Scanning laser vibrometer measurement of guided waves in rails

Abstract

Guided wave based inspection and monitoring systems for railway tracks operate at frequencies where as many as 40 modes of propagation may exist. During the development of such systems it is advantageous to be able to measure the amplitude of the individual modes of propagation. The availability of scanning laser vibrometer systems has made it possible to measure the displacement or velocity at a large number of points on the rail surface. The contribution of each mode of propagation may be estimated from the measured frequency responses by using a pseudo-inverse technique and mode shape information computed from a semi-analytical finite element model. Scanning laser measurements were performed in the field at distances of 10m and 500m from a transducer used to transmit the guided waves. A scan of 430 measurement points was used to measure 25 modes at 40kHz.