Quantitative defect characterization via guided waves

Pipeline inspection systems are being developed to locate critical defects, and more recently to characterize and size them efficiently in order to prevent potential leakage or catastrophic failure, as well as to assist in the decision process of wait or replace. Ultrasonic guided waves have demonstrated their abilities and advantages of detecting a variety of defect types that occur in a plate or hollow cylindrical structure compared to other conventional methods. Yet defect classification and sizing in these structures by guided waves is still a major issue. In this work, the Boundary Element Method (BEM) combined with the guided wave normal mode expansion technique is used to study the interaction of various guided wave packets of energy with various crack and corrosion boundaries in a structure. Attempts are made to find the best modes and frequency with the best chance of success in carrying out the defect classification and sizing process.; To simplify the problem, a plate model is used to approximate the wall structure of a hollow cylinder of large diameter-to-wall-thickness ratio. The dispersion relation and wave structures of Shear Horizontal (SH) waves along the circumference of a hollow cylinder is derived and calculated. A parametric study with the hollow cylinder wall thickness to diameter ratio is conducted. Together with similar studies on circumferential Lamb waves in a pipe wall, a quantitative criterion as when a hollow cylinder wall can indeed be treated as a plate is given.; A two-dimensional boundary element method (BEM) FORTRAN code was developed for surface breaking defect characterization and sizing in a plate using guided waves. A new code is developed for internal inclusion types of defect that are considered benign to see whether they will give false alarms. Parametric studies on different incident wave modes and frequencies establish a guideline on how to characterize a defect in a two-dimensional wave guide.; In reality, all defects are three-dimensional, i.e. their length, width and depth are finite. A 3-D BEM code was developed to study the effects of defect length, width and depth on the propagation of guided waves. A circular disk region on a plate enclosing the 3-D defect was meshed with constant quadrilateral boundary elements. Outside the region, 3-D normal modes of both Lamb and SH waves were used to describe both the incident and scattered wave field. The wave scattering coefficients in different angle directions were obtained for defect characterization.; Finally, the design and manufacturing of a guided wave Electromagnetic Acoustic Transducer (EMAT) is discussed. 2-D and 3-D defect characterization in a plate via SH waves generated by EMATs and Lamb waves by piezoelectric transducers was conducted. Comparisons of experimental and BEM results were presented to describe quantitative NDE potential.