Crack Propagation in Crystals
Dominic Holland and Michael Marder
Center for Nonlinear Dynamics

GIF Image (104 KB)
Quicktime movie (4.4 MB)

This first simulation shows crack initiation in silica. New crystal is constantly fed to the advancing crack, while cracked material is lopped off at the tail. However, at any stage the simulation involves about 34,000 atoms, with each atom having up to 40 interacting neighbors. In the close-ups shown here, the system is almost as deep as it is high (approximately 20 Angstroms), but with periodic front and back boundaries. Silicon atoms are blue and oxygen atoms are red, with shadings to brighter colors for higher kinetic energies. The strain is being increased very slowly along the vertical. Crack initiation occurs suddenly, with full speed of about 2000 m/s achieved almost instantaneously.

Fracture occurs by the breaking of atomic bonds. One of the best ways to analyze the atomic effects in dynamic fracture is by simulation, understanding of which is greatly facilitated by visualization.

This simulation was run on the High Performance Computing Facility Cray J916 parallel vector processor, using code developed at the Center for Nonlinear Dynamics.

Images were rendered on CNLD computer systems, then recorded onto video tape and converted into animation files in the Visualization Lab.

GIF Image (131 KB)
Quicktime movie (793 KB)

This second simulation shows stable crack propagation in silicon.

When a piece of material is not stretched too much, a crack traveling through the material can be atomically sharp, leaving perfectly flat surfaces behind. This is brittle fracture, with the crack behaving in a very stable manner. In this movie clip, a crack is traveling at 1.8 km/s through a piece of silicon that is strained so that about twice the minimum energy needed to create new surfaces is being supplied to the tip. Red indicates high kinetic energy. The movie clip is designed to be viewed with a program that "loops" from the end back to the beginning.

This simulation was run on the Texas Advanced Computing Center Cray T3E. Images were rendered at the Center for Nonlinear Dynamics, and recorded onto video tape and converted to animation files in the Visualization Lab.

GIF Image 1 (185 KB)
GIF Image 2 (184 KB)
Quicktime movie (1.9 MB)

Lastly, this third simulation shows unstable crack propagation in silicon.

At high strains, cracks go unstable because the energy flowing to the crack tip is too great -- the crack is no longer able to behave in a smooth, sharp way, but advances by blunting, emitting dislocations, jumping planes, branching, and creating rough surfaces. In the movie clip, a crack is traveling at approximately 3.6 km/s in silicon. It cannot go any faster. Higher strain would just mean more damage. About five times the minimum energy that is needed for fracture is being supplied by straining.