Year

2015

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronics Engineering

Abstract

The study of material failure has always been very important for human beings. Previous studies were mostly conducted at the macroscopic and microscopic scale based on continuum mechanics. However, the effect of nanostructural features on fracture has not been fully understood. Therefore, it is necessary to investigate the fracture mechanics at the atomic scale. Computational modelling, particularly atomistic (or molecular) simulation is becoming an increasingly important technology with which to analyse fracture. In this thesis, molecular dynamics (MD) simulations were carried out to investigate the fracture behaviours in Face Centred Cubic (fcc) nanocrystals.

Nanotwinned Copper (Cu) has an unusual combination of ultra-high yield strength and high ductility, and in fact a brittle-to-ductile transition was previously observed in nanotwinned Cu despite Cu being an intrinsically ductile metal. However, the atomic mechanisms responsible for brittle fracture and ductile fracture in nanotwinned Cu are still not clear, so in this thesis, MD simulations at different temperatures were carried out to investigate the fracture of a specimen of nanotwinned Cu with a single-edge-notched crack whose surface coincides with a twin boundary. Three temperature ranges were identified as being indicative of the distinct fracture regimes under tensile straining that are perpendicular to the twin boundary. Below 1.1 K, the crack propagated in a brittle fashion, but between 2 K and 30 K a dynamic brittle-to-ductile transition occurred, and above 40 K the crack propagated in a ductile mode. A detailed analysis has been carried out to understand the atomic fracture mechanism in each fracture regime.

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