Research On Defect Detection Mechanism Of Plate Weld Using Coded Ultrasonic Wave

Conventional plate weld defects are detected generally by pulse ultrasonic wave.At equivalent excitation voltage,this method has the shortage of low average transmitted sound power,low echo signal-to-noise ratio,and small detection range,which is not conducive to the effective detection of plate weld defects.In order to enhance the signal-to-noise ratio and distance resolution of echo signal of the ultrasonic detection system,some scholars have introduced coded ultrasonic excitation technology and pulse compression technology to improve this situation.However,the current coded ultrasonic detection methods still have problems of long main lobe duration,high side lobe level and narrow frequency band of echo pulse compression signals.Therefore,a composite coded ultrasonic excitation method based on NLFM Barker(Non-linear Frequency Modulation Barker)is proposed in this thesis,which is used for defects detection of plate weld.On the basis of comparison and analysis of the generation principle,time-frequency characteristics,and sensor response characteristics of conventional coded excitation signal,a NLFM(Non-linear Frequency Modulation)signal with explicit analytical expression is designed using nonlinear fitting method.The signal is used as a carrier to modulate the Barker code to obtain the NLFM Barker composite coded signal.The function expression of the composite coded signal is derived.Based on the Gaussian function model of the ultrasonic transducer,the sensor response model of the NLFM Barker composite coded signal is derived.And the response characteristics and influence factors of the composite coded signal are studied.The time-frequency performance,the sensor response characteristics and the pulse compression performance are researched and simulated to compare with other coded excitation signals,such as Barker,NLFM,and LFM Barker(Linear Frequency Modulation Barker).The coded ultrasonic defect detection system is built,and the ultrasonic waves generated by these four excitation signals are used to detect the five defects of plate weld.The detection performance of different coded ultrasonic waves in the weld defects are compared and analyzed.The main research contents and conclusions of the thesis are as follows:(1)Based on the analysis of the common types and characteristics of defects of plate weld,the research progress of current ultrasonic detection methods of plate weld defect and coded excitation signals are summarized.The overall design scheme of coded ultrasonic defect detection system is given.(2)By comparing and analyzing the generation principles,time-frequency characteristics,and sensor response characteristics of common coded excitation signals,a design method of NLFM based on nonlinear fitting is proposed.This method uses the amplitude and frequency characteristics of ultrasonic sensor as basic function,combines cubic spline interpolation with a nonlinear fitting algorithm,and derives an explicit analytical expression of NLFM signal.(3)In order to further enhance signal-to-noise ratio and detection distance resolution,and expand the frequency band distribution,NLFM Barker composite coded signal is designed.The composite coded signal not only has the characteristic of high signal-to-noise ratio of Barker,but also has the characteristics of rich frequency band and low side lobe level of NLFM.(4)Based on the Gaussian function model of the impulse response of the ultrasonic transducer,the sensor response model of the NLFM Barker composite coded signals is derived.The response characteristics and influencing factors of the composite coded signal are studied to provide theoretical basis for parameters selection of the composite coded signal.(5)A coded ultrasonic defect detection system is built.Barker,NLFM,LFM Barker and NLFM Barker four coded signals is used to detect five defects in the actual plate weld,and the corresponding signal-to-noise ratio and distance resolution are counted.The experimental results show that the power ratio of main-lobe to side-lobe is increased about 11 dB and 5dB compared with NLFM and LFM Barker respectively,and the level of peak side-lobe is reduced about 7.8dB,7dB,and 3.6dB compared to Barker code,NLFM,and LFM Barker code,respectively.The composite coded ultrasonic signal can improve the distance resolution and effectively detect the common five types of defects in plate weld sample.