The use of ultrasonic wave technology has been anticipated as an important technology in inspection and health monitoring of rails. Currently rail inspection cars and rail inspection tools equipped with special transducers such as wheel probes has been feasible in inspection of a railhead and a web. However, the bottom edges of rail, easily damaged regions due to contact to soil and fastening, are blind zones for such conventional ultrasonic inspection technique in which signals are input from a railhead, and the defects in bottom edges of rails can not be detected. Therefore, in this paper, inspection technique for defects in bottom edges of rails is discussed by using guided waves. Dispersion curves in a rail were numerically elucidated by a semi analytical finite element method. Furthermore, the suitable modes to inspection for bottom edges were selected by deriving dispersion curves and wave structures. Particularly ,among a thousand of propagating modes, a Ray leigh mode propagating on side surfaces of the bottom edges was found out to be the best mode for a rail with fastenings. And an inspection tool was designed and developed for generating and receiving the Ray leigh mode, and satisfactory defect echoes were obtained with fastenings.
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