| Wind turbine blades are one of the most critical components of wind turbine system.Blades account for about 15%-20%of the total turbine cost,and its quality is related to the energy conversion efficiency of wind power equipment.Spar caps and shear webs are respectively the main load-carrying structure and support structure when blades are running,and the two are bonded by epoxy resin structure adhesive to form the bonded joints.This is done in order to provide required bending stiffness to the blade.Internal quality of spar caps and its bonding strength with shear webs are a key factor affecting the service life of blades.Consequently,the inspection of bonding quality of spar caps and shear webs is a necessary step to ensure the service safety of wind turbine blades.Ultrasonic testing is the main method for non-destructive testing of the adhesively bonded joints.However,traditional ultrasonic detection results are easily affected by the coupling,instrument noise and other factors,resulting in that it is not easy for detection datas and acoustic characteristic parameters to be analyzed and extracted.The reliability of the results depend on the experience of inspectors.Modern ultrasound testing is moving toward quantification,imagination and automation.Propagation characteristics of ultrasonic waves in the adhesively bonded joints of blades were studied,and influence of defect size on the amplitude and frequency acoustic parameter was quantitatively analyzed in this paper.To meet the development need of modern ultrasonic testing,detection results were displayed in the form of images.The main research works and conclusions are as fllow.(1)The propagation characteristic of ultrasonic waves in spar caps of wind turbine blades was researched.The finite element software of ANSYS was used to establish the no defect spar caps model,crack and delamination damage spar caps models of different sizes,and the received echo detection signals were analyzed in energy and waveform.Research results indicate that the first-pulse amplitude curve for different location cracks satisfies the cubic polynomial well.The first-pulse amplitude scatter plot for rectangular delaminations with different aspect ratios exhibites linearly increasing segment,slowly increasing segment and stationary segment as the delaminations width increase.(2)The propagation characteristic of ultrasonic waves in adhesively bonded joints of spar caps and shear webs was researched.The finite element software of ANSYS was used to establish bonded joints models with good bonding strength and different debonding lengths for 10%,30%,50%and 70%interface debonding rates,and the received echo detection signals were analyzed in energy and waveform.In addition,the echo signals for different interface debonding rates were analyzed in frequency domain by STFT and constrained weighted least square algorithm.Research results indicate that the amplitude-ratio curve of debonding echoes exhibits linearly increasing segment,slowly increasing segment and stationary segment with the increase of debonding length.STFT-constrained weighted least square algorithm can well identify the debonding rate for different the adhesive interface.(3)The ultrasonic C-scan imaging system of wind turbine blades was improved through the above propagation characteristic analysis.The system was used to respectively perform C-scan experiments on the defective sample of adhesively bonded joints of spar caps and shear webs and the actual blades.Research results indicate that the relative errors of the defective contour boundary position along the X direction for the total lack of adhesive,partial lack of adhesive and porosity defects in adhesive are less than 5%,and the relative errors of the defective contour boundary position along the Y direction for them are less than 15%.Spar caps area,bonding area of shear webs and bonding width are visually displayed in the C-scan imaging. |
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