3D characterization of RCF crack networks

Paper in proceeding, 2015

Rolling contact fatigue (RCF) damage is becoming more frequent with increased traffic, accelerations, and loading conditions in the railway industry. Defects which are characterized by a two-lobe darkened surface and a V-shaped surfacebreaking crack are defined as squats. The origination and propagation of squats in railway rails is the topic of many recent studies; the associated crack networks develop with complicated geometry near the surface of rails, but can be difficult to detect and distinguish from normally existing head checks in its early stages, using in-field non-destructive testing. After cutting out damaged sections of rail, there are a number of options to characterize the damage. The aim of this study was to evaluate different methods to geometrically describe squat crack networks; through radiography, metallography, and topography measurements. The experiments were performed on squats from a rail section taken from field. First, highresolution and high-energy X-ray images exposed from a range of angles were combined using geometrical reconstruction, and a 3D representation of the complex crack network was achieved. This was complemented with repeated metallographic sectioning to determine the accuracy of prediction of the geometrical reconstruction. Another squat was opened after radiography which gave full access to the crack faces, and topography measurements were performed. The X-ray, 3D reconstruction method showed accurate crack geometry in the medium depths; the crack tips were not visible though, due to limitations in radiography in terms of detecting tightly closed cracks. Metallographic investigation of the cracks gave good interpretation of crack geometry along the sections examined, and gave the possibility to study microstructure and plastic deformation adjacent to the crack face. The topography measurements provided a more accurate description of the surface texture, including features such as surface ridges and beach marks.