Fatigue crack growth and localisation of strain in zircaloy-4
File(s)
Author(s)
Long, Daniel
Type
Thesis
Abstract
Short fatigue crack growth in engineering alloys is among the most prominent challenges in micromechanics. The multifarious nature of crack growth at microstructure length scales necessitates advanced modelling techniques to predict fatigue life accurately. A key contributor to this is crack path, which is also challenging to predict. To address these issues in the context of Zircaloy-4, this thesis presents (i) a critical appraisal of the stored energy density criterion within a crystal plasticity modelling framework for crack growth rate prediction, (ii) the development of new energy-based criteria for crack path, and (iii) a novel mechanistic analytical model for crack growth rate using a stored energy density fracture criterion. Subsequent chapters address environmental degradation issues and their impact on structural integrity, viz., new models for synergistic coupling of irradiation damage and thermomechanical loads, and measurement of fatigue crack growth and cyclic strain ratcheting effects in microstructures containing hydride precipitates. Stored energy density is shown to accurately capture major microstructure-driven differences in crack growth rate; direct comparisons of simulations with experiments enable estimation of the critical stored energy density for crack growth. This is later used to predict crack propagation rates via the new analytical model, which is shown to accurately capture experimental measurements while offering significant computer processing time reductions compared with state-of-the-art numerical methods. The synergistic coupling of irradiation damage and thermomechanical loads, compared with conventional post-irradiation testing, is shown to lead to localisation of quantities linked with crack nucleation including geometrically necessary dislocations and stress. Lastly, experimental measurements of fatigue cracking through hydrided microstructures reveal new crack propagation mechanisms, which on average, accelerate crack propagation rates. Measurements of static hydride precipitation strains demonstrate a discernible strain field directionality, while cyclic thermomechanical loads are shown to promote hydride strain ratcheting, leading to the development of geometrically necessary dislocation networks.
Version
Open Access
Date Issued
2024-01-29
Date Awarded
01/09/2024
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Dunne, Fionn
Pedrazzini, Stella
Sponsor
Rolls-Royce Group PLC (Firm)
Engineering and Physical Sciences Research Council
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)