Atomistic simulation of crack tip shielding effect due to embedded nanoparticles in amorphous carbon

It is now well documented in the literature that the inclusion of nanoparticles, such as graphene nanoplatelets (GNP), in matrix materials, such as epoxy, has resulted in significantly improved fracture toughness in mode I and mixed-mode. One of the mechanisms postulated to increase the effective crack initiation fracture toughness is the crack tip shielding effect due to nanoparticles in the fracture process zone. This effect is deemed to arise due to debonding of nanoparticles from the matrix material in the process zone, which in turn reduces the stress state at the tip of the primary crack via shielding. Thus, nanoparticles act to redistribute stress in the crack tip region, thereby lowering the near tip stress intensity factor, depending on their orientation relative to the crack. Therefore, higher far-field loads can be achieved before the critical stress intensity is reached at the crack tip. In this paper the K-test approach is used in conjunction with molecular dynamics (MD) to model fracture in an amorphous carbon matrix material, with embedded GNPs. Amorphous carbon matrix is deliberately selected to facilitate the computational efficiency of the solution process, because the fracture process zone size for amorphous carbon is relatively small from a MD simulation viewpoint. The effect of GNPs on the shielding of the crack tip, with varying orientation and location relative to the crack is investigated using detailed virial stress plots, the atomistic J-integral, and compared with linear elastic fracture mechanics (LEFM) results.