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Ragab, Raheeg (2024) Computational modelling of creep and cyclic visco-plasticity damage of CSEF steels at elevated temperatures. PhD thesis, University of Nottingham.

PDF (PhD thesis with corrections following viva Exam) (Thesis for reader access - any sensitive & copyright infringing material removed) - Repository staff only until 18 July 2026. Subsequently available to Repository staff only - Requires a PDF viewer such as GSview, Xpdf or Adobe Acrobat Reader Available under Licence Creative Commons Attribution. Download (9MB)

Abstract

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.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Sun, Wei Liu, Tao Li, Ming Tizani, Walid
Keywords:	CSEF, Weld, Creep, Fatigue, CDM, Creep Ductility, Cyclic Softening, Ratcheting, Notch Constraint, Viscoplasticity, SPCT, Mechanistic Modelling
Subjects:	T Technology > TN Mining engineering. Metallurgy
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	77886
Depositing User:	Ragab, Raheeg
Date Deposited:	18 Jul 2024 04:40
Last Modified:	18 Jul 2024 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/77886

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