| At present,reflective electromagnetic ultrasonic high-frequency guided wave detection technology is adopted for pipes with a wall thickness of 6 mm or more.This paper designs the electromagnetic ultrasonic testing system for pipelines with a wall thickness of 6 mm or less.And through the transmissive circumferential Lamb wave to solve the problem of low pipeline detection accuracy of pipe wall thickness of 6mm and below.Aiming at the problem that the reflection circumferential Lamb wave detection accuracy is not high,this paper designs a transmissive circumferential Lamb wave transducer.The dispersion equation of the circumferential Lamb wave is derived based on the working principle of the circumferential Lamb wave and the elastic dynamic equation,the solution of the dispersion equation is obtained by numerical solution and the dispersion curve is drawn.Finally,the operating frequency of the transducer is obtained by the dispersion curve.And the feasibility of circumferential Lamb waves in the pipeline is verified by comparison with Lamb waves in the plate.Secondly,the software and hardware structure of electromagnetic ultrasonic online flaw detection system is designed.The hardware circuit is mainly composed of three parts: transmitting and receiving circuit of electromagnetic ultrasonic transducer,control circuit and user interaction circuit.The transmitting and receiving circuit is used to generate a high frequency alternating current of the transducer and receive the echo signal,the control circuit mainly implements a control signal for generating a transmitting and receiving circuit,and parameter settings for the underlying hardware.User interaction circuit is used to implement user interaction function.The software part is designed with circumferential Lamb wave special flaw detection analysis software,realized the setting of parameters,display of echo signals,etc.Software optimization is implemented using a single-case design pattern and a multi-threaded design approach.Then,a wavelet denoising algorithm is designed to improve the signal-tonoise ratio of the detection signal of the online flaw detection system.The system uses threshold denoising,and analyze the soft and hard thresholds and the number of decomposition layers of the wavelet algorithm.Finally,the sym20 wavelet function for hard threshold processing with 6-layer decomposition is selected.And compared with the cumulative average algorithm,the validity and superiority of the wavelet algorithm are verified.Finally,the entire online flaw detection system was tested and verified.The experiment with axial cracks was carried out,and the axial crack defects of 1×10×0.5mm were successfully detected by the optimization of wavelet algorithm.The test results meet the non-destructive testing standard of the petrochemical pipeline of the People's Republic of China SH/T 3545-2011. |
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