Cracks dynamics in brittle crystals by 3D Molecular dynamics simulations

David Wang Auditorium, 3rd floor Dalia Maydan Bldg.
Mr. Guy Kovel, PhD Candidate

Mr. Guy Kovel, PhD Candidate (Haifa Campus)

Material science and Eng., Technion,

Recent fracture cleavage experiments of silicon crystal specimens indicate that atomistic scale effects at the crack front influence the macroscopic brittle cracks dynamical behavior, emphasizing the importance of atomistic work to the understanding of fracture mechanics.

However, atomistic simulations have yet to provide an accurate representation of brittle fracture experiments as a boundary value problem (BVP) with appropriate boundary conditions (BC). For example, atomistic simulations so far have shown the existence of “lattice trapping” effect, where dynamic brittle crack propagation requires energy of about 60% above the Griffith barrier of 2γs to initiate the crack, which then is bursting at high speed. In contrast, recent lab experiments show crack propagation occurring at energy close to the Griffith barrier and at velocities as low as 1 mm/sec, far lower than predicted by atomistic simulation. We attempt to understand and address some of the issues that contribute to this discrepancy.

Molecular Dynamic simulations of static and dynamic crack in the (110)[11 ̅0] cleavage system of a silicon-like brittle crystal were performed. We first examined the applicability of the square root singularity and the Irwin K-G relationship using Psedo-2D atomistic models. We show that a square root singularity exists at the crack front, and a minimal model size, that is much larger than the existing models, is required to acquire this relationship.

We also employed a more realistic full 3D boundary value problem, instead of the commonly used Pseudo-2D computational setup, that allows both a double kinking mechanism for crack propagation and the formation of a more realistic curved crack front, which reduces the ‘lattice trapping’ effect. We also examined the effects of computational specimen size on the speed-energy relations. Finally, we will discuss the issue of a required full 3D specimen, appropriate for atomistic problems containing a crack.

Prof. Dov Sherman