Dispersion Characteristics And Experimental Study Of Ultrasonic Guided Waves In Pipes

The pipe equipments are widely used in many departments,such as petrochemical,natural gas,electric power and urban water supply system.But the huge damage to the environment and economy are caused by the leakage of the pipe defects,corrosion and cracks.Compare to the conventional ultrasonic NDT methods for the pipe inspection,guided wave has great advantages,such as long range propagation,high efficiency,high sensitivity,low cost.Because of the dispersion and multi-mode characteristics of guided waves in pipes,the selection of the modes and frequency becomes the key problem for guided wave inspection.When the guided wave encounters the non-axisymmetric defect,the flexural mode defect echoes will be generated whether using the axisymmetric or non-axisymmetric guided wave.Together with the limited access pipes in some inspection situation,exciting the flexural mode guided wave using single transducer becomes the ideal method.So it is important to research the flexural mode guided wave excited by signal transducer.The analysis has been carried out on the dispersion characteristics of longitudinal modes, torsional modes and flexural modes in pipes.According to the dispersion curves of guided waves in the stainless steel pipe which is used in the experiment,the right modes and the exciting frequency are chosen.The exciting modes are verified through the group velocity measurement,then the defects inspection is taken in the pipe.Through the theoretical analysis and experimental study,the results include(1) The phase velocity and group velocity dispersion curves of longitudinal mode,torsional mode and flexural mode are plotted.And their dispersion characteristics are analyzed.The dispersion characteristics of the three modes are determined by the ratio of r/d,independent of the concrete value of r and d.The change of the ratio of r/d has little influence on the dispersion characteristics of torsional mode.For L(0,1),the larger the ratio of r/d,the closer the dispersion curves of L(0,1) and A₀;the larger the value of fd,the smaller the phase velocity difference of L(0,1) and A₀.L(0,2) and F(1,3) have the similar law to L(0,1).(2) The right modes of L(0,2) and F(n,3),and the right exciting frequency of 1MHz are chosen.The right incidence angle is determined to be 31°.The group velocity is 5.298km/s through the experiment measurement,1.61%error compared to the theoretical value.(3) Under this experimental condition in the present work,via hole with the diameter larger than 1.2mm (including 1.2mm) can be effectively inspected.