposed stiffness model can well characterize the stiffness degradation of composite I-beam under fatigue loadings as well as the bonding layer is the weak link of the composite I-beam.(3)A stiffness degradation model of composite blade is proposed based on the damage evolution process.According to the definition of blade stiffness in full-scale fatigue test,considering the stages and non-linear characteristics of blade stiffness degradation process,and based on the damage evolution process of composite laminates and two boundary constraints of damage accumulation model,a stiffness degradation model of composite blade is proposed from the macro-phenomenological point of view.And this stiffness degaradation model is defied as “product-level” stiffness degradation model for blade.It is validated by using full-scale fatigue test data of a 2.5MW blade.At the same times,this stiffness degradation model is compared with the “material-level” and “structure-level” stiffness degradation model for blade respectively.The results show that the “product-level” stiffness degradation model has the highest accuracy for describing the stiffness degradation process of this blade.Based on that,as the application and extension of the “product-level” stiffness degradation model for blade,the fatigue life of this blade is predicted under full-scale fatigue test conditions.(4)A stiffness degradation model of composite blade is established based on the stochastic process.Considering the randomness and monotonicity of blade stiffness degradation process,Gamma process and Inverse Gaussian(IG)process with st rict monotonicity are introduced from the statistical point of view respectively.A stiffness degradation model of composite blade is established based on the two stochastic processes(Gamma and IG),and the models are defined as “product-level” stiffness degradation model for blade.Using full-scale fatigue test data of a 2.5MW blad and fatigue failure criterion of blade,the rationality of the proposed models are verified by predicting the fatigue reliability life of th is blade.The results show that under the same test conditions,compared with Gamma process,IG process can characterize the blade stiffness degradation process more accurately under full-scale fatigue test conditions.