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Title: Investigation of cold dwell facet fatigue in titanium alloys utilising crystal plasticity and discrete dislocation plasticity modelling techniques
Author: Zheng, Zebang
ISNI:       0000 0004 7228 7784
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The focus of this project is the mechanistic basis of the load shedding phenomenon that occurs under the dwell fatigue loading scenario. A systematic study was carried out using modelling techniques at different length scales, from atomistic simulations to discrete dislocation plasticity and crystal plasticity, to investigate the effect of crystallographic orientations, localized dislocation behaviour, material intrinsic properties and external loading environments on the dwell sensitivity. A new mechanistic formalism for incorporating thermally activated dislocation escape into discrete dislocation plasticity modelling techniques is presented. The origin of the rate-sensitive behaviour of plasticity over strain rate regimes from 〖10〗^(-5) to 〖10〗^5 s^(-1) has been assessed with reference to three key mechanisms: dislocation nucleation, time of flight (dislocation mobility) and thermally activated escape of pinned dislocations. It is shown that nucleation and dislocation mobility explain rate-sensitive behaviour for strain rates in the range 〖10〗^2 to 〖10〗^5 s^(-1) while thermally-activated dislocation escape becomes the predominant rate-controlling mechanism at low strain rates. The new thermal activation DDP model was then use to investigate the soft-hard-soft rogue grain combination which is commonly associated with load shedding in Ti-6Al, Ti-6242 and Ti-6246 alloys. The application of Stroh’s dislocation pile-up model of crack nucleation to facet fracture was quantitatively assessed. Crystal plasticity modelling has been utilised to extract the thermal activation energies for pinned dislocation escape for Ti alloys based on independent experimental data. The activation energies determined are then utilised within the polycrystalline DDP model to predict the load shedding in the aforementioned alloys. The grain morphology and grain boundary penetrability effect were also studied but the key property controlling the load shedding is argued to be the time constant of the thermal activation process relative to that of the loading. The dwell sensitivity of Ti alloys was also found to be highly related to the temperature. The load shedding was found to diminish at very low or very high temperatures and maximum peak stress increase occurs at a higher temperature for a material with high activation energy. The deformation mechanisms under dwell fatigue loading are categorised into three scenarios and discussed separately.
Supervisor: Dunne, Fionn ; Balint, Daniel Sponsor: China Scholarship Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral