Surface wave modes in rails

Rail breaks caused by rolling contact fatigue defects are of growing concern to the railway industry. Very often critical defects cannot be detected reliably by conventional inspection methods. Low-frequency surface waves with a high penetration depth have the potential to overcome such difficulties. In this paper, the properties of surface wave modes in both new and worn rails are investigated and the implications for rail inspection are discussed. The dispersion curves of the dominant surface wave modes were determined up to a frequency of 350 kHz using a finite element model and show excellent agreement with experimental data. One surface wave mode was identified to be nondispersive and not significantly affected by cross-section changes due to wear at frequencies above 180 kHz. It exhibits a relatively homogeneous energy distribution in the upper half of the rail head and is therefore suitable for inspection purposes. The problem is that there exist several other surface modes with very similar propagation properties. The interference of these multiple modes, which may be generated either directly by the exciter or by mode conversion at defects, means that the received amplitude is position dependent. For this reason, accurate defect sizing will be difficult. (c) 2006 Acoustical Society of America.