Critical behavior in spallation failure of metals

Using molecular dynamics with an accurate many-body potential, we studied the rapid expansion of Ta metal following the high compression (50 to 100 GPa) induced by high velocity (2 to 4 km/s) impact. We find that catastrophic failure in this system coincides with a critical behavior characterized by a void distribution of the form N(v)∝V-τ, with τ∼2.2. This corresponds to a threshold in which percolation of the voids results in tensile failure. We define an order parameter (φ, the ratio of the volume of the largest void to the total void volume) which changes rapidly from ∼0 to ∼1 when the metal fails and scales with as φ∝(ρ-ρc)β with exponent β∼0.4, where ρ 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.