Measurement of vertical and longitudinal rail displacements using digital image correlation

Abstract

Excessive rail displacements can result in reduced rail traffic speeds and increased risk of derailments. A number of methods exist for the measurement of vertical rail displacements, including using geophones, high-speed cameras, and rail vehicle mounted systems. The advantage of rail vehicle mounted methods is that large lengths of track can be assessed. However, there are some instances where the measurement of absolute rail deformations is essential, particularly in poor subgrade conditions where significant long-term settlements are possible. Vehicle-mounted monitoring strategies cannot capture the so-called “running rail” phenomena, where the passage of a train can push the rails longitudinally. Monitoring rail displacements using digital image correlation (DIC) has the potential to capture both of these phenomena unlike other technologies. The objectives of this paper are to evaluate the use of a system of synchronized high-speed cameras to measure absolute longitudinal and vertical rail displacements using DIC, to observe what factors influence the relative magnitudes of these displacements, and to investigate whether DIC measurements can be used to evaluate the stiffness and damping parameters required to develop the displacement–time response of a rail foundation system. The DIC system was evaluated at two sites: one with a high-quality subgrade and one with a peat subgrade. The DIC system was able to capture the absolute vertical and longitudinal displacements due to the passage of trains at both sites. The data from one of the sites, with the high-quality subgrade, were used to develop parameters for system stiffness and damping.