The Influence of RCF Crack Propagation Angle and Crack Shape on the ACFM Signal

摘要

The alternating current field measurement (ACFM) technique can be used to detect and predict the pocket length of rolling contact fatigue (RCF) cracks in rails, based on a sizing algorithm developed for small cracks that have a planar semi-ellipse shape. RCF cracks usually grow at a shallow angle into the rail (vertical angle) that depends on train speed, axle loads and rail grade. Knowledge of the vertical angle is important, as this allows the crack vertical depth to be determined from the pocket length, which then determines the amount of material (rail depth) to be removed by grinding if RCF cracks are to be eliminated. No method for determining vertical angle for small cracks using electromagnetic type sensors is currently available. In the present work the relationship between ACFM signals and RCF crack vertical angle has been studied. The Bz trough-peak ratio is being proposed as a new method to determine the crack vertical angle. Results from computer simulations, using Comsol Multiphysics, and experimental measurements on calibration samples and rails removed from service containing multiple RCF cracks are reported. In addition, RCF cracks become complex in shape as they grow, which will affect the vertical angle predictions. X-ray tomography and progressive milling has been used to determine the crack shapes and angles for RCF cracks in rails removed from service. The crack profiles have been reconstructed and modelled to study the effect of crack asymmetry on the crack vertical angle prediction.