| 1. |
- Cao, Yu, 1969, et al.
(author)
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Mechanical Behaviour of a Rephosphorized Steel for Car Body Applications — Effects of Temperature, Strain Rate and Pre-treatment
- 2011
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In: Journal of Engineering Materials and Technology, Transactions of the ASME. - : ASME International. - 1528-8889 .- 0094-4289. ; 133:2, s. 021019-1 - 021019-11-
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Journal article (peer-reviewed)abstract
- Temperature and strain rate effects on the mechanical behaviour of a commercial rephosphorized, interstitial free steel have been investigated by uniaxial tensile testing, covering applicable temperatures (-60 – +100°C) and strain rates (1•10-4 – 1•102 s-1) experienced in automotive crash situations. The effect of prestraining to 3.5 % with or without successive annealing at 180°C for 30 min has also been evaluated. These treatments were used to simulate pressing of the plates and the paint-bake cycle in the production of car bodies. Yield and ultimate tensile strengths, ductility including uniform and total elongation and area reduction, thermal softening effect at high strain rate and strain rate sensitivity of stress were determined and discussed in all cases. It was found that the Voce equation [σ = σs- (σs -σ0) exp (ε/ε0)] can be fitted to the experimental true stress-true plastic strain data with good precision. The parameter values in this equation were evaluated and discussed. Furthermore, temperature and strain rate effects were examined in terms of thermal and athermal components of the flow stresses. Finally, a thermal activation analysis was performed.
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| 2. |
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| 3. |
- Ahlström, Johan, 1969, et al.
(author)
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3D characterisation of RCF crack networks
- 2014
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In: MATEC Web of Conferences. - : EDP Sciences. - 2261-236X. ; 12
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Conference paper (peer-reviewed)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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| 4. |
- Ahlström, Johan, 1969
(author)
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Lcf loop shape in near pearlitic steels – influence of temperature
- 2013
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In: Proceedings of 7th International Conference on Low Cycle Fatigue, Aachen, Germany, September 9–11, 2013. - 9783981451627 ; , s. 81-86
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Conference paper (peer-reviewed)abstract
- Specimens of near pearlitic railway wheel steels have been exposed to isothermal, uniaxial low cycle fatigue tests at constant total strain amplitudes from 0.4 – 1.0%. Temperatures have been varied from -60°C to 500°C. The data recorded has been analysed by fitting polynomial expressions to different parts of the loop. A technique used for extracting the unloading stiffness was presented at the previous conference, LCF6 in Berlin 2008. This procedure has now been extended to characterize the entire loop. The degree of micro plasticity and its effect on unloading stiffness, the strain energy and the cyclic stress development have been characterized. An increased cyclic hardening during the main part of the fatigue life was identified on cycling at 300°C at total strain amplitude of 0.6%. At 500°C the viscous behaviour dominates over the cyclic strain hardening yielding decreasing stress amplitude.
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| 5. |
- Ahlström, Johan, 1969, et al.
(author)
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Modeling of Distortion during Casting and Machining of Aluminum Engine Blocks with Cast-in Gray Iron Liners
- 2012
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In: Materials Performance and Characterization. - 2165-3992. ; 9:5, s. 1-19
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Journal article (peer-reviewed)abstract
- A method for quantifying distortions arising in engine blocks of aluminium with liners of grey iron has been developed based on virtual finite element based simulation tools. The key processing steps comprise the convection cooling step, associated with shrinkage and residual stress build-up, and the machining step. The engine block has been modelled using the finite element method and machining has been simulated using the element-removal technique. The mechanical behaviour of aluminium has been characterised by tensile tests at different temperatures and strain rates. The results were used for calibration of material models capturing the viscoplastic behaviour at higher temperatures. Finally, following the key process steps, results from a complete process analysis are reported comprising the initial stress build-up based on the cooling associated with initial out-of-roundness and predictions of final residual stress state in the block and out-of-roundness of the liners after various machining operations.
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| 6. |
- Ahlström, Johan, 1969, et al.
(author)
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Modelling of distortion during casting and machining
- 2010
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In: Conference proceedings ICTPMCS-2010, 31 May – 2 June 2010, Shanghai, China. ; , s. 8 pp-
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Conference paper (peer-reviewed)abstract
- A method for quantifying distortions arising in engine blocks of aluminium with liners of grey iron has been developed. The engine block has been modelled using the finite element method and machining has been simulated using the element-removal technique. The mechanical behaviour of aluminium has been characterised by tensile tests at different temperatures and strain rates. The results were used for calibration of material models capturing the viscoplastic behaviour at higher temperatures.
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| 7. |
- Ahlström, Johan, 1969
(author)
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Residual stresses in martensite after multiple heating events
- 2014
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In: 5th International Conference on Thermal Process Modeling and Computer Simulation, ICTPMCS 2014; Gaylord Palms Resort and Convention CenterOrlando; United States; 16 June 2014 through 18 June 2014. ; , s. 192-198
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Conference paper (peer-reviewed)abstract
- The study presented relates to the technical problem concerning repeated friction heating in a railway wheel-rail contact surface, caused by partial or full slip. Wheels and rails are commonly made from medium carbon pearlitic steels. The temperature reached during wheel skidding is sometimes high enough to cause austenite formation in a thin layer, and thereafter rapid self-cooling yields martensite. Volume expansion occurs during transformation to martensite. On repeated heating to temperatures around 100-500°C, the martensite is tempered, and a simultaneous volumetric shrinkage takes place. This causes residual stress fields and discontinuities in strength that, in combination with high stress levels, can lead to initiation of cracks. The FE modelling presented in this paper is done to characterize resulting stress and strain fields due to repeated heating of martensite. The model utilizes temperature and structure dependent material properties, including thermal expansion and flow stress. Specimens were modelled as axisymmetric coins with controlled temperature at the center of the top surface. Much of the results, and parts of the descriptive text, are reproduced from a published article with a wider scope [1] with permission from Elsevier. The main conclusion from the modelling work in this study is that there is a large difference in residual stress gradients depending on starting structure. Exposed to a local heating pulse, as-quenched martensitic coins develop a deep tensile residual stress field, with a low gradient. Tempered martensitic coins, on the other hand, can develop high tensile stresses in the surface, but rapidly declining with depth. The explanation lies in the different mechanisms of residual stress formation.
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| 8. |
- Ahlström, Johan, 1969, et al.
(author)
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Short-time tempering kinetics of quench hardened pearlitic steels
- 2010
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In: Conference proceedings ICTPMCS-2010, 31 May – 2 June 2010, Shanghai, China. ; , s. 6 pp-
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Conference paper (peer-reviewed)abstract
- In many industrial processes such as welding, and operating environments like railway wheel/rail contact, certain material volumes are exposed to very short temperature pulses that endure for times of the order of 1 s or shorter. When pearlitic steel is exposed to temperatures above the effective austenitisation temperature, martensite forms upon rapid cooling. In the current work tempering of a martensitic carbon steel was performed, with a laser heat source to create short time top-hat temperature pulses, and also with conventional salt bath experiments for longer tempering times. Temperatures were varied in the range of 500 to 700°C, and times between 0.05 to 3000 s. It was found that the martensitic test samples showed a very rapid initial softening upon tempering. During the first 0.1 second the hardness decrease was measured to 35-55% in the tested temperature range. Thereafter the additional hardness decrease was limited to ~10-15% of the original hardness, even after 30 min of tempering. Thus the use of time-temperature models for predicting tempering properties, that relies on constant or linear dependence of the activation energy cannot be extrapolated to very short time processes.
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| 9. |
- Cvetkovski, Krste, 1983, et al.
(author)
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Analysis of wear debris in rolling contact fatigue cracks of pearlitic railway wheels
- 2014
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In: Wear. - : Elsevier BV. - 0043-1648. ; 314:1-2, s. 51-56
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Journal article (peer-reviewed)abstract
- In the current study a severe subsurface crack network in a railway wheel has been studied and compared to typical rolling contact fatigue cracks found in the wheel tread surface. Microstructural characteristics, chemical composition and microhardness within the cracks and around crack faces were examined. While the two damage types are principally different, both showed similar crack characteristics, with short cracks branching along the main crack paths and a discontinuous sheared layer of wear debris and metallic flakes within them. Analyses of the wear debris showed that it does not originate from external contamination or being the result of corrosion primarily. Instead it has most likely been produced by shear deformation and wear mechanisms within crack faces caused by mixed-mode crack growth. Although microstructural appearance at lower magnification seemed to differ from the bulk material, at high magnification a lamellar structure was observed consisting of layers of deformed metallic flakes and particles of the base metal. Auger electron spectroscopy was used to analyze these sheared layers; higher concentration of oxygen was measured in between flakes, indicating the presence of oxides and flakes being of similar chemical composition as the base material. A possible explanation is that these layers are created due to high shear forces and friction between crack faces in the service of the wheel. With continued rolling the material being sheared by the cyclic relative motion of the crack faces disintegrates into smaller wear debris particles with concurrent oxidation.
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| 10. |
- Cvetkovski, Krste, 1983, et al.
(author)
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Characterisation of plastic deformation and thermal softening of the surface layer of railway passenger wheel treads
- 2013
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In: Wear. - : Elsevier BV. - 0043-1648. ; 300:1-2, s. 200-204
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Journal article (peer-reviewed)abstract
- Microstructure, plastic deformation and annealing properties of the material in the outermost tread layer of used passenger railway wheels were characterised. Large deformation existed up to 5 mm depth below the surface and material flow, i.e. shear strain, was present both in tangential- and transverse directions of the wheel. Close to the field side and flange root moderate rolling contact fatigue was observed, where also the largest work hardening and shear deformation were measured. Annealing trials at temperatures around 500 degrees C showed softening of the deformed surface layer at lower temperatures and at higher rate than for non-deformed material.
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