Guided elastic waves in structures with an arbitrary cross-section

by Lee, Chong Myoung.

iii A train is one of the oldest and most important transportation methods for moving people and goods. A train accident can causes serious casualties and property damage. Many factors could lead to a train disaster and the defects in rail are one of the major problems. Detection of defects and proper maintenance action for a rail is therefore essential. There are two kinds of typical defects in a rail head. They are shelling and transverse defects. Shelling is a horizontal plane defect generated by the sliding and/or rolling the wheel over the rail from shear reversal and is usually located just below the top surface of the rail. The transverse defects are usually generated and grown inside the rail head from the shelling region down into the head. Shelling is not fatal but transverse defects are. Conventional ultrasonic tests (the normal incident technique and the oblique incident technique) have difficulties in detecting the transverse defects under the shelling, because most of the ultrasonic energy is reflected from the shelling. For this reason, the guided wave ultrasonic technique is potentially a suitable method for detecting defects under the shelling. The cross-sectional area of the shelling is much smaller than that of the transverse defects in the guided wave propagation direction. iv The purpose of this study is therefore to find an appropriate guided wave mode and frequency for the detection of a transverse defect under the shelling. The phase velocity and group velocity dispersion curves are calculated numerically using a semianalytical finite element method (SAFE). From the phase velocity dispersion curves, the spacing of the elements in an electromagnetic acoustic transducer (EMAT), an important feature in EMAT design, is determined to generate the appropriate guided waves for reliable defect detection. Characteristics of the guided wave propagation is also explored at various points in the phase velocity dispersion curves using ABAQUS/Explicit, a commercially available finite element method (FEM) package with a simulation of a Lamb type EMAT. Finally, the aspect of the wave scattering of guided waves from several types of defects along with the shelling located in the rail head is examined. This research provides a new modeling technique to simulate the EMAT loading and can suggest guide lines for a new inspection technique for finding defects in the rail head under shelling using EMATs. Furthermore, the research area can be extended to a study of various types of defects, different location of the defects, different loading positions, various welding zones, and overall changes in rail boundary conditions.