Design criteria for rolling contact fatigue resistance in back-up rolls

Detection of surface breaking defects, such as rolling contact fatigue (RCF) cracks, is an on-going topic of research within the context of rail inspection. At present, detection and classification of RCF cracks using an ACFM sensor is based on low speed walking stick systems where negligible sensor lift-off changes result in a reasonably stable background signal and hence the defects can be automatically detected using a threshold method. However, in the case of high speed inspection systems, the inevitable lift-off variations (e.g. owing to the dynamics of the train bogie etc.) lead to a varying background ACFM signal which renders the threshold method ineffective. A novel method for the detection of isolated RCF cracks, namely combined threshold and signature match (CTSM), has previously been applied to ACFM scans over RCF cracks. The method proved to be effective for automatic detection of isolated RCF cracks, however, it was observed to perform poorly in response to areas of multiple RCF cracks, which often appear in clusters. This paper investigates the application of an enhanced CTSM algorithm to the detection of clustered RCF cracks. The algorithm has been applied to low speed scans over sections of rails removed from service containing real RCF cracks and to ACFM scans obtained at high speed (up to 48 km/h) over a spinning rail rig containing clusters of artificial cracks. Results suggest that the extended CTSM algorithm is effective in automatically detecting multiple RCF cracks with a high detection rate (> 90%). Further, in the case of widely spaced (> 5 mm) multiple cracks, the algorithm can also provide extra characterisation information about the number and position of cracks within a cluster.

In this paper, the spalling causes of backup rolls in a roughing mill of a hot strip rolling mill were investigated. The roughing mill withstood extreme service conditions with long service cycles for backup and work rolls, a large variety of the same width rolling campaigns, severe and non‐uniform wear contours of backup and work rolls. Analysis of cyclic stress applied on backup rolls indicated that contact stress played a dominant role in rolling contact fatigue which led to spalling on the backup roll surface. A 3D finite element model of roll system was established and contact stresses between rolls in whole service periods of backup and work rolls were calculated. The simulation results showed that roll wear had significant impact on contact stress distribution. In particular, the severe and non‐uniform U‐typed wear contours of work rolls led to produce huge contact stress peak in two sides of the rolls, where existed large cracks by eddy current inspection of post‐service backup rolls. Backup rolls stood thousands of times stress cycle in a service period, they were liable to fatigue damage in these areas. A new backup roll contour was developed to improve contact stress distribution and no spalling accidents were happened any longer after applied to the roughing mill.

Characterization of intersecting and bifurcating rolling contact fatigue (RCF) cracks in railway rails using ACFM sensor

A fine-grain layer is produced in high-speed train wheels after machining. In this article, the influence of the fine-grain layer on rolling contact fatigue (RCF) is studied using a GPM-30 fatigue tester. The results show that there is a 1-μm-thick fine-grain layer after grinding treatment. The fine-grain layer can improve the RCF life of wheel samples. The formation of RCF cracks on the sample with a fine-grain layer is divided into three stages: (1) the initiation of fine fatigue cracks is preferred in the fine-grain layer during RCF; (2) RCF cracks in the fine-grain layer propagate gradually to lead to flaking of the fine-grain layer; (3) when the fine-grain layer is flaked, RCF cracks initiate and propagate at the surface of the original microstructure. However, RCF cracks of the sample without a fine-grain layer directly initiate and propagate at the surface of the original microstructure. Therefore, the fine-grain layer can hinder the formation of RCF cracks at the surface of the original microstructure to improve RCF life.

There is no method available to monitor localised stress levels on in-service railway track, although residual stresses in the rail crown have an important influence on the initiation and evolution of rolling contact fatigue (RCF) cracking. The MAPS technology has the ability to meet this need both in the short term with manual inspections and in the longer term with development of an inspection vehicle. This paper presents an update ( 2 ) of the MAPS measurement of stress levels and distributions in service rail but with a scope limited to specific studies into the early stages of rolling contact fatigue leading to gauge corner cracking. Data presented includes fine-scale stress distributions on rail cross-sections with RCF cracks together with stress distributions and stress depth profiles on rail heads, indicating both the earliest stages of rolling contact fatigue and established RCF cracking. The evidence suggests MAPS measurement data can act as indicators to the future emergence, presence and criticality of the RCF in rails, and thereby enhance maintenance regimes improving the safe service life of the rail network.

The effect of slip ratio on the rolling contact fatigue property of railway wheel steel

The accumulated damage process of rolling contact fatigue (RCF) of plasma-sprayed coatings was investigated. The influences of surface roughness, loading condition, and stress cycle frequency on the accumulated damage status of the coatings were discussed. A ball-ondisc machine was employed to conduct RCF experiments. Acoustic emission (AE) technique was introduced to monitor the RCF process of the coatings. AE signal characteristics were investigated to reveal the accumulated damage process. Result showed that the polished coating would resist the asperity contact and remit accumulated damage. The RCF lifetime would then extend. Heavy load would aggravate the accumulated damage status and induce surface fracture. Wear became the main failure mode that reduced the RCF lifetime. Frequent stress cycle would aggravate the accumulated damage status and induce interface fracture. Fatigue then became the main failure mode that also reduced the RCF lifetime.

A study on the rolling contact fatigue mechanism of ZC-L cast wheel steel

• The effect of grinding on the long-term degradation of in-service rails is evaluated. • Both ground and non-ground rails exhibit WEL and microcracks after long-term service. • Grinding is beneficial to mitigating rolling contact fatigue cracks and removing plastic deformation. • Ratcheting is the dominant crack initiation mechanism under the examined conditions. Rail grinding has been widely applied in railway networks worldwide to remove or prevent rolling contact fatigue (RCF) cracks. However, some concerns have arisen regarding grinding, that it may introduce initial damage to the rail and largely shorten the RCF life. This work aims to better understand the effect of grinding on the long-term degradation of in-service rails, particularly concerning White Etching Layer (WEL) and RCF cracks. Seven rail samples were selected and taken from the Belgian and Swedish railway networks, with different grinding histories, accumulated loads, and steel grades. The mechanical and microstructural properties of these samples were examined through the hardness test and optical microscopy. WEL and microcracks were observed in both ground and non-ground rails, suggesting that rail grinding does not create additional defects nor negatively impact the rail surface after long-term service. Macrocracks were observed only in rail samples that had undergone zero or a single grinding cycle, confirming the beneficial role of rail grinding in mitigating RCF cracks. Ratcheting is the dominant crack initiation mechanism under the examined conditions, while WEL may also contribute to crack formation, given that macrocracks predominantly occur at the transition between the WEL and the pearlite.

Analysis on asymmetrical RCF cracks characterisation using an ACFM sensor and the influence of the rail head profile

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.

Experimental and theoretical investigation of the phenomenon of filling the RCF crack with liquid

Mechanism and improvement of the rolling contact fatigue of the surface layer with heterogeneous microstructures of the rail steel treated by laminar plasma jet

Bearings used in the intermediate and high speed stages of offshore wind turbine gearboxes run in harsh working conditions and may fail prematurely due to rolling contact fatigue (RCF). The microstructural alterations linked with such premature bearing failures are often described as white etching crack failures (WEC). Understanding these white etching bearing failures requires the study of RCF phenomenon at multiple scales ranging from macroscopic to microscopic and at different stages such as crack initiation and propagation. This paper presents a 2D numerical framework to evaluate subsurface RCF crack initiation originating from non-metallic inclusions in bearings. The global finite element (FE) model simulates parts of the contact bodies such as roller elements and raceway to represent the contact zone. A submodel containing a non-metallic inclusion is derived from the global FE model of a rolling contact. The inclusion is elliptical in shape and its position, dimensions and orientation can be changed. A moving Hertzian pressure distribution is modeled to simulate the rolling pass and the stress history around the inclusion/matrix interface. A multi-axial critical plane approach is adopted to calculate fatigue initiation damage. After explaining the numerical framework, this paper highlights the functionalities based on a number of case studies, focusing on effects of normal load and surface traction between the roller and inner raceway, and on inclusion characteristics. The results give us an insight into the underlying physics behind the mechanism of subsurface initiated RCF. Investigation of the link between RCF and white etching cracking is ongoing.

Propagation of surface initiated rolling contact fatigue cracks in bearing steel