| 1. |
- Ahlström, Johan, 1969, et al.
(författare)
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3D characterisation of RCF crack networks
- 2014
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Ingår i: MATEC Web of Conferences. - : EDP Sciences. - 2261-236X. ; 12
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Konferensbidrag (refereegranskat)abstract
- Rolling contact fatigue (RCF) damage is becoming more frequent with increased traffic 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 often so-called 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 that is difficult to characterise using most non-destructive methods. The cracks can be examined with repeated metallographic sectioning, but the process is time-consuming and destructive. In order to reduce time, as well as information and material loss, high-resolution and high-energy X-ray imaging of railway rails was done in the current study. Combining the exposures from a range of angles using image analysis, a 3D representation of the complex crack network is achieved. The latter was complemented with metallographic sectioning to determine the accuracy of prediction of the geometrical reconstruction.
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| 2. |
- Jessop, Casey, 1988, et al.
(författare)
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3D characterization of RCF crack networks
- 2015
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Ingår i: 10th International Conference on Contact Mechanics of Wheel / Rail Systems, CM 2015, Colorado Springs, United States, 30 August - 3 September 2015.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 3. |
- Jessop, Casey, 1988, et al.
(författare)
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3D characterization of rolling contact fatigue crack networks
- 2016
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 366:SI, s. 392-400
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Jessop, Casey, 1988
(författare)
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Damage and defects in railway materials: influence of mechanical and thermal damage on crack initiation and propagation
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With rising societal concern for the environment, railways offer travelers a sustainable alternative to other forms of fossil-fueled transportation. In order to be competitive with airlines, for example, the railway industry must provide safe, efficient, and affordable service. In the current state, frequent delays on many networks and some rare accidents lead travelers to question the reliability and safety of the rail transport system. This problem is a complex mishmash of different matters which must be addressed from various perspectives. This study focuses on the materials in wheels and rails themselves. Railway wheel and rail materials are subjected to extreme stress in the field; increasing speeds, loads, traffic, at times in harsh weather conditions aggravate this. The combination of loadings affects the materials in a multitude of ways, for example through mechanical and thermal damage. One of the most common types of mechanical damage in the railway industry is rolling contact fatigue (RCF), the effect of which is frequently manifested in the surface of railway components, as cracks. Thermal damage, on the other hand, can affect both the surface and bulk of the material. The aim of this project is to properly characterize mechanical and thermal defects in railway components, and evaluate their effects on crack initiation and propagation, and on mechanical properties. This has been approached through extensive characterization of field samples with a certain type of cracks called squat cracks, which in some cases may lead to rail break. The squat crack networks were examined through a variety of methods, followed by the recreation of similar defects in the laboratory. Finally the effect of such defects on the microstructure, crack initiation and propagation in laboratory experiments was evaluated. Squat crack networks were characterized using several methods; detection limits of each technique have been clarified, and it was concluded that using a combination of methods, the network can be accurately described on many scales. In a second part, well-defined thermal damage on rail surfaces called white etching layers (WELs) similar to those found in field were produced using laser welding equipment, and the effect of these WEL spots on crack initiation and fatigue life has been shown. The WELs reduce fatigue life by providing a crack initiation site; both by stress and strain concentration and by decreasing ductility. The effect of thermal damage on bulk properties was also investigated using microscopy techniques including electron backscatter diffraction (EBSD) and differential-aperture X-ray microscopy (DAXM). It was found that the variation in local misorientation and residual strains decrease with increasing annealing temperature. Additionally, a method to examine crack face friction has been identified, and using this method, similar crack face features to those observed in cracks from field are created in the lab. The results from the friction experiments can be used as input towards crack propagation experiments.
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| 5. |
- Jessop, Casey, 1988
(författare)
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Damage and thermally induced defects in railway materials
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A major concern for the railway industry today is the problem of rolling contact fatigue (RCF) damage in wheels and rails as a result of increased traffic, accelerations, and loading conditions. The wheel/rail contact condition that occurs in railway applications is complex, and it is essential to understand the associated damage from combined mechanical and thermal loadings in order to predict component life and develop appropriate maintenance systems. The crack initiation and propagation processes of RCF have been extensively studied from both theoretical and experimental points of view; however, certain mechanisms for crack initiation and propagation are still not well understood. Thermally damaged surface layers, often called white etching layers (WELs), seem to contribute to crack initiation. One hypothesis is that such cracks can develop into so-called squats or studs in the rail, and to RCF clusters in wheels. The work presented in this thesis can be separated into two main objectives: the first was to properly describe the three-dimensional (3D) geometry of the network of RCF defects in rails using several characterization techniques, and the second was to study crack initiation from thermally damaged surface spots, which mimic a WEL, using low-cycle fatigue (LCF) tests.The following methods were used to geometrically describe squat crack networks: high-intensity X-ray radiography complemented with geometrical reconstruction, serial sectioning and metallography in conjunction with microscopy, X-ray tomography, and topography measurements. The experiments were performed on squats from rail sections taken from the field. It was concluded that the different methods are complementary, and observations made using one method can supplement and explain the deficiencies of other methods. Secondly, investigation of crack initiation from thermally damaged surface spots was carried out via strain-controlled LCF experiments using a MTS 809 servo-hydraulic test machine. The effect of initial thermal damage, observed in the form of a small martensite spot (or WEL) on test bars, on fatigue crack initiation was examined and evaluated relative to smooth specimens. Comparisons between the WELs observed in field samples and those produced artificially on test bars and railheads were made regarding microstructure and residual stresses. The presence of initial thermal damage was found to slightly reduce fatigue life.
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| 6. |
- Jessop, Casey, 1988, et al.
(författare)
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Damage evolution around white etching layer during uniaxial loading
- 2020
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Ingår i: Fatigue and Fracture of Engineering Materials and Structures. - : Wiley. - 8756-758X .- 1460-2695. ; 43:1, s. 201-208
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Tidskriftsartikel (refereegranskat)abstract
- Rolling contact fatigue cracks and thermally induced defects are common problems in the railway industry especially as demands for increasing loads, speeds, and safety continue to rise. Often, the two types of defects are found together in the field, however, whether one causes the other to occur is not completely agreed upon. The effect of thermal damage, in the form of a martensite spot on pearlitic steel test bars, on the fatigue life in uniaxial low cycle fatigue experiments was investigated by the authors. However, the focus of the current work was to characterize the damage evolution from the low cycle fatigue (LCF) tests and correlate the crack initiation and propagation with the initial thermal damage. Residual stress measurements, digital image correlation, and X-ray tomography were used to characterize the effects of the thermal damage before, during, and after fatigue testing, respectively. It was found that the thermal damage causes strain accumulation and crack initiation at the interface between the two materials. The strain evolution was visualized using digital image correlation (DIC), clearly showing the strain concentrations at the top and bottom of the white etching layers (WEL), where the residual stresses are also most tensile. X-ray tomography confirmed the planar crack growth from the martensite spot.
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| 7. |
- Jessop, Casey, 1988, et al.
(författare)
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Friction between pearlitic steel surfaces
- 2018
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Ingår i: Proceedings - 11th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems. - 9789461869630 ; , s. 431-437
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Konferensbidrag (refereegranskat)abstract
- Experiments rubbing two pearlitic rail steel surfaces against each other were done using an axial-torsion test rig. After the experiments were completed, the surfaces were observed under stereomicroscope, and the wear debris were examined. The aim of the experiments was to evaluate the variation of friction coefficient between two surfaces of pearlitic rail steel, mimicking the friction which occurs between the faces of a crack during Rolling Contact Fatigue (RCF) loading. For both dry and wet conditions, the sliding velocity, the angles of rotation, and the average normal force were varied. It was found that the most significant effect of changing angles of rotation is on the formation of ridges on the contact surface with large angles, leading to a higher friction coefficient. The presence of water reduces the friction coefficient and leads to less deformation and wear in the contact surface.
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| 8. |
- Jessop, Casey, 1988, et al.
(författare)
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Friction between pearlitic steel surfaces
- 2019
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 432-433
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Tidskriftsartikel (refereegranskat)abstract
- Experiments rubbing two pearlitic rail steel surfaces against each other were done using an axial-torsion test rig. After the experiments were completed, the surfaces were observed under stereomicroscope, and the wear debris were examined. The aim of the experiments was to evaluate the variation of friction characteristics between two surfaces of pearlitic rail steel, mimicking the friction which occurs between the faces of a crack during rolling contact fatigue (RCF) loading. For both dry and wet conditions, the sliding velocity, the angles of rotation, and the average normal force were varied. It was found that the most significant effect of changing angles of rotation is on the formation of ridges on the contact surface with large angles, leading to a higher friction coefficient. The presence of water reduces the friction coefficient and leads to less deformation and wear in the contact surface.
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| 9. |
- Yildirim, C., et al.
(författare)
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3D mapping of orientation variation and local residual stress within individual grains of pearlitic steel using synchrotron dark field X-ray microscopy
- 2021
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462. ; 197
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Tidskriftsartikel (refereegranskat)abstract
- Assessing the local residual stress and orientation with nanometer resolution within embedded steel grains has remained challenging. Here we use an advanced synchrotron technique, dark field X-ray microscopy to map 3D lattice variations, including both the crystallographic orientation and lattice strain, within two pro-eutectoid ferrite grains in pearlitic steel. We found an orientation variation up to 0.5° and compressive elastic strain up to 1.8 × 10−3 are present in the as-manufactured sample. There is no direct correlation between the measured compressive strain and lattice orientation. The origin of the variations and their influence on the manufacturing process and mechanical properties are discussed.
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| 10. |
- Zhang, Yubin, et al.
(författare)
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Stress relief during annealing of railway wheel steel characterized by synchrotron X-ray micro-diffraction
- 2022
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Ingår i: 42nd Risø International Symposium on Materials Science. - : Institute of Physics Publishing (IOPP). ; 1249
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Konferensbidrag (refereegranskat)abstract
- Railway wheels in service experience rolling contact fatigue loading, but also need to resist frictional heating on braking, yielding temperatures up to 500 degrees C. The combination of mechanical and thermal loads leads to changes in the mechanical properties of the material. The focus of this study is to investigate the effect of annealing on local microstructure and residual stresses in railway wheel pearlitic steel (medium carbon steels, similar to 0.55 wt.% C) using synchroton X-ray Laue micro-diffraction. It is found that the local residual stress releases to a large extent after annealing at 500 degrees C. The stress formation and relief mechanisms and their relationship to the local microstructure are discussed.
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