Why does the cross section of a metal after a hypervelocity impact look like that?

Question

Look at this image of a cross section of a piece of (I assume) metal after a hypervelocity impact: I understand where the crater shape comes from, but what puzzles me is the cavity near the bottom: How did this form? Is this a usual feature that forms under such impacts? What was in there before the piece was cut in half?

Answer 1

I believe that this is a form of spallation. The impact causes a stress wave which travels through the material. These waves reflect off the back and sides of the plate, leading to a region of even greater stress than found elsewhere in the material, located slightly above the "bottom" of the plate. You can see a simulation of this here (YouTube link here). In the video, notice how the leading edge of the stress wave reflects off the back of the plate and interferes with the lagging portions of the wave, leading to a region of greater stresses than seen in any part of the wave prior to its reflection. If these stresses exceed the tensile strength of the material, then it will fracture in this characteristic sort of "internal cavitation" way.

You can find a little more info here. They have additional photos of this phenomenon, and also attribute it to spallation. From the above link:

Spallation occurs when a dynamic compressive pulse impinges on a free surface, where it is reflected as a rarefaction pulse. The associated tensile stress may lead to internal spallation fractures ...