Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures
The change in the energy landscape has raised the need for new materials development and improved constitutive material modelling to support energy decarbonisation efforts. Among the emergent high-temperature materials, which have found increasing applications for boilers and turbine component...
| Main Author: | |
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| Format: | Thesis (University of Nottingham only) |
| Language: | English |
| Published: |
2024
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| Subjects: | |
| Online Access: | https://eprints.nottingham.ac.uk/77886/ |
| _version_ | 1848801031408844800 |
|---|---|
| author | Ragab, Raheeg |
| author_facet | Ragab, Raheeg |
| author_sort | Ragab, Raheeg |
| building | Nottingham Research Data Repository |
| collection | Online Access |
| description | The change in the energy landscape has raised the need for new materials development
and improved constitutive material modelling to support energy decarbonisation
efforts. Among the emergent high-temperature materials, which have found increasing
applications for boilers and turbine components in the power generation sector, are 9
12% Cr tempered martensitic Creep Strength Enhanced Ferritic (CSEF) steels. The
present thesis deals with the computational modelling of creep and cyclic visco
plasticity damage of CSEF steels (particularly Grade 91 and FV566 steels) under
elevated temperatures. One of the key challenges concerning the creep performance
of CSEF steels is related to the wide variability in the creep ductility of steel casts.
Nonetheless, consideration of creep ductility in creep damage modelling of CSEF
steels and the design of pressure parts operating at high temperatures has received little
attention thus far. To address this challenge, a phenomenological ductility-based
continuum damage mechanics model (CDM) was proposed in this study, which
captures the influence of material creep ductility on creep damage and rupture lives.
The model holds a key advantage over existing models in that it requires fewer
material constants to be identified and calibrated. The proposed model was
implemented into an ABAQUS user-defined subroutine to simulate the creep
deformation and damage behaviour of CSEF steel weldment and to predict creep crack
growth behaviour in Grade 91 vessels weldment. Uniaxial creep tests and feature-type
cross-weld creep tests were utilized to calibrate the proposed model and identify the
relevant creep damage properties for the weld constituents including the base metal
(BM), weld metal (WM) and heat-affected zone (HAZ). The capability of the model
was then examined through multi-axial notch bar creep tests and full-scale
components tests. The proposed model not only demonstrated good predictive
capabilities but also offered an improved understanding of creep failure behaviour in
Grade 91 welded joints. Based on the modelling results, the highly localised stresses
and stress triaxialities in the HAZ region of the weld were found to play a key role in
the occurrence of ‘Type IV’ failure in Grade 91 steel welded structures.
Since next-generation powerplants are expected to operate intermittently, a refined
understanding of the cyclic deformation and damage mechanisms of tempered
martensitic CSEF steels is crucial. One of the thesis's key aims is to examine important
aspects of the high-temperature cyclic visco-plasticity behaviour. Specifically, it
focuses on the ratcheting (cyclic creep) and constraint (multi-axiality) effects which
have not been extensively studied for 12% Cr CSEF steels. To achieve this aim, a
hybrid methodology is adopted comprising cyclic mechanical tests, microstructural
characterisation and physically based cyclic visco-plasticity damage modelling.
Within the experimental program, fully reversed, load-controlled uniaxial and multi
axial saw-tooth (SWT) low-cycle fatigue tests were carried out on martensitic steel
(FV566) at 600oC. The mechanical tests were complemented by detailed
microstructural characterisation of the tested samples to unveil the key mechanisms
contributing to the cyclic visco-plasticity damage in the FV566 steel at high
temperatures. Regarding the computational work, an improved microstructure
informed visco-plasticity modelling framework is introduced, which accounts for the
softening mechanisms due to dislocation annihilation and lath coarsening. The developed model was embedded in a user material subroutine in ABAQUS and
implemented to simulate the uniaxial and multi-axial ratcheting behaviour and the
associated microstructural evolutions. The VP model reasonably predicted the
ratcheting behaviour under load-controlled cycling. Moreover, under multi-axial stress
states, the model was able to predict crack initiation at the notched bar root. Based on
this investigation, the micromechanics origin of the softening was elucidated.
Additionally, the micro-damage modelling results in conjunction with the physical
characterisation offered an improved mechanistic understanding of notch constraint in
multi-axial LCF tests.
Accurate material properties determination is crucially important for reliable creep life
assessment of CSEF steels. However, this can be challenging for the heat-affected
zone of the weld, particularly when applying conventional (standard size) creep testing
methods due to the small volume of material available for sampling. To overcome this
challenge, small specimen creep testing techniques such as small punch creep tests
have been proposed as alternative means of creep properties characterization.
However, creep data conversion from such tests can be very complicated. This is
particularly true for the SPCT which exhibits several non-linear deformation
mechanisms such as friction, plasticity etc. As such, simplified empirical relations for
the analysis of the SPCTs and data interpretation are often employed. However, such
approaches lack theoretical underpinnings, thereby limiting the potential of the SPCT
as a standardised material characterisation method. To address this limitation, a novel
mechanistic-based model was proposed in this study for the first time, which describes
creep deformation and damage in the SPCT. The theoretical framework was
established based on the membrane stretching theory and continuum damage
mechanics-based constitutive model. The accuracy of the proposed analytical model
was verified using finite element analysis. The analytical solutions demonstrated
excellent capabilities and advantages over the existing models. Further, the potential
applications of the new model for creep properties determination of martensitic steels
from SPCT data were demonstrated. |
| first_indexed | 2025-11-14T21:00:59Z |
| format | Thesis (University of Nottingham only) |
| id | nottingham-77886 |
| institution | University of Nottingham Malaysia Campus |
| institution_category | Local University |
| language | English |
| last_indexed | 2025-11-14T21:00:59Z |
| publishDate | 2024 |
| recordtype | eprints |
| repository_type | Digital Repository |
| spelling | nottingham-778862025-02-28T15:20:30Z https://eprints.nottingham.ac.uk/77886/ Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures Ragab, Raheeg The change in the energy landscape has raised the need for new materials development and improved constitutive material modelling to support energy decarbonisation efforts. Among the emergent high-temperature materials, which have found increasing applications for boilers and turbine components in the power generation sector, are 9 12% Cr tempered martensitic Creep Strength Enhanced Ferritic (CSEF) steels. The present thesis deals with the computational modelling of creep and cyclic visco plasticity damage of CSEF steels (particularly Grade 91 and FV566 steels) under elevated temperatures. One of the key challenges concerning the creep performance of CSEF steels is related to the wide variability in the creep ductility of steel casts. Nonetheless, consideration of creep ductility in creep damage modelling of CSEF steels and the design of pressure parts operating at high temperatures has received little attention thus far. To address this challenge, a phenomenological ductility-based continuum damage mechanics model (CDM) was proposed in this study, which captures the influence of material creep ductility on creep damage and rupture lives. The model holds a key advantage over existing models in that it requires fewer material constants to be identified and calibrated. The proposed model was implemented into an ABAQUS user-defined subroutine to simulate the creep deformation and damage behaviour of CSEF steel weldment and to predict creep crack growth behaviour in Grade 91 vessels weldment. Uniaxial creep tests and feature-type cross-weld creep tests were utilized to calibrate the proposed model and identify the relevant creep damage properties for the weld constituents including the base metal (BM), weld metal (WM) and heat-affected zone (HAZ). The capability of the model was then examined through multi-axial notch bar creep tests and full-scale components tests. The proposed model not only demonstrated good predictive capabilities but also offered an improved understanding of creep failure behaviour in Grade 91 welded joints. Based on the modelling results, the highly localised stresses and stress triaxialities in the HAZ region of the weld were found to play a key role in the occurrence of ‘Type IV’ failure in Grade 91 steel welded structures. Since next-generation powerplants are expected to operate intermittently, a refined understanding of the cyclic deformation and damage mechanisms of tempered martensitic CSEF steels is crucial. One of the thesis's key aims is to examine important aspects of the high-temperature cyclic visco-plasticity behaviour. Specifically, it focuses on the ratcheting (cyclic creep) and constraint (multi-axiality) effects which have not been extensively studied for 12% Cr CSEF steels. To achieve this aim, a hybrid methodology is adopted comprising cyclic mechanical tests, microstructural characterisation and physically based cyclic visco-plasticity damage modelling. Within the experimental program, fully reversed, load-controlled uniaxial and multi axial saw-tooth (SWT) low-cycle fatigue tests were carried out on martensitic steel (FV566) at 600oC. The mechanical tests were complemented by detailed microstructural characterisation of the tested samples to unveil the key mechanisms contributing to the cyclic visco-plasticity damage in the FV566 steel at high temperatures. Regarding the computational work, an improved microstructure informed visco-plasticity modelling framework is introduced, which accounts for the softening mechanisms due to dislocation annihilation and lath coarsening. The developed model was embedded in a user material subroutine in ABAQUS and implemented to simulate the uniaxial and multi-axial ratcheting behaviour and the associated microstructural evolutions. The VP model reasonably predicted the ratcheting behaviour under load-controlled cycling. Moreover, under multi-axial stress states, the model was able to predict crack initiation at the notched bar root. Based on this investigation, the micromechanics origin of the softening was elucidated. Additionally, the micro-damage modelling results in conjunction with the physical characterisation offered an improved mechanistic understanding of notch constraint in multi-axial LCF tests. Accurate material properties determination is crucially important for reliable creep life assessment of CSEF steels. However, this can be challenging for the heat-affected zone of the weld, particularly when applying conventional (standard size) creep testing methods due to the small volume of material available for sampling. To overcome this challenge, small specimen creep testing techniques such as small punch creep tests have been proposed as alternative means of creep properties characterization. However, creep data conversion from such tests can be very complicated. This is particularly true for the SPCT which exhibits several non-linear deformation mechanisms such as friction, plasticity etc. As such, simplified empirical relations for the analysis of the SPCTs and data interpretation are often employed. However, such approaches lack theoretical underpinnings, thereby limiting the potential of the SPCT as a standardised material characterisation method. To address this limitation, a novel mechanistic-based model was proposed in this study for the first time, which describes creep deformation and damage in the SPCT. The theoretical framework was established based on the membrane stretching theory and continuum damage mechanics-based constitutive model. The accuracy of the proposed analytical model was verified using finite element analysis. The analytical solutions demonstrated excellent capabilities and advantages over the existing models. Further, the potential applications of the new model for creep properties determination of martensitic steels from SPCT data were demonstrated. 2024-07-18 Thesis (University of Nottingham only) NonPeerReviewed application/pdf en cc_by https://eprints.nottingham.ac.uk/77886/1/PhD%20Thesis%20%28with%20corrections%29_R%20Ragab.pdf Ragab, Raheeg (2024) Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures. PhD thesis, University of Nottingham. CSEF Weld Creep Fatigue CDM Creep Ductility Cyclic Softening Ratcheting Notch Constraint Viscoplasticity SPCT Mechanistic Modelling |
| spellingShingle | CSEF Weld Creep Fatigue CDM Creep Ductility Cyclic Softening Ratcheting Notch Constraint Viscoplasticity SPCT Mechanistic Modelling Ragab, Raheeg Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures |
| title | Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures |
| title_full | Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures |
| title_fullStr | Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures |
| title_full_unstemmed | Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures |
| title_short | Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures |
| title_sort | computational modelling of creep and cyclic visco-plasticity damage of csef steels at elevated temperatures |
| topic | CSEF Weld Creep Fatigue CDM Creep Ductility Cyclic Softening Ratcheting Notch Constraint Viscoplasticity SPCT Mechanistic Modelling |
| url | https://eprints.nottingham.ac.uk/77886/ |