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Understanding microstructurally-sensitive fatigue crack nucleation in superalloys

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Title: Understanding microstructurally-sensitive fatigue crack nucleation in superalloys
Authors: Chen, Bo
Item Type: Thesis or dissertation
Abstract: This PhD project is concerned with fatigue crack nucleation in a range of superalloys, including single, oligo, and polycrystalline. An integrated experimental, characterisation and computational crystal plasticity study of cyclic plastic beam loading has been carried out on these alloys in order to assess quantitatively the mechanistic drivers for fatigue crack nucleation. The experimentally validated modelling provides knowledge of key microstructural quantities (accumulated slip, stress and GND density) at experimentally observed fatigue crack nucleation sites and it is shown that while each of these quantities is potentially important in crack nucleation, none of them in its own right is sufficient to be predictive. However, the local (elastic) stored energy density, measured over a length scale determined by the density of SSDs and GNDs, has been shown to predict crack nucleation sites in the mechanical tests. In addition, the stored energy is also able to correctly identify where secondary fatigue cracks are observed to nucleate in experiments. Further, experimental and crystal plasticity modelling studies have also been carried out to investigate non-proportionality and stress state effects in fatigue in a 316 stainless steel and Ni-based alloy RR1000 which have been shown to have substantial effects on fatigue life. Stored energy density has provided a consistent and unifying explanation for the experimental observations of fatigue life in differing loading regimes (both proportional and non-proportional). A single fatigue property (the critical stored energy density, equating to new surface energy) determined from axial fatigue data alone has been shown to provide good qualitative and reasonable quantitative prediction of the experimental observations of the complex loading, providing a mechanistic explanation for the fatigue behaviour.
Content Version: Open Access
Issue Date: Sep-2018
Date Awarded: Jan-2019
URI: http://hdl.handle.net/10044/1/77799
DOI: https://doi.org/10.25560/77799
Copyright Statement: Creative Commons Attribution NonCommercial Licence
Supervisor: Dunne, Fionn
Britton, Ben
Sponsor/Funder: Imperial College London
China Scholarship Council
Department: Materials
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Materials PhD theses