Experimental Study On Modal Analysis And Defect Detection Of Circumferential Lamb Waves In Thick Wall Pipes

Thick-walled pipes occupies a very important position in the nuclear power plant,which are likened to the “blood vessels”.The health of thick-walled pipes plays a vital role in national energy strategy development and social stability.Common non-destructive testing techniques are difficult to quickly scan and detect defects in thick-walled pipes.The circumferential Lamb wave propagates along the circumferential direction of the pipe.Rapid scanning in the axial direction,the circumferential Lamb wave can detect the full range of thick-walled pipes.Based on the theory of circumferential Lamb wave,this paper has carried out research work on the circumferential Lamb wave detection technology of thick-walled pipes from the aspects of theoretical dispersion characteristics,propagation characteristics and modal component identification and defect detection.Firstly,based on the basic theory of circumferential Lamb wave,the circumferential Lamb wave dispersion curve of thin-walled pipe and thick-walled pipe is compared.The dispersion curves,the relationship between the frequency and the incident angle and the modal displacement distribution of the circumferential Lamb waves in the thick pipe with different size parameters are calculated.The dispersion characteristics of the circumferential Lamb waves are studied theoretically.It is found that the dispersion curve of the circumferential Lamb wave is closely related to the wall thickness and is not sensitive to the change of the outer diameter;Combined with literature data and analysis results,the excitation frequency of 0.5MHz,the incident angle of 30 ~ 60 ° was finally selected to carry out detection method research;The change of pipe wall thickness will affect the vibration displacement distribution characteristics of each mode.The displacement distribution of CL3 ~ CL6 modal is more uniform,which is helpful to find the defects in the whole wall thickness range.Secondly,in view of the complexity of propagation of circumferential Lamb waves in thick-walled pipes,the propagation characteristics of the circumferential Lamb wave are studied in detail from the influence of the detection parameters on the propagation signal and the propagation characteristics of the circumferential Lamb wave in the different thick-walled pipes.Considering the magnitude and the time domain resolution of the signal,the number of cycles of the excitation signal for subsequent experiments is taken as 10.The incident angle has a great influence on the propagation signal of the circumferential Lamb wave.As the incident angle decreases,the wave width of the signal increases,and dispersion becomes serious with the number of modes corresponding to the 0.5MHz frequency component increases.The propagation characteristics of the circumferential Lamb wave are studied by analyzing the two waveform parameters of wave packet amplitude and wave packet width for three thick-walled pipes with different wall thicknesses.The amplitude of the circumferential Lamb wave propagation in the thick-walled pipes fluctuates with the increase of the propagation distance,and the width of the wave packet increases almost linearly.Then,based on the continuous wavelet transform,the propagation mode of the circumferential Lamb wave propagating in the 25 mm pipe with different angle of incidence is fitted and identified,and the attenuation of each mode is analyzed.The main propagation modes of different angles of incidence in thick-walled pipes are different,and the amplitude of the main modal wave is fluctuating with increasing propagation distance.Finally,a series of defects are designed on the three thick-walled pipe test blocks.Research on key issues of the detection capability and the detection sensitivity of circumferential Lamb waves in thick-walled pipes,defect echo modal analysis and defect echo propagation mechanism has been carried out.The experimental results show that the detection of the inner and outer wall defects of the thick-walled pipe can be realized by proper selection of the incident angle.The detection sensitivity of different angles of incidence is different.For the inner wall defect,only 6mm defect can be detected at the angle of incidence of 37°.For the outer wall defect,the incident angle of 55° is the most sensitive to the outer wall defect.In the pipe with the wall thickness of 25 mm,the outer wall defect can be found at least 1mm.The wall thickness is smaller and the outer wall defect detection sensitivity of the pipe is higher.In the propagation distance away,the outer wall defect echo will occur in the dispersion and modal conversion.And defect size will affect the distribution of modal components.The propagation mechanism of the defect echo at 25 mm pipe with the angle of incidence of 37° is analyzed by using geometric acoustics principle.It is found that there are shear wave components in the defect echo,and the shear wave occupies the main energy,which causes the amplitude of the defect echo to fluctuate regularly with the increase of propagation distance.