Research On Defects Detection In Weld Zone Of Pipe Based On Ultrasonic Guided Waves

Periodic inspection is very important for the safety of pipeline. Ultrasonic guided waves have many advantages comparing with traditional technologies, such as long detecting distance and high efficiency. In recent years, there are lots of researches on pipe defects detection based on ultrasonic guided waves, but less on defects in pipe weld zone, in or immediately adjacent to the weld, because of the complexity of ultrasonic guided waves and welding joint. However, defects in weld zone, considerably affect the quality of pipeline. So it has great practical significance to study the detection method of defects in pipeline weld zone based on ultrasonic guided waves.In this research, crack and hole defects in or immediately adjacent to the weld, crack in weld metal of pipe with double welds and filled water or buried were detected using ultrasonic guided waves by numerical simulation and experimental method. Effects of different defects on weld echoes were studied. This research provides bases for detecting defects in weld zone of pipeline. Main researches and results are given as follows.1) Dispersion curves of ultrasonic guided waves in vacuum, water filled or buried pipe were analyzed on the basis of formation mechanism and frequency dispersion theories. How to select the excitation signal and the excitation frequency of guided waves were confirmed. The structure and defects of pipe weld zone were studied. The algorithm of separating symmetric and asymmetrical modes was proposed to detect defects in pipe weld zone.2) Based on the ANSYS finite element analysis theory, the analysis models of pipe with defects in weld zone was established, combined with the complexity of weld structure and ultrasonic guided waves. The modes transition in the defect is studied through analyzing the axial and circumferential displacement nephogram. The asymmetrical modes in weld echoes were studied using the separation algorithm of symmetrical and asymmetrical modes. The variety of asymmetrical modes with different defect degree of section was analyzed.3) The excitation and receiver experiment system of guided waves were established, and the scheme was designed. The single weld with crack or hole defect and two welds with crack were studied respectively. The asymmetric modes in weld echoes were studied using modal separation algorithm. The study found that asymmetric modes in weld echo with defects increasing significantly compared with weld without defects. It can be almost determined that there are defects in weld zone when the amplitude ratio of asymmetrical mode and symmetric mode is greater than 0.2. And the amplitude ratio increases with the increasing of degree of section defect. Taking into account the actual situation, experiments of detecting crack in weld metal of filled water or buried pipe using guided waves were conducted.In this research, profound researches on detecting defects in weld zone using ultrasonic guided waves were conducted by numerical simulation and experimental method, proving it is feasible of applying guided waves to detect defects in weld zone of pipe, and providing theory and engineering application bases for judging the defects of pipeline weld zone.