This paper presents a new framework for determining the Stillinger-Weber (SW) potential parameters for modeling fracture in graphene and carbon nanotubes. In addition to fitting the equilibrium material properties, the approach allows fitting the potential to the forcing behavior as well as the mechanical strength of the solid, without requiring ad hoc modification of the nearest-neighbor interactions for avoiding artificial stiffening of the lattice at larger deformation. Consistent with the first-principles results, the potential shows the Young's modulus of graphene to be isotropic under symmetry-preserving and symmetry-breaking deformation conditions. It also shows the Young's modulus of carbon nanotubes to be diameter-dependent under symmetry-breaking loading conditions. The potential addresses the key deficiency of existing empirical potentials in reproducing experimentally observed glass-like brittle fracture in graphene and carbon nanotubes. In simulating the entire deformation process leading to fracture, the SW-potential costs several factors less computational time compared to the state-of-the-art interatomic potentials that enables exploration of the fracture processes in large atomistic systems which are inaccessible otherwise.

Using density functional theory simulations, we examine the electronic structure of an isolated monovacancy defect in graphene under symmetry-breaking deformation. Results show that the defect experiences a second-order Jahn–Teller reconstruction at a critical strain of 1.7%. It stabilizes the orientation of the JT bond relative to the loading direction and breaks the threefold degeneracy of the defect structure. We call it Jahn–Teller re-reconstruction (JTRR), and it is mechanically reversible. The reversibility and stabilization of the orientation depend on the direction cosine between the JT bond and the loading direction. Also, a change in the loading direction by 90° can change the orientation of the JT bond by 120°. An atomic-scale analysis suggests that the maximum bond force arising from “the derivative of the kinetic energy of electrons” defines the critical strain. JTRR alters the electron occupation in the individual electronic orbitals at the defect site. The electronic charge redistribution and the density of states at the defective sites reveal that the pz orbitals dominate the reconstruction process. Furthermore, JTRR changes the magnitude of the magnetic moment at the defective site from 1.36 μB to 1.22 μB. This unravels a new way of controlling the magnetic behavior of monovacancy by applying symmetry-breaking mechanical strain. Results also show that passivation of the dangling bond can subside or eliminate the reconstruction process depending on the number of valence electrons available in the passivating atom.

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Applying a combination of atomistic and continuum scale simulations, we show that stress-localization forms the fundamental basis for toughening in “carbon nanotube reinforced amorphous silica” (CNT−aSiO2). Depending on the cohesive strength of the interface, a propagating crack renders three distinct types of failure conditions: (i) with stronger cohesive interactions both silica and nanotube undergo catastrophic failure, (ii) with moderate cohesive interactions the nanotube debonds from the matrix and undergoes severe mechanical deformation but fracture remains in the matrix, and (iii) with lower cohesive strengths the nanotube debonds from the matrix easily and allows quicker failure of the matrix, compared to the previous two failure conditions. For either of the cases, continued propagation of the crack requires renucleation at the opposite side of the nanotube. However, the renucleation criteria are mostly unaffected by the strength of interfacial interactions. Also, the effective toughness of the nanocomposite increases nonlinearly with increasing interfacial strength and the maximum possible toughness enhancement is strictly controlled by the strength of elastic interactions between the nanotube and the matrix. The overall toughening behavior of the nanocomposite is governed primarily by stress-localization at the nanotube–silica intersections along the projected crack path direction in the composite. The observations highlight the dramatic role of site-selective interatomic interactions that can affect the macroscopic mechanical behavior of the nanocomposite substantially.