A multi-scale approach to the development of high-rate-based microstructure-aware constitutive models for magnesium alloys

Thesis

Computational tools are at the forefront of modern day engineering as they offer low cost, timely, and informed approaches to product design, with finite element analysis packages being particularly favoured in industry for predicting the behaviour of physical systems. When high-rate structural components are designed using such approaches, it is typical to allocate constitutive models that reflect experimentally measured material behaviour to the parts being analysed. A common shortcoming of this approach, especially in industrial applications, is the phenomenological nature of the models, which can only make predictions of bulk-level mechanical behaviour, and do not offer insight into the mechanisms driving material behaviour.

Over the last few decades, numerous methodologies to enable the consideration of the lower length-scale phenomena driving the bulk-level mechanical behaviour of polycrystalline-based metals have been proposed, which can be categorised into two branches, mean-field and full-field. As the mechanisms driving plasticity in such materials are found interior to a grain, but the orientation of a collection of grains are found to dictate anisotropic bulk-level mechanical behaviour, crystal plasticity-considered approaches have become popular in literature. Mean-field crystal plasticity-based models provide insight into the relative activity of available micro- and meso-scale deformation mechanisms, as well as their effect on homogenised bulk-level behaviour. Whereas, full-field approaches enable consideration of the heterogeneity of the stress field manifesting in the polycrystalline structure, and thus the relative activity of the deformation mechanisms in individual grains.

Presented in this thesis is a hierarchical full-field-considered approach to the development of high-rate-based microstructure-aware constitutive models for magnesium alloys. At the heart of the full-field polycrystalline-based simulations is a rate-dependent explicitly-run crystal plasticity-based user-subroutine that describes plasticity through both dislocation glide and homogenised twinning mechanisms. The reason for incorporating the user-subroutine into an explicit framework being that they are better suited for high-rate-based analyses. Two magnesium alloys, WE43-T6 plate and AZ31B-O sheet, were selected as model materials to present the developed capabilities as accurate predictions of their high-rate behaviour are difficult to make owing to the low symmetry possessed by the material system's unit cell, and the presence of textures. It was shown through an example where armour plates are blast-loaded, that by fully resolving micro- and meso-structural features, the potential to delve into the phenomena underpinning high-rate bulk-level mechanical behaviour becomes possible, thus offering the ability to design them to improve a material system for its component's specific application.

Authors: Wason, D

• DOI: 10.5287/ora-qmkq56byo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: magnesium alloys; crystal plasticity finite element modelling; multi-scale modelling; mesostructure; high-rate
• Subjects: Materials science; Structural engineering; Computational mechanics