Research On Simulation And Methodology Of Weld Defect Testing Based On Ultrasonic Lamb Wave

There are problems such as near-surface blind zone and low efficiency in the testing of Large girth and straight weld by conventional ultrasonic method.Different from general ultrasonic testing,ultrasonic Lamb wave has small attenuation and long propagation distance,and it can overcome the drawbacks of conventional ultrasonic testing requiring point-by-point scanning,and does not require time and effort to clear and restore large areas of anti-corrosion paint and cold-resistant coatings,improving the testing efficiency and reducing the testing cost greatly.In this paper,LS-DYNA was used to numerically simulate the propagation of ultrasonic Lamb wave in the weld and the effect of ultrasonic Lamb waves and defects.The center frequency of the excitation signal is 0.5MHz and the thickness of the aluminum plate is 4mm.Firstly,the effect of welding profile on Lamb wave propagation process was studied.When the weld reinforcement increases,the peak energy of the acoustic field increases first and then decreases,reaching the maximum at weld reinforcement 1.2mm.When the weld reinforcement reaches 2.0mm,the echo wave packet widens in time domain.When the weld width is 15 mm,the peak energy of the sound field reach the highest,and the weld width does not affect echo mode.Secondly,the propagation process of the Lamb wave on the curved plate was simulated.It is found that the curvature change of the plate have little influence on the propagation process and echo signal.Lastly,Boolean operations were used to construct transverse groove defects and hole defects at different positions in the simulation study.The defects were tested using A0 and A1 modes with high SNR and single mode.The results showed that the echo amplitude of the A0 mode have a linear relationship with the size of the defect,which can be used to characterize the size of the defect.A1 mode echo amplitude and size does not have a positive correlation,but the echo signals are sensitive to small defects.In the Lamb wave detection experiment,artificial defects of different types and sizes were made on the specimens with thickness of 4 mm,weld reinforcement of 1.6 mm,and width of 12 mm,which was tested by Lamb wave.The simulation results of the measured signal of transverse groove artificial defects have a good consistency,thus confirming the correctness of the simulation.In order to improving the ability to identify defects,the effect of probe frequency on Lamb wave echo signals was studied,and the optimum detection frequency was determined to be 1.5 MHz.The results show that A2 mode can identify the bottom defect with 3.0mm aperture and 4.0mm hole depth effectively and the testing distance is 500 mm,and the wave height accounted for 12% of full screen.Short-time Fourier transform(STFT)and wavelet transform(WT)were used to perform time-frequency analysis on echo signals.The optimal HAMMING window size and wavelet center frequency factor of CMOR wavelet base were determined.The results show that compwered with STFT,WT has less interference frequency information and the frequency band of the echo signal is narrow.WT identify defects that are 2% higher than the full screen effectively and improve the ability to recognize target signals under clutter interference.