Research On Fatigue Damage Of Wind Turbine Blades With Defect Based On Infrared Technology

Wind energy is one of the clean and renewable energy in the world.The blades are one of the important components of the wind turbine.Due to the artificial and manufacturing technical factors,the wind turbine blades are prone to generate original defects during the manufacturing process.The defects are fiber fracure,delamination,bubber,and microcrack.These defects have grown and connected by fatigue when wind turbine blades are in service.The macro-mechanical properties of wind turbine blades have deterioration.The wind turbine blades may cause fatigue fracture,and the entire wind turbine cannot run normally.Therefore,it is very important to monitor the fatigue damage of wind turbine blades with original defects.It is beneficial to predict the failure of wind turbine blade in advance.The wind turbine can ensure the safety and usability.The economic losses are caused by the shutdown.The fatigue damage is an irreversible nonlinear thermodynamics process.The fatigue damage evolution process of wind turbine blades with various original defects is studied by materials science,fatigue science,thermodynamic theory and infrared thermography.And the fatigue damage is studied by microscopic and macroscopic scales.The research provides theoretical support for online,real-time health monitoring of wind turbine blades.This paper analyzes and studies the fatigue damage process of blade with the original defect by the experimental and theoretical aspects.The fatigue damage of the wind turbine blades with delamination and bubber were studied by infrared thermography.The surface temperature changes and infrared thermal image sequence diagram of wind turbine blade were monitoring during the fatigue process.The effects of defect depth and defect type on fatigue damage of wind turbine blades were found.The fatigue damage of blades with delamination and bubber gradually increased with the fatigue time.The surface temperature of wind turbine blade gradually was increased.The outer layer defect of the specimen caused more fatigue damage to the blades than the inner layer defect of the specimen.Delamination defects have a greater impact on fatigue damage of blade than bubber defects.Fatigue limit is an important parameter of fatigue,which is closely related to fatigue damage.It is the most common method to predict the fatigue limit using the two-line method of the infrared thermography.The accuracy of fatigue limit prediction of the double-line type method is affected by inaccurate surface temperature during fatigue.Therefore,the angle normalized double-line method was proposed to predict the fatigue limit of wind turbine blades with defect.The wind turbine blades with delamination and bubber were carried out by staged fatigue tests.The fatigue limits of wind turbine blades with defect were tested by the lifting method,the double-line type method and the angle normalized double-line method.The test result was found that the fatigue limit predicted by the angle normalized double line method was consistent with the lifting method.And the error of angle normalized double line method is smaller than that of the double-line method.Primary defects will evolve into formation and propagation of the microcracks under fatigue loading.The crack tip temperature field can reflect the heat dissipation in the crack propagation process.The temperature measurement accuracy of the infrared camera is not accurate due to the test conditions.The fatigue damage of wind turbine blades is directly affects the accurate assessment by imprecise accuracy temperature.Firstly,the heat dissipation temperature calculation model of defect blade with micro crack was studied during the fatigue process.Secondly,the accuracy and feasibility of the temperature calculation model were verified using test results.Finally,the factors affect the temperature measurement accuracy of the infrared camera were analyzed on calculated numerical solution and test value of temperature field,and the adjustment methods of the infrared camera were proposed.The temperature of the wind turbine blade with micro crack defects in fatigue test was monitored by infrared thermography.The temperature field model of wind turbine blades with micro-cracks was correct by comparing the numerical solution of the ANSYS finite element and the experimental results.It proved that the temperature field mathematical model of wind turbine blade with micro-crack defect was accurate and feasible.The data processing method was very small to improve the temperature measurement accuracy of the thermography in the fatigue test.The temperature measurement accuracy of the thermal thermography was increased to 4% through the specimen emissivity,test distance,ambient temperature and ambient radiation.Fatigue is accompanied by irreversible energy dissipation.Another part of energy is converted into internal energy storage,which mainly changes the internal micro-morphology of the material.The internal energy storage changes of wind turbine blades with delamination and microcrack were analyzed based on different fatigue tests.The macro and micromorphology were used to analyze the fatigue damage mechanism of wind turbine blade with the original defects.The internal energy storage of the wind turbine blade gradually was increased by fatigue tests.When the internal energy storage was reached a certain level,the internal energy storage was increased slowly.Different fatigue damage forms were caused by internal energy storage at different moments.It was found that micro-morphology maps at different moments of wind turbine blades with microcrack and delamination.The initiation and evolution mechanisms of fatigue damage to wind turbine blades with defect were studied.The studied provides theoretical support for the fatigue damage mechanism of the wind turbine blades.The critical point of fatigue damage is an important criterion for fatigue fracture.Accurate assessment of critical points is helpful to prevent fatigue fracture of wind turbine blades.The fatigue damage model of wind turbine blades with entropy as the fatigue damage parameter was established.The critical point of fatigue damage was determined in the fatigue test.The cumulative entropy production curve of wind turbine blades containing fiber break and bubble had three stages in the fatigue test.The third stage starting point of the cumulative entropy production curve was determined as the critical point of fatigue damage by fatigue tests.The accuracy and feasibility of critical fatigue damage points were verified by other fatigue tests.The cumulative entropy production at the critical point and cumulative entropy production at the fatigue fracture point were both constant values.They were not affected by parameters of the fatigue test.The ratio of cumulative entropy production at the critical point and cumulative entropy production at the fatigue fracture point was 0.5.The fatigue life of wind turbine blades with defect at the critical point was account about 80% fatigue life.The cumulative entropy production has a constant characteristic in fatigue tests,and it would not be affected by factors such as working conditions of wind turbine blades and blade material properties.Entropy can be used as a representation of fatigue damage,and it has an important advantage for evaluating fatigue damage of wind turbine blades.