Rail rolling contact fatigue (RCF) cracks are complex in shape, spacing and propagation angle (vertical angle) into the rail due to variations in train speed, axle loads and rail grades. RCF cracks often present in the form of clusters and initially propagate into the rail with a shallow angle. It is important to know the crack depth (which varies with the propagation angle) to aid maintenance planning. Currently, electromagnetic (EM) sensors can be used to measure crack length but there is no EM method reported for determining the crack propagation angle. In this work a novel method to predict the vertical angle for single and multiple RCF cracks (light to moderate category classified by Network Rail) using an alternating current field measurement (ACFM) pencil probe has been developed. The results show, through modelling and experimental verification, that the Bz trough-peak ratio can be used to determine the RCF crack vertical angle. The influence of the sensor lift-off, the angle at which the probe is scanned across the cracks, and the presence of uniform surface length cracks on the ACFM response for cracks with different vertical angles are discussed.
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