Study On Propagation Characteristics Of Lamb Wave And Guided Wave In Pipes

Comparing to conditional nondestructive testing technology,ultrasonic guided wave technology(UGWT) is a new nondestructive testing technology with characteristics of fast, long-distance,large-scale and low-cost.UGWT has wide application prospect because it is easy,high efficient,accurate and reliable.Guided wave can test materials omnidirectionally,but further investigation on excitation and propagation of the wave are needed because of sound field and multi-mode of guided wave.The propagation property of Lamb waves in aluminum plate and guided wave in the stainless steel pipe is investigated to provide experimental basis to practical testing in this paper.The frequency dispersion equations of Lamb wave in aluminum plate and guided wave in the stainless pipe were calculated by the Matlab software,then the dispersion curves and wave structure curves of Lamb wave in aluminum plate and guided wave in the stainless pipe were plotted.Further,suitable modes of guided wave were selected to test defects in the aluminum plate and stainless steel pipe.Guided wave modes were excited by variable angle probe and identified by group velocity curves.The experimental results show that sound field in aluminum plate is not uniform.On the acoustic beam direction,sound pressure is higher and acoustic energy is centralized,but it becomes weaker when deviating from the acoustic beam direction.S₀ mode of Lamb wave is very sensitive to crossholes in the aluminum plate.The wave packet divides into several wave packets while there is crosshole in the aluminum plate,and the amplitude of reflection wave from boundary decreases significantly.The amplitude of defect wave increase with increasing diameter of crosshole.Also S₀ mode of Lamb wave is sensitive to cracks in the aluminum plate.The amplitude of defect wave does not increase at crack length 9mm and 17mm. Amplitude distribution of F(n,3) mode wave on axial direction is oscillatory.One probe system was used to test different cross-holes in the stainless steel pipe.Cross-holes(φ＞1mm) can be detected by F(n,3) mode wave under the condition of frequency-thickness product(fd) 0.88MHz·mm and detecting frequency 1MHz.