Crack Detection in Pipes with Different Bend Angles Based on Ultrasonic Guided Wave

Pipeline plays an indispensable role in process industries,because the progressing crack-like defects of in it may result in serious accidents and significant economic losses.Therefore,it is essential to detect the cracks occurred in pipelines.The axial crack-like defects in elbows with different angle are inspected by using the T(0,1)mode guided waves,in which different configurations including 45°,90°,135°and 180°(straight pipe)are considered respectively.Firstly,the detection sensitivity for different defect location is experimentally investigated.After that,finite element simulation is used to explore the propagation behaviors of T(0,1)mode in different bend structures.Simulation and experiment results show that the crack in different areas of the elbow can affect the detection sensitivity.It can be found that the detection sensitivity of crack in the middle area of the elbow is higher compared to the extrados and intrados of the elbow.Finally,the mode conversion is also investigated when the T(0,1)crosses the bend,and the results show that bend is a key factor to the mode conversion phenomenon which presents between the T(0,1)mode and F(1,2)mode.

DETECTION OF LONGITUDINAL DEFECT IN PIPES USING TORSIONAL MODES

The multi-modes and disperse characteristics of torsional modes in pipes are investigated theoretically and experimentally. At all frequencies, both phase velocity and group velocity of the lowest torsional mode T（0,1） are constant and equal to shear wave velocity. T（0,1） mode at all frequencies is the fastest torsional mode. In the experiments, T（0,1） mode is excited and received in pipes using 9 thickness shear vibration mode piezoelectric ceramic elements. Furthermore, an artificial longitudinal defect of a 4 m long pipe is detected using T（0,1） mode at 50 kHz. Experimental results show that it is feasible for longitudinal defect detection in pipes using T（0,1） mode of ultrasonic guided waves.

The effect of pipe bend on low-frequency longitudinal mode guided wave propagation

The pipe bend significantly changes the propagation characteristics of guided wave,and makes the interpretation of the received signals difficult.Therefore,better understanding of guided wave propagating in bended pipe is essential for the inspection of pipeline comprising bends.First of all,the different features of dispersion curves derived with the semi-analytical finite element method for guided wave in bended pipes are summarized.Secondly,based on the dispersion curves for guided wave in bended pipes,experiments are performed to investigate the mode conversions of L（0,1） mode guided wave traveling through pipe bends.It is found that,except for the mode conversion from L（0,1） to F（1,1）,the L（0,1） reflections of bends are also observed in some cases,which are proven to be the mode converted negative L（0,1）mode guided wave,and the negative L（0,1） mode guided wave becomes more obvious with the decrease of excitation frequency and bending radius.The findings of this paper will provide some insight for guided wave behavior in bended pipe,and generalize the application of guided wave inspection in practical pipelines.