Research On The Characterization Of Defects In Pipes Based On Longitudinal Modes Waveguide

Pipes are important carriers of the flowing medium, which has been widely used in modern industry. Non-destructive testing based on ultrasonic guided wave is an important technical means to ensure the safe operation of pipes. Defect characterization is an important topic for guided wave-based pipeline inspection. Since abundant defect related information is contained in the echo of guided waves, a comprehensive analysis of defects reflection helps to identify the geometric characterization of pipe defects. The interaction of longitudinal mode waveguide with defect results in a double mode conversion:the L(0,2) mode and L(0,1) mode. The L(0,1) mode converted from L(0,2) mode after the interaction with defects provides significant information about the defect. This thesis investigates the influence of double mode conversion on defects feature recognition using finite element analysis and experiment al validations.Governing equation of dispersion characteristics in hollow cylinders is deduced to, analyze multi-modal and dispersion characteristics of guided waves. The characteristics of wave structures are analyzed for three modes of guided waves such as longitudinal, flexural, and torsion waveguide. Methods of ultrasonic guided wave excitation and reception are examined in numerical simulation and experiments, including how to select detection frequency and excitation signals, how to place piezoelectric wafer and select monitoring points.Numerical dispersion about guided wave propagation in a hollow pipe is studied by using Finite Element Method. The convergence of simulation process is analyzed by setting the axial length of the grid and the time step parameter. Meanwhile the influence of different monitoring points on the received signals investigated by simulation. The numerical results show that when the distance between the monitoring point and the pipe end is equal to a quarter of guide wavelength at incentive center frequency, the amplitude of the received echo signal is the largest.Reflection characteristics of L(0,1) and L(0,2) guided wave are studied when the defects is extended along the pipe radial, circumferential and axial. The results suggest that the double mode conversion only occurs when the L(0,2) mode interacts with non-through-thickness defects; For the certain extent of circumferential and radial defect, the reflection coefficient of L(0,2) and L(0,1) presents a sine and cosine wave with defect axial extension; For the certain extent of circumferential and axial defect, the reflection coefficient of L(0,1) mode increases at first and then start to decrease when the depth of defects is increased; The reflection of the L(0,1) is as strong as the L(0,2) mode when the defect depth is less than half of the wall thickness. Results of this research provide a reference for the assessment of defect feature in pipeline inspection.