Atomistic Measures of Materials Strength

We examine the role of atomistic simulations in multiscale modeling of mechanical behavior of stressed solids. Theoretical strength is defined through modes of structural instability which, in the long wavelength limit, are specified by criteria involving elastic stiffness coefficients and the applied stress; more generally, strength can be characterized by the onset of soft vibrational modes in the deformed lattice. Alternatively, MD simulation of stress-strain response provides a direct measure of the effects of small-scale microstructure on strength, as illustrated by results on SiC in single crystal, amorphous, and nanocrystalline phases. A Hall-Petch type scaling is introduced to estimate strength of laboratory specimens containing microstructural flaws of certain critical size. A preliminary simulation of Cu thin film nano-indentation is described as a means of probing the ideal shear strength. The challenge of formulating a local measure of the driving force for defect motion is briefly discussed.