The System And Method Of Ultrasonic Testing For Weld Based On Linear Frequency Modulation Technique

With the development of industry, coarse-grained materials and welded structureswith large thickness are widely used in the field of pressure vessel, petrochemical and soon. Nondestructive testing need be used to ensure the reliability and the security ofwelded structures. In all of the methods of nondestructive testing, ultrasonic testingplays an essential role. But there is a conflict between the long distance propagation andthe time resolution in the traditional ultrasonic testing when testing materials with greatattenuation.This paper provides a new real-time ultrasonic system with the LFM (LinearFrequency Modulation) excitation signal to solve the conflict. The developed system issmall in size, so it is convenient to be used at the project site. Meanwhile, since thesystem is open, it can be easily updated according to the specific needs of customers.The ultrasonic chirp excitation signal generation part of the system is based ondirect-digital-synthesizer technique whose excitation signal can be changed in real time.The bandwidth of excitation signal is in the range of1MHz and20MHz and the pulsewidth is in the range of1μs and1ms. And the ultrasound echo acquisition andprocessing part of the system uses dual-channel to collect data simultaneously and usespulse compression technology to adapt the changes of excitation signals. The samplingrate of the part is50MSPS.In the experimental part, TOFD testing with LFM pulse excitation is used in thispaper. The parameters of LFM pulse which can affect on the compression result isdetermined. Moreover, the developed system was successfully applied to testing defectsin specimens, compared with the results of conventional pulse reflection ultrasonicmethod. The system is a more powerful tool to increase signal-to-noise ratio and thedetection accuracy. The average error of defect depth measurement is less than0.3mmin specimen. The average error of defect depth measurement is less than0.5mm in weld.The relative error is less than5%.