Research On Stiffness And Strength Coupled Model Of Wind Turbine Blade Composites

Wind turbine blades(hereinafter called the blade)are the key part of wind turbines to capture wind energy,and their fatigue performance is crucial to the reliability of the whole wind turbine.At this stage,almost all large-scale commercial blades use composites as the main material,but due to the anisotropy of composite laminates,a variety of different fatigue damage modes are generated within them.With the accumulation of fatigue damage,different fatigue damage modes ar e induced and coupled with each other,resulting in signif icant degradation of their stiffness,strength,and other mechanical properties of composites.At present,the research on the stiffness degradation law and strength degradation law of blade composite laminates is mainly based on the stiffness degradation model and strength degradation model.Due to the inherent limitations of stiffness degradation models and strength degradation models,in this dissertation,a coupled model of stiffness and strength of composite laminates for blades is proposed to provide a theoretical basis and method to study fatigue performance of laminates and improve their fatigue life,which has certain academic significance.The main research work is as follows:(1)A stiffness degradation model during the life cycle of composite laminates for blades is proposed.Based on the fatigue damage accumulation model of composite laminates,the stage and non-linear characteristics of the damage evolution process of composite laminates are fully considered,thus a stiffness degradation model during the life cycle of composite laminates for blades is established,and the correctness and superiority of the proposed model are verified by using fatigue test data of composite laminates successively.Comparing the existing stiffness degradation models,the results show that the accuracy of the proposed model is relatively higher,and the fitting accuracies are higher than 0.9962,which can reasonably and accurately describe the law of stiffness degradation of composite laminates.(2)A coupled model of stiffness and strength of composite laminates for blades is proposed.To elucidate the intrinsic relationship between stiffness degradation and strength degradation,a coupled model between the cumulative damage characterized by residual stiffness and the cumulative damage characterized by resid ual strength is proposed based on the assumption that the residual strength and residual stiffness at the same moment depend on the same damage state within the material,based on the two boundary conditions of composite laminates damage and the stiffness and strength degradation processes.The results show that the fatigue damage accumulation rate characterized by stiffness degradation is greater than that characterized by strength degradation under general test conditions.(3)A strength degradation model of composite laminates for blades is proposed.Based on the stiffness degradation model established in(1)and the coupled model established in(2),a new strength degradation model is finally o btained,which combines the respective advantages of the stiffness degradation model and the strength degradation model.This new strength degradation model furtherly reduces the dependence on strength data in the modeling process and reduces the time-cost required for strength testing to a greater extent.