Field measurement of guided wave modes in rail track

During the development of guided wave systems for rail monitoring it is useful to be able to measure the amplitude of the individual modes of propagation. The availability of scanning laser vibrometers makes it possible to measure the displacement at a large number of points on the rail surface. This paper describes scanning laser vibrometer measurements performed in the field and the estimation of the amplitudes of the propagating waves from these measurements. The process makes use of mode shape information obtained from semi-analytical finite element analysis and a pseudoinverse technique. Field measurements are performed by exciting the rail with a piezoelectric transducer and performing scans at various distances from the excitation. Results of two scans performed at short-range and on the same section of rail are compared to assess the repeatability of the process. Results from scans performed at 200m and 400m from the transducer are then used to estimate the attenuation of selected modes of propagation.