Large-Scale Molecular Dynamics Simulations of Three-Dimensional Ductile Fracture

We have performed massively parallel 3D molecular dynamics simulations with up to 35 million atoms to investigate ductile fracture. We have obtained mechanistic information at the atomistic level inaccessible to experiment. We observe dislocation loops emitted from the crack front|the rst time this has been seen in computer simulations. The sequence of dislocation emission (crack blunting process) strongly depends on the crystallographic orientation of the crack front and diiers strikingly from anything previously conjectured. We nd that dislocation emission modes, jogging or blunting, are very sensitive to boundary conditions and interatomic potentials. Understanding the fracture behavior of materials is crucial to the development of new materials with high strength and toughness. Cracks and dislocations are the two major defects determining these mechanical properties. While continuum theory can successfully describe the long-range strain elds of cracks and dis-locations, atomistic simulations are required to characterize their core regions. In the past, because only a small number of atoms could be simulated, various boundary treatments were employed to provide a static crack stress eld e.g., 1, 2]]. In addition to their lack of dynamic response, those boundary treatments are only valid until the rst dislocation travels a fraction of the computational cell away from the crack tip. Furthermore, intrinsic three-dimensional (3D) features of dislocations and cracks can not be properly investigated with only a few atoms allocated in the direction of the crack front. With the advent of massively parallel computers, simulations of many millions of atoms are now feasible 3, 4, 5], allowing us to avoid using, at least for early times, complicated boundary treatments. We have developed a 3D MD code, SPaSM, (Scalable Parallel Short range Molecular dynamics) 4], designed for very large scale simulations on a variety of parallel computing platforms. In addition to the computation of the many-body trajectory, SPaSM allows us to visualize, lter, and analyze the huge amount of data produced from a simulation of millions of atoms. To demonstrate the eeectiveness of this new powerful tool and to investigate fracture behavior, we have simulated the process of dislocation emission from a 3D crack. Crack blunting by dislocation emission enhances the ductility of crystalline materials 6]. There are two kinds of 3D dislocation emission processes: blunting and jogging. In the blunting connguration the dislocation is on a slip plane inclined to the crack plane and containing the crack front. The emission causes the crack to blunt in …