| With the large-scale construction of civil engineering and infrastructure,many major projects and complex equipment have appeared in our country's industrial and civil fields.Plate structure is a common structure in these projects and equipment,and steel plate is the largest output product in our country's steel industry.During the production and service of the steel plate,cracks or holes will be generated on the surface and inside of the steel plate due to initial defects,environmental erosion and external forces.The existence of these defects will affect the mechanical strength and stability of the structure,so its inspection is very important.Electromagnetic ultrasonic testing is a relatively new non-destructive testing technology,which uses electromagnetic coupling to excite ultrasonic waves in the structure.Compared with traditional piezoelectric ultrasonic detection technology,electromagnetic ultrasonic has the characteristics of non-contact and rich excitation waveform types.However,the electromagnetic ultrasonic transducer's low energy conversion efficiency and complex energy conversion mechanism have affected the application of electromagnetic ultrasonic detection technology.In this paper,electromagnetic ultrasonic technology is used to detect defects in plate-like structures.Aiming at the problem of low electromagnetic ultrasonic detection efficiency,the study and verification are carried out from three aspects: electromagnetic ultrasonic transducer,defect detection method and signal processing method.Finally,the orthogonal test method is used to design an electromagnetic ultrasonic transducer that meets the performance requirements,and a damage location method based on equivalent time and gridding iterative algorithm is proposed.The specific content and research results are as follows:First,through the COMSOL finite element software to simulate and analyze the transduction mechanism of the electromagnetic ultrasonic transducer,and explore the influence of the transducer parameterson the amplitude of the shear wave(excitation current size,number of cycles and lift-off distance).Through the built electromagnetic ultrasonic testing system,the parameters of the transverse wave probe are optimized based on the steel plate thickness test to achieve the maximum energy conversion efficiency in the work.Secondly,perform finite element simulation and experimental research on 7 kinds of steel plates with different defect depths.The electromagnetic ultrasonic transducer is used to realize non-contact and non-destructive testing,and the influence of the depth of the defect on the amplitude and energy of the echo signal is studied.The signal amplitude and energy change are used to characterize the influence of the defect depth on the echo signal.Based on the detection signal of the defect-free steel plate,the damage index AI value and EI value based on the ultrasonic attenuation theory are constructed.Research shows that these two indicators increase with the increase of defect depth,and the shear wave probe can well detect the depth of steel plate defects.Finally,aiming at improving the energy conversion efficiency of the electromagnetic ultrasonic Lamb wave transducer and suppressing multi-mode clutter,the electromagnetic ultrasonic Lamb wave transducer suitable for ferromagnetic thin steel plates is designed.The orthogonal experimental design method is used to optimize the parameters of the electromagnetic ultrasonic transducer probe(permanent magnet height,width,wire width,lift-off distance and current size),and the influence law of related parameters on the intensity of the detected signal is obtained.The "false positives" that appear during damage positioning and the phenomenon of modal conversion and boundary reflection when Lamb waves encounter defects cause aliasing of received signals,an iterative algorithm for locating damage based on equivalent time and gridding is proposed.This method can achieve precise damage location. |
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