Small thin-wall tubes are used in aircraft and nuclear power structures in large amounts. In order to prevent catastrophic accidents caused by break of the tubes, this kind of tubes are required by standards being tested by high sensitive and high reliable nondestructive methods, especially by UT. However, the commonly used UT method, i.e. focused beam immersion testing, can not be used for in- situ inspection. In addition the speed of the method is slow. It is hopeful to overcome the shortcomings by using circumferential guided waves in stead of immersed focused beam. However circumferential guided waves in small thin-wall tubes are not investigated at all both theoretically and practically. Therefore this study is aimed at the properties of circumferential guided waves in small thin-wall tubes and their application to UT of the tubes.
Theoretical analyses on circumferential guided waves in small thin-wall tubes are made as the 1st major part of this study. The wave equations for the waves are solved under the boundary conditions that the inner and outer surfaces both are free. The phase velocities dispersion equations are obtained as results. The dispersion equations are solved numerically and then the phase and group velocities dispersion curves are plotted. The common propagation characteristics and properties of circumferential guided waves in tubes of different size and wall thickness are revealed. The variations of velocities dispersion curves with the size and the wall thickness of the tubes are investigated. The displacements of the circumferential guided waves of the same mode at different frequencies and those of different modes at the same frequency are calculated as function of the radial position r of the points where the waves propagate. Detailed comparisons between circumferential guided waves of 1st mode in small thin-wall tubes and Rayleigh wave at the same frequencies are made and the conclusion is that they are very similar to each other in many aspects, for example propagation velocities as well as the amplitude and phase variations of the vibration displacements with the radial position r.
Experimental studies on the waves are conducted as the second major part of this study. Angle beam probes of 50 degree with frequency of 10 MHz and 5 MHz are used to generate the waves. Circumferential guided waves of 1st mode are generated in the tubes. Echoes from the 0.07mm deep ID and OD notches are observed with higher signal/noise ratio. The comparisons between the measured and calculated group velocities and the comparisons between the calculated and experimentally observed dispersion characteristics and variations of displacement amplitude with the radial position of the points are made, Based on the comparisons, it is determined that the waves generated are the circumferential guided waves of 1st mode. The experiments verify that the amplitudes of the echoes increase with the increase of the notches depth and the wave frequencies and decrease with the increase of the distance from the notches to the outer surface of the tubes. During the experiments, a series of special echoes are observed. It is revealed that the echoes are generated by the reflection at the couplant surface. The echoes are valuable for the reliability of the UT on the tubes. NDT practice and metallographic examination on the sections of the tubes where ultrasonic flaw signals happen verifies that the circumferential guided wave method is effective, sensitive and reliable. The sensitivity of UT on the tubes by the method is up to the national standard required sensitivity for UT of tubes used in aircraft and nuclear reactors. In addition, this method is flexible and suitable for in-suit inspection.It can be used for in-suit inspection as a complement to the commonly used immersion focused beam method.
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