A large crater is formed in a thick aluminum target by a lexan (plastic) projectile similar to the one shown in the photograph (above right). Note the petaled lip surrounding the crater created during the melting phase of the aluminum. The impact velocity in this test was about 6.92 km/s.

During a hypervelocity impact between two metal objects, the metals can behave for short periods of time like fluids. This phenomenon, called hydrodynamic flow, is not a result of temperature melting the metal, but rather is caused by extremely high stresses concentrations generated within the metal during impact. Impacts with sufficient velocity and energy can induce shock waves in the impacted metals which are accompanied by extremely high stresses (millions of pounds per square inch). The stresses are so high, in fact, that they cause the metal to flow, like a fluid.

The result of hydrodynamic flow is seen in the above photograph by the crater in the thick aluminum target. Note the lip surrounding the crater. Impact into this thick, semi-infinite target is very much like a drop of fluid hitting a liquid surface.