Transverse and longitudinal crack detection in the head of rail tracks using Rayleigh wave-like wideband guided ultrasonic waves Stuart B Palmer, Steve Dixon, Rachel S Edwards and Xiaoming Jian University of Warwick, Dept of Physics, Coventry CV4 7AL, UK ABSTRACT We have developed a ‘pitch-catch’ low frequency-wideband Rayleigh wave EMAT system with a centre frequency of approximately 200kHz, extending to around 500kHz and study here its applicability to crack detection in the head of rail tracks. On the head of a rail, the generated waves are strictly speaking a type of guided wave mode as the propagation surface is not a flat halfspace. They propagate along the surface of the rail penetrating down to a depth of several millimetres. We have used this approach to demonstrate detection of gauge corner and longitudinal cracking in the rail head. On samples containing machined slots we have shown that crack depth can be estimated by measuring the proportion of the ultrasonic wave at a particular frequency that passes underneath the crack. The approach that we have used is fundamentally different to and has several advantages over conventional ultrasonic contact methods and should ultimately facilitate testing the rail head more thoroughly at higher speeds. Keywords: 1. INTRODUCTION Ultrasonic inspection of rails is usually restricted to low speeds of around 20-30mph 1 , which limits the viability of testing many tracks regularly. Furthermore many of the most serious defects that can develop in the rail head can be very difficult to detect using the currently available inspection equipment. One of the reasons for slow inspection speeds using conventional NDT is the need for couplant between the transducer and the track using either liquid or dry couplant materials. EMATs have been used 2,3 or suggested 4 to measure both rail tracks and wheels by other workers and the use of non-contact ultrasonic measurements are still being investigated by a number of international research groups 5-8 . In this paper we discuss the use of EMATs on rail for longitudinal and transverse crack defect detection and depth gauging. Ultrasonic surface waves that are similar in behaviour to Rayleigh waves are an obvious candidate for surface breaking crack detection, or indeed for defects that lie just under the surface within the Rayleigh wave penetration depth 9-12 . There are different approaches that may be used to detect a crack using a pitch-catch method. Strictly speaking, a Rayleigh wave only exists on a flat surface and as the surface of the rail head is curved the surface waves that propagate along or around the rail surface are a type of guided wave mode. Nevertheless these are still Rayleigh-like and for the purposes of this paper we will refer to them as Rayleigh waves. If a defect lies between the Rayleigh wave generator and detector then it will to some degree block the Rayleigh wave. The amplitude of a Rayleigh wave displacement decays with depth into the sample and most of the energy associated with a particular frequency lies within a depth equal to one wavelength at that frequency. Almost all of the energy lies within a depth corresponding to two wavelengths 9 . The different frequency components will effectively probe to different depths below the sample surface. In a measurement where we attempt to propagate a Rayleigh wave through a region containing a surface breaking crack, the crack depth can be estimated by the amount of Rayleigh wave energy or amplitude that is transmitted through or underneath that region. Closed or partially closed cracks can obviously complicate the analysis and increase the amount of Rayleigh wave energy transmitted through the crack compared to an open crack. Nondestructive Evaluation and Health Monitoring of Aerospace Materials, Composites, and Civil Infrastructure IV, edited by Shull, Gyekenyesi, Mufti, Proc. of SPIE Vol. 5767 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.598142 70