Design of Ultrasonic Signal and Welding Inspection Based on Combined Coded Excitation

Author:Zhang Jia Ying

Supervisor:lin shang yang gang tie


Degree Year:2018





Welding defects impact on the reliability of weld structure on service,due to process variation,corrosion,and fatigue loading during weld manufacturing and servicing.Reliability evaluation and residual life prediction for weld are based on accurate measurement of welding defects position and size.So coded excitation ultrasonic testing with high time resolution and signal-to-noise ratio(SNR)has a good application prospect in the field of weld inspection.The conflict of testing distance and time resolution in conventional ultrasonic testing is solved by coded excitation.And coded excitation improves the measurement accuracy of welding defects.However,suitability and reliability of coded excitation are limited by chirp bandwidth of excitation signal,peak sidelobe level of testing signal,and response of transducers.Further researches are needed that are on influence law of excitation signal parameters to testing results,decreasing frequency and bandwidth requirement of transducers,and application in high attenuation and strong noise detection environment.In order to solve the above issues,the signal combined frequency,phase and amplitude is designed using digital coding method.The proposed signal is employed as ultrasonic testing excitation signal,which is binary Barker code with linear frequency modulation based on Blackman windowed amplitude weighting.Simulation program based on the k-space pseudospectral method is developed to verify feasibility of the proposed combined coded excitation in ultrasonic testing.The interaction between transducer and coded signal is simulated,and attenuation of ultrasound propagation is simulated.Weld ultrasonic testing imaging system is designed and developed.The system excites coded signal with required parameters and does pulse compression with received signal.The characteristic parameters are given to evaluate ultrasonic testing signal,including main lobe width(MLW),peak sidelobe level(PSL)and SNR.With simulation analysis and experimental research,it is revealed that is influence law of excitation signal parameters to testing results in combined coded excitation ultrasonic testing.When the excitation has smaller pulse width and higher central frequency,testing result has higher time resolution.And parameters of excitation signal has less impact on PSL and SNR of testing signal.Chirp bandwidth of excitation signal has less impact on characteristic parameters of testing signal.It is found that combined coded excitation can improve time resolution and SNR of ultrasonic testing compared with phase coded or frequency coded excitation.So ultrasonic testing with combined coded excitation acquires higher sensitivity and more accurate measurement of defects position and size.When the central frequency of transducer is lower,pulse-echo method with combined coded excitation can identify the flaws which have 0.8mm relative depth.And in defects size measurement with TOFD ultrasonic testing,combined coded excitation method can measure flaw of 2mm height.The error of combined coded excitation decreases 35%,30% and 20% separately compared with conventional ultrasonic excitation,frequency coded and phase coded excitation testing.It is shows that testing signal characteristic parameters of coded excitation are all improved by combined coded excitation in the detection environment of lower frequency and smaller bandwidth.And the accuracy measurement of defects is also improved.Combined coded excitation has stronger applicability in weld ultrasonic testing,because its time resolution relies less on transducers’ central frequency and bandwidth.In order to increase reliability of combined coded excitation ultrasonic testing,it is needed to suppress sidelobe level and improve SNR.The methods “Amplitude weighting,delay and accumulation” and “ Complementary sequence,two times excitation” are proposed,which can decrease flaw probability of missing or misjudgement.The simulation and experiment results show that the two sidelobe suppression method make the PSL under-57 dB and SNR above 58 dB.And “Complementary sequence,two times excitation” method also improves time resolution of testing results.When testing austenitic stainless steel weld with high attenuation and strong noise,combined coded excitation with sidelobe suppression methods increases 19 dB of SNR,38% of time resolution and decrease about 50% of measurement error compared with conventional ultrasonic testing.