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, with t;2.2. This corresponds to a threshold in which percolation of the voids results in tensile failure. We define an order parameter (f , 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 f}(r2rc) b with exponent b;0.4, where r 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.