Anisotropic fracture toughness in graphene

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University of Delaware
Fracture is a significant problem in the industrialized world and a theoretical and practical basis for design against fracture is needed. In the real engineering world, presence of minute cracks or defects are unavoidable and inevitable at nano to sub-micron length scales. Also, anisotropy of materials at the nanoscale is ubiquitous; every material is anisotropic at the nanoscale. Theories describing the correlation between toughness and anisotropy are not well developed in the case of fracture and it is difficult to study anisotropy, specifically at the atomistic level. What is the role of anisotropy in fracture toughness and what are the complex mechanistic processes that take place during fracture? This thesis aims to answer these questions and provide fundamental insights into the fracture mechanisms at the nanoscale. ☐ This research delves into the fracture mechanics and crack propagation behavior of monolayer graphene in the atomistic scale using molecular dynamics simulations in LAMMPS, a classical molecular dynamics code. Graphene is selected due to its simplistic crystallographic structure, coupled with its two-dimensionality and its brittle fracture behavior during deformation. Uniaxial tension in the Y-direction is applied with a constant strain rate on the material to investigate the crack propagation, determine the stress-strain behavior of the material and understand the fracture toughness. An edge crack is created as a 'defect' within the material. By considering multiple chiral angles of graphene from 0 degrees (zigzag configuration) to 30 degrees (armchair configuration) and varying strain rates, the anisotropy of fracture toughness and crack energetics is investigated in this thesis research. ☐ For 1.0 10^{-5} ps^{-1} applied strain rate, the strength of graphene is found to have a 19.7% increase from 0 degrees to 30 degrees chiral angle, thereby having a chirality dependence. Similarly, the fracture toughness is found to have a 43.3% increase with change of chirality from zigzag to armchair. However, regardless of angle and strain rate, an existing crack always propagates at a zigzag direction which is at an angle from the remote loading. It is found that this angle plays an important role and governs the mechanisms that eventually affects the relationship between anisotropy and the toughness. For example, the change in the total energy in front of the crack tip is dependent on this direction angle of macroscopic loading. During propagation, two physical processes influence the chirality dependent behavior (bifurcation at the crack tip and self-healing of nucleated nanocracks). ☐ In conclusion, the key finding from this thesis is that toughness is anisotropic at the nanoscale and this anisotropy is governed by a complex mechanistic process. This understanding could be exploited to control the crack path in designing materials with improved toughness for applications in composites, electronics, semiconductors and photovoltaics.
Applied sciences, Anisotropy, Crack propagation, Fracture mechanics, Fracture toughness, Graphene, Toughness