<Original Paper> Journal of the Korean Society for Nondestructive Testing, Vol. 32, No. 6: 696-702, 2012 ISSN 1225-7842 / eISSN 2287-402X http://dx.doi.org/10.7779/JKSNT.2012.32.6.696 Rail Inspection Using Noncontact Laser Ultrasonics Nak-Hyeon Kim*, Hoon Sohn* ✝ and Soon-Woo Han** Abstract In this study, a noncontact laser ultrasonic system is proposed for rail defect detection. An Nd-Yag pulse laser is used for generation of ultrasonic waves, and the corresponding ultrasonic responses are measured by a laser Doppler vibrometer. For the detection of rail surface damages, the shape of the excitation laser beam is transformed into a line. On the other hand, a point source laser beam is used for the inspection of defects inside a rail head. Then, the interactions of propagating ultrasonic waves with defects are examined using actual rail specimens. Amplitude attenuation was mainly observed for a surface crack, and reflections were most noticeable from an internal damage. Finally, opportunities and challenges associated with real-time rail inspection from a high-speed train are discussed. Keywords: Noncontact Laser Ultrasonics, Rail Inspection, Damage Detection, High-Speed Train [Received: November 7, 2012, Revised: December 3, 2012, Accepted: December 7, 2012] *Department of Civil and Environmental Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Deajeon 305-701, Korea **Korea Railroad Research Institute, 176 Cheoldo Bangmulgwan-ro, Uiwang, Gyeonggi-do, 437-757, Korea ✝Corresponding Author: hoonsohn@kaist.ac.kr ⓒ 2012, Korean Society for Nondestructive Testing 1. Introduction In this study, we propose a noncontact laser ultrasonic system for detection of surface and subsurface detects in rail heads. High-speed trains in Korea are gaining popularity as effective and timely means of transportation. As the speed and frequency of the high-speed trains increase, the possibility for rail damage also increases. To inspect rail damage, several techniques have been developed including visual inspection, conventional NDT techniques, specialized inspection trains, and noncontact scanning techniques [1]. The majority of inspection still relies on visual inspection, which is labor intensive and time consuming. Conventional NDT techniques are very reliable, but their applications are often limited to local inspection because they require couplants such as water or contact type transducers. For long range rail inspection, specialized inspection vehicles equipped with visual cameras and/or laser scanning devices have been developed and commercialized. Here, vision-based devices can only defect surface damage, and rapid image processing can be a daunting task. The laser system often utilizes noncontact acoustic transducers for measurement of leaky guided waves, but the stand-off distance between the acoustic transducer and the rail surface is limited less than 8 cm in static configuration [2]. In this study, a noncontact laser based rail inspection system is proposed. Because the proposed system utilizes laser beams for both ultrasonic generation and sensing, the stand-off distance from the rail surface can be significantly increased. Furthermore, the possibility of detecting subsurface damages as well as surface defects has been demonstrated. This paper is organized as follows. First, the configuration of the proposed laser ultrasonic system is described in Section 2. Then, the experimental setup and test results are reported in Sections 3 and 4, respectively. Finally, conclusions and discussion for high-speed train operation have been provided.