Research On The Early Damage Of Micro-Cracks In Wind Turbine Blade Based On The Relationship Between The Stress And Temperature Field

Blades are the most crucial component of a wind turbine to obtain wind energy.With the development of wind energy in recent years,blade size tends to be larger to meet the demand for wind energy.However,the manufacturing process is quite easy to leave air bubble defects in large blades and the further evolution of air bubble defects will cause the micro-cracks initiation and propagation under the complex loads in the upper air,which will result in the malignant accidents such as blades fracture.It has been estimated that the micro-cracks in the wind turbine blades have become a potential safety hazard in large-scale wind farms.Thus,clarifying the formation mechanism of the micro-cracks,identifying damage modes,and predicting damage evolution in the wind turbine blades can reduce unnecessary downtime loss and maintenance cost.In this paper,the initiation,propagation,and fracture of the micro-cracks in wind turbine blades were evaluated from three aspects: theory,simulation,and test to make an accurate assessment of the wind turbine before its fracture.In this paper,taking the wind turbine blade with air bubble defects as the research object,failure theory of rheological materials with defects and rheological fracture theory were introduced.On the basis of these theories,the deformation and the change rate of energy release were calculated respectively,the correlation between temperature field and stress field during the evolution of air bubble defect was derived eventually.At the same time,the fatigue tensile tests of specimens with air bubble defects were carried out,and the stress during the evolution of air bubble defects can be revealed by using the temperature variation rule of infrared thermal imager combined with the theoretical formula.The errors of test and theory are less than 5% and the 45°layer is much easier to accelerate the propagation of the micro-cracks.The micro-cracks evolved from air bubble defects will gradually expand into macroscopic cracks under the wind,and this process accounts for most of the fatigue fracture of wind turbine blades.Therefore,the finite element simulation and tensile test of the micro-cracks propagation were carried out,the temperature of the whole process was recorded by the infrared thermal imager,the relationship between the temperature and stress at different locations near the microcracks was analyzed,and the influence of defect distribution on temperature and stress field around the micro-crack was clarified.The temperature field calculated by using the simulatedstress field shows a good correlation with the experimental results,which indicates that the existence of air bubble defects leads to the initiation of micro-cracks and expands with the increase of load.Finally,the fracture morphology of the specimens were observed by optical microscope,which verified that the functional relationship between the temperature and stress during the initiation and propagation of micro-cracks proposed in this paper have a good correlation with the actual situation.According to the function,the damage identification of micro-cracks in the infrared thermal images of the initiation and propagation process was carried out,and the method of using temperature rise to identify the fatigue damage degree was put forward.At the same time,the infrared thermal images during the whole propagation process of micro-crack can also be used to determine the effect of different early damage types on the fracture,and the extreme point of the temperature appears between the unstable propagation and fracture of the specimen.