| 1. |
- Caprioli, Sara, 1978
(författare)
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Combined thermal and mechanical loading of railway wheel treads. A numerical study of material response and cracking under braking conditions
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The scope of the thesis is prediction of the material response owing to combined thermal and mechanical loading of a railway wheel tread. The long-term objective is to improve models for prediction of surface-initiated Rolling Contact Fatigue (RCF) under the combined effect of mechanical loading due to rolling (and sliding) contact and thermal loading due to braking and/or wheel--rail friction.Paper A and Paper B present elastic-plastic Finite Element (FE) simulations to evaluate the impact of simultaneous thermal and mechanical loadings of the wheel tread. Thermal and mechanical loads are combined in a three-dimensional (3D) sequentially coupled analysis where nodal temperatures from a transient thermal analysis are applied as predefined fields in a structural analysis. The mechanical contact load is prescribed as a moving Hertzian contact stress distribution on the wheel tread. The interfacial shear stress distribution corresponds to full slip or partial slip conditions. Paper A outlines the necessity of 3D analyses when dealing with the combined effect of thermal and mechanical loading for realistic predictions of the material response. It is further quantified how a combination of high traction and thermal loading has a very detrimental influence on the material damage. Paper B identifies feasible mesh discretizations and load application schemes that lead to sound predictions of the material response at reasonable computational efforts. Paper B also quantifies differences in predicted material responses under full slip and partial slip conditions. For a given total tangential force, partial slip conditions result in larger plastic strain magnitudes in a thin layer near the contact surface. Paper C presents a numerical analysis of the formation of tread cracking due to severe tread braking. The analysis combines two-dimensional (2D) FE-simulations to evaluate thermal stresses with an analytical evaluation of resulting Stress Intensity Factors (SIFs). A criterion for static fracture is used to identify critical crack size for when existing surface cracks are prone to propagate and resulting crack lengths after propagation. Conclusions are that fully functional brake systems are not likely to induce thermal crack propagation under normal stop braking. However with pre-existing defects a severe drag braking due to malfunctioning brakes may cause very deep cracking. Results from the 2D analysis are compared to full 3D FE-simulations. 2D results are found to be on the conservative side.
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| 2. |
- Caprioli, Sara, 1978, et al.
(författare)
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Influence of short thermal cracks on the material behaviour of a railway wheel subjected to repeated rolling
- 2014
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Ingår i: Advanced Materials Research. - 1662-8985 .- 1022-6680. - 9783038350088 ; 891-892, s. 1139-1145
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Konferensbidrag (refereegranskat)abstract
- An analysis of whether and how the occurrence of shallow (radial) thermal cracks promotes additional plastic deformation of a mechanically loaded wheel tread is carried out. The study employs numerical simulations of a 2D slice of an elastoplastic railway wheel tread containing thermal (radial) cracks. The cracked wheel material is subjected to repeated passes of a frictional rolling contact load. The effect of the existing thermal cracks on bulk deformation and subsequent rolling contact promoted growth is quantified. Results indicate that thermal cracks of a depth of 0.1 mm have a negligible effect, whereas 1.0 mm cracks significantly decrease the bulk resistance of the wheel material. Further, it is shown how the magnitude of stress, strain and deformation depends on the direction of applied traction.
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| 3. |
- Caprioli, Sara, 1978, et al.
(författare)
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Numerical evaluation of the material response of a railway wheel under thermomechanical braking conditions
- 2012
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Ingår i: Proceedings of the 9th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2012). ; , s. 460-467
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Konferensbidrag (refereegranskat)abstract
- The material response of a railway wheel subjected to thermomechanical rolling contact is evaluated. Thermal and mechanical loads are combined in a three-dimensional sequentially coupled analysis where nodal temper- atures from a transient thermal analysis are applied as predefined fields in a structural analysis featuring an elastic-plastic material model. The mechanical contact load is prescribed as a moving Hertzian contact stress distribution with a surface shear stress distribution corresponding to full or partial slip conditions. Modelling aspects studied in detail are feasible model resolution at the contact patch, the influence of sequences of thermal and mechanical loads, and the influence of wheel–rail interfacial shear distributions. The study identifies fea- sible mesh sizes and load application strategies to obtain a good accuracy at reasonable computational efforts. Further, differences in predicted material response under full slip and partial slip conditions are assessed. It is found that for a given total tangential force, partial slip conditions result in larger plastic strain magnitudes in a thin layer near the contact surface.
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| 4. |
- Caprioli, Sara, 1978, et al.
(författare)
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Numerical evaluation of the material response of a railway wheel under thermomechanical braking conditions
- 2014
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 314:1-2, s. 181-188
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Tidskriftsartikel (refereegranskat)abstract
- The material response of a railway wheel subjected to thermomechanical rolling contact is evaluated. Thermal and mechanical loads are combined in a three-dimensional sequentially coupled analysis where nodal temperatures from a transient thermal analysis are applied as predefined fields in a structural analysis featuring an elastic-plastic material model. The mechanical contact load is prescribed as a moving Hertzian contact stress distribution with a surface shear stress distribution corresponding to full or partial slip conditions. Modelling aspects studied in detail are feasible model resolution at the contact patch, the influence of sequences of thermal and mechanical loads, and the influence of wheel--rail interfacial shear distributions. The study identifies feasible mesh sizes and load application strategies to obtain a good accuracy at reasonable computational efforts. Further, differences in predicted material response under full slip and partial slip conditions are assessed. It is found that for a given total tangential force, partial slip conditions result in larger plastic strain magnitudes in a thin layer near the contact surface
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| 5. |
- Caprioli, Sara, 1978
(författare)
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Short rolling contact fatigue and thermal cracks under frictional rolling - A comparison through simulations
- 2015
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Ingår i: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944. ; 141, s. 260-273
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Tidskriftsartikel (refereegranskat)abstract
- The relative severity of radial (thermal) and inclined rolling contact fatigue surface cracks of equal depth in a railway wheel is investigated by three-dimensional elastoplastic finite element analyses of a cracked wheel sector subjected to contact loading. Response is quantified by relative displacements of nodes close to crack tip and at the crack mouth. Highly inclined cracks give the highest magnitudes of crack tip shear displacements, which is the dominant deformation mode. Braking conditions are found to open the crack mouth. Initially higher temperatures on thermal cracks cause increased crack tip deformation and opening of the crack, whereafter subsequent mechanical load cycles impose crack closure.
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| 6. |
- Caprioli, Sara, 1978, et al.
(författare)
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Thermal Cracking of a Railway Wheel Tread due to Tread Braking – Critical Crack Sizes
- 2011
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Ingår i: Proceedings 2011 International Heavy Haul Association Conference. ; , s. 8-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A numerical study of tread cracking due to thermal loading induced by tread braking is carried out. The analysis features a computationally efficient approach combining 2D FE-simulations with an analytical evaluation of resulting stress intensity factors. The analysis identifies critical sizes for when existing surface cracks are prone to propagate under thermal loading and resulting crack lengths after propagation. The results imply that fully functional brake systems are not likely to induce thermal crack propagation under normal stop braking, but that with pre-existing defects a severe drag braking due to malfunctioning brakes may cause very deep cracking. Further the analysis concludes the cracking to be a static phenomenon related to the most severe brake cycle, i.e. later brake cycles of similar or lower severity will not cause any significant propagation of existing cracks. Preliminary 3D FE-simulations that have been performed indicate the presented results to be conservative.
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| 7. |
- Caprioli, Sara, 1978, et al.
(författare)
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Thermal cracking of a railway wheel tread due to tread braking -- critical crack sizes and influence of repeated thermal cycles
- 2013
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Ingår i: Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit. - : SAGE Publications. - 0954-4097 .- 2041-3017. ; 227:1, s. 10-18
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Tidskriftsartikel (refereegranskat)abstract
- A numerical study of tread cracking due to thermal loading induced by tread braking is carried out. The analysis features a computationally efficient approach combining two-dimensional finite-element simulations with an analytical evaluation of resulting stress intensity factors. The analysis identifies critical sizes for when existing surface cracks are prone to propagate under thermal loading and resulting crack lengths after propagation. The results imply that fully functional brake systems are not likely to induce thermal crack propagation under normal stop braking, but that with pre-existing defects, a severe drag braking due to malfunctioning brakes may cause very deep cracking. Furthermore, the analysis concludes the cracking to be a static phenomenon related to the most severe brake cycle, i.e. later brake cycles of similar or lower severity will not cause any significant propagation of existing cracks. Additional three-dimensional finite-element simulations are used to validate the model, and they indicate two-dimensional results to be on the conservative side.
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| 8. |
- Caprioli, Sara, 1978, et al.
(författare)
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Thermal cracking of a railway wheel tread due to tread braking – critical crack sizes and influence of repeated thermal cycles
- 2011
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Ingår i: International Heavy Haul Association Special Technical Session (IHHA STS 2011). ; , s. 8-
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Konferensbidrag (refereegranskat)abstract
- A numerical study of tread cracking due to thermal loading induced by block braking is carried out. The analysis features a computationally efficient approach combining 2D FE-simulations with an analytical evaluation of resulting stress intensity factors. The analysis identifies critical sizes for when existing surface cracks are prone to propagate under thermal loading and resulting crack lengths after propagation. The results imply that fully functional brake systems are not likely to induce thermal crack propagation under normal stop braking, but that with pre-existing defects a severe drag braking due to malfunctioning brakes may cause very deep cracking. Further the analysis concludes the cracking to be a static phenomenon related to the most severe brake cycle, i.e. later brake cycles of similar or lower severity will not cause any significant propagation of existing cracks. Finally it should be noted that preliminary FE-simulations that have been performed indicate the presented results to be conservative.
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| 9. |
- Caprioli, Sara, 1978, et al.
(författare)
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Thermal cracking of railway wheels: Towards experimental validation
- 2016
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Ingår i: Tribology International. - : Elsevier BV. - 0301-679X. ; 94, s. 409-420
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Tidskriftsartikel (refereegranskat)abstract
- Thermal cracking of railway wheel treads is investigated using a combined experimental and numerical approach. Results from controlled brake rig tests of repeated stop braking cycles for a railway wheel in rolling contact with a so-called railwheel are presented. Test conditions are then numerically analysed using finite element (FE) simulations that account for the thermomechanical loading of the wheel tread. For the studied stop braking case, thermal cracks are found in the wheel tread after few brake cycles. Results from thermal imaging reveal a frictionally excited thermoelastic instability phenomenon called "banding" where the contact between brake block and wheel occurs only over a fraction of the block width. This condition results in locally high temperatures. The numerical analysis assesses ratcheting response of the wheel tread material under operational conditions corresponding to two types of banding patterns and also a case of uniform heating. Fatigue life predictions are estimated from the evaluated ratcheting response using a simplified accumulation rule. Predicted lives are found to be in reasonable agreement with experimental results.
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| 10. |
- Caprioli, Sara, 1978
(författare)
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Thermal impact on rolling contact fatigue of railway wheels
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Rolling contact fatigue (RCF) is a very common and costly damage mechanism for rails and wheels. This thesis investigates the influence of combined thermal and mechanical loading on RCF of railway wheels on the basis of numerical predictions. The established computational framework includes heat flux analyses, (two- and three-dimensional) elastoplastic finite element simulations and subsequent RCF life analyses. The computational framework is employed to quantify the influence of various operational parameters and modelling presumptions such as applied heat and tangential stress characteristics, load application schemes, mesh densities etc. Examples of results include quantifications of how partial slip conditions result in higher plastic strain magnitudes in a thin layer at the wheel tread surface, and differences in material responses between accelerating and braking wheels.The numerical model was extended to incorporate surface initiated cracks. With the extended model it is shown that 1 mm deep cracks have a substantial influence on the state of stress and strain in the bulk material between surface cracks. Further, comparisons between radial (thermal) and inclined (RCF) surface cracks show that the deformation of significantly inclined cracks (30 degrees) is more severe than that of radial cracks. Further, acceleration is found to give larger crack face displacements. However braking tends to induce tensile residual stresses that open the crack mouth, thus allowing fluid penetration that can promote crack growth. Also thermal loading is found to cause a significant crack mouth opening that is decreased by subsequent rolling contact.In a final study numerical RCF predictions are compared to full-scale experimental studies carried out at the Railway Technical Research Institute in Japan. Thermal loading tuned towards measurements by thermocameras and thermocouples are introduced in a truncated loading scheme corresponding to the test configuration. Estimated crack initiation life is found to be in good agreement with test results. The investigation also shows the significant influence of the employed material model. In addition to thermomechanical fatigue analyses, the case of purely thermal fracture has been investigated. This study quantified how the risk of fracture and resulting crack sizes depend on braking conditions and initial surface cracks. The results of this thesis are believed to be of importance in defining and enforcing sustainable operational conditions and maintenance actions. Further, this thesis provides tools to establish root causes and pertinent mitigating actions when thermomechanical wheel cracking nevertheless occurs.
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