| With the rapid development of China’s wind power industry,it is vital for the healthy operation of the main components in wind turbine blades during its long service life.It has become an important topic in the era of wind power operation and maintenance.Wind turbine blades are subjected to random loading during service,and the main spars are the most important load-bearing structure of the blades.The defects of the main spar during manufacturing are the root of early fatigue damage,which can affect the fatigue life of the blades directly.The fatigue life prediction of the main spar is obviously uncertain and challenging,because of the dispersion of defect types and sizes within the main spar and the randomness of the wind loads.In order to standardize the fatigue test standard of the main spar before leaving the factory and formulate a reasonable blade operation and maintenance strategy,the fatigue damage mechanism was clarified from the source of manufacturing defects to predict the fatigue life of the main spar under random loading in this paper.The main research work of this paper are as follows:(1)Based on the laws of thermodynamics,considering the orthogonal anisotropy of the main spar composite,the expression for fatigue damage energy was derived according to the relationship between the energy dissipation sources of each part during fatigue.By homogenizing each thermophysical property parameter of the defective material,the calculation models of fatigue damage energy for main spar laminates with void defects and delamination defects were constructed,respectively.It can be used to characterize the evolution of fatigue damage of the main spar with defects.They can be used to characterize the evolution of internal defects in the main beam into fatigue damage.Fatigue tests monitored by the infrared camera were carried out on the main spar specimens with different void and delamination defects.It was found that the temperature rise was relatively greater for specimens with void defects and less for specimens with larger void and delamination defects during the tests.It provides an effective basis for monitoring main spars with defects.Early damage evolution of the main spar can be predicted when the local thermal zone of the defect in the infrared thermogram is no longer expanding but the temperature increases sharply,which is a critical moment for assessing the damage state.(2)According to the calculation models of fatigue damage energy,the variation of fatigue damage energy and stiffness degradation of main spar specimens with void and delamination defects were explored during the tests.Through micro-morphology analysis,it was verified one by one that the critical change values of the two variables during fatigue could be used as a criterion for the early damage state of the main spar material with defects.Based on these two damage variables,the damage accumulation models were established,and the experimental data were curve-fitted,among which the fatigue damage energy damage model had higher fitting accuracy.A simple method was proposed to evaluate the early damage state of main spar with void and delamination defects based on the fatigue damage energy model.The finite element software was applied to simulate the change of fatigue damage energy of the specimens during fatigue loading.The agreement between the simulated and experimental results verified the accuracy and feasibility of identifying fatigue damage evolution states based on fatigue damage energy.(3)A rapid method for fatigue limit evaluation of the main spars with wrinkle defects is proposed based on fatigue damage energy.When the main spar laminates were in the stage of damage stabilization during fatigue,the internal defect evolution reached a quasiequilibrium state and the fatigue damage energy was basically constant.The two-curve method was used to fit the stabilized fatigue damage energy of the main spar specimens with wrinkle defects under different stress levels,and the stress amplitude corresponding to the inflection point of the two lines was the fatigue limit.Compared to the results estimated by the conventional method,the fatigue limit evaluated by applying fatigue damage energy had a higher prediction accuracy and did not require experiments until the specimen failed,which significantly shortened the experimental period.The effect of different wrinkle geometry on the mechanical property of stiffness degradation was quantified.Then,based on stiffness degradation and fatigue damage energy,a fatigue life prediction model of the main spar laminate with wrinkle defects under constant amplitude loading was developed.The S-N curves predicted by applying the proposed model were within 95% confidence interval of the traditional test results,it had the higher accuracy for assessing the fatigue life of the main spar with different wrinkle defects.(4)On the basis of the proposed fatigue life prediction model under constant amplitude loading,considering the probability distribution characteristics of the random variables in the model,the fatigue life prediction and probability distribution model for main spar laminates with wrinkle defects under random loading were derived.The fatigue life and probability distribution of the main spar laminates under random loading can be predicted from only a few experimental results under constant amplitude loading.The error between the theoretical predictions and the experimental results was within 10%,which greatly reduced the workload of random load experiments.The fatigue life of the 1.5 MW blade was predicted based on the fatigue load spectrum of the actual blade in service.Based on the generalized stress-life relationship of the blade material,the total fatigue damage of the blade was calculated by applying the fatigue damage accumulation model under random loading,from which the fatigue life of the blade was predicted.The agreement between the field test results and the calculated results verified the correctness of the proposed method for evaluating the fatigue life of blades. |
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