Guided Wave Propagation in Complex Curved Waveguides: Flexural Guided Waves and their Application for Defect Classification in Pipes
Open Access
- Author:
- Khajeh, Ehsan
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 04, 2013
- Committee Members:
- Joseph Lawrence Rose, Dissertation Advisor/Co-Advisor
Reginald Felix Hamilton, Committee Chair/Co-Chair
Clifford Jesse Lissenden Iii, Committee Member
Arthur Thompson Motta, Committee Member - Keywords:
- Guided Waves
Flexural
Ultrasonic
Inspection
Pipes
Defect
Dispersion Curves - Abstract:
- A new method, called ray-plate theory, is developed to investigate guided wave propagation in complex curved waveguides. This theory is applied to the study of guided waves in hollow circular cylinders. For the first time analytical relations are derived for longitudinal and torsional dispersion curves in hollow circular cylinders. Dispersion curves, derived using the ray-plate theory, are compared with the results of the Global Matrix Helmholtz decomposition method. It is shown that the predictions of the ray-plate theory are accurate.\\ Flexural modes are investigated in the context of the ray-plate theory. A new consistent understanding is presented for flexural modes in hollow circular cylinders. Several aspects of flexural modes and their relationship with axisymmetric modes are studied. Based on this understanding, helical loading is proposed for pure flexural mode excitation. Analytical relations are derived for the angle of the helical load in order to excite a specific flexural mode. Angular dispersion curves and the concepts of angular dispersivity and angular closeness are introduced for selecting suitable flexural modes for excitation. Finite element analysis and Fourier transforms are utilized to verify the pure excitation of the desired flexural mode at a predicted angle. It is shown that a pure torsional or longitudinal flexural mode is effectively excited by choosing the correct angle for helical load and applying a suitable load.\\ A beam-steering method is proposed in order to excite flexural modes. A linear phased array is used to steer guided waves in hollow circular cylinders. An analytical relation is derived for time delays to excite a desired flexural mode. Finite element analysis is utilized to examine the pure flexural mode excitation using a linear phased array and the reliability of theoretically predicted time delays. It is shown that the proposed beam-steering method can effectively excite pure flexural modes. Pure flexural mode excitation using helical loading and beam-steering are compared; advantages and disadvantages of each method are considered. \\ Experiments are performed to excite flexural modes experimentally. A ring of transducers with 8 channels are used to experimentally excite pure flexural modes using the beam-steering method in a 4[in] schedule 40 steel pipe. The experiments show that pure flexural modes are excited effectively using the beam steering method.\\ A method based on pure flexural modes is proposed for defect detection and classification in pipes. Pure flexural modes with different angles of propagation are utilized in the inspection process. While for a volumetric defect the reflection coefficient for flexural modes are somewhat similar, for a crack-like defect there is a specific flexural mode that shows the maximum reflection coefficient. This property is used to classify defects as volumetric or crack-like. In addition, the angle of the crack-like defect can be determined using the propagation direction of this flexural mode. It is demonstrated that pipeline inspection using flexural modes can be supplemental to the current axisymmetric and focused inspection in order to improve the probability of an accurate inspection process.