| The variable thick-walled pipe is used for nuclear power,petrochemical,military and other special equipment applications more and more widely.Traditionally,the method of reducing the damage of the variable pipe,one is the chemical composition;the second is through the simulation experiment,the variable pipe in some environments prone to cracks,corrosion and other defects were analyzed;third is the use of numerical or experimental Method,direct analysis of the deformation tube stress,strength and other material properties.Although the above method can analyze the strength of the reducer and the performance of the material is qualified to avoid damage caused by the reduction of the relevant accident,but the quality of the variable pipe can not be quickly detected.It is of great significance to study the effective non-destructive testing technology of the variable pipe and realize the rapid detection of the defect of the variable pipe.Compared with the axial guided wave,ultrasonic circumferential guided wave is more suitable for the detection of thick-walled pipeline,along the pipeline axial movement of ultrasonic guided wave transducer,can achieve a full-scale thick-walled pipe scan.In this paper,the detection method of variable thick-walled pipe is studied,and the basic experimental data and feasibility method are provided for the development of the technical standard of the modified thick-walled pipe.In this paper,firstly,the dispersion characteristics of the circumferential Lamb waves in the variable thick-walled pipe are analyzed,and the derivation process of the dispersion equation of the circumferential Lamb waves is given.The equivalent substitution method is used to reduce the equivalent thickness The thickness of the wall is not changed,and the difference between the diameter and the diameter of the pipe is obtained by solving the dispersion equation and the wave structure.Compared with the dispersion curve in the frequency range of 0 ~ 1MHz,we can see that with the decrease of the outer diameter of The variable thick-walled pipe,the group velocity of each mode decreases in the optimal dispersion characteristic,The higher the frequency of the whole variable diameter,the larger the excitation frequency is,and the larger the excitation frequency is.In the case where the excitation center frequency is constant,The higher the outer diameter is,the higher the order of the wave guide mode that can detect the whole wall thickness.According to the relationship between the frequency and the angle of entry,select the appropriate excitation angle,it is expected to stimulate the less modal of the circumferential lamb wave.Secondly,the numerical simulation of the propagation characteristics of the Lamb waves in the circumferential wall and the reflection on the defects are carried out.The finite difference method is used to simulate the effects of the circumferential thickness on the wall thickness of 30 mm,the large diameter of 250 mm,the small diameter of 100 mm,Propagation Characteristics and Scattering Characteristics of Reducing Thick Pipes with a Length of 600 mm.The distribution of the sound field in the thick-waled pipe with 30°?45°and60° oblique incidence is studied,and the defects of different sizes in the outer wall,the middle wall and the inner wall are detected.Inaddition,the propagation characteristics of the circumferential Lamb wave in different inclination and thickening wall are further studied.The results show that the different angle of incidence will affect the coverage of the sound field in the variable thick-walled pipe with different angle of incidence.The angle is too large to make the sound field of the inner wall weak,and the dispersion angle is too serious when the angle is too small.The size of the defect is too small,the effect is very weak,when the radius of the hole hole hole is greater than 3mm,the effect changes significantly;by comparing the circumferential Lamb waves in the equal diameter thick pipe,the variable thick-walled pipe,the 45° variable thick-walled pipe,60° variable thick-walled pipe on the propagation characteristics we can see that the larger the thickness of the thick-walled pipe tilt angle,the more difficult to spread in the circle.thirdly,based on the results of numerical simulation,a piezoelectric glass with a center frequency of 500 KHz was designed,and a plexiglass wedge with angle of 37°,45° and 55° was designed.An angle beam probe with a center frequency of 500 kHz was fabricated To stimulate the weekly lamb waves.It is determined that the excitation wave distance of the circumferential Lamb waves forming the stable amplitude is 300 mm when the circumferential Lamb wave propagates in the variable thick-walled pipe.On the basis,the continuous wavelet Time-frequency analysis is used to obtain the relationship between the velocity and frequency of the circumferential lamb waves at the three angles oblique incidence.Finally,the deformation wall of the variable thick-walled pipe was studied.The outer wall,the middle wall and the inner wall of the thick-walled pipe with wall thickness of 30 mm,large diameter of 250 mm,small diameter of 100 mm and axial length of 600 mm were completed Position processing a series of defects.The effects of different angles on the detection of different positions,different sizes and different types of defects were studied.and the diameter of the thick-walled pipe is different from that of the circumferential Lamb wave.The experimental results show that the detection sensitivity of the thick-walled pipe is about 1mm long,1mm wide and 1mm high.When the deformation is thick and the outer wall is defective,the circumferential lamb probe with a wedge angle of 55°.The detection of the guide wave detection effect is the best,when the detection of inner wall defects,the choice of wedge angle of 45° probe is more suitable;The circumferential Lamb wave in the variable thickness of the thick pipe in the spread of a strong directional,The reflected wave energy in the thick-walled pipe is helpful to locate the defect. |
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