Limitations of eddy current inspection in railway rail evaluation

At the moment, one of the most cost-effective ways to inspect a whole railway network for rolling-contact fatigue is to carry out an eddy current inspection. It has a great sensitivity to even the smallest near surface defects, but this can also be a downside as it can lead to indications of deep damage from an array of tiny cracks. This can lead to overestimation of damage severity. Moreover, sizing of cracks in rails is unreliable due to the large number of parameters that affect the eddy current inspection response. The depth of penetration is of major importance in eddy current inspection, since cracks that are over this limit cannot be sized reliably. Deep inspection is desirable, but it is unattainable in practice, since signal-to-noise ratio decreases while penetration increases. Also, deep inspection is more affected by variations in steel's electromagnetic properties. Furthermore, increasing inspection velocity will lead to decreased inspection depth. Most of the problems with eddy current inspection are related to the surface microstructure of the rail, and thus could be mitigated with a preliminary rail grinding. Therefore, eddy current inspection is best suited for maintenance quality control. However, eddy current inspection carried out with conventional equipment and without support from other methods gives only coarse estimate of the rails condition.