Dissertations

Dissertations in materials science (contributions to the digital magazine Innovative Materials)
4TU Delft
4TU Eindhoven
4TU Twente
4TU Wageningen

What really happens inside railways....

Jun Wu (TU Delft)

Each year, ProRail has to spend several million euros on periodic and ad-hoc maintenance work on railway tracks, in order to remove fatigue cracks before they lead to catastrophic rail fracture. One of the widely considered causes for the fatigue damage, which in railways applications occurs as Rolling Contact Fatigue (RCF), lies in the changes in the internal structure of the steel that are induced during the passage of trains: so-called white etching layers (WEL) form. The extremely high hardness of the WELs is considered to be the origin of its susceptibility to fracture and crack formation during the subsequent loading by passing train wheels. Until now, the prevalent models used to describe the RCF process in rails fail to predict the occurrence of the WEL, due to the fact that the origin and character of the WEL remained debatable.

'What really happens inside railways...' Innovative Materials, Volume 2 2019, p. 35. (Pdf file)


Railway steel at different length scales. From left to right: (a) macroscopic view of the rail/wheel contact at the metre scale; (b) signs of WEL formation at the rail surface, seen at the millimetre scale; (c) microscopic cross-section of railway steel, where the hard (820 HV Vickers hardness) WEL, as well as a fatigue crack, is visible at the top at the micrometre scale; (d) nano-twins and cementite traces in the WEL’s martensite at the nanometre scale; (e) distribution of alloying elements C, Si and Mn at the nanometre scale, with the WEL in the top three pictures and the original steel in the bottom three.

Dr. Jun Wu explores, together with prof.dr.ir. Jilt Sietsma and prof.dr.ir. Roumen Petrov, the root cause of the WEL formation via a series of microstructure characterizations, laboratory simulations, and theoretical modelling. The microstructural study, combining a wide variety of modern microscopy techniques, leads to the unambiguous conclusion that the WEL forms due to the significant heat generated during the wheel passages. The microstructure changes show that underneath train wheels the temperature repeatedly increases to over 700°C. The WEL structure was successfully reproduced under well-controlled laboratory conditions mimicking this rapid temperature rise followed by fast cooling. Furthermore, the characteristics of the WEL formation process were studied by phase field modelling. The insight provided by the thesis work of Jun Wu provides important guidance for the future design of new rail steels with higher resistance against WEL formation.

The Ph.D. research work of Jun Wu was performed under the joint supervision of Delft University of Technology and Ghent University. Jun Wu succeeded in defending his thesis first at Ghent University on 7 November 2018 and later, on 17 December 2018, at Delft University of Technology. This entitles him a joint degree from both universities. The title of his thesis is ‘Microstructure Evolution in Pearlitic Rail Steel due to Rail/Wheel Interaction’.

 The thesis can be found at: https://doi.org/10.4233/uuid:c536ca47-8981-4a9e-916f-396bcbca4bc5


Nano-scale failure in steel

Astrid Elzas (TU Delft)

Multiphase alloys such as advanced high strength steels can show unexpected failure due to dislocations piling up at internal boundaries between the soft iron matrix and the hard precipitates. The stress concentrations caused by the dislocation pile-ups might trigger interface decohesion followed by the formation of voids and, eventually, macroscopic cracks. To prevent such material failure, accurately predictive material models are needed. 


'Nano-scale failure in steel. Interface decohesion at iron/precipitate interfaces' Innovative Materials, Volume 1 2019, p 35. (Pdf file)

Dr.ir. Astrid Elzas studied together with Prof.dr. Barend Thijsse crack nucleation and interface decohesion on the nanoscale with large-scale molecular dynamics simulations, to understand which conditions lead to interface decohesion. Systematically varying different physical parameters allowed clear observation of the important physical effects in a controlled manner. Not only where the studied interfaces and the numbers of dislocations piling-up at the interfaces varied, also different loading modes where considered. Apart from pure shear and pure tensile loading, in this study also mixed loading, i.e. a tensile force under an angle with the interface, was considered. It is found that the interface structure, which changes during response to loading, is the key factor determining the material response.

Apart from the deeper and systematic understanding of interface decohesion resulting from this study, also interface/material-specific traction-separation relationships are developed which can be applied in larger scale material models to  improve the accuracy of this models with respect to the description of interface behaviour and with that lead to a better prediction of material failure.


Figure 1: Molecular dynamics simulation of interface decohesion at an interface between the soft iron matrix (red) and a hard precipitate (blue) of steel.



Figure 2: Schematic representation of various interfaces subjected to different loading modes, due to their orientation. Various relations between tractions and separations are found for the different interfaces and different loading modes.

On 10 January 2019 Astrid Elzas obtained her Doctorate cum laude for her thesis “Nano-scale failure in steel” at Delft University of Technology.

The thesis can be found at: https://doi.org/10.4233/uuid:f72f61f4-4508-4552-85b8-d89abbbee90e

Source: TU Delft

What really happens inside railways....

Jun Wu (TU Delft)

Each year, ProRail has to spend several million euros on periodic and ad-hoc maintenance work on railway tracks, in order to remove fatigue cracks before they lead to catastrophic rail fracture. One of the widely considered causes for the fatigue damage, which in railways applications occurs as Rolling Contact Fatigue (RCF), lies in the changes in the internal structure of the steel that are induced during the passage of trains: so-called white etching layers (WEL) form. The extremely high hardness of the WELs is considered to be the origin of its susceptibility to fracture and crack formation during the subsequent loading by passing train wheels. Until now, the prevalent models used to describe the RCF process in rails fail to predict the occurrence of the WEL, due to the fact that the origin and character of the WEL remained debatable.

'What really happens inside railways...' Innovative Materials, Volume 2 2019, p. 35. (Pdf file)


Railway steel at different length scales. From left to right: (a) macroscopic view of the rail/wheel contact at the metre scale; (b) signs of WEL formation at the rail surface, seen at the millimetre scale; (c) microscopic cross-section of railway steel, where the hard (820 HV Vickers hardness) WEL, as well as a fatigue crack, is visible at the top at the micrometre scale; (d) nano-twins and cementite traces in the WEL’s martensite at the nanometre scale; (e) distribution of alloying elements C, Si and Mn at the nanometre scale, with the WEL in the top three pictures and the original steel in the bottom three.

Dr. Jun Wu explores, together with prof.dr.ir. Jilt Sietsma and prof.dr.ir. Roumen Petrov, the root cause of the WEL formation via a series of microstructure characterizations, laboratory simulations, and theoretical modelling. The microstructural study, combining a wide variety of modern microscopy techniques, leads to the unambiguous conclusion that the WEL forms due to the significant heat generated during the wheel passages. The microstructure changes show that underneath train wheels the temperature repeatedly increases to over 700°C. The WEL structure was successfully reproduced under well-controlled laboratory conditions mimicking this rapid temperature rise followed by fast cooling. Furthermore, the characteristics of the WEL formation process were studied by phase field modelling. The insight provided by the thesis work of Jun Wu provides important guidance for the future design of new rail steels with higher resistance against WEL formation.

The Ph.D. research work of Jun Wu was performed under the joint supervision of Delft University of Technology and Ghent University. Jun Wu succeeded in defending his thesis first at Ghent University on 7 November 2018 and later, on 17 December 2018, at Delft University of Technology. This entitles him a joint degree from both universities. The title of his thesis is ‘Microstructure Evolution in Pearlitic Rail Steel due to Rail/Wheel Interaction’.

 The thesis can be found at: https://doi.org/10.4233/uuid:c536ca47-8981-4a9e-916f-396bcbca4bc5


Nano-scale failure in steel

Astrid Elzas (TU Delft)

Multiphase alloys such as advanced high strength steels can show unexpected failure due to dislocations piling up at internal boundaries between the soft iron matrix and the hard precipitates. The stress concentrations caused by the dislocation pile-ups might trigger interface decohesion followed by the formation of voids and, eventually, macroscopic cracks. To prevent such material failure, accurately predictive material models are needed. 


'Nano-scale failure in steel. Interface decohesion at iron/precipitate interfaces' Innovative Materials, Volume 1 2019, p 35. (Pdf file)

Dr.ir. Astrid Elzas studied together with Prof.dr. Barend Thijsse crack nucleation and interface decohesion on the nanoscale with large-scale molecular dynamics simulations, to understand which conditions lead to interface decohesion. Systematically varying different physical parameters allowed clear observation of the important physical effects in a controlled manner. Not only where the studied interfaces and the numbers of dislocations piling-up at the interfaces varied, also different loading modes where considered. Apart from pure shear and pure tensile loading, in this study also mixed loading, i.e. a tensile force under an angle with the interface, was considered. It is found that the interface structure, which changes during response to loading, is the key factor determining the material response.

Apart from the deeper and systematic understanding of interface decohesion resulting from this study, also interface/material-specific traction-separation relationships are developed which can be applied in larger scale material models to  improve the accuracy of this models with respect to the description of interface behaviour and with that lead to a better prediction of material failure.


Figure 1: Molecular dynamics simulation of interface decohesion at an interface between the soft iron matrix (red) and a hard precipitate (blue) of steel.



Figure 2: Schematic representation of various interfaces subjected to different loading modes, due to their orientation. Various relations between tractions and separations are found for the different interfaces and different loading modes.

On 10 January 2019 Astrid Elzas obtained her Doctorate cum laude for her thesis “Nano-scale failure in steel” at Delft University of Technology.

The thesis can be found at: https://doi.org/10.4233/uuid:f72f61f4-4508-4552-85b8-d89abbbee90e

Source: TU Delft

Dissertations

What really happens inside railways....

Jun Wu (TU Delft)

Each year, ProRail has to spend several million euros on periodic and ad-hoc maintenance work on railway tracks, in order to remove fatigue cracks before they lead to catastrophic rail fracture. One of the widely considered causes for the fatigue damage, which in railways applications occurs as Rolling Contact Fatigue (RCF), lies in the changes in the internal structure of the steel that are induced during the passage of trains: so-called white etching layers (WEL) form. The extremely high hardness of the WELs is considered to be the origin of its susceptibility to fracture and crack formation during the subsequent loading by passing train wheels. Until now, the prevalent models used to describe the RCF process in rails fail to predict the occurrence of the WEL, due to the fact that the origin and character of the WEL remained debatable.

'What really happens inside railways...' Innovative Materials, Volume 2 2019, p. 35. (Pdf file)


Railway steel at different length scales. From left to right: (a) macroscopic view of the rail/wheel contact at the metre scale; (b) signs of WEL formation at the rail surface, seen at the millimetre scale; (c) microscopic cross-section of railway steel, where the hard (820 HV Vickers hardness) WEL, as well as a fatigue crack, is visible at the top at the micrometre scale; (d) nano-twins and cementite traces in the WEL’s martensite at the nanometre scale; (e) distribution of alloying elements C, Si and Mn at the nanometre scale, with the WEL in the top three pictures and the original steel in the bottom three.

Dr. Jun Wu explores, together with prof.dr.ir. Jilt Sietsma and prof.dr.ir. Roumen Petrov, the root cause of the WEL formation via a series of microstructure characterizations, laboratory simulations, and theoretical modelling. The microstructural study, combining a wide variety of modern microscopy techniques, leads to the unambiguous conclusion that the WEL forms due to the significant heat generated during the wheel passages. The microstructure changes show that underneath train wheels the temperature repeatedly increases to over 700°C. The WEL structure was successfully reproduced under well-controlled laboratory conditions mimicking this rapid temperature rise followed by fast cooling. Furthermore, the characteristics of the WEL formation process were studied by phase field modelling. The insight provided by the thesis work of Jun Wu provides important guidance for the future design of new rail steels with higher resistance against WEL formation.

The Ph.D. research work of Jun Wu was performed under the joint supervision of Delft University of Technology and Ghent University. Jun Wu succeeded in defending his thesis first at Ghent University on 7 November 2018 and later, on 17 December 2018, at Delft University of Technology. This entitles him a joint degree from both universities. The title of his thesis is ‘Microstructure Evolution in Pearlitic Rail Steel due to Rail/Wheel Interaction’.

 The thesis can be found at: https://doi.org/10.4233/uuid:c536ca47-8981-4a9e-916f-396bcbca4bc5


Nano-scale failure in steel

Astrid Elzas (TU Delft)

Multiphase alloys such as advanced high strength steels can show unexpected failure due to dislocations piling up at internal boundaries between the soft iron matrix and the hard precipitates. The stress concentrations caused by the dislocation pile-ups might trigger interface decohesion followed by the formation of voids and, eventually, macroscopic cracks. To prevent such material failure, accurately predictive material models are needed. 


'Nano-scale failure in steel. Interface decohesion at iron/precipitate interfaces' Innovative Materials, Volume 1 2019, p 35. (Pdf file)

Dr.ir. Astrid Elzas studied together with Prof.dr. Barend Thijsse crack nucleation and interface decohesion on the nanoscale with large-scale molecular dynamics simulations, to understand which conditions lead to interface decohesion. Systematically varying different physical parameters allowed clear observation of the important physical effects in a controlled manner. Not only where the studied interfaces and the numbers of dislocations piling-up at the interfaces varied, also different loading modes where considered. Apart from pure shear and pure tensile loading, in this study also mixed loading, i.e. a tensile force under an angle with the interface, was considered. It is found that the interface structure, which changes during response to loading, is the key factor determining the material response.

Apart from the deeper and systematic understanding of interface decohesion resulting from this study, also interface/material-specific traction-separation relationships are developed which can be applied in larger scale material models to  improve the accuracy of this models with respect to the description of interface behaviour and with that lead to a better prediction of material failure.


Figure 1: Molecular dynamics simulation of interface decohesion at an interface between the soft iron matrix (red) and a hard precipitate (blue) of steel.



Figure 2: Schematic representation of various interfaces subjected to different loading modes, due to their orientation. Various relations between tractions and separations are found for the different interfaces and different loading modes.

On 10 January 2019 Astrid Elzas obtained her Doctorate cum laude for her thesis “Nano-scale failure in steel” at Delft University of Technology.

The thesis can be found at: https://doi.org/10.4233/uuid:f72f61f4-4508-4552-85b8-d89abbbee90e

Source: TU Delft

What really happens inside railways....

Jun Wu (TU Delft)

Each year, ProRail has to spend several million euros on periodic and ad-hoc maintenance work on railway tracks, in order to remove fatigue cracks before they lead to catastrophic rail fracture. One of the widely considered causes for the fatigue damage, which in railways applications occurs as Rolling Contact Fatigue (RCF), lies in the changes in the internal structure of the steel that are induced during the passage of trains: so-called white etching layers (WEL) form. The extremely high hardness of the WELs is considered to be the origin of its susceptibility to fracture and crack formation during the subsequent loading by passing train wheels. Until now, the prevalent models used to describe the RCF process in rails fail to predict the occurrence of the WEL, due to the fact that the origin and character of the WEL remained debatable.

'What really happens inside railways...' Innovative Materials, Volume 2 2019, p. 35. (Pdf file)


Railway steel at different length scales. From left to right: (a) macroscopic view of the rail/wheel contact at the metre scale; (b) signs of WEL formation at the rail surface, seen at the millimetre scale; (c) microscopic cross-section of railway steel, where the hard (820 HV Vickers hardness) WEL, as well as a fatigue crack, is visible at the top at the micrometre scale; (d) nano-twins and cementite traces in the WEL’s martensite at the nanometre scale; (e) distribution of alloying elements C, Si and Mn at the nanometre scale, with the WEL in the top three pictures and the original steel in the bottom three.

Dr. Jun Wu explores, together with prof.dr.ir. Jilt Sietsma and prof.dr.ir. Roumen Petrov, the root cause of the WEL formation via a series of microstructure characterizations, laboratory simulations, and theoretical modelling. The microstructural study, combining a wide variety of modern microscopy techniques, leads to the unambiguous conclusion that the WEL forms due to the significant heat generated during the wheel passages. The microstructure changes show that underneath train wheels the temperature repeatedly increases to over 700°C. The WEL structure was successfully reproduced under well-controlled laboratory conditions mimicking this rapid temperature rise followed by fast cooling. Furthermore, the characteristics of the WEL formation process were studied by phase field modelling. The insight provided by the thesis work of Jun Wu provides important guidance for the future design of new rail steels with higher resistance against WEL formation.

The Ph.D. research work of Jun Wu was performed under the joint supervision of Delft University of Technology and Ghent University. Jun Wu succeeded in defending his thesis first at Ghent University on 7 November 2018 and later, on 17 December 2018, at Delft University of Technology. This entitles him a joint degree from both universities. The title of his thesis is ‘Microstructure Evolution in Pearlitic Rail Steel due to Rail/Wheel Interaction’.

 The thesis can be found at: https://doi.org/10.4233/uuid:c536ca47-8981-4a9e-916f-396bcbca4bc5


Nano-scale failure in steel

Astrid Elzas (TU Delft)

Multiphase alloys such as advanced high strength steels can show unexpected failure due to dislocations piling up at internal boundaries between the soft iron matrix and the hard precipitates. The stress concentrations caused by the dislocation pile-ups might trigger interface decohesion followed by the formation of voids and, eventually, macroscopic cracks. To prevent such material failure, accurately predictive material models are needed. 


'Nano-scale failure in steel. Interface decohesion at iron/precipitate interfaces' Innovative Materials, Volume 1 2019, p 35. (Pdf file)

Dr.ir. Astrid Elzas studied together with Prof.dr. Barend Thijsse crack nucleation and interface decohesion on the nanoscale with large-scale molecular dynamics simulations, to understand which conditions lead to interface decohesion. Systematically varying different physical parameters allowed clear observation of the important physical effects in a controlled manner. Not only where the studied interfaces and the numbers of dislocations piling-up at the interfaces varied, also different loading modes where considered. Apart from pure shear and pure tensile loading, in this study also mixed loading, i.e. a tensile force under an angle with the interface, was considered. It is found that the interface structure, which changes during response to loading, is the key factor determining the material response.

Apart from the deeper and systematic understanding of interface decohesion resulting from this study, also interface/material-specific traction-separation relationships are developed which can be applied in larger scale material models to  improve the accuracy of this models with respect to the description of interface behaviour and with that lead to a better prediction of material failure.


Figure 1: Molecular dynamics simulation of interface decohesion at an interface between the soft iron matrix (red) and a hard precipitate (blue) of steel.



Figure 2: Schematic representation of various interfaces subjected to different loading modes, due to their orientation. Various relations between tractions and separations are found for the different interfaces and different loading modes.

On 10 January 2019 Astrid Elzas obtained her Doctorate cum laude for her thesis “Nano-scale failure in steel” at Delft University of Technology.

The thesis can be found at: https://doi.org/10.4233/uuid:f72f61f4-4508-4552-85b8-d89abbbee90e

Source: TU Delft