3D characterization of rolling contact fatigue crack networks

Artikel i vetenskaplig tidskrift, 2016

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 surface-breaking 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 their 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, X-ray tomography, and topography measurements. The experiments were performed on squats from rail sections taken from the field. First, high-resolution and high-energy X-ray images exposed through the entire rail head from a range of angles were combined using a semi-automated image analysis method for 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, which gave full access to the crack faces, and the topography was measured. A third squat was investigated by X-ray tomography after extraction of a section of the rail head. The high-energy X-ray, 3D reconstruction method showed accurate main crack geometry at medium depths; however significant drawbacks exist due to limitations in radiography in terms of detecting tightly closed cracks in very thick components. This includes the inability to detect the crack tips which is an important factor in determining the risks associated to a specific crack. 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. However this time-consuming method requires destruction of the specimen investigated. The X-ray tomography revealed the 3D crack network including side branches in a 10x10x30 mm3 sample, and provided topographic information without completely opening the squat. Topography measurements acquired by stylus profilometry provided an accurate description of the entire main crack surface texture, including features such as surface ridges and beach marks.