Measurement and modelling of ACFM response to multiple RCF cracks in rail and wheels

Abstract

This paper presents the experimental and model results of the response of an alternating current field measurement (ACFM) sensor to clusters of rolling contact fatigue (RCF) cracks typical of those found in rails and rail wheels. Both artificial and real cracks occurring in rails taken from service are considered. Currently, commercially available ACFM software is capable of producing an estimate of crack pocket length for isolated cracks, assuming they are regularly shaped. The results presented are part of continuing work to link the ACFM signal to the whole range of complex shaped RCF cracks that appear in rail and rail wheels, including those appearing in clusters. The challenges in accurately sizing clustered RCF cracks using the ACFM technique are discussed.

Characterisation of clustered cracks using an ACFM sensor and application of an artificial neural network

“…At the derailment site over 300 cracks were discovered, and the rail had fragmented into over 200 pieces. As at this site, RCF cracks in rails often appear in clusters, with the cracks typically between 2 and 20 mm apart [20]. Characterisation of clustered cracks using EM testing approaches has only rarely been reported [21].…”

“…Another common approach is the application of the finite element method (FEM) to solve the distribution of the magnetic field in the air above the defect and tabulation of the measured ACFM signal and respective crack size using the modelled data [32]. In this study, a uniform field COMSOL model previously developed for isolated RCF cracks [20,24,50,51] has been extended to allow a wider parametric study of clustered RCF cracks closely enough spaced that their ACFM signals overlap (see Figure 2). In the model the material properties of the air and the metal domains are: conductivity ( ) 50 and 5x10 6 S/m, respectively, and relative magnetic permeability ( ) 1 and 50, respectively.…”

“…A non-zero value of air conductivity is used to avoid singularity issues when solving the forward problem with the FEM solver. In the simulation step, the probe has been modelled using a uniform field approximation, validated in previous work [20,24,50,51] to simplify resolving the FEA model for each probe position and to speed up the computations. Therefore, in the model a uniform incident field has been applied to the conductor domain and the solution of the disturbed magnetic field intensity (i.e.…”

“…Crack aspect ratio ( ) describes the ratio of the major axis (a) to the minor axis (b) of an ellipse; the value of the major axis is given, assuming the minor axis is 1, thus a ratio of 1.5:1 is denoted R = 1.5. The crack vertical angle ( ) was fixed at 30 [20,24] while a crack width of °0…”

“…Network Rail, the GB infrastructure manager, has investigated the use of an ACFM sensor contained in a walking stick system [22,23]. Models have been developed to relate the ACFM signal to crack size (via the pocket length) for isolated RCF cracks [24] whilst more recent work has shown the ACFM response for real RCF cracks within clusters is very different to that of isolated cracks and that there is a non-linear relationship between the signal response and the geometry of the cracks in the cluster [20,24,25].…”