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Title: Prediction of long-term static and cyclic creep rupture and crack growth of grade 92 steels under different stress states
Author: Alang, Nasrul Azuan
ISNI:       0000 0004 7223 6047
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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During long-term exposure in creep or creep-fatigue environment, the creep strength of 9-12%Cr steel drops significantly due to the damage developed from metallurgical and microstructural changes occurring in complex subgrain structures. As creep time increases, failure strain of the material is reduced and the reduction is further enhanced under multiaxial condition, where a localised damage zone formed thus leading to crack initiation and growth under increased constraint. The present work focuses on high-temperature creep and fatigue behaviour, and life prediction of Grade 92 steel. Long-term data from literature is utilised to establish creep constitutive properties and to develop the predictive model. The expected scatter in the data is accounted for and it is shown that the important upper and lower bound trend under short and long-term tests can be identified irrespective of the variability of the data. Modelling approach is first applied to the stress-based Continuum Damage Mechanics (CDM) model coupled with a concept of representative stress to predict notched bar failures. Exponential-type predictive model which consists of two material constants, compared to standard CDM model with six is proposed as an alternative to the prediction. Furthermore, a semi-empirical constraint-based approach to predict creep rupture and crack growth under simple and complex stress state condition is presented. The model relates the Monkman-Grant failure strain to the constraint which arises from the geometry and time-dependent creep damage process in microscopic level. The model is consistent with the NSW (Nikbin-Smith-Webster) crack growth model which predicts the crack initiation and growth under the range of plane stress and strain. Finally, finite element (FE) analysis is performed on plain and notched bar to examine the influence of stress state on creep rupture and damage behaviour, and to validate the proposed predictive approach. The model is sufficiently simple yet reliable in predicting long-term failures.
Supervisor: Nikbin, Kamran ; Davies, Catrin Sponsor: Kementerian Pengajian Tinggi (Malaysia)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral