IBM and LLNL Scientists Show Supercomputer Advance in Predicting Materials Strength

"Computational microscope" can aid understanding of fracture, from small crystals to large earthquakes

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San Jose, CA, USA - 30 Apr 2002: An unprecedented billion-atom calculation has enabled a team of IBM and Lawrence Livermore National Laboratory (LLNL) scientists to demonstrate a major advance in using supercomputers to simulate the strength of materials.

Using one of the world's most powerful supercomputers as a computational microscope, the scientists can peer deep inside simulated materials to reveal how they break, as well as what makes them strong or weak, stiff or flexible. Calculating the strength of new materials is a critical issue in creating structures as small as microprocessors -- or as large as buildings or airplanes -- that will withstand real-world forces.

The scientists' results are also a major step toward using supercomputers to design new materials with customized properties, such as their levels of strength, hardness and toughness.

"The sudden unexpected fracture of a material can have devastating consequences, such as during an earthquake or the failure of an airplane structure," said Farid F. Abraham, the researcher from IBM's Almaden Research Center in San Jose, Calif., who led the team effort. "Today's supercomputers and our innovative software allow us to understand their properties much better and how they deform and break."

In the most extensive computer calculations of their type to date, the scientists used the ACSI White supercomputer, which was built last year by IBM for LLNL, to create and then deform simulated cubes of as many as 1 billion atoms. Creative computer visualization techniques revealed the inner workings of the atoms' response to the stress: stunning images and videos showing cracks moving at surprising supersonic speeds as well as the expanding tangle of defects deep inside the cube that can harden a tough, flexible material to the point of brittle fracture.

"Handling the data was a research project in itself," said Tomas Diaz de la Rubia, LLNL physicist. "Visualizing and navigating within huge datasets such as these is a milestone of the Accelerated Strategic Computing Initiative (ASCI) project that we have now achieved."

Details and results of the computer simulation experiments are published in two technical papers and the cover illustration of Tuesday's (April 30) online edition of the prestigious Proceedings of the National Academy of Sciences.

Technical details
The first paper describes a 20-million-atom simulation that shows how brittle-fracture cracks can travel far faster than theory had previously predicted. This result is expected to be important in helping scientists understand a wide ranges of fractures -- from shallow earthquakes to the sudden failure of fiber-reinforced composite materials, such as those used in airplanes. The second paper recounts a 1-billion-atom simulation of "work hardening" -- the process by which deformation strengthens a material but can embrittle it if overdone. Bending a paper clip back and forth is an example of work hardening: the metal is initially rather flexible, but it soon stiffens and breaks where it was repeatedly stressed. Work hardening also strengthens materials during forging, an important manufacturing technique used to make products as diverse as critical auto parts and golf clubs.

In each of the simulations, which required up to 10 days of around-the-clock computations, the supercomputer calculated the forces between each of the atoms and its neighbors and their positions as the edges of a notched cube or atoms were pulled apart. In the brittle fracture simulation, a crack formed at the notch and traveled rapidly through the material as stress concentrated at the crack tip and ripped apart the chemical bonds that held nearby atoms together. The IBM/LLNL scientists found that when the material is given the property of becoming stiffer, not weaker, as it is stressed --as occurs with certain polymers and rubbers -- the crack tip can shoot through the material at supersonic speeds (that is, faster than the speed of sound in that material). Such behavior was long thought to be impossible. But in recent years, supersonic crack speeds have been observed directly, or suspected, in both laboratory experiments and two devastating 1999 earthquakes in Turkey. The IBM/LLNL simulation gives a sound theoretical footing to such claims and will result in improved tools to understand and predict the behaviors of earthquakes and to design new materials that can resist brittle fracture.

In the work hardening computation, the simulated material was made to be tough, not brittle. That meant that the atoms would initially respond to stress by sliding past each other rather than simply breaking apart. The offset atoms create lines of misalignment in the periodic structure of the material that are called dislocations. In a soft metal under stress, such dislocations simply pass through the material as deformation occurs. But in a stronger or more complex material, various dislocations collide, which halts further atomic motion at each intersection. As deformation continues, these pinned dislocations accumulate, initially increasing the strength of the material because it can resist a greater force. But if the stress continues, the density of pinned dislocations can become so great that the material turns brittle and breaks.

In addition to Abraham and Diaz de la Rubia, co-authors were Robert Walkup of IBM's T.J. Watson Research Center, Yorktown Heights, N.Y.; Huajian Gao, a visiting scientist at IBM-Almaden now at the Max Planck Institute for Metals Research in Stuttgart, Germany; and Mark Duchaineau and Mark Seager of LLNL.

Over the past three decades, Abraham has been a pioneer in using the most powerful computers available to calculate and predict materials properties. In 1985, he was able to model 200,000 atoms -- a flat square having only 450 atoms on a side. In July 1994, he published a million-atom simulation, including the first-ever World Wide Web video linked from a scientific paper. ("Instability Dynamics of Fracture: A Computer Simulation" by F. F. Abraham, D. Brodbeck, R. A. Rafey and W. E. Rudge, Physical Review Letters, Vol. 73, No. 2, 11 July 1994, pp. 272-275.)

Ductile Starting conditions:
A cube of more than a billion atoms (1,008 atoms on a side) has two notches (each 90 atom layers deep) cut into the middle of opposing faces. The crystal is stretched outward by 4 percent in the horizontal (left-right) direction by a force pulling apart the cube. To show how the atoms respond to this stress, we render invisible those atoms with a normal, bulk-material potential energy. This visualization trick allows us to look deep inside the material and see only those atoms in deformed areas, such as dislocations and cracks in what began as a perfect crystal structure.

Ductile Simulation summary:
Outward forces pull this cube's left- and right-hand faces. At the tips of the two notches, atoms begin to slide past each other, forming dislocations, which travel through the material like a ripple in a rug. A spaghetti-like network of many dislocations is created as the material deforms. Colliding dislocations create immobile imperfections that are barriers to mobile dislocations. As additional force is applied, the three-dimensional train wreck of colliding dislocations hardens the material, eventually making it prone to brittle failure.

Ductile failure videos:
· Large (full, 480x360 pixels) (63 Mbyte)
· Medium (fewer frames, 360x240 pixels) (13 Mbyte)
· Small (partial, fewer frames, 360x240) (1.8 Mbyte)

Supersonic crack Starting conditions:
The figures show two simulations, each consisting of two weakly joined crystals containing a total of about 20 million atoms. In the bottom simulation (called the "harmonic" case), the attraction between the atoms remains constant as they are pulled apart, like a spring. In the top ("anharmonic") simulation, the atomic attraction increases as the material is stressed, as it does for rubbery polymers. A pre-existing crack exists on the left-side boundary between the joined crystals. In the simulation, a shear force is applied to the joined crystals (pushing to the left on the upper crystal and to the right on the lower crystal). The images track the displacement of the atoms and growth of the crack in reponse to the force.

Supersonic crack Simulation summary:
A shear force applied to each of the joined crystals causes the crack on the left to grow toward the right. The formation of two Mach cones in the top simulation indicates that the crack is traveling supersonically through the anharmonic material. In the lower simulation, the single Mach cone indicates the crack is traveling through the harmonic material at the speed of sound.

Supersonic crack propagation videos:
· Large (full, 780x430 pixels) (12 Mbyte)
· Small (fewer frames, 390x215 pixels) (2 Mbyte)

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