Discrete dislocation plasticity analysis of cyclic loading phenomena
File(s)
Author(s)
Waheed, Sana
Type
Thesis or dissertation
Abstract
Cyclic loading scenarios often arise in real-world situations. In this thesis, planar discrete dislocation plasticity simulations are utilised to provide new insights into the micromechanics of two particular cyclic deformation phenomena: the Bauschinger effect in small-scale confined films and dwell fatigue failure in large aero-engine components. The 2D discrete dislocation
plasticity (DDP) model incorporates material heterogeneity and dislocation penetration across interfaces with capabilities to mesh and simulate complex specimens efficiently and accurately.
For the Bauschinger effect in small-scale plasticity, an encapsulated film micropillar
geometry is used to investigate geometric and temperature effects on cyclic response. A unifying scaling relationship is introduced which, for the first time, incorporates effects of each of source density, dipole nucleation strength, film orientation, film thickness and plastic pre-strain before unloading on the size of the Bauschinger effect. Next, temperature-dependent plastic hysteresis in BCC niobium is shown to be arising from changes in dislocation mobility rather than changes in the extent of dislocation penetration
across grain boundaries.
Finally, microstructural effects on dwell-sensitivity in dual-phase titanium alloys are investigated. Corroborating evidence in the literature, it is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity. The mechanistic cause of these effects is argued
to be changes in dislocation mean free-path and the total amount of slip in the specimen. Comparing DDP results with corresponding crystal plasticity finite element (CPFE) simulations, it is shown that discrete aspects
of slip and hardening mechanisms have to be accounted for to capture experimentally observed material rate sensitivity.
Overall, this thesis provides increased understanding of dislocation mechanisms
operating during the chosen cyclic loading phenomena, highlighting the importance of considering micromechanics for an analysis of both small- and large-scale deformation.
plasticity (DDP) model incorporates material heterogeneity and dislocation penetration across interfaces with capabilities to mesh and simulate complex specimens efficiently and accurately.
For the Bauschinger effect in small-scale plasticity, an encapsulated film micropillar
geometry is used to investigate geometric and temperature effects on cyclic response. A unifying scaling relationship is introduced which, for the first time, incorporates effects of each of source density, dipole nucleation strength, film orientation, film thickness and plastic pre-strain before unloading on the size of the Bauschinger effect. Next, temperature-dependent plastic hysteresis in BCC niobium is shown to be arising from changes in dislocation mobility rather than changes in the extent of dislocation penetration
across grain boundaries.
Finally, microstructural effects on dwell-sensitivity in dual-phase titanium alloys are investigated. Corroborating evidence in the literature, it is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity. The mechanistic cause of these effects is argued
to be changes in dislocation mean free-path and the total amount of slip in the specimen. Comparing DDP results with corresponding crystal plasticity finite element (CPFE) simulations, it is shown that discrete aspects
of slip and hardening mechanisms have to be accounted for to capture experimentally observed material rate sensitivity.
Overall, this thesis provides increased understanding of dislocation mechanisms
operating during the chosen cyclic loading phenomena, highlighting the importance of considering micromechanics for an analysis of both small- and large-scale deformation.
Version
Open Access
Date Issued
2018-07
Online Publication Date
2019-01-17T08:37:36Z
Date Awarded
2018-11
Advisor
Balint, Daniel S.
Giuliani, Finn
Sponsor
Imperial College London
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)