Discrete dislocation plasticity analysis of cyclic loading phenomena

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.