Using classical molecular dynamics simulations and the virial description of atomic stress, this paper reveals that effective toughness and strength in defective diamond nanowires (NWs) are governed by the geometric confinement of atoms at the critical sites of the NWs. Results suggest existence of five characteristic regimes in defective NWs under applied deformation. They include the defective regime, the surface regime, the core regime, the surface-core intersection regime, and the defect-core intersection regime. The defective regime and the surface regime soften the NW and carry the maximum angular deformation at the expense of breaking local symmetry of atomic structure. On the other hand, the intersecting regimes tend to preserve the symmetry, carry the most aggressive linear deformation, and act as the critical sites for localization of atomic stress. In the defect-core intersecting regime, the differential response of its neighboring regimes to linear and angular deformations as well as the elastic fields emanating from its critical sites impose an intricate effect comprising geometric confinement and propensity to retain local symmetry. Consequently, the highest localization of elastic energy and atomic stress takes place at the critical sites of the defect-core regime and it controls the effective toughness and strength behavior of the defective nanowires. Although size-dependent variation in strength and toughness is controlled primarily by surface softening, defect-induced alteration of strength and toughness is governed by the geometric confinement. Furthermore, an atomistic analysis reveals that the localized stress fields grow radially outward from the defective regime for defective NWs, whereas in defect-free NWs they evolve inward from the surface.
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