Using molecular dynamics with an accurate many-body potential, we studied the rapid expansion of Ta metal following the high compression ͑50 to 100 GPa͒ induced by high velocity (2 to 4 km/s͒ impact. We find that catastrophic failure in this system coincides with a critical behavior characterized by a void distribution of the form N(v)ϰV Ϫ , with ϳ2.2. This corresponds to a threshold in which percolation of the voids results in tensile failure. We define an order parameter ( , the ratio of the volume of the largest void to the total void volume͒ which changes rapidly from ϳ0 to ϳ1 when the metal fails and scales with as ϰ( Ϫ c )  with exponent ϳ0.4, where is the total void fraction. We found similar behavior for FCC Ni suggesting that this critical behavior is a universal characteristic for failure of solids in rapid expansion. DOI: 10.1103/PhysRevB.63.060103 PACS number͑s͒: 62.50.ϩp, 62.20.Fe, 62.20.Mk, 64.60.Ht Despite much progress in characterizing and analyzing the mechanical processes controlling the strength of materials, there are still many uncertainties regarding the dynamic failure of metals, particularly for the atomistic phenomena underlying macroscopic failure. To study such process at the atomistic level, we developed a first principles-based force field ͑FF͒ for Ta. We then applied this FF to examine dynamic failure using a high velocity plate colliding with a static target. Upon impact compressive waves travel both into the target and projectile. When these waves reach the free surfaces they reflect, becoming tensile, and propagate back into the material. When the two tensile waves meet, the material is subjected to a high tension, and for sufficiently high impact velocity the material will fail dynamically. This process, known as spallation, leads to failure of the material through formation, growth, and coalescence of microvoids or cracks.Spallation has been studied extensively both experimentally and theoretically. The most widely used experimental methods are plate impact 1 and, more recently, high power pulsed laser shock generators. [2][3][4][5][6] Strain rates up to 10 8 sec Ϫ1 can be achieved with laser shocks, higher than those in plate impact experiments.A complete description of spallation involves understanding and linking processes taking place at very different time and space scales, ranging from atomic size ͑vacancy coalescence and initial void formation͒ to microns ͑plastic deformation and linkage of micron-sized voids͒. Consequently theoretical studies of spallation have used methods ranging from microscopic molecular-dynamics ͑MD͒ simulations, 7 to mesoscale micromechanical models and continuum descriptions. 1,8,9 We report here a MD study of the initial stages of spallation ͑length scales of nm and time scales of ps͒ that provides a detailed atomistic description for the time evolution of the process. We focus on characterizing the dynamical evolution of the void distribution.To simulate high velocity impact, we assign a specific relative velocity to previously thermaliz...