The researchers utilized the power of the year-old ASCI White supercomputer, which can complete more than 1013 computations a second, to model the behavior of up to one billion atoms. "The sudden unexpected fracture of a material can have devastating consequences, such as during an earthquake or the failure of an airplane structure," notes Farid F. Abraham of IBM's Almaden Research Center. To better understand the processes at work, he and his colleagues simulated cubes of various materials and tracked the responses of individual atoms during two types of failure. In the first, brittle fracture, bonds between neighboring atoms are ruptured, which happens when glass breaks. In the second, so-called work hardening, a material initially becomes flexible in response to applied pressure but breaks after repeated stress. A bent paper clip often succumbs to this phenomenon.
Each simulation required up to 10 days of computations, during which the supercomputer calculated the forces between each of the atoms and its neighbors. In the first experiment, which modeled a two-layer solid that experienced brittle fracture, the scientists found that a crack could travel through the material at speeds greater than the speed of sound. Although such supersonic crack speeds were previously thought to be impossible, they have been observed or suspected both in laboratory experiments and in recent earthquakes in Turkey. The new results lend further theoretical support to such observations.
In the second experiment, the supercomputer modeled atomic interactions as the edges of a simulated cube made of a ductile substance were pulled apart. The atoms in such a material initially respond to stress by sliding past one another, creating so-called dislocations of misaligned atoms. The computations revealed the formation and interaction of hundreds of atomic ripples. In some materials, dislocations can simply pass through the material, but if the stress continues in stronger substances, dislocations get pinned together, eventually causing the material to turn brittle and break.