Dissertations

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

Hierarchical behaviour and failure mechanisms in automotive steel grades

Behnam Shakerifard (TU Delft)

Sheet Metal Forming is an important requirement for metal sheet to be employed in automotive applications. Many experiments and simulations are performed in order to predict the forming limits of steel sheets, which are determined either by observation of the sheets’ failure or deformation localization during various forming processes. In order to enhance the formability at the macroscopic level, a deep understanding of micro-mechanisms of failure is necessary as well as the hierarchical connection through various length scales.

'Hierarchical behaviour and failure mechanismes in automotive steel grades'
Innovative Materials, Volume 4 2019, p. 21 (Pdf file)


Figure: An example of damage mechanisms at different length scales which lead toglobal failure: (a) a high strain rate (687s-1) tensile deformation of a bainitic steel loaded along the Rolling Direction (RD), (b) contiguous collection of ~1000 SEM micrographs of the fracture sample in (a), (c) the corresponding image processed version of (b) with clear distribution of voids and (d) etched microstructure of the fractured bainitic multiphase steel.

In the current research work, dr.ir. Behnam Shakerifard, under supervision of dr.ir. Jesus Galan Lopez and prof. dr. Leo Kestens, investigates damage initiation micro-mechanisms under static and dynamic loading by advanced experimental characterization techniques and crystal plasticity based modelling. This research is conducted on the 3rd generation of advanced high strength steels, which are promising candidates for the production of various components of the car Body-In-White.

The topology of 2nd phase constituents at micro and meso-scale have different local and global impact in bainitic multiphase steels. It is shown that earlier damage initiation and higher volume fraction of voids do not essentially lead to earlier macroscopic failure. Moreover, crystallographic orientations susceptible to damage initiation are identified by crystal plasticity modelling and experimentally validated by scanning electron microscopy observations. 

Bainitic multiphase steels exhibit a positive strain rate sensitivity, which is desirable for crash worthiness and for improving the forming behaviour. It is shown that any microstructural strengthening mechanisms in steels can decrease the strain rate sensitivity.

The PhD research work of Behnam Shakerifard was carried out at Delft University of Technology in collaboration with Ghent University. Behnam Shakerifard successfully defended his PhD thesis on 24th of June 2019. The title of his dissertation is ‘From Micro-mechanisms of Damage Initiation to Constitutive Mechanical Behaviour of Bainitic Multiphase steels’.

The thesis can be found at:
https://repository.tudelft.nl/islandora/object/uuid%3A230fffcb-b313-4f9f-bec0-e619bc62b0d4

Harvesting Energy from your Windows

Yuan Gao (TU Delft)

Globally, more than a third of energy consumption is attributable to the building sector. Reducing the consumption of building energy generated from fossil fuels helps alleviate the air pollution and global warming. Some European countries regulate that all new buildings shall be nearly zero energy buildings (ZEBs) by the end of 2020. Apparently, local energy harvesting is required to realize this goal. Among all realistic strategies, building-integrated photovoltaic (BIPV) is an obvious choice for those regions with adequate solar radiation. In congested urban areas, high-rise modern buildings possess more potential for harvesting solar energy around the window areas than roofs. Therefore, innovative design is required from the cell level to the system level regarding window-integrated photovoltaics.

'Harvesting Energy from your Windows'
Innovative Materials, Volume 4 2019, p. 25 (Pdf file)


Figures (a) - (c): (a) A demonstration house model of STPV-PDLC system. The window dimensions are around 10 * 5 cm. (b) The STPV-PDLC window reveals opaque in the “off” state when no voltage is applied to the PDLC film. (c) The STPV-PDLC window turns semi-transparent when an AC voltage is applied to the PDLC film.

Dr. Yuan Gao proposed, together with prof. Miro Zeman, prof. Kouchi Zhang, dr. Olindo Isabella and dr. Jianfei Dong, an innovative approach to collecting solar energy from window areas in buildings and meanwhile creating comfortable daylighting for human eyes. This idea is realized by designing and fabricating a semi-transparent photovoltaic (STPV) glazing window combined with a polymer-dispersed liquid-crystal (PDLC) film. The semi-transparent amorphous silicon solar cell shows an average transmittance of 20.04% with a power conversion efficiency (PCE) of 6.94%. According to the climate data of Delft, such a PCE is sufficient to power an equal-area PDLC film, which can switch from an opaque to a transparent state in a second by applying an alternating-current (AC) voltage. The prototype of a house model, containing the STPV-PDLC system, has been built to demonstrate the feasibility of such a combination.

The PhD research work of Yuan Gao was performed under the joint supervision of Delft University of Technology in the Netherlands and State Key Laboratory of Solid State Lighting in China. Yuan Gao succeeded in defending his thesis on 25 June 2019, at Delft University of Technology. The title of his thesis is ‘Photovoltaic Windows: Theories, Devices and Applications’.

The thesis can be found at:
https://doi.org/10.4233/uuid:7aa8438c-6106-4c0f-a33f-0ceb8782ad23

More: TU Delft website

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)

Figures (a) - (e): 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

TU Delft website:
First joint PhD degree Materials Science TU Delft and Ghent University

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

Hierarchical behaviour and failure mechanisms in automotive steel grades

Behnam Shakerifard (TU Delft)

Sheet Metal Forming is an important requirement for metal sheet to be employed in automotive applications. Many experiments and simulations are performed in order to predict the forming limits of steel sheets, which are determined either by observation of the sheets’ failure or deformation localization during various forming processes. In order to enhance the formability at the macroscopic level, a deep understanding of micro-mechanisms of failure is necessary as well as the hierarchical connection through various length scales.

'Hierarchical behaviour and failure mechanismes in automotive steel grades'
Innovative Materials, Volume 4 2019, p. 21 (Pdf file)


Figure: An example of damage mechanisms at different length scales which lead toglobal failure: (a) a high strain rate (687s-1) tensile deformation of a bainitic steel loaded along the Rolling Direction (RD), (b) contiguous collection of ~1000 SEM micrographs of the fracture sample in (a), (c) the corresponding image processed version of (b) with clear distribution of voids and (d) etched microstructure of the fractured bainitic multiphase steel.

In the current research work, dr.ir. Behnam Shakerifard, under supervision of dr.ir. Jesus Galan Lopez and prof. dr. Leo Kestens, investigates damage initiation micro-mechanisms under static and dynamic loading by advanced experimental characterization techniques and crystal plasticity based modelling. This research is conducted on the 3rd generation of advanced high strength steels, which are promising candidates for the production of various components of the car Body-In-White.

The topology of 2nd phase constituents at micro and meso-scale have different local and global impact in bainitic multiphase steels. It is shown that earlier damage initiation and higher volume fraction of voids do not essentially lead to earlier macroscopic failure. Moreover, crystallographic orientations susceptible to damage initiation are identified by crystal plasticity modelling and experimentally validated by scanning electron microscopy observations. 

Bainitic multiphase steels exhibit a positive strain rate sensitivity, which is desirable for crash worthiness and for improving the forming behaviour. It is shown that any microstructural strengthening mechanisms in steels can decrease the strain rate sensitivity.

The PhD research work of Behnam Shakerifard was carried out at Delft University of Technology in collaboration with Ghent University. Behnam Shakerifard successfully defended his PhD thesis on 24th of June 2019. The title of his dissertation is ‘From Micro-mechanisms of Damage Initiation to Constitutive Mechanical Behaviour of Bainitic Multiphase steels’.

The thesis can be found at:
https://repository.tudelft.nl/islandora/object/uuid%3A230fffcb-b313-4f9f-bec0-e619bc62b0d4

Harvesting Energy from your Windows

Yuan Gao (TU Delft)

Globally, more than a third of energy consumption is attributable to the building sector. Reducing the consumption of building energy generated from fossil fuels helps alleviate the air pollution and global warming. Some European countries regulate that all new buildings shall be nearly zero energy buildings (ZEBs) by the end of 2020. Apparently, local energy harvesting is required to realize this goal. Among all realistic strategies, building-integrated photovoltaic (BIPV) is an obvious choice for those regions with adequate solar radiation. In congested urban areas, high-rise modern buildings possess more potential for harvesting solar energy around the window areas than roofs. Therefore, innovative design is required from the cell level to the system level regarding window-integrated photovoltaics.

'Harvesting Energy from your Windows'
Innovative Materials, Volume 4 2019, p. 25 (Pdf file)


Figures (a) - (c): (a) A demonstration house model of STPV-PDLC system. The window dimensions are around 10 * 5 cm. (b) The STPV-PDLC window reveals opaque in the “off” state when no voltage is applied to the PDLC film. (c) The STPV-PDLC window turns semi-transparent when an AC voltage is applied to the PDLC film.

Dr. Yuan Gao proposed, together with prof. Miro Zeman, prof. Kouchi Zhang, dr. Olindo Isabella and dr. Jianfei Dong, an innovative approach to collecting solar energy from window areas in buildings and meanwhile creating comfortable daylighting for human eyes. This idea is realized by designing and fabricating a semi-transparent photovoltaic (STPV) glazing window combined with a polymer-dispersed liquid-crystal (PDLC) film. The semi-transparent amorphous silicon solar cell shows an average transmittance of 20.04% with a power conversion efficiency (PCE) of 6.94%. According to the climate data of Delft, such a PCE is sufficient to power an equal-area PDLC film, which can switch from an opaque to a transparent state in a second by applying an alternating-current (AC) voltage. The prototype of a house model, containing the STPV-PDLC system, has been built to demonstrate the feasibility of such a combination.

The PhD research work of Yuan Gao was performed under the joint supervision of Delft University of Technology in the Netherlands and State Key Laboratory of Solid State Lighting in China. Yuan Gao succeeded in defending his thesis on 25 June 2019, at Delft University of Technology. The title of his thesis is ‘Photovoltaic Windows: Theories, Devices and Applications’.

The thesis can be found at:
https://doi.org/10.4233/uuid:7aa8438c-6106-4c0f-a33f-0ceb8782ad23

More: TU Delft website

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)

Figures (a) - (e): 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

TU Delft website:
First joint PhD degree Materials Science TU Delft and Ghent University

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

Hierarchical behaviour and failure mechanisms in automotive steel grades

Behnam Shakerifard (TU Delft)

Sheet Metal Forming is an important requirement for metal sheet to be employed in automotive applications. Many experiments and simulations are performed in order to predict the forming limits of steel sheets, which are determined either by observation of the sheets’ failure or deformation localization during various forming processes. In order to enhance the formability at the macroscopic level, a deep understanding of micro-mechanisms of failure is necessary as well as the hierarchical connection through various length scales.

'Hierarchical behaviour and failure mechanismes in automotive steel grades'
Innovative Materials, Volume 4 2019, p. 21 (Pdf file)


Figure: An example of damage mechanisms at different length scales which lead toglobal failure: (a) a high strain rate (687s-1) tensile deformation of a bainitic steel loaded along the Rolling Direction (RD), (b) contiguous collection of ~1000 SEM micrographs of the fracture sample in (a), (c) the corresponding image processed version of (b) with clear distribution of voids and (d) etched microstructure of the fractured bainitic multiphase steel.

In the current research work, dr.ir. Behnam Shakerifard, under supervision of dr.ir. Jesus Galan Lopez and prof. dr. Leo Kestens, investigates damage initiation micro-mechanisms under static and dynamic loading by advanced experimental characterization techniques and crystal plasticity based modelling. This research is conducted on the 3rd generation of advanced high strength steels, which are promising candidates for the production of various components of the car Body-In-White.

The topology of 2nd phase constituents at micro and meso-scale have different local and global impact in bainitic multiphase steels. It is shown that earlier damage initiation and higher volume fraction of voids do not essentially lead to earlier macroscopic failure. Moreover, crystallographic orientations susceptible to damage initiation are identified by crystal plasticity modelling and experimentally validated by scanning electron microscopy observations. 

Bainitic multiphase steels exhibit a positive strain rate sensitivity, which is desirable for crash worthiness and for improving the forming behaviour. It is shown that any microstructural strengthening mechanisms in steels can decrease the strain rate sensitivity.

The PhD research work of Behnam Shakerifard was carried out at Delft University of Technology in collaboration with Ghent University. Behnam Shakerifard successfully defended his PhD thesis on 24th of June 2019. The title of his dissertation is ‘From Micro-mechanisms of Damage Initiation to Constitutive Mechanical Behaviour of Bainitic Multiphase steels’.

The thesis can be found at:
https://repository.tudelft.nl/islandora/object/uuid%3A230fffcb-b313-4f9f-bec0-e619bc62b0d4

Harvesting Energy from your Windows

Yuan Gao (TU Delft)

Globally, more than a third of energy consumption is attributable to the building sector. Reducing the consumption of building energy generated from fossil fuels helps alleviate the air pollution and global warming. Some European countries regulate that all new buildings shall be nearly zero energy buildings (ZEBs) by the end of 2020. Apparently, local energy harvesting is required to realize this goal. Among all realistic strategies, building-integrated photovoltaic (BIPV) is an obvious choice for those regions with adequate solar radiation. In congested urban areas, high-rise modern buildings possess more potential for harvesting solar energy around the window areas than roofs. Therefore, innovative design is required from the cell level to the system level regarding window-integrated photovoltaics.

'Harvesting Energy from your Windows'
Innovative Materials, Volume 4 2019, p. 25 (Pdf file)


Figures (a) - (c): (a) A demonstration house model of STPV-PDLC system. The window dimensions are around 10 * 5 cm. (b) The STPV-PDLC window reveals opaque in the “off” state when no voltage is applied to the PDLC film. (c) The STPV-PDLC window turns semi-transparent when an AC voltage is applied to the PDLC film.

Dr. Yuan Gao proposed, together with prof. Miro Zeman, prof. Kouchi Zhang, dr. Olindo Isabella and dr. Jianfei Dong, an innovative approach to collecting solar energy from window areas in buildings and meanwhile creating comfortable daylighting for human eyes. This idea is realized by designing and fabricating a semi-transparent photovoltaic (STPV) glazing window combined with a polymer-dispersed liquid-crystal (PDLC) film. The semi-transparent amorphous silicon solar cell shows an average transmittance of 20.04% with a power conversion efficiency (PCE) of 6.94%. According to the climate data of Delft, such a PCE is sufficient to power an equal-area PDLC film, which can switch from an opaque to a transparent state in a second by applying an alternating-current (AC) voltage. The prototype of a house model, containing the STPV-PDLC system, has been built to demonstrate the feasibility of such a combination.

The PhD research work of Yuan Gao was performed under the joint supervision of Delft University of Technology in the Netherlands and State Key Laboratory of Solid State Lighting in China. Yuan Gao succeeded in defending his thesis on 25 June 2019, at Delft University of Technology. The title of his thesis is ‘Photovoltaic Windows: Theories, Devices and Applications’.

The thesis can be found at:
https://doi.org/10.4233/uuid:7aa8438c-6106-4c0f-a33f-0ceb8782ad23

More: TU Delft website

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)

Figures (a) - (e): 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

TU Delft website:
First joint PhD degree Materials Science TU Delft and Ghent University

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

Hierarchical behaviour and failure mechanisms in automotive steel grades

Behnam Shakerifard (TU Delft)

Sheet Metal Forming is an important requirement for metal sheet to be employed in automotive applications. Many experiments and simulations are performed in order to predict the forming limits of steel sheets, which are determined either by observation of the sheets’ failure or deformation localization during various forming processes. In order to enhance the formability at the macroscopic level, a deep understanding of micro-mechanisms of failure is necessary as well as the hierarchical connection through various length scales.

'Hierarchical behaviour and failure mechanismes in automotive steel grades'
Innovative Materials, Volume 4 2019, p. 21 (Pdf file)


Figure: An example of damage mechanisms at different length scales which lead toglobal failure: (a) a high strain rate (687s-1) tensile deformation of a bainitic steel loaded along the Rolling Direction (RD), (b) contiguous collection of ~1000 SEM micrographs of the fracture sample in (a), (c) the corresponding image processed version of (b) with clear distribution of voids and (d) etched microstructure of the fractured bainitic multiphase steel.

In the current research work, dr.ir. Behnam Shakerifard, under supervision of dr.ir. Jesus Galan Lopez and prof. dr. Leo Kestens, investigates damage initiation micro-mechanisms under static and dynamic loading by advanced experimental characterization techniques and crystal plasticity based modelling. This research is conducted on the 3rd generation of advanced high strength steels, which are promising candidates for the production of various components of the car Body-In-White.

The topology of 2nd phase constituents at micro and meso-scale have different local and global impact in bainitic multiphase steels. It is shown that earlier damage initiation and higher volume fraction of voids do not essentially lead to earlier macroscopic failure. Moreover, crystallographic orientations susceptible to damage initiation are identified by crystal plasticity modelling and experimentally validated by scanning electron microscopy observations. 

Bainitic multiphase steels exhibit a positive strain rate sensitivity, which is desirable for crash worthiness and for improving the forming behaviour. It is shown that any microstructural strengthening mechanisms in steels can decrease the strain rate sensitivity.

The PhD research work of Behnam Shakerifard was carried out at Delft University of Technology in collaboration with Ghent University. Behnam Shakerifard successfully defended his PhD thesis on 24th of June 2019. The title of his dissertation is ‘From Micro-mechanisms of Damage Initiation to Constitutive Mechanical Behaviour of Bainitic Multiphase steels’.

The thesis can be found at:
https://repository.tudelft.nl/islandora/object/uuid%3A230fffcb-b313-4f9f-bec0-e619bc62b0d4

Harvesting Energy from your Windows

Yuan Gao (TU Delft)

Globally, more than a third of energy consumption is attributable to the building sector. Reducing the consumption of building energy generated from fossil fuels helps alleviate the air pollution and global warming. Some European countries regulate that all new buildings shall be nearly zero energy buildings (ZEBs) by the end of 2020. Apparently, local energy harvesting is required to realize this goal. Among all realistic strategies, building-integrated photovoltaic (BIPV) is an obvious choice for those regions with adequate solar radiation. In congested urban areas, high-rise modern buildings possess more potential for harvesting solar energy around the window areas than roofs. Therefore, innovative design is required from the cell level to the system level regarding window-integrated photovoltaics.

'Harvesting Energy from your Windows'
Innovative Materials, Volume 4 2019, p. 25 (Pdf file)


Figures (a) - (c): (a) A demonstration house model of STPV-PDLC system. The window dimensions are around 10 * 5 cm. (b) The STPV-PDLC window reveals opaque in the “off” state when no voltage is applied to the PDLC film. (c) The STPV-PDLC window turns semi-transparent when an AC voltage is applied to the PDLC film.

Dr. Yuan Gao proposed, together with prof. Miro Zeman, prof. Kouchi Zhang, dr. Olindo Isabella and dr. Jianfei Dong, an innovative approach to collecting solar energy from window areas in buildings and meanwhile creating comfortable daylighting for human eyes. This idea is realized by designing and fabricating a semi-transparent photovoltaic (STPV) glazing window combined with a polymer-dispersed liquid-crystal (PDLC) film. The semi-transparent amorphous silicon solar cell shows an average transmittance of 20.04% with a power conversion efficiency (PCE) of 6.94%. According to the climate data of Delft, such a PCE is sufficient to power an equal-area PDLC film, which can switch from an opaque to a transparent state in a second by applying an alternating-current (AC) voltage. The prototype of a house model, containing the STPV-PDLC system, has been built to demonstrate the feasibility of such a combination.

The PhD research work of Yuan Gao was performed under the joint supervision of Delft University of Technology in the Netherlands and State Key Laboratory of Solid State Lighting in China. Yuan Gao succeeded in defending his thesis on 25 June 2019, at Delft University of Technology. The title of his thesis is ‘Photovoltaic Windows: Theories, Devices and Applications’.

The thesis can be found at:
https://doi.org/10.4233/uuid:7aa8438c-6106-4c0f-a33f-0ceb8782ad23

More: TU Delft website

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)

Figures (a) - (e): 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

TU Delft website:
First joint PhD degree Materials Science TU Delft and Ghent University

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