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Title: Cold dwell facet nucleation in titanium alloy aero-engine components
Author: Cuddihy, Mitchell
ISNI:       0000 0004 7656 8823
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Cold dwell fatigue is a failure mode in titanium alloys, manifesting in significantly reduced fatigue life when load is held for a period of time (< 2 minutes), as opposed to continuously cycled, at ambient temperatures (-40 to 200°C). This reduced fatigue life is often characterised by the ratio of lives compared with an equivalent low-cycle fatigue test. The aims of this thesis is broadly to contribute to the fundamental understanding of the problem of cold dwell fatigue. In particular, this work will largely focus on facet nucleation and the local stress state ahead of an in-situ crack in high performance alloys. We firstly introduce and develop the micromechanics theory used to formulate the crystal plasticity framework used throughout this thesis. Implementation and calibration of the crystal plasticity model against experimentally obtained single crystal and polycrystal data, incorporating key factors such as rate sensitivity and length scale effects, has been carried out. The bulk of the facet nucleation studies deal with three distinct but interrelated topics in dwell fatigue; grain size effects, load excursions, and multiaxial loading, each of which provide insight for many topics currently under investigation in both laboratory and industrial settings. Finally a fracture mechanics based study of the stress state near an in-situ crack in single and bicrystal samples is carried out. This investigation examines in particular the effects of grain orientation and morphology on local stress state and stress intensity through incorporation of full elastic anisotropy. This study has direct relevance for the understanding of microstructurally sensitive short crack growth in hcp alloy systems, as well as cold dwell fatigue.
Supervisor: Dunne, Fionn ; Dye, David Sponsor: Rolls-Royce plc ; Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral